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Ultraprecision CMP for sapphire, GaN, and SiC for advanced optoelectronics materials
Current Applied Physics 12 (2012) S41eS46
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Ultraprecision CMP for sapphire, GaN, and SiC for advanced optoelectronics materials Hideo Aida a, *, Toshiro Doi b, Hidetoshi Takeda a, Haruji Katakura a, Seong-Woo Kim a, Koji Koyama a, Tsutomu Yamazaki b, Michio Uneda b a b
Namiki Precision Jewel Co., Ltd., 8-22, Shinden 3-chome, Adachi-ku, Tokyo 123-8511, Japan Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2011 Accepted 2 February 2012 Available online 20 February 2012
Chemical mechanical polishing (CMP) of sapphire, GaN, and SiC substrates, which are categorized as hard-to-process materials, is demonstrated with a colloidal silica slurry under acidic and alkaline slurry pH conditions. Atomic level surface flatness was achieved by CMP and was confirmed to be equivalent to an almost ideally minimized surface roughness. By comparing the Preston coefficients under different slurry conditions, differences in the CMP properties among the three substrate materials and difficulties in the CMP of the GaN and SiC substrates are presented. The difference in CMP properties between the (0001) and (000-1) planes of GaN and SiC due to their non-revers crystallographical symmetry is also presented. Oxidation processes that occur during CMP of GaN and SiC are also discussed. By comparing the removal rate among GaN, SiC, and their oxides, it was found that the rate-limiting step in the total CMP process for GaN and SiC was surface oxidation reaction of GaN and SiC. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Sapphire Gallium nitride Silicon carbide Chemical mechanical polishing Colloidal silica
1. Introduction A large amount of attention has been given recently to next generation optoelectronics devices, for which single crystal substrates such as sapphire, GaN, and SiC are likely to be used as key materials. Sapphire substrates are well-known as they are the most common substrate for fabricating III-nitride devices [1,2]. IIInitride devices fabricated on sapphire substrates have been applied, for example, to light emitting diodes (LEDs) for the light sources of liquid crystal displays (LCDs), and thus, a further expansion of the market volume is expected for sapphire substrates along with the rapidly growing III-nitride device market. On the other hand, some III-nitride applications, such as laser diodes (LDs) used as Blu-ray Disc (BD) light sources and high efficiency LEDs for general lighting systems, require lower dislocation densities for the epitaxial device films. Due to limited device performance of devices fabricated on sapphire substrates caused by a large number of crystal defects generated by the difference in thermal expansion and lattice mismatch, the realization of mass production of single crystal GaN substrates is highly desirable [3,4]. In addition to GaN substrates, SiC substrates have also recently attracted much attention as a next generation key material because * Corresponding author. Tel.: þ81 3 5390 7875; fax: þ81 3 5390 7624. E-mail address: [email protected] (H. Aida). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.02.016
they are considered to be a promising candidate for electronic devices that use transistors which can be operated in high-power, high-frequency regions that are currently inaccessible with conventional silicon-based transistor devices [5,6]. In most cases, c-plane (0001) of sapphire, GaN, and SiC substrates is used for device epitaxy. To use these crystal materials as substrates for device fabrication, the as-grown bulk single crystals have to undergo a wafering process to form a wafer shape. As it is essential to provide a complete damage-free surface on which the device is fabricated, chemical mechanical polishing (CMP) is applied to the surface as a final treatment to produce atomic level surface flatness by removing the damage on or near the surface due to mechanical polishing. Sapphire, GaN, and SiC are categorized as hard-to-process materials due to their extreme hardness and strong stability against chemicals. Among the hardto-process materials, sapphire substrates are already produced in market volumes, and the sapphire wafering process is regarded as mature technology; thus, difficulties in the mechanical processing due to its hardness are considered to be resolved. There are also many studies dealing with CMP with a colloidal silica, which is widely used for sapphire, and the sapphire removal mechanism is also well understood [7,8]. Consequently, the CMP of sapphire exhibits a removal rate as high as several micrometers per hour, and therefore, the strong stability of sapphire crystal against chemicals is no longer a pressing problem in the CMP process.
