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Effects of surface treatment on sapphire substrates
ARTICLE IN PRESS
Journal of Crystal Growth 274 (2005) 241–245 www.elsevier.com/locate/jcrysgro
Effects of surface treatment on sapphire substrates Yinzhen Wanga,b, Shiliang Liua,b, Guanliang Penga,b, Shengming Zhoua, Jun Xua, a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Qinghe Road No. 390, Jiading, Shanghai 201800, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 14 July 2004; accepted 19 September 2004 Communicated by M. Roth Available online 30 October 2004
Abstract The influence of mechanical polishing, chemo-mechanical polishing (CMP), as well as CMP and subsequent chemical etching on the properties of sapphire substrate surfaces has been studied. The sapphire substrates have been investigated by means of polarizing microscopy, atomic force microscopy (AFM), X-ray diffraction rocking curves (XRCs) and micro-Raman spectroscopy. The results show that CMP with subsequent chemically etching yields the best-quality sapphire substrate surfaces. r 2004 Elsevier B.V. All rights reserved. PACS: 81.65; 81.65.C; 82.80.C; 61.16.C; 61.10.N Keywords: A1. Substrate; A1. Surface; B1. Sapphire
1. Introduction Sapphire single crystals are widely used in a variety of modern high-technology applications, from commercial and military optical systems to high-power laser components, blue emitting diodes, laser diode devices, visible-infrared windows, substrates for semiconductor devices and Corresponding
author. Tel.: +86 21 69918606; +86 21 69918607. E-mail address: [email protected] (J. Xu).
fax:
other applications because of the advantageous combination of their optical and mechanical properties [1–6]. As an important dielectric substrate material, sapphire single crystal is used for many device purposes, such as SOS technology [7,8] and GaN heteroepitaxy [9,10]. In many of these applications, stringent surface quality requirements, i.e. surface finish and flatness, are required. The generation of high-quality surfaces with fine surface finish and low surface and subsurface damage is of critical importance. It has been established
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.09.074
ARTICLE IN PRESS 242
Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
that the crystal structure of epitaxial films is strongly influenced not only by the substrate material and its orientation, but to a great extent also by the surface preparation of the substrate. The use of polishing techniques, such as chemomechanical polishing (CMP), may produce high quality surface finishes at low cost and with fast material-removal rates. CMP has been widely and effectively applied to a variety of materials during the last two decades. In the semiconductor industry, CMP is currently the most popular method for wafer planarization including oxides (such as silica, sapphire), silicon, copper, polymers, etc. Planarized surfaces have become a key to the advanced processing of semiconductor devices/circuits/chips. Therefore, continuing studies of sapphire surfaces and interfaces are essential. In this work, we report on the effect of surface treatment on the properties of sapphire substrates.
2. Experimental procedure In this study, a c-plane (0 0 0 1)-oriented sapphire boule was grown by the temperature gradient technique (TGT). The raw material was high-purity 99.99% Al2O3 power. The sapphire boule appeared colorless and transparent after annealing. The (0 0 0 1)-oriented sapphire substrates obtained from the central region of the boule were cut perpendicularly to the growth
direction and treated in three different ways in order to carry out studies in (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching. The substrates were colorless, transparent, free of pores, bubbles and grains. Chemical etching was performed using a hot H2SO4:H3PO4 (3:1) solution. The substrates were then rinsed for 5 min each in acetone and alcohol. The above steps were repeated three times to complete the degreasing process. The samples were characterized by polarizing microscopy, atomic force microscopy (AFM) (Digital Instrument Nanoscope IIIa), X-ray diffraction rocking curves (XRCs) (PHILIPS X’PertMRD) and micro-Raman spectroscopy (SPEX 1877 Raman spectrometer).
