1. Field of the Invention
The present invention relates to a mullite-containing sintered body, a method for manufacturing the same, and a composite substrate.
2. Description of the Related Art
In general, a mullite sintered body is a material, which is produced by sintering an aluminum oxide (Al2O3) and a silicon oxide (SiO2) in a ratio of 3 to 2 and which has excellent thermal shock resistance, and is represented by 3Al2O3.2SiO2. Regarding such a mullite sintered body, a material produced by forming a powder, in which 30 percent by mass of yttria-stabilized zirconia (YSZ) powder is mixed into a mullite powder, and sintering the resulting compact is known, as disclosed in, for example, PTL 1. In PTL 1, a base substrate to be bonded to a GaN substrate is produced by cutting a mullite substrate from the mullite sintered body and polishing a principal surface of the mullite substrate. The thermal expansion coefficient of GaN is specified as 6.0 ppm/K in the range of room temperature to 1,000° C. and the thermal expansion coefficient of mullite is specified as 5.2 ppm/K. Therefore, in consideration of the two substrates being bonded to each other when the two substrates are used, it is preferable that the thermal expansion coefficient of mullite be increased so as to approach the thermal expansion coefficient of the GaN substrate. Consequently, the YSZ powder is mixed into the mullite powder and sintering is performed.
PTL 2 describes an example in which a composite substrate produced by directly bonding a functional substrate composed of lithium tantalate, lithium niobate, or the like to a support substrate composed of a mullite sintered body is used for acoustic wave devices, e.g., a surface acoustic wave element. Regarding such an acoustic wave device, the mullite substrate serving as the support substrate has a small thermal expansion coefficient of about 4.4 ppm/° C. (40° C. to 400° C.) and a large Young's modulus of 220 GPa or more. Therefore, expansion and shrinkage due to temperature changes of the acoustic wave device itself can be reduced and, as a result, the temperature dependence of frequency is improved to a great extent. In order to directly bond the functional substrate to the support substrate, it is required that each bonding surface have high smoothness. For example, PTL 2 mentions that the center line average roughness Ra is preferably 3 nm or less.