TiSi2 process for a quarter-micron complementary metal-oxide-semiconductor

ELSEVIER

Thin Solid Films 253 (1994) 395-401

TiN-capped TiSi2 formation in W/TiSi2 process for a quarter-micron complementary metal-oxide- semiconductor Y. Matsubura, M. Sekine, N. Kodama, K. Noguchi, K. Okumura ULSI Device Development Laboratories, NEC Corporation, 1120 Shimokuzawa, Sagamihara 229, Japan

Abstract

We have investigated the diffusion of nitrogen and oxygen into TiSi2 through a thermally formed TiN capped layer. We have observed that nitrogen diffuses through the TiN capped layer into the TiSi2 layer at temperatures above 200 °C, but oxygen diffuses into TiSi2 through the TiN capped layer at temperatures above 400 °C. Both oxygen and nitrogen in the TiSi2 film increase the C49-to-C54 phase transition temperature, and also degrade the morphology of selectively deposited tungsten films.

Keywords: Deposition process; Phase transitions; Silicides; Titanium

1. Introduction For very-large-scale integrated-circuit manufacturing, it is highly desirable to use refractory metal silicides such as TiSi2, CoSi2 and MoSi2 in order to reduce parasitic resistance in the circuits. Of those refractory silicides, TiSi2 has the lowest sheet resistance and therefore is the most attractive candidate for the silicide process to fabricate half-micron devices. However, there are several parameters such as diffusion area width, titanium sputter thickness and impurity concentration that can affect the TiSi 2 formation in terms of the silicide growth rate and the phase transition temperature. These factors narrow the process window and decrease the device yield. To achieve low sheet resistance for a quarter-micron complementary metal-oxide-semiconductor, we proposed a new silicide structure which consists of a tungsten layer selectively deposited on TiSi2 [ I]. The tungsten layer provides sufficiently low sheet resistance, making the sensitive phase transition process from TiSi 2 C49 to TiSi2 C54 unnecessary. The major role of TiSi 2 is as a barrier layer (against the diffusion of W and Si) rather than as a low resistance layer. This new silicide structure was accomplished by developing selective chemical vapour deposition of tungsten [2-4]. In the fabrication of W-covered TiSi2, the morphology of the tungsten layer must be improved in order to reduce the variation in sheet resistance. It is well known that TiSi 2 reacts with oxygen and nitrogen [5-8], and 0040-6090/94/$7.00 ~ 1994 - - Elsevier Science S.A. All rights reserved SSDI 0 0 4 0 - 6 0 9 0 ( 9 4 ) 0 4 6 3 9 - N

that oxygen and nitrogen can inhibit the formation of titanium silicide [9-11]. Furthermore, both oxygen and nitrogen degrade the morphology of selectively deposited tungsten films [3, 4]. In this paper, we examined the effect of a TiN cap that can prevent the diffusion of nitrogen and oxygen into TiSi2 [12, 13]. Although TiN is known to react with oxygen at lower temperature than TiSi 2 does [14], studies of ambient gas diffusion through the TiN barrier layer into TiSi2 have not been reported.

2. Experiments Si(100) p-type wafers with a resistivity of 13 f~ cm were used. Just prior to titanium deposition, the wafers were cleaned by dipping in a 1:100 dilute H F solution. Titanium film was then sputtered to a thickness of 10 nm. To form the TiSi> a two step rapid thermal annealing (RTA) process [9] was employed in a Dainippon Screen LA-W815-A. The temperature of the wafer was measured with a thermal couple attached to the back side of the wafer. The ramp rate was + 100 °C s -~. After annealing, the wafer was kept in the annealing chamber until the wafer temperature reached a pre-set value (cooling temperature, 200 or 400 °C). The first rapid thermal annealing (RTA1) was performed at 650 °C for 30 s in a nitrogen or argon ambient. Next, the TiN layer either was removed with a 15 min selec-

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tive etch (NH3OH:H202:H20 = 1:1:5) in the case of the conventional process or was not removed in the case of the TiN-capped process. Then, the second rapid thermal annealing (RTA2) was performed at different temperatures ranging from 700 to 900 °C in a nitrogen or argon ambient. Following a wet pre-treatment to remove the TiN layer and oxide layer, the tungsten film was deposited on the TiSi2 layer using conventional silane (Sill4) reduction of tungsten hexafluoride ( W F 6 ) [2-4]. The deposition temperature was set at 400 °C. After tungsten deposition, the tungsten morphology was observed by scanning electron microscopy (SEM). Before tungsten deposition, the depth profile of TiSi2 was analysed by secondary-ion mass spectrometry (SIMS) using a Cameca IMS-5F and Atomika ADIDA. The sample was sputtered by Cs + primary beam at an impact energy of 2.5 keV. The depth axis of the SIMS profile was calibrated from the film thickness measured by cross-sectional transmission electron microscopy (TEM). The diffusion depth of the impurity was determined to be the depth when the count number r e a c h e s 10 z counts s ~. The percentage of C49 phase present in the partially reacted sample was determined by its resistance. The TiSi2 sheet resistance was measured with a four-point probe on an unpatterned wafer. The fraction z R of C49 phase is defined as follows: Rm - - R °

