Wavefront modulation methods for EUV maskless lithography

摘要

Wavefront modulation methods based on a general multiple-scan imaging model are invented for EUV maskless lithography. The model includes the effects of both deterministic image blur caused by uniform linear scanning of the wafer and stochastic blur due to laser's random timing jitter. It is shown that the expected blurred image intensity is a linear function of a “double convolution” of the stationary image with the “scanning pupil” function and the probability density function of the laser's timing jitter. Consequently, the spectrum of the expected blurred image is the product of the stationary image spectrum and the spectrums of the “scanning pupil” function and the probability density function. An inverse-filtering method to modulate EUV wavefront is invented to reduce image blur by coating the EUV reflective mirror on the Fourier plane with a thin absorbing layer whose thickness profile will determine the amplitude and phase modulation of the incident wave. It is also proposed that the random image noise can be minimized with a Wiener-type filter and the placement errors can be reduced by increasing the scan times. Two processes are invented to fabricate the proposed filters.