Patent ID: 6930366

Claim:
A method for forming cavities in prefabricated silicon wafers comprising a first silicon layer ( 1 ), a second monocrystalline silicon layer, or a so-called a structural layer ( 3 ), oriented substantially parallel with said first silicon layer ( 1 ) and an insulating layer ( 2 ) situated between said first and second layers ( 1 , 3 ), the method comprising the steps of fabricating in at least one of said second silicon layers ( 1 , 3 ) windows ( 4 ) extending through the thickness of said layer, and etching cavities in said insulating layer ( 2 ) by means of etchants passed to the layer via said fabricated windows ( 4 ), characterized in that subsequent to the fabrication step of said windows ( 4 ) and prior to said etching step, a thin porous layer ( 5 ) is formed on the surface to be processed such that the etchants can be passed through said porous layer into said cavities ( 6 ) being etched, and after said cavities ( 6 ) are etched ready, at least one supplementary layer ( 7 ) is deposited in order to render to the material of said porous layer impermeable to gases.