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Table 1 CMP conditions for the sapphire, GaN, and SiC substrates. Platen rotation speed Carrier rotation speed Applied pressure Diameter of platen Polishing pad type Slurry type Slurry pH Abrasive particle size Abrasive concentration
rpm rpm kg/cm2 mm Suede type Colloidal silica 1.8, 10.5 nm %
50 25e80 0.15e0.4 300
40 20
The devoted efforts and experiences of previous researchers in the development of the mechanical processing of sapphire can be applied to the mechanical processing of GaN and SiC. However, the removal rate of GaN and SiC in CMP with a colloidal silica exhibits a range of ten to a hundred nanometers per hour due to their strong chemical inertness [9e12]. Assuming that the depth of the damaged layer to be removed by CMP and the removal rate are 1.5 mm and 50 nm/h, respectively, the processing time will be 30 h, which means this is not a practical method for commercial production. Therefore, understanding the mechanism involved in the CMP of GaN and SiC is important research in order to improve the CMP removal rate for these materials, which will lead to an acceleration in the realization of next generation optoelectronics devices on GaN and SiC substrates. In this study, we demonstrate CMP of GaN and SiC substrates with a colloidal silica slurry by comparison with the CMP performance for sapphire. First, a theoretical surface model is discussed to present evidence of an ideally minimized surface roughness that is manifest for the sapphire, GaN, and SiC substrates after CMP with a colloidal silica slurry. Then, the difference in the removal properties among the sapphire, GaN, and SiC substrates due to the slurry pH is investigated using acidic- and alkaline-based colloidal silica slurries. The difference in CMP properties between the (0001) and (000-1) planes of GaN and SiC due to their non-revers crystallographical symmetry is also presented. The difficulties in the CMP of GaN and SiC as compared to sapphire are presented together with evidence that the oxidation reaction is the rate-limiting step for GaN and SiC. Different oxidation processes between GaN and SiC under the chemical atmosphere provided by the slurries is also suggested in this study.
acidic solution (a mixture of H2SO4 and H2O2). The slurry pHs were 10.5 and 1.8 for the alkaline and acidic-colloidal silica slurries, respectively. c-plane substrates for sapphire, GaN, and SiC were used in this study. In the case where the front side of the substrate is the (0001) plane, the back side of the substrate is the (000-1) plane. The sapphire has a reverse crystallographical symmetry, whereas GaN and SiC crystals have a non-revers crystallographical symmetry. Thus the (0001) and (000-1) planes are not crystallographically equivalent for GaN and SiC. Ga- and Si-face is the (0001) plane for GaN and SiC substrates, respectively. Similarly, N- and C-face is the (000-1) plane for GaN and SiC, respectively. In this study, the CMP of both (0001) and (000-1) planes were examined for GaN and SiC substrates. 2-inch diameter as-lapped sapphire, GaN, and SiC substrates were pretreated with two steps of diamond polishing, diamond abrasive sizes of 2 and 0.5 mm, which was then followed by polishing with the colloidal silica under the CMP conditions summarized in Table 1. The removal rates of sapphire, GaN, and SiC were determined by weight loss. The surface quality was characterized by an atomic force microscope (AFM). 3. Results and discussion Fig. 1 shows AFM images of the sapphire, Ga-faced GaN, and Sifaced SiC substrate surfaces after CMP under conditions of 10.5, 0.4 kg/cm2, and 50 rpm for the slurry pH, downward pressure, and platen rotation speed, respectively. The removal rate was 2.2 mm/h, 17 nm/h, and 64 nm/h for sapphire, GaN, and SiC, respectively, and it should be noted that the removal rate of GaN and SiC is extremely low compared with that of sapphire. The surface roughness values after CMP for sapphire, GaN, and SiC are 0.08, 0.18, and 0.10 nm, respectively. It should be noted that the pits are seen in the AFM image of GaN surface as show in Fig. 1(b). They are threading dislocations created during the crystal growth of GaN and became visible during the CMP as the dislocations are opened by chemical etching effect during the CMP. The average surface roughness Ra is calculated by following expression under the definition of the average surface roughness illustrated in Fig. 2(a);
Ra ¼
1 L
ZL jf ðxÞjdx
(1)
0
2. Experimental Acidic- and alkaline-based colloidal silica slurries for CMP were prepared. The alkaline-based colloidal silica slurry was from a commercial source, while the acidic-based colloidal silica slurry was made by mixing the alkaline-based colloidal silica with an
If the surface is perfectly flat without any crystal misorientation, Ra will be zero. However, the sapphire, GaN, and SiC substrates used for epitaxy normally have a slight misorientation in that the actual surface is slightly miscut from the crystal c-plane. For example, a misorientation angle of around 0.15e0.20 is common
Fig. 1. AFM images of the (a) Sapphire, (b) Ga-faced GaN, and (c) Si-faced SiC substrates.
