Patent ID: 7948630
Filing Date: 2011-05-24
Classification: G01N,G03F

Abstract:
1. A method of auto focusing a workpiece for optical metrology using an illumination beam, a focus detection beam, and a focus detector, the focus detector having a capture range, a tilt angle, a plurality of sensors, and calibration data, the method comprising: (a) setting the tilt angle of the focus detector wherein the focus detector is a quadrant detector, the quadrant detector having quadrants on the right hand side and quadrants on the left hand side, the quadrant detector further having lower quadrants and upper quadrants, wherein the tilt angle is set such that the focus detection beam after having moved from the lower quadrants to the upper quadrants, the focus detection beam has also shifted to the right hand side or to the left hand side of the quadrant detector and further wherein the tilt angle is set such that the focus detection beam after having moved from the upper quadrants to the lower quadrants, the focus detection beam has also shifted to the right hand side or to the left half side of the quadrant detector; (b) directing the focus illumination beam on the workpiece, the focus illumination beam generating the focus detection beam; (c) obtaining a measurement of the focus detection beam using the focus detector, the focus detection beam projecting to a location on the focus detector based on the set focus detector tilt angle and position of the workpiece on the Z-axis; and (d) determining two or more focus parameters derived from measurements obtained from the plurality of sensors and calibration data, the two or more focus parameters used to generate an initial position of the workpiece on the Z-axis; (e) if the initial position of the workpiece is not within the capture range of the focus detector, using the two or more focus parameters to move the workpiece to within the capture range of the focus detector; and iterating (b), (c), (d), and (e) until the workpiece is within the capture range of the focus detector; (f) if the initial position of the workpiece is within the capture range of the focus detector: