CMUT cavity pressure measurement using an atomic force microscope

A method for non-destructively measuring the pressure inside the sealed cavity of a wafer-bonded capacitive micromachined ultrasonic transducer (CMUT) using an atomic force microscope (AFM) has been presented. The AFM measurements were performed using the dynamic mode (tapping mode) operation of an AFM to obtain the maximum deflection of the CMUT diaphragm center. This data was then post-processed using both finite element and analytical methods to determine the pressure inside the sealed CMUT cavity. The CMUT geometry was fabricated with a 1.63 μm thick, 28 μm sidelength metalized square polymer diaphragm, which is separated from a silicon backplate by a 0.75 μm thick vacuum cavity. The pressure inside the CMUT cavity was measured to be 836.82 kPa higher than the atmospheric pressure. The 3D finite element analysis predicted a maximum deflection of 227.2472 nm at the center of the diaphragm, caused by a measured pressure of 836.82 kPa. This result deviates by only 0.55% from the maximum deflection of 226 nm measured by AFM at the center of the diaphragm. The deviation is 6.19% from the mean deflection of 214 nm measured by AFM. `This method can be used to evaluate the vacuum quality inside a CMUT cavity to characterize the bonding process and measure the residual stress in a laminated diaphragm. This method can also be used to characterize any outgassing from the structural materials of a CMUT for the purpose of estimating its lifetime, or calibrating its operational condition as it ages, in order to enhance its reliability. Additionally, the method can be extended to measure the vacuum quality and outgassing of other types of micro-sized vacuum enclosures for their estimating their lifetime.