Mask overlay scaling error caused by exposure energy using a stepper

The wafer expansion scaling error in overlay accuracy using a stepper has been investigated. The scaling error depends on the exposure energy, mask aperture ratio and wafer surface reflectance. The scaling error is observed only along the X axis when using a global alignment method. From the experimental results, we present a mechanism which explains the scaling error. Then the occurrence of the scaling error according to the mechanism is simulated by the heat conduction and displacement analysis. The cause of the scaling error is proved to be local wafer expansion due to the accumulated heat generated by the exposure light energy.