Photochemical etching of silicon and silicon nitride using synchrotron radiation (SR) was studied in the presence of reactive species produced by a 2.45-GHz microwave discharge in a mixture of SF6 and Ar. SR irradiation enhanced the etching reaction. The difference in patterning was investigated by using two kinds of SR that have different energy dispersions. It was found that the effect of SR irradiation is to achieve area-selective etching on the sample surface and SR with shorter wavelength is useful for achieving a clear pattern transfer. The increments of the etched depth due to monochromatic SR irradiation depended on the photon energy and were compared with the total electron yield (TEY) spectra. The etch rate for the increments was found to be almost proportional to the light absorption of the sample fully exposed to reactive species. It was suggested by TEY measurements that a fluorinated overlayer is formed on the sample surface during etching.