Electrical Insulation of CMUT Elements Using DREM and Lapping

This paper demonstrates a novel fabrication process to implement a capacitive micromachined ultrasound transducer (CMUT) into a laparoscope. Due to the limited space in the laparoscope, a new method for electrically contacting the transducer elements is required. Using a state-of-the-art high aspect ratio dry etching process called Deposit, Remove, Etch, Multistep (DREM), it is possible to separate the bottom electrodes and use a common grounded top electrode, which allows for back side contacting of the device and mitigates the use of wire-bonds and external electromagnetic interference shielding to thereby reduce the footprint. DREM trenches that are 6.7 μm wide and approximately 143 μm deep have been etched into the substrate. The trenches are insulated using a thermally grown dry oxide before they are plugged through deposition of a high uniformity poly-silicon layer. To finalize the structure, the excess substrate is removed from the back side of the wafer until the bottom of the trenches are reached, and the elements become completely separated. This is done using lapping followed by polishing to reduce the surface roughness.