JOURNAL OF MATERIALS SCIENCE 28 (1993) 6279-6284 A study of the morphology and microstructure of LPCVD polysilicon EUN GU LEE Department of Materials Engineering, Chosun University, 375, Seosuk-Dong,Dong-Gu, Kwangju 501 - 759, Korea SA KYUN RHA Semiconductor Process Division (8th Laboratory), Goldstar Electron Co. Ltd, 16, Woomyeon-Dong, Seocho-Gu, Seoul 137-140, Korea The morphology and microstructure of polysilicon films deposited by low-pressure chemical vapour deposition (LPCVD) have been investigated as a function of deposition conditions. The deposition temperature was varied from 540-0 ~ As-deposited polysilicon films had a rough surface with (1 1 0) textured columnar grain structure, while as-deposited amorphous films had a smooth surface. The polysilicon film deposited at the amorphous to polycrystalline transition temperature had an extra-rough, rugged surface with (31 1 ) texture. At the trans- ition temperature, the grain structure tended to shift from the polycrystalline to the amorphous state with increasing deposition pressure and film thickness. It was found that nucleation of amorphous film during in situ annealing at the transition temperature without breaking the vacuum began to occur from surface silicon atom migration, in contrast to a heterogeneous nucleation during film deposition. 1. Introduction line transition temperature without breaking the Polycrystalline silicon (polysilicon) films formed by vacuum has been also discussed. low-pressure chemical vapour deposition (LPCVD) of silane (Sill4) are widely used in integrated circuits for various applications as MOS gates, interconnects, resistors, and emitter contacts. Other applications 2. Experimental procedure include photovoltaic conversion, thermal and mech- The experiments were carried out in an induction- anical sensors, and thin film transistors (TFT) for heated hot-wall horizontal reactor. Undiluted mono- large-area liquid-crystal displays (LCDs). The electri- silane (Sill4) gas as a silicon source was supplied from cal performance of the polysilicon is strongly deter- both sides of the tube and was evacuated using a mined by its microstructure, which depends on rotary pump. The deposition parameter, temperature deposition parameters [1-4]. Electrical properties for and pressure, were varied within the constraints of as-deposited polycrystalline or as-deposited amorph- equipment available and film quality. The starting ous silicon have been investigated by a number of wafers were CZ(1 0 0) p-type silicon, with 100 nm thick authors [5,7]. It was suggested that deposition tem- silicon dioxide (SiO2). The film thickness was meas- perature should be as low as possible to obtain high ured using an e!lipsometer, and was approximately conductivity and carrier mobility. 100 nm unless otherwise specified. All films were un- It is well known that the surface roughness of doped and were visually inspected with an oblique polysilicon degrades the electrical characteristics of light and a qualitative assessment of film quality based dielectric film on the polysilicon [6, 8, 9]. However, in on the hazy spot. order to obtain the sufficient storage capacitance re- The preferred orientation (texture) of the films was quired for M bit dynamic random access memory investigated using an X-ray diffractometer (XRD) with (DRAM) and beyond, some fabrication technologies a glancing incident angle to reduce the penetration having an uneven surface of hemispherically grained depth of X-rays and hence the diffraction from the (HSG) polysilicon film have been suggested for suhstrate silicon. The texture of the films was meas- increasing effective surface area [10, 11]. ured by comparing the intensity of diffraction peaks In the present work, we investigated the deposition with those obtained on randomly oriented poly- condition dependence of surface morphology and crystalline film. In order to quantify the texture of the microstructure of LPCVD polysilicon. The grain films, the relative unit (r.uh k Z) for each diffraction plane growth mechanism during the deposition and sub- (h k l) is normalized to the (! 