In situ characterization of ALD processes and study of reaction mechanisms for high-k metal oxide formation
Yoann Tomczak
Laboratory of Inorganic Chemistry
Department of Chemistry
Faculty of Science
University of Helsinki
Finland
ACADEMIC DISSERTATION
To be presented with the permission of the Faculty of Science of the University of Helsinki for public criticism in Auditorium A110 of the Departement of Chemistry, A.I Virtasen aukio
1, on 06.06.2014 at 12 o,clock noon
Helsinki 2014
2
© Yoann Tomczak
ISBN: 978-952-10-9925-0 (paperback) ISBN: 978-952-10-9926-7 (PDF version)
http://ethesis.helsinki.fi
University of Helsinki
Helsinki 2014
3
Supervisors
Professor Mikko Ritala and
Professor Markku Leskelä
Laboratory of Inorganic Chemistry Department of Chemistry
University of Helsinki Helsinki, Finland
Reviewers
Professor Martyn Pemble Tyndall National Institute and University College Cork
Cork, Ireland
Professor Charles H. Winter Wayne State University
Detroit MI, U.S.A
Opponent
Professor Annelies Delabie Katholieke Universiteit Leuven
Leuven, Belgium
4
Abstract
Atomic Layer Deposition (ALD) is a thin-film deposition method allowing the growth of highly conformal films with atomic level thickness and composition precision. For most of the ALD processes developed, the reaction mechanisms occurring at each step of the deposition remain unclear. Learning more about these reactions would help to control and optimize the existing growth processes and develop new ones more quickly. For that purpose, in situ methods such as quartz crystal microbalance (QCM) and quadrupole mass spectrometer (QMS) present numerous advantages: They provide direct information on the growth mechanisms during the process. Additionally, they do not require separate experiments or large amounts of precursors to test the efficiency of new processes and could be very effective means to monitor industrial processes in real time. This thesis explores the most common in situ analytical methods used to study ALD processes. A review on the ALD metal precursors possessing ligands with nitrogen bonded to the metal center and their reactivity is provided. The results section reports the reaction mechanisms of the following ozone and water based ALD processes using precursors with nitrogen bonded ligands for the deposition of high-k and other oxide films:
The tBuN=M(NEt2)3 (M = Nb, Ta) water or ozone processes for the deposition of Nb2O5 and Ta2O5: The influence of the nature of the metal on the adsorption mechanism of precursor with similar heteroleptic ligands was highlighted.
The Si2(NHEt)6, Al(CH3)3 and water process for the deposition of ternary AlxSiyOz oxide: A
theoretical model based on the in situ results was used to relate the film composition with the proposed reaction mechanisms.
The LiN[Si(CH3)3]2 – O3 process for the deposition of Li2SiO3 : Dissociative adsorption of the bimetal precursor was observed. Additionally, as the temperature increases the adsorption of HN[Si(CH3)3]2 becomes more efficient and thus more silicon is incorporated in the film, decreasing the Li/Si ratio and increasing the growth rate.
The Ti(OiPr)3(NiPr-Me-amd) and Ti(NMe2)2(OiPr)2 with water and ozone processes for the
deposition of TiO2: In the water based processes, the amidinate precursor showed adsorption with decomposition of its ligand while the alkoxy/alkylamide precursor adsorbed through ligand exchange. The ozone processes showed combustion reactions during both metal precursor and ozone pulses. Additionally, the evolution of the reaction mechanism with temperature and its influence on the growth characteristics are reported.
The [Zr(NEtMe)2(guan-NEtMe)2] – water process for the deposition of ZrO2: Decomposition of the guanidinate ligand upon reaching the surface was observed during adsorption of the metal precursor.
5
Preface The studies reported in this thesis were carried out in the Laboratory of Inorganic Chemistry at the
Department of Chemistry of the University of Helsinki during the years 2010-2014. I am very grateful
to my supervisors Prof. Mikko Ritala, Prof. Markku Leskelä and Dr. Kjell Knapas for their guidance,
support and responsiveness during these 4 years. In addition, I would like to acknowledge the
reviewer of this thesis, Prof. Charles H. Winter and Prof. Martyn Pemble.
I would like to thank my co-workers at the inorganic chemistry laboratory who have helped me during
the thesis: Dr. Timothee Blanquart, Dr. Jani Hämäläinen and Dr Jaakko Niinistö for their contribution
to process development and their help with technical issues; Dr. Suvi Haukka for the constructive
discussions we had on reaction mechanisms; Dr. Marianna Kemell, Mikko Heikkilä and Markku
Sundberg for their time and assistance with EDS, XRD and computational results, respectively. I also
want to thank all the other members of the ALD and Catlab groups for their welcome and the good
ambiance in the lab.
The research leading to this thesis was funded by the European Community’s Seventh Framework
Program (FP7/2007-2013) under Grant Agreement ENHANCE-238409 and was also supported by
the Finnish Centre of Excellence in Atomic Layer Deposition. I wish to thank Dr. Harish Parala, Prof.
Anjana Devi, Prof. Erwin Kessels and all the other supervisors involved in ENHANCE for the
exceptional opportunities and trainings they provided during the program.
I also want to thank all my ENHANCE research fellows for the good times we had together: Matthieu
Weber, Valentino Longo, Marco Gavagnin, Dr. Daniela Beckermann, Dr. Quentin Simon, Brunot
Gouat, Dr. Nabeel Aslam, Dr Manish Banerjee and Babu Srinivasan, whishing them a bright and
successful future.
Special thanks also to my parents, my brother Pierre and my friends Tibi, Alexis, Franky and Fred for
their psychological support.
Finally, I would like to thank my wife Sorina for her love and understanding during all these years.
Helsinki, May 2014
Yoann Tomczak
6
Table of contents Abstract ................................................................................................................................................... 4
Preface .................................................................................................................................................... 5
Table of contents .................................................................................................................................... 6
List of publications .................................................................................................................................. 8
List of acronyms used ............................................................................................................................. 9
1. Introduction ...................................................................................................................................... 10
2. Background ....................................................................................................................................... 11
2.1 Atomic layer deposition .............................................................................................................. 11
2.2 In situ analytical methods used for reaction mechanism studies in ALD ................................... 12
2.3 Precursors with nitrogen donor ligands used in ALD .................................................................. 15
2.3.1 Homoleptic alkylamides ....................................................................................................... 16
2.3.2 Disilazides ............................................................................................................................. 17
2.3.3 Amidinates ........................................................................................................................... 17
2.3.4 Guanidinates ........................................................................................................................ 18
2.3.5 Heteroleptic alkylamides ..................................................................................................... 18
2.4 Metal precursor adsorption mechanisms in ALD ........................................................................ 19
2.4.1 Chemisorption and dissociative adsorption ........................................................................ 20
2.4.2 Ligand exchange mechanism ............................................................................................... 21
2.4.3. Precursor adsorption through ligand decomposition ......................................................... 22
2.4.4. Partial combustion .............................................................................................................. 22
2.5 Reactions of precursors with nitrogen donor ligands ................................................................. 23
2.5.1 Alkylamides .......................................................................................................................... 23
2.5.2 Hetroleptic alkylamides ....................................................................................................... 24
2.5.3 Amidinates ........................................................................................................................... 25
2.5.4 Guanidinate .......................................................................................................................... 27
2.5.5 Disilazide .............................................................................................................................. 28
7
3. Experimental ..................................................................................................................................... 30
4. Results and discussion ...................................................................................................................... 32
4.1 Nb2O5 and Ta2O5 .......................................................................................................................... 32
4.2 AlxSiyO .......................................................................................................................................... 33
4.3 Li2SiO3 .......................................................................................................................................... 35
4.4 TiO2 .............................................................................................................................................. 37
4.5 ZrO2 ............................................................................................................................................. 39
5. Conclusions ....................................................................................................................................... 41
6. References ........................................................................................................................................ 43
8
List of publications
This work is based on the following publications which are referred to with Roman numerals:
I. Y. Tomczak, K. Knapas, M. Sundberg, M. Ritala, M. Leskelä “In situ reaction mechanism
studies on the new tBuN=M(NEt2)3 -Water and tBuN=M(NEt2)3 - Ozone (M=Nb,Ta) Atomic
Layer Deposition processes.” Chem. Mater.(2012), 24(9), 1555-1561
II. Y. Tomczak, K. Knapas, S. Haukka, M. Kemell, M. Heikkilä, M. Ceccato, M. Leskelä, M.
Ritala “In situ reaction mechanism studies on atomic layer deposition of AlxSiyOz from
trimethylaluminium, hexakis ethylaminodisilane and water.” Chem. Mater.(2012), 24(20),
3859-3867
III. Y. Tomczak, K. Knapas, M. Sundberg, M. Leskelä, M. Ritala “In situ reaction mechanism
studies on lithium hexadimethyldisilazide and ozone atomic layer deposition process for
lithium silicate.” Journal of Physical Chemistry C (2013), 117(27), 14241-14246
IV. Y. Tomczak, K. Knapas, M. Ritala, M. Leskelä “In situ reaction mechanism studies on the
Ti(NMe2)2(OiPr)2-D2O and Ti(OiPr)3(NiPr-Me-amd)-D2O Atomic Layer Deposition
processes” Journal of Vacuum Science and Technology A: Vacuum, Surfaces, and Films
(2014), 32(1), 01A121-01A121-7
V. T. Blanquart, J. Niinistö, N. Aslam, M. Banerjee, Y. Tomczak, M. Gavagnin, V. Longo, E.
Puukilainen, H.D. Wanzenböck, W.M.M. Kessels, A. Devi, S. Hoffmann-Eifert, M. Ritala,
and M. Leskelä “[Zr(NEtMe)2(guan-NEtMe)2] as a novel ALD precursor: ZrO2 film growth
and mechanistic studies” Chem. Mater.(2013), 25(15), 3088-3095
VI. M. Kaipio, T. Blanquart, Y. Tomczak, J. Niinistö, M. Gavagnin, V. Longo, V. Pallem, C.
Dussarrat, M. Ritala, M. Leskelä “Atomic layer deposition, characterization and growth
mechanism of high quality TiO2 thin films” submitted
The publications are reprinted with the permission from the copyright holders. For publications I-IV,
the author has planned the research with the help of the coauthors, performed the experiments, wrote
the first versions of the papers and completed them for submission with the help of the coauthors. In
publications V-VI, the author has performed the reaction mechanism studies and written that part of
the article.
9
List of acronyms used
AFM Atomic force microscopy
ALD Atomic layer deposition
amd Amidinate, RN(CR’)NR
Cp Cyclopentadienyl, -C5H5
CN Coordination number
CVD Chemical vapor deposition
DFT Density functional theory
DRAM Dynamic random access memory
DL Detection limit
EDX Energy-dispersive X-ray spectroscopy
Et Ethyl, -C2H5
FT-IR Fourier transform infrared spectroscopy
GPC Growth per cycle
GR Growth rate
guan Guanidinate, NR(CNR’R’’)NR
HMDS Hexamethyldisilazane iPr Isopropyl, -CH(CH3)2
Me Methyl, -CH3
ML Monolayer
NOx Nirous oxides: NO, NO2, N2O…
PEALD Plasma enhanced ALD
QCM Quartz crystal microbalance
QMS Quadrupole mass spectrometer
SE Spectroscopic ellipsometry
SEM Scanning electron microscopy tBu tert-butyl, C(CH3)3
ToF-ERDA Time-of-flight elastic recoil detection analysis
TPD Temperature-programmed desorption
HV High vacuum
XPS X-ray photoelectron spectroscopy
XRF X-ray fluorescence
XRR X-ray reflectometry
10
1. Introduction In the microelectronics industry, the size of the devices continuously decreases to follow Moore´s law.
To further enhance their efficiency, the geometry of MOSFETs is shifting from planar to three
dimensional. Consequently, thin film deposition methods used for producing these devices have to
adapt to produce thinner and more conformal films. In this context, Atomic Layer Deposition (ALD) is
a method of choice because it allows precise control over the deposited film thicknesses down to the
atomic level and exhibits perfect conformality even on structures with high aspect ratios.1-3 This
method permits the deposition of a wide range of materials, from pure noble metals to transition metal
oxides.4
Many types of metal precursors have been investigated for the deposition of high-k metal oxides:
halides, alkyls, alkoxides, alkylamides, amidinates and guanidinates are a few examples of the types
of ligands used in chemical vapor deposition (CVD) and its pulsed version, ALD. Good volatility,
thermal stability and reactivity of the metal precursors are important prerequisites for their use in
industry. In addition, the purity of the films deposited with these precursors and the compatibility of the
ALD method with the other steps of the device fabrication are critical. Among the wide variety of
precursors, those containing nitrogen donor ligands are of particular interest because of their good
volatility and the high reactivity of their metal-nitrogen bonds.
A deep understanding of the reaction mechanisms occurring during the ALD processes is needed in
developing new precursors and in optimizing the already existing ALD processes. In situ
measurements are best suited for mechanistic studies of ALD processes as they provide chemical
information continuously during the process. However, a strong theoretical knowledge of the nature of
the potential reactions taking place during the deposition is required to solve the reaction mechanisms
completely and to explain the main characteristics of the process.
The goal of this work is to give some insight on in situ reaction mechanism studies on the ALD of
high-k metal oxide thin films. A particular focus is applied on the reactivity of precursors with metal-
nitrogen bonds such as alkylamides, disilazides, amidinates and guanidinates. Several decomposition
pathways are explicated and their possible influence on the process characteristics discussed.
The structure of this thesis is the following: Chapter 2 presents a literature review on the reactivity of
different types of precursors with nitrogen donor ligands used in ALD. Chapter 3 shows the
experimental information on the in situ mechanistic studies performed in this work. Finally, the results
obtained for this thesis are summarized in Chapter 4.
11
2. Background
2.1 Atomic layer deposition
The general principle of ALD for metal oxide deposition is as follows: first the metal containing
precursor is pulsed in the reaction chamber where it reacts with the substrate surface. The excess of
unreacted precursor molecules and the reaction by-products are removed from the chamber during
the following purge by inert gas (generally N2 or Ar). The oxygen containing precursor is then pulsed
and reacts with the adsorbed metal species on the surface, removing their remaining precursor
ligands and regenerating the reactive surface sites. Finally, another pulse of purge gas is supplied to
remove the reaction by-products and the excess of oxygen containing precursor from the chamber
and the substrate surface is ready for another ALD cycle to begin. For thermal ALD of metal oxides,
the oxygen precursors used are mainly water and ozone. Plasma Enhanced Atomic Layer Deposition
(PEALD) uses pulses of oxygen activated with plasma to deposit metal oxides.5 However, the
complex reactions involved in PEALD are beyond the scope of this work and will not be examined in
this thesis.
Figure 1. Typical growth rate evolution of ALD processes with temperature.
