Silicon-on-insulator (SOI) etching method with variable selection ratio

The invention relates to an SOI (Silicon On Insulator) etching method with a variable selection ratio. Silicon on insulator materials are etched through the following steps: (1) providing a silicon on insulator substrate; (2) depositing a composite hard mask with a variable selection ratio to replace a traditional hard mask with an invariable selection ratio; (3) gluing; (4) plate making: defining a region to be etched by using a photoetching plate; (5) etching the photoresist in the defined area; (6) etching the composite hard mask; (7) removing photoresist; (8) etching the top layer silicon at a first selection ratio by using a second etching method; and (9) etching the buried oxide layer by using a third etching method according to the second selection ratio. The method is different from the traditional etching technology, can avoid damage to the side wall of the deep groove when the buried oxide layer is etched, and does not need to use an over-thick hard mask when the buried oxide layer with the thicknes