Optical rasterization algorithms for contrast devices utilizing different physical modulation principles in Optical Maskless Lithography

摘要

In Optical Maskless Lithography (OML), the die pattern to be printed is generated by a contrast device known as SpatialLight Modulator (SLM), consisting of a multitude of pixels. Each pixel is independently actuated so as to change itsoptical properties. Different physical principles can be used to modulate the light. For instance, liquid crystal pixelscan be used to vary the amplitude transmittance of a pixel, or mirrors actuated by tilting or pistoning can be used to varythe amount of light from each pixel reaching the image plane. Optical rasterization is an algorithm that, given thedescription of the pattern to be printed (e.g. a GDSII mask file), computes the states (e.g. pixel transmittance or pixelmicro-mirror tilt / piston) of the contrast device pixels that will reproduce the pattern at an optical image plane.The purpose of this paper is to present the Global Optimization (GO) rasterization algorithm based on matching thepupil field generated by the given mask, taking into account the constraints dictated by the modulation principle of thecontrast device. In particular, we discuss a relation of GO algorithm and a grid filter approach to rasterization inMaskless Lithography. Also, a global optimization algorithm allowing the minimization of light loss is formulated anddiscussed.We present simulated results of lithographic patterns at the 65 nm node imaged using both tilt mirror and piston mirrorcontrast devices. In contrast with the previously reported work, we demonstrate that for a particular case of an SLMwith piston mirror pixels, the presented GO rasterization algorithm results in aerial images that do not exhibit placementdrift with defocus. The variations in the rasterization procedure needed to account for contrast devices with differentphysical modulation principles are discussed.