Figure 1 shows the different possible evolutions of an ALD growth rate with temperature:2 on one
hand, the metal precursors can condense on the surface at low temperature, eliminating the perfect
conformality and increasing the growth rate. On the other hand, low temperature can limit the
reactivity of the precursors and thus decrease the growth rate. There is a temperature range where
the deposition is surface-controlled and the surface reactions are self-limiting. This is usually
described as the “ALD window” of the process. Therefore, the film thickness can be precisely
controlled by changing the number of deposition cycles without the need to control the precursor
doses strictly. The self-limiting behavior allows deposition of perfectly uniform and conformal films on
12
large area substrates and on complex structures with high aspect ratio. In ideal ALD processes, the
growth per cycle (GPC) is constant in this window. For real ALD processes, the growth per cycle is
highly dependent on the nature of the metal precursor used and the deposition parameters
(temperature, pressure, reactor geometry): for instance, the reactivity of the precursor and the density
of reactive sites on the surface are strongly influenced by the deposition temperature. Other
properties of the deposited film such as the GPC, the density of the film and the amount of impurities
incorporated are directly linked to these factors and can vary even within the “ALD window”. However,
the main characteristic of ALD remains a self-limiting behavior where the adsorption reactions on the
surface can stop even if some potential reactive sites are still unreacted. The ligands of already
adsorbed metal precursors can hinder the access of unreacted precursor molecules to adjacent
hydroxyl groups and/or other potential reactive sites. Within real processes, the steric hindrance of the
adsorbed surface ligands can also impact some properties of the deposition (GPC, density).
At higher temperature, secondary decomposition reactions start to play an increasing role in the film
deposition characteristics: in Figure 1 the dashed area corresponds to a temperature range where the
decomposition reactions of the metal precursor become thermodynamically and kinetically favored
and modify the adsorption mechanism. Decomposition reactions open additional reactive sites that
can destroy the self-limiting behavior of the deposition, increasing the growth rate and possibly
incorporating more impurities in the films. High temperature can also enhance desorption of
precursors and surface dehydroxylation, thus decreasing the growth rate.
Although ALD has been used for several decades, many of the underlying mechanisms of the film
deposition remain mostly unknown. Firstly, the nature of the elemental reactions occurring during an
ALD cycle is often simplified: the common thought is that a single precursor molecule reacts with one
or several reactive sites upon adsorption, forming volatile by-products that leave the surface without
further interactions. Secondly, the surface itself is ideally depicted as two dimensional with a discrete
number of identical reactive sites, often considered as reacting separately. This ideal vision of the
reaction is contradicted by empirical evidence hinting to a more complex reality: the reactivity of one
adsorbed precursor can be strongly influenced by the neighboring molecules. Reaction mechanism
studies help to understand the influence of the substrate, co-reactants and temperature on the
process characteristics such as growth per cycle, crystallinity and film composition.
2.2 In situ analytical methods used for reaction mechanism studies in ALD
In situ analytical methods allow the investigation of ALD chemistry and also several characteristics of
the growing films directly during the process. They minimize the alteration of the film due to exposure
of the sample to air and constitute the only means to observe reaction mechanisms accurately as they
are taking place. Most of the in situ analytical methods require specific set-up conditions and proper
interpretation to provide valuable information on the reaction mechanisms. The following describes
the most common in situ techniques used to investigate atomic layer deposition processes.
13
Fourier transform infrared spectroscopy (FT-IR) is a non-destructive technique which provides direct
information about the nature of the chemical bonds formed on the reaction surface during an ALD
process. To a smaller extent, FT-IR can also inform on the volatile by-products released at each step
of the growth.6 It has been used to study reaction mechanisms in ALD metal oxide6-10 and nitride11-13
processes. Recording in situ FT-IR spectra requires a significant acquisition time that lengthens the
ALD cycles but also ensures that each reaction reaches completion before switching to the next
reactant. The data obtained are often differential spectra qualitatively assessing the evolution of the
chemical bonds on the growing film. However, FT-IR does not directly inform on the exact nature of
the surface species (especially regarding carbon containing bonds) as one chemical bond can be
present in several different surface species. Accordingly, a careful comparison with similar molecules
and surfaces from the literature is necessary to correctly interpret the observed IR bands.
Quadrupole mass spectrometry (QMS) allows the investigation of the gaseous species present in the
reactor during the process.14-18 Volatile by-products released at each step of the ALD process are
ionized and fragmented by electrons from a filament source and then sorted by the quadrupole
analyzer. The quadrupole requires high vacuum (HV) conditions obtained by differential pumping
through an orifice with a turbomolecular pump. The signal measured is expressed versus mass to
charge ratio, m/z. The main limitation of this technique is the difficulty to attribute an m/z signal to a
single fragment: several by-products originating from different sources (ozone combustion,
hydrogenated ligands…) can produce fragments at the same m/z and thus the signal observed with
the QMS can be composed of multiple contributions. It is possible to discriminate these signals when
comparing their evolution with a signal belonging to only one fragment. However, in most cases the
interpretation remains problematic, especially with combustion reactions occurring during the process.
The quadrupole analyzer allows monitoring only a few masses at the same time during a short ALD
cycle. This limitation could be eliminated by using another type of analyzer such as a Time-of-Flight
(ToF).
Quartz crystal microbalance (QCM) allows the detection of minuscule mass changes on the reaction
surface: deposition of less than a monolayer can be observed with a great precision. When a certain
mass is deposited on a quartz crystal, its resonance frequency changes accordingly. The frequency
change is also dependent on other factors such as temperature and deposition area.
Figure 2 exhibits an example QCM data obtained when measuring one typical ALD cycle: During the
metal precursor pulse, the mass increases abruptly in the first few seconds before it reaches a
plateau. The self-limiting behavior of the studied ALD process is demonstrated by this mass
stabilization during the metal precursor pulse. The mass can sometimes decrease slightly during the
following purge, illustrating desorption of unreacted molecules from the surface or decomposition of
the adsorbed ligands. The oxygen precursor pulse (D2O in Figure 2) induces another mass change
when the remaining adsorbed ligands are released from the surface and the reactive sites are
regenerated. Depending on the mass of the ligand compared to the mass of the reactive surface
species (hydroxyl groups, active oxygen) this mass change can be an increase, for instance in the
14
tetramethylaluminum and water process.19 However as portrayed in Figure 2, in most processes the
mass decreases during the oxygen precursor pulse due to the removal of heavy ligands.
Depending on the process, additional phenomena can influence the quartz resonance frequency.
Sudden pressure and temperature change in particular can directly influence the QCM data. For
instance, ozone pulse triggers a significant artifact on the QCM data. Accordingly, longer stabilization
times are needed for the purge after an ozone pulse.
Figure 2. Typical QCM data obtained for one ALD cycle. m1 corresponds to the mass increase after
the metal precursor pulse. m0 corresponds to the overall mass change after the full cycle.
X-ray photoelectron spectroscopy (XPS) is a surface sensitive quantitative spectroscopic technique
that measures the elemental composition, empirical formula, chemical state and electronic state of the
elements present on the growing film. The XPS measurements do not occur exactly in situ as the ALD
deposition would damage the XPS set-up but rather in vacuo.20-24 The substrate is moved from the
reaction chamber between each cycle or half cycle to the XPS chamber for analysis without breaking
the vacuum. Additionally, the acquisition requires UHV in the XPS chamber. This limits the
contamination of the sample due to air exposure. In situ XPS is especially useful to observe the
quantitative evolution of an element in the film as well as its direct environment: the binding energies
measured with XPS for an atom are directly related to its neighbors. A change in the shape of an XPS
peak illustrates a modification of the environment of the corresponding atom. This feature allows
precise investigation on the interfacial layer formation and on the effect of the substrate on the initial
growth rate.25-27 When measuring thicker films, quantitative compositional analysis can also be done
with ex situ XPS by ion beam etching: consecutive XPS spectra are measured as a function of the
etching time and give a compositional depth profile of the grown film. This is especially useful to
observe migration of a specific element (for instance a dopant) in a multilayer structure.28
In situ spectroscopic ellipsometry (SE) monitors the change in polarization of a light beam upon
reflection on the surface.29-33 With a model-based analysis, SE gives information on the evolution of
the optical dielectrical function including the film refractive index and the thickness of the growing film
after each deposition cycle. However, SE does not provide direct chemical information on the reaction
15
mechanism. The set-up required is generally non-invasive and very versatile which makes in situ SE
a valuable alternative to QCM to measure film thickness.
X-ray fluorescence spectroscopy (XRF) is a spectroscopic technique used for elemental analysis.
Using the high intensity of synchrotron generated X-rays increases the sensitivity of the XRF to
monitor the number of metal atoms deposited on the surface during each ALD cycle. This technique is
especially useful to study initial nucleation on planar substrates and nanoporous films.34, 35 However, it
is unsuitable for the detection of light atoms such as carbon, oxygen and nitrogen. High-resolution
synchrotron x-ray diffraction (XRD) is an in situ method that studies the crystallography of the growing
films.36 This technique is particularly important to investigate the nucleation of crystalline films and the
influence of reaction temperature on the nucleation of a specific crystalline phase. The limited access
to synchrotron facilities and the high cost of the experiments limit the use of these techniques to study
reaction mechanism of ALD processes routinely.
In situ measurements sometimes do not provide sufficient information to fully solve the reaction
mechanisms occurring during the ALD deposition: in complex adsorption and decomposition
reactions, several competitive pathways can possibly lead to the same by-products. In such cases,
computational modeling of the key process steps can help to assess the energetically most favorable
reaction pathways. Among all the atomic-scale simulation methods available, Density functional
theory (DFT) is the most commonly used to study ALD processes. This quantum mechanical method
allows the modeling of systems up to 1000 atoms, occupying roughly one cubic nanometre. It can
describe classical two-centre covalent bonds, non-classical multi-centre bonds, metallic bonding in
conductors and polar / ionic bonding. This is especially valuable when assessing structural
differences in metal precursors which could account for a difference in the reactivity of these
precursors. In addition, DFT can represent accurately atomic layer arrangement in the ALD deposited
semiconductors and insulators. Recently, Elliott reviewed the atomic-scale simulation studies on ALD
processes.37
2.3 Precursors with nitrogen donor ligands used in ALD
The traditional precursors used to deposit transition metal oxides possess halides, alkyls, β-
diketonate or alkoxy ligands. During the last decades, several new types of ligands have been
experimented to expand the deposition possibilities (ALD window, evaporation temperature, growth
per cycle…). Particularly, precursors possessing metal-nitrogen bonds present interesting advantages
in ALD. They are highly reactive with water, ozone and surface hydroxyls and usually lead to higher
growth rates than alkoxides and halides. In addition, the easy conversion of the M-N bonds into M-O
bonds leaves very small N contamination in the metal oxide films grown with such precursors.
16
2.3.1 Homoleptic alkylamides
Figure 3. Types of ligand with metal-nitrogen bonds employed in ALD processes (Ri correspond to
alkyl groups and L to another type of ligand such as alkoxide, cyclopentadienyl, amidinate…).
An alkylamide ligand consists of a nitrogen atom linked to the metal center of the precursor and to
small alkyl groups (see Figure 3). Homoleptic alkylamide precursors have been used for the ALD of a
wide range of materials: aluminium oxide,38-42 tungsten oxide43 and antimony oxide44-46 as well as
most of the group 4 and 5 transition metal oxides (namely titanium,47-59 hafnium,60-75 zirconium76-84 and
tantalum85-91) have been deposited thermally with water and ozone but also with oxygen plasma. The
same alkylamide precursors allow the deposition of transition metal nitrides (TiN,92-96 ZrNx,97-99 HfNx,98,
100-102 TaNx103-107 and WxN108) with ammonia, nitrogen plasma or hydrogen plasma.
The most common alkylamide precursors possess dimethylamide, diethylamide or ethylmethylamide
ligands. They are liquids with reasonably high vapor pressures and low evaporation temperatures due
to the small size of their alkyl moieties. Heavier and bulkier ligands would reduce their volatility
without necessarily increasing the stability of the precursor. The M-N bonds present in the homoleptic
alkylamides are generally weaker than the corresponding M-O bonds which render alkylamide ligands
highly reactive with surface hydroxyl groups, water and ozone, even at low temperatures. However,
they also induce poor thermal stability both in the gas phase and on the surface. Thermogravimetric
17
analysis of homoleptic alkylamides show mass losses in a single step and with low mass residue,
indicating evaporation of the precursors without decomposition.109
The nature of the metal center also plays an important role in the deposition. Late transition metals
are less oxiphilic and the M-N bonds of their metal precursor are stronger than early transition metals.
For instance among the group IV and V transition metals, titanium alkylamides seem to yield
significantly lower growth rates than their niobium and hafnium counterparts.109 Although the Ti-N
bond energy is less than those of the other group IV and V metal-nitrogen bonds, the small size of the
Ti atom can hinder nucleophilic attacks and account for the lower growth rates observed. For most of
the homoleptic alkylamides, the maximum temperature to obtain a proper ALD film is around 250
°C.67, 72, 76, 109 This upper limit is seemingly independent of the size of the alkylamide ligands but varies
with the metal: vanadium alkylamides decompose already at 200 °C110 when zirconium alkylamides
remain stable up to 250 °C.76 The amount of impurities in the films deposited with the homoleptic
alkylamides is small, usually below 2 at.% for carbon and 1 at.% for nitrogen.67, 72, 76, 109 However,
there are some discrepancies depending on the oxygen source, the metal and the ligands with no
clear trend.
2.3.2 Disilazides
Disilazide precursors, also known as silylamides, possess ligands consisting of a nitrogen atom
bonded to the metal center and to two trimethylsilyl groups (Figure 3). These precursors have been
used in ALD in conjunction with water to deposit zirconium oxide,111 hafnium oxide,7, 111 lanthanum
oxide112-115 and praseodymium oxide.116-118 In addition, ALD with lutetium and lithium disilazide
precursors and ozone as the oxygen source led to the formation of lutetium and lithium silicates.119-121
The reaction mechanism of the lithium disilazide and ozone process is discussed in publication III.
Zn(N(SiMe3)2)2 was also used to grow ZnSe but resulted in an important incorporation of nitrogen into
the deposited films.122 Disilazide precursors start to decompose partially at relatively low temperature
(200 °C) and thus the oxide films deposited from these precursors contain a significant amount of
silicon impurities.
2.3.3 Amidinates
Amidinate ligands are bonded to a metal center through two nitrogen atoms that themselves are
linked together by a carbon atom (Figure 3). The substituents of the ligand can be chosen to tailor the
volatility and the melting point of the precursor. The chelating effect of the amidinate ligand increases
the thermal stability of these precursors but the M-N bonds allow high reactivity even at low
temperatures. Amidinates are also non-pyrophoric but maintain high reactivity toward water. They
allow the ALD of metal films such as iron,123 cobalt,124-127 nickel,123 ruthenium128-130 and copper123, 124,
131-135 when used in combination with ammonia or hydrogen, with or without plasma. Many oxide films
(Al2O3,42 CoOx,123 Y2O3,136 La2O3,137-139 PrOx,116 Er2O3,140) have also been deposited with amidinate
precursors and water or ozone. The surface reactions involved in the adsorption of amidinates are
18
complex (see publications IV and VI) and can potentially lead to a significant incorporation of
impurities in the growing films.
