As-doped polysilicon system

Thin Solid Fihns, 208 (1992) 168 171

168

Influence of TiAs precipitate formation on morphology degradation of the TiSi2/As-doped polysilicon system* Hyung-Ho Park Semiconductor Technology Division, Electronics and Telecommunications Research Institute, Daeduk Science Town, Dae/eon 305-606 (South Korea)

Jeong Yong Lee Department fff Materials Science, Korea Advanced Institute qf Science and Technology, Daeduk Science 7)re'n, Da~jeon 305-701 (South Korea)

Kyoung-Ik Cho, Mun-Cheol Paek and Oh-Joon Kwon Semiconductor Technology Division, Electronics and Telecommunications Research Institute, Daeduk Science Town, Daejon 305-606 (South Korea)

Chi-Kyu Choi Department qf Ph)'si~w, Cheju National ~#tiversity, Ctuju 690-756 (South Korea)

Kee-Soo Nam Semiconductor Technology Division, Electronics and Telecommunications Research hlstitute, Daeduk Science Town, Dae,ieon 305-606 (South Korea) (Received May 1, 1991: revised July 10, 1991; accepted July 30. 1991)

Abstract The formation of TiAs precipitates between TiSi 2 with C54 structure and arsenic-doped polysilicon and the influence of TiAs and silicon resulting from the reaction TiSi 2 + As ~ TiAs + 2Si on layer morphology degradation have been studied. The formation of TiAs precipitates has been revealed by X-ray diffraction in a sample annealed at 900 ':C for 60 min and a sequential increase in sheet resistance with increasing annealing time has been observed. Cross-sectional scanning electron microscopy of the sample annealed for 60 rain has shown irregular-shaped protrusions. Point analyses by Auger electron spectroscopy and cross-sectional transmission electron microscopy of the same sample have shown the presence of TiAs precipitates and extra silicon near the TiSi 2 polysilicon interface under the protrusion area. From the results it has been found that TiAs precipitates and extra silicon resulting from the reaction TiSi 2 + As ~ TiAs + 2Si lead to morphology degradation of this system.

1. Introduction

Refractory metal silicides have recently attracted much attention as metal-oxide semiconductor (MOS) gates and interconnects in silicon integrated circuits because of their low resistivities and high thermal stabilities [1-4]. Among the silicides studied, TiSi2 is a potential candidate because it has the lowest resistivity ( 1 0 - 2 0 ~Ocm at 20 °C) and a low formation temperature (about 500'~C) [5]. Usually, this silicide is formed by co-sputtering, co-evaporation or annealing of titanium metal on silicon [1-6]. The last technique is used for the Salicide (self-aligned silicide) process, but in application the silicide will always be in

*Presented at the 18th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, U.S.A., April 22 26, 1991.

0040-6090/92/$5.00

contact with doped silicon. Diffusion of dopants into the silicide and formation of m e t a l - d o p a n t compounds such as TiAs and TiB2 will change the silicon doping level [7]. This then can lead to a flat-band voltage shift in MOS devices or to an increase in contact resistance. Two types of studies for dopant-silicide interaction have been performed [8, 9]. In the first type, codeposition or co-evaporation of metal and silicon on heavily doped silicon followed by annealing is used to study the dopant redistribution during silicidation. In the second type, dopant implantation on formed silicide followed by annealing has been done to study the thermal stability of the silicide to dopants. These studies have revealed precipitate formation near the interface between TiSi2 and polysilicon. However, for the first type the possibility of reaction between the metal and dopants during silicidation should be considered

¢', 1992

Elsevier Sequoia. All rights reserved

169

H.-H. Park et al./ Morphology degradation of TiSi2/As-daped polysilicon

and for the second type damage on the silicide due to high energy ion implantation cannot be avoided. In this study TiSi2 film has been deposited and annealed under conditions necessary to form the C54-TiSi2 phase. After that, polysilicon has been deposited on the silicide and dopant ions have been implanted into the polysilicon. With this procedure the intrinsic thermal stability of C54-TiSi2 to arsenic after annealing has been studied.

