Appendix – Applications to Optoelectronics Materials
• Hideo Aida
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doi:10.1016/B978-1-4557-7858-4.00013-3
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A.1. Importance and expectation of next-generation optoelectronics materials
Human beings have a long history of processing close-at-hand materials and creating tools for daily tasks. For example, stone accessories and knives were created in the Stone Age, and subsequent advances in technology have seen commonly utilized materials change from stone to ceramics and glass, with a simultaneous change in processing technologies; lens processing in the Middle Ages is one such example. Materials such as ceramics and glass have a proven degree of durability and water resistance and have been used for routine daily applications for a long time. This familiarity has prompted the ongoing development of reinforced ceramics and crystal materials for applications in which strength, stability, workability, and transparency are required or become required for a society at a given time. The relatively recent development of semiconductors, which ushered in the current information-technology generation, also demanded a range of functional requirements such as optimal electrical conduction properties, insulating properties, electric-field characteristics, and transparent conductive properties in addition to strong abrasion and corrosion resistance to fabricated devices. At present, to realize next-generation compound semiconductor devices capable of operating under even more severe environments, materials with a higher heat conductivity, heat resistance, and chemical stability are required.
Against this background, sapphire, GaN, and diamond are promising single-crystal materials for next-generation optoelectronics devices. Sapphire is commonly used as a substrate for growing group-III nitride semiconductor device films, but it is becoming an essential substrate for the growth of devices such as white light-emitting diodes (LEDs). GaN substrates are expected to make next-generation environmentally resistant high-speed and high-voltage devices possible, and it is attracting attention as a substrate for power electronics. The use of diamond as a substrate is expected to realize further technical advantages in semiconductor devices because it is the ultimate semiconductor substrate. Diamond substrates are still under development, with a range of issues in regard to its practical use as a large-area substrate, but active studies of its practical use are likely in the near future. A comparison of the device performance obtained for devices grown on GaN, diamond, and existing semiconductor materials such as Si, GaAs, and InP is shown in Figure A.1. These new materials (GaN and diamond) are expected to result in improved device performances.
Figure A.1.
Device performance expected with next-generation optoelectronics materials
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A.2. Difficulties in CMP of next-generation optoelectronics materials
Characteristics of the chemical mechanical polishing (CMP) processing for the next-generation single-crystal optoelectronics materials of sapphire, GaN, and diamond in terms of the hardness of each material and its chemical stability are presented here. CMP involves removing material from a surface through mechanical and chemical action without introducing processing strain into the material. The mechanical action between the abrasive and the workpiece is related to the type of material, hardness, quality, shape, and diameter of abrasives and the concentration of such abrasives in a slurry; whereas the chemical action relates to the pH of the chemical solvent and the etching performance for the workpiece and so on. Because the CMP processing characteristics are the result of a complex interplay of various factors, a wide range of choices is available for the abrasive compound used in CMP.
The most versatile abrasive compound is a colloidal silica slurry. It is possible to obtain a surface free from processing-induced strain by using a colloidal silica slurry even if the material is a chemically stable crystal[1]. Colloidal silica is generally used for the CMP of sapphire substrates [1] and [2], and its effectiveness for the CMP of GaN has recently been reported [3].
Doy [1] derived the following relationship between the processing rate MRR, hardness H of the workpiece, and processing pressure p:
equation(A.1)
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from which it can be seen that a higher processing pressure is required for a harder workpiece. As shown in Table A.1, sapphire is harder than Si, but we see that the values for GaN are not as accurate as the others because crystal growth is still under development. The data in the literature for the hardness for GaN, for example, suggest that it is somewhere close to or twice that of silicon. Improvements in the growth techniques of bulk crystals are likely to lead toward the true value of the hardness. The most recent study [5] gives the GaN hardness as twice that of silicon; the general consensus is that GaN is slightly softer than sapphire. Based on equation (A.1), the processing pressure will be an important determining parameter in CMP, especially for the hard materials.
Table A.1.
Vickers Hardness of Si, Sapphire, and GaN (GPa)
Sapphire 28 [4], 22.5
GaN 20–21.2 [5], 21 [6], 18–20 [7], 18 ± 1 [8], 12 ± 2 [9], 12.3 [10], 11 [11], [12] and [13]
Si 12 [11], [12] and [13], 9 [7]
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The CMP processing rate can also be represented the formula proposed by Preston [14]:
equation(A.2)
MRR=k⋅p⋅v,
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where k is a constant that depends on the conditions of the abrasive environment (i.e., Preston’s constant), and v is the relative speed between the workpiece and the lap. The processing efficiency is thus determined by the interplay between k, v, and p. Note that k is mainly determined by the action between the workpiece and the abrasive compound, while p and v are factors determined by the removal conditions dictated by the machine parameters. Therefore, the design of the abrasive machine is important, but the processing equipment offer little flexibility when a material to which a high pressure has to be apply is used because of the high hardness mentioned earlier. For this reason, careful attention must be paid to the chemical action, but for materials such as sapphire, GaN, and diamond, which have a high thermal/chemical stability, the improvement in the removal rate by chemical action is limited.
The processing rates for Si, sapphire, and GaN are compared in Figure A.2. The removal rate of sapphire is 1/10 of that of silicon, and sapphire is widely regarded as a hard-to-work substrate. In the case of GaN, however, the best obtainable processing rate is only 1/100 of that of sapphire. Thus, if sapphire is a hard-to-process material, then GaN could well be said to be a super-hard-to-process material. For diamond, a CMP removal rate has yet to be established, but preliminary experimental data estimate the removal rate to be 1/1,000 to 1/10,000 of that of silicon.
Figure A.2.
Comparison of the CMP removal rates of Si, sapphire, and GaN [1] and [3]