Wafer-scale Fabrication of Nanometer Silicon Posts for Capacitive Micromachined Ultrasonic Transducers with Substrate-Embedded Springs

A capacitive micromachined ultrasonic transducer (CMUT) with substrate-embedded springs has been demonstrated high transmit efficiency with non-flexural parallel-plate piston movement. In this paper, we introduce whole wafer-scale uniform nanometer silicon posts with a fabrication technique of combination between DRIE and RIE. In order to maintain our fabrication cost-effective, a photomask with a minimum feature size of 2.0 μm was used for whole wafer-scale photolithography, and a size-reduced RIE process followed by DRIE was conducted for achieving the sub-micron or nanometer post area. Furthermore, In-situ nanomechanical tests of the fabricated silicon posts were conducted using a pico-indenter (PI 85L Pico-Indenter, Bruker) monitored under a scanning electron microscopy. The length and the diameter of the silicon post after size-reduction by RIE are measured as 5.6 μm and .74\ \mu m$, respectively. The uniformity across the whole 4-inch wafer is less than 5%. The loading-unloading graph by In-situ nanomechanical experiments confirmed that the silicon posts could consistently achieve above 6% elastic strain. We are currently applying this nanometer springs to the CMUTs with substrate-embedded springs.