228 Thin Solid Fihns, 221 (1992) 228-232

Stoichiometry effects in TiN diffusion barriers

H. J o s w i g a n d W . P a m l e r Siemens AG, Semiconductor Division, Otto-Hahn-Ring 6, 8000 Munich 83 (Germany)

(Received May 5, 1992; accepted June 26, 1992)

Abstract

TiN diffusion barriers were deposited via reactive sputtering in an Ar -N 2 gas mixture. Several TiN films with variation in the stoichiometry were produced and used as a diffusion barrier between a silicon substrate and an aluminium alloy film in order to prevent A1-Si interdiffusion ("spiking"). The quality of the barrier films during further anneal steps was investigated by detection of the diffusion processes and interfacial reactions using Auger electron spectroscopy (AES) depth profiling and by testing the spiking directly on wafers with contact hole structures ("spiking tests").

A correlation between TiN stoichiometry and barrier quality was established; the use of nitrogen-deficient TiN films resulted in a high degree of interdiffusion and a clear tendency towards spiking.

In order to counteract barrier failure due to substoichiometric TiN a barrier post-treatment in N 2 was performed and characterized with AES. With this post-treatment an improvement in the barrier performance for substoichio- metric TiN films could be achieved.

1. Introduction

In advanced semiconductor technologies, diffusion barrier layers are used to prevent interdiffusion of alu- minium and silicon in contacts of electrical circuits ("spiking") [1-4]. For this purpose often a Ti/TiN sandwich is used [5-7]. In most cases the TiN layer is deposited via reactive sputtering from a titanium target, but also other methods such as chemical vapour deposi- tion [8], sputter deposition from a TiN target [9], or nitridation of a titanium film via rapid thermal anneal- ing in a suitable atmosphere [10] have been reported.

Several workers have studied the barrier mechanism of such a Ti/TiN sandwich and have discussed possible failure mechanisms of the barrier layer [5, 11, 12].

This work concentrates on the role of the TiN stoi- chiometry with respect to barrier reliability. This seems especially important in view of a recent and very de- tailed analytical study [11] showing that the TiN stoi- chiometry within a submicron contact hole is difficult to control for reactive sputter deposition. We then focus on a possible barrier post- treatment which should allow us to-counteract stoichiometry-induced barrier failures.

2. Experimental details

Titanium and TiNx were produced via sputtering in a commercial batch-type d.c. magnetron sputter system with a rotating substrate table. TIN,. was deposited by reactive sputtering from a titanium target in an A r - N 2

gas mixture. The stoichiometry of the T iN, films de- pends mainly on the sputter power and the gas mixture. Stoichiometric (x = 1) as well as substoichiometric films (x < 1) have been deposited by varying the nitrogen-to- argon ratio in the sputter gas. The film thicknesses were in the range 2 0 - 5 0 n m for titanium and 100-150nm for TiN. Aluminium alloy films A l - l w t . % S i - 0.5wt.%Cu of 800 nm thickness were deposited on top of the Ti/TiN films using d.c. magnetron sputtering in a single-wafer sputter equipment. Thus the vacuum was always broken between the barrier and aluminium de- position. It was shown in refs. 13 and 14 that air exposure is a crucial step in achieving a high barrier quality. During aluminium deposition, the wafers were heated with quartz lamps in the sputter chamber to a temperature of around 180 °C as measured with a thermocouple glued onto test wafers.

Two types of investigation of the barrier quality have been performed in this work: firstly, we have evaluated spiking tests on wafers with contact holes; secondly, interdiffusion on planar films was measured via Auger electron spectroscopy (AES) depth profiling.

Spiking tests were performed on Si{100} wafers equipped with arrays of contact holes. The wafers had received an ion implantation of arsenic (1016cm-2; 50 keV) which was activated by an anneal at 900 °C for 60 min in N2. Then a borophosphosilicate glass (BPSG) film 1.3 lam thick was deposited. Contact holes of 0.9 gm diameter through the BPSG film were fabrica- ted using standard photoli thographic techniques and dry etching processes. Then the barrier layer and the

0040-6090/92/$5.00 ,~ 1992 --- Elsevier Sequoia. All rights reserved

H. Joswig, W. Pamler ] Stoichiometry in TiN diffusion barriers 229

aluminium metallization were deposited as described above. This process sequence ended with a 450 °C anneal in forming gas.

