High numerical aperture Hartmann wavefront sensor with pinhole array extended source

In situ aberration measurement of projection objective is necessary for lithography tool. For 90 nm technology node, aberration measurement accuracy of 1 nm rms is required. In this paper, a high numerical aperture Hartmann wavefront sensor with pinhole array extended source is proposed. The sensor uses source mask with pinhole array on the object plane of projection objective to filter the aberration of illumination optics as well as provide sufficient power required by Hartmann sensor. A coupling objective, which is installed at the confocal position of the projection objective under test, transforms the high numerical aperture spherical waves to plane waves. A null mask, which has similar structure with source mask, can be inserted at the image plane of projection objective. With the null mask installed and source mask uninstalled, the systematic measurement errors mainly caused by coupling objective can be calibrated by the relative measurement process. In this paper, some design considerations of source mask and null mask are presented. Using partial coherent light propagation and Fourier optics theory, the proper spacing and quantity of pinholes on either source mask or null mask are calculated. Finally, measurement accuracy of the sensor is evaluated using three-dimensional electromagnetic simulation of 193nm high numerical aperture converging beam propagation through pinhole with different pinhole parameters. Simulation results show that, measurement accuracy of the sensor is better than 0.5 nm rms in theory after systematic errors calibration.