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For optimum optical lithography it is critical to maintain the wafer plane in the aerial image plane of the stepper since the aerial image plane is the stepper focus plane. There are a variety of off-line measurement techniques to independently verify the stepper focus, including short step focus, pin dot, clearing gradients and phase shift monitors. However, these techniques generally require operator inspection. This paper exploits the short step focus routine and provides an alternative technique for automated inspection and analysis. The short step focus routine involves microstepping the test image across the wafer in nine different steps of focus. The test image is comprised of a large array of resolution structures (equal lines and spaces) of various geometries in both vertical and horizontal orientations. The focus increments and exposure energies are user defined. The operator conducts a visual inspection to determine optimal focus and image tilt which can then be adjusted on the stepper as necessary. The industry's progression to smaller critical dimensions, meaning higher NA tools and smaller depth of focus, implies less tolerance for errors in the determinations of stepper focus. In general, the use of traditional operator dependent visual inspection techniques suffer from subjective interpretation and require long inspection times. An alternative technique that is readily adapted to a standard test reticle and metrology tool is introduced and will show how optimal focus and image tilt from the short step focus routine can be automatically calculated using RS1 programming language (RPL) and fed back to the stepper. In order to successfully implement the KLA 5107 optical metrology system to automate the inspection and analysis, several challenges had to be overcome: (1) pattern recognition -- because the test image pattern is staggered, (2) resolution capabilities of the optical metrology tool, and (3) the development of an algorithm for data interpretation. The sensitivity of this technique to resist thickness, top and bottom critical dimensions of the test image, side-wall angles, and exposure energies are explored. The capabilities of the optical metrology tool compared with an E-beam metrology system are discussed, and a comparison of this automated technique with other prevailing techniques is discussed.
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