In order to provoke spiking, pieces of these wafers underwent anneals at a higher temperature (500- 570 °C). After these stress anneals the samples were inspected for spiking. For this purpose the films were removed via a sequence of wet-etch steps (phosphoric acid, RCA clean (H=O:H202:NH4OH 7:2:1) and buffered HF), and the etch pits due to spiking were observed by optical microscopy or scanning electron microscopy (SEM).

Planar specimens were selected for analysis of the TiN composition and the interfacial diffusion effects by AES in a Perkin-Elmer PHI-660 scanning Auger instrument. A 5 keV primary electron beam of about 1 I~A was used for excitation of the Auger spectra. Depth profiles were aquired by sputtering with 2 keV Xe ÷ ions mostly incident at 72 ° from the surface normal. These bombard- ment conditions were selected so as to achieve a high depth resolution [15]. For avoiding the severe sputter- induced roughening of aluminium during depth profiling [16] and the resulting degradation of depth resolution, the aluminium films were removed by wet chemical etching. Although some TIN,. Auger spectra were cali- brated in terms of atomic composition by utilizing a TiN standard characterized by Rutherford backscattering spectrometry [ 17], the depth profiles shown in this paper contain the raw intensities which are sufficiently indica- tive of the processes going on. It has to be noted that the overlapping Ti LMV and N KLL Auger lines near 385 eV have not been separated.

3. Results

Typical AES depth profiles of an as-deposited substo- ichiometric and stoichiometric Ti/TiN film are depicted in Figs. l(a) and l(b) respectively. A striking difference between both films is the depth homogeneity. Whereas the composition of the stoichiometric TiN film is uni- form (Fig. l(b)), the substoichiometric TiN film shows an oscillating nitrogen-to-titanium ratio as a function of depth (Fig. l(a)). Values of the nitrogen-to-titanium ratio range between 0.45 and 0.6 throughout the film. These oscillations have already been reported [11, 18] and are believed to result from the rotations o f the substrate table under the target. This particular arrange- ment leads to a non-continuous film deposition of various sublayers. Between the deposition of these sub- layers the substrates are moving through the sputter gas atmosphere, allowing the reactive surface of the freshly deposited film to react with the sputter gas. In the case of a non-saturated TiN film, the surface of the sublayer will absorb some additional nitrogen which leads to the

L U

F " B - - J

< Q .

n, e r "

U J

,<

(a)

;;",~",,,, ~ N+Ti Ti

L0 Ti " " V" ] I

10 20 30 40 50 60

SPUTTER TIME (rnin) 70

Q..

&

L0 <

(b)

f ~ Si

- i ! : ............

A t .......................... ..o.. .......................... J B J i -

,o ~o 3o 4 SPUTTER TIME (min)

Fig. I. Auger depth profiles of (a) substoichiometric and (b) stoichio- metric TIN,. films on titanium-coated silicon. The samples are as deposited without any further annealing processes. The data labelled N + Ti are due to the overlapping Ti LMV and N KLL Auger peaks. As this peak overlap has not been corrected for, the N + Ti intensity does not drop to zero in the titanium layer.

depth inhomogeneity of the film as shown in Fig. l(a). It should be noted that the 450 °C forming gas anneal has not caused enough diffusion to smooth out the vertical differences in film composition~

Substoichiometric as well as stoichiometric films were examined with respect to their diffusion barrier perfor- mance in spiking tests. We compared a series of Ti/ TiNx barriers of various compositions with respect to their barrier properties during a stress anneal at 550 °C for 20 min. Figure 2 shows optical micrographs of contact hole arrays after this anneal and after lift-off of the films down to the substrate. The location of the contacts can still be detected even after film removal because of the overetch used during opening of the contact holes. The dark features seen in the array of contacts are spikes. Some of them are depicted in an SEM picture in Fig. 3, exhibiting the typical pyramidal shape along the crystallographic planes. It is obvious from Fig. 2 that a clear correlation exists between the amount of nitrogen deficiency in the TiNx film and the frequency of spikes. The stoichiometric TiN barrier has withstood the additional temperature stress without spiking, whereas the nitrogen-deficient TiNx films show a large propensity towards barrier failure.

