Apparatus for measuring defects in a glass sheet
A method of measuring the topography of a large, thin, non-flat specular substrate in a production environment with minimal movement of a majority of the measurement apparatus. A gimbal-mounted reflecting element is used to steer a short coherence length probe beam such that the probe beam is substantially perpendicular to a local surface of the substrate. The probe beam and the reference beam are combined and the resulting interference pattern used to characterize defects on the local surface.
1. A method of characterizing a topography of a substrate comprising;
providing a radiation beam comprising a coherence length less than about 300 μm;
splitting the radiation beam into a probe beam and a reference beam;
intercepting the probe beam with a first reflective element;
steering the probe beam with the first reflective element to perpendicularly irradiate a specular local surface of a non-flat substrate, and wherein the probe beam is reflected from the local surface;
intercepting and reflecting with the first reflector the probe beam reflected from the local surface;
intercepting and reflecting the reference beam with a second reflective element such that optical path lengths of the probe beam and the reference beam are substantially equal;
collecting the probe beam reflected from the first reflective element and the reference beam reflected from the second reflective element;
combining the reflected reference beam and the probe beam reflected from the first reflective element to produce an interference pattern;
detecting the interference pattern across a two-dimensional array of pixels; and
using a phase shifting interferometer to determine a topography of the local surface.
2. The method according to claim 1 wherein the first reflective element is steered by a galvometer.
3. The method according to claim 1 wherein the first reflective element comprises a plurality of individually steerable mirrors.
4. The method according to claim 1 wherein the first reflective element is gimbaled.
5. The method according to claim 1 wherein the non-flat substrate comprises a first side and a second side, the first side being nearest the interferometer, and wherein the local surface is on the first side.
6. The method according to claim 1 wherein the non-flat substrate comprises a first side and a second side, the first side being nearest the interferometer, and wherein the local surface is on the second side.
7. The method according to claim 1 wherein the substrate comprises glass.
8. The method according to claim 1 wherein a thickness of the substrate is equal to or less than about 0.7 mm.
9. The method according to claim 1 further comprising translating the second reflective element parallel to a longitudinal axis of the reference beam.