3. Results and discussion Fig. 1 shows the results of surface microscopy of variously treated samples. Image 1(a) taken after mechanical polishing gives a clear indication of scratches. Mechanical polishing alone, theoretically, may achieve the planarization, but it is not desirable because of the extensive associated damage of material surfaces. CMP surfaces (Fig. 1(b)) show no indications of scratches and are smooth, and the CMP technique can provide planarization without damaging the material surfaces. In the case of CMP with subsequent chemical etching (Fig. 1(c)), the surface contains
Fig. 1. Surface images of sapphire substrate (microscope magnification 50 ) after: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
ARTICLE IN PRESS Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
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Fig. 2. AFM images of sapphire substrate surfaces after: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
no etch pits; it is even smoother and more lucent than in the case of mere CMP. Fig. 2 shows AFM images of the various sample surfaces. Similarly to the optical microscopy results, it is apparent that there are scratches on the sample surface after mechanical polishing (Fig. 2(a)). There are no scratches on the wafer surface after CMP treatment, and the root-mean square (RMS) roughness decreases (Fig. 2(b)). The sample surface is significantly smoother and the surface roughness value is only 0.230 nm RMS when treated by CMP with subsequent chemical etching (Fig. 2(c)). The above results suggest that sapphire substrates subjected to the latter surface treatment exhibit a very smooth morphology and are of highest quality. Fig. 3 shows the corresponding X-ray rocking curves, which clearly indicate that the substrate surface crystallinity has been also improved substantially using the CMP and subsequent chemical etching treatment. The XRC full-width at half-maxima (FWHM) of samples subjected to mechanical polishing, CMP and CMP with sub-
sequent chemical etching are 30.2, 20.3 and 11.2 arcsec, respectively. The sequential narrowing of the rocking curve width is expected, and it indicates furthermore the improvement of the substrate surface quality. Fig. 4 shows the Raman scattering spectra taken in a Z(YY)-Z orientation of the differently treated samples surfaces. The Raman spectra were acquired in a strict 1801 backscattering geometry using an Ar+ laser source (wavelength 514.5 nm) at room temperature. The laser power supplied through the objective lens was estimated to be less than 2.0 mW incident on the sample into a focused spot about 2.5 mm in diameter within 0.1 mm depth. The spectrum 4(c) corresponding to the surface treated by CMP with subsequent chemical etching contains seven bands located at 376.049 (Eg(ext)), 413.671 (A1g), 428.1947 (Eg(ext)), 440.2804 (Eg(int)), 573.384(Eg(int)), 642.1803(A1g) and 747.9213 cm 1 (Eg(int)), which is in good agreement with the spectrum of the natural crystal [11], and the reflected Raman scattering signal is strong. The spectrum 4(b) of the CMP-treated
ARTICLE IN PRESS Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
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Fig. 4. Raman scattering spectra of the sapphire substrate surfaces with different surface treatments: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
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Fig. 3. X-ray rocking curves of sapphire substrate after: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
sample contains only five bands, and their intensity is lower in comparison with spectrum 4(c). The Raman spectrum obtained from the surface subjected to mechanical polishing alone shows only two bands, and their intensity decreases further, which indicates that the surface scattering signal is weak. In the spectra 4(b) and 4(a), some vibrational modes disappear depending on the crystal surface morphology, which indicates that in the thin damaged surface layer some chemical bonds are destroyed. It is noteworthy that the Raman scattering spectra taken in the middle depth of all samples are similar to spectrum 4(c). These results suggest that the stronger the Raman scattering signal is from the substrate surface and the more complete the Raman spectrum, the higher the quality of the sapphire substrate surface is. 4. Conclusion Based on the results of polarized optical microscopy, sapphire wafers treated by CMP with subsequent chemical etching exhibit a significantly better surface smoothness as compared to wafers treated by mechanical polishing or CMP alone.
ARTICLE IN PRESS Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
Also, the lowest surface roughness of 0.230 nm RMS and the narrowest FWHM of 11.2 arcsec have been measured for such wafers by the AFM and XRD rocking curves methods, respectively. The strongest Raman scattering signal and a complete Raman spectrum have been obtained only from surfaces treated by CMP with subsequent chemical etching. The latter surface treatment is thus suggested as an improved sapphire substrate preparation technique. References [1] F. Schmid, C.P. Khattak, D.M. Felt, Am. Ceram. Soc. Bull. 73 (1994) 39.
245
[2] M.B. Smith, K. Schmid, F. Schmid, C.P. Khattak, J.C. Lambropoulos, Proc. SPIE 3705 (1999) 85. [3] O. Ambacher, J. Phys. D 31 (1998) 2653. [4] J.W. Orton, C.T. Foxon, Rep. Prog. Phys. 61 (1998) 1. [5] S. Nakamura, Ann. Rev. Mater. Sci. 28 (1998) 125. [6] J.L. Li, S.R. Nutt, K.W. Kirby, J. Am Ceram. Soc. 82 (1999) 3260. [7] G.W. Cullen, J. Crystal Growth 9 (1971) 107. [8] S. Lam, W.K. Lee, A.C.K. Chan, P.K.T. Mok, P.K. Ko, M. Chan, Jpn. J. Appl. Phys. 43 (2004) 2176. [9] J.J. Nickl, W. Just, R. Bertinger, Mater. Res. Bull. 9 (1974) 1413. [10] B. Beaumont, P. Vennegues, P. Gibart, Phys. Stat. Sol. (b) 227 (2001) 1. [11] C. Karr Jr., Infrared and Raman Spectroscopy of Lunar and Terrestrial Minerals, Academic Press, New York, 1975.
