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Applied Surface Science 349 (2015) 757–762

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

haracterization of HfOxNy thin film formation by in-situ plasmanhanced atomic layer deposition using NH3 and N2 plasmas

oung Bok Lee1, Il-Kwon Oh1, Edward Namkyu Cho, Pyung Moon, Hyungjun Kim,lgu Yun ∗

epartment of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea

r t i c l e i n f o

rticle history:eceived 6 February 2015eceived in revised form 11 May 2015ccepted 11 May 2015vailable online 18 May 2015

a b s t r a c t

The structural and electrical characteristics of in-situ nitrogen-incorporated plasma enhanced atomiclayer deposition (PE-ALD) HfOxNy thin films using NH3 and N2 plasmas as reactants were compara-tively studied. The HfOxNy test structures prepared using NH3 and N2 plasmas were analyzed by X-rayphotoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high resolution transmission electronmicroscopy (HR-TEM) to investigate the chemical composition, crystallinity, and cross-sectional layers

eywords:fON thin filmlasma enhanced atomic layer depositionH3 plasma2 plasma

nterfacial layer

including the interfacial layer, respectively. By utilizing NH3 and N2 plasmas, the nitrogen-incorporatedHfOxNy thin films fabricated by in-situ PE-ALD showed a high dielectric constant and thermal stability,which suppresses the interfacial layer and increases the crystallization temperature. The high leakagecurrent densities of the HfOxNy thin film test structures fabricated using NH3 and N2 plasmas caused bylowering the energy bandgap and band offset are related to the Hf N bond ratio and dielectric constant.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

HfO2 dielectric thin films have been considered to be one of theost promising high-k materials to replace conventional SiO2 thin

lms in complementary metal-oxide-semiconductor (CMOS) tech-ology applications due to their high dielectric constant resulting in

decrease in the equivalent oxide thickness (EOT) [1–4]. However,roblems associated with the technology include formation of an

nterfacial layer with a low dielectric constant, low crystallizationemperature, and the threshold voltage shift caused by the fixedharge [5–8]. Thus, incorporation of nitrogen into HfO2 films haseen researched to supplement the disadvantages of HfO2 dielec-ric thin films. It was reported that the incorporation of nitrogennto HfO2 films is useful to increase the crystallization tempera-ure, can inhibit the formation of the interfacial layer, and improvehe electrical properties of devices [9–11]. However, the incorpora-ion of nitrogen into hafnium-based thin films is generally carriedut by high temperature annealing or plasma nitridation in nitro-

en ambient. Thus, these methods require a high temperature andultiple steps [12,13]. Furthermore, it is difficult to control the

itrogen profile with atomic accuracy. The degradation of electrical

∗ Corresponding author. Tel.: +82 2 2123 4619; fax: +82 2 313 2879.E-mail address: iyun@yonsei.ac.kr (I. Yun).

1 These authors contributed equally to this work.

ttp://dx.doi.org/10.1016/j.apsusc.2015.05.066169-4332/© 2015 Elsevier B.V. All rights reserved.

characteristics can be induced by incorporating nitrogen at theinterface between dielectric thin films and the Si substrate [14,15].Generally, there are several nitridation methods including chemicalvapor deposition (CVD) [16], re-oxidation of HfN films formed byphysical vapor deposition (PVD) [17], and thermal annealing withNH3 [18] for HfOxNy thin films that have a high dielectric constantand thermal stability [9–11].

In this study, nitrogen-incorporated HfOxNy thin films preparedby using in-situ plasma enhanced atomic deposition (PE-ALD) at alow processing temperature were investigated. The in-situ PE-ALDprocess can uniformly control a higher nitrogen profile with atomicaccuracy. In addition, the process does not require a high tempera-ture or multiple steps. The structural and electrical characteristicsof the in-situ PE-ALD HfOxNy thin films fabricated using NH3 andN2 plasmas as reactants were also evaluated.

