Investigation of TiN thin film oxidation depending on the substrate temperature atvacuum breakFabien Piallat, Remy Gassilloud, Pierre Caubet, and Christophe Vallée Citation: Journal of Vacuum Science & Technology A 34, 051508 (2016); doi: 10.1116/1.4960648 View online: http://dx.doi.org/10.1116/1.4960648 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/34/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Effect of conductive TiN buffer layer on the growth of stoichiometric VO2 films and the out-of-planeinsulator–metal transition properties J. Vac. Sci. Technol. A 32, 041502 (2014); 10.1116/1.4874844 Electrical properties of TiN on gallium nitride grown using different deposition conditions and annealing J. Vac. Sci. Technol. A 32, 02B116 (2014); 10.1116/1.4862084 Effect of substrate temperature on structural and electrical properties of liquid-delivery metal organic chemicalvapor deposited indium oxide thin films on silicon J. Vac. Sci. Technol. B 26, 909 (2008); 10.1116/1.2905238 Effects of nitrogen content on microstructure and oxidation behaviors of Ti–B–N nanocomposite thin films J. Vac. Sci. Technol. A 24, 340 (2006); 10.1116/1.2172949 Effect of temperature on Ti and TiN films deposited on a BN substrate J. Vac. Sci. Technol. A 16, 1901 (1998); 10.1116/1.581125

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Investigation of TiN thin film oxidation depending on the substratetemperature at vacuum break

Fabien Piallata)

STMicroelectronics, 850 rue Jean Monnet, 38920 Crolles, France; CEA, LETI, Campus Minatec,F-38054 Grenoble, France; and LTM-CNRS, 17 rue des Martyrs, 38054 Grenoble, France

Remy GassilloudCEA, LETI, Campus Minatec, F-38054 Grenoble, France

Pierre CaubetSTMicroelectronics, 850 rue Jean Monnet, 38920 Crolles, France

Christophe Vall�eeLTM-CNRS, 17 rue des Martyrs, 38054 Grenoble, France

(Received 30 June 2016; accepted 28 July 2016; published 11 August 2016)

Due to the reduction of the thickness of the layers used in the advanced technology nodes, there is a

growing importance of the surface phenomena in the definition of the general properties of the

materials. One of the least controlled and understood phenomenon is the oxidation of metals after

deposition, at the vacuum break. In this study, the influence of the sample temperature at vacuum break

on the oxidation level of TiN deposited by metalorganic chemical vapor deposition is investigated. TiN

resistivity appears to be lower for samples which underwent vacuum break at high temperature. Using

X-ray photoelectron spectrometry analysis, this change is correlated to the higher oxidation of the TiN

layer. Moreover, angle resolved XPS analysis reveals that higher is the temperature at the vacuum

break, higher is the surface oxidation of the sample. This surface oxidation is in turn limiting the

diffusion of oxygen in the volume of the layer. Additionally, evolution of TiN layers resistivity was

monitored in time and it shows that resistivity increases until a plateau is reached after about 10 days,

with the lowest temperature at vacuum break resulting in the highest increase, i.e., the resistivity of the

sample released to atmosphere at high temperature increased by a factor 1.7 whereas the resistivity of

the sample cooled down under vacuum temperature increased by a factor 2.7. VC 2016 AmericanVacuum Society. [http://dx.doi.org/10.1116/1.4960648]

I. INTRODUCTION

Titanium nitride is one of the most used materials in the

semiconductor industry, for many applications, from metal

layer in the metal gate to hard mask for lithography.

Depending on the chosen application, one of the many advan-

tages of TiN can be privileged, either the low resistivity,1 the

suitable work function for metal gate in CMOS application,2

the good adhesion with copper,3 the low etching rate or high

selectivity compared to SiO2,4 or the barrier properties against

copper diffusion.5 TiN appears to be the most cited metal in

the literature, and each of its properties has been deeply inves-

tigated and lots of references prove the influence of the com-

position on its properties,1,2,6,7 mainly considering the

nitrogen content, and only few references concerning the

influence of the oxidation can be found.6–8 However, due to

the fact that the oxide of titanium is an insulator, whereas the

nitride is a conductor, one cannot overlook the effect of TiN

oxidation. This effect is even of higher importance when thin

layers (<10 nm) are taken into consideration. Oxidation of

TiN is taking place at room temperature due to the negative

Gibbs energy of formation of TiO2 from TiN and O2 (Ref. 9)

TiNþ O2 ! TiO2 þ 1=2 N2 DGðformationÞ

¼ �578:8 kJ:mol�1ðat 20 �CÞ:(1)

In some of the previous works,10 it was supposed that

exposure to room air or voluntary oxidation would “stuff”

the grain boundaries and hence improve the barrier proper-

ties. But it also showed that high level of oxidation results in

the formation of an insulating layer.

