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Evaluation of subsurface damage in GaN substrate induced by mechanical polishing with diamond abrasives
Applied Surface Science 292 (2014) 531–536
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Evaluation of subsurface damage in GaN substrate induced by mechanical polishing with diamond abrasives Hideo Aida a,b,∗ , Hidetoshi Takeda a , Seong-Woo Kim a , Natsuko Aota a , Koji Koyama a , Tsutomu Yamazaki b , Toshiro Doi b a b
NJC Institute of Technology, Namiki Precision Jewel Co., Ltd., 3-8-22 Shinden, Adachi, Tokyo 123-8511, Japan KASTEC, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 1 December 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: GaN substrate Mechanical polishing Chemical mechanical polishing Subsurface damage Cathodoluminescence Brittle materials
a b s t r a c t The relationship between the depth of the subsurface damage (SSD) and the size of the diamond abrasive used for mechanical polishing (MP) of GaN substrates was investigated in detail. GaN is categorized as a hard, brittle material, and material removal in MP proceeds principally to the fracture of GaN crystals. Atomic force microscopy and cathodoluminescence imaging revealed that the mechanical processing generated surface scratches and SSD. The SSD depth reduced as the diamond abrasive size reduced. A comparison of the relationship between the SSD depth and the diamond abrasive size used in the MP of GaN with the same relationship for typical brittle materials such as glass substrates suggests that the MP of GaN substrates proceeds via the same mechanism as glass. © 2013 Elsevier B.V. All rights reserved.
1. Introduction GaN and related alloys are among the most promising materials for optoelectronic applications and high-power, high-frequency devices [1–3]. The most common process for growing these alloys is heteroepitaxy on a foreign substrate because there is a lack of optimal substrates that have perfect matching conditions for the thermal expansion coefficients and lattice constants between the substrate and grown films. Mass production of GaN substrates, therefore, is highly desired, and significant effort has been devoted to the growth and synthesis of bulk GaN crystals; the corresponding technologies have seen substantial improvement in recent years [4–8]. The development of wafering processes for GaN substrates, however, is not as advanced as GaN-crystal growth and synthesis technology, since GaN is categorized as a brittle, hard, and inert material, which makes the wafering process somewhat difficult [9–14]. The main step in the wafering process consists of mechanical removal, usually by cutting, grinding, or mechanical polishing (MP). Generally, mechanical removal of a brittle material proceeds principally to fracture, and thus cracks and scratches appear on the surface. In addition, subsurface damage (SSD) layers are produced.
∗ Corresponding author. Tel.: +81 353907875. E-mail address: [email protected] (H. Aida). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.005
SSD in a typical brittle material such as glass has been reported to weaken the material strength, enhance the electric field inside the cracks, and influence the operational durability and lifetime of fabricated semiconductor components [15–17]. Therefore, minimizing the amount of SSD is essential for glass substrates. This, however, is not the case for GaN; SSD must be completely removed in order to grow high-quality device films on GaN substrates by the homoepitaxial growth of nitride films [18]. Chemical mechanical polishing (CMP) is normally applied as the final step in the wafering process to completely remove the SSD induced by MP. Considering the chemical inertness [14] and associated low CMP removal rate of GaN [13], minimizing the SSD depth is an important task for improving the CMP performance and producing a surface finish ideal for the subsequent homoepitaxy of III-nitridebased device films. To the best of our knowledge, however, there have been very few reports describing precise structural research of SSD induced by MP of GaN substrates. In our previous paper, we detailed the MP of a GaN substrate with a diamond abrasive and proposed that two types of SSD layers were formed in the GaN substrate by MP with a diamond abrasive [13]. The first type was a highly damaged layer that resulted in an etching effect during CMP. The second type, which existed underneath a layer of the first type, consisted of relatively weaker damage, and thus no etching effect was observed, but the existence of nonradiative recombination sites were observed in cathodoluminescence (CL) images. Further detailed studies are necessary to
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H. Aida et al. / Applied Surface Science 292 (2014) 531–536 Table 2 CMP conditions.
Fig. 1. The removal rate as a function of the average abrasive size.