2.3.4 Guanidinates
Guanidinate ligands resemble amidinates except their central bridge carbon is linked to a third
nitrogen atom (see Figure 3). Homoleptic guanidinate precursors present a high thermal stability but
also a high temperature of evaporation, generally above 120 °C. Similarly to amidinates, guanidinates
are versatile ligands with many tunable properties depending on the number and the nature of their
substituents. Guanidinates are novel promising precursors in MOCVD and PEALD of nitrides (ZrNx,
TiN, WxN,141TaNx) and metals (copper,142-144 ruthenium145). They have also been experimented in
thermal ALD for deposition of various oxides such as Gd2O3,146 ZrO2,147 HfO2,148 and lanthanide
oxides.149, 150 The thin films obtained with the guanidinate precursors show comparable nitrogen and
carbon contamination comparable to the films grown with the amidinate precursors (below 2%).
2.3.5 Heteroleptic alkylamides
Heteroleptic precursors possess metal center surrounded by several types of ligands. The
combination of these different ligands allows the tailoring of the precursor reactivity and its thermal
properties (vapor pressure, evaporation temperature, and thermal stability). Moreover, combination of
different metal-ligand bonds modify the electron distribution around the metal center and thus the
surface reaction mechanisms of heteroleptic precursors compared to homoleptic ones.
With heteroleptic alkylamides, several deposition characteristics depend on the nature of the other
ligands. Only a couple of ALD processes involving alkoxy/alkylamide precursors have been reported
in the literature: the deposition of titanium and hafnium oxides exhibits self-limiting growth rate and
excellent purity even above 300 °C.151, 152 In addition, many patents of different combinations of
alkoxy/alkylamides precursors have been filed by precursor manufacturers.153
Oxide ALD processes using zirconium and hafnium monocyclopentadienyl alkylamides result in
relatively high growth rates (0.8-0.9 Å/cycles) and low amounts of carbon and nitrogen remaining in
the films.154 Similarly to the homoleptic titanium alkylamides, titanium oxide ALD processes involving
monocyclopentadienyl alkylamides exhibit a lower GR109 (0.3-0.4 Å/cycle) than other metal oxide
processes. Precursors containing more than one Cp ligands have been evaluated for PEALD of
zirconium carbides155 and MOCVD of hafnium oxide.156 Generally, cyclopentadienyl alkylamides tend
to be thermally less stable than the other types of heteroleptic cyclopentadienyl precursors.
Decomposition reactions involving the alkylamide ligands are directly responsible for this decrease in
thermal stability. Such reactions will be investigated later in this thesis (Chapter 3).
Alkylamide/imido precursors have mostly been investigated in ALD in combination with ammonia to
deposit tantalum13, 157-160, tungsten,161-164 and molybdenum165-168 nitrides. Hausmann et al.91 deposited
19
Ta2O5 using EtN=Ta(NEt2)3 and water. More recently, Blanquart et al.169, 170 also used
alkylamide/imido precursors with water and ozone for the deposition of niobium and tantalum oxides.
In situ reaction mechanism studies of these processes are displayed in publication I.
Several alkylamide/amidinate and alkylamide/guanidinate precursors have been synthetized for use in
ALD and CVD: thermogravimetric studies of hafnium and zirconium precursors indicate significant
differences in volatility and thermal stability depending on the size of the ligands.171-173 So far, only a
couple of studies have reported precise growth characteristics for the deposition of transition metal
oxides. The ALD processes174 using (EtMeN)2Zr(guan-NEtMe)2 as the metal precursor exhibit high
growth rates (0.9 Å/cycle with water and 1.15 Å/cycle with ozone) and low carbon and nitrogen
contamination (< 3 at.%). The reaction mechanisms of these processes are investigated in publication
V. Similar processes, namely (Me2N)Hf(guan-NMe2)2 and water171 and dysprosium/gadolinium
alkylamides/guanidinates with water or ozone, have also been investigated and shown promising
results.175
2.4 Metal precursor adsorption mechanisms in ALD
The reaction of metal precursor molecules during a typical ALD cycle can be decomposed in several
simple steps according to the following equation:
gas-MLn surf-(MLn) surf-(MLx) surf-MOx bulk-MOx
where MLn is the metal-containing precursor with n ligands L, x is the number of ligands eliminated
from the surface so that 0 < x < n, and “bulk-MOx” refers to a metal atom with a local environment
(coordination number) corresponding to the structure of the film material. Firstly, the precursor
molecule adsorbs on the surface without chemically reacting with surface species. Secondly, the
adsorbed molecule can react with the surface and lose some of its ligands either by exchange with
the reactive surface sites, decomposition or dissociation. In parallel, the coordination number (CN) of
each metal center expands gradually during the adsorption to reach the configuration they would
possess in the bulk of the film. This gradual modification of the CN acts as a driving force of the
deposition and can be responsible for the crystalline state of the deposited film.37, 176 Unfortunately,
some of these steps are often difficult to clearly assess with in situ measurements and computational
studies are required to investigate their potential influence on the reaction mechanism.
During the ALD cycle, the metal precursors can adsorb by different mechanisms. These possible
mechanisms are summed up in Figure 4. Firstly, the precursor can chemisorb on the surface without
reaction with surface species (path A). The precursor molecule can also dissociate with its metal
center bonding to one reactive site and some of its intact ligands linking to the surface (path B).
However, most of the metal precursors possess highly reactive metal-ligand bonds which can interact
with surface reactive sites. For example, the metal precursors often chemisorb though the exchange
of their ligands with reactive surface sites such as hydroxyl groups (path C). The ligands are then
released intact in hydrogenated form. They can also decompose upon reacting with the surface and
20
release smaller volatile by-products (path D). Finally, for ozone based processes another type of
adsorption is possible: adsorbed oxygen atoms can react with metal precursors and partially combust
their ligands and release combustion by-products (path E).
Figure 4. Possible adsorption mechanisms of metal precursors that occur during an ALD cycle.
2.4.1 Chemisorption and dissociative adsorption
An extreme case of metal precursor adsorption is when the chemisorption occurs without reactions
between the metal precursors and the reactive surface sites. The removal of the precursor ligands is
then solely due to reaction with the oxygen containing precursors. The QCM results for such
mechanisms show a relatively small m0/m1 ratio because the precursor molecules stay intact on the
surface at the end of the metal precursor pulse (high m1). The QMS data exhibit the release of by-
products only during the oxygen precursor pulse.
Alternatively, the precursor molecule can dissociate and thus one or several of its ligand will bond to
neighboring reactive sites instead of remaining kinked to the adsorbing molecule. Dissociative
adsorption can occur due to the high reactivity of the metal-ligand bond, the lack of suitable reactive
sites or thermal energy necessary for ligand exchange reactions. Such dissociative adsorption was
observed for the deposition of copper amidinates on metallic surfaces at low temperature.134
21
2.4.2 Ligand exchange mechanism
Figure 5. Ligand exchange mechanism for the ALD of metal oxide with water.
Ligand exchange is the most common adsorption mechanism in metal oxide ALD. During the metal
precursor pulse, the precursor ligands react with surface hydroxyl groups and leave the surface
without decomposition: the hydrogen from the surface –OH groups is transferred to the ligand, a bond
is formed between the metal center and the oxygen of the surface –OH group and the hydrogenated
ligand is released to the gas phase (Figure 5). The number of molecules deposited before reaching
saturation is related to the number of available reactive sites. For ALD of oxides, the reactive sites
involved in ligand exchange reactions are almost exclusively hydroxyl groups. During the water pulse,
the remaining ligands are hydrogenated and the surface –OH groups regenerated to start a new
cycle.
The exchange reactions release gaseous by-products that can be detected by QMS. However, the
nature and repartition of by-products originating from ligand exchange can vary greatly from process
to process. An extreme case would be all the ligands reacting with the reactive surface sites and
being eliminated as by-products during the metal precursor pulse. For most processes, the release of
these specific by-products is divided between both metal and oxygen precursor pulses. In situ
analytical techniques are then necessary to quantify precisely this repartition and to draw accurate
half-reactions for the reaction mechanism. In this respect, the combination of QCM and QMS allows
the discrimination of by-products originating from different sources. This is especially interesting while
studying heteroleptic precursors with two or more types of ligands that behave differently upon
reaching the surface.
22
2.4.3. Precursor adsorption through ligand decomposition
More complex reactions can occur when a precursor reaches the surface. Amidinate, guanidinate and
disilazide precursors tend to decompose partially during their adsorption on reactive surfaces. These
decomposition mechanisms can take place without direct reaction with surface reactive groups
(hydroxyls) and lead to the release of numerous gas phase by-products that can further readsorb on
the surface and continue to react. This behavior leads to multiple pathways of adsorption in which
desorption, readsorption and rearrangement on the surface are as important in the surface saturation
as the initial reactions. For instance, gas thermolysis of amidinate precursors at normal pressure
shows carbodiimide deinsertion at a temperature as low as 120 °C (see pathway A in Figure 7).177-179
Moreover, dissociation of the acetamidinate ligand around -73°C on a metal surface leads to
adsorbed alkene, cyanamides and alkylamide groups on the surface. The removal of such adsorbed
species can be problematic with water as the oxygen co-reactant and ultimately lead to high carbon
and nitrogen contamination of the deposited films. The reaction mechanisms governing the
decomposition of amidinate ligands are discussed in detail in a later section.
The nature of the surface and temperature are the key factors in the thermodynamics and kinetics of
these surface reactions: for instance on an oxide surface, the density of hydroxyl groups decreases at
higher temperature, which can limit the initial adsorption of the precursor molecule through chemical
bonding or change the reaction mechanism by favoring decomposition of the metal precursor and
CVD type reactions.
Adsorption of the precursor with decomposition of the ligands is not always detrimental: in the case of
disilazides, the decomposition of the precursors through reaction with surface hydroxyl groups can be
used to passivate a reactive surface with trimethylsilyl groups.180, 181 Disilazides can also constitute
potential bimetal precursors for deposition of ternary silicon oxide.121, 182
2.4.4. Partial combustion
According to in situ FT-IR studies, surface impurities and hydroxyl groups can be removed during the
ozone pulse.10 Additionally, the number of hydroxyl groups on the surface diminishes with increasing
deposition temperature. Carbonate and formate surface groups originating from partial combustion of
alkyl substituent by ozone can replace the hydroxyls groups as nucleation sites.9, 10 Also, the
combustion of ligands by ozone creates H2O that can be readsorbed.183 Another alternative reactive
species on the surface is adsorbed oxygen radicals: similarly to ozone molecules, these active
oxygen species can partially combust ligands leading to the decomposition of the metal precursors
during their adsorption. Even though no in situ characterization techniques could formally prove their
presence, active oxygen radicals remain the best explanation available at this point for the
combustive adsorption reaction mechanism of several ozone based processes (see publication VI).
23
2.5 Reactions of precursors with nitrogen donor ligands
2.5.1 Alkylamides
The lone electron pair of the nitrogen atom in the alkylamide ligand is capable of initiating a
nucleophilic attack onto electron-deficient species. Additionally, the weaker M-N bonds strength
compared to the corresponding M-O bonds explain the superior reactivity of alkylamide compared to
alkoxy ligands: with heteroleptic precursors containing both types of ligands, adsorption of the
precursor occurs primarily through reaction with the alkylamide ligands (see publication V).
The length of the alkyl chain within the alkylamide ligand influences greatly the vapor pressure of the
precursor. However, there is no direct correlation with the growth rate of the process. For the same
type of alkylamide ligand, the growth rate depends mainly on the metal. The difference in the strength
of the metal-nitrogen bonds also results in a variation of the number of alkylamide ligands reacting
with surface groups. For instance in publication I, QMS and QCM results show that only one
deuterated ligand (DNEt2) is released during the adsorption of tBuN=Nb(NEt2)3 while 1,7 DNEt2 are
released during the adsorption of tBuN=Ta(NEt2)3 . In addition, the gradual expansion of the metal CN
during the adsorption step can also depend on the strength of the M-N bonds present in the ligands
remaining on the surface.
Alkylamide ligands can undergo partial decomposition reactions in the gas phase or on the surface
after adsorption of the precursor molecule. For the majority of metal alkylamides, gas phase
thermolysis studies reveal that decomposition occurs around 250-300 °C, depending on the length of
the alkyl chains.76 This decomposition often defines the upper limit of the ALD window as the
subsequent parasitic reactions occurring during adsorption above this threshold temperature avert a
saturative growth. Oppositely, surface decomposition reactions do not necessarily destroy the self-
limiting character of a process. They can be slow and remain marginal within the ALD window, or can
lead to unreactive surface intermediates that passivate the surface against further adsorption of the
precursor. However, these reactions can also lead to increased surface roughness and amount of
impurities deposited in the film.
Figure 6 exhibits some initial pathways for the surface decomposition of alkylamide precursors: an
alkylamide ligand can undergo β-hydride elimination that releases an alkylimine in the gas phase and
forms of a M-H bond as observed with in situ FT-IR184 (path A). For instance with Hf(NEtMe)4, the β-
hydride elimination above 270 °C leads to the release of ethylmethyleneimine or methylethyleneimine
and the formation of Hf-H surface species. These Hf-H species can later react with H2O to regenerate
surface –OH groups and release H2.184 QMS and FT-IR results and DFT calculations both indicate
that β-hydride elimination can lead to further intramolecular hydrogen transfer and release of
hydrogenated alkylamide185, 186 (path B). Although possible, such a H transfer between two alkylamide
moieties linked to the same metal center is unlikely compared to hydrogen incorporation from another
surface species (path C), especially on oxide surfaces covered with –OH groups.186, 187 Another
possible pathway requires the scission of a C-N bond and β-hydrogen migration within the same
24
alkylamide ligand to release volatile hydrogenated alkyls, mostly methane (path D). This forms an
imino surface group that was observed experimentally with FT-IR185 and predicted by DFT
calculations186 and can be involved in further decomposition reactions. Most of these reaction
pathways are more noticeable on H terminated metal or silicon surfaces than on hydroxylated oxide
surfaces.184, 188 At higher temperatures (> 300 °C), additional dehydrogenation reactions increase
surface species decomposition thus eliminating the self-limiting character of the deposition and to
higher growth rate and leaving more impurities in the films.75
Figure 6. Potential decomposition pathways for an adsorbed alkylamide molecule (R1 to R5 represent
hydrogen atoms or alkyl substituents).
