Nucleation of Nb on Cu
A-M Valente-Feliciano
A. Lukaszew (College William & Mary)
L. Phillips, C. Reece, J. Spradlin (JLab)
Thin Film Nucleation overview
The Nb – Cu system
Hetero-epitaxy
Fiber growth
Conclusions
Outline
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Nucleation: Why do we care?
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
The thickness of interest for SRF applications corresponds to the RF penetration depth, i.e. the very top 40 nm of the Nb film. However the final surface is dictated from its origin, i.e. the substrate, the interface, and deposition technique (ion energy, substrate temperature…)
Heterogeneous nucleation.Nucleation driven by nucleation centers such as defect, impurities on the substrate surface or the orientation of the underlying substrate in the case of hetero-epitaxy.
Crystalline Cu vs. Amorphous CuO Substrate
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
CI=
CI=
Standard films Oxide-free films
0.5 µm 0.5 µm
Columnar grains, size ~ 100 nm
In plan diffraction pattern: powder diagram
(110) fiber texture substrate plane
Equi-axed grains, size ~ 1-5mm
In plan diffraction pattern: zone axis [110]
Heteroepitaxy
Nb (110) //Cu(010) , Nb (110) //Cu(111),Nb (100) //Cu(110)
Courtesy of CERN, P. Jacob,FEI
CERN magnetron sputtered 1.5GHZ Nb/Cu films (coated with Ar)
Standard Oxide-free
RRR 11.5 ± 0.1 28.9 ± 0.9
TC 9.51 ± 0.01 K 9.36 ± 0.04 K
Ar cont. 435 ± 70 ppm 286 ± 43 ppm
Texture -110 (110), (211), (200)
Hc1 85 ± 3 mT 31 ± 5 mT
Hc2 1.150 ± 0.1 T 0.73 ± 0.05 T
a0 3.3240(10)Å 3.3184(6) Å
a/a 0.636 ± 0.096 % 0.466 ± 0.093 %
Stress -706 ± 56 MPa -565 ± 78 MPa
Grain size 110 ± 20 nm > 1 µm
Oxide –free films closer to bulk but Rres, standard < Rres, oxide-free
Condensation from the vapor involves incident atoms becoming bonded adatoms whichdiffuse over the film surface until they are trapped at low energy lattice sites. This atomicodyssey involves 4 basic processes:
ShadowingSurface diffusionBulk diffusionDesorption
The last 3 are quantified by the characteristic diffusion and sublimation activation energies(scaling with the melting point). Shadowing arises from the line of sight impingement ofarriving toms. The dominance of one or more of these processes as function of substratetemperature is manifested by different structural morphologies.
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Film Nucleation & GrowthThin film growth from the gas phase=non-equilibrium process phenomenon governed by a competition between kinetics & thermodynamics. 3 stages can be distinguished in the nucleation & growth of films:
• Production of ionic, molecular or atomic species in the gas phase.• Transport of these species to the substrate• Condensation of the species onto the substrate directly or either by chemical or
electrochemical reaction.
