First and second laser inputs are directed to a location on a semiconductor
wafer to be interrogated. A first signal analyzer receives a first light
signal at a first second-harmonic wavelength generated by the first laser
input and converts the signal to a first electronic signal, thus
monitoring the intensity of the first second-harmonic wavelength as a
function of contamination. A second surface optical signal analyzer
provides a similar function at a second second-harmonic wavelength
generated by the second laser input and provides a second electronic
signal. A third surface optical signal analyzer receives a third light
signal at a sum-frequency wavelength generated by the first laser input
and the second laser input on the semiconductor wafer to be interrogated
and converts the light signal to a third electronic signal. An electronic
signal analyzer compares the first, second and third electronic signals
for determining the level of semiconductor wafer contamination.