2.5.2 Hetroleptic alkylamides
In typical ALD conditions, the reactivity of alkoxy/alkylamide precursors is governed by ligand
exchange. The higher reactivity of the M-N bonds of the alkylamide ligands compared to the M-O
bonds of the alkoxy ligands implies preferential reaction of the chemisorbed precursor through an
exchange of the alkylamide ligands (see publication V). Concerning amidinate/alkylamide and
guanidinate/alkylamide precursors, the surface reactions underline the decomposition of the
amidinate or guanidinate ligand.
Within the “ALD window”, heteroleptic cyclopentadienyl/alkylamide precursors can possibly react with
surface hydroxyl groups and release their hydrogenated alkylamide ligands, leaving the surface
covered with Cp groups. The presence of these bulky ligands on the surface limits further access of
precursor molecules to other reactive sites. Alternatively, the Cp ligands can also exchange with
surface hydroxyl groups and leave the surface as a whole hydrogenated Cp. The remaining
alkylamide ligands on the surface can lead to a more closely packed saturated surface and increase
the density and/or the growth rate of the film. At higher temperature, the Cp ligands start to
decompose and form reactive by-products that can readsorb on the surface. These reactive species
25
increase significantly the amount of carbon impurities in the film and remove the self-limiting character
of the surface reactions.
Similarly to the other heteroleptic alkylamides, the main mechanism for adsorption of imido/alkylamide
precursors is ligand exchange with the available reactive surface sites. Depending on the metal, the
charge repartition between the metal and the nitrogen of the imido ligand can allow its reaction with
surface hydroxyl groups during the metal precursor adsorption (see publication I). At high
temperatures, unimolecular decomposition reactions involving the imido ligand can occur: γ-H
migration would lead to the formation of an M=NH group and the release of alkene by-product.
Alternatively, β-H transfer from another alkylamide ligand would simply release a volatile alkylamide.
Both of these by-products have been observed with QMS during temperature-programmed desorption
(TPD) experiments.187
2.5.3 Amidinates
Metal amidinate precursors have been selected for ALD because of their volatility and reactivity with
hydrogen, water, ozone and ammonia. However, the adsorption of amidinates observed with in situ
analytical methods (QCM, QMS, and XPS) involves decomposition of the amidinate ligands when
they chemisorb and react with the surface sites. These decomposition reactions are critical to the
success of a deposition process. Although multiple and highly complex, several of these pathways
have been characterized with in situ analytical methods and/or calculation modeling and are
discussed in the following sections.
Hydrogen transfer decomposition pathways
Similarly to the alkylamides, decomposition of amidinates often involves hydrogen transfer within the
same ligand. Starting from 120 up to 250°C, some QMS studies during the gas phase thermolysis of
copper amidinates have led to a suggestion of carbodiimide deinsertion which is the reverse reaction
for the amidinate synthesis and leads to the formation of a metal-alkyl bond.179 This M-C bond
formation could be a major cause of carbon contamination in the deposited films. However, the
cleavage of a C-C bond at low temperature appears unprobable and carbodiimide by-products were
never observed during actual ALD cycles involving amidinates. Therefore, carbodiimide deinsertion of
amidinate ligands seems very unlikely. Additional XPS and TPD measurements189 of copper
amidinates point toward several other possible pathways displayed in Figure 7: β-hydrogen transfer
within the amidinate ligand can occur through a four member ring reaction (path A), forming a N-H
bond and releasing alkene by-products. Further β-hydrogen transfer leads to the removal of the
hydrogenated substituent R3 and formation of a N≡C triple bond (path D). Formation of this nitrile
species can also result from a six member ring with a δ-hydrogen transfer reaction (path C). Path B
illustrates another possible hydrogen transfer producing a M-H bond as calculated by DFT for cobalt
and nickel amidinates.190 The resulting moieties formed are not reactive with the other metal
precursors thus maintaining the self-limiting behavior of the process. They can later be removed from
the surface by the following oxygen precursor pulse. These decomposition reactions occur on the
surface but can also take place in the gas phase at high temperatures (>350 °C). In this case, the
26
deposition loses its self-limiting behavior and a drastic increase in the amount of impurities
incorporated in the film is observed.189
Figure 7. Potential amidinate hydrogen transfer decomposition pathways for Cu, Co and Ni amidinates.
Mechanisms leading to incorporation of impurities in the film
The decomposition of amidinate is quite complex and involves many intermediates and competitive
reaction steps. Depending on deposition temperature, the nature of the surface and the co-reactants,
some of these reactions can dominate and lead to more or less impurities being incorporated in the
film. The detrimental reaction pathways that have been characterized with in situ surface or gas
phase techniques are displayed in Figure 8.
For copper amidinates, XPS and TPD results189 indicate that upon the initial adsorption of the metal
precursor, the two nitrogen atoms which had a similar chemical environment in the gas phase, start to
differentiate into two distinct types: the amido type of nitrogen which is directly bonded to the metal
atom and the imido type of nitrogen which is double bonded to the central carbon and only weakly
bonded or not bonded to the surface. Surface decomposition starts with a scission of the N-C bond
from either the amido (path A) or the imido side (path B) with a H transfer from the alkyl substituent to
the nitrogen atom, allowing the alkyl substituent to readsorb on the surface as a bridged molecule.134,
189 These bridged alkyl groups can later be removed by a strong co-reactant (ozone, hydrogen
plasma) or dehydrogenate further and lead to carbon impurities in the film. The hydrogen formed this
way can later react with another adsorbed amidinate ligand and release alkylamidine in the gas phase
(path B in Figure 7). The remaining smaller amidinate-like species on the surface can continue to
decompose through β-hydrogen transfer as shown above (path D). Alternatively, another C-N bond
scission would produce bridged imino and alkylamide surface groups. These potential pathways do
27
not involve additional reaction steps with the surrounding reactive surface sites: a hydroxylated
surface would facilitate some reaction steps such as the formation of alkylamidine or surface alkoxy
groups while on the other hand a metal surface would enhance dehydrogenation and decomposition
reactions.
Figure 8. Potential copper amidinate decomposition pathways leading to incorporation of impurities in
the film.
Part of the amidinate ligand remaining on the surface after adsorption can also be hydrolyzed during
the water pulse and release alkylamine, as measured with QMS. This could be problematic while
studying heteroleptic precursors with alkylamide/amidinate ligands because the gaseous fragments of
hydrogenated alkylamides can originate from both the exchange of alkylamide ligands and the
decomposition of amidinate ligands. This renders quantitative interpretation of the reaction
mechanism extremely difficult (see publication V). The same issues arise with ozone processes as
the combustion by-products of amidinates and alkylamides are often the same (NOx). Generalization
of these reaction pathways to all amidinates should be avoided as the chemistry is likely strongly
dependent upon the nature of the metal. Additional in situ studies need therefore to be conducted on
a broader range of amidinates to clarify their surface reaction mechanisms.
2.5.4 Guanidinate
Guanidinate precursors possess a chemical structure very similar to the amidinates with the two
chelate N atoms bonded to the metal center. The presence of an additional alkylamide group linked to
the bridge carbon atom participates in the delocalization of the π bonding of the chelate nitrogen
atoms. For conjugation of this dialkylamide moiety with the NCN unit to occur, it should be sp2-
hybridized. A minimal torsion angle between the NR2 p-orbital and the NCN bridge is also required for
an optimal overlap with the chelate π-orbital of the nitrogen atoms.
28
Sublimation of homoleptic lanthanide guanidinates occurs in the same temperature range (120-165
°C at 0.05 Torr)149 as their amidinate counterparts although the mass of the guanidinate ligands is
higher. This indicates that steric crowding, electronic saturation and intermolecular interactions also
have a strong influence on the thermal behaviour of these precursors. Therefore, the increase in
thermal stability of guanidinate ligands allows deposition of conformal films up to 280°C.146, 191
The decomposition of the guanidinate ligand upon adsorption shares several reaction pathways with
the amidinates: β-hydrogen elimination can occur and form a M-H bond with a release of a
dehydrogenated guanidinate ligand in the gas phase. Carbodiimide deinsertion is also more favorable
for guanidinates compared to the alkyl deinsertion from the amidinates and would result in the transfer
of the exocyclic alkylamide moiety to the metal center. Two of these alkylamide moieties can
subsequently react together to release a hydrogenated alkylamide and an alkylimine.177 With copper
amidinates, this reaction proceeds with a disproportionation of Cu(I) in the guanidinate dimer into
Cu(0) and Cu(II) on the surface above 150 °C.177, 189 These adsorption reactions require many steps
and rearrangement on the surface, and thereby may explain why the saturation of mass observed by
in situ QCM is more gradual for guanidinates and amidinates than for precursors with simpler
adsorption chemistry (V).
Thin films deposited with the guanidinate precursors tend to contain less carbon and nitrogen
impurities than the films deposited at the same conditions from the amidinates. This can be explained
by the increased stability of the guanidinate ligands which diminishes detrimental decomposition
reactions such as dehydrogenation in the gas phase. In addition, carbodiimide deinsertion leads to
the formation of amido complexes. These complexes and the presence of the dialkylamide moiety can
both facilitate the removal of intermediate surface species through nucleophilic attack by reactive
surface groups or water.192
2.5.5 Disilazide
The metal centre of disilazide precursor initially exchanges with a surface –OH group to form
hexamethyldisilazane (HN[Si(CH3)3]2 or HMDS). Therefore, the subsequent reactivity of disilazide
precursors is linked to the behavior of its hydrogenated ligand. Hexamethyldisilazane further reacts
with a hydroxylated surface in a two-step mechanism: first it exchanges with a surface hydroxyl group
and dissociates into a surface-bound trimethylsilyl and a reactive intermediate trimethylsilylamine
(H2NSi(CH3)3). The second step consists of the reaction of the trimethylsilylamine with a neighbouring
–OH group to form another trimethylsilyl surface species (–Si(CH3)3) and release ammonia.193, 194
These timethylsilyl species on the surface are unreactive toward water. As shown in Figure 9, HMDS
reacts mainly with isolated –OH groups and leaves the strongly and weakly H-bonded surface –OH
groups unoccupied.193, 194 The reaction is initially fast as non-hydrogen bonded single and geminal
silanols are consumed. However, further adsorption is hindered because of slower reaction with
geminal silanols that already have one hydroxyl group silylated. At high deposition temperatures,
dehydrogenation of the trimethylsilyl groups can occur and trigger parasitic adsorption reactions,
leaving higher amounts of carbon and silicon impurities in the films.
29
Figure 9. Ligand exchange reactions that occur during the chemisorption of hexamethyldisilazane on
a hydrogenated silicon oxide surface.
30
3. Experimental All the in situ measurements and the depositions were performed with a specially modified
commercial flow-type F-120 SAT ALD reactor manufactured by ASM Microchemistry Ltd that is
integrated with QCM and QMS instruments. Argon or N2 (Oy AGA Ab, 99,999%) were used as the
carrier and purging gas. The gas species present in the reactor during the ALD cycle were
investigated using a Hiden HAL/3F 501 RC QMS with a Faraday cup detector and ionization energy
of 50 or 70 eV. The pressure in the QMS chamber was around 1*10-5 mbar, obtained by differential
pumping through a 100 μm orifice with a turbomolecular pump backed with a mechanical pump. The
mass changes on the substrate were recorded using a Maxtek TM 400 QCM with a sampling rate of
20 Hz. During a typical deposition cycle, the pressure inside the reactor was around 3-5 mbar. 3500
cm2 of soda lime glass substrates was required to obtain good signal intensities with the QMS. All the
in situ reaction mechanism studies were performed on the same oxide surface as the material being
deposited. For instance, tBuN=Nb(NEt2)3 and D2O or O3 processes were studied on Nb2O5 surface.
This allows the study of the ALD reaction mechanisms under stable growth conditions.
The quadrupole mass spectrometer allows the investigation of volatile by-products released during
the ALD process. The evolution in intensity of a few signals at different m/z ratio can be investigated
simultaneously. In order to confirm the actual release of by-products for these signals, background
pulses are necessary. The electron source in the QMS fragments all molecules, i.e also unreacted
metal precursors producing fragments that can correspond to signals with the same m/z ratio as
surface by-products. Accordingly, comparing the intensities of these signals during the background
and ALD pulses indicates the signal intensities measured during the ALD pulses belong partly or fully
to the fragments of by-products originating from the surface reactions. Five reference pulses of the
oxygen containing precursor and the metal precursor are respectively performed before and after the
studied cycles. They are separated by adequate purge time to match the length of the studied ALD
full cycles. This ensures a similar partial pressure of the metal and oxygen precursors during the
background pulses and the ALD cycle pulses.
Quartz crystal microbalance monitors the mass changes occurring on the quartz surface through
variations in the piezoelectric resonance frequency. The resonance frequency of the quartz crystal is
directly proportional to its mass and other factors such as temperature, surface area and shear
modulus. Ten ALD cycles are usually recorded and the frequency changes after the metal precursor
pulse and after a full ALD cycle are measured. Using a ratio of these two frequency changes removes
the influence of external factors and thus corresponds to mass ratios. For reaction mechanisms
involving ligand exchange, the experimental ratios are often compared to calculated mass ratios for
integer number of ligands exchanged. This provides indications on the number and type of ligands
reacting during the adsorption of the metal precursor.
The tBuN=Nb(NEt2)3 and tBuN=Ta(NEt2)3 precursors (I) were furnished by ATMI while (iPrO)3Ti(NiPr-
Me-amd), (iPrO)2Ti(NMe2)2 (IV, VI) and Si2(NHEt)6 (II) were obtained from Air Liquide. The
LiN[Si(CH3)3]2 (III) and Al(CH3)3 (II) were bought from Sigma. Finally, (EtMeN)2Zr(guan-NEtMe2)2 (V)
31
was synthetized by Manish Banerjee, a colleague from the ENHANCE program at the Ruhr-University
Bochum. Table 1 sums up the experimental details of each mechanistic study performed for this
thesis.
Table 1. Summary of processes studies in situ.
Precursor Subl. /Evap. Temp. (°C) at 2-5 mbar
Oxygen sources
Deposition temperatures tested (°C)
Material deposited Publication
tBuN=Nb(NEt2)3 65 (liquid) H2O, O3 250 Nb2O5 I tBuN=Ta(NEt2)3 65 (liquid) H2O, O3 250 Ta2O5 I
Si2(NHEt)6 / Al(CH3)3
65 (liquid) H2O 200 AlxSiyOz II
LiN[Si(CH3)3]2 65 (liquid) O3 200-300 Li2SiO3 III (iPrO)2Ti(NMe2)2 35 (liquid) H2O, O3 275-325 TiO2 IV,VI
(iPrO)3Ti(NiPr-Me-amd) 55 (liquid) H2O, O3 275-325 TiO2 IV,VI
(EtMeN)2Zr(guan-NEtMe2)2
120 (solid) H2O, O3 255 ZrO2 V
32
4. Results and discussion
4.1 Nb2O5 and Ta2O5 Nb2O5 and Ta2O5 thin films possess high-permittivity and have potential applications in DRAM
devices, capacitor dielectrics and as catalyst-supporting oxide materials. The reaction mechanisms of tBuN=Nb(NEt2)3 and
tBuN=Ta(NEt2)3 using D2O or ozone as the co-reactants in ALD were investigated
in situ with QCM and QMS at 250 °C (I).
