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Proceedings Paper

Gray scaling in high performance mask making
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Paper Abstract

This paper is a review of different gray scaling techniques used in mask making. It shows that high address resolution and high throughput can be combined with the lithographic performance necessary for the most advanced applications. In the semiconductor industry, the demand for better performance in terms of clock-frequency and circuit density continues to push Moore’s law. One effect is shrinking design grids to cope with the tighter requirements on resolution, CD control, and aggressive OPC. For mask making this means that the address resolution of the mask writing equipment must be improved for every tool generation. The address resolution in the mask writer can be increased in two ways; either by decreasing the physical grid, or by introducing a virtual grid, here referred to as gray scaling. A decreased physical grid puts a high penalty on throughput, a performance parameter of utmost importance for reasonable mask costs and cycle times. With gray scaling, a fine address grid is created, while keeping a large physical grid for high throughput. In earlier publications, a single pass gray scaling technique has been shown to reduce image quality in terms of image log-slope. This paper shows that the effects are smaller when using other approaches to gray scaling and that it is kept to a minimum in the SLM-based DUV Sigma7300 mask writer, which uses partial coherent imaging and multiple writing passes. Analysis shows that for this combination of SLM and partially coherent light, the reduction in image log-slope due to gray scaling is less than 8%. In addition, the systematic averaging of four displaced writing passes makes the loss isotropic and independent of grid position. A detailed error analysis shows that a small address grid is more important for composite CD uniformity than the loss in image log-slope.

Paper Details

Date Published: 28 June 2005
PDF: 12 pages
Proc. SPIE 5853, Photomask and Next-Generation Lithography Mask Technology XII, (28 June 2005); doi: 10.1117/12.617527
Show Author Affiliations
Hans Martinsson, Micronic Laser Systems AB (Sweden)
Tor Sandstrom, Micronic Laser Systems AB (Sweden)

Published in SPIE Proceedings Vol. 5853:
Photomask and Next-Generation Lithography Mask Technology XII
Masanori Komuro, Editor(s)

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