Hetero-epitaxy of Nb on Cu
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
DC magnetron sputtered @ 150 °CLukaszew A. et al. , W&M
[W. M. Roach et al. , PRSTAB 15, 062002 (2012)][Masek &Matolin, Vacuum 61(2001) 217-221]
bcc Nb on fcc Cu system hetero-epitaxial relationships: [110]Nb || [100]Cu 4 domains[100]Nb || [110]Cu [110]Nb || [111]Cu 6 domains
Growth of Nb on CuOThe Cu oxide (CuO) is amorphous, the Nb growth is then not driven by the substrate orientation
Vacuum ~10-11 TorrSubstrate cleaned by in-situ Ar + etching andannealing @ 600 °C
Topography STM maps of Nb islands deposited onCu(111) substrates at 300 K (RT) with coverages from0.1 to 0.4 AL. Randomly distributed 2 AL high islandsprimarily observed on substrate and close to theterrace edges at very low coverage..Irregular shaped islands → amorphousmicrostructure at low temp. (RT)[Study of Nb epitaxial growth on Cu (111) at sub-monolayer level) C. Clavero et al., JAP 112, 074328(2012)]
Nb Film Nucleation at RT
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Nb/Cu hetero-epitaxy by MBE at RTA. Lukaszew et al. , W&M
STM/STS studies-Proximity effects
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Annealing at 350 °C
Annealing @ 350 °C leads to rearrangement of Nb atoms into crystalline islands.Atomic resolution topography STM images of islands with hexagonal and rhomboidal shape
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Annealing at 600 °C
Further annealed Nb islands, which exhibit hexagonal and triangular shapes, with two distinctive heights, namely 1 and 2 AL
Topography STM map for a nominally 1 AL thick Nb film / Cu (111): 3D Volmer-Weber growth mode with islands up to 3 AL height
𝛾𝑓,𝑁𝑏 2.983 𝐽.𝑚−2 > 𝛾𝑠,𝐶𝑢 1.934 𝐽.𝑚−2
Lattice mismatch 9%
Further annealing @ 600 °C leads to coalescence to larger islands with reconstruction on their surface
MBE Growth @ 350 °C
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Film NucleationMolecular Dynamics Simulation
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Initially circular Nb nano-islands→Hexagonal shape as observed in experimentWith 350 °C annealing
Higher temperatures → intermixing
[C. Clavero, M. Bode, G. Bihlmayer, S. Bl€ugel, and R. A. Lukaszew, Phys.Rev. B 82(8), 085445 (2010)]
large-scale atomic/molecular massively parallel simulator (LAMMPS) code Visualization with OVITO.
Energetic Condensation
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Additional energy provided by fast particles arriving at a surface ⇒number of surface & sub-surface processes ⇒changes in the film growth process:
Generalized Structure Zone Diagram
residual gases desorbed from the substrate surface
chemical bonds may be broken and defects created thus affecting nucleation processes & film adhesion
enhanced mobility of surface atoms stopping of arriving ions under the
surface
morphology microstructure stress
As a result of these fundamental changes, energetic condensation allows the possibility of controlling the following film properties:
Density of the film Film composition Crystal orientation may be controlled to give the possibility of low-temperature epitaxy
Condensing (film-forming) species : hyper-thermal & low energies (>10 eV).
⇒ Changes in
A. Anders, Thin Solid Films 518 (2010) 4087
derived from Thornton’s diagram for sputtering (1974)
Nb on Cu single crystals
A-M Valente-Feliciano - AVS Conference 2011– Nashville TN,11/01/2011
RRR=88 RRR=242RRR=76
In the same run, Nb/fine grain Cu RRR=82Nb/large grain Cu RRR=169
Structural Properties of Niobium Thin Films Deposited on Metallic Substrates by ECR Plasma Energetic CondensationJoshua K. Spradlin, Anne-Marie Valente-Feliciano, Larry Phillips, Charles E. Reece, Xin Zhao (JLAB, Newport News, Virginia), Kang Seo (NSU, Newport News), Diefeng Gu (ODU, Norfolk, Virginia) - to be submitted PRST-AB
A-M Valente-Feliciano - TFSRF 2012, 07/18/2012