Several studies present the evolution of TiN when pur-

posely oxidized at temperatures ranging from room tempera-

ture up to 600 �C.11–14 An in-depth work was completed by

Tompkins in the 1990s (Refs. 10 and 15) to determine the

mechanism of oxidation of TiN layers. He reported that first

an initiation period is taking place during which the oxida-

tion proceeds at a slower rate. The material formed during

this first phase is an oxynitride, which then transforms into

an oxide, the nitrogen being pushed further inside the layer

and replaced by oxygen migrating from the surface.10,11,13,15

The phenomenon described in the referenced articles was

observed for controlled conditions under vacuum, and there

is no reference to oxidation taking place at low temperature

in the uncontrolled atmosphere of a clean room (i.e., air at

20 �C and containing 40% of humidity), neither to oxidation

taking place in conditions similar to the atmosphere, i.e.,

760 Torr, 20% O2, and 20 �C. However, oxidation in the

clean room atmosphere is of importance as it corresponds to

the real case conditions of the production lines. If the exam-

ple of 200 mm production line is taken, the substrate temper-

ature at the vacuum break varies, indeed the first wafer of aa)Electronic mail: fabien.piallat@gmail.com

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lot is cooling down under vacuum whereas the last wafer

might undergo vacuum break at high temperature if no cool-

ing down step is planned before vacuum break. This varia-

tion might result in a wafer to wafer variation which can

lead to differences in the yield of the fabricated devices.

In this article is presented the case of oxidation taking

place directly after vacuum break from the deposition tool.

Thin layers of <10 nm TiN were taken out of the vacuum at

different temperatures directly after deposition in order to

comprehend the oxidation mechanism and highlight the

parameter which can allow its reduction. To do so, spectra

obtained from XRR, XPS, and resistivity measurement are

compared after vacuum break. Furthermore, evolution of the

resistivity was monitored over a long period of time to

ensure that the final resistivity reached was stable.

II. EXPERIMENT

A. Sample preparation

All depositions were done in an Altatech AltaCVD

FASTVR

(Fast Atomic Sequential Technique) chamber,16 on

300 mm silicon substrate with a silicon thermal oxide of

100 nm for resistivity four-points probe measurements.

Tetrakis(diethylamino)titanium precursor was used for

deposition together with NH3 reactant gas, in a CVD mode

growth at 360 �C.

To allow the wafer to cool down after deposition, a wait-

ing step was introduced before the vacuum break. This step

was done in the transfer chamber, which was pressurized to

200 mTorr, with a 30 sccm N2 flow. The step time was tuned

to obtain a wafer temperature at the vacuum break ranging

from 140 �C (no waiting time) to the temperature of the

clean room 20 �C (20 min waiting time). Four samples are

presented here, with a vacuum break temperature of: 140,

70, 45, and 20 �C. During the cool down, the wafer was

placed on lift-pins to limit the contacts and resulting in a

heat exchange mainly by radiation. The wafer temperature

was measured on ten points at vacuum break with an infrared

thermometer; the average temperature is used hereafter.

B. Characterizations

Thickness, density, and roughness were measured by

XRR, in 43 points using a Jordan Valley JVX6200 (h¼ [0;

3.5�]), and 4 points probe technique was used to measure the

films resistivity in 25 points using Napson WS-3000.

Evolution of the resistivity was monitored over two months,

using different locations for the measurements to avoid an

increase of the resistivity linked to the local degradation of

the layer. XPS measurements were performed with a Theta

300 XPS tool from Thermo Scientific, once the resistivity of

the layers stabilized. A high resolution monochromatic Al

Ka x-ray source (1486.6 eV photons) was used with pass

energy of 100 eV and a resolution of 0.1 eV. No carbon or

oxide removal was performed on the samples before XPS

characterization; thus, due to oxidation and atmospheric con-

tamination, high levels of C and O were observed at the

extreme surface of the samples. Carbon C1s, located at

285 eV (Ref. 17) was used to remove any possible shift in

the binding energy due to sample charging. Observation of

Ti, Si, C, N, and O chemical environments was extracted

from the Ti2p, Si2p, C1s, N1s, and O1s core level energy

regions, respectively. Using a numerical procedure, spectral

FIG. 1. (Color online) Variation of XRR spectra with sample temperature at vacuum break. Full spectra (a) and zoom on the falling angle (b) and first arch (c).