Table 1 MP conditions. Platen rotation speed (min−1 )
50
Carrier rotation speed (min−1 ) Applied pressure (kg/cm2 ) Diameter of platen (mm) Platen (abrasive material) Average abrasive particle size (m)
40 0.1 380 Metal plate (diamond) 0.05, 0.25, 0.5, 1.5, 3, 9
fully understand the two SSD types based on the mechanisms of mechanical removal of GaN. In this paper, we study the relationship between the diamond abrasive size and the depth of the SSD induced in the GaN surface. To estimate the SSD depth, we analyze the CMP process time required to obtain a completely SSD-free GaN substrate after MP with a diamond abrasive. The results are discussed in comparison to SSD research dealing with glass substrates, which is recognized as a typical brittle material, to obtain additional insight into the two types of SSD layers induced in GaN by MP with diamond abrasives. 2. Experimental procedure A liquid phase epitaxy (LPE)-grown GaN substrate, 10 × 15 mm2 in size, was used in this study. An as-grown GaN crystal was fixed to a ceramic plate in such a way that a Ga-faced surface was obtained, and grinding was then performed to remove the as-grown surface roughness and make the substrates flat. Grinding was followed by several MP process steps conducted using progressively smaller sized diamond abrasives. The detailed MP conditions are given in Table 1. The removal rate of the Ga-faced GaN by MP with a diamond abrasive was measured as a function of the diamond abrasive size.
Platen rotation speed (min−1 )
50
Carrier rotation speed (min−1 ) Applied pressure (kg/cm2 ) Diameter of platen (mm) Polishing pad type Slurry type Slurry pH Abrasive particle size (nm) Abrasive concentration (%) Removal rate (nm/h)
40 0.4 300 Suede type Colloidal silica 10.5 40 40 17
The surface roughness after MP was measured using an atomic force microscope (AFM). After MP with the diamond abrasives, CMP was conducted to estimate the depth of the induced SSD. A colloidal silica slurry was used for CMP of the Ga-faced GaN under the conditions summarized in Table 2. Because the removal rate of the GaN substrate under the conditions given in Table 2 was known to be 17 nm/h [13], the SSD depth can be estimated from the CMP process time required to obtain an atomically flat surface completely free of SSD. The SSD induced by the polishing process can be imaged using CL [13,19], as polishing damage generates dislocations networks at or near the surface region of the substrate, which appear as dark areas in the CL image because they act as nonradiative recombination sites. Therefore, the extent of the damage induced by different sized diamond abrasives was evaluated visually by CL imaging. The CL was excited by accelerated electrons with the energy of 5 kV, corresponding to the maximum penetration depth of 0.22 m [20]. The estimated depth of the SSD was investigated as a function of the diamond abrasive size used for MP. 3. Results and discussion The effect of MP by a diamond abrasive on the GaN surface was studied by changing the abrasive size. Fig. 1 shows the GaN substrate removal rate as a function of the average size of the diamond abrasive. The larger diamond abrasive led to the faster removal rate but also to the rougher surface finishing. The AFM images (50 × 50 m2 ) in Fig. 2 show the GaN surface finished by 1.5, 0.5-, 0.25- and 0.05-m diamond abrasives in average size. The number of heavy scratches reduced as the diamond abrasive size decreased. In the case of the 0.05-m diamond abrasive, there were few visible scratches. Fig. 3 shows the surface roughness of the GaN substrate after MP as a function of the average size of diamond abrasive (0.05 to 9 m). The surface roughness decreased with a decrease in the size of diamond abrasive, and in a similar manner to other brittle materials, there was a tradeoff between the removal rate and surface roughness; although the removal rate decreased as the size of the diamond abrasive decreased, the smaller abrasive led to a finer surface finishing.
Fig. 2. AFM images (50 × 50 m2 ) of the GaN surface finished by diamond abrasives of (a) 1.5, (b) 0.5, (c) 0.25, and (d) 0.05 m in average size.
H. Aida et al. / Applied Surface Science 292 (2014) 531–536
Fig. 3. Surface roughness as a function of the average abrasive size.
The extent of SSD induced by MP was visually analyzed using CL imaging in Fig. 4. For the GaN finished with a diamond abrasive larger than 0.5 m, the CL image is completely black, indicating that the surface is entirely covered by nonradiative recombination sites. A clear reduction in the amount of SSD, however, can be seen with the smaller diamond abrasive; the GaN surfaces finished with 0.25- and 0.05-m diamond abrasives in average size show slight emission from the subsurface, indicating a lower amount of SSD.