The behaviour of a substoichiometric TiN diffusion barrier during high temperature anneal is further eluci- dated by AES depth profiling. Figure 4(a) shows the

230 H. Joswig, W. Pamler / Stoichiometry in TiN diffusion barriers

i -- - - i t

N]Ti=0.35 N/Ti=0.73 N]Ti= 1.0

Fig. 2. Microscope picture of contact arrays after a spiking test (for details see text). Three samples differing in TiN stoichiometry of the diffusion barrier are shown. Note the strong propensity towards spiking with decreasing nitrogen content of the TiN film.

depth profile of a substoichiometric Ti/TiN diffusion 8 LU

barrier after a stress anneal at 550 °C for 20 min has o been performed. (The depth profile of this film before 2 6. ._J the stress anneal is given in Fig. l(a).) The sharp ~_ 7 interfaces of the Si/Ti/TiN structure which were present < 4, 13_ in the as-deposited films have disappeared after the high d. temperature anneal. Obviously, interfacial reactions ~ 2 have taken place. From the direction of the substrate, silicon has diffused far into the TiN, reacting with the 0 titanium film and also with the "free" titanium within (a) the substoichiometric TiN to form titanium silicides.

L . , ~ . . . . . . . . . . . . . . . s7 . . . . . . . .

10 20 30 40 50

S P U T T E R T I M E (min)

60

Fig. 3. SEM picture of spikes (enlargement of Fig. 2). All films have been removed by chemical etching; the silicon substrate surface is shown. Spikes show a typical growth along the crystallographic planes of the crystalline silicon.

<

4o

(b)

A

' 56 ' 6~) ' 70 ' 86 9o 100

E N E R G Y (eV)

Fig. 4. (a) Auger depth profile of the substoichiometric Ti/TiN,. film of Fig. l(a) after aluminium deposition and a stress anneal at 550 °C for 20 rain (the aluminium overlayer has been removed by chemical etching for achieving a better depth resolution). (b) Typical Si LVV Auger line shapes measured at depths labelled by arrows A, B and C in (a).

The shape of the Si LVV Auger line varies in a charac- teristic way throughout the broad T i N - S i interface region (Fig. 4(b)) and can be used as a fingerprint to characterize the phases formed [19, 20]. The shape of the near-substrate peak marked in Fig. 4(b) corre- sponds to that of TiSi2 while the spectrum closer to TiN at position B is most probably due to a more titanium- rich silicide such as TiSi or TisSi 3. The most important observation, however, is that a considerable amount of aluminium has diffused into the TiN from the top and is easily detected in the Auger depth profile (Fig. 4(a)). This suggests failure of the diffusion barrier as con- firmed by the high density of spikes observed for the substoichiometric TiN layer.

H. Joswig, W. Parnler / Stoichiometry in TiN diffusion barriers 231

It has to be concluded in accordance with other work [11, 21] that the stoichiometry of the TiN film is of crucial importance in achieving a high barrier quality. It is, however, quite difficult to control tightly the TiN composition down to a submicron scale on a structured wafer. In a previous paper Pamler and Kohlhase [22] analysed the composition of reactively sputtered TiN in contact holes and found that the composition in a contact hole is likely to differ from what is deposited on a planar surface. The deviations in composition within a contact hole might be an intrinsic effect of the deposi- tion method applied. For example, effects such as gas phase scattering, directionality of sputtered species, and dependence of sticking coefficients on the momentum and direction of the film constituents play a role in reactive sputtering and may lead to a composition non-uniformity on non-planar substrates.