H. Aida et al. / Current Applied Physics 12 (2012) S41eS46
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Fig. 2. (a) Definition of the surface roughness Ra, and (b) The surface model of a one bilayer step-and-terrace structure. The symbols w, h, and q are the terrace width, step height, and misorientation angle, respectively. Fig. 3. Theoretically calculated Ra for the sapphire, GaN, and SiC substrates with ideal one bilayer step-and-terrace structures as a function of the misorientation angle.
in III-nitride epitaxy on sapphire substrates [13e15]. In this case, the surface model is a step-and-terrace structure due to the misorientation. When the surface, consisting of a one bilayer structure corresponding to the periodical step-and-terrace structure, is modeled as shown in Fig. 2(b), the relationship between the misorientation angle q, terrace width w, and step height h is given by the following equation:
tanðqÞ ¼
h w
(2)
where h corresponds to the height of one bilayer and is 0.216, 0.259, and 0.250 nm for sapphire, GaN, and SiC, respectively. Therefore, the substrate Ra for each crystal is calculated theoretically as a function of misorientation angle and one bilayer height as follows;
1 Ra ¼ L
ZL jf ðxÞj dx ¼ 0
1 h h$cosq 2 ¼ $n$ w$ 2 4 n$w=cosq
GaN, and SiC as a function of downward pressure and platen rotation speed, respectively, under different pH slurry conditions. For all substrates, the removal rate exhibited a linear relationship with downward pressure and platen rotation speed. It should be noted that different relationships between the CMP removal rate and the slurry pH were seen among sapphire, GaN, and SiC: a high removal rate is obtained with an acidic slurry for the GaN substrate but with the alkaline slurry for the sapphire and SiC substrates. In general, the stock removal V of the material is expressed by Preston’s equation [16]:
V ¼ k$p$v$t
(4)
where, k is a system dependent parameter, p is the downward pressure, v is the relative polishing speed between the polishing plate and work piece, and t is the process time. In terms of the material removal rate (MRR), Eq. (4) can be modified as:
(3)
where, n is the number of step-and-terrace structures within the measuring surface length L. Fig. 3 plots the calculated Ra as a function of misorientation angle for the sapphire, GaN, and SiC substrates using Eq. (3). The theoretical Ra for the ideal one bilayer step-and-terrace structure is almost constant as the misorientation angle is negligibly small, and the theoretical Ra values are 0.054, 0.065, and 0.062 nm for sapphire, GaN, and SiC, respectively. As presented in Fig. 1, the actual surface roughness achieved after CMP is comparable to the theoretical Ra value for sapphire and SiC, indicating that the surface consists almost only of one bilayer step-and-terrace structures. The actual Ra value of GaN exhibits slightly higher than that of theoretical one. This is caused by the pits created on the surface due to the etching effect of dislocations. A line profile which is escaping the pits shows Ra value as low as 0.10 nm as shown in Fig. 4. Therefore, we consider that ideally minimized surface roughness surfaces are obtained by CMP with colloidal silica for the sapphire, Ga-face of GaN, and Si-face of SiC substrates. The difference in the removal properties during CMP among sapphire, Ga-face of GaN, and Si-face of SiC substrates was investigated in detail. Figs. 5 and 6 show the removal rate of sapphire,
MRR ¼ V=t ¼ k$p$v
(5)
CMP of sapphire, GaN, and SiC is shown to obey Prestonian behavior. When the relative velocity and chemical environment remain unchanged, the removal rate is simply a function of downward pressure. Similarly, when the downward pressure and chemical environment remain unchanged, the removal rate is a function of the relative velocity. Therefore, for each pH condition, k is summarized in Table 2 for a downward pressure of 0.4 kg/cm2 and for a platen rotation speed of 50 rpm. As shown in Table 2, for sapphire and SiC, the alkaline slurry exhibits a k around 2.5 times
Fig. 4. Line profile of Ga-faced GaN surface scanned by AFM.
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Fig. 5. Removal rate as a function of downward pressure for (a) Sapphire, (b) Ga-faced GaN, and (c) Si-faced SiC.