(1)

-R-Rmax__R0

where R m is the measured resistance, R0 is the sheet resistance corresponding to the C54 TiSi2 resistivity of 18 la~ cm after RTA2 at 870 °C for 10 s and Rma x is the sheet resistance corresponding to the C49 TiSiz resistivity of 80 la~ cm before RTA2. We determined the C49to-C54 phase transition temperature as the temperature at which zR = 0.5.

3. Results and discussion

3. I. TiN-capped TiSiz formation process 3.1. I. Effect of the rapid thermal annealing ambient The effect of the RTA ambient was investigated. The cooling temperature of 200 °C was chosen to prevent oxidation of TiSi2. Fig. 1 shows scanning electron micrographs of tungsten deposited on TiSi2 at different conditions. Fig. l(a) shows a smooth surface morphology when both RTA1 and RTA2 were performed in an argon atmosphere. Fig. l(b) shows that the tungsten morphology deteriorated when RTA1 was performed in nitrogen and RTA2 was performed in argon. Fig. l(c) shows the morphology when both RTA1 and RTA2 were performed in nitrogen. The grain size of the

Fig. 1. SEM micrographs after tungsten deposition on TiSi2: (a) both RTAs in argon, (b) RTA1 in nitrogen and RTA2 in argon, (c) both RTAs in nitrogen. RTAI was at 650 ' C for 30 s and RTA2 was at 850 ' C for 10 s.

tungsten was larger than the case of RTA2 in argon which was shown in Fig. l(b). Fig. 2 shows cross-sectional scanning electron micrographs of the same sample as in Fig. 1. When both RTA1 and RTA2 were in argon (Fig. 2(a)), the W/ TiSi2 bilayer structure was not clearly identified because tungsten diffused into Si substrate through TiSiz. However, when RTA1 was performed in nitrogen, the TiSi 2 layer remained (Figs. 2(b) and 2(c)). When the RTA2 ambient was changed from argon (Fig. 2(b)) to nitrogen (Fig. 2(c)), the morphology of tungsten also degraded. This implies that nitrogen in TiSi2 inhibited the reaction between WF 6 and TiSi2 and degraded the surface morphology. The reaction between WF 6 and TiSi 2 results in the formation of two solid compounds: W and TiF3 [3]. The formation of TiF 3 was suppressed when RTAI was performed in nitrogen because of either the nitrogen dissolved in the silicide, or the titanium nitride in silicide [5]. Fig. 3 shows SIMS depth profiles of TiSi2 after RTA2. When both RTA1 and RTA2 were in argon

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Y. Matsubara et al. / Thin Solid Films, 253 (1994) 395-401

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Fig. 2. Cross-sectional SEM views after tungsten deposition on TiSi2: (a) both RTAs in argon, (b) RTA1 in nitrogen and RTA2 in argon, (c) both RTAs in nitrogen. RTAI was at 650 oC for 30 s and RTA2 was at 850 °C for 10 s.

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(C)

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~

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(Fig. 3(a)), an oxidized layer (TiOx) was observed on the TiSi 2 surface. Examination by cross-sectional T E M revealed that an oxidized layer o f a b o u t 4 n m was formed on TiSi2 layer o f a b o u t 20 nm. Other workers have also reported a similar observation [15]. A possible reason for the formation o f TiOx is f r o m the absorption o f oxygen in the annealing ambient during the R T A process. W h e n R T A 1 is performed in nitrogen, a T i N layer is formed on TiSi2. It is k n o w n that T i N layer is mainly the p r o d u c t o f a reaction o f Ti with nitrogen in the ambient, rather than with nitrogen in the starting films [15]. A cross-sectional transmission electron mic r o g r a p h revealed a 7 n m layer o f T i N on TiSi2 film o f about 17nm. The S I M S depth profile in Fig. 3(b) shows that a high count o f oxygen was observed in two regions: the surface o f the T i N layer and the transition region between T i N and TiSi 2. In the transition region between T i N and TiSi2, oxygen atoms were snowplowed towards the T i N layer during the silicide reac-

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Fig. 3. SIMS depth profiles of TiSi2 after RTA2: (a) both RTAs in argon, (b) RTAI in nitrogen and RTA2 in argon, (c) both RTAs in nitrogen. RTA1 was at 650 °C for 30 s and RTA2 was at 850 °C for 10s. tion [15, 16]. On the contrary, the count o f nitrogen decreased continuously in TiN. This nitrogen diffused into TiSi2 during both silicidation annealing and the cooling period. A snowplow effect o f nitrogen was not observed. Fig. 3(c) shows that, when both R T A 1 and R T A 2 were performed in nitrogen, the depth o f nitrogen in

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Y. Matsubara et al. / Thin Solid Films, 253 (1994) 395 401

TiSi2, defined at 102counts s -~, shifts about 2 nm toward the substrate compared with the results in Fig. 3(b). This indicates that, during the RTA2 in nitrogen, the TiN layer cannot act as a barrier to the diffusion of nitrogen into TiSi2.

