Fabrication and characterization of a micromachined acoustic sensor with integrated optical readout

Implementation and characterization of a micromachined acoustic sensor with integrated optoelectronic readout is described. The mechanical part of the sensor is surface micromachined on a quartz substrate and consists of an aluminum membrane which is electrostatically actuated by a back electrode shaped in the form of a diffraction grating. Optical detection is performed by measuring the reflected diffraction orders when the grating is illuminated through the quartz substrate. This scheme provides interferometric displacement detection sensitivity as well as a compact optical interconnect to a custom designed silicon photodetector array fabricated with n-well CMOS technology. The array also contains optical apertures formed by post-CMOS deep reactive ion etching for backside illumination. A compact hybrid packaged sensor array is formed by bonding the silicon photodetector array to the quartz substrate, resulting in an integrated acoustic sensor volume of 2.5 mm³. Experimental characterization has been performed on integrated sensors with 200-mm-diameter, 1-mm-thick aluminum membranes. The results show a minimum detectable membrane displacement of 2.08×10^-4 Å/√Hz at 20 kHz and 1.35×10^-4 Å/√Hz at 100 kHz with 61-mW laser power detected on the integrated photodetector. Operation of the device with a pulsed vertical cavity surface emitting laser as the light source and differential detection of diffraction orders for noise reduction are demonstrated to show the potential for low-power, low-cost micromachined acoustic sensors.