Nb films on polycrystalline Cu substrates
RRR=169 RRR=82
Nb/large grain Cu(substrate heat treated ex-situ @ 1000°C, 12h)
Nb/fine grain Cu
150 x 150 μm, 1 μm resolution, CI Avg. 0.71
120 x 150 μm, 1 μm resolution,CI Avg. 0.23
50 x 75 μm, 1 μm resolution,CI Avg. 0.16
Bias -120V, 2mmBias 0V , 4mm
Typical Cu substrate
Effect of Bias Voltage for Nb/Cu
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Ab-normal growthvith bias
vs.columnar growth
without bias
Fiber Growth
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
1 0 0 n m
LAADF
Nb Cu
CL=12CM
5 n m
ABF
Nb Cu
interface
T bake = 200 °CT coating = 200 °CENb ions = 64 eVThickness = 1.4 μm RRR = 21Tc= 9.41 ± 0.16 K
Fiber Growth
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
1 0 0 n m
Nb
Cu
2 0 n m
LAADF
Nb Cu
CL=12CM
CL=12CM
2 nm
ABF
Nb
Cu
Amorphous interface
T bake = 200 °CT coating = 200 °CENb ions = 184 eV then 64 eV
Thickness = 1.7 μm RRR = 15Tc= 9.46 ± 0.19 KΔ=1.71 meV
1 0 n m
Spectrum Image
Spatial Drift
EELS plot for Cu/Nb signal across interface
11.5 nm
Interface thickness(e-1 of highest density)
Incident ion energy: 64 eVNb: 7.6 nmCu: 8.9 nm
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Fiber Growth
Hetero-epitaxy, low Tcoating
A-M Valente-Feliciano - TFSRF 2014 - 07/10/20142 n m
Nb Cu
Continuous crystalline
1 0 0 n m
ABF
Nb Cu
interface
CL=12CM
CL=12CM
T bake = 200 °C (long bake)T coating = 200 °CENb ions = 184 eVThickness = 1.8 μm RRR = 58Tc= 9.43 ± 0.13 KΔ=1.56 meV
Nb Cu
0V (64 eV)120 V (184 eV)
EELS plot for Cu/Nb signal across interface
Interface thickness(e-1 of highest density)
Ion energy 64 eV: Nb: 3.89 nmCu: 1.89 nm
Ion energy 184 eV:Nb: 3.67 nmCu: 7.33 nm
86: 6 nm
87: 11.4 nm
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Low versus high energy
Hetero-epitaxy, High Tcoating
A-M Valente-Feliciano - TFSRF 2014 - 07/10/20142 0 n m
interface
CL=12CM
Nb Cu
T bake = 500 °CT coating = 360 °CENb ions = 184 eV then 64 eVVery thick filmThickness = 4.5 μm RRR = 305Tc= 9.37 ± 0.12 KΔ=1.53 meV (1.38 meV?)
interface
5 0 n m
Spectrum Image
EELS plot for Cu/Nb signal across interface
Interface thickness(e-1 of highest density)
Nb: 12.5 nmCu: 20.1 nm
31.8nm
1 0 n m
Continuous crystalinterface
Substrate roughness and defects
Cu
Nb
E-beam evaporated Pt
Ion beam coated Pt
Nb
Cu
Section cut by FIB
Whatever the inherent nature of the film, the roughness of the substrate will dictate the minimum roughness of the film (the final roughness depends as well on the coating technique and other refinements).Any defect (scratch, pin-hole) is duplicated and enhanced in the film as it grows.
CONCLUSIONS
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
First atomic layers of Nb film constitute an adaptation layer to the substrate, (differs from relaxed cubic Nb structure) , a template for subsequent growth of more or less “relaxed” Nb film.
Deposition at high energy : sub-implantation , enhanced with higher temperatures (on-going studies)
Interface engineering : introduction of interlayers, buffer layer to “erase the influence “ of the substrate (nature, grain boundaries…)
Nb on Cu substratesInfluence of energy as function of substrate nature,
coating temperature
Nb/ single crystals Cu Nb/ large grain Cu Nb/ large grain Cu
Nb(100) always higher RRRNb/large grain Cu can achieve higher RRR due to underlying relaxed (heat treated) substrateFor Nb/fin grain Cu, less variation in RRR as preferred orientation (110)
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Nb/a-Al2O3 - Early stages of growth
• Using Reflection high energy
electron diffraction (RHEED), we
observed a hexagonal Nb
surface structure for the first 3
atomic layers followed by a
strained bcc Nb(110) structure and
the lattice parameter relaxes after
3 nm.
• RHEED images for the hexagonal
phase at the third atomic layer.
Patterns repeat every 60 deg.
