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Micro-Raman spectroscopy characterization of polycrystalline silicon films fabricated by excimer laser crystallization
ARTICLE IN PRESS Optics and Lasers in Engineering 47 (2009) 612–616
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Micro-Raman spectroscopy characterization of polycrystalline silicon films fabricated by excimer laser crystallization Chil-Chyuan Kuo Department of Mechanical Engineering, Mingchi University of Technology, No. 84, Gungjuan Road, Taishan, Taipei Hsien 243, Taiwan
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 May 2008 Received in revised form 23 June 2008 Accepted 26 June 2008 Available online 3 August 2008
The rapid recrystallization of amorphous silicon films utilizing excimer laser crystallization (ELC) is presented. The resulting poly-Si films are characterized by Raman spectroscopy. Polycrystalline silicon (poly-Si) films with higher crystallinity can be realized by a dehydrogenation process before ELC. Raman spectra as a function of various excimer laser energy densities are demonstrated. The crystallinity and residual stress of the poly-Si films are determined and discussed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Micro-Raman spectroscopy Polycrystalline silicon Excimer laser crystallization
1. Introduction The technology of low-temperature polycrystalline silicon (poly-Si) thin-film transistors (LTPS TFTs) has been widely investigated for applications in active-matrix flat-panel displays (AMFPD), giant microelectronics and photovoltaics [1–5]. Various methods for fabricating high-quality poly-Si films with large grain size, low defect density, and high carrier mobility have been reported. These include solid-phase crystallization (SPC) [6], excimer laser crystallization (ELC) [7], and metal-induced lateral crystallization (MILC) [8]. Among these technologies, ELC has been studied as a promising option. It has been employed by industries to fabricate high-quality poly-Si films on commercially available inexpensive glass substrates for the development of high-performance LTPS TFTs in AMFPD, since the excimer laser has larger beam size and better density homogeneity than other lasers [9–11]. ELC is a rapid crystallization technique that is characterized by melting of thin films and solidification within several tens of nanoseconds (ns) [12]. The major advantage of this technique is that it provides a low thermal process budget during laser crystallization compared with SPC [13,14]. A full overview of well-known crystallization techniques can be found elsewhere [15]. Crystallinity is the key index for evaluating the quality of polySi films after ELC, which is governed by grain boundaries (GBs) and intra-grain defect density [16]. In general, poly-Si films with higher crystallinity are of higher quality. Scanning electron Tel.: +886 2 29089899x4524; fax: +886 2 29063269.
E-mail address: [email protected] 0143-8166/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2008.06.018
microscopy (SEM) [17], transmission electron microscopy (TEM) [18], X-ray diffraction (XRD) [19] spectroscopy, and spectroscopic ellipsometry (SE) [20,21] are certainly the most appropriate techniques for evidencing microstructural studies of poly-Si films fabricated by ELC. SEM and TEM are suitable for evaluating the grain size of poly-Si, but these techniques are time consuming and involve destructive characterization. XRD is suitable for evaluating the residual stress of poly-Si, but this technique lacks spatial resolution. Although much microstructural information of poly-Si films can be obtained directly by SE, the major drawback of this technique is that specific software is needed for subsequent analysis. Thus, the analysis of crystallinity and residual stress of poly-Si films by Raman spectroscopy is the most effective and simplest approach compared with the methods described above because it involves no specimen preparation and non-destructive characterization, and yields fast, high spatial resolution, and high sensitivity for microstructural evaluations of poly-Si films. In this work, the resulting poly-Si films crystallized by ELC are characterized by FE-SEM, micro-Raman spectroscopy, and highresolution TEM (HR-TEM). Raman spectra as a function of various excimer laser energy densities are investigated. Effect of concentration of hydrogen remaining in the specimen on the quality of poly-Si films is discussed. The issue of residual stress in poly-Si films after ELC is also discussed.