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The CMP time needed after MP with a diamond abrasive to obtain a damage-free surface was studied as a function of the average size of the diamond abrasive. CL imaging was also used to determine whether residual SSD was completely removed by CMP, namely, to estimate the required CMP time. Fig. 5 shows examples of CL images that were taken during CMP of the GaN substrate finished by MP with a diamond abrasive of 0.5 or 0.05 m in average size. In both cases, black lines that correspond to the subsurface mechanical damage created by surface fractures caused by the diamond abrasive were removed by CMP. The intrinsic hardness of the SiO2 in glass phase is 10.2 GPa [21]. On the other hand, the reported hardness of GaN differs among the literatures but it ranges from 18 to 21 GPa [22–25]. Therefore, it was found that the much softer particle of SiO2 can remove the harder GaN. It took 150 (35) h to achieve a completely SSD-free substrate for GaN finished by MP with a diamond abrasive of the average size of 0.5 (0.05) m. There are still some black spots in the CL images even after CMP has been completed, but we note that these black spots indicate the existence of threading dislocations that were created during the GaN crystal growth and are not polishing-related SSD. Fig. 6 summarizes the required CMP time as a function of the average size of the diamond abrasive. Since the CMP time for the GaN substrates finished with diamond abrasives larger than 0.5 m was extremely long, the figure only shows the results for diamond abrasives smaller than 0.5 m. As we can see in the figure, the required CMP time was reduced when the GaN substrate was pretreated by a smaller diamond abrasive. Since the average CMP
Fig. 4. CL images (60 × 60 m2 ) of the GaN surface finished by diamond abrasives of (a) 1.5, (b) 0.5, (c) 0.25, and (d) 0.05 m in average size.
Fig. 5. Change in the CL images during the CMP process. CMP of the GaN substrate finished by MP with the average size of (a) 0.5-m and (b) 0.05-m diamond abrasives.
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Fig. 6. Required CMP time and estimated depth of the subsurface damaged (SSD) layer introduced by the diamond abrasive as a function of the average abrasive size used in MP as a pretreatment of CMP.
removal rate is 17 nm/h under the applied CMP conditions, the required time can be converted to an equivalent SSD depth, which is also shown in Fig. 6. The SSD depth in the GaN substrate MP with a diamond abrasive of the average size of 0.5 m can be estimated as 2550 nm, which was reduced to 595 nm with a 0.05-m of the diamond abrasive. To understand the mechanical removal mechanism of GaN, it is worth comparing the SSD depth in GaN with that in glass, since glass is one of the most typical brittle materials and its polishing properties are well researched [25–31]. There is an estimation method of the depth of SSD for glass substrates: the SSD can be related to the abrasives used in lapping. Although there are many reports dealing with the lapping of glass, the most relevant reports present the following two equations for the relationship between the SSD depth and the size of the abrasive [29–31]; 0.3d0.68 < SSD < 2d0.85 ,
(1)
0.2d < SSD < 1.6d,
(2)
where d is the abrasive size, and the SSD depth is expressed in micrometers. This model describes the brittle removal mode case with the relatively smaller abrasives (a few micrometers or smaller) in lapping. The above model simply considers the size of abrasive. Here, we apply this model to the present case of MP of GaN. Fig. 7(a) and (b) shows the relationship between the SSD depth in the GaN substrate and the average size of the diamond abrasive together with the calculated range for the depth of SSD on the basis of the models in Eqs. (1) and (2), respectively. The fit line (i) in Fig. 7 shows the depth of SSD estimated in Fig. 6 as a function of the average size of the diamond abrasive. As seen in Fig. 7, the ratio of the SSD depth to the average size of the abrasive is much larger for GaN substrates of the present study than that reported for glass substrates. However, we note that the CL imaging method conducted in our study revealed the maximum depth of the SSD. As mentioned earlier, we proposed in our previous study that there are two types of SSD layers [13]. For further discussion, we also investigated the depth of the strong SSD layer only, using the same method as in our previous paper; these results are also plotted in Fig. 7 as a fit line (ii). Then, the depth of the strong SSD layer in the GaN substrate shows fairly good agreement with the estimated SSD depth in the glass substrates as given by Eqs. (1) and (2). On the other hand, the total SSD depth, including the weak SSD layers, takes a larger value than that calculated on the basis of the glass models (Eqs. (1) and (2)). We note that there exists a size distribution of the abrasives; the size distributions of the abrasive used in the study was shown in Fig. 8. The maximum size of diamond
Fig. 7. Comparison of the SSD layer depth with the reported models for typical brittle materials: SSD depth range calculated on the basis of (a) Eq. (1) and (b) Eq. (2). The fits (i) and (ii) show estimations of the total SSD depth plotted with marker () and strongly damaged SSD depth plotted with maker () as a function of the average abrasive size, respectively.