1 10 100
0.29
0.30
0.31
0.32
0.33
0.34
0.35
0.36
0.23 2.3 23
Nb thickness (nm)
Latt
ice
para
me
ter
(nm
)
Nb atomic layers
bulk Nb bcc
[111]Nb ll[0001]Al O2 3
[1120]Nb ll [0001]Al O2 3
a
hcp Nb
bcc Nbhcp
+b
cc N
b
a
0 deg 30 deg 60 deg
A-M Valente-Feliciano - SRF Conference 2011– Chicago, 07/26/2011
Hetero-epitaxy
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
184 eV continuous500 C360 CRRR= 182Tc = 9.39 ± 0.08 KΔ = 1.62 meV
Zone Structure Model
• Zone 1
-lack of surface mobility
-random direction of incoming
vapor atoms
-shadowing
loose fibrous structure, voids, porosity
• Zone T - transition between Zones 1 and 2 (Thornton)
- more tightly packed fibrous grain structure but not fully dense
• Zone 2 - Ts ~ < 0.3 Tm - fully dense columnar grain structure with long columns extending from substrate to film surface
• Zone 3 - Ts ~ > 0.45 Tm – no longer columnar –recrystallized with random orientation
J. A. Thornton, J. Vac. Sci. Technol.11, 666, 1974
Film Nucleation & Growth
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
The film nucleation depends first and foremost on the nature of the materialdeposited (metal... )
Niobium as most metals usually grows in the island mode.
(i) 3-D or island growth mode, also known as Volmer–Weber (VW) modeThe adatoms have a strong affinity with each other and build 3-D islands that grow in all directions, including the direction normal to the surface. The growing islands eventually coalesce and form a contiguous and later continuous film.
(ii) 2-D or layer-by-layer growth, also known as Frank–van derMerwe (FVDM) modeThe condensing particles have a strong affinity for the substrate atoms: they bond to the
substrate rather than to each other.
(i) a mixed mode that starts with 2-D growth that switches into island mode after one or more monolayers; this mode is also known as the Stranski–Krastanov (SK) mode.
Thin Film Growth Modes
Effect of ion energy on film growth
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
A. Anders / Thin Solid Films 518 (2010) 4087–4090
L. Hultman, J.E. Sundgren, in: R.F. Bunshah (Ed.), Handbook of Hard Coatings, Noyes, Park Ridge, NY, 2001, p. 108.D.K. Brice, J.Y. Tsao, S.T. Picraux, Nucl. Instrum. Methods Phys. Res. B 44 (1989) 68.G. Carter, Phys. Rev. B 62 (2000) 8376.M.M.M. Bilek, D.R. McKenzie, Surf. Coat. Technol. 200 (2006) 4345.D.R. McKenzie, M.M.M. Bilek, Thin Solid Films 382 (2001) 280W. Eckstein, Computer Simulation of Ion-Solid Interactions, Springer-Verlag, Berlin,1991.D.K. Brice, J.Y. Tsao, S.T. Picraux, Nucl. Instrum. Methods Phys. Res. B 44 (1989) 68.
Non-penetrating ions (or atoms) in the film bulk Promotion of surface diffusion of atoms. Between the surface displacement energy and bulk displacement energy: epitaxial
growth is promoted because no defects are created in the film bulk . Atomic displacement cascades if Ekin > Ebulk displacement, (12–40 eV)
Penetrating particles :very short (∼100 fs) ballistic phase with displacement cascades followed by a thermal spike phase (∼1 ps) (mobility of atoms in the spike volume very high) ~ transient liquidlarge amplitude thermal vibrations still facilitate diffusion (migration of interstitials inside grains & adatoms on the surface). The driving force is the gradient of the chemical potential, leading to minimization of volume free energy and surface free energy density with contributions of interface and elastic strain energies and often resulting in a film where grains have a preferred orientation.As Ekin increase, e.g. by biasing, the sputtering yield is increased and the net deposition rate is reduced. Film growth ceases as the average yield approaches unity, for most elements between 400 eV and 1400 eV Surface etching as Ekin is further increased.
Inci
den
t io
n e
ne
rgy
Kinetic energy of arriving positive ions - an initial component from the plasma, E0
- a change due to acceleration in the sheath,
Ekin=E0+QeVsheath where Q =ion charge state number, e =elementary charge, and Vsheath is the voltage
drop between plasma and substrate surface.