2. Experiment The substrate chosen for the experimental study is inexpensive Corning Glass 1737. As a starting material for ELC, hydrogenated
ARTICLE IN PRESS C.-C. Kuo / Optics and Lasers in Engineering 47 (2009) 612–616
90-nm-thick a-Si thin films (a-Si:H) are deposited on glass substrates covered with a 300-nm-thick SiO2 layer using tetraethyl-ortho-silicate plasma-enhanced chemical vapor deposition. The thickness of the obtained films is measured by a scan profiler. In order to limit concentration of hydrogen in a-Si:H films, the specimens are then dehydrogenated by thermal treatment at 500 1C for 2 h for removing excess hydrogen, preventing ablation caused by sudden hydrogen eruption during ELC. After dehydrogenation, the specimens are then held in self-closing tweezers at the end of a cantilever beam fixed on a motorized X–Y linear translation stage (resolution ¼ 0.625 mm). The movement of the focusing lens (focus ¼ 100 mm) fixed on a motorized Z-axis linear translation stage is precisely controlled to adjust the desired laser energy density for crystallization. To enhance the efficiency of laser crystallization, the movement of the three-axis (X–Y–Z) motorized translation stages can accurately be manipulated using the self-developed man–machine interface. A XeF Lambda Physik excimer laser (COMPex 102) operating at 351 nm with 25 ns fullwidth at half-maximum (FWHM) pulse duration is employed for crystallization at room temperature (RT) in ambient air. A Joule meter (Vector H410 SCIENTECH) is employed to calibrate the output energy of excimer laser pulse before ELC. A series of incremental excimer laser energy densities up to 250 mJ/cm2 is employed for laser crystallization. It is estimated that the measured excimer laser energy densities have an error of around 10% because the pulse-to-pulse variation in excimer laser energy is estimated to be 10%. Fig. 1 shows the schematic illustration of backside ELC and specimen structure. The working principle of backside ELC denotes that the a-Si films are being irradiated indirectly with the excimer laser beam passing through first the glass substrate and SiO2 films. A stainless-steel slit mask of around 2 15 mm2 is employed to transform the incident Gaussian beam into a rectangular beam spot. The size of the beam spot can be controlled by changing the working distance between the focusing lens and the specimen surface. After ELC, the microstructural analyses of recrystallized Si films are performed by FE-SEM (Oxford JEOL JSM-6500F), micro-Raman spectroscopy (Renishaw inVia) and HR-TEM (JEOL JEM-2010). Before FE-SEM observation, specimens are treated by Seccoetching (50% HF: H2O: CrO3 ¼ 200 cc: 100 cc: 1.5 g) for highlighting the GBs and intra-grain defects [22]. The surface of the specimen is coated with thin Au films to prevent the charging effect during SEM measurements. The acceleration electron beam energy for FE-SEM and HR-TEM experiments are 15 kV (resolution 1.5 nm) and 200 kV (point resolution 0.23 nm), respectively. Crystallinity of poly-Si films is characterized by micro-Raman spectroscopy. To avoid any specimen being heated before and
613
during micro-Raman scattering measurements, Raman spectra are obtained at RT in back-scattering mode operated at a low power of 3 mW. The system is equipped with an optical microscope for focusing the 514.5 nm (spot size ¼ 5 mm) excitation line from the Ar+ laser beam on the specimens for evaluating fluctuations in crystallinity. The resolution of micro-Raman measurements is 2 cm1. Raman spectra are deconvoluted in the amorphous and crystalline contributions fitting the spectra with a Lorentzian curve to determine the wavenumber of the crystalline peak maximum [23].
3. Results and discussion Three major distinct regrowth regimes (partial melting, nearcomplete melting, and complete melting), which vary primarily with excimer laser energy density and thickness of precursor Si film, have been reported [24,25]. In the partial melting regime, there is an increase in the grain size of poly-Si films with increasing excimer laser energy density. In particular, the grain size of poly-Si films reaches the maximum because of super lateral growth (SLG) in the near-complete melting regime. In the complete melting regime, recrystallization is governed by homogeneous nucleation from a highly undercooled liquid Si, resulting in fine grains distributed randomly throughout the Si films. In order to understand the effects of concentration of hydrogen remaining in the specimen, some specimens are crystallized without a dehydrogenation process. Fig. 2 shows the wavenumber as a function of Raman intensity for specimens with and without dehydrogenation. A broad FWHM of Raman spectrum is observed for the without dehydrogenation specimen because hydrogen released from the films forms microbubbles that can explode, eventually resulting in ablation of films [26]. Therefore, the crystallinity of poly-Si films is low for the specimen without being dehydrogenated. On the other hand, a narrow FWHM of Raman spectrum of approximately 6 cm1 is clearly observed for the dehydrogenated specimen. This result indicates that the crystallinity of poly-Si films can be enhanced by a dehydrogenation process before ELC. The Raman peak at 520 cm–1 obtained from a single crystal wafer is used for reference of all micro-Raman scattering measurements [27]. Fig. 3 shows Raman spectra of poly-Si films crystallized at different laser energy densities. As can be seen, the symmetrical Raman peaks are centered at different wavenumbers of 510–520 cm1, revealing that a-Si films have changed into polySi films after ELC. It is interesting to note that the Raman intensity first increases with increasing excimer energy density from 100 to
16000
Excimer laser
Dehydrogenation
Intensity (arb. units)
14000
Glass
12000
Without dehydrogenation
10000 8000 6000 4000 2000
SiO
300 nm
a-Si
90 nm
Fig. 1. Schematic illustration of backside ELC and specimen structure.
0 500
505
515 520 510 Wavenumber (cm-1)
525
530
Fig. 2. Raman spectra of poly-Si films (with and without dehydrogenation) crystallized by XeF laser pulse. Two samples are crystallized at a laser energy density of 190 mJ/cm2.
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16000 100 mJ/cm2 150 mJ/cm2 175 mJ/cm2 190 mJ/cm2 200 mJ/cm2 225 mJ/cm2
12000 10000 8000
18000 16000 Intensity (arb. units)
Intensity (arb. units)
14000
6000 4000 2000 0 500
14000 12000 10000 8000 6000
505
510 515 520 Wavenumber (cm-1)
525
530
4000 75
100
125 150 175 200 Laser energy density ( mJ/cm2)
75
100
125 150 175 200 Laser energy density (mJ/cm2)
225
75
100
125 150 175 200 Laser energy density (mJ/cm2)
225
Fig. 3. Raman spectra of poly-Si films crystallized at different laser energy densities.
