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.
In Optical Maskless Lithography (OML), the die pattern to be printed is generated by a contrast device known as Spatial Light Modulator (SLM), consisting of a multitude of pixels. Each pixel is independently actuated so as to change its optical properties. Different physical principles can be used to modulate the light. For instance, liquid crystal pixels can be used to vary the amplitude transmittance of a pixel, or mirrors actuated by tilting or pistoning can be used to vary the amount of light from each pixel reaching the image plane. Optical rasterization is an algorithm that, given the description of the pattern to be printed (e.g. a GDSII mask file), computes the states (e.g. pixel transmittance or pixel micro-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 the pupil field generated by the given mask, taking into account the constraints dictated by the modulation principle of the contrast device. In particular, we discuss a relation of GO algorithm and a grid filter approach to rasterization in Maskless Lithography. Also, a global optimization algorithm allowing the minimization of light loss is formulated and discussed.
We present simulated results of lithographic patterns at the 65 nm node imaged using both tilt mirror and piston mirror contrast devices. In contrast with the previously reported work, we demonstrate that for a particular case of an SLM with piston mirror pixels, the presented GO rasterization algorithm results in aerial images that do not exhibit placement drift with defocus. The variations in the rasterization procedure needed to account for contrast devices with different physical modulation principles are discussed.
We review the current status of optical maskless lithography technology. The optical maskless systems presented in literature are either aimed for low-cost, large-feature, low-end production, or high-end performance directly competing with state of the art optical scanners. In the latter case, optical maskless systems are based on piston or tilting micromirror spatial light modulators (SLMs). Similar performance can be achieved with both mirror types, but tilting mirrors offer lower manufacturing complexity, the possibility of using larger mirrors, less complex rasterizing algorithms, and lower demands on the data path. This indicates that the tilting mirror arrangement might be more appealing for high-performance, high-capacity, economical optical maskless lithography. With the latest technology in SLM and rasterizing technology, an optical maskless tool can match regular mask-based scanners in all imaging modes at the 65-nm node, including binary, weak and strong phase-shifting, phase edge, and chromeless phase lithography. Optical maskless lithography can further provide an almost complete transparency with current lithography technology in terms of design rules, optical proximity correction (OPC) models, and illumination settings. Any difference is due not to the SLM, but to the reticle process and electromagnetic properties.
The business case for Maskless Lithography is more compelling than ever before, due to more critical processes, rising mask costs and shorter product cycles. The economics of Maskless Lithography gives a crossover volume from Maskless to mask-based lithography at surprisingly many wafers per mask for surprisingly few wafers per hour throughput. Also, small-volume production will in many cases be more economical with Maskless Lithography, even when compared to “shuttle” schemes, reticles with multiple layers, etc. The full benefit of Maskless Lithography is only achievable by duplicating processes that are compatible with volume production processes on conventional scanners. This can be accomplished by the integration of pattern generators based on spatial light modulator technology with state-of-the-art optical scanner systems. This paper reports on the system design of an Optical Maskless Scanner in development by ASML and Micronic: small-field optics with high demagnification, variable NA and illumination schemes, spatial light modulators with millions of MEMS mirrors on CMOS drivers, a data path with a sustained data flow of more than 250 GPixels per second, stitching of sub-fields to scanner fields, and rasterization and writing strategies for throughput and good image fidelity. Predicted lithographic performance based on image simulations is also shown.
Maskless lithography imaging based on SLM tilt mirror architecture requires illumination of light on a non-planar reflective topography. While the actual mirror dimensions can be much larger than the wavelength of light, the spacing between mirrors and the tilt range of interest are on the order of the wavelength. Thus, rigorous electromagnetic solution is required to capture light scattering effects due to the non-planar topography. We combine high NA imaging simulation with rigorous simulation of light scattering from the mirrors to study its effects on 65nm maskless lithography imaging. We vary mirror size, mirror tilt arrangemetn, feature type and illumination settings and compare the rigorous light scattering imagign resutls wtih standard imaging simulations using Kirchoff approximation. While electromagnetic scattering effects are present in the form of lateral standing waves and edge streamers in reflected light near-field intensity, they have negligible effects on SLM imaging for mirror sizes more than 1μm<sup>2</sup>. The effects of mirror tilt arrangement on diffraction orders aer used to study the through-focus behavior of alternating rows arrangement used in the SIGMA maskwriters as well as alternative arrangements. The good imaging properties of the alternating rows arrangement used in the SIGMA maskwriters as well as alternative arrangements. The good imaging properties of the alternating rows arrangement are confirmed and a multipass overlay scheme giving further image fidelity improvements is suggested.