Using AutoCAD to Generate PostScript Files for Photomasks
AutoCAD is a widely-available computer-aided design package that is very useful for engineering layout of geometric features, including a high degree of detail, complexity, organization, and replication. These aspects make it a very convenient tool for low-cost photolithography mask design and offers a very short learning curve, particularly for those with prior mechanical design background. There are multiple translators that convert AutoCAD's DXF file format into industry-standard maskmaking formats such as GDS and CIF. Less common, but often useful is PostScript (PS or EPS) representation, which can be used to drive a wide variety of high-resolution printers. This document offers some tips on how to create EPS files using AutoCAD, applied toward photomask design in particular.
A low-cost alternative to making traditional chromium-based photolithography masks is printing onto transparent plastic film using high-resolution graphics (>3000 dpi), as used in magazine publishing. These low-cost alternatives offer coarse masks that may be quite useful for early prototyping and relatively simple devices at a small fraction of the cost (e.g. a few tens of dollars, compared to many hundreds of dollars for the glass-chromium type). The trade-off is resolution. In practice the transparency-film masks do not achieve much better than 30-40 microns as a minimum feature size (line or gap width), whereas conventional photomasks typically produce on the order of 1 micron. Office laser printers (1200 dpi and up) can often be competitive in terms of apparent resolution, but opacity is far inferior to professional graphics. The most common file format for the low-cost masks is PostScript.
AutoCAD has a function PSFILL, that applies a filled property to closed polylines. Upon issuing the command, the user is prompted to select a polyline, followed by the pattern with which it should be filled. Of the few options available, "Grayscale" is the one most suited to photomasks. A Grayscale value of 0 corresponds to white, and a value of 100 corresponds to black. PSFILL is distinct from and more accurate than applying AutoCAD hatches to polylines. Hatches are an overlay geometric feature, and do not necessarily match the underlying polyline perfectly. Printed masks suffer from imperfect fits and will contain defects if based on hatching; therefore, PSFILL should always be used instead. Also note that PSFILL may not be applied to multiple polylines, so it is important to plan ahead for any array patterns, etc.
Opaque vs. Clear
Managing regions that are opaque and clear will often require careful planning of overlapped and nested polylines. This can be most simply described by analogy to painting on a canvas. Several scenario cases are presented below, and for distinction any undefined background is shown in gray. The order of feature creation (just as in painting on a canvas) dictates which polyline lies below or above other polylines. In AutoCAD 2003, changing the "Display Order" command under the "Tools" has no effect on the PostScript output and is not useful for rearranging overlay. When printed on transparencies, black is printed opaque and white is left clear. However, it is possible to print PostScript files in negative tone (swapping opaque vs. clear), and this may be a helpful option depending on the particular strategy for mask layout.
Black Rectangle First,
White Triangle Second
White Triangle First,
Black Rectangle Second
Text may be typed directly into the AutoCAD file. The characters are made to appear opaque or clear in the final PostScript file by changing their color properties. However, it can often be confusing to use the pre-defined "black" and "white" colors, because depending on display mode sometimes these colors can be automatically reversed. A workaround solution to achieve expected results more reliably is to assign text color *almost* completely white or *almost* completely black, by editing the color palette "Properties" for the text. There will be essentially no detectable difference in the final PostScript file.
Generating the PostScript File
AutoCAD (at least since version 2000) has an "Export" command under the "File" menu to export (Encapsulated) PostScript files, with the ".EPS" suffix. The most important detail when using this function is correct scaling. In the "Export" dialog box for AutoCAD 2003, there is an "Options..." feature under the "Tools" menu (upper right corner). Typically it will be desirable to assign a one-to-one correspondence between "Output Units" and "Drawing Units" for "Scale", with appropriate "Size Units" (inch or millimeter) pre-selected. It is very important to verify that the output scale matches design intent by inspecting the PostScript file after it is exported.
Viewing & Verification
Software capable of viewing PostScript files is necessary to verify the results. One of the most popular free utilities for PostScript viewing is a program called GSview from (http://www.ghostscript.com/). A number of commercial graphics packages such as Adobe Illustrator have convenient WYSIWYG (what-you-see-is-what-you-get) viewing of PostScript files. Many other applications including the Microsoft Office clipboard also have PostScript viewing capability.
The Stanford Nanofabrication Facility has a helpful list of printing service providers in the Silicon Valley area at http://snf.stanford.edu/Process/Masks/mylar.html. SNF furthermore has a broad overview of a few different maskmaking options at http://snf.stanford.edu/Process/Masks/Masks.html.
San Josť State University | Mechanical & Aerospace Engineering | 2003 Nov 08 by S. J. Lee