2. Experiments

The p-type Si(1 0 0) substrates were pre-cleaned at 80 ◦Cfor 10 min in a standard Radio Corporation of America(RCA) solution [1:1:5 (v/v/v) NH4OH/H2O2/H2O]. The HfOxNy

films were subsequently deposited by PE-ALD using TDMAH

[tetrakis(dimethylamino)hafnium] as the Hf precursor, which wasevaporated at 50 ◦C in stainless-steel bubbler to obtain sufficientvapor pressure at a temperature of 250 ◦C. Two types of counterreactants were used for comparison: NH3 and N2 plasmas. ALD

758 Y.B. Lee et al. / Applied Surface Science 349 (2015) 757–762

bricat

pstPttPewtpbpX(ncRtgcadt2

3

st

Fig. 1. XRD spectra of the as-deposited HfOxNy films on Si substrates fa

rocesses were developed based on optimized process conditionsuch as the precursor and the reactant exposure time, the purgingime and the substrate temperature. The saturation conditions ofE-ALD HfOxNy using NH3 plasma are 2 s of the precursor exposureime, 1 s of the reactant exposure time, and 5 s of the purgingime at the temperature of 250 ◦C, which is the same process asE-ALD HfOxNy using N2 plasma except for 1.5 s of the reactantxposure time. After deposition, post-deposition annealing (PDA)as carried out in N2 ambient at temperatures varying from 400

o 800 ◦C for 60 s by applying a rapid thermal annealing (RTA)rocess for the X-ray diffraction (XRD) analysis. The chemicalonding structures of the HfOxNy films were investigated by X-rayhotoelectron spectroscopy (XPS) with a monochromatic Al K�-ray source. High resolution transmission electron microscopy

HR-TEM) was additionally performed to investigate the thick-esses of the HfOxNy films and interfacial layers. For electricalharacterization, MOS capacitor structures were fabricated andu was deposited on the HfOxNy films by magnetron sputteringhrough a shadow mask with an area of 3.14 × 10−4 cm2 as aate electrode in order to evaluate the electrical properties. Theapacitance–voltage (C–V) measurements were performed using

Keithley 590 C–V analyzer at 1 MHz and the leakage currentensity–electric field (J–E) characteristics were calculated fromhe current–voltage (I–V) characteristics measured by a Keithley36 source measure unit.

. Results and discussion

Fig. 1 shows the XRD patterns of the HfOxNy films on Si sub-trates fabricated using NH3 and N2 plasmas at various annealingemperatures. As seen in Fig. 1(a), no diffraction peaks were

Fig. 2. XPS depth profiles of the atomic concentrations of Hf, N, O, and Si

ed using (a) NH3 and (b) N2 plasma at various annealing temperatures.

observed up to an annealing temperature of 600 ◦C for the HfOxNy

films using NH3 plasma indicating that the film consists of an amor-phous phase [19]. However, at a temperature of 800 ◦C, the filmexhibits some weak crystallization peaks, which can be attributedto the monoclinic HfOxNy (1 1 1) and (2 0 0) planes [20,21]. Asobserved in Fig. 1(b), the amorphous structure of the HfOxNy filmproduced using N2 plasma remains at annealing temperatures upto 800 ◦C, resulting in a higher crystallization temperature thanthat of the HfOxNy films fabricated using NH3 plasma. Comparedto the HfO2 films, the crystallization temperature of the nitrogen-incorporated HfOxNy films using NH3 and N2 plasmas increased byover 200 ◦C, indicating enhanced thermal stability for crystalliza-tion [20].

In order to evaluate the nitrogen concentration and chemicalbonding states of the HfOxNy films deposited by in-situ PE-ALDusing NH3 and N2 plasmas, XPS depth profile measurements wereperformed. The shift of the whole spectrum was calibrated by theSi 2p peak at 99.3 eV.

Fig. 2 shows the depth profiles of the atomic concentrations ofHf, N, O, and Si in the films. A large amount of nitrogen is mainly dis-tributed in the bulk films and not at the Si/HfOxNy interface. Fromthe XPS depth profiles, the nitrogen concentrations are approxi-mately 40 atomic conc. %, indicating that the in-situ PE-ALD processcan uniformly control the higher nitrogen profile without requir-ing a high temperature or multiple steps. Thus, it is found fromthe results of the electrical characteristics shown in the below thatthe nitrogen-rich HfOxNy film shows high dielectric constant and

electrically insulating characteristics.

As shown in Fig. 3(a), the XPS spectra of Hf 4f shows a doubletshape according to the spin-orbit splitting into Hf 4f5/2 and Hf 4f7/2[13,22]. The fitted data of the Hf 4f peaks of the HfOxNy films appear

in the HfOxNy films produced by using (a) NH3 and (b) N2 plasma.