Journal of Crystal Growth 274 (2005) 241–245 www.elsevier.com/locate/jcrysgro
Effects of surface treatment on sapphire substrates Yinzhen Wanga,b, Shiliang Liua,b, Guanliang Penga,b, Shengming Zhoua, Jun Xua, a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Qinghe Road No. 390, Jiading, Shanghai 201800, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 14 July 2004; accepted 19 September 2004 Communicated by M. Roth Available online 30 October 2004
Abstract The influence of mechanical polishing, chemo-mechanical polishing (CMP), as well as CMP and subsequent chemical etching on the properties of sapphire substrate surfaces has been studied. The sapphire substrates have been investigated by means of polarizing microscopy, atomic force microscopy (AFM), X-ray diffraction rocking curves (XRCs) and micro-Raman spectroscopy. The results show that CMP with subsequent chemically etching yields the best-quality sapphire substrate surfaces. r 2004 Elsevier B.V. All rights reserved. PACS: 81.65; 81.65.C; 82.80.C; 61.16.C; 61.10.N Keywords: A1. Substrate; A1. Surface; B1. Sapphire
1. Introduction Sapphire single crystals are widely used in a variety of modern high-technology applications, from commercial and military optical systems to high-power laser components, blue emitting diodes, laser diode devices, visible-infrared windows, substrates for semiconductor devices and Corresponding
author. Tel.: +86 21 69918606; +86 21 69918607. E-mail address: [email protected] (J. Xu).
fax:
other applications because of the advantageous combination of their optical and mechanical properties [1–6]. As an important dielectric substrate material, sapphire single crystal is used for many device purposes, such as SOS technology [7,8] and GaN heteroepitaxy [9,10]. In many of these applications, stringent surface quality requirements, i.e. surface finish and flatness, are required. The generation of high-quality surfaces with fine surface finish and low surface and subsurface damage is of critical importance. It has been established
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.09.074
ARTICLE IN PRESS 242
Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
that the crystal structure of epitaxial films is strongly influenced not only by the substrate material and its orientation, but to a great extent also by the surface preparation of the substrate. The use of polishing techniques, such as chemomechanical polishing (CMP), may produce high quality surface finishes at low cost and with fast material-removal rates. CMP has been widely and effectively applied to a variety of materials during the last two decades. In the semiconductor industry, CMP is currently the most popular method for wafer planarization including oxides (such as silica, sapphire), silicon, copper, polymers, etc. Planarized surfaces have become a key to the advanced processing of semiconductor devices/circuits/chips. Therefore, continuing studies of sapphire surfaces and interfaces are essential. In this work, we report on the effect of surface treatment on the properties of sapphire substrates.
2. Experimental procedure In this study, a c-plane (0 0 0 1)-oriented sapphire boule was grown by the temperature gradient technique (TGT). The raw material was high-purity 99.99% Al2O3 power. The sapphire boule appeared colorless and transparent after annealing. The (0 0 0 1)-oriented sapphire substrates obtained from the central region of the boule were cut perpendicularly to the growth
direction and treated in three different ways in order to carry out studies in (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching. The substrates were colorless, transparent, free of pores, bubbles and grains. Chemical etching was performed using a hot H2SO4:H3PO4 (3:1) solution. The substrates were then rinsed for 5 min each in acetone and alcohol. The above steps were repeated three times to complete the degreasing process. The samples were characterized by polarizing microscopy, atomic force microscopy (AFM) (Digital Instrument Nanoscope IIIa), X-ray diffraction rocking curves (XRCs) (PHILIPS X’PertMRD) and micro-Raman spectroscopy (SPEX 1877 Raman spectrometer).