9
o ¼ oc þ 5:2 10 P
(1)
where oc indicates the wavenumber of Raman peak maximum of the single crystal wafer, o indicates the wavenumber of Raman peak maximum of the observed poly-Si Raman spectrum, and P indicates the residual stress in poly-Si films after ELC. The unit of P is Pascal (Pa). The positive value and negative value for P indicate the compressive stress and tensile stress in poly-Si films, respectively. After calculation using this equation, the poly-Si films fabricated by ELC are tensile stressed. This phenomenon is attributed to the difference in thermal expansion coefficient between poly-Si films (7.5 106 K1) and the glass substrate
250
6.9 6.8 FWHM (cm-1)
6.7 6.6 6.5 6.4 6.3 6.2 6.1 6 250
5.5 5.3 5.1 Δω (cm-1)
190 mJ/cm2 and eventually becomes saturated. Conversely, the Raman intensity decreases at excimer laser energy densities exceeding 190 mJ/cm2. Moreover, the FWHM first decreases with increasing laser energy density from 100 to 190 mJ/cm2 and eventually increases. These results are summarized in Fig. 4. Do denotes the deviation in wavenumber of Raman peak maximum between the single crystal wafer (oc) and the observed Raman spectrum (o) extracted under the same micro-Raman measurement conditions. As can be seen, both the FWHM of micro-Raman scattering spectrum and Do reach the minimum level at a laser energy density of 190 mJ/cm2, whereas intensity of micro-Raman scattering spectrum reaches the maximum level at the same laser energy density. The higher the intensity of micro-Raman scattering spectra, the smaller the FWHM, and the smaller the FWHM, the greater the defect (microtwinning and dislocation) [28]. The FWHM for this laser energy density is approximately 6.1 cm1, whereas the FWHM is approximately 4.2 cm1 for a single crystal wafer [29]. Such difference has been interpreted by the grain size effects or attributed to the existing internal strain [30]. The sharp peaks of the micro-Raman spectra are centered at a range of 514–517 cm1. The crystalline volume fraction of poly-Si films is approximately 100% because no transverse-optical photon made of a-Si is found in the spectrum, which indicates that the films are sufficiently crystallized. This result is exceedingly different from the poly-Si films fabricated by SPC [13,14]. Thus, it can be said that ELC is a very powerful technology for producing poly-Si films with high crystallinity by melt–regrowth of silicon films. This is confirmed by the corresponding cross-sectional HR-TEM images, as shown in Fig. 5. As can be seen, the precursor a-Si films are totally transformed into poly-Si films with different grain sizes after ELC at excimer laser energy densities above the surface melting threshold. The Raman spectra obtained are also employed to determine the stress level of poly-Si films. The residual stress in the poly-Si after ELC can be determined by [31]
225
4.9 4.7 4.5 4.3 4.1 3.9 250
Fig. 4. (a), (b), and (c) represent intensity, FWHM, and Xo as a function of laser energy density, respectively. Do denotes the deviation in wavenumber of Raman peak maximum between the single crystal wafer (oc) and the observed Raman spectrum (o) extracted under the same Raman measurement conditions.
(4.2 106 K1) [32]. It should be noted that this phenomenon is opposed to the most common compressive stress in poly-Si films fabricated by low-pressure chemical vapor deposition (LPCVD) [33]. The tensile stress is reduced from 1019 Mpa for laser energy density of 100 mJ/cm2 to 807 Mpa for laser energy density of 190 mJ/cm2. On the other hand, the tensile stress is increased from 807 Mpa for laser energy density of 190 mJ/cm2 to 913 Mpa for laser energy density of 225 mJ/cm2. This result reveals that tensile stress caused by melting and recrystallization of a-Si films is observed in poly-Si films fabricated by ELC. Moreover, tensile stress is also found in poly-Si films annealed by a continuouswave laser [34]. Fig. 6 shows FE-SEM micrograph of Secco-etched poly-Si films irradiated at various excimer laser energy densities. The average
ARTICLE IN PRESS C.-C. Kuo / Optics and Lasers in Engineering 47 (2009) 612–616
615
Fig. 5. Cross-sectional HR-TEM images of excimer laser-crystallized Si thin films at various energy densities. (a) E ¼ 100 mJ/cm2, (b)E ¼ 175 mJ/cm2, (c) E ¼ 190 mJ/cm2. Large-grained poly-Si is formed due to SLG behavior. (d) E ¼ 210 mJ/cm2.
100nm
100nm
100nm
100nm
100nm
100nm
Fig. 6. FE-SEM micrograph of Secco-etched poly-Si films irradiated at various excimer laser energy densities.
