A method for determining an endpoint for etching a layer includes steps of
estimating the etch endpoint and, during etch, directing radiant energy at
two or more wavelengths onto the layer to be etched, detecting the last
intensity maximum reflected at a first wavelength prior to the estimated
etch endpoint, and detecting the intensity maximum reflected at a second
wavelength first occurring after the last intensity maximum at the first
wavelength. Also, a method for determining an endpoint for etching a layer
having an approximate initial thickness by steps of, during etch,
directing radiant energy at three or more wavelengths onto the layer to be
etched; selecting first, second, and third wavelengths; approximating an
etch rate from the time interval between a first detected intensity
minimum and an adjacent intensity maximum reflected at the third
wavelength, estimating an etch endpoint from the approximate initial
thickness of the layer and the approximate etch rate; detecting the last
intensity maximum reflected at the first wavelength prior to the estimated
etch endpoint; and detecting the intensity maximum reflected at the second
wavelength first occurring after the last intensity maximum at the first
wavelength. The material making up the layer is at least partly
transparent to both the first and the second wavelength. The first
wavelength is longer than both the second wavelength and the third
wavelength. In some embodiments the third wavelength is longer than the
second wavelength. The endpoint is at the point of intensity maximum of
the second wavelength or is at a point following an interval thereafter.
