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Shadow Masks

How and when to use and how to get a shadowmask.

A shadowmask is just a thin, micromachined template that can be used instead of photoresist for patterning your substrate.  Photoresist allows better resolution and wider margin in terms of pattern layout.  And as resist adheres directly to the substrate (rather than just laying on top of the substrate as in a shadowmask) there are a lot more processing options available.

 

 

 

 

 

 

From Beth Pruitt (thanks!):

"If you're going to use this more than once, a [silicon shadowmask] wafer can be quite fragile and instead of spending the $$ on the photomask and the time on the STS, if you can afford it (about $750 for a bi-layer Be-Cu mask I think) try Photosciences... I've attached guidelines [below] I created from my interactions with them, I don't know why they don't have them, but these design rules will help you talk to them about what you can and can't do. there are other companies that create the mask by plating it up and they can achieve submicron features in theory, their masks are closer to $5K though."

Photo Sciences guidelines for Shadowmasks, as defined by Beth Pruitt.

Contact:
Roger Horstman
Email:roger1psi@aol.com
Phone: 408-748-7673

Design Format: Files can be provided in GDS, dxf, dwd format. There is a setup and conversion charge to create a master mask from your design file.

Size: To work with the EV alignment fixture, the footprint is circular and should be no larger than 100mm but at least 97mm in diameter. Your pattern should extend no further than about 85mm in diameter. These limits are for the vacuum lines on the chuck. Photo Sciences can build masks with tight spacing and small features, but they may be too fragile to be practical, the more supporting material you have, more robust the mask will be.

Single metal masks:

  • isotropic etch from one or both sides defines pattern as shown. Resolution not as good as bi-metal but they are less expensive. Get good resolution with thinner masks, but they are more flimsy.

 

Bi-Metal Masks:

  • Edge definition is crisper while mask can be thicker and more robust.

Resolution: (see cartoon below)

  • Depends which kind of mask you choose and mask thickness. Typically can resolve 2.5 micron open features within about +/-1microns, call for tighter tolerances. Features will have rounded corners from the etch process.
  • "Bridges" in the mask must be at least 50microns and will be more robust if over 100microns. Long bridges -- greater than about 2:1 aspect ratio won't hold up well, short runs of tight spacing are ok if there is enough material around them.
  • Alignment Marks: better to have an all open design for alignment to existing crosses than flimsy crosses with "bridges"

 

More from Neville Mehenti (thanks!)

1. Photo Etch Technology (www.stencil.com) 978-805-5005
They make their shadow masks in stainless steel, and the masks are 2-25 mil thick (50-625 microns). They define their features either through chemical etching (if feature is >4 mil), or laser cutting. As for data preparation, most drawing formats are accepted, and if you have a simple geometry or array, a hand drawing with dimensions is acceptable as well. A 15x15 inch steel patterned sheet can cost in the ballpark of $250 if it is chemically etched. (Yes, so you can get about 16 masks from this one sheet).

2. Photo Sciences 408-748-7673

 

This company charges a bit more, but can do a better job if you need smaller features. For features over 5 mil, they use stainless steel. But for smaller features (a few microns), they can make fine edges by using a bimetal Ni/Be-Cu mask, but the structural integrity of the mask then becomes a concern. They take all drawing formats, but may charge for data conversion. A 4x4 inch mask made in stainless steel can cost about $500. Roger Horstman is the guy to talk to at this company, he is very helpful.

 

 

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