A method of forming hidden cavities in bonded silicon wafers using dry etch Bisbenzocyclobutene (BCB) is presented. The cavities are formed by vacuum bonding of partially-cured patterned BCB over a Silicon on Insulator (SOI) wafer and over a bare silicon wafer in a low temperature process. The vacuum bonding process parameters are determined through an iterative process that involves SEM inspection of the BCB layer at the bonding surface to ensure a void and wrinkle free strong bond. The cavities can be as small as 28 μm wide with a support margin of only 10 μm and a height of 800 nm. Cavities of other dimensions can also be realized following the same procedure. The cavities can be used to realize MEMS microphones, capacitive micromachined ultrasonic transducers (CMUT), resonant cavities, and also for protective encapsulation of MEMS and microelectronic dies.