Micro-Raman stress characterization of polycrystalline silicon films grown at high temperature

Transcription

1 Materials Science and Engineering B 112 (2004) Micro-Raman stress characterization of polycrystalline silicon films grown at high temperature R.C. Teixeira a,b, I. Doi a,b, *, M.B.P. Zakia b, J.A. Diniz a,b, J.W. Swart a,b a School of Electrical and Computer Engineering (FEEC) Unicamp, C.P. 6101, Campinas, SP, Brazil b Center for Semiconductor Components (CCS) Unicamp, C.P. 6101, Campinas, SP, Brazil Abstract In this paper the residual stress of polycrystalline silicon (poly-si) grown at high temperature in a vertical LPCVD reactor has been studied using micro-raman spectroscopy. The samples were prepared on Si(1 0 0) n-type substrates coated with 100 nm of SiO 2. The films were deposited in the temperature range of C at pressures of 5 and 10 Torr. The as-deposited poly-si films are highly crystalline and show tensile stress. Micro-Raman measurements show that the residual stress is reduced as the deposition temperature is increased and, above 800 8C, tensile stress is reduced to less than 150 MPa. These results indicate that high quality, high crystalline and low strained poly-si films can be obtained in this type of reactor using higher deposition temperature. # 2004 Elsevier B.V. All rights reserved. Keywords: Chemical vapor deposition; Stress; Polycrystalline silicon; Thin film 1. Introduction Since the mid-1970s, polycrystalline silicon (poly-si) have been used in microelectronics to improve the characteristics of MOS, TFT and related devices, and in the last decade poly-si has also been used as active layer in microelectro-mechanical systems (MEMS) [1]. The most usual technique for obtaining poly-si thin films is the horizontal low pressure chemical vapor deposition (h-lpcvd) [1]. Although this process can attain a high deposition rate and good step coverage, the poly-si thin films deposited by h-lpcvd exhibit a high compressive stress [1 4]. Stress characteristics are of great importance in the polysilicon technology. For application in the MOS devices, stressed films can lead to a high surface state density in the poly/oxide interface and in MEMS it causes bending and/or buckling of suspended structures and may even break poly-si structures [5]. Poly-Si thin films are also reported to detach from the Si substrate when a high stress level is involved [6] and may even crack the layer below it [2]. Further thermal steps are used for stress relaxation [2 4]. * Corresponding author. Tel.: ; fax: address: ioshiaki@fee.unicamp.br (I. Doi). Stress in thin films can have different origins such as the film formation process, material geometry, thermal steps (due to different thermal expansion coefficients), defects in the crystalline matrix (twinning, dislocations and grain boundaries) and/or thin film microstructure (crystallinity, grain size and orientation) [2,6]. There are many stress measurement methods like X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM) but these techniques are time consuming and lacks spatial resolution or are destructive [6]. Micro-Raman spectroscopy (Raman) is another technique that can be applied in stress characterization. This method is currently recognized as a powerful tool in identifying stress and strain in polycrystalline Silicon. As the method is contactless, no sample preparation is required. Other advantages of Raman measurements are that it is non-destructive, fast, has high spatial resolution and has high sensitivity for stress measurement [2,6,7]. In this work, Raman measurements were used to evaluate stress level and cristallinity (crystalline fraction P c ) of poly-si thin films deposited on 1000 Å thermally oxidized Silicon substrates using Silane in a Hydrogen carrier gas flow. A vertical LPCVD system (v-lpcvd) is used for deposition. The deposition temperature ranges employed /$ see front matter # 2004 Elsevier B.V. All rights reserved. doi: /j.mseb