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2. Experimental details

~SB.oO The experiments were carried out on (100) p-type silicon wafers with 4 0 - 6 0 fl cm resistivity, After a standard cleaning process the wafers were thermally oxidized to form SiO2 300 nm thick at 925 °C in an oxygen furnace. A TiSi2 film 100 nm thick was deposited in an argon atmosphere and annealed rapidly at 850 °C for 1 min in a nitrogen atmosphere to form C54-TiSi2 [10]. The formation of the C54 structure has been confirmed by X-ray diffraction (XRD) and sheet resistance measurements made using a four-point probe. Polysilicon 200 nm thick was deposited by low pressure chemical vapour deposition (LPCVD) at 625 °C and arsenic ions were implanted into the polysilicon film to a dose of 3 x 10 ~ c m -2, For a diffusion barrier for arsenic, S i O 2 300 nm thick was deposited by LPCVD [8]. Diffusion anneals were carried out at 900°C for 10, 30 and 60 min. The films have been characterized by means of XRD, scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). A typical sample structure is shown in Fig. 1.

3. Results and discussion

Figure 2 shows an X-ray diffraction pattern of the sample annealed at 900 °C for 60 min after removing the surface SiO2 film using dilute H F solution. The (102) and (103) diffraction peaks of TiAs resulting from the reaction between TiSi2 and arsenic are clearly

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observable. This agrees well with the result of Beyers et al. [8]. For the sample annealed for 10 min only diffraction peaks of C54-TiSi 2 and silicon were observed and it has been very difficult to identify the diffraction peaks of TiAs for the sample annealed for 30 min. Probst et al. [9] showed that the formation of TiAs precipitates could change the dopant profile and increase the contact resistance at the silicide silicon interface. In our experiments, sheet resistance measurements using a four-point probe have been carried out after removing the surface SiO2 in order to observe any change in the electrical properties of the TiSi 2 films. Sequentially increasing sheet resistances of 2.4_+0.3, 3.7 _+0.5 and 4,7 + 0.3 f~/~ with increasing annealing time were measured respectively. From this sequential increase the formation of TiAs precipitates could be expected with the sample annealed for 30 min. There is good agreement between the X-ray diffraction and sheet resistance measurements for the sample annealed for 60 min. Figure 3(a) shows a cross-sectional SEM image of the sample annealed for 60 min. Intermediate SiO2 and TiSi2 layers appear dark because these layers are etched by dilute HF solution. Irregular-shaped protrusions result from morphology degradation of the polysilicon

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Fig. 3. Cross-sectionalSEM images of sample annealed at 900 'C for 60 min: (a) protrusion area; (b) area free from protrusions.

H.-tt. Park el al./ Morpholog), degradation o1" TiSie/As-doped polysilicon

170

layer. For the sample annealed for 10 min only the flat silicon layer was observed, while ball-shaped protrusions were observed for the sample annealed for 30 rain. Two explanations could be possible for these protrusions. One is a solid phase epitaxy (SPE) reaction of silicon through the silicide layer as a result of the high temperature annealing. This SPE phenomenon is a wellknown process in silicide/silicon systems. Especially for the TiSi2/Si system, surface roughening and thickness variation of the silicon layer after annealing above 850 ~'C have been reported [11]. Another possibility is the local distribution of TiAs precipitates and extra silicon from the reaction TiSi2 + As --+ TiAs + 2Si. Figure 3(b) shows a cross-sectional SEM photograph of a region where protrusions are not present after annealing for 60 min. From this it can be concluded that the morphology degradation shown in Fig. 3(a) does not result from an SPE reaction. Because the SPE reaction is not a local phenomenon, if it does occur, the thickness variation should be observed over the whole silicon layer instead only locally. Indeed, with an annealed, undoped sample the morphology degradation shown in Fig. 3(a) was not observed. In order to investigate the relationship between this morphology degradation and the formation of TiAs and extra silicon, point analyses by AES and cross-sectional TEM have been carried out. AES depth analyses of the area with protrusions and the fiat area showed that the ASLM M peak appears only for the former after etching about 240 nm depth from the polysilicon layer ( Fig. 4). Figures 5(a) and 5(b) show a cross-sectional TEM image of the sample annealed for 60 min and its schematic diagram respectively. The region labelled A represents an SiO2 layer and the region labelled B I corresponds to a deposited polysilicon layer. The layer identified as B2 can be distinguished from B1 since the thickness of B1 is uniformly 200 nm. The TiSi2 layer

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(b) Fig. 5. (a) Cross-sectional TEM image and (b) its schematic diagram of sample annealed at 900 C for 60 min.

