in situ observation of self cleansing phenomena during ultra high vacuum anneal of transition metal nitride thin films prospects for non destructive photoelectron spectroscopy

In-situ observation of self-cleansing phenomena during ultra-high vacuum anneal of transition metal nitride thin films: Prospects for non-destructive photoelectron spectroscopy G.Greczyn

In-situ observation of self-cleansing phenomena during ultra-high vacuum anneal

of transition metal nitride thin films: Prospects for non-destructive photoelectron spectroscopy

G Greczynski and L Hultman

Citation: Appl Phys Lett 109, 211602 (2016); doi: 10.1063/1.4968803

View online: http://dx.doi.org/10.1063/1.4968803

View Table of Contents: http://aip.scitation.org/toc/apl/109/21

Published by the American Institute of Physics

In-situ observation of self-cleansing phenomena during ultra-high vacuum anneal of transition metal nitride thin films: Prospects for non-destructive photoelectron spectroscopy

G.Greczynskiand L.Hultman

Thin Film Physics Division, Department of Physics (IFM), Link€ oping University, SE-581 83 Link€ oping, Sweden

(Received 4 October 2016; accepted 11 November 2016; published online 23 November 2016)

Self-cleansing of transition metal nitrides is discovered to take place during ultra-high vacuum

annealing of TiN, NbN, and VN thin films Native oxide layers from air exposure disappear after

isothermal anneal at 1000C Also, for TiN, the Ti 2p and N 1s X-ray photoelectron spectra (XPS)

recorded after the anneal are identical to those obtained fromin-situ grown and analyzed epitaxial

TiN(001) These unexpected effects are explained by oxide decomposition in combination with

N-replenishing of the nitride during recrystallization The finding opens up new possibilities for

true bonding assignments through non-destructive XPS analyses, thus avoiding artefacts from Ar

etching.V C 2016 Author(s) All article content, except where otherwise noted, is licensed under a

Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

[http://dx.doi.org/10.1063/1.4968803]

The importance of X-ray photoelectron spectroscopy

(XPS) in materials science cannot be overestimated Over the

last decades, XPS became an essential research technique for

assessing the surface chemistry and composition of

com-pounds in bulk or thin film form Many classes of samples

require some form of cleaning prior to spectra acquisition to

remove surface oxides and contaminants, which is typically

donein-situ by etching with 0.5–4 keV Arþions The latter is

performed despite well-known destructive effects of ion

bom-bardment like cascade mixing, chemical reduction, recoil

implantation, segregation, surface roughening, preferential

sputtering, redeposition, and forward implantation of surface

species, to name the predominant.1 This destructive type of

surface treatments is particularly adherent to XPS studies of

refractory ceramic thin films grown by physical vapor

deposi-tion (PVD), for applicadeposi-tions such as protective layers on

high-speed cutting tools2,3engine parts,4,5as well as diffusion

bar-riers in electronics.68Interpretation of XPS results obtained

in such way always poses a number of questions as to what

extent the actual XPS spectra reflect the native material to be

studied instead of the ion-beam modified surface layer, which

thickness is comparable to the XPS probing depth

Here, we report on a surprising observation that allows

for a non-destructive acquisition of high-quality XPS spectra

characteristic of a native material from samples that have

been exposed to atmosphere High-temperature anneal of

transition metal (TM) nitride (TM ¼ Ti, V, Nb) thin films

under ultra-high vacuum (UHV) conditions leads to surface

self-cleansing as evidenced by XPS Heat treatments

per-formed in the vacuum vessel with the base pressure better

than 1.5 1010Torr (2 108Pa) and in the temperature

range of 200–1000C cause a gradual removal of the native

oxide layer following even 2-years-long storage in air The

effect is believed to be triggered by the recrystallization

pro-cess, the latter evidenced by an increased grain size and

reduced residual stress from defect annihilation The

accom-panying surface reconstruction leads to the release of CO,

CO2, ONH, and H2O species The Ti 2p and N 1s XPS

spectra acquired from TiN samples after the 1000C anneal are identical to those obtained from in-situ grown epitaxial TiN/MgO(001) The oxygen concentrations following the heat treatment are lower than those recorded from

