Composite substrate and method of manufacturing the same

A method of manufacturing a composite substrate, comprising:

• preparing a first substrate which is formed of a first silicon having a dopant;
• forming a semiconductor layer formed by an epitaxial growth a second silicon on a main surface of the first substrate;
• following the forming of the semiconductor layer, bonding the semiconductor layer and a second substrate of insulating;
• and
• after the bonding, selectively etching the semiconductor layer from a side of the first substrate up to a middle portion in a thickness direction of the semiconductor layer by using an etchant, wherein
• the etchant has an etching rate with respect to silicon which decreases by a not less than an certain value in a dopant concentration of a threshold lower than a dopant concentration of the first substrate,
• in the forming the semiconductor layer, the semiconductor layer is formed so as to comprise a first region in a thickness direction of the first substrate, the first region being in contact with the first substrate and in which a dopant concentration thereof decreases to the threshold as a distance from the first substrate increases, and
• in the bonding the semiconductor layer and the second substrate, bonding the semiconductor layer and the second substrate is performed by activating main surfaces of both which are bonded to each other and then bringing the main surfaces of both into contact with each other at a normal temperature which is 200° C. or lower.