Impact of Graybeam Method of Virtual Address Reduction on Image Quality

摘要

In both optical and electron beam raster scan imaging for mask making or direct write on wafer, graybeam techniques are used to create a small virtual address grid while maintaining a large physical address grid. Using simple simulations of aerial image formation as the summation of Gaussian spots, several important conclusions about the use of graybeam are made. Graybeam results in a non-linear variation in edge position with gray level, with the non-linearity increasing with larger physical address grid size. While this edge position deviation from non-linearity can be calibrated out of the writing tool, the calibration curve is process dependent. One problem with the use of graybeam is the reduction of image quality as expressed by the image log-slope. For the case of a physical address grid equal to half of the spot size, the worst case graybeam level has an image log-slope at the edge that is 20% less than the best case.