The tBuN=Nb(NEt2)3 and D2O process exhibits a straightforward ligand exchange reaction as
demonstrated by the formation of D2NtBu and DNEt2 as the main by-products observed with QMS:
one out of three -NEt2 ligands is exchanged during the precursor pulse when the tert-butylimido ligand
remains on the surface. During the following D2O pulse, the remaining ligands on the surface are
released in deuterated form and the surface –OD groups are regenerated, leaving the surface ready
for another cycle to begin.
–OD(s) + tBuN=Nb(NEt2)3 (g) (-O-)Nb(tBuN)(NEt2)2 (s) + DNEt2 (g)
(-O-)Nb(tBuN)(NEt2)2 (s) + 2,5 D2O (g) (-O-)2,5Nb(OD) (s) + D2NtBu (g) + 2 DNEt2 (g)
Interestingly, for the tBuN=Ta(NEt2)3 and D2O process, the QCM and QMS results indicate that in
average 1,7 -NEt2 ligands and 0,3 =NtBu ligands are exchanged during the tBuN=Ta(NEt2)3 pulse:
2,3 –OD (s) + tBuN=Ta(NEt2)3 (g) (-O-)2,3Ta(tBuN)0,7(NEt2)1,3 (s) + 1,7 DNEt2 (g) +0,3 D2NtBu (g)
(-O-)2,3Ta(tBuN)0,7(NEt2)1,3 (s) + 2,5 D2O (g) (-O-)2,5Ta (OD)2,3 (s) + 1,3 DNEt2 (g) + 0,7 D2NtBu (g)
This is surprising as the two precursors contain exactly the same ligands, differing only by the nature
of their group V metal centers. As shown by our quantum chemical calculations, the discrepancy
between the reaction mechanisms can be explained by the differences in the polarity between the Ta-
N and Nb-N bonds: the double bond present in the tantalum precursor is more polar than the one in
the niobium precursor and hence is more likely to react with the surface –OD groups.
Concerning the ozone processes, QCM results point to a molecular adsorption of tBuN=Nb(NEt2)3 on
the surface during the metal precursor pulse while in the case of tBuN=Ta(NEt2)3 a release of at least
two ligands could be deduced. The QMS data indicate typical ozone combustion by-products, such as
CO2 (m/z = 44), CO (m/z = 28), H2O (m/z =18) and NO (m/z = 30) or N2O (m/z = 44, 30). A major
fragment of both types of ligands (m/z = 58) is also observed during the metal precursor and ozone
pulses. Further quantification of these by-products is rendered impossible due to an overlap of
different species with the same m/z value. However, by comparing the shape of the m/z = 58 signal
during the tBuN=Ta(NEt2)3 and tBuN=Nb(NEt2)3 precursor pulses (Figure 10) differences in the
adsorption mechanism of these precursors can be underlined: the sharp rise of the m/z = 58 signal at
the beginning of the tBuN=Ta(NEt2)3 pulse corresponds to a release of by-products originating from
ligand exchange reactions with reactive surface groups during the adsorption of the precursor. This
contribution is absent in the case of tBuN=Nb(NEt2)3 that adsorbs molecularly on the surface.
33
Figure 10. Comparison of the shapes of the signals at m/z = 58 during a metal precursor pulse in the
ozone processes (top curve is tBuN=Nb(NEt2)3 and bottom one is tBuN=Ta(NEt2)3).
4.2 AlxSiyO Binary SiO2 has been difficult to grow by ALD with only water as the oxygen source. By contrast, in
the ALD of ternary metal-silicon-oxides, the other metal oxide activates the ALD reactions of SiO2: as
shown in Figure 11, the growth stops after a few cycle of the Si2(NHEt)6-D2O process whereas it is
sustained in the Al(CH3)3-D2O-Si2(NHEt)6-D2O process. The origins of that peculiarity and the study of
reaction mechanisms beneath are important to optimize and develop processes for the deposition of
silicon based oxides.195, 195-204 Therefore, the reaction mechanisms of the Al(CH3)3-D2O-Si2(NHEt)6-
D2O ALD process were studied in situ with a QCM and QMS at 200 °C (II). Two additional pulsing
sequences were also investigated to assess the specific surface reactivity of Si2(NHEt)6 and Al(CH3)3.
Additional information on the reaction mechanisms was obtained from ex situ characterization of the
deposited films by XRR, XPS, EDX and FT-IR.
Figure 11. QCM data illustrating the supporting effect of aluminium oxide in silicon oxide growth.
34
The main by-products observed with QMS were CDH3 and NDHC2H5. They point to a ligand exchange
reaction with surface hydroxyl groups during the adsorption of Si2(NHEt)6 and Al(CH3)3. On average,
1,3 methyl ligands of Al(CH3)3 and 3,8 –NHEt ligands of Si2(NHEt)6, were released during the metal
precursor pulses, the rest being eliminated during the following D2O pulses. However, the low
reactivity of Si2(NHEt)6 with D2O leaves a small amount of –NHEt ligands present on the surface after
the last D2O pulse. The FT-IR spectra of the film showed the presence of Si–CH3 bonds which
indicates the incorporation of methyl groups from TMA into the film. This could be the result of either
opening of Si-O-Si bridge by TMA or exchange of –CH3 ligands with the few –NHEt ligands remaining
on the surface when the TMA pulse started.
Several pathways were considered for the adsorption of Si2(NHEt)6 on a hydroxylated surface (Figure
12). The high growth rate observed for the Al(CH3)3-D2O-Si2(NHEt)6-D2O process (2.2 Å/cycle) and
the detection of Si-H bonds in the films support the pathway B where both Si atoms in the dimeric
Si2(NHEt)6 precursor molecule bond to the surface with a cleavage of their Si-Si bond. This pathway
also explains why the Si2(NHEt)6-D2O process could not sustain itself: the adsorption of Si2(NHEt)6
forms Si-D surface groups that cannot be eliminated by the D2O pulse. This surface poisoning added
to the low reactivity of silicon precursors renders the deposition of binary SiO2 with water impossible.
However, in the Al(CH3)3-D2O-Si2(NHEt)6-D2O process the high reactivity of TMA induces a better
regeneration of the reactive surface sites and allows the deposition to take place.
Figure 12. Possible reaction pathways for the attachment of Si2(NHEt)6 on a hydroxylated surface.
By integrating the QCM, EDX and XPS results into a quantitative reaction model, the following
reaction mechanism could be drawn:
1.4 Al(CH3)3 + 1.8 –OD + 0.2 –Si(NHC2H5)
–O1.8-Al1.4(CH3)2.4 + 1.8 CH3D + 0.2 –Si(NHC2H5)
35
–O1.8-Al1.4(CH3)2.4+ 0.2 –Si(NHC2H5)
–O1.8-Al1,4(CH3)2.2(NHC2H5)0.2 + 0.2 –Si(CH3)
–O1.8-Al1,4(CH3)2.2(NHC2H5)0.2 + 0.2 –Si(CH3) + 2.4 D2O
–O1.8-Al1,4(OD)2.4 + 2.4 CH3D + 0.2 NDHC2H5 + 0.2 –Si(CH3)
–O1.8-Al1,4(OD)2.4 + 0.5 [Si(NHC2H5)3]2 + 0.2 –Si(CH3)
–O4.2-Al1,4SiD0,5(NHC2H5)1.1 + 0.2 –Si(CH3) + 1.9 NDHC2H5
–O4.2-Al1,4SiD0,5(NHC2H5)1.1 + 0.2 –Si(CH3) + 0.9 D2O
–O4.2-Al1,4SiD0,5(NHC2H5)0.2OD1.8 + 0.2 –Si(CH3) + 0.9 NDHC2H5
This mechanism illustrates the incorporation of Si-D and Si-CH3 bonds in the film and the incomplete
reaction of surface –NHEt ligands with D2O but cannot account for the high amount of carbon
impurities measured. This study demonstrates the importance of the growth surface in ternary oxide
deposition and that parasitic reactions can strongly influence the quality and repeatability of an ALD
process.
4.3 Li2SiO3
Lithium silicates can potentially be used in lithium ion batteries as solid state electrolytes, leading to
all solid state batteries with improved efficiency and safety. LiN[Si(CH3)3]2 was previously used with
ozone by our group to deposit amorphous Li2SiO3.205 Interestingly, the growth rate of the ALD process
and the composition of the deposited film (Li/Si ratio) seem to be highly dependent on the reaction
temperature. To investigate these characteristics, the reaction mechanisms were studied in situ with
QCM and QMS at several temperatures (III). In addition, the chemical bonds present in the deposited
films were identified with ex situ FT-IR.
QMS results reveal typical combustion by-products such as CO2 (m/z = 44), CO (m/z = 28), H2O (m/z
=18) and NO (m/z = 30) being released during the ozone pulse. Signals corresponding to the
fragments of the ligands were detected during the LiN[Si(CH3)3]2 pulse but their low intensities imply
that there are no direct ligand exchange reactions with the hydroxyl groups on the surface. The mass
ratios measured with QCM indicate a non-stoichiometric adsorption of the ligand thus confirming its
decomposition through complex reactions upon reaching the surface. Accordingly, several reaction
pathways were drawn (Figure 13) and DFT calculations were performed to assess the reactivity of
possible reaction intermediate.
36
Figure 13. Possible reaction pathways upon adsorption of LiN[Si(CH3)3]2 on an –OH covered surface.
The FT-IR results suggest that lithium is incorporated into the film as Li+ ions and show the presence
of Li2CO3 and Si-CH3 in the deposited films. The latter originates from an incomplete combustion of –
Si(CH3)3 surface groups during the ozone pulse.
The Li/Si ratio decreases and the growth rate increases at higher deposition temperatures as the
adsorption of HN[Si(CH3)3]2 (intermediate 1 in Figure 13) becomes more efficient and thus more
silicon is incorporated in the film. Faster desorption of other non-reactive intermediates can also play
a role by leaving more surface –OH groups available for further adsorption reactions.
The deposited lithium silicate film continues to react with LiN[Si(CH3)3]2 molecules beyond the
common monolayer adsorption reaction after the end of the ALD cycles. This is due to the high
reactivity of lithium oxide with H2O, especially in binary form but apparently also in lithium silicates: a
“bulk reactivity” occurs where the film bulk absorbs water according to the following reaction (the
underlined species denote the film bulk):
Li2O + H2O 2 LiOH
The LiOH thereby formed can further react with LiN[Si(CH3)3]2 molecules. In addition, CO2 created by
combustion of the LiN[Si(CH3)3]2 by ozone during the process can also react with the Li-OH bonds
present in the film, forming lithium carbonate and water:
2 Li-OH + CO2 Li2CO3 + H2O or Li2O + CO2 Li2CO3
37
These parasitic reactions induce reactions not only on the surface of the growing film but also within
the bulk. Since this publication, other groups have also observed this “bulk reactivity” for several other
lithium containing ternary oxides.182, 206, 207 Accordingly, the films produced with the LiN[Si(CH3)3]2 - O3
process tend to deteriorate rapidly when exposed to air. To prevent these parasitic reactions in the
bulk, the films should be capped to prevent their deterioration by exposure to water and CO2
molecules.
4.4 TiO2
Titanium dioxide (TiO2) thin films have promising applications as dielectric materials for the next
generation dynamic random access memories (DRAMs) and can also be used as photocatalysts. Two
processes for depositing TiO2 with heteroleptic precursors and D2O were studied in situ with QCM
and QMS at 275 °C: Ti(OiPr)3(NiPr-Me-amd) has three isopropoxy ligands and an amidinate ligand
whereas Ti(NMe2)2(OiPr)2 possesses two isopropoxy ligands and two alkylamide ligands. Comparison
of these precursors helps to understand the synergy between different types of ligands and their
effect on the adsorption reaction mechanisms (IV).
For the Ti(NMe2)2(OiPr)2-D2O process, the QCM mass ratios measured suggest either one –NMe2
ligand or one –OiPr ligand exchanged with surface –OD groups, or Ti(NMe2)2(OiPr)2 simply adsorbing
on the surface without ligand exchange. However, the main signals observed with QMS belonged to
fragments of deuterated ligands and allowed the discrimination between these different possibilities:
as only DNMe2 fragments were observed during the metal precursor pulse the adsorption of
Ti(NMe2)2(OiPr)2 occurs through an exchange of one alkylamide ligand:
Ti(NMe2)2(OiPr)2 + –OD -O-Ti(NMe2)(OiPr)2 + DNMe2
-O-Ti(NMe2)(OiPr)2 + 2 D2O TiO2 + –OD + DNMe2 + 2 DOiPr
These results show the importance of monitoring both the surface changes and the gas phase
species during the ALD process in order to draw a coherent reaction mechanism.
Figure 14. Comparison of QCM data during the Ti(OiPr)3(NiPr-Me-amd) and Ti(NMe2)2(OiPr)2 pulses
on hydroxylated surface.
38
Moreover, differences in the conclusions of the QCM and QMS results often reveal more complex
surface reactions: for the Ti(OiPr)3(NiPr-Me-amd)-D2O process the QMS results indicated one out of
three isopropoxy ligands being exchanged during the metal precursor pulse but this did not correlate
with the mass ratios measured with QCM. In addition, the lower intensity of the signals from (NiPr-Me-
amd) and the shape of the QCM mass increase suggested decomposition of the (NiPr-Me-amd)
ligand upon reaching the surface (Figure 14). This partial decomposition was also supported by our
calculation model: the relatively high growth rate observed for this process is incompatible with the
whole (NiPr-Me-amd) ligand remaining on the surface.
In this work, the influence of the other ligands on the release of isopropoxy ligand was demonstrated:
in Ti(NMe2)2(OiPr)2, an alkylamide ligand primarily reacts with the surface and the isopropoxy ligands
did not take part in the precursor adsorption whereas with Ti(OiPr)3(NiPr-Me-amd), the decomposition
of the amidinate ligand allowed one –OiPr ligand to exchange on the surface. For comparison, during
the adsorption of Ti(OiPr)4 two hydrogenated –OiPr ligands are exchanged with the surface hydroxyl
groups208
Figure 15. Comparison of the QCM data for one cycle of the (Me2N)2Ti(OiPr)2-O3 process at 275°C
(dashed line) and 325°C (full line).