Y.B. Lee et al. / Applied Surface Science 349 (2015) 757–762 759

fOxN

tbt4HN1iaTtHtuoia[HNotcrcp

HuttppIr(d

Fig. 3. XPS spectra of (a) Hf 4f and (b) N 1s for H

o consist of two sub-peaks corresponding to Hf N and Hf O Nonds [13,22]. For the HfOxNy film produced using NH3 plasma,he Hf N bond-related binding energy peaks corresponding to Hff5/2 and Hf 4f7/2 are located at 16.8 and 14.8 eV, respectively. Thef N bond-related binding energy peaks for the HfOxNy film using2 plasma corresponding to Hf 4f5/2 and Hf 4f7/2 are also located at6.7 and 14.7 eV, respectively. It has been reported that the bind-

ng energies of Hf O bonds for HfO2 films corresponding to Hf 4f5/2nd Hf 4f7/2 are located at ∼19.2 and 17.5 eV [22,25], respectively.he shift to lower binding energies compared to HfO2 films andhe different shape of the Hf 4f spectral peaks observed for thefOxNy films indicate the presence of Hf N [22,25]. Fig. 3(b) shows

he XPS spectra of the N 1s peaks for the HfOxNy films producedsing NH3 and N2 plasmas. The binding energies for the N Hf bondsf the HfOxNy films made using NH3 and N2 plasmas correspond-ng to N 1s are located at 396.4 and 396.0 eV, respectively, whichre in good agreement with the reported values for HfOxNy films22,25]. The deconvolution results suggest that the shapes of thef 4f and N 1s peaks compared to the HfOxNy films made usingH3 and N2 plasmas are slightly distorted due to a small differencef the nitrogen concentration. The relative Hf N bond ratios forhe Hf 4f state level are 49.4 and 44.8% for the HfOxNy films fabri-ated using NH3 and N2 plasmas, respectively. Since the ionizationates for N2 plasma is smaller than those of NH3 plasma, it can beoncluded that NH3 plasma process is more effective than the N2lasma process.

Fig. 4 shows cross-sectional HR-TEM images of the as-depositedfOxNy films produced using NH3 and N2 plasmas and HfO2 filmsing O2 plasma (as a reference) on the Si substrate to investigatehe interfacial layer thickness and microstructure. The thickness ofhe interfacial layer (IL) for the HfOxNy film generated using NH3lasma is about 1.3 nm whereas the IL thickness for the HfOxNy filmroduced using N2 plasma is about 1.7 nm. The thicknesses of the

L for the HfOxNy films fabricated using NH3 and N2 plasmas areeduced compared to separately prepared Al2O3 (4.7 nm) and HfO22.6 nm) films because nitrogen incorporation suppresses oxygeniffusion and inhibits interfacial reactions with the Si substrate [11].

y films produced by using NH3 and N2 plasmas.

The microstructures of the as-grown HfOxNy films produced usingNH3 and N2 plasmas were confirmed to be an amorphous phase,which are well matched with the results of the XRD analyses.

A Ru/HfOxNy/p-Si(1 0 0) MOS capacitor test structure was fabri-cated to evaluate the electrical characteristics of the in-situ PE-ALDHfOxNy thin films grown using NH3 and N2 plasmas. Fig. 5(a) showsthe C–V curves of the HfOxNy MOS capacitor test structures usingNH3 and N2 plasmas, which were normalized to calibrate for thethickness difference. The dielectric constants of the HfOxNy filmsfabricated using NH3 and N2 plasmas extracted from the 1 MHzC–V measurements are 34.2 and 25.8, respectively. The EOT of theHfOxNy films produced using NH3 and N2 plasmas are lower thanthat of HfO2 due to the higher dielectric constant. It was reportedthat the higher nitrogen concentration in the deposited HfOxNy

films results in the higher dielectric constant [9]. The C–V hystere-sis of the HfOxNy MOS capacitor test structures containing filmsproduced using NH3 and N2 plasmas are 200 and 300 mV, respec-tively. In addition, the interface trap densities (Dit) of the HfOxNy

MOS capacitor test structures containing films fabricated using NH3and N2 plasmas were extracted using conductance method andthey were determined to be 14.4 × 1011 and 9.65 × 1011/cm2 eV,respectively. The differences of the C–V hysteresis and Dit val-ues between the NH3 and N2 plasma processed HfOxNy films arerelated to the incorporated nitrogen concentration difference inthe bulk of the HfOxNy films and the interface with the Si sub-strate [21]. The results indicates that the nitrogen will stabilizethe incomplete bonding in the HfOxNy interface layer fabricatedusing N2 plasma even it contains slightly more the trapped oxidecharges.