larger than that for the acidic slurry. On the other hand, for GaN, the acidic slurry exhibits a k that is around 3 times larger than that for the alkaline slurry. The k values for GaN and SiC are w2 orders of magnitude smaller than those of sapphire, showing the extreme difficulty in CMP for these two materials. This means that the change in the removal rate gained by changing the downward pressure or relative polishing speed is much lower for GaN and SiC than for sapphire, indicating the limitations in improving the removal rate by increasing the downward pressure and linear velocity during CMP. Comparing the k values for GaN and SiC, CMP of GaN is around 1.5 times more difficult than CMP of SiC even if suitable pH conditions are selected for these materials. Another important aspect to be discussed is asymmetric crystallographical properties of GaN and SiC. Figs. 7 and 8 show the removal rate of N-face of GaN and C-face of SiC as a function of
downward pressure and platen rotation speed, respectively, under different pH slurry conditions. As compared to the (0001) face, higher removal rates were observed for (000-1) plane of GaN and SiC. The difference in the removal rate between (0001) and (000-1) plane is much larger for GaN than SiC substrate as it is reported that the Ga- and N-faces show totally different chemical etching properties [9,10,17e20]. In addition, the (000-1) faced GaN and SiC substrates exhibit higher removal rates with alkaline- and acidiccolloidal silica slurry, respectively, which is completely opposite to the CMP properties obtained for (0001) faced GaN and SiC. These results suggest the totally different CMP mechanism between GaN and SiC. However, interestingly, although nitrides and carbides are different, it has been reported that the CMP of both GaN and SiC proceeds via a surface oxidation reaction [21e25]. For example,
Fig. 6. Removal rate as a function of platen rotation speed for (a) Sapphire, (b) Ga-faced GaN, and (c) Si-faced SiC.
H. Aida et al. / Current Applied Physics 12 (2012) S41eS46
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Table 2 Preston’s coefficient k as a function of slurry pH.
Sapphire GaN (0001) SiC (0001)
pH
k at 50 rpm ( 1011 cm2/kg)
k at 0.4 kg/cm2 (1/rpm)
1.8 10.5 1.8 10.5 1.8 10.5
56.7 162.4 3.6 1.2 2.5 5.9
27.8 59.4 2.0 0.7 1.4 3.2
GaN is reported to be polished by the surface oxidation mechanism of forming Ga2O3 followed by the dissolution of Ga2O3 into the chemical solution. Similarly, in the case of SiC, SiO2 is formed during CMP. To discuss the CMP removal rate of the oxide layer for each material, bulk Ga2O3 [26] and SiO2 were polished under the same CMP conditions. Table 3 summarizes the removal rates of Ga2O3 and SiO2 as a function of the slurry pH. A removal rate of several microns per hour was obtained for both Ga2O3 and SiO2, showing the two orders of magnitude difference in the removal rates from GaN and SiC. This indicates that the oxidation process is the ratelimiting step in the CMP processing of GaN and SiC. Therefore, this suggests the importance of developing a novel oxidation approach during CMP to increase the removal rate for GaN and SiC, such as a photoenhanced wet etching [21,22,27e30], catalystreferred etching (CARE) method [21e25], and atmospherecontrolled CMP with closed bell-jar CMP machine [31e34] etc. In addition, the observed difference in the trend of the removal rate against the slurry pH between GaN and SiC suggests different oxidation mechanisms between the substrate material and silica particles under the chemical solution of the slurry. Although it is unclear why the Ga-face of GaN removal rate is higher under acidic conditions and that of Si-face of SiC is higher under alkaline conditions, which is also totally opposite from the CMP property of the (000-1) plane, further study to understand these phenomena may provide us with valuable information to help solve the difficulties of CMP of GaN and SiC.
Fig. 8. Removal rate as a function of platen rotation speed for (a) N-faced GaN and (b) C-faced SiC.
Table 3 CMP removal rate for Ga2O3 and SiO2 with 20% colloidal silica under the conditions of 0.4 kg/cm2 and 50 rpm for the applied downward pressure and platen rotation speed, respectively.