The reaction mechanisms of the same precursors with ozone were also studied (VI). The in situ
results obtained at 275 and 325 °C were compared to explain the increase in GR observed in the
Ti(NMe2)2(OiPr)2-O3 process above 300 °C (see publication VI). The QMS results showed the release
of typical combustion by-products (CO2, CO, H2O, NO) during both the ozone pulse and the metal
precursor pulse in the two processes. Accordingly, partial combustion reactions seem to occur also
during the metal precursor adsorption. Various reactive surface sites are possibly involved in these
39
adsorption reactions: active oxygen radicals formed during the previous ozone pulse could remain on
the surface and partially combust the metal precursor ligands. Surface carbonates, formates and
hydroxyl groups originating from readsorption of combustion by-products (H2O and CO2) can also
react with metal precursor through ligand exchange.
Figure 15 shows a comparison of the QCM results obtained for the Ti(NMe2)2(OiPr)2-O3 process at
two temperatures. At 325 °C, the mass increases strongly and continuously during the whole
Ti(NMe2)2(OiPr)2 pulse and then steadily decreases during the following purge. This behavior supports
partial decomposition of the ligands remaining after the Ti(NMe2)2(OiPr)2 pulse at 325 °C while at 275
°C they remain intact. Additionally, the percentage of all by-products with QMS observed during the
metal precursor pulse increased at 325 °C, suggesting that more ligands react and leave the surface
during the adsorption of Ti(NMe2)2(OiPr)2. The higher GR at 325 °C can thus be explained by an
increased reactivity of Ti(NMe2)2(OiPr)2 molecules and a partial decomposition of the ligands.
4.5 ZrO2
ZrO2 is a promising high-k dielectric material for DRAM and MOSFETS applications. The ALD
processes available to deposit this material often use homoleptic metal precursors with poor thermal
stability. To circumvent this issue, heteroleptic precursors have been experimented and heteroleptic
Cp-alkyls, Cp-alkylamides and Cp-halides have shown good thermal stability. In addition, novel
guanidinate precursors, although relatively complex, produce high quality films but their surface
chemistry is mostly unknown. Accordingly, we studied the reaction mechanism of Zr(NEtMe)2(guan-
NEtMe)2 with D2O in situ with the combined QCM-QMS at 225 °C (V).
Figure 16 displays a comparison of the mass increases measured by QCM for the adsorption of
Zr(NEtMe)2(guan-NEtMe)2 and Zr(NEtMe)4 on a hydroxylated surface: in the case of
Zr(NEtMe)2(guan-NEtMe)2, the mass increase is less sharp and a longer time is needed to reach
saturation. This behavior shows the importance of the guanidinate ligands in the adsorption of
Zr(NEtMe)2(guan-NEtMe)2: much like the amidinate precursor of the previous study, the guanidinate
ligand most likely decomposes rapidly upon reaching the surface. This occurs through complex
surface reactions which induce gradual desorption of by-products and reorganization of the remaining
adsorbed molecules thus explaining the slower saturation. Oppositely, the adsorption of the
homoleptic Zr(NEtMe)4 occurs through simple exchange reactions of the alkylamide ligands with
surface –OH groups leading to a much faster saturation. The apparent two-step saturation is probably
due to differences in the mass transport of the metal precursors flowing through different parts of the
reaction chamber cross section: A small portion of the precursor molecules can bypass the glass
array and is responsible for the first step of the QCM mass increase but is not enough to reach
saturation. The main pulse arrives later and completes the saturation of the quartz crystal in the
second step.
40
Figure 16. Comparison of QCM data during the Zr(NEtMe)2(guan-NEtMe)2 and Zr(NEtMe)2 pulses on
hydroxylated surface.
The QMS results indicate a release of by-products from decomposition of the guan-NEtMe ligands
only during the metal precursor pulse. In addition, by-products originating from the deuterated –
NEtMe ligands are only observed during the water pulse. This supports the decomposition of the
guanidinate ligand during the adsorption of Zr(NEtMe)2(guan-NEtMe)2 and the removal of the
remaining alkylamide ligands during the following water pulse. The high growth rate observed for this
process (0.8 Å/cycle) also indicates close packing of the adsorbed molecules on the surface. A partial
decomposition of the guanidinate ligands upon adsorption of Zr(NEtMe)2(guan-NEtMe)2 leaves more
neighboring reactive surface sites available to react and thus lead to a higher number of precursor
molecules adsorbed on the surface.
Despite this partial decomposition of the Zr(NEtMe)2(guan-NEtMe)2 precursor during the process, the
films deposited exhibit remarkably low amount of impurities and the conformality is maintained even in
high aspect ratio trenches. These results emphasize the importance of using new heteroleptic
precursors to produce good quality thin films within a broader temperature range compared to the
commonly used homoleptic precursor. Although the reactions mechanisms occurring in these ALD
processes can be complex, in situ measurements allow their understanding and better control.
41
5. Conclusions This thesis is dedicated to the in situ analysis of some common reaction mechanisms occurring during
ALD processes. The reactivity of metal precursors possessing different types of nitrogen bonded
ligands is discussed. A particular emphasis was put on the typical adsorption mechanisms of metal
precursors such as ligand exchange and ligand decomposition. The in situ studies of homoleptic and
heteroleptic precursors for the deposition of binary and ternary high-k metal oxides conducted in this
thesis are regrouped in Table 2.
For the deposition of Nb2O5 and Ta2O5, the influence of the metal center on the adsorption
mechanism of tBuN=Nb(NEt2)3 and tBuN=Ta(NEt2)3 was highlighted: with the tantalum center the
imido ligand takes part in the adsorption of the precursor whereas it does not react during the
adsorption of the niobium precursor. The differences in polarity and charge repartition between the
two metal-nitrogen bonds explained this variation in the adsorption mechanism of the two otherwise
similar precursors.
The in situ results on the ALD of AlxSiyOz ternary oxides stressed the importance of TMA as a co-
reactant for the growth of silicate with Si2(NHEt)6 and water: without TMA, the low reactivity of the
silicon precursor and the formation of unreactive Si-D bonds on the surface stops the growth. TMA
allows the partial removal of these Si-D bonds and can potentially increase the number of reactive
sites through Si-O-Si bridge opening, allowing the deposition to continue.
The growth characteristics of the LiN[Si(CH3)3]2 – O3 process were linked to the reaction mechanisms
taking place during and after the ALD deposition: the evolution of the GPC, Li/Si ratio and carbon
content in the film with temperature were explained by the increasing efficiency of the adsorption of
HN[Si(CH3)3]2 and the more complete combustion by ozone. Parasitic reactions with water and CO2
occurring on the surface and within the bulk of the grown film were also explicated.
Adsorption through ligand exchange was characterized for alkylamide and isopropyl ligands in several
heteroleptic precursors (tBuN=Nb(NEt2)3, tBuN=Ta(NEt2)3, Ti(NMe2)2(OiPr)2…) within water processes.
Decomposition of metal precursors during their adsorption was also observed for the
Zr(NEtMe)2(guan-NEtMe)2 – D2O and Ti(OiPr)3(NiPr-Me-amd) – D2O processes. Even with hydroxyl
groups present on the surface, the amidinate and guanidinate ligands seem to decompose upon
reaching the surface. Regarding ozone processes, partial combustion was found to occur during both
metal and ozone pulses in the TiO2 processes studied. An increase in the GR with Ti(NMe2)2(OiPr)2
but not with Ti(OiPr)3(NiPr-Me-amd) above 300 °C was explained by the decomposition of the -NMe2
ligands during the metal precursor pulse.
42
Table 2. Summary of in situ reaction mechanisms studied in this thesis.
Metal Precursor Oxygen Precursor
Material Deposited Reaction Mechanism Publications
tBuN=Nb(NEt2)3 D2O Nb2O5 Ligand exchange I tBuN=Nb(NEt2)3 O3 Nb2O5
Molecular adsorption / combustion I
tBuN=Ta(NEt2)3 D2O Ta2O5 Ligand exchange I tBuN=Ta(NEt2)3 O3 Ta2O5
Ligand exchange / combustion I
Al(CH3)3 / Si2(NHEt)6 D2O AlxSiyOz Ligand exchange II LiN[Si(CH3)3]2 O3 Li2SiO3 Combustion III
Ti(NMe2)2(OiPr)2 D2O TiO2 Ligand exchange IV
Ti(OiPr)3(NiPr-Me-amd) D2O TiO2 Ligand decomposition /
ligand exchange IV
Zr(NEtMe)2(guan-NEtMe)2
D2O ZrO2 Ligand decomposition /
ligand exchange V
Ti(NMe2)2(OiPr)2 O3 TiO2 Combustion VI Ti(OiPr)3(NiPr-Me-amd) O3 TiO2 Combustion VI
43
6. References
(1) Ritala, M.; Leskelä, M. Handb. Thin Film Mater. 2002, 1, 103.
(2) Leskelä, M.; Ritala, M. Thin Solid Films 2002, 409, 138.
(3) Ritala, M.; Niinistö, J. Chem. Vap. Deposition 2009, 158.
(4) Miikkulainen, V., Leskelä, M., Ritala, M.; Puurunen, R. L. J. Appl. Phys. (Melville, NY, U. S. ) 2013, 113, 021301/1.
(5) Kessels, E., Profit, H., Ports, S.; van de Sanden, R. At. Layer Deposition Nanostruct. Mater. 2012, 131.
(6) Li, K., Li, S., Li, N., Klein, T.; Dixon, D. Abstracts, Joint 66th Southwest and 62nd Southeast Regional Meeting of the American Chemical Society, New Orleans, LA, United States, December 1-4 2010, SESW.
(7) Kang, S., Rhee, S.; George, S. M. J. Vac. Sci. Technol. , A 2004, 22, 2392.
(8) Du, X., Du, Y.; George, S. M. J. Vac. Sci. Technol. , A 2005, 23, 581.
(9) Goldstein, D. N., McCormick, J. A.; George, S. M. J. Phys. Chem. C 2008, 112, 19530.
(10) Wang, Y., Dai, M., Ho, M., Wielunski, L. S.; Chabal, Y. J. Appl. Phys. Lett. 2007, 90, 022906/1.
(11) Yokoyama, S., Goto, H., Miyamoto, T., Ikeda, N.; Shibahara, K. Appl. Surf. Sci. 1997, 112, 75.
(12) Snyder, M. Q., McCool, B. A., DiCarlo, J., Tripp, C. P.; DeSisto, W. J. Thin Solid Films 2006, 514, 97.
(13) Burton, B. B., Lavoie, A. R.; George, S. M. J. Electrochem. Soc. 2008, 155, D508.
(14) Rahtu, A., Alaranta, T.; Ritala, M. Langmuir 2001, 17, 6506.
(15) Rahtu, A., Kukli, K.; Ritala, M. Chem. Mater. 2001, 13, 817.
(16) Knapas, K.; Ritala, M. Chem. Mater. 2008, 20, 5698.
(17) Knapas, K., Hatanpää, T., Ritala, M.; Leskelä, M. Chem. Mater. 2010, 22, 1386.
(18) Knapas, K., Rahtu, A.; Ritala, M. Langmuir 2010, 26, 848.
(19) Rahtu, A., Alaranta, T.; Ritala, M. Langmuir 2001, 17, 6506.
(20) Swaminathan, S.; McIntyre, P. C. ECS Trans. 2010, 33, 455.
(21) Lee, S. Y., Jeon, C., Kim, S. H., Kim, Y., Jung, W., An, K.; Park, C. Jpn. J. Appl. Phys. 2012, 51, 031102/1.
(22) Li, K., Li, S., Li, N., Klein, T. M.; Dixon, D. A. J. Phys. Chem. C 2011, 115, 18560.
(23) McDonnell, S., Dong, H., Hawkins, J. M., Brennan, B., Milojevic, M., Aguirre-Tostado, F. S., Zhernokletov, D. M., Hinkle, C. L., Kim, J.; Wallace, R. M. Appl. Phys. Lett. 2012, 100, 141606/1.
(24) Brennan, B., Qin, X., Dong, H., Kim, J.; Wallace, R. M. Appl. Phys. Lett. 2012, 101, 211604/1.
44
(25) Tallarida, M., Karavaev, K.; Schmeisser, D. J. Appl. Phys. 2008, 104, 064116/1.
(26) Strehle, S., Schmidt, D., Albert, M.; Bartha, J. W. Chem. Vap. Deposition 2011, 17, 37.
(27) Shin, B., Clemens, J. B., Kelly, M. A., Kummel, A. C.; McIntyre, P. C. Appl. Phys. Lett. 2010, 96, 252907/1.
(28) Wu, Y., Hermkens, P. M., van de Loo, B. W. H., Knoops, H. C. M., Potts, S. E., Verheijen, M. A., Roozeboom, F.; Kessels, W. M. M. J. Appl. Phys. (Melville, NY, U. S. ) 2013, 114, 024308/1.
(29) Dakshinamurthy, S.; Bhat, I. J. Electron. Mater. 1998, 27, 521.
(30) Kumagai, H.; Toyoda, K. Reza Kagaku Kenkyu 1994, 16, 56.
(31) Langereis, E., Heil, S. B. S., Knoops, H. C. M., Keuning, W., van de Sanden, M. C. M.; Kessels, W. M. M. J. Phys. D: Appl. Phys. 2009, 42, 073001/1.
(32) Langereis, E., Heil, S. B. S., Knoops, H. C. M., Keuning, W., Van de Sanden, M. C. M.; Kessels, W. M. M. Bull. - Soc. Vac. Coaters 2010, 36.
(33) Lamagna, L., Wiemer, C., Perego, M., Spiga, S., Rodriguez, J., Santiago Coll, D., Grillo, M. E., Klejna, S.; Elliott, S. D. Chem. Mater. 2012, 24, 1080.
(34) Dendooven, J., Pulinthanathu Sree, S., De Keyser, K., Deduytsche, D., Martens, J. A., Ludwig, K. F.; Detavernier, C. J. Phys. Chem. C 2011, 115, 6605.
(35) Devloo-Casier, K., Dendooven, J., Ludwig, K. F., Lekens, G., D'Haen, J.; Detavernier, C. Appl. Phys. Lett. 2011, 98, 231905/1.
(36) Methaapanon, R., Geyer, S. M., Brennan, S.; Bent, S. F. Chem. Mater. 2013, 25, 3458.
(37) Elliott, S. D. Semicond. Sci. Technol. 2012, 27, 074008/1.
(38) Katamreddy, R., Inman, R., Jursich, G., Soulet, A.; Takoudis, C. J. Electrochem. Soc. 2006, 153, C701.
(39) Wade, C. R., Silvernail, C., Banerjee, C., Soulet, A., McAndrew, J.; Belot, J. A. Mater. Lett. 2007, 61, 5079.
(40) Katamreddy, R., Inman, R., Jursich, G., Soulet, A., Nicholls, A.; Takoudis, C. Thin Solid Films 2007, 515, 6931.
(41) Majumder, P., Katamreddy, R.; Takoudis, C. Electrochem. Solid-State Lett. 2007, 10, H291.