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fitting was performed to extract the peak contributions in the

acquired energy regions. Individual line shapes were simu-

lated with a combination of Lorentzian and Gaussian func-

tions. The background subtraction was performed using a

Shirley function calculated from a numerical iterative

method.

III. RESULTS AND DISCUSSION

A. Physicochemical characterization after deposition

In Fig. 1 are presented the XRR spectra of the four sam-

ples (a), a zoom on the falling angle (b), and a zoom on the

first arch (c), which correspond to the density and the thick-

ness of the layer, respectively.

Overall, the spectra in Fig. 1(a) show little variation,

thanks to the 1% wafer to wafer repeatability of the process.

Yet, a slight modification of the falling angle is observed at

0.51� on Fig. 1(b), revealing a decrease of the TiN density

with the decreasing temperature at vacuum break. Considering

that titanium oxide has a lower density than titanium nitride,

this variation suggests a higher oxide content for the sample

released to air at low temperature than the one released at high

temperature.

Additionally, a clear difference of the first arch location

can be observed between each spectrum [Fig. 1(c)]. This

change shows a decrease of the TiN thickness with the lon-

ger cool down step.

To fit the experimental spectra, a mathematical model

made of two distinct layers was used. This model permits to

simulate a surface oxidation, but does not take into account

the possible oxygen gradient. The TiN and TiO2 layers thick-

nesses and densities obtained from the spectra fitting are

reported Table I.

The decrease of the extracted density with the sample tem-

perature decrease can be linked to the increase of the oxide

layer at the sample surface, due to the lower density of TiO2

[3.9 g cm�3 (Ref. 18)] compared to the higher density of TiN

[bulk density of crystalline TiN 5.22 g cm�3 (Ref. 18)].

The total thickness shows a reduction by 8% between the

two extreme samples. This reduction appears on both the

volume nitride and the surface oxide.

Surprisingly, the thickness and density of the nitride layer

show a similar trend of reduction with decreasing tempera-

ture. The calculation of the area density (g cm�2) implies

that as the temperature decreases there is less material on the

substrate. This evolution is also true if the oxide surface

layer is taken into consideration.

Furthermore, the distinction between TiN layer and TiO2

surface material are less pronounced at the lower vacuum

break temperature, as the TiN and TiO2 densities are follow-

ing opposite trends. The fact that TiO2 density is getting

closer to TiN density suggests that there is no clear separa-

tion between the two materials.

Yet, one has to remember that the use of two layers is

only a model allowing extraction of thickness and density

from the XRR spectra, and in reality, it is most likely that an

oxynitride with a gradient from oxide to nitride is formed.

A representation of the TiO2/TiN layer deduced from the

XRR measurements is presented Fig. 2.

The difference of density and the separation between the

two layers between the high temperature vacuum break sam-

ple and the low temperature sample can be supported by the

following two hypotheses. First, in the case of the high tem-

perature, thanks to the energy brought by the temperature an

oxide layer is formed at the surface of the sample, limiting

further oxidation in the depth of the material. Second behav-

ior is observed for the sample at low temperature, where the

oxygen is able to diffuse in the volume of the material, creat-

ing a gradient which can continue to evolve with time. The

XPS analysis and resistivity measurements will be analyzed

TABLE I. Thickness and density of TiN and TiO2 surface oxide depending on substrate temperature at vacuum break.

Vacuum break temperature (�C) TiN thickness (nm) TiN density (g cm�3) TiO2 thickness (nm) TiO2 density (g cm�3) Total thickness (nm)

140 5.84 (60.24) 4.01 1.31 (60.04) 2.79 7.15 (60.28)

70 5.72 (60.28) 3.92 1.29 (60.06) 2.93 7.01 (60.34)

45 5.59 (60.18) 3.88 1.22 (60.04) 3.33 6.81 (60.22)

20 5.44 (60.22) 3.83 1.19 (60.04) 3.15 6.63 (60.26)

FIG. 2. (Color online) Location the oxide surface deduced from XRR measurements variation: (a) at high temperature vacuum break and (b) at low

temperature.

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and discussed in the next paragraphs to support these two

hypotheses.

XPS spectra of Ti2p, N1s, O1s, and C1s core level are

presented Figs. 3(a)–3(d), respectively. As mentioned ear-

lier, the XPS measurements were done after the oxidation

appeared to be stabilized (see the next paragraph).

No significant change of the chemical environment can

be observed in the Ti2p, N1s, or C1s. Only in the O1s core

level, Fig. 2(c), the O–O or O–C bond shows a shift of their

bonding energy toward higher energy. On the O1s spectra,

the peak with highest intensity located at 530.4 eV corre-

sponds to the Ti–O bonding environment,17 whereas the sec-

ond peak, at 532.2–532.4 eV corresponds to the C–O and/or

O–O environments.17,19

As mentioned in the Introduction, TiN oxidation is highly

favorable even at low temperature, hence the Ti2p spectra, Fig.