Ga2O3 SiO2
pH
Removal rate (mm/h)
1.8 10.5 1.8 10.5
8.0 5.6 7.1 5.8
4. Conclusion We have demonstrated CMP of sapphire, GaN, and SiC substrates with a colloidal silica slurry under different slurry pH conditions. Atomic level surface flatness was achieved with CMP. By discussing a theoretical surface model, evidence of an ideally minimized surface roughness for the sapphire, GaN, and SiC substrates after CMP was presented. By comparing the Preston coefficients as a function of the slurry pH for sapphire, GaN, and SiC, difficulties in the CMP of GaN and SiC as compared to sapphire were understood. An interesting trend in the CMP properties against the slurry pH was observed: sapphire and SiC exhibit a larger k under alkaline slurry conditions, whereas GaN exhibits a higher k under acidic slurry conditions. The removal rate of Ga2O3 and SiO2 were two orders of magnitude higher than that of GaN and SiC, indicating that the oxidation process is the rate-limiting step in the total CMP processing time of GaN and SiC. References
Fig. 7. Removal rate as a function of downward pressure for (a) N-faced GaN and (b) Cfaced SiC.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Ultraprecision CMP for sapphire, GaN, and SiC for advanced optoelectronics materials Hideo Aida a, *, Toshiro Doi b, Hidetoshi Takeda a, Haruji Katakura a, Seong-Woo Kim a, Koji Koyama a, Tsutomu Yamazaki b, Michio Uneda b a b
Namiki Precision Jewel Co., Ltd., 8-22, Shinden 3-chome, Adachi-ku, Tokyo 123-8511, Japan Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2011 Accepted 2 February 2012 Available online 20 February 2012
Chemical mechanical polishing (CMP) of sapphire, GaN, and SiC substrates, which are categorized as hard-to-process materials, is demonstrated with a colloidal silica slurry under acidic and alkaline slurry pH conditions. Atomic level surface flatness was achieved by CMP and was confirmed to be equivalent to an almost ideally minimized surface roughness. By comparing the Preston coefficients under different slurry conditions, differences in the CMP properties among the three substrate materials and difficulties in the CMP of the GaN and SiC substrates are presented. The difference in CMP properties between the (0001) and (000-1) planes of GaN and SiC due to their non-revers crystallographical symmetry is also presented. Oxidation processes that occur during CMP of GaN and SiC are also discussed. By comparing the removal rate among GaN, SiC, and their oxides, it was found that the rate-limiting step in the total CMP process for GaN and SiC was surface oxidation reaction of GaN and SiC. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Sapphire Gallium nitride Silicon carbide Chemical mechanical polishing Colloidal silica
1. Introduction A large amount of attention has been given recently to next generation optoelectronics devices, for which single crystal substrates such as sapphire, GaN, and SiC are likely to be used as key materials. Sapphire substrates are well-known as they are the most common substrate for fabricating III-nitride devices [1,2]. IIInitride devices fabricated on sapphire substrates have been applied, for example, to light emitting diodes (LEDs) for the light sources of liquid crystal displays (LCDs), and thus, a further expansion of the market volume is expected for sapphire substrates along with the rapidly growing III-nitride device market. On the other hand, some III-nitride applications, such as laser diodes (LDs) used as Blu-ray Disc (BD) light sources and high efficiency LEDs for general lighting systems, require lower dislocation densities for the epitaxial device films. Due to limited device performance of devices fabricated on sapphire substrates caused by a large number of crystal defects generated by the difference in thermal expansion and lattice mismatch, the realization of mass production of single crystal GaN substrates is highly desirable [3,4]. In addition to GaN substrates, SiC substrates have also recently attracted much attention as a next generation key material because * Corresponding author. Tel.: þ81 3 5390 7875; fax: þ81 3 5390 7624. E-mail address: [email protected] (H. Aida). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.02.016
they are considered to be a promising candidate for electronic devices that use transistors which can be operated in high-power, high-frequency regions that are currently inaccessible with conventional silicon-based transistor devices [5,6]. In most cases, c-plane (0001) of sapphire, GaN, and SiC substrates is used for device epitaxy. To use these crystal materials as substrates for device fabrication, the as-grown bulk single crystals have to undergo a wafering process to form a wafer shape. As it is essential to provide a complete damage-free surface on which the device is fabricated, chemical mechanical polishing (CMP) is applied to the surface as a final treatment to produce atomic level surface flatness by removing the damage on or near the surface due to mechanical polishing. Sapphire, GaN, and SiC are categorized as hard-to-process materials due to their extreme hardness and strong stability against chemicals. Among the hardto-process materials, sapphire substrates are already produced in market volumes, and the sapphire wafering process is regarded as mature technology; thus, difficulties in the mechanical processing due to its hardness are considered to be resolved. There are also many studies dealing with CMP with a colloidal silica, which is widely used for sapphire, and the sapphire removal mechanism is also well understood [7,8]. Consequently, the CMP of sapphire exhibits a removal rate as high as several micrometers per hour, and therefore, the strong stability of sapphire crystal against chemicals is no longer a pressing problem in the CMP process.
