Patent ID: 7728961

Claim:
A method for operating a surface height sensing system for detecting a location of a portion of a workpiece surface along a direction approximately parallel to the optical axis of an objective lens, the method comprising: outputting a workpiece illuminating beam from a light source; providing the workpiece illumination beam with a controllable degree of collimation; inputting light from the workpiece illuminating beam having the controllable degree of collimation to the objective lens; outputting the light from the workpiece illuminating beam from the objective lens such that it is focused at an illumination focus height near a proximate portion of the workpiece surface; inputting reflected workpiece illuminating beam light from the workpiece surface to the objective lens, and transmitting the reflected light through the objective lens to provide a focus-detection light beam; inputting the focus detection light beam to a wavefront sensor, and operating the wavefront sensor to sample at least two local ray angles at separated regions along a wavefront of the input focus detection light beam to provide at least two output signals that depend on a characteristic of the input focus detection light beam that depends at least partially on a difference between a location of the illumination focus height along the direction approximately parallel to the optical axis of an objective lens and a location of the proximate portion of the workpiece surface along the direction approximately parallel to the optical axis of an objective lens; and performing operations that detect the location of the proximate portion of the workpiece surface along a direction approximately parallel to the optical axis of the objective lens based at least partially on the at least two output signals provided by the wavefront sensor, wherein: the operations that detect the location of the proximate portion of the workpiece surface comprise adjusting the controllable degree of collimation provided to the workpiece illumination beam until the at least two output signals from the wavefront sensor correspond to the location of the illumination focus height approximately coinciding with the location of the proximate portion of the workpiece surface.