Simple estimation of lens aberration with pinhole aperture on the backside of photomask

A novel method for monitoring lens aberration in projection optics of a stepper is developed utilizing pinhole aperture formed on backside of photo mask. With the pinhole aperture, illumination beam to a mark pattern on the front side of a photo mask becomes semi-coherent with an incident angle which is determined by lateral distance between the pinhole and the mark. When the mark pattern generates diffraction beams within narrow angle region, imaging is carried out by using localized area of pupil. As a result, Hartmann test structure is effectively realized by this configuration. By elaboration of mask pattern, measurement error is significantly reduced resulting in sufficient accuracy for monitoring lens aberration in current scan stepper. Simulations by optical image calculation reveal that measurement error is less than 10m wave in RMS and 40m wave in maximum local deviation for an aberration which is expressed with first 35 polynomials of Zernike series. In preliminary experiments, measured aberration seems to be reasonable. This method should provide a simple, easy and cost effective tool for monitoring of lens aberration.