diffusion is the anneal temperature and the thickness of the top semiconductor layer 10 for example the minimum annealing conditions to dissolve 2 nm of interfacial sio 2 with 100 nm of top si lt 111 gt layer 10 in a ar or h 2 atmosphere are 1 100 u00b0 c for 2 hr or 1 200 u00b0 c for 10 min or 1 250 u00b0 c for 4 min the temperature and the duration of the heat treatment are then chosen for inciting an amount of oxygen of the oxide layer 30 to diffuse through the top layer 10 then the thickness of the oxide layer 30 decreases by a predetermined value additionally the thickness of the top layer 10 may also have been chosen when forming it for inciting the diffusion particularly the thickness of the top layer 10 and the temperature of the heat treatment determine the mean reduction rate of the oxide layer 30 more the thickness less the rate more the temperature more the rate for example the thickness and temperature may be predetermined such that at least about 0 5 angstroms per minute of oxide layer 30 mean reduction rate is reached to this purpose for a temperature of about 1200 u00b0 c a thickness of a si single crystal layer 10 is chosen less than 2500 angstroms only the duration of the heat treatment is then necessary to control for accurately reducing the thickness of the oxide layer 30 by a predetermined value alternatively the thickness of the top layer 10 has been chosen for reducing the oxide layer 30 by a predetermined value.