X-ray photoelectron analysis of thin film Tinx

186

of Surface Science 20 (1984)186- 192 North-Holland, Amsterdam

Apphcations

COMMUNICATION

X-RAY PHOTOELECTRON Nguyen L&xx-utoty MIInaukre.

Received

VAN HIEU /or

2 February

OF THIN FILM TiN,r

and David LICHTMAN

Surface Studres

Wrsconsin

ANALYSIS

53201,

ond

Depurtmertt

of

Ph.r‘.m,

Unrwx~,~

of

Wmonsrn

Mdwuukee.

USA

1984; accepted

for publication

12 July 1984

XPS depth profiles from chemical vapor deposited titanium nitride films (TiN,) indicate that the surface is oxidized to such an extent that TiO, is the only oxidized species observed. For the sputter cleaned sample, the binding energyy values of 454.80 and 460.60 eV were obtained for Ti of the film, which has a Ti-to-N 2~~~1 and Ti 2~,,,, respectively. In addition, the stoichiometry ratio of unity (approximately), is obtained from the surface layer up to 900 A deep into the sample. This result is further supported by the appearance of the sample surface which had acquired a uniform gold color, characteristic of pure TiN.

Titanium nitride belongs to a class of refractory transition-metal compounds which exhibit a number of unique physical and electrical properties such as high melting temperatures conductivity been

[l]. Because

extensively

studied

and great hardness,

of its technological by several

as well as good metallic

applications,

investigators.

titanium

In early

1966,

nitride has Nelson

re-

ported the use of titanium nitride as a conducting film and a diffusion barrier in semiconductor technology [2]. Since then a great deal has been accomplished in this area. Nicolet

and the Caltech

group studied titanium

applied them to solar cell contacts [3,4]. titanium nitride films as a high-temperature [5,6] and as a gate electrode

Wittmer contact

in MOS structures

nitride films and

also reported the use of barrier for silicon devices

[7].

In the present work, we have studied the oxidation states and the stoichiometric ratio of titanium nitride films, prepared by chemical vapor deposition. using XPS depth profiles. The experimental system used to obtain the XPS results, which will be presented below, has been described in detail previously [8]. The XPS measurements initially consisted of an overall scan of the OGlOOO eV binding energy range with a pass energy of 100 eV to check for all main surface components. Next, detailed scans covering 20 eV with a pass energy of 25 eV were made for the core levels of the Ti 2p and N Is photoelectron lines. Fig. 1, showing an XPS survey scan from an as-received titanium nitride sample, reveals the presence of titanium, nitrogen and typical impurity peaks of carbon and oxygen. For the latter impurity species, the 0 1s binding energy 0378-5963/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

N.

Van HEW, D. Lichtman

/

X-ray

photoelectron

analysts

ot

thin

film

TiN,

187

value of 530.0 eV was obtained. This value corresponds to the binding energy of the 0 1s photoelectron line for the oxidized surfaces of transition metals [9]. For the purpose of chemical state identification, a detailed scan of the Ti 2p photoelectron line from this sample was obtained and is presented in fig. 2. The values of the binding energies of Ti 2p,,, and Ti 2p,,, are 458.80 and 464.50 eV, respectively, which agree well with the binding energy values of 458.50 and 464.20 eV of the published Ti 2p,,, and Ti 2p,,, photoelectron lines from TiO, [lo]. Also, from fig. 2, one notices a small shoulder at the binding energy of 454.80 eV, which will be discussed below. All binding energy values were charge-corrected to the C 1s photoelectron line, assuming the C Is line for adventitious hydrocarbons to be 284.60 eV [lo-131. Based on the above results, we conclude that the titanium nitride sample surface is oxidized to such an extent that the TiO, is the only oxidized species observed. This conclusion is not surprising, since titanium metal is very reactive with oxygen. For titanium metal exposed to air atmosphere, several monolayers of oxygen are absorbed at room temperature and the main oxide layer is TiO, [14,15]. Besides sample oxidation from exposure to the air and residual gases in the UHV chamber, the sample can be oxidized from exposure to contamination in the annealing furnace. Usually, the samples were annealed at 900 ‘C for 15 min in N,. Also, it is interesting to mention that there are two important factors that govern the resistivity of the titanium nitride film: the oxygen residue and the nitrogen doping level. The resistivity increases with

700

600

BINDING

500

ENERGY

400

300

200

100

(eV)

Fig. 1. An XPS survey spectrum of an as-received titanium nitride sample.

