Rotating-compensator ellipsometer

In an automatic ellipsometer for measuring characteristics of a sample and including:

• a monochromatic light source for directing a light beam of a known wavelength onto said sample at a known angle of incidence so that the beam is reflected from the surface of the sample;
• a first polarizing element in the path of incident light beam and adapted to be fixed at a predetermined angle relative to the plane of incidence of the beam;
• adjusting means for setting the azimuth of said first polarizing element at different discrete predetermined angles relative to said plane of incidence;
• a second polarizing element disposed in the path of the reflected light beam;
• a photoresponsive device in the path of said reflected light beam and responsive to said beam, after it has passed through said second polarizing element, to produce an electrical signal which is a function of the intensity of the reflected light beam;
• an angular encoder;
• an analog-to-digital converter;
• and
• a data analyzing means;
• the improvement comprising:
• a rotating optical compensator disposed in the path of the light beam;
• said second polarizing element being fixed at a known angle relative to said plane of incidence;
• said angular encoder being associated with said rotating compensator for providing outputs consisting of first pulses for each revolution of said rotating compensator and respective second pulses for each chosen fraction of each of the revolutions;
• said analog-to-digital converter being responsive to the outputs of said angular encoder and to the signal from said photoresponsive device for digitizing said signal in accordance with said second pulses;
• and
• said data analyzing means being responsive to the digitized signal from said analog-to-digital converter and to said first pulses for analyzing the digitized signal.