| abstract |
The alumina purity of the translucent polycrystalline alumina constituting the handle substrate is 99.9% or more, and the porosity of the translucent polycrystalline alumina is 0.01% or more and 0.1% or less. The number of pores having a size of at least 0.5 mu m included in the surface area on the bonding surface side of the handle substrate is not more than 0.5 times the number of pores having a size of not less than 0.1 mu m and not more than 0.3 mu m including the surface region. |