(42) Brazeau, A. L.; Barry, S. T. Chem. Mater. 2008, 20, 7287.
(43) Dezelah, C. L.,IV, El-Kadri, O. M., Szilagyi, I. M., Campbell, J. M., Arstila, K., Niinistö, L.; Winter, C. H. J. Am. Chem. Soc. 2006, 128, 9638.
(44) Elam, J. W., Baker, D. A., Hryn, A. J., Martinson, A. B. F., Pellin, M. J.; Hupp, J. T. J. Vac. Sci. Technol. , A 2008, 26, 244.
(45) Elam, J. W., Baker, D. A., Martinson, A. B. F., Pellin, M. J.; Hupp, J. T. J. Phys. Chem. C 2008, 112, 1938.
(46) Prasittichai, C.; Hupp, J. T. J. Phys. Chem. Lett. 2010, 1, 1611.
45
(47) Kääriäinen, T. O., Lehti, S., Kääriäinen, M. ; Cameron, D. C. Annu. Tech. Conf. Proc. - Soc. Vac. Coaters 2011, 54th, 357.
(48) Xie, Q., Musschoot, J., Deduytsche, D., Van Meirhaeghe, R. L., Detavernier, C., Van den Berghe, S., Jiang, Y., Ru, G., Li, B.; Qu, X. J. Electrochem. Soc. 2008, 155, H688.
(49) Xie, Q., Jiang, Y., Detavernier, C., Deduytsche, D., Van Meirhaeghe, R. L., Ru, G., Li, B.; Qu, X. J. Appl. Phys. 2007, 102, 083521/1.
(50) Alessandri, I., Zucca, M., Ferroni, M., Bontempi, E.; Depero, L. E. Small 2009, 5, 336.
(51) Gougousi, T.; Lacis, J. W. Thin Solid Films 2010, 518, 2006.
(52) Alessandri, I.; Depero, L. E. ACS Appl. Mater. Interfaces 2010, 2, 594.
(53) Bontempi, E., Zanola, P., Gelfi, M., Zucca, M., Depero, L. E., Girault, B., Goudeau, P., Geandier, G., Le Bourhis, E.; Renault, P. Nucl. Instrum. Methods Phys. Res. , Sect. B 2010, 268, 365.
(54) Gerasopoulos, K., Chen, X., Culver, J., Wang, C.; Ghodssi, R. Chem. Commun. (Cambridge, U. K. ) 2010, 46, 7349.
(55) Rose, M., Bartha, J. W.; Endler, I. Appl. Surf. Sci. 2010, 256, 3778.
(56) Lee, C., Kim, J., Son, J. Y., Choi, W.; Kim, H. Appl. Catal. , B 2009, 91, 628.
(57) Lee, C., Kim, J., Gu, G. H., Jo, D., Park, C. G., Choi, W.; Kim, H. Thin Solid Films 2010, 518, 4757.
(58) Rodriguez-Reyes, J. C. F.; Teplyakov, A. V. J. Phys. Chem. C 2008, 112, 9695.
(59) Lee, C., Kim, J., Son, J. Y., Maeng, W. J., Jo, D., Choi, W.; Kim, H. J. Electrochem. Soc. 2009, 156, D188.
(60) Afanas'ev, V. V., Badylevich, M., Stesmans, A., Brammertz, G., Delabie, A., Sionke, S., O'Mahony, A., Povey, I. M., Pemble, M. E., O'Connor, E., Hurley, P. K.; Newcomb, S. B. Appl. Phys. Lett. 2008, 93, 212104/1.
(61) Chang, C. , Chiou, Y. , Chang, Y. , Lee, K. , Lin, T. , Wu, T. , Hong, M.; Kwo, J. Appl. Phys. Lett. 2006, 89, 242911/1.
(62) Chang, Y. C., Chiu, H. C., Lee, Y. J., Huang, M. L., Lee, K. Y., Hong, M., Chiu, Y. N., Kwo, J.; Wang, Y. H. Appl. Phys. Lett. 2007, 90, 232904/1.
(63) Chen, Z., Sarkar, S., Biswas, N.; Misra, V. J. Electrochem. Soc. 2009, 156, H561.
(64) Chiou, Y., Chang, C.; Wu, T. J. Mater. Res. 2007, 22, 1899.
(65) Cho, M., Kim, J. H., Hwang, C. S., Ahn, H., Han, S.; Won, J. Y. Appl. Phys. Lett. 2007, 90, 182907/1.
(66) Delabie, A., Brunco, D. P., Conard, T., Favia, P., Bender, H., Franquet, A., Sioncke, S., Vandervorst, W., Van Elshocht, S., Heyns, M., Meuris, M., Kim, E., McIntyre, P. C., Saraswat, K. C., LeBeau, J. M., Cagnon, J., Stemmer, S.; Tsai, W. J. Electrochem. Soc. 2008, 155, H937.
(67) Deshpande, A., Inman, R., Jursich, G.; Takoudis, C. J. Vac. Sci. Technol. , A 2004, 22, 2035.
(68) Hackley, J. C., Demaree, J. D.; Gougousi, T. J. Vac. Sci. Technol. , A 2008, 26, 1235.
46
(69) Kamiyama, S., Miura, T.; Nara, Y. Electrochem. Solid-State Lett. 2006, 9, G285.
(70) Kim, J. C., Cho, Y. S.; Moon, S. H. Jpn. J. Appl. Phys. 2009, 48, 066515/1.
(71) Kirsch, P. D., Quevedo-Lopez, M. A., Li, H. -., Senzaki, Y., Peterson, J. J., Song, S. C., Krishnan, S. A., Moumen, N., Barnett, J., Bersuker, G., Hung, P. Y., Lee, B. H., Lafford, T., Wang, Q., Gay, D.; Ekerdt, J. G. J. Appl. Phys. 2006, 99, 023508/1.
(72) Kukli, K., Pilvi, T., Ritala, M., Sajavaara, T., Lu, J.; Leskelä, M. Thin Solid Films 2005, 491, 328.
(73) Lee, T., Ko, H., Ahn, J., Park, I., Sim, H., Park, H.; Hwang, H. Jpn. J. Appl. Phys. , Part 1 2006, 45, 6993.
(74) Liu, X., Ramanathan, S., Longdergan, A., Srivastava, A., Lee, E., Seidel, T. E., Barton, J. T., Pang, D.; Gordon, R. G. J. Electrochem. Soc. 2005, 152, G213.
(75) Nyns, L., Delabie, A., Swerts, J., Van Elshocht, S.; De Gendt, S. J. Electrochem. Soc. 2010, 157, G225.
(76) Hausmann, D. M., Kim, E., Becker, J.; Gordon, R. G. Chem. Mater. 2002, 14, 4350.
(77) Kim, S. K.; Hwang, C. S. Electrochem. Solid-State Lett. 2008, 11, G9.
(78) Kim, Y., Koo, J., Han, J., Choi, S., Jeon, H.; Park, C. J. Appl. Phys. 2002, 92, 5443.
(79) Henkel, C., Abermann, S., Bethge, O.; Bertagnolli, E. Semicond. Sci. Technol. 2009, 24, 125013/1.
(80) Meyer, J., Goerrn, P., Bertram, F., Hamwi, S., Winkler, T., Johannes, H., Weimann, T., Hinze, P., Riedl, T.; Kowalsky, W. Adv. Mater. (Weinheim, Ger. ) 2009, 21, 1845.
(81) Yun, S. J., Lim, J. W.; Lee, J. Electrochem. Solid-State Lett. 2004, 7, F81.
(82) Yun, S. J., Lim, J. W.; Lee, J. H. Electrochem. Solid-State Lett. 2005, 8, F47.
(83) Bethge, O., Abermann, S., Henkel, C., Straif, C. J., Hutter, H.; Bertagnolli, E. J. Electrochem. Soc. 2009, 156, G168.
(84) Hausmann, D. M.; Gordon, R. G. J. Cryst. Growth 2003, 249, 251.
(85) Kim, I., Tuller, H. L., Kim, H.; Park, J. Appl. Phys. Lett. 2004, 85, 4705.
(86) Maeng, W. J.; Kim, H. Electrochem. Solid-State Lett. 2006, 9, G191.
(87) Maeng, W. J., Park, S.; Kim, H. J. Vac. Sci. Technol. , B: Microelectron. Nanometer Struct. --Process. , Meas. , Phenom. 2006, 24, 2276.
(88) Maeng, W. J., Lim, S. J., Kwon, S.; Kim, H. Appl. Phys. Lett. 2007, 90, 062909/1.
(89) Kim, H., Sohn, S., Jung, D., Maeng, W. J., Kim, H., Kim, T. S., Hahn, J., Lee, S., Yi, Y.; Cho, M. Org. Electron. 2008, 9, 1140.
(90) Potts, S. E., Keuning, W., Langereis, E., Dingemans, G., van de Sanden, M. C. M.; Kessels, W. M. M. J. Electrochem. Soc. 2010, 157, P66.
(91) Hausmann, D. M., de Rouffignac, P., Smith, A., Gordon, R.; Monsma, D. Thin Solid Films 2003, 443, 1.
47
(92) Banerjee, P., Perez, I., Henn-Lecordier, L., Lee, S. B.; Rubloff, G. W. Nat. Nanotechnol. 2009, 4, 292.
(93) Jeon, S.; Park, S. J. Electrochem. Soc. 2010, 157, H1101.
(94) Kim, H. K., Kim, J. Y., Park, J. Y., Kim, Y., Kim, Y. D., Jeon, H.; Kim, W. M. J. Korean Phys. Soc. 2002, 41, 739.
(95) Kim, J. Y., Kim, Y.; Jeon, H. Jpn. J. Appl. Phys. , Part 2 2003, 42, L414.
(96) Kim, J. Y., Kim, H. K., Kim, Y., Kim, Y. D., Kim, W. M.; Jeon, H. J. Korean Phys. Soc. 2002, 40, 176.
(97) Kim, J.; Yong, K. Electrochem. Solid-State Lett. 2004, 7, F35.
(98) Becker, J. S., Kim, E.; Gordon, R. G. Chem. Mater. 2004, 16, 3497.
(99) Cho, S., Lee, K., Song, P., Jeon, H.; Kim, Y. Jpn. J. Appl. Phys. , Part 1 2007, 46, 4085.
(100) Kim, E.; Kim, D. Electrochem. Solid-State Lett. 2006, 9, C123.
(101) Consiglio, S., Zeng, W., Berliner, N.; Eisenbraun, E. T. J. Electrochem. Soc. 2008, 155, H196.
(102) Jeong, W., Ko, Y., Bang, S., Lee, S.; Jeon, H. J. Korean Phys. Soc. 2010, 56, 905.
(103) Wu, Y. Y., Kohn, A.; Eizenberg, M. J. Appl. Phys. 2004, 95, 6167.
(104) Bae, N., Na, K., Cho, H., Park, K., Boo, S., Bae, J.; Lee, J. Jpn. J. Appl. Phys. , Part 1 2006, 45, 9072.
(105) Kim, H., Detavenier, C., Van der Straten, O., Rossnagel, S. M., Kellock, A. J.; Park, D. -. J. Appl. Phys. 2005, 98, 014308/1.
(106) Xie, Q., Musschoot, J., Detavernier, C., Deduytsche, D., Van Meirhaeghe, R. L., Van den Berghe, S., Jiang, Y., Ru, G., Li, B.; Qu, X. Microelectron. Eng. 2008, 85, 2059.
(107) Langereis, E., Knoops, H. C. M., Mackus, A. J. M., Roozeboom, F., van de Sanden, M. C. M.; Kessels, W. M. M. J. Appl. Phys. 2007, 102, 083517/1.
(108) Dezelah, C. L., El-Kadri, O. M., Kukli, K., Arstila, K., Baird, R. J., Lu, J., Niinistö, L.; Winter, C. H. J. Mater. Chem. 2007, 17, 1109.
(109) Katamreddy, R., Wang, Z., Omarjee, V., Rao, P. V., Dussarrat, C.; Blasco, N. ECS Trans. 2009, 25, 217.
(110) Blanquart, T., Niinistö, J., Gavagnin, M., Longo, V., Heikkilä, M., Puukilainen, E., Pallem, V. R., Dussarrat, C., Ritala, M.; Leskelä, M. RSC Adv. 2013, 3, 1179.
(111) Nam, W.; Rhee, S. Chem. Vap. Deposition 2004, 10, 201.
(112) Triyoso, D. H., Hegde, R. I., Grant, J., Fejes, P., Liu, R., Roan, D., Ramon, M., Werho, D., Rai, R., La, L. B., Baker, J., Garza, C., Guenther, T., White, B. E., Jr.; Tobin, P. J. J. Vac. Sci. Technol. , B: Microelectron. Nanometer Struct. --Process. , Meas. , Phenom. 2004, 22, 2121.
(113) Lim, B. S., Rahtu, A., de Rouffignac, P.; Gordon, R. G. Appl. Phys. Lett. 2004, 84, 3957.
(114) Inman, R., Schuetz, S. A., Silvernail, C. M., Balaz, S., Dowben, P. A., Jursich, G., McAndrew, J.; Belot, J. A. Mater. Chem. Phys. 2007, 104, 220.
48
(115) Wang, T.; Ekerdt, J. G. Chem. Mater. 2010, 22, 3798.
(116) de Rouffignac, P.; Gordon, R. G. Chem. Vap. Deposition 2006, 12, 152.
(117) Jones, A. C., Aspinall, H. C., Chalker, P. R., Potter, R. J., Kukli, K., Rahtu, A., Ritala, M.; Leskelä, M. Mater. Sci. Eng. , B 2005, B118, 97.
(118) Kukli, K., Ritala, M., Pilvi, T., Sajavaara, T., Leskelä, M., Jones, A. C., Aspinall, H. C., Gilmer, D. C.; Tobin, P. J. Chem. Mater. 2004, 16, 5162.
(119) Scarel, G., Wiemer, C., Tallarida, G., Spiga, S., Seguini, G., Bonera, E., Fanciulli, M., Lebedinskii, Y., Zenkevich, A., Pavia, G., Fedushkin, I. L., Fukin, G. K.; Domrachev, G. A. J. Electrochem. Soc. 2006, 153, F271.
(120) Scarel, G., Wiemer, C., Fanciulli, M., Fedushkin, L., Fukin, G. K., Domrachev, G. A., Lebedinskii, Y., Zenkevich, A.; Pavia, G. Z. Anorg. Allg. Chem. 2007, 633, 2097.
(121) Hämäläinen, J., Munnik, F., Hatanpää, T., Holopainen, J., Ritala, M.; Leskelä, M. J. Vac. Sci. Technol. , A 2012, 30, 01A106/1.
(122) Rees, W. S., Jr., Green, D. M., Anderson, T. J., Bretschneider, E., Pathangey, B., Park, C.; Kim, J. J. Electron. Mater. 1992, 21, 361.
