Fig. 2: Fabrication of silicon nanocolumn and snCMUT arrays.

The FEA simulation of moving plate in CMUT compared to conventional CMUT (a) and snCMUT (b). The snCMUT achieves a high average displacement through the parallel motion of the plate induced by nano-silicon springs. c Schematic illustrations of fabrication steps for top-down approaches of silicon nanocolumn: i) PR patterning by conventional photolithography on a 4-inch silicon wafer after oxidation for growth silicon dioxide, ii) dry etching of silicon dioxide layer by RIE, iii) Defining silicon microcolumn via DRIE, iv) shrink down of silicon microcolumn via RIE to produce silicon nanocolumn. SEM images of fabricated silicon microcolumn (d) and silicon nanocolumns with diameters of 800 nm (e-i), 600 nm (e-ii), and 300 nm (e-iii). Etch time and opening area (W) of RIE in (e-i, ii, and iii) are 5, 6, and 8 min, 10, 5, and 4 µm, respectively. The scale bar is 2 µm. f The diameter of the silicon nanocolumn as the etch time and opening area. Data are presented as means ± standard deviation (n = 10 independent samples). g SEM image of silicon nanocolumn while pico-indentation (left) and the compressed displacement of silicon nanocolumns as pico-indentation force and diameters of silicon nanocolumns (right). The scale bar is 2 µm. h Photograph of fabricated snCMUT array. i–k SEM images of fabricated snCMUT. Perspective view of snCMUT (i). The piston top plate was cut via FIB to evaluate the dimension of the silicon nanocolumn and the vacuum gap. The cross-sectional SEM image of the silicon nanocolumn (j) and the 180 nm vacuum gap (k). During the cutting of the piston top plate via FIB, by-products from ion milling were redeposited onto the silicon nanocolumn. The scale bar is 50 µm in i, 1 µm in j, and 100 nm in (k). l, m, TEM images of wafer bonding surface (l) and thermally grown silicon dioxide surface (m). Despite the narrow cross-sectional area of the silicon nanocolumn, it was well bonded with the top SOI wafer. The scale bar is 5 nm. All the representative cross-sectional SEM and TEM images of the silicon nanocolumn were obtained from repeated experiments more than 4 times.