Fig. 5(b) shows the J–E characteristics of the HfOxNy MOS capac-itor test structures containing films produced using NH3 and N2plasmas. The leakage current densities of the test structures withfilms fabricated using NH3 and N2 plasmas at −1 MV/cm were

4.87 × 10−3 and 1.13 × 10−5 A/cm2, respectively. These high leak-age current densities of both the NH3 and N2 plasma-processedHfOxNy films can be primarily attributed to the decrease of theenergy bandgap and band offset as a result of the high nitrogen

760 Y.B. Lee et al. / Applied Surface Science 349 (2015) 757–762

FN

iafilT

RPN is larger than that of N2–RPN. However, the leakage currentof NH3–RPN is better than that of N2–RPN, which was explainedby the increase of interface layer. In our work, the interface layer

Table 1Summary of the HfOxNy thin film electrical properties produced using NH3 and N2

plasmas.

Plasma type Dielectricconstant

Dit

(cm−2 eV−1)Hysteresis(mV)

Leakage currentdensity at

ig. 4. Cross-sectional HR-TEM images of the Si/HfOxNy films produced using (a)H3 and (b) N2 plasma and (c) HfO2 film using O2 plasma (as a reference).

ncorporation in the HfOxNy films [22,23]. The difference of leak-ge current densities for the NH and N plasma processed HfO N

3 2 x y

lms are correlated with the Hf–N bond ratio for the Hf 4f stateevel, as demonstrated in the XPS and dielectric constant results.he electrical properties of the HfOxNy MOS capacitor test structure

Fig. 5. (a) C–V and (b) J–E characteristics of the Ru/HfOxNy/p-Si(1 0 0) containingfilms produced using NH3 and N2 plasmas.

containing films fabricated using NH3 and N2 plasmas are summa-rized and compared in Table 1.

However, the results shown in the above exhibit a differenttrend with the previous report [11]. Previous report presentedremote plasma nitridation (RPN) of HfOxNy film, which is the ini-tial HfO2 film formation followed by ex-situ RPN process using N2and NH3 plasmas [11]. In our work, the in-situ HfOxNy film forma-tion using N2 and NH3 plasmas was presented, which is a differentprocess for film formation. As a result, the nitrogen concentrationsare different, i.e. the N concentration of previous result is in therange of 10–15%, which is much smaller than that of our work(∼40%).

The originality of our work is the in-situ HfOxNy film formationusing N2 and NH3 plasmas to obtain high N concentration. Sincethe previous report explained that for N2–RPN case, the nitrogencan suppress the oxygen diffusion resulting in the reduction of theinterface layer (IL) of 1.0 nm whereas, for NH3–RPN case, by thereaction of NH2

+/− and NH0 with Si and HfOxNy, the Hf-silicate ofHfSiOxNy layer was formed in the interface layer resulting in theincrease of IL (∼1.9 nm). In addition, EOT of NH3–RPN (1.45 nm) issmaller than EOT of N2–RPN (1.75 nm) and the capacitance of NH3-

−1 MV/cm (A/cm2)

NH3 plasma 34.2 14.4 × 1011 200 4.87 × 10−3

N2 plasma 25.8 9.7 × 1011 300 1.13 × 10−5

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[19] M-H. Cho, Y. Roh, C. Whang, K. Jeong, H. Choi, S. Nam, et al., Dielectric charac-teristics of Al2O3–HfO2 nanolaminates on Si (1 0 0), Appl. Phys. Lett. 81 (2002)1071–1073.

Y.B. Lee et al. / Applied Surf

f N2–plasma (1.7 nm) is larger than that of NH3 plasma (1.3 nm),hich shows a different result with previous report [11].