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Table 1 CMP conditions for the sapphire, GaN, and SiC substrates. Platen rotation speed Carrier rotation speed Applied pressure Diameter of platen Polishing pad type Slurry type Slurry pH Abrasive particle size Abrasive concentration
rpm rpm kg/cm2 mm Suede type Colloidal silica 1.8, 10.5 nm %
50 25e80 0.15e0.4 300
40 20
The devoted efforts and experiences of previous researchers in the development of the mechanical processing of sapphire can be applied to the mechanical processing of GaN and SiC. However, the removal rate of GaN and SiC in CMP with a colloidal silica exhibits a range of ten to a hundred nanometers per hour due to their strong chemical inertness [9e12]. Assuming that the depth of the damaged layer to be removed by CMP and the removal rate are 1.5 mm and 50 nm/h, respectively, the processing time will be 30 h, which means this is not a practical method for commercial production. Therefore, understanding the mechanism involved in the CMP of GaN and SiC is important research in order to improve the CMP removal rate for these materials, which will lead to an acceleration in the realization of next generation optoelectronics devices on GaN and SiC substrates. In this study, we demonstrate CMP of GaN and SiC substrates with a colloidal silica slurry by comparison with the CMP performance for sapphire. First, a theoretical surface model is discussed to present evidence of an ideally minimized surface roughness that is manifest for the sapphire, GaN, and SiC substrates after CMP with a colloidal silica slurry. Then, the difference in the removal properties among the sapphire, GaN, and SiC substrates due to the slurry pH is investigated using acidic- and alkaline-based colloidal silica slurries. The difference in CMP properties between the (0001) and (000-1) planes of GaN and SiC due to their non-revers crystallographical symmetry is also presented. The difficulties in the CMP of GaN and SiC as compared to sapphire are presented together with evidence that the oxidation reaction is the rate-limiting step for GaN and SiC. Different oxidation processes between GaN and SiC under the chemical atmosphere provided by the slurries is also suggested in this study.
acidic solution (a mixture of H2SO4 and H2O2). The slurry pHs were 10.5 and 1.8 for the alkaline and acidic-colloidal silica slurries, respectively. c-plane substrates for sapphire, GaN, and SiC were used in this study. In the case where the front side of the substrate is the (0001) plane, the back side of the substrate is the (000-1) plane. The sapphire has a reverse crystallographical symmetry, whereas GaN and SiC crystals have a non-revers crystallographical symmetry. Thus the (0001) and (000-1) planes are not crystallographically equivalent for GaN and SiC. Ga- and Si-face is the (0001) plane for GaN and SiC substrates, respectively. Similarly, N- and C-face is the (000-1) plane for GaN and SiC, respectively. In this study, the CMP of both (0001) and (000-1) planes were examined for GaN and SiC substrates. 2-inch diameter as-lapped sapphire, GaN, and SiC substrates were pretreated with two steps of diamond polishing, diamond abrasive sizes of 2 and 0.5 mm, which was then followed by polishing with the colloidal silica under the CMP conditions summarized in Table 1. The removal rates of sapphire, GaN, and SiC were determined by weight loss. The surface quality was characterized by an atomic force microscope (AFM). 3. Results and discussion Fig. 1 shows AFM images of the sapphire, Ga-faced GaN, and Sifaced SiC substrate surfaces after CMP under conditions of 10.5, 0.4 kg/cm2, and 50 rpm for the slurry pH, downward pressure, and platen rotation speed, respectively. The removal rate was 2.2 mm/h, 17 nm/h, and 64 nm/h for sapphire, GaN, and SiC, respectively, and it should be noted that the removal rate of GaN and SiC is extremely low compared with that of sapphire. The surface roughness values after CMP for sapphire, GaN, and SiC are 0.08, 0.18, and 0.10 nm, respectively. It should be noted that the pits are seen in the AFM image of GaN surface as show in Fig. 1(b). They are threading dislocations created during the crystal growth of GaN and became visible during the CMP as the dislocations are opened by chemical etching effect during the CMP. The average surface roughness Ra is calculated by following expression under the definition of the average surface roughness illustrated in Fig. 2(a);
Ra ¼
1 L
ZL jf ðxÞjdx
(1)
0
2. Experimental Acidic- and alkaline-based colloidal silica slurries for CMP were prepared. The alkaline-based colloidal silica slurry was from a commercial source, while the acidic-based colloidal silica slurry was made by mixing the alkaline-based colloidal silica with an
If the surface is perfectly flat without any crystal misorientation, Ra will be zero. However, the sapphire, GaN, and SiC substrates used for epitaxy normally have a slight misorientation in that the actual surface is slightly miscut from the crystal c-plane. For example, a misorientation angle of around 0.15e0.20 is common
Fig. 1. AFM images of the (a) Sapphire, (b) Ga-faced GaN, and (c) Si-faced SiC substrates.