Arþ-etched surfaces, without destructive effects from ion bombardment

Polycrystalline (TM)N thin films of thickness 200

6 10 nm are grown on Si(001) substrates at 410C by reac-tive high power pulsed magnetron sputtering (HIPIMS)911

in a CC800/9 CemeCon AG system using rectangular 8.8 50 cm2target and Ar:N2(4:1) gas mixture HIPIMS is operated at the average power of 1.3 kW, the pulsing fre-quency of 600 Hz, and the duty cycle of 12% Substrate bias

is applied in the form of 200-ls-long pulses synchronized with HIPIMS cathode and the amplitude of60 V.12,13The target-to-substrate distance is 6 cm, while the total pressure during deposition is 3 mTorr (0.4 Pa) Following film growth, the samples are allowed to cool down to 180C before the deposition chamber is ventilated, which allows for

a better control of surface chemistry upon air exposure.14 h2h X-ray diffraction (XRD) scans reveal that all films possess a single-phase NaCl-crystal structure TiN layers exhibit no preferred orientation, while NbN and VN films are 002- and 111-oriented, respectively Rutherford back-scattering spectroscopy (RBS) gives N/Ti¼ 0.93 6 0.01, N/Nb¼ 1.13 and N/V ¼ 0.85, while from time-of-flight energy elastic recoil detection analyses (ToF-E ERDA),15 the O bulk concentrations are0.5 at % in all layers Annealing employing an e-beam heater is performed in the UHV chamber directly connected to the XPS instrument, with a base pressure <1.5 1010Torr (2 108Pa), which raises during the treatment to a maximum of 3.8 108Torr (5.1 106Pa) due to sample outgassing After the anneal, samples are allowed to cool for ca 0.5 h and then transferred

in UHV to the analysis chamber for XPS characterization Core level XPS spectra are acquired using an Axis Ultra DLD instrument from Kratos Analytical, with a base pres-sure of 1.1 109Torr (1.5 107Pa) and monochromatic

Al Ka source (h¼ 1486.6 eV) The binding energy (BE)

scale is calibrated by examining the sputter-cleaned Au, Ag,

and Cu samples according to the recommended ISO

stand-ards for monochromatic Al Ka sources and setting the Fermi

level cut-off recorded from TM(N) surfaces at zero

energy.16,17Spectra deconvolution and quantification is

per-formed using CasaXPS software employing Shirley-type

background,18and manufacturer’s sensitivity factors.19

The set of normalized Ti 2p core-level spectra acquired

from air-exposed polycrystalline TiN (poly-TiN) film as a

function of thein-situ annealing temperature Tais shown in

Fig.1(a) We focus here on the stronger 2p3/2 peaks of the

spin-orbit split doublet, located in the BE region from 455.0

to 460.0 eV, which give the clearest signature of Ti chemical

states compared to their 2p1/2counterparts Since the sample

has been stored in air for a longer period of time, the

spec-trum of the as-deposited film, apart from the Ti-N

compo-nent at 455.0 eV, possess a clear signature of Ti-OxNy and

Ti-O2 peaks at 456.6 and 458.2 eV, respectively.20–24 The

formation of native oxides and oxynitrides is regularly

encountered in the XPS practice asin-situ XPS capability is

rare and for vast majority of samples exposure to ambient

atmosphere during transport to spectrometer cannot be

avoided Surprisingly, already after 1 h anneal at 200C

a significant decrease of Ti-OxNy and Ti-O2 signals is

observed, which continues with further increasingTaup to

1000C (a maximum temperature available in our setup) Eventually, Ti 2p signal obtained after the 1000C in-situ anneal is essentially identical to that of epitaxial stoichiomet-ric TiN/MgO(001) films (epi-TiN) grown and analyzed in-situ in a UHV XPS system, as discussed below.25,26 In par-ticular, the satellite features on the high BE side above the primary peaks,27–29 exhibit high intensities comparable to those obtained from epi-TiN We have also established in control experiments that (i) a direct 1 h long anneal at

1000C gives essentially the same result as the temperature ramp, and (ii) a prolonged treatment at 200C (3 h) leads to marginally small change as compared to the 1 h anneal The corresponding set of normalized N 1s spectra from as-deposited poly-TiN films shown in Fig.1(b)is fully con-sistent with the evolution of the Ti 2p signals The spectrum consists of a main peak at 397.2 eV due to Ti-N, a pro-nounced feature at 396.1 eV assigned to Ti-OxNy forma-tion,20 and an asymmetrical tail on the high BE side Following a 200C anneal, the intensity of the Ti-OxNypeak

is drastically reduced, in accordance with the evolution of the Ti 2p spectrum In addition, there is a shoulder on the low BE side of the main N 1s peak, which we assign to Ti-OxNy species with an y/x ratio significantly higher than for the as-deposited sample, thus possessing stoichiometry closer to that of TiN This interpretation is supported by changes observed upon further anneals, which lead to