Therefore the aim of formation of a completely ni- trided TiN film may require a post-treatment after barrier deposition [23]. We examined in this work whether this could be achieved by a high temperature (600-700 °C) anneal in a nitrogen atmosphere. A sub- stoichiometric barrier of the type shown in Fig. l(a) was deposited and post-annealed in nitrogen. An AES depth profile of such a post-treated layer is depicted in Fig. 5(a). Apparently nitrogen has diffused into the nitro- gen-deficient TiN, film, forming stoichiometric TiN at

LU t:3 ]....

. .d Q .

< 12.

n " I l l

<

(a)

,,-~:~, . . . . . . . . . .

0 . . . . . . I0 20 3O 40 ,~0

SPUI-fER TIME (min)

• ~ Si . i S. ~ • ~ , I .~ ,~ ,~

%/

"~'ss's'~ i i i /./t

0 i

eO 70

8

LLI " " . . . . . . - . . . . N+Ti

:~fl- ~\ Si . / 4 Ti \ t "~'

f ~ i s ./ D <I: AI O .s'

10 20 ~0 40 50 60

SPUTTER TIME (min)

Fig. 6. Auger depth profile of the post-annealed, substoichiometric Ti/TiN~ film of Fig. 5 after a stress anneal at 550 °C for 20 min.

the film surface. The nitrogen content of the film de- creases with increased distance from the surface. Oscil- lations in the nitrogen-to-titanium ratio--as detected in the as-deposited films--have been smoothed out by diffusion. A strong interaction between silicon and tita- nium is observed in the vicinity of the substrate, indi- cating silicide formation. The marked double-stepped profile indicates the presence of at least two silicide phases: TiSi2 at the plateau near the substrate, and titanium-rich silicides at the plateau near the TiN. This conclusion is supported by the marked changes in the Si LVV line shapes observed for both steps in the depth profile (Fig. 5(b)).

The performance of the barrier layers after this post- treatment was tested by deposition of aluminium and stressing the films at 550 °C for 20 min. AES depth profiles (Fig. 6) showed these films to be very stable. In fact, comparison with Fig. 5(a) reveals that stress an- nealing has had no influence on the depth profiles of the films. It is especially worth noting that no aluminium was detected within the TiN layer. This is a clear sign of an improvement of the barrier quality achieved by the post-treatment used.

4. Conclusions

...J < z (.9 CO r'r" U J O <

B

A

i i i i i i i i i i i i 40 50 60 70 80 90 100

(b) ENERGY (eV)

Fig 5. (a) Auger depth profile of the substoichiometric Ti/TiN x film of Fig. l(a) after annealing in nitrogen (600 °C for 30 min) and subsequent aluminium deposition (the aluminium has been removed by chemical etching). (b) Typical Si LVV Auger line shapes measured for each plateau (arrows A and B), and for the substrate (arrow C).

The influence of the stoichiometry of a Ti/TiN diffu- sion barrier on the barrier properties has been investi- gated. Spiking tests on submicron structures as well as investigations using AES depth profiling show that nitrogen-deficient TiN films perform very poorly as diffusion barriers between aluminium and silicon.

A post-treatment by annealing in nitrogen at elevated temperatures (600-700 °C) shows the potential for im- proving the barrier quality of a substoichiometric TiN film by forming a nitrogen-saturated TiN layer at the TiN surface. This is of interest for device manufactur- ing because stoichiometry control during reactive sput- tering is difficult as structures are shrinking into the submicron size.

232 H. Joswig, W. Pamler / Stoichiometrv in TiN diffusion barriers

Acknowledgments

W e t h a n k R. K a k o s c h k e , A. K o h l h a s e , P. Kf i che r

and M. M/ ind l fo r f ru i t fu l d i scuss ions and K. B o t h e

and M. S c h 6 n i n g e r for s u p p o r t wi th the e x p e r i m e n t a l

work . Th i s w o r k has been s u p p o r t e d by the F e d e r a l

D e p a r t m e n t o f R e s e a r c h and T e c h n o l o g y o f the F e d e r a l

R e p u b l i c o f G e r m a n y . T h e a u t h o r s a l o n e are respons i -

ble for the con ten t s .

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