A novel approach to the design and fabrication of released membrane thermal sensors has been developed. Its advantages are to facilitate processing over the suspended membranes after their liberation, and to allow the design of 1 mu m to 100's of mu m deep cavities under the membrane. It consists in sacrificial etching of a layer up to an open silicon area hidden under the membrane, followed by front-side bulk micromachining of silicon under the membrane, to the depth desired. This approach is compatible to complementary metal-oxide-semiconductor commercial fabrication, and only needs one protective coarse lithography step and a few etching without alignment. This approach may be used in the fabrication of low temperature heated membrane gas sensors and thermal pressure sensors, as of thermal sensors in general, since thermal isolation may be optimized to high values. (C) 1998 American Vacuum Society. [S0734-2101(98)05102-1].