For the C–V characteristics in our work, higher k values com-ared with the previous report [11] were obtained. Although theyid not mention the dielectric constant of HfOxNy film in the report,ll of calculated values from accumulation capacitance are below5, which are much smaller than our results (larger than 25). Inddition, the C–V hump effect was not observed in our work, indi-ating that the HfOxNy film has less damage for the interface layerompared with the previous report [11]. For the I–V characteris-ics, N can be replaced with the replacement of oxygen or vacancyf oxygen (VO), which can remove the trap state resulting in theeduction of leakage current by hopping mechanism. In our work,he leakage current of the film using N2 plasma is smaller thanhat of NH3 plasma due to the larger Hf O N ratio verified by XPS.ince HfOxNy film shows insulator characteristic, the film using N2lasma is more effective to form Hf O N bonding than that of NH3lasma.

Based on the previous studies for crystallization of HfOxNy filmormation, the crystallization processes for N2 and NH3 plasmasere different depending on the nitrogen and oxygen concentra-

ions. Contrary to the previous work, the nitrogen concentrationf our work is much larger (∼40%) than those of previous studies10–15%).

It was reported that when the N concentration of HfOxNy filmas increased, the crystallization of HfOxNy was suppressed [24]. Itas also reported that the crystallization temperatures of HfO2 andfN films are relatively higher than that of HfOxNy film [25]. When

he O concentration is increased and N concentration is decreasedn HfN film, the crystallization of the film is also suppressed [26]. Inur work, the deposited HfOxNy film contains high N concentrationround 40% and small O concentration around 10%. In other words,he deposited film in our work is HfN film doped with oxygen, ratherhan HfO2 film doped with nitrogen. Thus, the crystallization is sup-ressed as N concentration is decreased. Since the film using N2lasma contains relatively smaller N concentration and larger Ooncentration than the film using NH3 plasma (see Fig. 2), the crys-allization of the film using N2 plasma was more suppressed thanhat of NH3 plasma, resulting in lower leakage current and largerysteresis.

In addition, from the XPS results shown in Fig. 3, the area ofe-convoluted peaks are different and the area of Hf O N peakor N2 plasma is larger than that for NH3 plasma. It is explainedhat the film using N2 plasma has larger nitrogen ratio bondedith Hf O than that using NH3 plasma, which shows deficiency

f the phase separation. For the case of NH3 plasma, even though Noncentration is relatively larger than that of N2 plasma, the bond-ng is dominantly formed with Hf N bonding indicating that thehase separation is easier than the case of N2 plasma. This indi-ates another evidence of the suppression of the crystallization ofhe film.

Finally, it is observed that the growth rate of the film using NH3lasma is much larger than that using N2 plasma. This is due tohe hydrogen ion and radical separated from NH3 plasma, whichncreases the plasma density. As the plasma density is increased,

ore numbers of the radicals and ions chemically react on the filmurface resulting in the increase of growth rate. In general, for thelm deposition using N2 plasma, H2 plasma is also utilized for theeposition process to promote the film growth characteristics since

radical can promote Hf N bonding by the reduction of precursorigand easily [27]. In our work, since the N2 plasma is solely utilizedor the comparison of the case for using NH3 plasma, the growthate using N2 plasma is lower than that of using NH3 plasma.

Therefore, it can be concluded that N2 plasma process is pre-

erred for leakage current suppression and NH3 plasma process isreferred for dielectric constant enhancement.

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ience 349 (2015) 757–762 761

4. Conclusion

In this study, nitrogen-incorporated HfOxNy thin films depositedusing NH3 and N2 plasmas as a reactant during in-situ PE-ALD werestructurally and electrically investigated. The XPS analyses showedthat a large amount of nitrogen was mainly distributed in the bulkfilms and not at the Si/HfOxNy interface. The interfacial layer thick-nesses of the HfOxNy films fabricated using NH3 and N2 plasmaswere about 1.3 nm and 1.7 nm, respectively. As observed in thecross-sectional HR-TEM images, the nitrogen-incorporated HfOxNy

films had reduced IL thicknesses compared to Al2O3 and HfO2.The electrical characteristics of the MOS capacitor test structuresshowed that the in-situ PE-ALD nitrogen-incorporated HfOxNy thinfilms fabricated using NH3 and N2 plasmas exhibited high dielec-tric constants. In addition, the leakage current densities for bothtest structures caused by the lower energy bandgap and band offsetwere correlated with the Hf N bond ratio and dielectric constant.

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