MECHANICAL LIMITATIONS IN THE PERFORMANCE OF INTEGRATED ACOUSTIC SENSORS

The sensitivity of silicon micromachined sensors based on diaphragm st ructures has been lower than anticipated due to a number of factors. O ne of the most fundamental of these is the mechanical response of the diaphragm. In this study measurements of diaphragm deflection are made for both silicon and silicon nitride diaphragms. Interferometric meas urements are used for small deflections and profilometer measurements for large deflections. The results clearly demonstrate the increasing importance of membrane stresses as the thickness is decreased. Small-d eflection measurements for 2.5 and 5 mum thick Si diaphragms indicate a stress of about 3 X 10(7) Pa. Reducing the thickness by a factor of two increases the maximum deflection by a factor of about three, rathe r than the factor of eight predicted for an unstressed plate. SiN diap hragms have a higher stress, and show similar behavior. The large-defl ection measurements are believed to be the first measurements of diaph ragm response at a deflection-to-thickness ratio significantly larger than one, demonstrating the ability of these structures to withstand l arge overpressures. Silicon nitride deposited by PECVD offers advantag es over silicon in etch selectivity and control over internal stress.