2 are from 750 to 900 8C at 5 Torr pressure and from 700 to 900 8C at 10 Torr pressure. The as-deposited poly-si thin films presented low tensile stress (<150 MPa) without any subsequent heat treatments in opposition to typical compressive stress found for poly-si films obtained by horizontal chemical vapor deposition [1 3]. R.C. Teixeira et al. / Materials Science and Engineering B 112 (2004) Experimental procedures Poly-Si thin films were deposited on 2 in V cm n- type h1 0 0i silicon substrates covered with a 1000 Å thick thermal silicon dioxide in a PMC200 LPCVD Vertical Reactor by thermal decomposition of silane. The wafers were sliced into two parts before oxidation. Gas flow rates used were 4800 sccm of hydrogen (99.99%) as carrier gas and 40 sccm of silane (99.999%) as precursor gas. The deposition temperature was monitored using an infrared pyrometer in the range from 750 to 900 8C for 5 Torr and from 700 to 900 8C for 10 Torr with 50 8C steps. After cutting off the silane flow the hydrogen flow was kept during further 6 min with the temperature used in deposition for the total removal/consumption of reactive species. The thickness of the obtained films were measured by scan profiler and they were about 7000 Å. Stress characteristics and crystallinity of the poly-si thin films were determined from the spectra obtained by Raman spectroscopy using a Jobin Yvon T64000 spectrometer, equipped with a liquid nitrogen cooled charge coupled detector. The excitation wavelength was the 5145 Å line of an Argon laser in the backscattering configuration. The laser spot on the sample was kept at a power density low enough to avoid temperature effects (about 6 mw over the sample) [8]. The Raman spectra were deconvoluted in the amorphous and crystalline contributions fitting the spectra with a Lorentzian curve to determine the crystalline peak s wave number [1,9,10]. The obtained fitting parameters were used to determine the stress level and crystallinity of the samples. 3. Results and discussion The Raman peak at x cm = 520 cm 1 taken on a Si single crystal is used as a reference for all our measurements. The measured Raman spectra were deconvoluted in the amorphous (480 cm 1 ) and crystalline (520 cm 1 ) contributions using a Lorentzian fit [1,9,10]. Fig. 1 shows some Raman spectra of the deposited poly-si thin films. As shown in the figure, the peak position shifts from one deposition condition to another (the top spectrum s peak is shifted to right when compared to the bottom spectrum). The area ratio for the amorphous and crystalline contribution and peak intensity also varies greatly. Using wavenumber shift of the poly-si crystalline peak the stress level can be determined by Eqs. (1) and (2) as Fig. 1. Raman spectra of poly-si thin films deposited by v-lpcvd. (Top) thin film deposited at 900 8C and 10 Torr. (Bottom) thin film deposited at 750 8C and 5 Torr. For comparision the vertical scales were kept as obtained and these are the same for both spectra. follows [6,7,11]: e ¼ 250ðMPa=cmÞD$ (1) D$ ¼ x CP x CM (2) where D$ is the deviation of the polycrystalline fit peak (x cp ) relative to the single crystal fit peak (x cm ) extracted in the same Raman measurement conditions and e is the stress magnitude in MPa. Fig. 2 shows the stress behavior of the poly-si samples calculated from Raman spectra using Eqs. (1) and (2). We verify that the films have a tensile stress for all the deposition temperatures and pressures used in this study. We observe that stress varies inversely with deposition temperature, with a minimum of 145 MPa for 5 Torr and 72.5 MPa for 10 Torr deposition process. We can also notice that above 800 8C the stress tends to saturation. This result is quite different from the compressive stress found in literature for LPCVD deposited poly-si [1 4]. Although the tensile stress for poly-si films have been reported, these were found in amorphous Si thin films annealed after ion implantation [3] or heavily doped poly-si [12] and such a low tensile stress in undopped, as-deposited poly-si is only

3 162 R.C. Teixeira et al. / Materials Science and Engineering B 112 (2004) Fig. 2. Temperature dependence of stress for poly-si thin films deposited at 5 and 10 Torr, denoted by full squares and open circles, respectively. achieved after multiple cycles of heat treatments by rapid thermal annealing (RTA) [4]. Conventional furnace annealing of the LPCVD poly-si also reduces stress, but it still remains compressive [2 4]. Poly-Si thin films obtained in the v-lpcvd have low tensile stress without any further thermal step requirements, which makes this very suitable for freestanding surface-micromachined structures [3]. Many factors are involved in the stress characteristics and one of these is the internal microstructure of the deposited poly-si films. As indicated by XRD measurements (Fig. 3) deposition parameters have a high influence on the thin film microstructure. This figure shows that different deposition conditions for the v-lpcvd leads to poly-si films with very different grain orientations. Increasing the deposition temperature or pressure, the formed structure orientation changes from the h2 20i obtained at 5 Torr/850 8C and 10 Torr/800 8C, to a strongly h1 11i oriented film deposited at 10 Torr/850 8C. The formation of h1 1 1i orientated structures in the deposited films, as the consequence of the enlargement of the silicon bond length, favors the relaxation of compressive stress in poly-si films [3,4]. This is the opposite of what we see in Fig. 2; the relaxation in tensile stress means a reduction in the bond length. Therefore, the tensile stress and its drastic reduction with deposition temperature and pressure observed for the v-lpcvd deposited poly-si, cannot be attributed to this structural changes. Other parameter that affects the residual stress in the poly-si films is the grain size of the formed crystallite. Dimensions determined by Scherrer formula from obtained XRD data indicates grain size from 25 nm at 5 Torr/800 8C to 70 nm at 10 Torr/850 8C. In this grain size dimension order small changes in the structure affects the strain value of the formed films. Thus, the variation in the residual stress observed in the samples deposited in this temperature range may be partially attributed to the change in grain size of the Fig. 3. XRD spectra of poly-si deposited by vertical LPCVD. (Top) 10 Torr/850 8C; (middle) 10 Torr/800 8C; (bottom) 5 Torr/850 8C.

