Abstract:
A condenser microphone suited for on-chip integration with CMOS signal conditioning circuits has been developed. The microphone structure consists of a polyimide diaphragm, sensitive to sound, and a perforated polyimide backplate. The structure is similar to previous single-chip microphone designs; however, with the introduction of polyimide, it is possible to realize a low-temperature (<300 (degrees)C) fabrication process, consisting of only evaporation, spin-on, and silicon etching processes, which are all compatible with preprocessed CMOS circuits. Microphones with sizes of 1.6 and 2.1 mm, and an air gap of 1.5 (mu)m were fabricated. The devices have been characterized using an acoustical setup, in which the response of the microphone is compared to that of a known reference microphone. The frequency responses are flat within (plus or minus)2 dB in the frequency range from 100 Hz to 15 kHz, and the open-circuit sensitivities at 1 kHz with a bias voltage of 15 V were -46.1 dBV/Pa (1.6 mm) and -41.8 dBV/Pa (2.1 mm). The A-weighted noise was measured to be -112 dBV for both microphones, yielding an open-circuit equivalent noise level of 28 and 24 dBA SPL.