3. Results and discussion Fig. 1 shows the results of surface microscopy of variously treated samples. Image 1(a) taken after mechanical polishing gives a clear indication of scratches. Mechanical polishing alone, theoretically, may achieve the planarization, but it is not desirable because of the extensive associated damage of material surfaces. CMP surfaces (Fig. 1(b)) show no indications of scratches and are smooth, and the CMP technique can provide planarization without damaging the material surfaces. In the case of CMP with subsequent chemical etching (Fig. 1(c)), the surface contains
Fig. 1. Surface images of sapphire substrate (microscope magnification 50 ) after: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
ARTICLE IN PRESS Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
243
Fig. 2. AFM images of sapphire substrate surfaces after: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
no etch pits; it is even smoother and more lucent than in the case of mere CMP. Fig. 2 shows AFM images of the various sample surfaces. Similarly to the optical microscopy results, it is apparent that there are scratches on the sample surface after mechanical polishing (Fig. 2(a)). There are no scratches on the wafer surface after CMP treatment, and the root-mean square (RMS) roughness decreases (Fig. 2(b)). The sample surface is significantly smoother and the surface roughness value is only 0.230 nm RMS when treated by CMP with subsequent chemical etching (Fig. 2(c)). The above results suggest that sapphire substrates subjected to the latter surface treatment exhibit a very smooth morphology and are of highest quality. Fig. 3 shows the corresponding X-ray rocking curves, which clearly indicate that the substrate surface crystallinity has been also improved substantially using the CMP and subsequent chemical etching treatment. The XRC full-width at half-maxima (FWHM) of samples subjected to mechanical polishing, CMP and CMP with sub-
sequent chemical etching are 30.2, 20.3 and 11.2 arcsec, respectively. The sequential narrowing of the rocking curve width is expected, and it indicates furthermore the improvement of the substrate surface quality. Fig. 4 shows the Raman scattering spectra taken in a Z(YY)-Z orientation of the differently treated samples surfaces. The Raman spectra were acquired in a strict 1801 backscattering geometry using an Ar+ laser source (wavelength 514.5 nm) at room temperature. The laser power supplied through the objective lens was estimated to be less than 2.0 mW incident on the sample into a focused spot about 2.5 mm in diameter within 0.1 mm depth. The spectrum 4(c) corresponding to the surface treated by CMP with subsequent chemical etching contains seven bands located at 376.049 (Eg(ext)), 413.671 (A1g), 428.1947 (Eg(ext)), 440.2804 (Eg(int)), 573.384(Eg(int)), 642.1803(A1g) and 747.9213 cm 1 (Eg(int)), which is in good agreement with the spectrum of the natural crystal [11], and the reflected Raman scattering signal is strong. The spectrum 4(b) of the CMP-treated
ARTICLE IN PRESS Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
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Fig. 4. Raman scattering spectra of the sapphire substrate surfaces with different surface treatments: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
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Fig. 3. X-ray rocking curves of sapphire substrate after: (a) mechanical polishing, (b) CMP and (c) CMP with subsequent chemical etching.
sample contains only five bands, and their intensity is lower in comparison with spectrum 4(c). The Raman spectrum obtained from the surface subjected to mechanical polishing alone shows only two bands, and their intensity decreases further, which indicates that the surface scattering signal is weak. In the spectra 4(b) and 4(a), some vibrational modes disappear depending on the crystal surface morphology, which indicates that in the thin damaged surface layer some chemical bonds are destroyed. It is noteworthy that the Raman scattering spectra taken in the middle depth of all samples are similar to spectrum 4(c). These results suggest that the stronger the Raman scattering signal is from the substrate surface and the more complete the Raman spectrum, the higher the quality of the sapphire substrate surface is. 4. Conclusion Based on the results of polarized optical microscopy, sapphire wafers treated by CMP with subsequent chemical etching exhibit a significantly better surface smoothness as compared to wafers treated by mechanical polishing or CMP alone.
ARTICLE IN PRESS Y. Wang et al. / Journal of Crystal Growth 274 (2005) 241–245
Also, the lowest surface roughness of 0.230 nm RMS and the narrowest FWHM of 11.2 arcsec have been measured for such wafers by the AFM and XRD rocking curves methods, respectively. The strongest Raman scattering signal and a complete Raman spectrum have been obtained only from surfaces treated by CMP with subsequent chemical etching. The latter surface treatment is thus suggested as an improved sapphire substrate preparation technique. References [1] F. Schmid, C.P. Khattak, D.M. Felt, Am. Ceram. Soc. Bull. 73 (1994) 39.
245
[2] M.B. Smith, K. Schmid, F. Schmid, C.P. Khattak, J.C. Lambropoulos, Proc. SPIE 3705 (1999) 85. [3] O. Ambacher, J. Phys. D 31 (1998) 2653. [4] J.W. Orton, C.T. Foxon, Rep. Prog. Phys. 61 (1998) 1. [5] S. Nakamura, Ann. Rev. Mater. Sci. 28 (1998) 125. [6] J.L. Li, S.R. Nutt, K.W. Kirby, J. Am Ceram. Soc. 82 (1999) 3260. [7] G.W. Cullen, J. Crystal Growth 9 (1971) 107. [8] S. Lam, W.K. Lee, A.C.K. Chan, P.K.T. Mok, P.K. Ko, M. Chan, Jpn. J. Appl. Phys. 43 (2004) 2176. [9] J.J. Nickl, W. Just, R. Bertinger, Mater. Res. Bull. 9 (1974) 1413. [10] B. Beaumont, P. Vennegues, P. Gibart, Phys. Stat. Sol. (b) 227 (2001) 1. [11] C. Karr Jr., Infrared and Raman Spectroscopy of Lunar and Terrestrial Minerals, Academic Press, New York, 1975.