Ellipsometry study of the adsorbed surfactant thickness on Si{110} and Si{100} and the effect of pre-adsorbed surfactant layer on etching characteristics in TMAH

A comprehensive review on convex and concave corners in silicon bulk micromachining based on anisotropic wet chemical etching

“…surface followed by {110} and {100} surfaces [135][136][137]. The etch rate of {100} is almost unaffected because the layer of surfactant molecules on {100} surface is not able to protect the surface from the etchant owing to its lower adsorptivity.…”

“…In the surfactantadded TMAH and IPA-added KOH, the etch rates of {110} and its vicinal planes are suppressed to considerably low level. Due to this factor, the undercutting at extruded corners, curved and non-<110> edges on {100} surface is significantly reduced [136][137][138][139][140][141][142][143][144][145][146]. It may be emphasized here that the surfactant-added TMAH is not suitable for the fabrication of microstructures on {110} wafer as the etch rate of {110} orientation is very low as presented in Figure 12.…”

A simple and robust model to explain convex corner undercutting in wet bulk micromachining

“…The Si{110} is more hydrophobic (lower H-density) than Si{110}, resulting in the formation of a more densely packed surfactant layer [22]. Several studies have been performed to confirm the orientation dependent adsorption of surfactant molecules [18,19,22]. They explain that the maximum adsorption is for {111} surface followed by {110} and {100} surfaces.…”

“…Triton-X-100, polyethylene glycol (PEG), NC-200, etc.) is added in the etchant [16][17][18][19][20][21][22][23]. In order to explain the mechanism behind the corner undercutting, several models have been proposed [12,[24][25][26][27].…”

“…This is a very simple way of explaining why a convex corner is more vulnerable to undercutting whereas there is no-undercutting at the concave corner. Now the last concern is, why does the undercutting at convex corner decrease dramatically in surfactant-added TMAH [16][17][18][19][20][21][22][23]. The summary of the etching characteristics of the pure and very small amount of surfactant (0.1% by volume) added TMAH solution is presented in Figure 4.…”