grain size of poly-Si films increases with the increase in excimer laser energy density from 100 to 190 mJ/cm2 and decreases at higher excimer laser energy density ranging from 200 to 225 mJ/cm2. As can be seen, the average grain size is in the range of 100–900 nm and the grains are generally elliptical in shape. Notably, a large grain with a diameter of 900 nm formed by SLG is clearly observed in poly-Si films crystallized at an excimer laser energy density of 190 mJ/cm2. This result is in good agreement with the observation in Fig. 4. A distinct phenomenon of minimum level of both FWHM and Do and maximum level of Raman intensity is observed because large grain formed by SLG is known as high quality for single-grain TFTs application [35]. Note that the smaller the FWHM and Do, the greater the crystallinity of poly-Si films. When the excimer laser energy density is increased above 225 mJ/cm2, ablation of Si films is triggered. Thus, crystallized poly-Si films cannot be employed for fabricating LTPS TFTs. Lu et al. [36] confirmed that ablation is mainly caused by explosive boiling or explosive transformation of the superheated liquid into a mixture of particles or droplets and vapor, which are then ejected out of the Si films. Sugawara et al. [37] suggested that lasers operating at a higher wavelength can
overcome this drawback since photons would travel deeper into the thin films compared with the excimer laser. Thus, excimer laser energy density of 250 mJ/cm2 is employed to be the maximum excimer laser energy density used for this work. Agglomeration phenomenon in Si films is also observed after ELC. Agglomeration is the dewetting of the underlying SiO2 by Si films and is easily induced when the interfacial energy between Si films and SiO2 increases. This result is consistent with the observation by He et al. [38]. Thus, avoiding the agglomeration effect of Si films for fabricating high-performance LTPS TFTs by ELC is required. In general, the energy density for SLG has a very narrow process window, typically about 1.5% [39] and 2.5% [40]. A novel optical method for enlarging the process window of phasemodulated ELC has been proposed [41]. Moreover, the process window has been experimentally observed to reach approximately 5% by in-situ time-resolved optical measurements in my previous investigation [42]. These results reported here are believed to give a possible solution for realization of highperformance LTPS TFTs on a large-area glass substrate under the optimum experimental conditions by ELC.
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4. Conclusions The rapid recrystallization of a-Si films utilizing ELC has been investigated. The crystallized poly-Si films have been characterized extensively by Raman spectroscopy for structural investigations. Poly-Si films with higher crystallinity can be obtained by a dehydrogenation process before ELC because the quality of poly-Si is significantly affected by the concentration of hydrogen in the specimen. At an excimer laser energy density of 100–190 mJ/cm2, the crystallinity of poly-Si films increases with the increase in excimer laser energy density. A large grain formed by SLG is clearly observed in poly-Si films crystallized at an excimer laser energy density of 190 mJ/cm2, which is consistent with the observation of minimum level of both FWHM and Do and maximum level of the Raman intensity. Existence of tensile in the poly-Si films has been observed. This result is quite different from the most common compressive stress in poly-Si films fabricated by LPCVD. It is noted that the micro-Raman analysis is an accurate and reliable approach for determining the residual stress and crystallinity of poly-Si films.
Acknowledgements The author would like to thank Professors W.C. Yeh and J.Y. Jeng of the National Taiwan University of Science and Technology for assistance with related experimental apparatus. The skillful technical assistance of Mr. C. B. Chen of the National Taiwan University of Science and Technology is highly appreciated. References [1] Per I, Widenborg, Armin G, Aberle. J Cryst Growth 2007;306:177. [2] Gall S, Schneider J, Klein J, Hubener K, Muske M, Rau B, et al. Thin Solid Films 2006;511:7. [3] Ji C, Anderson WA. Sol Energy Mater Sol Cells 2005;85:313. [4] Persidis E, Baur H, Pieralisi F, Schalberger P, Fruehauf N. Solid-State Electron 2008;52:455. [5] Tsai CC, Lee YJ, Wang JL, Wei KF, Lee IC, Chen CC, et al. Solid-State Electron 2008;52:365. [6] Toyama T, Muhida R, Harano T, Sugano T, Okajima M, Okamoto H. J Appl Phys 2003;42:L1347. [7] Vikas R, Ishihara R, Hiroshima Y, Abe D, Inoue S, Shimoda T, et al. Jpn J Appl Phys 2006;45:4340.
