A method and apparatus for measuring wafers, thin films, or other planar
layers are disclosed. This invention utilizes a tunable, monochromatic
light source reflected from or transmitted through the layer to be
measured. The wavelengths of light are selected such that the light is
partially transmitted through the material to be measured so as optical
interference is seen among the interfaces of the layer(s). The
wavelengths are also controlled to sufficiently small increments to
resolve these interference features. This apparatus relates to the need
to monitor wafer thinning, film deposition, and other semiconductor
device related processes.