3.1.2. Effect of cooling temperature The effect of cooling temperature on the tungsten morphology was investigated by SEM. Tungstendeposited TiSi 2 films that were annealed in nitrogen with different RTAI and RTA2 temperatures are shown in Fig. 4. The cooling temperature of both RTA1 and RTA2 was 200 or 400 °C. The morphology of tungsten films deposited after RTA1 was independent of the cooling temperature, which is 200 °C (Fig. 4(a)) or 400 °C (Fig. 4(b)). However, when tungsten was deposited after RTA2, the morphology shown in Fig. 4(c) at 200 °C was slightly better than the morphology shown in Fig. 4(d) at 400 °C. Fig. 5 shows SIMS depth profiles of TiSi2 after RTA2. When both RTA1 and RTA2 cooling temperatures are 200 °C (Fig. 5(a)), the nitrogen concentration in TiN layer is higher than in the case of the 400 °C cooling temperature (Fig. 5(b)). The diffusion depth of oxygen in TiSi2 was shallower than that of nitrogen. This nitrogen diffusion when both RTA1 and RTA2 were in nitrogen occurred during the RTA2 process and the cooling process. On the contrary, the oxygen concentration in the TiN layer for a cooling temperature of 400 °C is higher than for a 200 °C cooling temperature. Furthermore, the diffusion depth of oxygen in TiSi2 was deeper than that of nitrogen. The oxygen profile was flat in TiN, implying that oxygen in TiN saturated at about 30% [16]. In conclusion, the degraded morphology of tungsten films deposited after RTA2 is caused by the diffused oxygen and nitrogen into TiSi 2 to a depth of 10 nm.

3.1.3. C49-to-C54 phase transition temperature In order to evaluate the influence of the annealing ambient on the amount of silicide formed, the C49-toC54 phase transition temperature of TiSi 2 was measured. Fig. 6 shows the fraction undergoing the transition from C49 to C54 TiSi2 as a function of RTA2 temperature. The phase transition temperature depends on both the annealing and the cooling temperature. When both RTA1 and RTA2 were performed in nitrogen and the cooling temperature was 400 ~'C, the transition temperature increased by about 50 °C as the Ti thickness is reduced from 50 to 10 nm. However, when the cooling temperature was reduced from 400 to 200 °C, the phase transition temperature decreased by about 10 °C. When the RTA2 ambient was changed from nitrogen to argon, the transition temperature decreased by another 10 °C. When the RTA1 ambient was changed from nitrogen to argon, the phase transition temperature further decreased by about 10 °C.

Fig. 4. SEM micrographs of tungsten deposited on TiSi2 after RTA1 with RTA1 cooling temperature of (a) 200 ~'C, and (b) 400 '~C:after RTA2 with both RTAI and RTA2 coolingtemperature of (c) 200C, and (d) 400 'C. The SIMS depth profile has a good correlation with the shift in transition temperature. When the cooling temperature was 400 °C (Fig. 5(b)), the high concentration of oxygen in TiSi2 increased the transition temperature. On the other hand, when RTA1 and/or RTA2 were performed in nitrogen at above 200 °C (Fig. 3), the nitrogen in TiSi2 increased the transition temperature. Thus the annealing ambient and annealing temperatures have a considerable influence on the phase transition temperature of TiSi2. Also TiSi2 formation in nitrogen ambient has an advantage for self-aligned TiSi2 because it can reduce TiSi2 overgrowth [15], but it

Y. Matsubara et al. / Thin Solid Films, 253 (1994) 395-401

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created a region o f high nitrogen concentration in TiSi 2 near the interface. The second process step is the unloading step after R T A 1 . Since the wafer temperature was still above 400 °C, oxygen in the air can diffuse t h r o u g h T i N into the TiSi 2 layer. The third process step is the R T A 2 in nitrogen ambient. Nitrogen will diffuse directly into TiSi2. These three process steps will increase the oxygen and nitrogen concentrations in the TiSi2 layer and subsequently increase the phase transition temperature. 3.2. N o n - c a p p e d TiSi 2 f o r m a t i o n process

