Si(111) and (100) surfaces etched in a trimethyl-2-hydroxyethylammonium hydroxide (choline) solution at 70degreesC have been studied using spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). The SE data indicate that a surface native oxide is etched gradually and then lifted off in places with immersion in the solution. Since the solution attacks bulk silicon more harshly than the surface native oxide, the (111) and (100) surfaces of partly remaining SiOx become very rough. The AFM image confirms a roughened overlayer on the surface. Just after the surface native oxide is etched away completely, the SE data yield the spectrum of a nearly flat (111) surface. In the case of the (100) surface, the roughened surface overlayer is observed even after long etching. This surface is caused by the formation of pyramids with different sizes. A correlation between the SE-estimated and AFM-determined roughness values has been discussed. The behaviors of silicon etching in other alkaline solutions (NaOH and tetramethylammonium hydroxide) are also presented and discussed.