[8] Kirimura H, Uraoka Y, Fuyuki T, Okuda M, Yamashita I. Appl Phys Lett 2005;86:262106. [9] Kawachi G, Nakazaki Y, Ogawa H, Jyumonji M, Akita N, Hiramatu M, et al. Jpn J Appl Phys 2007;46:51. [10] Im JS, Sposili RS, Crowder MA. Appl Phys Lett 1997;70:3434. [11] Mariucci L, Carluccio R, Pecora A, Foglietti V, Fortunato G, Legagneux P, et al. Thin Solid Films 1999;337:137. [12] Higashi S, Sameshima T. Jpn J Appl Phys 2001;40:480. [13] Aoyma T, Koike Y, Okajima Y, Konoshi N, Suzuki T, Miyata K. IEEE Trans Electron Dev 1991;38:2058. [14] Giust GK, Sigmon TW. IEEE Trans Elec Dev 2000;47:207. [15] Voutsas AT. Appl Surf Sci 2003;250:208. [16] Rezek B, Nebel CE, Stutzmann M. Jpn J Appl Phys 1999;38:L1083. [17] Oh JH, Ahn KW, Kang DH, Park WH, Jang J, Chang YJ, et al. Solid-State Electron 2008;52:482. [18] Stradal J, Scholma G, Li H, van der Werf CHM, Rath JK, Widenborg PI, et al. Thin Solid Films 2006;501:335. [19] Alford TL, Shetty PK, Theodore ND, Tile N, Adams D, Mayer JW. Thin Solid Films 2008;516:3940. [20] Yu G, Soga T, Shao CL, Jimbo T, Umeno M. Appl Surf Sci 1997;113:489. [21] Wakagi M, Fujiwara H, Collins RW. Thin Solid Films 1998;313:464. [22] Rezek B, Nebel CE, Stutzmann M. Jpn J Appl Phys 1999;38:L1083. [23] Voutsas AT, Hatalis MK, Boyce J, Chiang A. J Appl Phys 1995;78:6999. [24] Im JS, Kim HJ, Thompson MO. Appl Phys Lett 1993;63:1969. [25] Im JS, Kim HJ. Appl Phys Lett 1994;64:2303. [26] Lengsfeld P, Nickel NH, Fuhs W. Appl Phys Lett 2000;76:1680. [27] Kang TD, Lee H, Park SJ, Jang J, Lee S. J Appl Phys 2002;92:2467. [28] Kitahara K, Ohnishi K, Katoh Y, Yamazaki R, Kurosawa T. Jpn J Appl Phys 2003;42:6742. [29] Lyon SA, Nemanich RJ, Johnson NM, Biegelsen DK. Appl Phys Lett 1982;40: 316. [30] Kitahar K, Moritani A, Hara A, Okabe M. Jpn J Appl Phys 1999;38:L1312. [31] Higashi S, Ando N, Kamisako K, Sameshima T. Jpn J Appl Phys 2001;40:731. [32] Tada H, Kumpel AE, Lathrop RE, Slanina JB, Nieva P, Zavracky P, et al. J Appl Phys 2000;87:4189. [33] Zhang X, Zhang TY, Wong M, Zohar Y. J Electromech Syst 1998;7:356. [34] Nakashima S, Oima S, Mitsuishi A, Nishimura T, Fukumoto T, Akasaka Y. Solid State Commun 1981;40:765. [35] Ishihara R, Danciu D, Tichelaar F, He M, Hiroshima Y, Inoue S, et al. J Cryst Growth 2007;299:316. [36] Lu QM, Mao SS, Mao X, Russo RE. Appl Phys Lett 2002;80(2002):3072. [37] Sugawara Y, Uraoka Y, Yano H, Hatayama T, Fuyuki T, Mimura A. Jpn Appl Phys 2007;46:7607. [38] He M, Ishihara R, Metselaar W, Beenakker K. Thin Solid Films 2007;515:2872. [39] Ishihara R, Burtsev A, Alkemade PFA. Jpn J Appl Phys 2000;39:3872. [40] Pribat D, Legagneux P, Collet C, Plais F, Huet O, Reita C. Proceedings of the active-matrix liquid-crystal displays (AM-LCD), Tokyo, Japan, July 9–10, 1999, p.9. [41] Taniguchi Y, Endo T, Azuma K, Matsumura M, Ishiwara H. Jpn Appl Phys 2008;47:947. [42] C. C. Kuo, W. C. Yeh, C. B. Chen, and J. Y. Jeng. Proceedings of the 2005 IEEE/ ASME international conference on advanced manufacture, Taipei, Taiwan, R O C, November 28–December 2, 2005, p.337.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Micro-Raman spectroscopy characterization of polycrystalline silicon films fabricated by excimer laser crystallization Chil-Chyuan Kuo Department of Mechanical Engineering, Mingchi University of Technology, No. 84, Gungjuan Road, Taishan, Taipei Hsien 243, Taiwan
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 May 2008 Received in revised form 23 June 2008 Accepted 26 June 2008 Available online 3 August 2008
The rapid recrystallization of amorphous silicon films utilizing excimer laser crystallization (ELC) is presented. The resulting poly-Si films are characterized by Raman spectroscopy. Polycrystalline silicon (poly-Si) films with higher crystallinity can be realized by a dehydrogenation process before ELC. Raman spectra as a function of various excimer laser energy densities are demonstrated. The crystallinity and residual stress of the poly-Si films are determined and discussed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Micro-Raman spectroscopy Polycrystalline silicon Excimer laser crystallization
1. Introduction The technology of low-temperature polycrystalline silicon (poly-Si) thin-film transistors (LTPS TFTs) has been widely investigated for applications in active-matrix flat-panel displays (AMFPD), giant microelectronics and photovoltaics [1–5]. Various methods for fabricating high-quality poly-Si films with large grain size, low defect density, and high carrier mobility have been reported. These include solid-phase crystallization (SPC) [6], excimer laser crystallization (ELC) [7], and metal-induced lateral crystallization (MILC) [8]. Among these technologies, ELC has been studied as a promising option. It has been employed by industries to fabricate high-quality poly-Si films on commercially available inexpensive glass substrates for the development of high-performance LTPS TFTs in AMFPD, since the excimer laser has larger beam size and better density homogeneity than other lasers [9–11]. ELC is a rapid crystallization technique that is characterized by melting of thin films and solidification within several tens of nanoseconds (ns) [12]. The major advantage of this technique is that it provides a low thermal process budget during laser crystallization compared with SPC [13,14]. A full overview of well-known crystallization techniques can be found elsewhere [15]. Crystallinity is the key index for evaluating the quality of polySi films after ELC, which is governed by grain boundaries (GBs) and intra-grain defect density [16]. In general, poly-Si films with higher crystallinity are of higher quality. Scanning electron Tel.: +886 2 29089899x4524; fax: +886 2 29063269.