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In the n o n - c a p p e d TiSi 2 formation process, the T i N layer thermally formed on TiSi2 by R T A 1 was removed by wet etching. The cooling temperature o f both R T A 1 and R T A 2 was 400 °C. D u r i n g R T A 2 , TiSi2 was processed in a nitrogen control ambient. W h e n the wafer was unloaded in air, the TiSi2 surface was oxidized. Fig. 7 shows scanning electron micrographs o f tungsten deposited on TiSi 2. The R T A 1 ambient was nitrogen, and the R T A 2 ambient was argon (Fig. 7(a)) or nitrogen (Fig. 7(b)). In either case, the tungsten film was not uniformly formed on TiSi 2. Fig. 8 shows depth profiles o f TiSi2 formed with both R T A I and R T A 2 in nitrogen. The nitrogen concentration in TiSi2 after R T A 2 (Fig. 8(b)) is higher than that after R T A 1 (Fig. 8(a)), while the oxygen concentration was approximately the same. Fig. 9 shows the fraction o f transition as a function o f R T A 2 temperature. The phase transition

Fig. 5. SIMS depth profiles after RTA2 with cooling temperature of (a) 200 °C, and (b) 400 °C, RTA1 was at 650 °C for 30 s in nitrogen and RTA2 was at 850 °C for 10 s in nitrogen. will let extra oxygen and nitrogen diffuse t h r o u g h the T i N into TiSi2 layer. There are three process steps in which the wafer temperature was over 200 °C and the ambient was either nitrogen or the air. The first process step is the RTA1 in the nitrogen ambient. This process

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Fig. 6. Fraction of transition as a function of RTA2 temperature in TiSi2 without wet etching before RTA2. RTA1 was at 650 °C for 30 s in nitrogen or in argon and RTA2 time was 10 s in nitrogen or in argon.

Fig, 7. SEM micrographs after tungsten deposition for non-capped TiSi2 formation process: (a) RTA1 in nitrogen and RTA2 in argon, (b) both RTAs in nitrogen. RTA1 was at 650 °C for 30 s in nitrogen and RTA2 was for 10 s. Cooling temperature of both RTA1 and RTA2 was 400 °C.

Y. Matsubara et al. / Thin Solid Films, 253 (1994) 395 401

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temperature in the case of RTA2 in nitrogen was about 20 °C higher than the case of RTA2 in argon. Direct diffusion of nitrogen into TiSi2 during the RTA2 process increased the transition temperature. Therefore, in the non-capped TiSi 2 formation process, direct diffusion of oxygen and nitrogen from the

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We have investigated the diffusion of nitrogen and oxygen into TiSi2 through the thermally formed TiN layer. It was found that annealing TiN-capped TiSi2 in nitrogen at temperatures above 200 °C results in the diffusion of nitrogen into TiSi2 through TiN. Oxygen diffusion into TiSi2 through TiN was also observed at temperatures above 400 °C. Both oxygen and nitrogen in TiSi~ have a considerable influence on the phase transition temperature. Furthermore, oxygen and nitrogen in TiSi2 degraded the morphology of tungsten films selectively deposited on TiSi2. It is suggested that the control of nitrogen and oxygen diffusion into silicide is necessary to improve the morphology of tungsten layer and to form a very fine silicide with thickness of under 30 nm.

900

Fig. 9. Fraction of transition as a function o f RTA2 temperature for non-capped TiSi~ formation process. RTA1 was at 650 °C for 30 s in nitrogen and RTA2 was for 10 s in nitrogen or in argon. Cooling temperature of both RTAI and RTA2 was 400 C .

The authors would like to thank Professor S. S. Lau (University of California at San Diego) for fruitful advice and discussions. All RTA processes were carried out with the strong support of K. Kanatsu, H. Kiyama, T. Chiba and A. Yasue (Dainippon Screen Co. Ltd.). The authors also wish to thank M. Yamanaka, T. Kitano, S. Saito and K. Ikeda for helping with the SIMS analysis, K. Fujii and K. Kikuta for depositing the titanium films, and N. Nishio, D. T. C. Huo, M. Nakamae, M. Ogawa and M. Kamoshida for continuous encouragement.

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[13] A. C. Berti and V. Bolkhovsky, Proc. V-MIC Conf., Santa Clara, CA, June 9 10, 1992, University of South Florida, FL, 1992, p. 267. [14] T. Yamazaki, K. Goto, T. Fukano, Y. Nara, T. Sugii and T. Ito, IEDM Tech. Dig., Washington, DC, December 5-8, 1993, IEEE, New Jersey, 1993, 906. [15] T. Brat, C. M. Osburn, T. Finstad, J. Liu and B. Ellington, J. Electrochem. Soc., 133 (1986) 1451. [16] G. G. Bentini, M. Servidori, C. Cohen, R. Nipoti and A. V. Drigo, J. Appl. Phys., 53 (1982) 1525.