N. Van Hieu, D. Lichtmm

188

/ X-rq

photoelectron

unu@sis of thin film TIN,

increasing oxygen contamination, and is a complex function of the nitrogen doping level [16]. XPS, in conjunction with argon ion bombardment, was then used in analyzing the film composition as a function of depth. XPS results from the titanium nitride film sample, after it had been sputtered for 0 min. 3 min and 30 min, are presented in figs. 3a, 3b and 3c, respectively. Fig. 3b (after 3 min sputtering) shows that the Ti 2s, Ti 2p and N Is intensities have increased while the 0 Is intensity has decreased. However, the C 1 s peak has completely disappeared, which suggests that the carbon contaminant from the titanium nitride film surface is probably due to adsorbed hydrocarbons. An XPS spectrum for the titanium nitride film after it had been sputtered 30 min is shown in fig. 3c. It can be seen that the impurity oxygen peak was reduced to a minimum value, but not removed completely. The 0 1s signal, under argon ion bombardment, shifted toward the higher binding energy of 531.0 eV, the value characteristic for adsorbed oxygen on transition metal surfaces [9]. A detailed scan of the Ti 2p photoelectron line from the sputter cleaned sample (30 min sputtering time) was taken and is presented in fig. 4. The values of the binding energies of Ti 2~,,~ and Ti 2p,,, from this “cleaned” titanium nitride sample are 454.80 and 460.60 eV, respectively. In comparison with fig. 2 (oxidized sample), one notices that the binding energy value of 454.80 eV of the small shoulder observed coincides with that of Ti 2p,,, from the sputter cleaned titanium nitride sample. This result leads to the following

BEFORE

SPUTTERING TN 2b,2 458.80

BINDING Fig. 2. A detailed nitride

sample.

XPS spectrum

ENERGY

of the Ti 2p,,,,,,,

(eV) p hotoelectron

line from

an as-received

titanium

N. Van H~eu, D. Lichtman

conclusion.

/ X-ray photoelectron

The small shoulder appearing

2 is probably

associated

oxidized layer, TiO,. The stoichiometric

ana!vsis of thin film TiN,

189

on the low binding energy side in fig.

with Ti from TIN which lies under a contaminated

Ti-to-N

ratio was determined

as a function

of depth into

~ptmering Tome: 0

minutes

jputierlng

Time

:

,putterlnq

(a

,

3 minutes

Time:

30 mnutes

L

I

v

550

1

450

BINDING Fig. 3. XPS depth

profiles

min, (b) 3 min, (c) 30 min.

I

L

I

u

ENERGY

for the titanium

350

((;I I J 250

(eV) nitride

sample

which

was argon ion-sputtered

for: (a) 0

N.

190

Van Hieu,

D. Ltchtmun

/ X-q

photoelectron

unu~w.s

of thrn ftlm

TlN,

the sample by monitoring the XPS detailed scans of Ti 2p and N Is (see fig. 5). The atomic concentrations of Ti and N were determined by measuring the peak areas of Ti 2p,,, and N Is, and also by using published sensitivity factors [17]. An ion energy of 3 keV and ion current density of 16 PA/cm* were used in these experiments. From fig. 5, one obtains the stoichiometric Ti-to-N ratio of - 0.97 for the outermost layer. The average Ti-to-N ratio from the surface layer up to 750-900 A within the sample is about 0.9. It should be emphasized that the sputter depth and the Ti-to-N ratio presented here are estimates, since the ion beam sputter rate (28 f 3 A/min) used was obtained from calibrated sputtering of a layer of known thickness of SiO, deposited on a Si substrate, and it is well known that sputtering rates for different elements can be quite different [18-221. In addition, composition changes due to preferential sputtering in both alloys [18,23-251 and oxides [22,26-281 are well known. The observed value of 0.97 of the Ti-to-N ratio for the outermost layer can be approximately assigned a stoichiometric Ti-to-N ratio of unity. This result (T/N = 1) is further supported by the appearance of the titanium nitride sample surface which had acquired a uniform gold color characteristic of pure TIN [29,30]. Further information related to the band structure of TIN,, both theoretical calculations and UPS experimental results, can be found in a recent paper [31], TIN is cubic with the NaCl structure. In summary, XPS depth profiles from chemical vapor deposited titanium nitride films indicate that the surface is oxidized to such an extent that the

AFTER

SPUTTERING

TN%p3,*

T12p 460

Fig. 4. A detailed XPS spectrum argon ion-sputtered for 30 min.

14 60

of the Ti 2p,,,.,,,,

p hotoelectron

line from the sample which was

N. Van Hieu, D. Lichtman / X-ray

photoelectron

anulysis of thin film TiNx

191

x-

0.7 0 F

!

0.6-

P!? 0.5t p

0.4-

0.3

1

)

4

0.2

0.1, L

’

0

3

6

I

1

II

9

12

15

SPUTTERING Fig. 5. The Ti-to-N

I

I8

21

24

27

30

TIME (MINUTES)

ratio as a function

of sputtering

TiO, is the only oxidized species observed.

time.

For the sputter cleaned TIN sample,

binding energy values of 454.80 and 460.60 eV were obtained for Ti 2p,,, and Ti 2p,,,, respectively. In addition, the stoichiometry of TIN, which has a Ti-to-N ratio of unity (approximately), is obtained from the surface layer up to - 900 A deep into the sample.

Acknowledgement The authors would like to thank titanium nitride films.

Dr. Fabio

Pintchovski

for providing

the

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