E-mail address: [email protected] 0143-8166/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2008.06.018
microscopy (SEM) [17], transmission electron microscopy (TEM) [18], X-ray diffraction (XRD) [19] spectroscopy, and spectroscopic ellipsometry (SE) [20,21] are certainly the most appropriate techniques for evidencing microstructural studies of poly-Si films fabricated by ELC. SEM and TEM are suitable for evaluating the grain size of poly-Si, but these techniques are time consuming and involve destructive characterization. XRD is suitable for evaluating the residual stress of poly-Si, but this technique lacks spatial resolution. Although much microstructural information of poly-Si films can be obtained directly by SE, the major drawback of this technique is that specific software is needed for subsequent analysis. Thus, the analysis of crystallinity and residual stress of poly-Si films by Raman spectroscopy is the most effective and simplest approach compared with the methods described above because it involves no specimen preparation and non-destructive characterization, and yields fast, high spatial resolution, and high sensitivity for microstructural evaluations of poly-Si films. In this work, the resulting poly-Si films crystallized by ELC are characterized by FE-SEM, micro-Raman spectroscopy, and highresolution TEM (HR-TEM). Raman spectra as a function of various excimer laser energy densities are investigated. Effect of concentration of hydrogen remaining in the specimen on the quality of poly-Si films is discussed. The issue of residual stress in poly-Si films after ELC is also discussed.
2. Experiment The substrate chosen for the experimental study is inexpensive Corning Glass 1737. As a starting material for ELC, hydrogenated
ARTICLE IN PRESS C.-C. Kuo / Optics and Lasers in Engineering 47 (2009) 612–616
90-nm-thick a-Si thin films (a-Si:H) are deposited on glass substrates covered with a 300-nm-thick SiO2 layer using tetraethyl-ortho-silicate plasma-enhanced chemical vapor deposition. The thickness of the obtained films is measured by a scan profiler. In order to limit concentration of hydrogen in a-Si:H films, the specimens are then dehydrogenated by thermal treatment at 500 1C for 2 h for removing excess hydrogen, preventing ablation caused by sudden hydrogen eruption during ELC. After dehydrogenation, the specimens are then held in self-closing tweezers at the end of a cantilever beam fixed on a motorized X–Y linear translation stage (resolution ¼ 0.625 mm). The movement of the focusing lens (focus ¼ 100 mm) fixed on a motorized Z-axis linear translation stage is precisely controlled to adjust the desired laser energy density for crystallization. To enhance the efficiency of laser crystallization, the movement of the three-axis (X–Y–Z) motorized translation stages can accurately be manipulated using the self-developed man–machine interface. A XeF Lambda Physik excimer laser (COMPex 102) operating at 351 nm with 25 ns fullwidth at half-maximum (FWHM) pulse duration is employed for crystallization at room temperature (RT) in ambient air. A Joule meter (Vector H410 SCIENTECH) is employed to calibrate the output energy of excimer laser pulse before ELC. A series of incremental excimer laser energy densities up to 250 mJ/cm2 is employed for laser crystallization. It is estimated that the measured excimer laser energy densities have an error of around 10% because the pulse-to-pulse variation in excimer laser energy is estimated to be 10%. Fig. 1 shows the schematic illustration of backside ELC and specimen structure. The working principle of backside ELC denotes that the a-Si films are being irradiated indirectly with the excimer laser beam passing through first the glass substrate and SiO2 films. A stainless-steel slit mask of around 2 15 mm2 is employed to transform the incident Gaussian beam into a rectangular beam spot. The size of the beam spot can be controlled by changing the working distance between the focusing lens and the specimen surface. After ELC, the microstructural analyses of recrystallized Si films are performed by FE-SEM (Oxford JEOL JSM-6500F), micro-Raman spectroscopy (Renishaw inVia) and HR-TEM (JEOL JEM-2010). Before FE-SEM observation, specimens are treated by Seccoetching (50% HF: H2O: CrO3 ¼ 200 cc: 100 cc: 1.5 g) for highlighting the GBs and intra-grain defects [22]. The surface of the specimen is coated with thin Au films to prevent the charging effect during SEM measurements. The acceleration electron beam energy for FE-SEM and HR-TEM experiments are 15 kV (resolution 1.5 nm) and 200 kV (point resolution 0.23 nm), respectively. Crystallinity of poly-Si films is characterized by micro-Raman spectroscopy. To avoid any specimen being heated before and
613
during micro-Raman scattering measurements, Raman spectra are obtained at RT in back-scattering mode operated at a low power of 3 mW. The system is equipped with an optical microscope for focusing the 514.5 nm (spot size ¼ 5 mm) excitation line from the Ar+ laser beam on the specimens for evaluating fluctuations in crystallinity. The resolution of micro-Raman measurements is 2 cm1. Raman spectra are deconvoluted in the amorphous and crystalline contributions fitting the spectra with a Lorentzian curve to determine the wavenumber of the crystalline peak maximum [23].
