Application of Triton surfactant adsorption on Si surface for fabrication of 45° micromirrors

Anisotropic etching process of Si (100) and (110) planes in low concentrated potassium hydroxide (KOH) solutions containing Triton X-100 surfactant is studied in this paper. Addition of a little amount of the surfactant to the etchant considerably reduces etch rates of both the planes, though the etch rate ratio R(100)/R(110) > 1 is obtained. The (110) surface roughness is significantly decreased when Triton is added to the solution, too. Therefore, the {110} sidewall planes could be used as micromirrors inclined at 45° towards the (100) substrate. The {110} surface roughness is low in a wide range of Triton concentration, which gives some flexibility in the choice of surfactant concentration for fabrication of smooth micromirrors. Better understanding of etching processes with surfactants could help select a composition of the etching solution which yields little rough {110} planes. For that reason, the adsorption of Triton molecules on Si surface is investigated using contact angle measurements. The results show that the (110) surface is more hydrophilic as well as better wetted by the surfactant solution than the (100) one, though both the planes are rather hydrophobic. This suggests that a little more hydrophilic surface should be more advantageous to the surfactant adsorption. The explanation, based on literature reports and theoretical considerations, is proposed and associated with the etching results.