Fig. 8. Histogram of the abrasive size distribution for the diamond abrasives. (a) 0.5, (b) 0.25, and (c) 0.05 m in average sized diamond abrasives, respectively.
abrasive included in the diamond abrasives of the average size of 0.5-, 0.25-, and 0.05-m was found to be 1.6, 0.7, and 0.26 m, respectively. It is reasonable to reconsider the relation between the total SSD depth and the maximum size of the abrasive, which is shown in Fig. 9. The range for the depth of SSD on the basis of the model in Eqs. (1) and (2) is also calculated and shown in Fig. 9. Under the reconsideration with the maximum size of the abrasive, it was found that the depth of the weak SSD layer in the GaN
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of the uniformity of the distributions of diamond abrasives used for the MP to reduce the depth of the weak SSD. In addition, a useful new insight that the MP of GaN substrates proceeds via the same mechanism as glass and that the SSD layer is generated by the same SSD generation mechanism as that in glass was obtained for GaN substrate, which suggests that the approaches already confirmed to be useful to reduce SSD layer in MP of the typical brittle materials would be also useful for MP of GaN substrate to reduce the depth of SSD layer. 4. Conclusion The relationship between the SSD depth and diamond abrasive size used for MP of GaN substrates was investigated. Since GaN is categorized as a hard, brittle material, material removal in MP proceeds principally to the fracture of GaN crystals. We found that there was a tradeoff between the removal rate and surface roughness, namely, the removal rate decreased as the diamond abrasive size decreased, but a smaller abrasive size led to a finer surface finish; this trend is similar to other brittle materials. We also clarified by CL imaging that the required CMP time can be reduced if the GaN substrate is mechanically polished by a smaller diamond abrasive. Thus, a smaller diamond abrasive contributes to a reduction in the SSD depth. By comparing the relationship between the SSD depth and abrasive size with that reported for typical brittle materials such as glass substrates, we came to the conclusion that the MP of GaN substrates proceeds via the same mechanism as that for typical brittle materials. Acknowledgements
Fig. 9. Comparison of the SSD layer depth with the reported models for typical brittle materials: SSD depth range calculated on the basis of (a) Eq. (1) and (b) Eq. (2). The fits (i) and (ii) show estimations of the total SSD depth plotted with marker () and strongly damaged SSD depth plotted with maker () as a function of the maximum abrasive size, respectively.
