Film thicknesses of W, TiN, and TaSiN in a full-metal gate stack with photoresist masks were measured in real time during plasma etching with an in situ thickness monitor. The accuracy of such measurement was approximately AE1 nm. The monitor functions on the basis of the interference of plasma optical emission, reflected from the surface and base of the film. Although W and TiN have large absorption indexes, their thicknesses were more accurately measured when they became thinner. In particular, W thickness was estimated more accurately when it was smaller than approximately 10 nm.