3. Results and discussion Three major distinct regrowth regimes (partial melting, nearcomplete melting, and complete melting), which vary primarily with excimer laser energy density and thickness of precursor Si film, have been reported [24,25]. In the partial melting regime, there is an increase in the grain size of poly-Si films with increasing excimer laser energy density. In particular, the grain size of poly-Si films reaches the maximum because of super lateral growth (SLG) in the near-complete melting regime. In the complete melting regime, recrystallization is governed by homogeneous nucleation from a highly undercooled liquid Si, resulting in fine grains distributed randomly throughout the Si films. In order to understand the effects of concentration of hydrogen remaining in the specimen, some specimens are crystallized without a dehydrogenation process. Fig. 2 shows the wavenumber as a function of Raman intensity for specimens with and without dehydrogenation. A broad FWHM of Raman spectrum is observed for the without dehydrogenation specimen because hydrogen released from the films forms microbubbles that can explode, eventually resulting in ablation of films [26]. Therefore, the crystallinity of poly-Si films is low for the specimen without being dehydrogenated. On the other hand, a narrow FWHM of Raman spectrum of approximately 6 cm1 is clearly observed for the dehydrogenated specimen. This result indicates that the crystallinity of poly-Si films can be enhanced by a dehydrogenation process before ELC. The Raman peak at 520 cm–1 obtained from a single crystal wafer is used for reference of all micro-Raman scattering measurements [27]. Fig. 3 shows Raman spectra of poly-Si films crystallized at different laser energy densities. As can be seen, the symmetrical Raman peaks are centered at different wavenumbers of 510–520 cm1, revealing that a-Si films have changed into polySi films after ELC. It is interesting to note that the Raman intensity first increases with increasing excimer energy density from 100 to
16000
Excimer laser
Dehydrogenation
Intensity (arb. units)
14000
Glass
12000
Without dehydrogenation
10000 8000 6000 4000 2000
SiO
300 nm
a-Si
90 nm
Fig. 1. Schematic illustration of backside ELC and specimen structure.
0 500
505
515 520 510 Wavenumber (cm-1)
525
530
Fig. 2. Raman spectra of poly-Si films (with and without dehydrogenation) crystallized by XeF laser pulse. Two samples are crystallized at a laser energy density of 190 mJ/cm2.
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16000 100 mJ/cm2 150 mJ/cm2 175 mJ/cm2 190 mJ/cm2 200 mJ/cm2 225 mJ/cm2
12000 10000 8000
18000 16000 Intensity (arb. units)
Intensity (arb. units)
14000
6000 4000 2000 0 500
14000 12000 10000 8000 6000
505
510 515 520 Wavenumber (cm-1)
525
530
4000 75
100
125 150 175 200 Laser energy density ( mJ/cm2)
75
100
125 150 175 200 Laser energy density (mJ/cm2)
225
75
100
125 150 175 200 Laser energy density (mJ/cm2)
225
Fig. 3. Raman spectra of poly-Si films crystallized at different laser energy densities.
9
o ¼ oc þ 5:2 10 P
(1)
where oc indicates the wavenumber of Raman peak maximum of the single crystal wafer, o indicates the wavenumber of Raman peak maximum of the observed poly-Si Raman spectrum, and P indicates the residual stress in poly-Si films after ELC. The unit of P is Pascal (Pa). The positive value and negative value for P indicate the compressive stress and tensile stress in poly-Si films, respectively. After calculation using this equation, the poly-Si films fabricated by ELC are tensile stressed. This phenomenon is attributed to the difference in thermal expansion coefficient between poly-Si films (7.5 106 K1) and the glass substrate
250
6.9 6.8 FWHM (cm-1)
6.7 6.6 6.5 6.4 6.3 6.2 6.1 6 250
5.5 5.3 5.1 Δω (cm-1)
190 mJ/cm2 and eventually becomes saturated. Conversely, the Raman intensity decreases at excimer laser energy densities exceeding 190 mJ/cm2. Moreover, the FWHM first decreases with increasing laser energy density from 100 to 190 mJ/cm2 and eventually increases. These results are summarized in Fig. 4. Do denotes the deviation in wavenumber of Raman peak maximum between the single crystal wafer (oc) and the observed Raman spectrum (o) extracted under the same micro-Raman measurement conditions. As can be seen, both the FWHM of micro-Raman scattering spectrum and Do reach the minimum level at a laser energy density of 190 mJ/cm2, whereas intensity of micro-Raman scattering spectrum reaches the maximum level at the same laser energy density. The higher the intensity of micro-Raman scattering spectra, the smaller the FWHM, and the smaller the FWHM, the greater the defect (microtwinning and dislocation) [28]. The FWHM for this laser energy density is approximately 6.1 cm1, whereas the FWHM is approximately 4.2 cm1 for a single crystal wafer [29]. Such difference has been interpreted by the grain size effects or attributed to the existing internal strain [30]. The sharp peaks of the micro-Raman spectra are centered at a range of 514–517 cm1. The crystalline volume fraction of poly-Si films is approximately 100% because no transverse-optical photon made of a-Si is found in the spectrum, which indicates that the films are sufficiently crystallized. This result is exceedingly different from the poly-Si films fabricated by SPC [13,14]. Thus, it can be said that ELC is a very powerful technology for producing poly-Si films with high crystallinity by melt–regrowth of silicon films. This is confirmed by the corresponding cross-sectional HR-TEM images, as shown in Fig. 5. As can be seen, the precursor a-Si films are totally transformed into poly-Si films with different grain sizes after ELC at excimer laser energy densities above the surface melting threshold. The Raman spectra obtained are also employed to determine the stress level of poly-Si films. The residual stress in the poly-Si after ELC can be determined by [31]
225
4.9 4.7 4.5 4.3 4.1 3.9 250
Fig. 4. (a), (b), and (c) represent intensity, FWHM, and Xo as a function of laser energy density, respectively. Do denotes the deviation in wavenumber of Raman peak maximum between the single crystal wafer (oc) and the observed Raman spectrum (o) extracted under the same Raman measurement conditions.
