AFM study of surface finish improvement by ultrasound in the anisotropic etching of Si⟨100⟩ in KOH for micromachining applications

A new procedure for the production of smooth and defect-free surfaces in the anisotropic etching of silicon in alkaline solutions is described. It has been found that etching in an ultrasonic bath results in the facilitated detachment of hydrogen bubbles at the surface which is suggested to be one of the causes for surface roughening. In the presence of mild ultrasound radiation a significant improvement in surface finish of the {100} orientation has been observed. The inclusion in the bath of oxygen and/or isopropanol results in root mean square roughness values smaller than 20 nm. The effectiveness of bath additives has been related to changes of the contact angle between the liquid/gas/etching interface. Quantitative determination of roughness and surface imaging have been obtained by atomic force microscopy; dissolution rates were studied by profilometry. Etching under ultrasound conditions and the use of additives does not change the etch kinetics, indicating that additional convection only changes hydrogen bubble detachment.