Extended range focus detection apparatus

A focus sensor for providing focus detection signals that depend on the location of a workpiece surface within a focus detection range along a direction approximately parallel to the optical axis of an objective lens, the focus sensor comprising:

• an illumination source;
• a collimation lens configured to input radiation from the illumination source and output an illumination beam having a fixed degree of collimation;
• an objective lens configured to input the illumination beam and to focus the illumination beam at a nominal focal plane along an optical axis of the apparatus and to receive and transmit a reflected beam reflected from a workpiece surface located along the optical axis;
• a focus detector arrangement positioned along the optical axis to receive light from the reflected beam that is transmitted by the objective lens;
• and
• a first beamsplitting surface positioned along the illumination beam between the objective lens and the collimation lens, and along the reflected beam between the objective lens and the dual range focus detector arrangement,
• wherein the focus detector arrangement is a dual range focus detector arrangement comprising:
• a broad range focus detector configuration that is configured according to a first focus detection principle, and configured to provide at least one broad range focus detector signal that varies monotonically over a first focus detection range of the focus sensor, the broad range focus detector configuration including a broad range sub-aperture located proximate to the optical axis, and a broad range photodetector arranged to receive transmitted light from the broad range sub-aperture and output the at least one broad range focus detector signal;
• and
• a high resolution focus detector configuration that is configured according to a second focus detection principle that is different than the first focus detection principle, and configured to provide at least one high resolution focus detection signal over a second focus detection range that is smaller than, and located within, the first focus detection range, the high resolution focus detector configuration including at least one respective high resolution sub-aperture located proximate to the broad range sub-aperture and located off the optical axis, and a respective high resolution photodetector arranged to receive transmitted light from the respective high resolution sub-aperture and output a high resolution focus detector signal.