substrate was distributed near the upper limit of the estimated SSD depth in the glass substrates as given by Eqs. (1) and (2). Therefore, it is thought that the weak SSD layer is generated by the same SSD generation mechanism as that in glass. In addition, the consideration with Figs. 7 and 9 indicates that the strong SSD was induced by the high density of average sized diamond abrasives while the weak one was concluded to be most likely induced by the few amounts of maximum sized diamond abrasives. If GaN substrate is used for non-electrical applications such as a basal heat sink, then a residual weak SSD layer would not have any adverse effects, but even if the layer is too weak to show etching effects, a serious degradation in the device performance would arise if the substrate were to be used for opto-electronics devices such as light-emitting diodes since such a layer can still trap electrons and acts as non-radiative recombination sites, as observed in the CL images. Therefore, the weak SSD layer has to be completely removed. However, as we mentioned, CMP of GaN is extremely difficult due to the low removal rate, and in this regard, minimizing the total depth of the SSD layer is important; this will result in a drastic breakthrough in resolving the difficulties in CMP of GaN substrates and contribute to development in industries relying on GaN substrates. As shown in Fig. 7, the depth of the weak SSD layer is around 4 times deeper than that of the strong SSD layer. The revealed mechanism for the cause of the weak SSD layer suggests the importance
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Evaluation of subsurface damage in GaN substrate induced by mechanical polishing with diamond abrasives Hideo Aida a,b,∗ , Hidetoshi Takeda a , Seong-Woo Kim a , Natsuko Aota a , Koji Koyama a , Tsutomu Yamazaki b , Toshiro Doi b a b
NJC Institute of Technology, Namiki Precision Jewel Co., Ltd., 3-8-22 Shinden, Adachi, Tokyo 123-8511, Japan KASTEC, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 1 December 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: GaN substrate Mechanical polishing Chemical mechanical polishing Subsurface damage Cathodoluminescence Brittle materials
a b s t r a c t The relationship between the depth of the subsurface damage (SSD) and the size of the diamond abrasive used for mechanical polishing (MP) of GaN substrates was investigated in detail. GaN is categorized as a hard, brittle material, and material removal in MP proceeds principally to the fracture of GaN crystals. Atomic force microscopy and cathodoluminescence imaging revealed that the mechanical processing generated surface scratches and SSD. The SSD depth reduced as the diamond abrasive size reduced. A comparison of the relationship between the SSD depth and the diamond abrasive size used in the MP of GaN with the same relationship for typical brittle materials such as glass substrates suggests that the MP of GaN substrates proceeds via the same mechanism as glass. © 2013 Elsevier B.V. All rights reserved.
1. Introduction GaN and related alloys are among the most promising materials for optoelectronic applications and high-power, high-frequency devices [1–3]. The most common process for growing these alloys is heteroepitaxy on a foreign substrate because there is a lack of optimal substrates that have perfect matching conditions for the thermal expansion coefficients and lattice constants between the substrate and grown films. Mass production of GaN substrates, therefore, is highly desired, and significant effort has been devoted to the growth and synthesis of bulk GaN crystals; the corresponding technologies have seen substantial improvement in recent years [4–8]. The development of wafering processes for GaN substrates, however, is not as advanced as GaN-crystal growth and synthesis technology, since GaN is categorized as a brittle, hard, and inert material, which makes the wafering process somewhat difficult [9–14]. The main step in the wafering process consists of mechanical removal, usually by cutting, grinding, or mechanical polishing (MP). Generally, mechanical removal of a brittle material proceeds principally to fracture, and thus cracks and scratches appear on the surface. In addition, subsurface damage (SSD) layers are produced.
∗ Corresponding author. Tel.: +81 353907875. E-mail address: [email protected] (H. Aida). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.005
SSD in a typical brittle material such as glass has been reported to weaken the material strength, enhance the electric field inside the cracks, and influence the operational durability and lifetime of fabricated semiconductor components [15–17]. Therefore, minimizing the amount of SSD is essential for glass substrates. This, however, is not the case for GaN; SSD must be completely removed in order to grow high-quality device films on GaN substrates by the homoepitaxial growth of nitride films [18]. Chemical mechanical polishing (CMP) is normally applied as the final step in the wafering process to completely remove the SSD induced by MP. Considering the chemical inertness [14] and associated low CMP removal rate of GaN [13], minimizing the SSD depth is an important task for improving the CMP performance and producing a surface finish ideal for the subsequent homoepitaxy of III-nitridebased device films. To the best of our knowledge, however, there have been very few reports describing precise structural research of SSD induced by MP of GaN substrates. In our previous paper, we detailed the MP of a GaN substrate with a diamond abrasive and proposed that two types of SSD layers were formed in the GaN substrate by MP with a diamond abrasive [13]. The first type was a highly damaged layer that resulted in an etching effect during CMP. The second type, which existed underneath a layer of the first type, consisted of relatively weaker damage, and thus no etching effect was observed, but the existence of nonradiative recombination sites were observed in cathodoluminescence (CL) images. Further detailed studies are necessary to
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H. Aida et al. / Applied Surface Science 292 (2014) 531–536 Table 2 CMP conditions.
Fig. 1. The removal rate as a function of the average abrasive size.