(4.2 106 K1) [32]. It should be noted that this phenomenon is opposed to the most common compressive stress in poly-Si films fabricated by low-pressure chemical vapor deposition (LPCVD) [33]. The tensile stress is reduced from 1019 Mpa for laser energy density of 100 mJ/cm2 to 807 Mpa for laser energy density of 190 mJ/cm2. On the other hand, the tensile stress is increased from 807 Mpa for laser energy density of 190 mJ/cm2 to 913 Mpa for laser energy density of 225 mJ/cm2. This result reveals that tensile stress caused by melting and recrystallization of a-Si films is observed in poly-Si films fabricated by ELC. Moreover, tensile stress is also found in poly-Si films annealed by a continuouswave laser [34]. Fig. 6 shows FE-SEM micrograph of Secco-etched poly-Si films irradiated at various excimer laser energy densities. The average
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Fig. 5. Cross-sectional HR-TEM images of excimer laser-crystallized Si thin films at various energy densities. (a) E ¼ 100 mJ/cm2, (b)E ¼ 175 mJ/cm2, (c) E ¼ 190 mJ/cm2. Large-grained poly-Si is formed due to SLG behavior. (d) E ¼ 210 mJ/cm2.
100nm
100nm
100nm
100nm
100nm
100nm
Fig. 6. FE-SEM micrograph of Secco-etched poly-Si films irradiated at various excimer laser energy densities.
grain size of poly-Si films increases with the increase in excimer laser energy density from 100 to 190 mJ/cm2 and decreases at higher excimer laser energy density ranging from 200 to 225 mJ/cm2. As can be seen, the average grain size is in the range of 100–900 nm and the grains are generally elliptical in shape. Notably, a large grain with a diameter of 900 nm formed by SLG is clearly observed in poly-Si films crystallized at an excimer laser energy density of 190 mJ/cm2. This result is in good agreement with the observation in Fig. 4. A distinct phenomenon of minimum level of both FWHM and Do and maximum level of Raman intensity is observed because large grain formed by SLG is known as high quality for single-grain TFTs application [35]. Note that the smaller the FWHM and Do, the greater the crystallinity of poly-Si films. When the excimer laser energy density is increased above 225 mJ/cm2, ablation of Si films is triggered. Thus, crystallized poly-Si films cannot be employed for fabricating LTPS TFTs. Lu et al. [36] confirmed that ablation is mainly caused by explosive boiling or explosive transformation of the superheated liquid into a mixture of particles or droplets and vapor, which are then ejected out of the Si films. Sugawara et al. [37] suggested that lasers operating at a higher wavelength can
overcome this drawback since photons would travel deeper into the thin films compared with the excimer laser. Thus, excimer laser energy density of 250 mJ/cm2 is employed to be the maximum excimer laser energy density used for this work. Agglomeration phenomenon in Si films is also observed after ELC. Agglomeration is the dewetting of the underlying SiO2 by Si films and is easily induced when the interfacial energy between Si films and SiO2 increases. This result is consistent with the observation by He et al. [38]. Thus, avoiding the agglomeration effect of Si films for fabricating high-performance LTPS TFTs by ELC is required. In general, the energy density for SLG has a very narrow process window, typically about 1.5% [39] and 2.5% [40]. A novel optical method for enlarging the process window of phasemodulated ELC has been proposed [41]. Moreover, the process window has been experimentally observed to reach approximately 5% by in-situ time-resolved optical measurements in my previous investigation [42]. These results reported here are believed to give a possible solution for realization of highperformance LTPS TFTs on a large-area glass substrate under the optimum experimental conditions by ELC.
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4. Conclusions The rapid recrystallization of a-Si films utilizing ELC has been investigated. The crystallized poly-Si films have been characterized extensively by Raman spectroscopy for structural investigations. Poly-Si films with higher crystallinity can be obtained by a dehydrogenation process before ELC because the quality of poly-Si is significantly affected by the concentration of hydrogen in the specimen. At an excimer laser energy density of 100–190 mJ/cm2, the crystallinity of poly-Si films increases with the increase in excimer laser energy density. A large grain formed by SLG is clearly observed in poly-Si films crystallized at an excimer laser energy density of 190 mJ/cm2, which is consistent with the observation of minimum level of both FWHM and Do and maximum level of the Raman intensity. Existence of tensile in the poly-Si films has been observed. This result is quite different from the most common compressive stress in poly-Si films fabricated by LPCVD. It is noted that the micro-Raman analysis is an accurate and reliable approach for determining the residual stress and crystallinity of poly-Si films.
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