Back Cavity Formation on 50µm Thick Bumped PMUT Wafers

PMUT (Piezo Micromachined Ultrasound transducers) fabricated on silicon wafers require the definition of the membrane for the membrane to flex. One way this is done is by etching from the back side of the wafer so that an array of cavities can be formed with the ultrasound wave being emitted from the back side (Fig1). However, for high frequencies where a Bumped thin wafer is required, handling of the bumped thin wafer in a MEMS (Micro electro-Mechanical Systems) foundry is problematic. This has led to the development of a process flow utilising a Plasma Dicer in the Back-End Processing for the cavity Formation. In this paper the full process flow for the Cavity formation along with measurements of the cavities formed in 50um thick bumped wafers are outlined.