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Fig. 2. (a) Definition of the surface roughness Ra, and (b) The surface model of a one bilayer step-and-terrace structure. The symbols w, h, and q are the terrace width, step height, and misorientation angle, respectively. Fig. 3. Theoretically calculated Ra for the sapphire, GaN, and SiC substrates with ideal one bilayer step-and-terrace structures as a function of the misorientation angle.
in III-nitride epitaxy on sapphire substrates [13e15]. In this case, the surface model is a step-and-terrace structure due to the misorientation. When the surface, consisting of a one bilayer structure corresponding to the periodical step-and-terrace structure, is modeled as shown in Fig. 2(b), the relationship between the misorientation angle q, terrace width w, and step height h is given by the following equation:
tanðqÞ ¼
h w
(2)
where h corresponds to the height of one bilayer and is 0.216, 0.259, and 0.250 nm for sapphire, GaN, and SiC, respectively. Therefore, the substrate Ra for each crystal is calculated theoretically as a function of misorientation angle and one bilayer height as follows;
1 Ra ¼ L
ZL jf ðxÞj dx ¼ 0
1 h h$cosq 2 ¼ $n$ w$ 2 4 n$w=cosq
GaN, and SiC as a function of downward pressure and platen rotation speed, respectively, under different pH slurry conditions. For all substrates, the removal rate exhibited a linear relationship with downward pressure and platen rotation speed. It should be noted that different relationships between the CMP removal rate and the slurry pH were seen among sapphire, GaN, and SiC: a high removal rate is obtained with an acidic slurry for the GaN substrate but with the alkaline slurry for the sapphire and SiC substrates. In general, the stock removal V of the material is expressed by Preston’s equation [16]:
V ¼ k$p$v$t
(4)
where, k is a system dependent parameter, p is the downward pressure, v is the relative polishing speed between the polishing plate and work piece, and t is the process time. In terms of the material removal rate (MRR), Eq. (4) can be modified as:
(3)
where, n is the number of step-and-terrace structures within the measuring surface length L. Fig. 3 plots the calculated Ra as a function of misorientation angle for the sapphire, GaN, and SiC substrates using Eq. (3). The theoretical Ra for the ideal one bilayer step-and-terrace structure is almost constant as the misorientation angle is negligibly small, and the theoretical Ra values are 0.054, 0.065, and 0.062 nm for sapphire, GaN, and SiC, respectively. As presented in Fig. 1, the actual surface roughness achieved after CMP is comparable to the theoretical Ra value for sapphire and SiC, indicating that the surface consists almost only of one bilayer step-and-terrace structures. The actual Ra value of GaN exhibits slightly higher than that of theoretical one. This is caused by the pits created on the surface due to the etching effect of dislocations. A line profile which is escaping the pits shows Ra value as low as 0.10 nm as shown in Fig. 4. Therefore, we consider that ideally minimized surface roughness surfaces are obtained by CMP with colloidal silica for the sapphire, Ga-face of GaN, and Si-face of SiC substrates. The difference in the removal properties during CMP among sapphire, Ga-face of GaN, and Si-face of SiC substrates was investigated in detail. Figs. 5 and 6 show the removal rate of sapphire,
MRR ¼ V=t ¼ k$p$v
(5)
CMP of sapphire, GaN, and SiC is shown to obey Prestonian behavior. When the relative velocity and chemical environment remain unchanged, the removal rate is simply a function of downward pressure. Similarly, when the downward pressure and chemical environment remain unchanged, the removal rate is a function of the relative velocity. Therefore, for each pH condition, k is summarized in Table 2 for a downward pressure of 0.4 kg/cm2 and for a platen rotation speed of 50 rpm. As shown in Table 2, for sapphire and SiC, the alkaline slurry exhibits a k around 2.5 times
Fig. 4. Line profile of Ga-faced GaN surface scanned by AFM.
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Fig. 5. Removal rate as a function of downward pressure for (a) Sapphire, (b) Ga-faced GaN, and (c) Si-faced SiC.