Table 1 MP conditions. Platen rotation speed (min−1 )
50
Carrier rotation speed (min−1 ) Applied pressure (kg/cm2 ) Diameter of platen (mm) Platen (abrasive material) Average abrasive particle size (m)
40 0.1 380 Metal plate (diamond) 0.05, 0.25, 0.5, 1.5, 3, 9
fully understand the two SSD types based on the mechanisms of mechanical removal of GaN. In this paper, we study the relationship between the diamond abrasive size and the depth of the SSD induced in the GaN surface. To estimate the SSD depth, we analyze the CMP process time required to obtain a completely SSD-free GaN substrate after MP with a diamond abrasive. The results are discussed in comparison to SSD research dealing with glass substrates, which is recognized as a typical brittle material, to obtain additional insight into the two types of SSD layers induced in GaN by MP with diamond abrasives. 2. Experimental procedure A liquid phase epitaxy (LPE)-grown GaN substrate, 10 × 15 mm2 in size, was used in this study. An as-grown GaN crystal was fixed to a ceramic plate in such a way that a Ga-faced surface was obtained, and grinding was then performed to remove the as-grown surface roughness and make the substrates flat. Grinding was followed by several MP process steps conducted using progressively smaller sized diamond abrasives. The detailed MP conditions are given in Table 1. The removal rate of the Ga-faced GaN by MP with a diamond abrasive was measured as a function of the diamond abrasive size.
Platen rotation speed (min−1 )
50
Carrier rotation speed (min−1 ) Applied pressure (kg/cm2 ) Diameter of platen (mm) Polishing pad type Slurry type Slurry pH Abrasive particle size (nm) Abrasive concentration (%) Removal rate (nm/h)
40 0.4 300 Suede type Colloidal silica 10.5 40 40 17
The surface roughness after MP was measured using an atomic force microscope (AFM). After MP with the diamond abrasives, CMP was conducted to estimate the depth of the induced SSD. A colloidal silica slurry was used for CMP of the Ga-faced GaN under the conditions summarized in Table 2. Because the removal rate of the GaN substrate under the conditions given in Table 2 was known to be 17 nm/h [13], the SSD depth can be estimated from the CMP process time required to obtain an atomically flat surface completely free of SSD. The SSD induced by the polishing process can be imaged using CL [13,19], as polishing damage generates dislocations networks at or near the surface region of the substrate, which appear as dark areas in the CL image because they act as nonradiative recombination sites. Therefore, the extent of the damage induced by different sized diamond abrasives was evaluated visually by CL imaging. The CL was excited by accelerated electrons with the energy of 5 kV, corresponding to the maximum penetration depth of 0.22 m [20]. The estimated depth of the SSD was investigated as a function of the diamond abrasive size used for MP. 3. Results and discussion The effect of MP by a diamond abrasive on the GaN surface was studied by changing the abrasive size. Fig. 1 shows the GaN substrate removal rate as a function of the average size of the diamond abrasive. The larger diamond abrasive led to the faster removal rate but also to the rougher surface finishing. The AFM images (50 × 50 m2 ) in Fig. 2 show the GaN surface finished by 1.5, 0.5-, 0.25- and 0.05-m diamond abrasives in average size. The number of heavy scratches reduced as the diamond abrasive size decreased. In the case of the 0.05-m diamond abrasive, there were few visible scratches. Fig. 3 shows the surface roughness of the GaN substrate after MP as a function of the average size of diamond abrasive (0.05 to 9 m). The surface roughness decreased with a decrease in the size of diamond abrasive, and in a similar manner to other brittle materials, there was a tradeoff between the removal rate and surface roughness; although the removal rate decreased as the size of the diamond abrasive decreased, the smaller abrasive led to a finer surface finishing.
Fig. 2. AFM images (50 × 50 m2 ) of the GaN surface finished by diamond abrasives of (a) 1.5, (b) 0.5, (c) 0.25, and (d) 0.05 m in average size.
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Fig. 3. Surface roughness as a function of the average abrasive size.
The extent of SSD induced by MP was visually analyzed using CL imaging in Fig. 4. For the GaN finished with a diamond abrasive larger than 0.5 m, the CL image is completely black, indicating that the surface is entirely covered by nonradiative recombination sites. A clear reduction in the amount of SSD, however, can be seen with the smaller diamond abrasive; the GaN surfaces finished with 0.25- and 0.05-m diamond abrasives in average size show slight emission from the subsurface, indicating a lower amount of SSD.
