Then, {TAIL ~ the resultant surfaces} of {HEAD ~ first and second semiconductor} substrates 11 and 12 are attached to each other in an adequately clean atmosphere and this structure is then heat treated at, for example, above 1000° C. to obtain composite substrate structure 10, with the mirror finished surfaces bonded with oxide film 13 therebetween  
Then , { TAIL ~ the resultant surfaces } of { HEAD ~ first and second semiconductor } substrates 11 and 12 are attached to each other in an adequately clean atmosphere and this structure is then heat treated at , for example , above 1000 ° C. to obtain composite substrate structure 10 , with the mirror finished surfaces bonded with oxide film 13 therebetween
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