FIG 1 (a) Ti 2p, (b) N 1s, (c) O 1s, and (d) C 1s core-level XPS spectra acquired from polycrystalline TiN/ Si(001) films as a function of anneal temperature Detailed deconvolution of all core level spectra recorded from TiN surfaces with various degrees of oxidation is presented in Ref 20

gradual decrease of this feature, until complete

disappear-ance after the Ta¼ 1000C treatment With increasing Ta,

the presence of a small satellite peak at399.8 eV emerges,

in agreement with previous reports.20

A complementary Ta sets of O 1s and C 1s spectra are

shown in Figures 1(c) and 1(d), respectively The anneal at

200C, leads to a drastic decrease in the Ti-OxNy, O-C¼O,

C-O, and C-C/C-H signal intensities This is expected as these

C-containing species are likely due to adventitious carbon

accu-mulating on the surface upon air exposure and typically weakly

bonded to the oxides.30 With increasing Ta, the O 1s spectra

confirm a gradual loss of TiO2, and eventually only residual

amounts of oxygen are found on the surface following the

1000C anneal, likely due to redeposition of released

oxygen-containing species during the relatively long acquisition time of

2 h C 1s spectrum recorded after the final anneal step indicates,

in addition to residual amounts of hydrocarbons, also a small

contribution at 282.1 eV, assigned to TiC unintentionally

formed during film growth due to reaction with residual gases.31

Very low intensity of this peak accounts for the fact that no TiC

contribution is indicated in the corresponding Ti 2p spectrum

In Figures2(a)and2(b)Ti 2p and N 1s spectra recorded

from UHV-annealed (Ta¼ 1000C)poly-TiN films are

com-pared to those acquired from (i)in-situ grown and analyzed

epi-TiN from Ref.32, and (ii)poly-TiN in-situ capped with a

1.5-nm-thick Al layers in the deposition system immediately

after the film growth and prior to air-exposure.33 The latter

provides a good barrier towards oxidation and allows

non-destructive XPS with data quality characteristic of films

grown and analyzed in-situ.34 In addition, spectra recorded

frompoly-TiN films treated in a conventional way, i.e., with

Arþion etch, are included to illustrate the detrimental effects

of ion bombardment.35 Clearly, the Ti 2p spectrum from

UHV-annealedpoly-TiN is essentially identical to that obtained

fromepi-TiN samples and in agreement to Al-capped poly-TiN

films In particular, the intensity of the satellite features is

simi-lar, but higher than after sputter etching with low energy Ar

ions EAr þ¼ 0.5 keV incident at a shallow angle of w ¼ 70

with respect to the surface normal The latter case constitutes a

good example of Arþ-induced sputter damage, for a sputter

beam modified surface layer thickness36is comparable to the

XPS probing depth.37These destructive effects are even more

pronounced forEAr þ¼ 4 keV and w ¼ 45, with almost a

com-plete smearing out of the satellite peaks Changes in the

corre-sponding N 1s spectra, confirm destructive effects of Arþetch,

which are manifested by broadening of the Ti-N peak and

smearing out of the satellite feature on the high BE side (better

visible in the inset of Fig.2(b)) In fact, the spectra obtained

after 1000C UHV anneal have a superior quality to that seen

forin-situ Al-capped poly-TiN This is because inelastic

scat-tering in nm-thick capping layers is avoided, which results in a

noticeably lower background level on the high BE side of both

Ti 2p and N 1s signals, as well as narrower peaks

To get an insight into the mechanism behind the intriguing

removal of surface native oxides by high-TaUHV annealing,

we examined other members of the TM(N) family, focusing

on the widely-applicable group IVb–VIb Representative

examples are included in Figures3(a)–3(c)where the strongest

metal core level signals obtained from polycrystalline NbN,

VN, and ZrN films are shown Spectra are recorded in the

as-deposited state after 1000C UHV anneals, and following an

Arþ-etch (EAr þ¼ 0.5 keV, w ¼ 70) The 3d spectrum of NbN (Fig.3(a)) is of particular interest as, in contrast to the Ti 2p spectra of poly-TiN, no satellite features are observed on the high BE side of the main peaks34,38hence, any residual oxide left on the surface after UHV anneal may be directly observed Clearly, Nb 3d5/2and Nb 3d3/2oxide peaks, originally present