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The CMP time needed after MP with a diamond abrasive to obtain a damage-free surface was studied as a function of the average size of the diamond abrasive. CL imaging was also used to determine whether residual SSD was completely removed by CMP, namely, to estimate the required CMP time. Fig. 5 shows examples of CL images that were taken during CMP of the GaN substrate finished by MP with a diamond abrasive of 0.5 or 0.05 m in average size. In both cases, black lines that correspond to the subsurface mechanical damage created by surface fractures caused by the diamond abrasive were removed by CMP. The intrinsic hardness of the SiO2 in glass phase is 10.2 GPa [21]. On the other hand, the reported hardness of GaN differs among the literatures but it ranges from 18 to 21 GPa [22–25]. Therefore, it was found that the much softer particle of SiO2 can remove the harder GaN. It took 150 (35) h to achieve a completely SSD-free substrate for GaN finished by MP with a diamond abrasive of the average size of 0.5 (0.05) m. There are still some black spots in the CL images even after CMP has been completed, but we note that these black spots indicate the existence of threading dislocations that were created during the GaN crystal growth and are not polishing-related SSD. Fig. 6 summarizes the required CMP time as a function of the average size of the diamond abrasive. Since the CMP time for the GaN substrates finished with diamond abrasives larger than 0.5 m was extremely long, the figure only shows the results for diamond abrasives smaller than 0.5 m. As we can see in the figure, the required CMP time was reduced when the GaN substrate was pretreated by a smaller diamond abrasive. Since the average CMP
Fig. 4. CL images (60 × 60 m2 ) of the GaN surface finished by diamond abrasives of (a) 1.5, (b) 0.5, (c) 0.25, and (d) 0.05 m in average size.
Fig. 5. Change in the CL images during the CMP process. CMP of the GaN substrate finished by MP with the average size of (a) 0.5-m and (b) 0.05-m diamond abrasives.
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Fig. 6. Required CMP time and estimated depth of the subsurface damaged (SSD) layer introduced by the diamond abrasive as a function of the average abrasive size used in MP as a pretreatment of CMP.
removal rate is 17 nm/h under the applied CMP conditions, the required time can be converted to an equivalent SSD depth, which is also shown in Fig. 6. The SSD depth in the GaN substrate MP with a diamond abrasive of the average size of 0.5 m can be estimated as 2550 nm, which was reduced to 595 nm with a 0.05-m of the diamond abrasive. To understand the mechanical removal mechanism of GaN, it is worth comparing the SSD depth in GaN with that in glass, since glass is one of the most typical brittle materials and its polishing properties are well researched [25–31]. There is an estimation method of the depth of SSD for glass substrates: the SSD can be related to the abrasives used in lapping. Although there are many reports dealing with the lapping of glass, the most relevant reports present the following two equations for the relationship between the SSD depth and the size of the abrasive [29–31]; 0.3d0.68 < SSD < 2d0.85 ,
(1)
0.2d < SSD < 1.6d,
(2)
where d is the abrasive size, and the SSD depth is expressed in micrometers. This model describes the brittle removal mode case with the relatively smaller abrasives (a few micrometers or smaller) in lapping. The above model simply considers the size of abrasive. Here, we apply this model to the present case of MP of GaN. Fig. 7(a) and (b) shows the relationship between the SSD depth in the GaN substrate and the average size of the diamond abrasive together with the calculated range for the depth of SSD on the basis of the models in Eqs. (1) and (2), respectively. The fit line (i) in Fig. 7 shows the depth of SSD estimated in Fig. 6 as a function of the average size of the diamond abrasive. As seen in Fig. 7, the ratio of the SSD depth to the average size of the abrasive is much larger for GaN substrates of the present study than that reported for glass substrates. However, we note that the CL imaging method conducted in our study revealed the maximum depth of the SSD. As mentioned earlier, we proposed in our previous study that there are two types of SSD layers [13]. For further discussion, we also investigated the depth of the strong SSD layer only, using the same method as in our previous paper; these results are also plotted in Fig. 7 as a fit line (ii). Then, the depth of the strong SSD layer in the GaN substrate shows fairly good agreement with the estimated SSD depth in the glass substrates as given by Eqs. (1) and (2). On the other hand, the total SSD depth, including the weak SSD layers, takes a larger value than that calculated on the basis of the glass models (Eqs. (1) and (2)). We note that there exists a size distribution of the abrasives; the size distributions of the abrasive used in the study was shown in Fig. 8. The maximum size of diamond
Fig. 7. Comparison of the SSD layer depth with the reported models for typical brittle materials: SSD depth range calculated on the basis of (a) Eq. (1) and (b) Eq. (2). The fits (i) and (ii) show estimations of the total SSD depth plotted with marker () and strongly damaged SSD depth plotted with maker () as a function of the average abrasive size, respectively.
