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Electrical Method Of Determining Absolute Distortion In A 1X Wafer Stepper Independent Of Stage Precision

Absolute distortion measurement in stepping metrology is critical in optimizing wafer stepper performance. The conventional reduction wafer stepper method of determining absolute distortion relies on a single reticle field of adjustable size and on a high precision stage. Data to determine distortion is collected from "box in a box" registration structures by optical or electrical measurements. This method is inappropriate, or compromising, in determining distortion of a commercial 1X wafer stepper. The 1X wafer stepper has multiple reticle fields of fixed size and achieves accurate pattern overlay thru site by site alignment without a high precision stage. The vendor of the 1X stepper has developed a method of determining absolute distortion by imaging two reticles of identical distortion on the stepper and then superimposing the image of one reticle onto the other with a known offset. The site metrology system of the stepper is used to collect data to determine distortion. This paper evaluates a new method of determining distortion of a 1X wafer stepper. The new method combines the vendor concept of superimposed reticle images with data collection and analysis techniques of electrical "box in a box" registration structures thus eliminating any stage contributions to distortion. Results presented are compared with the distortion results obtained by the vendor.