at 207.3 and 210.2 eV in the spectrum from the as-deposited film, are completely gone after the 1000C anneal The remaining doublet with the 3d5/2-3d3/2 peaks at 204.0 and 206.8 eV, corresponds well with nitride spectra obtained from stoichiometric NbN films grown in-situ by N2 þion implanta-tion.38Corresponding data for Arþ-etched samples reveal sput-ter damage with low BE peaks shifted with respect to the original nitride peaks, and confirm the benefits of our non-destructive XPS by means of UHV anneal

The V 2p spectra recorded from VN layers (Fig.3(b)) are interesting for their proximity to the O 1s peak UHV-annealed films exhibit main spin-split 2p3/2-2p1/2 peaks at 513.4 and 520.9 eV, with no traces of oxygen, which is spectacular taking into account that the O-free surface is not obtained by sputter-etching, in which case remaining O 1s signal is still detected at

531 eV, due in large part to recoil mixing or redeposition of the sputtered oxygen In analogy to the Ti 2p signal of poly-TiN, the intensity of the satellite peaks of UHV-annealed VN

is significantly higher than for the sputter-cleaned samples and agrees with the results published for oxide-less films mechani-cally cleanedin-situ in the UHV system.39

However, not all TM(N) undergo self-cleansing upon

1000C UHV annealing An exemption is ZrN Fig 3(c)

shows Zr 3d spectra that are dominated by 3d5/2-3d3/2oxide peaks at 182.4 and 184.7 eV, with a low intensity of corre-sponding nitride peaks at 180.0 and 182.4 eV The spectrum

is similar to that of the as-deposited ZrN, thus different from oxide-free spectra of sputter-cleaned films As the O/Zr ratio decreases by only 10% upon anneal, the identical heat treat-ment to that performed on poly-TiN, has no effect on ZrN Other examples where the oxide was not removed by

1000C anneal in UHV are TaN and HfN

In previous experiments designed to investigate the TiN surface chemistry upon contact with oxygen-containing

FIG 2 A comparison of XPS (a) Ti 2p, and (b) N 1s spectra acquired from polycrystalline TiN/Si(001) films following the 1000C anneal to those from (i) in-situ grown and analyzed epi-TiN [Ref 32 ], and (ii) poly-TiN grown and capped in-situ with 1.5-nm-thick Al layer that prevents oxide formation and pre-serves the native surface [see Ref 33 ] In addition, corresponding spectra from TiN films conventionally treated with Arþsputter-etched prior to XPS analyses are also shown All spectra are normalized to the highest-intensity feature.

atmospheres (dry air or O2),23,24,40,41 XPS studies confirmed

a severe loss of N and TiO2formation at temperatures as low

asTa 400C (Refs.42and24) according to TiNþ O2!

TiO2þ 1/2N2 Typical XPS spectrum after such few hours

long treatment is characteristic of a pure TiO2, indicating that

the thickness of the reacted layer exceeds the XPS probing

depth In contrast, our results show that the UHV anneal of

air-exposedpoly-TiN leads to an extensive oxygen loss, with

O/Ti ratio decreasing from 0.83 for the as-deposited film to

0.06 after 1000C anneal, which is accompanied by

insignifi-cant N loss The XPS-derived N/Ti ratio following the

1000C treatment is 0.84, much higher than 0.74 obtained

from poly-TiN etched with low-energy (0.5 keV) Arþ ions,

and only 10% lower than the bulk N/Ti ratio of 0.93

Furthermore, as shown in Figure4, XRD measurements

following UHV anneal indicate a substantial reduction in the

full-width-at-half-maximum (FWHM) accompanied by an

increased intensity of 002 and 111 Bragg reflections for TM(N) films, where self-cleansing is observed (TiN, NbN, and VN) For example, FWHM of the TiN(002) decreases upon anneal from 0.89 to 0.36, accompanied by an increase in the peak intensity by a factor of 3.5 This corre-sponds to recrystallization where the average crystallite size increases from 11 to 26 nm, using Scherrer’s formulae.43 Simultaneously, film recovers from a growth-induced com-pressive stress state (indicated by the shift of 002 and 111 XRD peaks towards higher diffraction angles, see Fig 4), from2.8 GPa measured on the as-grown films to 0.3 GPa for the Ta¼ 1000C sample, as estimated by the sin2w method.44XRD shows no or very little change for layers that retain the native oxide, such as ZrN (see Fig.4(d)), in which case a stoichiometric signature in both bonding and lattice parameter is preserved