1 1) plane as follows. sequent in situ annealing at the amorphous to crystal- U.rhkl = Ihkz/Il l l (1) 0022-2461 9 1993 Chapman & Hall 6279 where lhk t and 11 1 1 are intensities of(hkl) and (1 1 1) planes, respectively. Film morphology and micro- structure were investigated using plane and cross- sectional scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 10 2 3. Results and discussion r- E i= 101 C 0 E a = 34 kcal mo1-1 8 a 10 ~ .0 , I , I ,\" 1.1 1.2 Deposition temperature (10 3 K 1) .3 pressure. Figure 1 Arrhenius plot of the deposition rate for 0.25 torr silane Fig. 1 shows the Arrhenius plots of growth rates in the temperature range 540-0~ under 0.25 torr. The silane growth is controlled by surface reaction and has an apparent activation energy of 34 kcal mol - 1, which agrees well with 32-39.9kcalmo1-1 reported by Harbeke et al. I-4]. The surface morphology of the films at various deposition temperatures is shown in Fig. 2. It was observed that the film deposited at 560 ~ has a smooth surface and the film deposited at 570~ has some nuclei, which start to grow in the amorphous phase. The film deposited at 580~ has hemispherical grains (HSG), while at 590-600 ~ it has an extra-rough, rugged surface with a greater surface profile variation, and at 620 ~ has a rough surface. The surface morphologies of the specimens annealed at 1000 ~ for 4 h were similar to those of as- deposited specimens. These facts imply that the sur- face morphology of the film is strongly dependent on the deposition temperature, but is almost independent of annealing conditions. Crystal structures of the films were analysed using XRD. Three X-ray diffraction peaks showing (1 1 1), (1 1 0),, and (3 1 1) reflections were detected for the samples deposited at and above 570 ~ Fig. 3 shows the ratios of(1 1 0) and (3 1 1) intensity to (1 1 1) intens- ity as a function of deposition temperature. For a Figure 2 Scanning electron micrographs of films deposited at (a) 560 oc, (b) 570 ~ (c) 580 ~ (d) 590 ~ (e) 600 ~ and (f) 620 ~ 6280 Figure 2 Contd. 5 9\" 1 0 D 540 560 580 600 620 Deposition temperature (~ C) 0 Figure 3 Relative unit of(Q) (1 1 0) and ([21) (3 1 1) intensity to (! 1 l) intensity as a function of deposition temperature. randomly oriented polysilicon film, the values of 11 i o/It 1 1 and 13 1 1/11 1 a are 0.6 and 0.35, respect- ively. The film deposited below 560 ~ is amorphous, because no XRD peaks were detected. It was observed that the (3 1 1) component dominates for the films deposited in the temperature range 580-600~ and the (1 1 0) component is a major peak for the films deposited above 620~ The dramatic increase in (1 10) texture above 620 ~ indicates that the prefer- ence for (1 1 0) texture is a growth phenomenon rather than nucleation behaviour. Comparing XRD data with film morphology, the undoped film deposited at the transition temperature has a rugged surface with small (3 1 1) texture. Bisaro et al. [12] also observed (3 1 1) texture for the film deposited at the transition temperature which agrees with ours, except for the surface smoothness of the films. They observed the surface of partially crystalline film to be rather fiat compared with that of totally crystalline film. In order to investigate the surface morphology and grain structure, transmission electron micrographs were taken. Fig. 4 shows plane transmission electron micrographs of the films obtained at various depos- ition temperatures. Large grains and microcrystallites were observed at 570 ~ The HSG polysilicon is a single grain with (1 1 1) twin boundaries, as shown in the selected-area diffraction pattern (SAD). It was found that with increasing deposition temperature, the grain diameter decreases and grain density increases. From the cross-sectional transmission electron micro- graph shown in Fig. 5, it was observed that small grains start to grow from the oxide substrate and a large grain protrudes from the amorphous silicon surface at 570 ~ With increasing deposition temper- ature, many grain boundaries impede lateral grain growth, resulting in an HSG growth at 580 ~ and further increasing the temperature results in cylindri- cal and/or columnar grain growth above 590 ~ In order to examine an initial grain-growth phe- nomenon, the film thickness was varied. Fig. 6 shows scanning electron micrographs of 50, 100, and 200 nm thick films deposited at 580 ~ It was observed that in the 200 nm thick film, cylindrical grains observed in the 50 nm thick film disappear and an amorphous film with hemispherical grains appears. With increasing film thickness, the surface morphology changes from cylindrical to hemispherical shape and the amorphous phase increases. These results suggest that the cylin- drical grain is the nucleation step which appears in an early stage of film growth. The surface morphology was also affected by depos- ition pressure. Fig. 7 shows surface morphology of the films with various silane pressures at 590 ~ It was observed that with decreasing silane pressure, the shape of the grain changes from hemispherical to cylindrical. It can be said that lowering the process pressure lowers the transition temperature. It is well known that the surface silicon migration rate is en- hanced when the pressure is reduced. These results suggest that the surface morphology is related to the deposition rate and silicon atom migration rate at the deposition temperature. It has been reported 1-13, 14] 6281 Figure 4 Plane transmission electron micrographs