4 R.C. Teixeira et al. / Materials Science and Engineering B 112 (2004) Fig. 4. Crystallinity of the poly-si thin film as function of the deposition temperature for 5 Torr (squares) and 10 Torr (circles) deposition. crystallites. The increase in the grain size means also that grain boundaries are reducing in the poly-si thin film. Grain boundaries are known to compressively stress poly-si films [2,13]. To make a better analysis of this characteristic in the poly-si thin films, we calculate the crystalline fraction (P c ) from Raman spectra using Eq. (3) [9,10]. In this equation I a and I c are the amorphous and crystalline areas of the deconvoluted Raman spectra, and g = 0.8 is a correction factor due to the different scattering cross section of amorphous and crystalline phases. As shown in the Fig. 4, the crystalline fraction rises rapidly with temperature deposition, achieving a maximum value of and 98.13% for 5 and 10 Torr, respectively. P c ¼ I c (3) I c þ gi a Assuming that crystalline fraction increases as grain boundaries volume reduces, the increase in crystalline amount in the sample is a indicative of a enlargement of the Silicon bond length. This means that the stress becomes more tensile, contrary of what we observe in Fig. 2. Since the amorphous silicon bond length is larger than the crystalline Fig. 5. FWHM of the crystalline peak fit as a function of temperature for 5 Torr (squares) and 10 Torr (circles) pressure deposition.

5 164 R.C. Teixeira et al. / Materials Science and Engineering B 112 (2004) one (2.55 Å for amorphous against 2.35 Å for crystalline [1]) the amorphous phase may be stretching the silicon bond length at the borders of the crystalline grains. Higher temperature of deposition produces thin films with smaller amorphous phase, leading to a reduction in the stretching and thus to a less tensile film. However, tensile stress presented in Fig. 2 reduces faster than the crystalline fraction rises in Fig. 4, indicating another factor is involved in the stress relaxation. Analyzing the full width at half maximum (FWHM) of the crystalline fraction from the Raman spectra, we find that this parameter also reduces as the deposition temperatures increased (Fig. 5). The larger the FWHM, the greater the defects amount [7,11] namely dislocations and twinning as the main defects found within the poly-si crystal grains [6,7]. Confinement effects also causes broadening of the FWHM, but this is present only if the grain size is less than 20 nm for Si crystals [7], so this effect may be neglected for v-lpcvd deposited poly-si as XRD indicates grain sizes bigger than 25 nm. The reduction of the FWHM indicates that a more perfect crystalline lattice is obtained, contributing for the stress relaxation found. 4. Conclusions In this paper we studied stress dependence with temperature of poly-si thin films deposited in a vertical LPCVD system using micro-raman spectroscopy. It was found that the poly-si layers are tensile stressed, as opposed to the most common compressive stress observed for horizontal LPCVD deposited films. Low stressed and high crystalline films were deposited above 800 8C for both 5 and 10 Torr pressure, without any further thermal step. The best values achieved are 145 MPa and 5 cm 1 for 5 Torr and 72.5 MPa and 4.85 cm 1 for 10 Torr, for stress and FWHM, respectively. Tensile stress relaxation is supposed to be due to the reduction of amorphous phase around crystallites and better quality of Silicon crystal grains. These characteristics make the poly-si thin films obtained in the v-lpcvd suitable for freestanding surface-micromachined structures. Acknowledgements The authors are grateful to Dailto Silva from IG/UNI- CAMP for the Raman measurements, Prof. Dr. Lisandro P. Cardoso from IFGW/UNICAMP for the XRD measurements and Regina M.A.G. Floriano and Jose G. Filho from CCS/UNICAMP for their help in the preparation of the samples for poly-si deposition. CAPES, CNPq and FAPESP supported this work. References [1] T.I. Kamins, Polycrystalline Silicon for Integrated Circuit and Displays, 2nd ed., Kluwer Academic Publishers, [2] G. Kaltsas, A.G. Nassiopoulos, M. Siakavellas, E. Anastassakis, Sens. Actuators A 68 (1998) [3] P.J. French, D. Poenar, R. Malleé, P.M. Sarro, J. Electromech. Syst. 5 (3) (1996). [4] X. Zhang, T.Y. Zhang, M. Wong, Y. Zohar, J. Electromech. Syst. 7 (4) (1998) [5] M.S. Benrakkad, M.A. Benitez, J. Esteve, J.M. López-Vellegas, J. Samitier, J.R. Morante, J. Michomech. Microeng. 5 (1995) [6] I. de Wolf, Semiconductor Sci. Technol. 11 (1996) [7] T. Jawhari, Analysis 28 (1) (2000) [8] M. Siakavellas, E. Anastassakis, G. Kaltsas, A.G. Nassiopoulos, Microelectron. Eng (1998) [9] P. Lengsfeld, Sucessive Laser Crystalization of Doped and Undoped a- Si:H; Doctor Thesis, Technischen Universität, Berlin, [10] A.T. Voutsas, M.K. Hatalis, J. Boyce, A. Chiang, J. Appl. Phys. 78 (12) (1995) [11] P. Münster, M. Sarret, T. Mohanned-Brahinm, N. Coulon, J.Y. Mevelle, Philos. Mag. B 82 (15) (2002) [12] N.H. Nickel, P. Lengsfeld, I. Sieber, Phys. Rev. B 61 (23) (2000) [13] Clark, S. available at on 20/10/2003, 1996.