In Optical Maskless Lithography, the die pattern to be printed is generated by a contrast device, known as a Spatial Light Modulator. The contrast device consists of a multitude of micro-mirror pixels that are independently actuated. Different physical principles can be utilized to change the optical properties of the pixels. Rasterization in Optical Maskless Lithography is an algorithm that, given the description of a pattern to be printed (e.g. an OPC'd GDS-II or OASIS mask file), computes the necessary states of the contrast device pixels. A Global Optimization rasterization algorithm for Optical Maskless Lithography was recently developed and successfully tested. Utilizing optimization techniques, this algorithm enables contrast devices to match the imaging and placement performance of conventional masks thru focus and dose. The algorithm has been demonstrated for contrast devices based on various light modulation principles, including tilt, phase-step tilt, and piston mirror devices.
This paper enhances the Global Optimization algorithm by significantly improving both computational time and memory requirements. These enhancements enable the algorithm to be implemented on an Optical Maskless Lithography scanner for printing die patterns of full size and complexity. The enhanced method is demonstrated on 130 nm node and 90 nm node SRAM layout test cases to validate the capability of Optical Maskless Lithography to reproduce realistic patterns. Simulations of the dose/focus process window in resist for rasterized patterns are presented, along with the ability of the rasterized images to match the CD and placement error performance of a conventional mask to below the level of process noise. In addition, the rasterization algorithm enhancements are verified experimentally on a calibrated tilt mirror spatial light modulator mounted to a 193 nm aerial image test stand.