Fig. 8. Histogram of the abrasive size distribution for the diamond abrasives. (a) 0.5, (b) 0.25, and (c) 0.05 m in average sized diamond abrasives, respectively.
abrasive included in the diamond abrasives of the average size of 0.5-, 0.25-, and 0.05-m was found to be 1.6, 0.7, and 0.26 m, respectively. It is reasonable to reconsider the relation between the total SSD depth and the maximum size of the abrasive, which is shown in Fig. 9. The range for the depth of SSD on the basis of the model in Eqs. (1) and (2) is also calculated and shown in Fig. 9. Under the reconsideration with the maximum size of the abrasive, it was found that the depth of the weak SSD layer in the GaN
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of the uniformity of the distributions of diamond abrasives used for the MP to reduce the depth of the weak SSD. In addition, a useful new insight that the MP of GaN substrates proceeds via the same mechanism as glass and that the SSD layer is generated by the same SSD generation mechanism as that in glass was obtained for GaN substrate, which suggests that the approaches already confirmed to be useful to reduce SSD layer in MP of the typical brittle materials would be also useful for MP of GaN substrate to reduce the depth of SSD layer. 4. Conclusion The relationship between the SSD depth and diamond abrasive size used for MP of GaN substrates was investigated. Since GaN is categorized as a hard, brittle material, material removal in MP proceeds principally to the fracture of GaN crystals. We found that there was a tradeoff between the removal rate and surface roughness, namely, the removal rate decreased as the diamond abrasive size decreased, but a smaller abrasive size led to a finer surface finish; this trend is similar to other brittle materials. We also clarified by CL imaging that the required CMP time can be reduced if the GaN substrate is mechanically polished by a smaller diamond abrasive. Thus, a smaller diamond abrasive contributes to a reduction in the SSD depth. By comparing the relationship between the SSD depth and abrasive size with that reported for typical brittle materials such as glass substrates, we came to the conclusion that the MP of GaN substrates proceeds via the same mechanism as that for typical brittle materials. Acknowledgements
Fig. 9. Comparison of the SSD layer depth with the reported models for typical brittle materials: SSD depth range calculated on the basis of (a) Eq. (1) and (b) Eq. (2). The fits (i) and (ii) show estimations of the total SSD depth plotted with marker () and strongly damaged SSD depth plotted with maker () as a function of the maximum abrasive size, respectively.
substrate was distributed near the upper limit of the estimated SSD depth in the glass substrates as given by Eqs. (1) and (2). Therefore, it is thought that the weak SSD layer is generated by the same SSD generation mechanism as that in glass. In addition, the consideration with Figs. 7 and 9 indicates that the strong SSD was induced by the high density of average sized diamond abrasives while the weak one was concluded to be most likely induced by the few amounts of maximum sized diamond abrasives. If GaN substrate is used for non-electrical applications such as a basal heat sink, then a residual weak SSD layer would not have any adverse effects, but even if the layer is too weak to show etching effects, a serious degradation in the device performance would arise if the substrate were to be used for opto-electronics devices such as light-emitting diodes since such a layer can still trap electrons and acts as non-radiative recombination sites, as observed in the CL images. Therefore, the weak SSD layer has to be completely removed. However, as we mentioned, CMP of GaN is extremely difficult due to the low removal rate, and in this regard, minimizing the total depth of the SSD layer is important; this will result in a drastic breakthrough in resolving the difficulties in CMP of GaN substrates and contribute to development in industries relying on GaN substrates. As shown in Fig. 7, the depth of the weak SSD layer is around 4 times deeper than that of the strong SSD layer. The revealed mechanism for the cause of the weak SSD layer suggests the importance
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