Evaluation of new attenuating phase-shifting mask techniques

The creation of defect free and accurately biased phase shifting masks (PSMs) with correctly designed layouts is a difficult task. The behavior of PSMs, with their complicated structures and topographies, is highly sensitive to process parameters. Attenuating PSMs are becoming an important and popular mask technology, because of the manufacturing and layout difficulties associated with other PSM types. They are flexible (being useful for both isolated and repeating structures) and relatively simple to manufacture in comparison with other PSM methods (not requiring extra layers of glass or very small mask openings). In addition, the mask layout is not further complicated by multiple phase openings. However, attenuating PSMs are complicated vertical devices. The height of the attenuating material (usually chrome) and the anisotropically etched, phase shifting glass beneath the opening (See Fig. 1) have significant impact upon imaging performance. The light scattering from the vertical glass walls alters the transmitted light intensity and the defocus behavior. The percentage of light transmitted through the chrome also affects the final image considerably. Questions have also been raised about the repairability of defective masks. Recently, both DuPont Co. and Mitsubishi Corp. have introduced new attenuating PSMs where the absorber material creates the iv radians phase shift in the attenuated light [l][2] (See Fig. 2). For the creation of these new masks, anisotropic etching of the glass mask substrate is not necessary and thus one manufacturing process step is eliminated. The absorber itself is thin, approximately the thickness of the chrome for a binary transmission mask. Therefore, mask edge light scattering is greatly reduced. Mitsubishi has reported the ability to manufacture these masks with satisfactorily high film uniformity, low defect rate and high chemical durability. The successful repair of pinhole and pindot defects in the attenuating material has also been demonstrated[2]. In this work, we have been investigating performance for the standard chrome attenuating PSM and the new DuPont and Mitsubishi attenuating PSMs using the vector 2D photolithography simulator METROPOLE[3]. METROPOLE, which was developed at Carnegie Mellon University, uses the so-called waveguide method [4][5] to solve Maxwell's equations rigorously and model non-vertical light propagation, defocused reduction images, arbitrary material optical properties and light scattering from complex mask topographies. Aerial image profiles on the wafer of isolated lines are evaluated with respect to: the percent of light transmitted through the attenuator, mask opening size, attenuator height, mask bias and defocus behavior. In addition, the transmitted light intensity results were compared to those from the scalar PSM simulator SPLAT[6] to check the correctness of scalar models for performing accurate attenuating PSM biasing calculations. Defective and repaired attenuating PSM were also analyzed for possible performance degradations caused by the repair process. This research differs from previous scalar modeling studies of attenuated PSMs in that a vector simulator can properly include light scattering effects caused by the often substantial vertical structure of the attenuated PSM.