Method and apparatus for imaging semiconductor device properties

An optical path length imaging system comprising

• a) a radiant energy point source for generating radiant energy;
• b) a first focuser for focusing the radiant energy generated by the point source onto a target and scanning the target with the focused radiant energy;
• c) a collector for collecting the focused radiant energy that is scattered from said target;
• d) a splitter for splitting the collected radiant energy into two paths;
• e) a second focuser for focusing each of the two paths of the collected radiant energy onto separate focal spots;
• f) a pair of spatial filters which pass a portion of the focused radiant energy in relation to the optical path to said target one spatial filter for each of the two paths of the collected radiant energy with one of the spatial filter being placed at one of the focal spots, wherein the spatial filters are offset from each other along the path of the focused radiant energy;
• g) detectors for separately detecting the focused radiant energy which passed through each of the spatial filters and producing signals proportional to the quantity of detected focused radiant energy present;
• and
• h) a processor for generating an image signal by combining said produced signals, wherein said image signal is related to the distance traveled by the radiant energy from the focusing means to the target and back to the collecting means.