of films deposited at (a) 570 ~ (b) 580 ~ and (c) 590 ~ (d) The selected-area diffraction (SAD) pattern of (a). that apparent activation energy of crystal growth rate the surface sinks in around the grain. Microcrystallites for undoped film is about 67 kcalmol-1 and larger already formed during deposition are also observed than that of the deposition rate. At a fixed temper- on the substrate. The density of the hemispherical ature, a film will be partially crystallized if the crystal grains on the surface increases with increasing in situ growth rate is comparable to the deposition rate. If the annealing temperature and time. This result is in crystal growth rate far exceeds the deposition rate, the agreement with the results of Watanabe et al. [11] and microstructure will be cylindrical and/or columnar Sakai et al. [15] obtained by using the ultra-high grain structure. vacuum annealing of the amorphous film after remov- Although the surface morphology of as-deposited ing a native oxide. They observed that the hemispher- film is almost unchanged with subsequent annealing ical grains are formed on the amorphous silicon sur- above 1000 ~ after exposuring in air, it is possible to face and protrude from the original amorphous silicon change the morphology by in situ annealing without plane. This result suggests that the nucleation process breaking the vacuum. Fig. 8 shows that cross-sectional during annealing is different from that of deposition; transmission electron micrographs of the film depos- grain growth starts to occur from surface silicon atom ited at 560~ and annealed at 570~ for 10 min migration during in situ annealing, while grain growth without breaking the vacuum. In contrast to grain starts from a nucleation on the substrate during de- growth from the substrate in Fig. 5, a large grain is position. It is believed that native oxide on the films observed on the surface in the amorphous phase and exposed in air inhibits surface silicon atom migration. 6282 4. Conclusions Microstructure of polysilicon films has been investig- ated as a function of deposition temperature, pressure, and film thickness and the results obtained are sum- marized as follows. Figure 5 Cross-sectional transmission electron micrograph of films deposited at (a) 570 ~ (b) 580 ~ and (c) 590 ~ Figure 6 Scanning electron micrographs of films deposited at 580 ~ with (a) 50 nm, (b) 100 nm, and (c) 200 nm thickness. 1. Film deposited at amorphous to polycrystalline transition temperature has a rugged surface with (3 1 1) texture. 2. At a fixed deposition temperature, the grain structure tends to shift from the polycrystalline to the 6283 Figure8 Cross-sectional transmission electron micrographs of filmsdeposited at 560°C and in situ annealed at 570°C for 10min without breaking the vacuum. amorphous state with increasing deposition pressure and film thickness. The surface morphology and microstructure are related to deposition rate and silicon atom migration rate. 3. Nucleation during deposition starts to occur from the substrate, while nucleation during in situ 6284 Figure 7 Scanning electron micrographs of films deposited at 590°C under (a) 0.44 torr, (b)0.25 torr, and (c)0.17 tort silane partial pressure. annealing at the transition temperature without breaking the vacuum, begins to occur from surface silicon atom migration. References 1. T. 1. KAMINS, M. M. MANDURAH and K. C. SARAS- WAT, J. Electrochem. Soc. 125 (1978) 927. 2. Y. WADA and S. NISHIMATSU, ibid. 125 (1978) 1499. 3. T.I. KAMINS, ibid. 127 (1980) 686. 4. G. HARBEKE, L. KRAUSBAUER, E. F. STEIGMEIER, A. E. WIDMER, H. F. KAPPERT and G. NEUGEBAUER, ibid. 131 (1984) 675. 5. M.T. DUFFY and G. HARBEKE, in \"Proceedings of the 9th International Conference on CVD\" (1984) p. 400. 6. M. HENDRIKS and C. MAVERO, J. Eleetrochem. Soc. 138 (1991) 1466. 7. E.G. LEE and H. B. 1M, ibid. 138 (1991) 3465. 8. T. ONO, T. MORI, T. AJIOKA and T. TAKAYASHIKI, IEEE 1EDM Tech. Dig. (1985) 380. 9. S. MORI, Y. KANEKO, N. ARAI, Y. OHSHIMA, H. ARAKI, .K. NARITA, E. SAKA GAMI and K. YOSHIK- AWA, IEEE IRPS Tech. Dig. (1990) 132. 10. M. YOSHIMARU, J. MIYANO, N. INOUE, A. SAKAMOTO, S. YOU, H. TAMURA and M. INO, IEEE IEDM Tech. Dig. (1990)659. 11. H. WATANABE, T. TATSUMI, T. NIINO, A. SAKAI, S. ADACHI, N. AOTO, K. KOYAMA and T. KIKKAWA, in \"Extended Abstracts of the 23rd Conference on SSDM\" (1991) p. 478. 12. R. BISARO, Ji MAGARINO, N. PROUST and K. ZELLAMA, J. Appl. Phys. 59 (1986) 1167. 13. A. LIETOILA, A. WAKITA, T. W. SIGMON and J. F. GIBBONS, ibid. 53 (1982) 4399. 14. R. BISARO, J. MAGARINO, K. ZELLAMA, S. SQUEL- ARD, P. GERMAIN and J. F. MORHANGE, Phys. Rev. B 31 (1985) 3568. 15. A. SAKAI, H. ONO, K. ISHIDA, T. NIINO and T. TATSUMI, Jpn. Appl. Phys. Lett. 30 (1991) L941. Received 2 June 1992 and accepted28 April 1993
