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Development of High-Quality Attenuated Phase-Shift Masks by Toshihiro Ii and Masao Otaki, Toppan Printing Co., Ltd.
Along with the year-by-year acceleration of semiconductor device miniaturization, the frequency of technology roadmap renewal has increased by a factor of three from once every three years to once a year. It is not possible to cope with fabrication of high-density semiconductor devices simply by reducing the pattern size of the mask because aggressive pattern shrink on the photomask will lead to deterioration of the resist pattern when transferred to the silicon wafer surface in the exposure process. To tackle this problem, OPC (optical proximity correction) features are added to the photomask, which results in increased complexity and miniaturization in the photomask-making process. Consequently, the volume of mask-pattern data is growing drastically, and the time required for mask-defect inspection and mask-writing processes keeps increasing. Moreover, the need to achieve outstanding process accuracy raises the costs of mask production and inspection tools. Such a drop in mask throughput and the increase in cost of materials and tools seriously affects the costs of photomasks.
Step-and-scan-exposure systems adopted ArF lithography in 1999, however, ArF lithography systems, including resist, are still under evaluation and development. From 2000 to 2001, the feature sizes of semiconductor devices will be further reduced to 0.13 Âľm while it is clear that KrF lithography will remain dominant. Challenges of the photomask
Photomask technology is currently facing a number of challenges in various fields. The major challenges are listed below: 1) material 2) volume of mask-pattern data 3) mask exposure and mask-fabrication process 4) inspection and measurement (quality assurance) 5) cost and delivery
Cost will be particularly important in the future of mask production. The drop in mask throughput mentioned above is a primary factor affecting cost. Improvement of accuracy is another challenge along with mask pattern miniaturization. For accuracy, it is critical to come up with a solution to the fluctuation of line width, which constitutes a more serious problem as mask patterns get finer. Fluctuations of mask-pattern dimensions have a multiplied pattern profile impact on the wafer surface, which is called MEEF (Mask Error Enhancement Factor). To be more specific, a change of pattern dimensions on the photomask is multiplied by a factor of two to three times when the pattern is transferred to the wafer surface in the exposure process. The PSM (phase-shift mask) is capable of considerably reducing the MEEF effect, tolerating fluctuation of mask pattern dimensions to some extent. In this sense, the PSM is effective in suppressing mask costs. In general, accuracy of mask fabrication is primarily determined by mask writer and manufacturing process procedure. As for the PSM, however, material selection is a dominant factor for accuracy. For inspection and measurement of the photomask, the major challenges are improvement of detection sensitivity in defect inspection and the establishment of PSM inspection Autumn 2000
Yield Management Solutions
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