larger than that for the acidic slurry. On the other hand, for GaN, the acidic slurry exhibits a k that is around 3 times larger than that for the alkaline slurry. The k values for GaN and SiC are w2 orders of magnitude smaller than those of sapphire, showing the extreme difficulty in CMP for these two materials. This means that the change in the removal rate gained by changing the downward pressure or relative polishing speed is much lower for GaN and SiC than for sapphire, indicating the limitations in improving the removal rate by increasing the downward pressure and linear velocity during CMP. Comparing the k values for GaN and SiC, CMP of GaN is around 1.5 times more difficult than CMP of SiC even if suitable pH conditions are selected for these materials. Another important aspect to be discussed is asymmetric crystallographical properties of GaN and SiC. Figs. 7 and 8 show the removal rate of N-face of GaN and C-face of SiC as a function of
downward pressure and platen rotation speed, respectively, under different pH slurry conditions. As compared to the (0001) face, higher removal rates were observed for (000-1) plane of GaN and SiC. The difference in the removal rate between (0001) and (000-1) plane is much larger for GaN than SiC substrate as it is reported that the Ga- and N-faces show totally different chemical etching properties [9,10,17e20]. In addition, the (000-1) faced GaN and SiC substrates exhibit higher removal rates with alkaline- and acidiccolloidal silica slurry, respectively, which is completely opposite to the CMP properties obtained for (0001) faced GaN and SiC. These results suggest the totally different CMP mechanism between GaN and SiC. However, interestingly, although nitrides and carbides are different, it has been reported that the CMP of both GaN and SiC proceeds via a surface oxidation reaction [21e25]. For example,
Fig. 6. Removal rate as a function of platen rotation speed for (a) Sapphire, (b) Ga-faced GaN, and (c) Si-faced SiC.
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Table 2 Preston’s coefficient k as a function of slurry pH.
Sapphire GaN (0001) SiC (0001)
pH
k at 50 rpm ( 1011 cm2/kg)
k at 0.4 kg/cm2 (1/rpm)
1.8 10.5 1.8 10.5 1.8 10.5
56.7 162.4 3.6 1.2 2.5 5.9
27.8 59.4 2.0 0.7 1.4 3.2
GaN is reported to be polished by the surface oxidation mechanism of forming Ga2O3 followed by the dissolution of Ga2O3 into the chemical solution. Similarly, in the case of SiC, SiO2 is formed during CMP. To discuss the CMP removal rate of the oxide layer for each material, bulk Ga2O3 [26] and SiO2 were polished under the same CMP conditions. Table 3 summarizes the removal rates of Ga2O3 and SiO2 as a function of the slurry pH. A removal rate of several microns per hour was obtained for both Ga2O3 and SiO2, showing the two orders of magnitude difference in the removal rates from GaN and SiC. This indicates that the oxidation process is the ratelimiting step in the CMP processing of GaN and SiC. Therefore, this suggests the importance of developing a novel oxidation approach during CMP to increase the removal rate for GaN and SiC, such as a photoenhanced wet etching [21,22,27e30], catalystreferred etching (CARE) method [21e25], and atmospherecontrolled CMP with closed bell-jar CMP machine [31e34] etc. In addition, the observed difference in the trend of the removal rate against the slurry pH between GaN and SiC suggests different oxidation mechanisms between the substrate material and silica particles under the chemical solution of the slurry. Although it is unclear why the Ga-face of GaN removal rate is higher under acidic conditions and that of Si-face of SiC is higher under alkaline conditions, which is also totally opposite from the CMP property of the (000-1) plane, further study to understand these phenomena may provide us with valuable information to help solve the difficulties of CMP of GaN and SiC.
Fig. 8. Removal rate as a function of platen rotation speed for (a) N-faced GaN and (b) C-faced SiC.
Table 3 CMP removal rate for Ga2O3 and SiO2 with 20% colloidal silica under the conditions of 0.4 kg/cm2 and 50 rpm for the applied downward pressure and platen rotation speed, respectively.
Ga2O3 SiO2
pH
Removal rate (mm/h)
1.8 10.5 1.8 10.5
8.0 5.6 7.1 5.8
4. Conclusion We have demonstrated CMP of sapphire, GaN, and SiC substrates with a colloidal silica slurry under different slurry pH conditions. Atomic level surface flatness was achieved with CMP. By discussing a theoretical surface model, evidence of an ideally minimized surface roughness for the sapphire, GaN, and SiC substrates after CMP was presented. By comparing the Preston coefficients as a function of the slurry pH for sapphire, GaN, and SiC, difficulties in the CMP of GaN and SiC as compared to sapphire were understood. An interesting trend in the CMP properties against the slurry pH was observed: sapphire and SiC exhibit a larger k under alkaline slurry conditions, whereas GaN exhibits a higher k under acidic slurry conditions. The removal rate of Ga2O3 and SiO2 were two orders of magnitude higher than that of GaN and SiC, indicating that the oxidation process is the rate-limiting step in the total CMP processing time of GaN and SiC. References
Fig. 7. Removal rate as a function of downward pressure for (a) N-faced GaN and (b) Cfaced SiC.
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