Abstract: A light detector is configured such that a light receiving portion having APDs and a peripheral portion are provided on a first principal surface of a p-type semiconductor substrate, and further includes a back electrode provided on a second principal surface of the semiconductor substrate and a p-type first separation portion provided between the light receiving portion and the peripheral portion. The APD has, on a first principal surface side, an n-type region and a p-epitaxial layer contacting the n-type region in a Z-direction. The peripheral portion has an n-type MISFET provided at a p-well and an n-well provided to surround entire side and bottom portions of the p-well.

Abstract: An optical disc consists of a base plate, a reflective thin layer and a protective thin layer. As the base plate, polymethylmethacrylate is used. Since defects in the protective thin layer are a function of defects in the base plate, in order to reduce defects in the protective thin layer, defects in the base plate are reduced. For reducing defects in the base plate, a copolymer of methyl methacrylate and methyl acrylate is used.