3(a), is composed of N–Ti–O, Ti–O, and Ti–N in order of

intensity, located at 456.9, 458.2, and 455.5 eV, respectively.17

On the N1s spectra, Fig. 3(b), two environments are visible,

the N–Ti and with lower intensity the N–N bonding environ-

ment, located at 396.4 and 399.4 eV, respectively. Figure 3(d),

the C1s spectra is composed by two environments, the C¼O

and C–O located at 289 and 285.2 eV, respectively. The C¼O

does not appear on the O1s spectra due to the relatively low

amount of these bonds compared to the O–O and C–O bonds.

Evolution of the peaks’ intensity does not represent a

change in the amount of the specie in the sample. However,

the composition of the samples can be extracted using the

relative sensitivity factors of each species. The compositions

of the four samples are presented in Fig. 4, in relative unit

compared to Ti content.

The display of the species concentration in Ti relative

content, Fig. 4, reveals that the four samples are made of a

slightly super-stoichiometric TiN, with small change in the

Ti/N ratio toward stoichiometric ratio at high temperature.

High levels of oxygen and carbon contaminations are

observed for the lower temperature samples. This tendency

is closely correlated to the sample temperature at vacuum

break. Due to the absence of shift in the bonding energy of

the Ti2p spectra, Fig. 3(a), and as the relative maximum of

the Ti–N and Ti–O bonds vary simultaneously, one can sup-

pose that the higher oxygen content of the low temperature

sample is located in interstitial position in the TiN material.

FIG. 3. (Color online) chemical environments of TiN layers with XPS spectra of (a) Ti2p, (b) N1s, (c) O1s, and (d) C1s.

FIG. 4. (Color online) XPS extracted volume composition of TiN sample

depending on temperature at vacuum break.

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The interstitial position of oxygen in the TiN matrix was

already suggested,20 and up to 42% of oxygen in TiN was

reported21 without modification of the Ti–N bonding

environment.

Evolution of the carbon content is similar to the one of

oxygen. This carbon is bonded to oxygen and was reported

to be mainly from CxOyHz compounds.19

A summary of the TiN thickness, density, composition,

and resistivity is given Table II, depending on the sample

temperature at vacuum break.

The oxygen concentration extracted from the XPS spectra

is in-line with values reported elsewhere.21 The resistivity,

given in the last column of Table II, shows a significant

decrease with the sample temperature increase at vacuum

break. This variation is in good agreement with the relative

increase of O and C content with the sample temperature

decrease, column 5 and 6 of the table.

Additionally, an angular resolved XPS analysis of the

samples was performed to estimate the surface composition

of the samples. At the smallest angle, i.e., 23�, only the first

2 or 3 nm of the TiN layer are probed using this technique.

On the other hand at the higher angle, i.e., 76�, the volume

of the layer is analyzed. The Ti2p and O1s spectra from the

TiN surface are presented Fig. 5.

As observed Fig. 3(a), no change of the bonding energy

of the several bonds appearing in the Ti2p spectra can be

seen. However, compared to Fig. 3(a), the Ti–N bond is

almost not visible due to the high intensity of the Ti–O bond.

This variation between the surface, Fig. 4(a), and the vol-

ume, Fig. 3, suggests that most of the oxidation is present at

the surface of the layer.

The O1s spectra, Fig. 5(b), present a significant increase

of the C–O or O–O bonds compared to the volume spectra

presented Fig. 3(c). Thus, most of the oxygen trapped in the

TiN and not bonded to Ti is present at the surface of the sam-

ple. As seen Fig. 3(c), the samples 140 and 70 �C show a

shift of the C–O or O–O bond energy toward the lower

energy.

Concentrations of each species present at the TiN surface

are presented in Ti relative content, in Fig. 6. This figure is

the equivalent of the volume composition of TiN presented

earlier in Fig. 4.

As reflected in the spectra shown Fig. 5, the surface of the

samples is mainly composed of titanium-oxide and C–O

contamination. The nitrogen content at the samples surface

is lower than in the volume, with a N/Ti ratio of 0.92 instead

of 1.1. A control of the gas flows during the process did not

reveal any change at the end of the deposition; thus, the

lower nitrogen content surface is not due to the process.

Furthermore, the amount of oxygen and carbon at the sam-

ples surface is twice higher than the overall content of the

layer. It implies that part of the nitrogen from the TiN layer

is replaced by oxygen and carbon at the vacuum break.

