Manufacturing and applications of surface-micromachined and SOI-based cMUTs

An inventive process involving a porous silicon membrane acting as an etching filter has been adopted for the fabrication of various types of capacitive Micromachined Ultrasonic Transducers (cMUTs). It has proven to be a useful processing method in both surface and SOI micromachining technologies. The method allows HF etching and drying of the sacrificial layers through thin porous polysilicon layers with superb yield. We have demonstrated both SOI-based and surface-micromachined cMUTs using this technology.The SOI-based cMUTs designed at VTT were engineered for different environmental measurement technologies at frequencies below 1 MHz. The coupling surface is a smooth single-crystal silicon plane passivated for hard environmental conditions. Due to its robust construction the component can stand high level mechanical shocks, and a good contacting and passivation technology enables the insertion of these cMUTs into demanding environmental conditions.The surface micromachined cMUTs are not readily useful in any environmental conditions due their lack of mechanical strength. However, they have a very good coupling factor into the air. Also, the manufacturing process developed at VTT is very simple, reliable and has an excellent yield. This makes the surface-micromachined cMUTs a perfect choice for several low-cost applications at the high frequency range above 2 MHz.In addition to the fabrication technologies, we present data on cMUT application in gas flow rate measurement based on the phase difference detection method.