| Disclosed are apparatus and methods for determining height of a semiconductor structure. The system includes an illumination module for directing one or more source lines or points towards a specimen having multiple surfaces at different relative heights and a collection module for detecting light reflected from the surfaces. The collection module contains at least two detectors with one slit or pinhole in front of each detector that that are positioned to receive light reflected from one of the surfaces. A first detector receives reflected light from a slit or pinhole that is positioned before a focal point, and a second detector receive reflected light from a slit or pinhole that is positioned after the focal point so that the first and second detector receive light having different intensity values unless the surface is at an optimum focus. The system includes a processor system for determining a height based on the detected light received by the detectors from two of the surfaces. |
