Date of this Version
To sense characteristics of a sample, an ellipsometer includes a pivotal diffraction grating positioned to receive white light from the analyzer without further focusing of the light after the light leaves the sample. The diffraction grating is focused on the sensor at a predetermined angle with a precision of at least plus or minus one-half degree using an alignment-sensing means positioned between the analyzer and diffraction grating. The sensor includes an aperture through which the incident beam of light is transmitted, light-sensitive areas on opposite sides of said sensor and a comparator for comparing the signal from said light-sensitive areas. Equality of the light from the light-sensitive areas indicates that the incident beam of light is perpendicular to the diffraction grating. A second diffraction grating and a second sensing means may receive the specularly reflected white light from a first diffraction grating and reflect it to another diffraction grating to investigate another spectrum of light. A spectroscopic ellipsometer uses a stacked photodiode in which the first photodiode and second photodiode have overlapping spectral ranges.