Based on the striking correlation between the removal

of surface oxides and recrystallization, we propose that the latter process triggers the self-cleansing Diffusion of N to the film surface together with C from residual gases or adventitious C results in reactions that disintegrate the native oxide by the release of gaseous species CO/N2 (28 amu), ONH (31 amu), H2 (2 amu), H2O (18 amu), and

CO2 (44 amu), as indicated by the residual gas analyzer (RGA) The out-diffusing N also replenishes the nitride surface for possible reconstruction, hence the N/Ti ratio in the surface region of annealed films is comparable to that

in the bulk Thus, the activation energy for recrystalliza-tion of a given nitride is a key parameter that steers the effectiveness of the UHV anneal treatment in removing surface oxides

The type and stoichiometry of the surface oxide is corre-spondingly expected to play a crucial role as there is a clear correlation between the effectiveness of the 1000C UHV anneal and the heat of oxide formation DfH0 All oxides studied here that were not affected by the high-Tatreatment have DfH0 in the range1098 kJ/mol (ZrO2) to 2046 kJ/ mol (Ta2O5), which is significantly higher than420 kJ/mol (NbO), 431 kJ/mol (VO), and 944 kJ/mol (TiO2), where oxides are effectively removed.45 In the case of poly-TiN

FIG 3 (a) Nb 3d, (b) V 2p and O 1s, and (c) Zr 3d core-level XPS spectra

acquired from polycrystalline transition metal nitride thin films in the

as-deposited state, following the 1000C anneal, and after the Arþion etch All

spectra are normalized to the highest-intensity feature.

FIG 4 h2h X-ray diffraction scans obtained from (a) TiN, (b) NbN, (c)

VN, and (d) ZrN polycrystalline thin films in the as-deposited state and fol-lowing the 1000  C anneal in UHV.

film, the fact that native oxide is not a pure-phase

stoichio-metric compound, but rather a mixture of TiO2and TiOxNy

(cf Fig.1(a)), with the latter compound located closer to the

TiN interface,14may facilitate the cleansing process

The scenario presented above is corroborated by

annealing experiments performed with epi-TiN(111) and

epi-TiN(001) films (not shown) Although the Ti 2p spectra

quality improved after 1000C treatment, qualitative

analy-ses indicated that the removal of surface oxygen is not as

efficient as for the poly-TiN samples, with the O/Ti ratio

varying from 1.44 to 0.53, and from 1.04 to 0.31, for

epi-TiN(111) and epi-TiN(001), respectively Greatly reduced

number of grain boundaries in the case of epitaxial layers,

removes the more easily thermally-activated sites for

inter-diffusion and recrystallization, considering that 1000C is

only 1/3 of the melting temperature of TiN

In summary, while performing in-situ annealing

experi-ments of magnetron-sputtered TM(N) films in UHV, we

unex-pectedly observe that a few hours at 1000C yield oxide-free

surfaces characteristic of the native nitride A direct

compari-son reveals that the core level XPS spectra recorded from TiN

samples following the treatment are identical to those

obtained fromin-situ grown epitaxial TiN/MgO(001) Thus,

the method enables a non-destructive acquisition of

high-quality XPS spectra representative of a native material even

after prolonged atmosphere exposure, which is a clear

advan-tage over the commonly used Arþ-ion etching The combined

XRD and RGA analyses indicate that the effect is thermally

activated with recrystallization of the nitride leading to the

disintegration of the surface oxide as well as adventitious

car-bon by the release of CO, CO2, ONH, and H2O species

The authors gratefully acknowledge the financial

support of the VINN Excellence Center Functional

Nanoscale Materials (FunMat) Grant No 2005-02666, the

Swedish Government Strategic Research Area in Materials

Science on Functional Materials at Link€oping University

(Faculty Grant SFO-Mat-LiU 2009-00971), the Knut and

Alice Wallenberg Foundation Scholar Grant 2011.0143, and

the A˚ forsk Foundation Grant No 16-359 We thank Dr

Daniel Primetzhofer for help with ToF-E ERDA and RBS

measurements and Dr Andrejs Petruhins for providing

epitaxial TiN samples

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