Comparing Fig. 6 with Fig. 4, one can see the trend of

oxygen and carbon content is opposite in the volume and at

the surface. The higher is the temperature of the sample at

vacuum break, the higher is the oxygen content at the surface

of the sample. As given in Eq. (1), oxidation of TiN is a

favorable reaction already at room temperature, thus increas-

ing the temperature will result in higher oxidation. Thus, it is

possible to suppose that the higher is the sample tempera-

ture, the more dense will be the oxide layer formed on the

TiN layer, which in turns results in the formation of a barrier

limiting the diffusion of oxygen in the volume of the layer.

This hypothesis will be compared to the evolution of the

resistivity, i.e., of the oxidation, over time.

TABLE II. Physical characteristics of TiN layers with vacuum break at different temperatures.

Vacuum break

temperature (�C)

Total

thickness (nm)

TiN

density (g cm�3)

Ti content

(at. %)

N content

(at. %)

O content

(at. %)

C content

(at. %)

Resistivity

(mX cm)

140 7.15 4.01 23.1 24.2 30.5 22.2 2.92

70 7.01 3.92 22.1 23.6 31.1 23.2 3.52

45 6.81 3.88 21.2 23.1 32.3 23.4 3.61

20 6.63 3.83 20.8 22.7 32.3 24.2 4.01

FIG. 5. (Color online) XPS Ti2p and O1s spectra of TiN surface and depending on substrate temperature using ARXPS.

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B. Evolution of TiN characteristics with oxidation time

The four samples, introduced previously, were kept for 2

months in the clean room environment, and the resistivity

was measured weekly. The resistivity normalized to the

resistivity measured at T0 is presented Fig. 7.

Whatever the temperature of the sample is, it appears that

the trend of the resistivity evolution is similar, with a plateau

reached after about 750 h of exposition to clean room air.

However, this figure also shows that the lower is the tem-

perature of the sample at vacuum break the higher is the oxi-

dation of the sample and thus the higher is the resistivity

increase.

Overall, it appears that reducing the TiN layer tempera-

ture before vacuum break results in a high uptake of oxygen

and carbon. This oxidation is impacting some of the proper-

ties of the layer, such as the density, chemical bonding and

resistivity. On the other hand, a vacuum break with sample

at high temperature leads to the formation of a surface pas-

sivation layer, which limits the amount of oxygen and car-

bon diffusion in the volume of the layer, and so prevents an

important resistivity increase.

IV. SUMMARY AND CONCLUSIONS

From this study, it comes into view that the temperature

of the substrate at the vacuum break has an impact on the

oxidation behavior of MOCVD deposited TiN layer. The

first noticeable change is the density and thickness variation

of the TiO2/TiN model used for fitting to the XRR spectrum.

Decrease of the sample temperature results in a decrease of

the layers’ densities and increase of the TiO2 thickness. The

separation between an oxidized surface layer and the volume

TiN is more distinct for the high temperature samples. This

higher content of oxygen of the low temperature samples

was then confirmed by XPS. Even though no shift in the

binding energy of the oxygen is observed, a variation of the

peak intensity confirms the oxygen content fluctuation. The

increase of oxidation level can account for the resistivity

modification, due to the insulating properties of titanium

oxide. Thus, higher oxide level ends in higher resistivity of

the material. The higher is the temperature of the substrate at

vacuum break, the lowest is the oxygen diffusion and the

resistivity. Angle resolved XPS (ARXPS) analysis revealed

that vacuum break at high temperature favors the formation

of a surface oxide, acting as a passivation layer against diffu-

sion of oxygen in the volume of the layer. Finally, the mea-

surement of the resistivity over time exposed an increase of

the resistivity with time over the first month, before reaching

a plateau. The resistivity increase is influenced by the sample

temperature, with a gain factor ranging from 2.7 for the low

temperature sample down to 1.9 on high temperature sample.

From these findings, it is clear that to avoid the oxidation of

thin TiN films it is better to take the material out of the vac-

uum while it is still hot. Moreover, a treatment for the

removal of surface oxide is easier than the treatment to

remove oxygen located in the volume of the TiN.

ACKNOWLEDGMENTS

This work was achieved with the help of the LETI DTSI

silicon platform and ST Crolles 300 mm fabline in the frame of

ST/LETI joined development program. It was also supported

by the French Government program “Investissements

d’Avenir” managed by the National Research Agency (ANR)

under the Contract No. ANR-10-IQPX-33.

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FIG. 6. (Color online) XPS extracted surface composition of TiN sample

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FIG. 7. (Color online) Evolution of the resistivity with exposure to atmo-

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