(123) Lim, B. S., Rahtu, A.; Gordon, R. G. Nat. Mater. 2003, 2, 749.
(124) Li, Z., Gordon, R. G., Farmer, D. B., Lin, Y.; Vlassak, J. Electrochem. Solid-State Lett. 2005, 8, G182.
(125) Lee, H., Kim, W., Lee, J. W., Kim, J., Heo, K., Hwang, I. C., Park, Y., Hong, S.; Kim, H. J. Electrochem. Soc. 2010, 157, D10.
(126) Lee, H.; Kim, H. Electrochem. Solid-State Lett. 2006, 9, G323.
(127) Kim, J., Lee, H., Lansalot, C., Dussarrat, C., Gatineau, J.; Kim, H. Jpn. J. Appl. Phys. 2010, 49, 05FA10/1.
(128) Wang, H., Gordon, R. G., Alvis, R.; Ulfig, R. M. Chem. Vap. Deposition 2009, 15, 312.
(129) Li, H., Farmer, D. B., Gordon, R. G., Lin, Y.; Vlassak, J. J. Electrochem. Soc. 2007, 154, D642.
(130) Farmer, D. B.; Gordon, R. G. J. Appl. Phys. 2007, 101, 124503/1.
(131) Li, Z., Rahtu, A.; Gordon, R. G. J. Electrochem. Soc. 2006, 153, C787.
(132) Kucheyev, S. O., Biener, J., Baumann, T. F., Wang, Y. M., Hamza, A. V., Li, Z., Lee, D. K.; Gordon, R. G. Langmuir 2008, 24, 943.
(133) Seitz, O., Dai, M., Aguirre-Tostado, F. S., Wallace, R. M.; Chabal, Y. J. J. Am. Chem. Soc. 2009, 131, 18159.
(134) Ma, Q., Guo, H., Gordon, R. G.; Zaera, F. Chem. Mater. 2010, 22, 352.
(135) Dai, M., Kwon, J., Halls, M. D., Gordon, R. G.; Chabal, Y. J. Langmuir 2010, 26, 3911.
(136) De Rouffignac, P., Park, J.; Gordon, R. G. Chem. Mater. 2005, 17, 4808.
(137) Lee, B., Park, T. J., Hande, A., Kim, M. J., Wallace, R. M., Kim, J., Liu, X., Yi, J. H., Li, H., Rousseau, M., Shenai, D.; Suydam, J. Microelectron. Eng. 2009, 86, 1658.
49
(138) Abermann, S., Henkel, C., Bethge, O., Pozzovivo, G., Klang, P.; Bertagnolli, E. Appl. Surf. Sci. 2010, 256, 5031.
(139) Kwon, J., Dai, M., Halls, M. D., Langereis, E., Chabal, Y. J.; Gordon, R. G. J. Phys. Chem. C 2009, 113, 654.
(140) Päiväsaari, J., Putkonen, M., Sajavaara, T.; Niinistö, L. J. Alloys Compd. 2004, 374, 124.
(141) Rische, D., Parala, H., Baunemann, A., Thiede, T.; Fischer, R. Surf. Coat. Technol. 2007, 201, 9125.
(142) Coyle, J. P., Monillas, W. H., Yap, G. P. A.; Barry, S. T. Inorg. Chem. 2008, 47, 683.
(143) Coyle, J. P., Johnson, P. A., DiLabio, G. A., Barry, S. T.; Mueller, J. Inorg. Chem. 2010, 49, 2844.
(144) Chen, T., Xu, C., Baum, T. H., Hendrix, B. C., Cameron, T., Roeder, J. F.; Stender, M. PCT Int. Appl. 2007, 2006-US62709; 2006-810578P, 47.
(145) Kukli, K., Ritala, M., Kemell, M.; Leskelä, M. J. Electrochem. Soc. 2010, 157, D35.
(146) Thiede, T. B., Krasnopolski, M., Milanov, A. P., de los Arcos, T., Ney, A., Becker, H., Rogalla, D., Winter, J., Devi, A.; Fischer, R. A. Chem. Mater. 2011, 23, 1430.
(147) Kukli, K., Niinistö, J., Tamm, A., Ritala, M.; Leskelä, M. J. Vac. Sci. Technol. , B: Microelectron. Nanometer Struct. --Process. , Meas. , Phenom. 2009, 27, 226.
(148) Milanov, A., Bhakta, R., Baunemann, A., Becker, H., Thomas, R., Ehrhart, P., Winter, M.; Devi, A. Inorg. Chem. 2006, 45, 11008.
(149) Edelmann, F. T. Chem Soc Rev 2009, 38, 2253.
(150) Edelmann, F. T. Chem. Soc. Rev. 2012, 41, 7657.
(151) Blanquart, T., Niinistö, J., Gavagnin, M., Longo, V., Pallem, V. R., Dussarrat, C., Ritala, M.; Leskelä, M. Chem. Mater. 2012, 24, 3420.
(152) Seo, M., Min, Y., Kim, S. K., Park, T. J., Kim, J. H., Na, K. D.; Hwang, C. S. J. Mater. Chem. 2008, 18, 4324.
(153) Pallem, V. R.; Dussarrat, C. PCT Int. Appl. 2012, 2011-US31360; 2010-981872, 38.
(154) Niinistö, J., Kukli, K., Tamm, A., Putkonen, M., Dezelah, C. L., Niinistö, L., Lu, J., Song, F., Williams, P., Heys, P. N., Ritala, M.; Leskelä, M. J. Mater. Chem. 2008, 18, 3385.
(155) Potts, S. E., Carmalt, C. J., Blackman, C. S., Abou-Chahine, F., Leick, N., Kessels, W. M. M., Davies, H. O.; Heys, P. N. Inorg. Chim. Acta 2010, 363, 1077.
(156) Carta, G., Rossetto, G., Sitran, S., Zanella, P., Crociani, L., Zherikova, K. V., Morozova, N. B., Gelfond, N. V., Semyannikov, P. P., Yakovkina, L. V., Smirnova, T. P.; Igumenov, I. K. Proc. - Electrochem. Soc. 2005, 2005-09, 260.
(157) Sreenivasan, R., Sugawara, T., Saraswat, K. C.; McIntyre, P. C. Appl. Phys. Lett. 2007, 90, 102101/1.
(158) Kim, J. Y., Lee, K. W., Park, H. O., Kim, Y. D., Jeon, H.; Kim, Y. J. Korean Phys. Soc. 2004, 45, 1069.
50
(159) Kumar, S., Greenslit, D., Chakraborty, T.; Eisenbraun, E. T. J. Vac. Sci. Technol. , A 2009, 27, 572.
(160) Choi, B. H., Lim, Y. H., Lee, J. H., Kim, Y. B., Lee, H.; Lee, H. K. Microelectron. Eng. 2010, 87, 1391.
(161) Li, Z., Gordon, R. G., Farmer, D. B., Lin, Y.; Vlassak, J. Electrochem. Solid-State Lett. 2005, 8, G182.
(162) Becker, J. S., Suh, S., Wang, S.; Gordon, R. G. Chem. Mater. 2003, 15, 2969.
(163) Becker, J. S.; Gordon, R. G. Appl. Phys. Lett. 2003, 82, 2239.
(164) Rugge, A., Becker, J. S., Gordon, R. G.; Tolbert, S. H. Nano Lett. 2003, 3, 1293.
(165) Miikkulainen, V., Suvanto, M.; Pakkanen, T. A. Chem. Mater. 2007, 19, 263.
(166) Miikkulainen, V., Suvanto, M.; Pakkanen, T. A. Thin Solid Films 2008, 516, 6041.
(167) Miikkulainen, V., Suvanto, M.; Pakkanen, T. A. Chem. Vap. Deposition 2008, 14, 71.
(168) Miikkulainen, V., Suvanto, M., Pakkanen, T. A., Siitonen, S., Karvinen, P., Kuittinen, M.; Kisonen, H. Surf. Coat. Technol. 2008, 202, 5103.
(169) Blanquart, T., Longo, V., Niinistö, J., Heikkilä, M., Kukli, K., Ritala, M.; Leskelä, M. Semicond. Sci. Technol. 2012, 27, 074003/1.
(170) Blanquart, T., Niinistö, J., Heikkilä, M., Sajavaara, T., Kukli, K., Puukilainen, E., Xu, C., Hunks, W., Ritala, M.; Leskelä, M. Chem. Mater. 2012, 24, 975.
(171) Xu, K., Milanov, A. P., Parala, H., Wenger, C., Baristiran-Kaynak, C., Lakribssi, K., Toader, T., Bock, C., Rogalla, D., Becker, H., Kunze, U.; Devi, A. Chem. Vap. Deposition 2012, 18, 27.
(172) Xu, K., Milanov, A. P.; Devi, A. ECS Trans. 2009, 25, 625.
(173) Eleter, M., Daniele, S., Brize, V., Dubourdieu, C., Lachaud, C., Blasco, N.; Pinchart, A. ECS Trans. 2009, 25, 151.
(174) Blanquart, T., Niinistö, J., Aslam, N., Banerjee, M., Tomczak, Y., Gavagnin, M., Longo, V., Puukilainen, E., Wanzenböck, H. D., Kessels, W. M. M., Devi, A., Hoffmann-Eifert, S., Ritala, M.; Leskelä, M. Chem. Mater. 2013, 25, 3088.
(175) Xu, K., Ranjith, R., Laha, A., Parala, H., Milanov, A. P., Fischer, R. A., Bugiel, E., Feydt, J., Irsen, S., Toader, T., Bock, C., Rogalla, D., Osten, H., Kunze, U.; Devi, A. Chem. Mater. 2012, 24, 651.
(176) Shirazi, M.; Elliott, S. D. Chem. Mater. 2013, 25, 878.
(177) Coyle, J. P., Johnson, P. A., DiLabio, G. A., Barry, S. T.; Mueller, J. Inorg. Chem. (Washington, DC, U. S. ) 2010, 49, 2844.
(178) Barry, S. T. Coord. Chem. Rev. 2013, 257, 3192.
(179) Brazeau, A. L., Wang, Z., Rowley, C. N.; Barry, S. T. Inorg. Chem. 2006, 45, 2276.
(180) Park, K.; Yao, Q. Abstracts of Papers, 234th ACS National Meeting, Boston, MA, United States, August 19-23, 2007 2007, ORGN.
51
(181) Yoshimura, Y., Kumamoto, H., Baba, A., Takeda, S.; Tanaka, H. Nucleic Acids Res. Suppl. 2003, 3, 17.
(182) Ostreng, E., Sonsteby, H. H., Sajavaara, T., Nilsen, O.; Fjellvåg, H. J. Mater. Chem. C 2013, 1, 4283.
(183) Comstock, D. J.; Elam, J. W. Chem. Mater. 2012, 24, 4011.
(184) Li, K., Li, S., Li, N., Klein, T.; Dixon, D. Abstracts, Joint 66th Southwest and 62nd Southeast Regional Meeting of the American Chemical Society, New Orleans, LA, United States, December 1-4 2010, SESW.
(185) Kan, B., Boo, J., Lee, I.; Zaera, F. J. Phys. Chem. A 2009, 113, 3946.
(186) Rodriguez-Reyes, J. C. F.; Teplyakov, A. V. J. Appl. Phys. 2008, 104, 084907/1.
(187) Kim, T.; Zaera, F. J. Phys. Chem. C 2011, 115, 8240.
(188) Kelly, M. J., Han, J. H., Musgrave, C. B.; Parsons, G. N. Chem. Mater. 2005, 17, 5305.
(189) Ma, Q., Lee, I.; Zaera, F. Abstracts of Papers, 239th ACS National Meeting, San Francisco, CA, United States, March 21-25, 2010 2010, COLL.
(190) Wu, J., Li, J., Zhou, C., Lei, X., Gaffney, T., Norman, J. A. T., Li, Z., Gordon, R.; Cheng, H. Organometallics 2007, 26, 2803.
(191) Barry, S. T., Gordon, P. G., Ward, M. J., Heikkilä, M. J., Monillas, W. H., Yap Glenn, P. A., Ritala, M.; Leskela, M. Dalton Trans 2011, 40, 9425.
(192) Kim, T., Yao, Y., Coyle, J. P., Barry, S. T.; Zaera, F. Chem. Mater. 2013, 25, 3630.
(193) Haukka, S.; Root, A. J. Phys. Chem. 1994, 98, 1695.
(194) Gun'ko, V. M., Vedamuthu, M. S., Henderson, G. L.; Blitz, J. P. J. Colloid Interface Sci. 2000, 228, 157.
(195) Senzaki, Y., Park, S., Tweet, D., Conley, J. F., Jr.; Ono, Y. Mater. Res. Soc. Symp. Proc. 2004, 811, 217.
(196) Rittersma, Z. M., Roozeboom, F., Verheijen, M. A., van Berkum, J. G. M., Dao, T., Snijders, J. H. M., Vainonen-Ahlgren, E., Tois, E., Tuominen, M.; Haukka, S. J. Electrochem. Soc. 2004, 151, C716.
(197) Kamiyama, S., Miura, T., Nara, Y.; Arikado, T. Electrochem. Solid-State Lett. 2005, 8, G215.
(198) Kamiyama, S., Miura, T.; Nara, Y. Electrochem. Solid-State Lett. 2005, 8, F37.
(199) Duenas, S., Castan, H., Garcia, H., Barbolla, J., Kukli, K., Ritala, M.; Leskelä, M. Span. Conf. Electron Devices, Proc. , 5th 2005, 45.
(200) Senzaki, Y., Park, S., Bartholomew, L.; Chatham, H. Proc. - Electrochem. Soc. 2004, 2004-01, 264.
(201) Nam, W.; Rhee, S. Stud. Surf. Sci. Catal. 2006, 159, 373.
(202) Zhang, H.; Solanki, R. J. Electrochem. Soc. 2001, 148, F63.
52
(203) Gladfelter, W. L.; Zhong, L. Abstracts of Papers, 227th ACS National Meeting, Anaheim, CA, United States, March 28-April 1, 2004 2004, INOR.
(204) Zhong, L., Daniel, W. L., Zhang, Z., Campbell, S. A.; Gladfelter, W. L. Chem. Vap. Deposition 2006, 12, 143.
(205) Hämälainen, J., Ihanus, J., Sajavaara, T., Ritala, M.; Leskelä, M. J. Electrochem. Soc. 2011, 158, P15.
(206) Ostreng, E., Vajeeston, P., Nilsen, O.; Fjellvåg, H. RSC Adv. 2012, 2, 6315.
(207) Miikkulainen, V., Nilsen, O., Laitinen, M., Sajavaara, T.; Fjellvåg, H. RSC Adv. 2013, 3, 7537.
(208) Rahtu, A.; Ritala, M. Chem. Vap. Deposition 2002, 8, 21.