Energetic particle bombardment promotes competing processes of defect generation and annihilation.
Kinetic energy → displacement and defects followed by re-nucleationRelease of potential energy & post-ballistic thermal spike → atomic scale heating, annihilation of defects.
- Epot/Ekin per incident particle as well as the absolute value of the kinetic energy will shift the balance and affect the formation of preferred orientation and intrinsic stress .Maximum of intrinsic stress for Ekin ~100 eV; the actual value depends on the material and other factors. insertion of atoms under the surface yet still very little annealing .
At higher temperature (higher homologous temperature or temperature increase due to the process itself) the grains are enlarged because the increase of adatom mobility dominates over the increased ion-bombardment-induced defects and re-nucleation rates.
All energy forms brought by particles to the surface will ultimately contribute to broad, non-local heating of the film and shift the working point of process conditions to higher homologous temperature.
Effect of ion energy and substrate temperature
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
[M. M. Bilek, D. R. McKenzie “A comprehensive model of stress generation and relief processes in thin films deposited with energetic ions” Surf. And Coat. Techno. 200 (2006) 4345-4354]
Energetic Condensation Processes
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Atomic Scale Heating
Sub-implantation- Energy transfer to knock-on atoms ~10E-13 sec.
- Collisional cascade, thermalization ~10E-11 sec.
- Fills sub-surface voids
At high energies the rate of defect creation can exceed defect annealing
Secondary Electron Emission
self-sputtering & sticking coefficient
Not all ions are incorporated on/into the substrate, rather, depending on the energy and incident angle of the arriving ion and the kind of substrate material, some ions may ‘‘bounce’’ back as neutralized atoms and therefore contribute to the density of neutral atoms rather than to film growth. Sticking probability for incoming energetic ions.
When an arriving atom becomes incorporated into the substrate, the collision cascade under the surface can lead to the expulsion of one (or more) surface atoms(sputtering). If the arriving ion and the sputtered atom are of the same material, one speaks of self-sputtering. This reduces the effective film deposition rate, and in case the yield exceeds unity, no film is grown.
Emitted secondary electrons are in the same electric field of the sheath that accelerated the (positive) ions, but now it accelerates the (negative) electrons in the opposite direction. The electrons may interact with the arc plasma, especially with the colder plasma electrons, as well as with the background.
Each ion delivers significant kinetic and potential energy, and both contribute to what may be called atomic scale heating (ASH).
Nb/CuO – High Resolution TEM
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
150mm x 150mm, 1mmCI =0.49500x
150mm x 150mm, 1mmCI =0.24500x
150mm x 150mm, 1mmCI =0.21500x
Nb(110)//Cu(100) Nb(100)//Cu(110) (Nb(110)//Cu(111)
Nb hetero-epitaxy on Cu
0.733° 0.357 ° 0.432°
CI =0.58CI =0.59CI =0.51
Lattice mismatch with (100) 8.5% 50x 1500 mm x 1500mm, 25 mm step
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Thin film growth from the gas phase=non-equilibrium process phenomenon governed by a competition between kinetics & thermodynamics. 3 stages can be distinguished in the nucleation & growth of films:
• Production of ionic, molecular or atomic species in the gas phase.• Transport of these species to the substrate• Condensation of the species onto the substrate directly or either by chemical or electrochemical
reaction.
The vapor atoms are continuously depositing on the surface. Depending on the atom’s energy and the position at which it its the surface, the impinging atom could re-evaporate from the surface or adsorb to it, becoming an adatom. Adsorption occurs either by forming a van der Waal’s bond with a surface atom -physisorption- or by forming a covalent/ionic bond with a surface atom-chemisorption. When the atoms are physisorbed they can migrate on the surface and interact with each other as well as with the substrate atoms. These interactions determine the morphology of the growing film.
Film Nucleation & Growth
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014
Competing Processes
• Addition to film – impingement (deposition) on surface
• Removal from film: – reflection of impinging atoms
– desorption (evaporation) from surface
sticking coefficient = mass deposited / mass impinging