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Fig. 6. High resolution TEM lattice images of sample annealed at 900 C for 60 rain: (a) TiAs: (b) silicon.

designated as C is discontinuous where the B2 layer is formed and this discontinuous region is marked with dots. High resolution TEM has been used to identify these B2 and dotted areas which appear only after annealing. Figure 6(a) shows a high resolution TEM micrograph for this dotted area. Using a spacing of Si{lll} planes of 0.313 nm as an internal magnification standard, a vertical interplanar distance of 0.218 nm and an inclined interplanar distance of 0.182rim have been measured. These values correspond to the (104) and (110) interplanar distances of TiAs (dl04 = 0.2183 rim, and dj~0 = 0.1823 nm). The angle between the two lattice planes has been measured to be 53 ~ in the image, which agrees very well with the interplanar angle o f ( 1 0 4 ) and ( l l 0 ) of TiAs (53.24). Thus it is concluded that the dotted area of Fig. 5 consists mainly of TiAs.

H.-H. Park et al./ Morphology degradation of TiSi~/As-doped polysilicon

171

d o p e d polysilicon by high t e m p e r a t u r e a n n e a l i n g a n d t h a t the layer m o r p h o l o g y is d e g r a d e d by d i s t r i b u t i o n o f the e x t r a silicon p r o d u c e d over the T i A s precipitates. ~,~

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Fig. 7. (a) Diffraction pattern and (b) its indexed pattern of sample annealed at 900 C for 60 rain.

F i g u r e 6(b) shows a high r e s o l u t i o n T E M mic r o g r a p h for region B2. T h e lattice p l a n e i m a g e with a 0.313 n m spacing is well developed, This 0.313 n m int e r p l a n a r d i s t a n c e c o r r e s p o n d s to the S i ( l 11) i n t e r p l a n a r spacing ( d ~ : 0.31355 nm). T h u s it is evident that the B2 layer is silicon. F i g u r e s 7(a) a n d 7(b) are the selected a r e a diffraction p a t t e r n for the B2 a n d d o t t e d areas a n d the c o r r e s p o n d ing indexed diffraction p a t t e r n respectively. The ring p a t t e r n o b s e r v e d on the inner p a r t c o r r e s p o n d s t o { 111 } o f polysilicon a n d the closed circles can be indexed as (1120), ( 1 0 i 4 ) a n d (2134) o f TiAs. T h e zone axis o f these closed diffraction spots is [4401]. The i n t e r p l a n a r angles between (1120) a n d (2134) a n d between (2134) a n d (1014) are m e a s u r e d as 25 "~) a n d 29 ° respectively. These values are in g o o d a g r e e m e n t with the c a l c u l a t e d values for the T i A s unit cell (24.04 °, 29.19°). T h e o p e n circles in the p a t t e r n c o r r e s p o n d to diffraction spots o f (1013) a n d (20,~6) for a n o t h e r crystal o f T i A s differently oriented. T h e a b o v e results show t h a t T i A s p r e c i p i t a t e s are f o r m e d in the i n t e r l a y e r between TiSi2 a n d arsenic-

T h e f o r m a t i o n o f T i A s precipitates between C54TiSi2 a n d a r s e n i c - d o p e d (3 × 1016 cm -2) polysilicon by a n n e a l i n g at 900 ~C for 60 min has been confirmed by X R D , S E M , A E S a n d T E M . T h r o u g h high r e s o l u t i o n T E M it has been f o u n d that T i A s precipitates f o r m e d at the T i S i 2 - p o l y s i l i c o n interface a n d extra silicon dist r i b u t e d over the T i A s induce layer m o r p h o l o g y d e g r a dation,

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