Mathies:Mask Making

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Overview

This section includes information on making chrome, emulsion, and transparency masks

  1. Chrome masks, 5" square glass with a layer of chromium + protective coating, are used where the area to be exposed is fairly small. These are used for patterning the channel, fluidic bus, or manifold layers of our chips.
  2. Emulsion masks are also 5" square glass. On these, the patterned layer is an emulsion of silver salts in a gelatin or colloidal. We mainly use emulsion masks to pattern heaters, RTDs, or electrodes on the metal layers of the chips. On these layers, the photoresist must be patterned to protect (rather than expose) the desired features, and thus the area to be exposed is a large percentage of the total chip area.
  3. Transparency masks are good for very quick turnaround if your fab does not require high resolution or very small features.


Transparency masks

Instructions

1. In AutoCAD make sure you have a box or circle of known width (12 cm works well) to represent your mask frame. Make sure there are no objects in outside this frame.

2. Save your mask CAD file in DXF format and open it up in Canvas 7.

3. In Canvas 7, make sure your layout settings are correct:

  • From the menu, select Menu:Layout:Document Layout. In the layout window, delete all layers but the layer that contains your mask by selecting them and dragging them to the trash icon.
  • In Menu:File:Preferences, select the Units tab and select “mm” for line width units.
  • In Menu:Layout:Document Preferences, select “centimeters” as your units.
  • In Menu:Layout:Rulers, make sure “1 unit = 1 cm.”
  • All of your mask objects will be in one big group with the dimensions of your mask frame. Open the “Object Specs” window via Menu:Object:Object Specs…. Check the “Keep Proportions” box. Then change the length/width to the correct width of your mask frame (12 cm) and click “OK.” Keep this window open – it will come in handy later.

4. Modify your mask objects for printing:

  • Ungroup your objects to edit them.
  • Make your mask black and white. Make sure to invert your colors if you want to etch channels. i.e. channels should be white and the background should be black.
  • Important: Canvas will change the widths of your polylines, so you will have to select your polylines and change them to the correct widths in the “Weight” box in the “Strokes” palette window (Menu:Windows:Palettes:Strokes). If you have arcs in your polylines, choose the “Round” line end-capping option at the bottom right of the “Strokes” palette so that you don’t get tiny gaps in your polylines.
  • Fill all of your polygons so that they are solid.

5. Optimimize your mask for printing:

  • Group all of your objects together again.
  • Check to see whether you need to flip your mask. This will vary depending on whether you need to align to a chrome mask or another transparency mask. Use the “Effects” Menu to do this.
  • Rotate your mask so that your long channels/lines are horizontal. This helps because inkject printers have better vertical resolution/uniformity, and so your horizontal channels will appear smoother and sharper.

6. Print out a rough copy of your mask on HP LaserJet 4050 to check if the sizes of the mask is correct, to spot any glaring errors, and to make a copy for your records.

7. Get a sheet of Epson InkJet transparency. Do not use laser printer or copier transparency.

8. Now you are going to run some test patterns to make sure the printer is in tip-top shape before you waste a bunch of ink and a $2.00 transparency. I have a test pattern file called “transparencyTestPattern.” Basically, it’s a black box with some lines in it that I print in the upper right hand corner of a blank transparency. Printing this uses very little ink but can tell you if you are getting good resolution, good opacity, and smooth, clean lines with no smudging. If you aren’t, then you’re wasting time, ink and transparency printing your whole mask and/or doing lithography with it. You can use this test pattern file (//Vader/Users/Emory/transparency masks/) or make your own to suit your needs. After you create/open your test pattern file, set up the printer like so:

9. Printer setup:

  • Open Menu:File:Print and click “Page Setup” to select "Epson 890" as your printer. In the main print window, click “Printer Properties.”
  • Make sure you have enough Color and Black ink to print.
  • Select Media Type: "Premium Glossy Photo Paper"
  • Select Mode: “Custom” and click on “Advanced…”
  • Choose "1440 dpi" as resolution.
  • Make sure “High Speed” and “Edge Smoothing” are turned off. “Microweave” will be turned on automatically.
  • Click OK to get out of the Advanced menu.
  • Click on the “Utility” tab and then “Nozzle” check. Make sure a piece of scrap paper is in the paper feed. If all the lines are connected, move on. If not, clean the nozzle and do the nozzle check again. (If you still cannot clean your nozzles after several cleanings, go to “Start Menu:Settings:Printers” and right click on “Epson 890” and select “Properties.” Print a test page to clean out the nozzles.)
  • Click OK several times to save your changes and go back to the main print window.
  • In the “Transparency Rendering” box, choose “complete area” and Color Mode: “RGB.” You can also choose “Black and White” instead of RGB.

10. Print a Test Pattern:

  • Put the transparency in the paper feed with the notched corner on the upper right hand corner. This will make the “sticky-side” (Charlie’s definition) up, which is the side that will be printed on.
  • Click “Print…” to print your test pattern. Make sure that your lines/channels are clean and continuous and that any turns are not jagged.
  • Stick your finger behind a solid black portion of your mask. If you can see the outlines of your finger through the black ink, your mask is not opaque enough.
  • If your test pattern looks like crap, check the Troubleshooting section of this manual to fix things before you proceed.
  • If you need to print another test pattern, re-insert the same transparency and in Canvas, move your test pattern down so that it doesn’t overlap with the old one.

11. Print your mask:

  • When you are satisfied with the quality of your test pattern, open up your optimized mask file again. You will need to repeat steps 8 and 9 for this file because Canvas will probably not remember your printing preferences. If you put your test pattern in the right place, you can re-use the same transparency with no problems. If you tweaked your settings to get a good test pattern, use the same settings for your mask. Print your mask, do a little dance, and pray to the gods of inkjet transparency masks that everything comes out alright.


Tips:

  • Don’t try to make channels/lines less than 200 um wide. You will get very inconsistent or poor results.
  • Print with your long channels/line horizontal to get smooth lines.
  • Print 1440 dpi with no fast printing or edge smoothing.
  • Transparency masks work GREAT for metal heaters, but not so great for glass etching.


Troubleshooting:

  • I get lots of smudging on my masks. It seems like its from the mask pattern itself.

Make sure you’re not printing at 2880 dpi. This will put down too much ink.

  • I get lots of smudging on my masks. It doesn’t seem like it’s coming from the mask itself.

Make sure the nozzles are clean and print a test patterns (step 8h.) until you get nice sharp text. If that doesn’t help, stick a blank sheet of paper in the paper feed and press the middle printer button (it has a droplet icon on it) to repeatedly run the paper through the printer until any ink on the rollers is cleaned off. If all else fails, position your mask in Canvas on a portion of the page where the smudging does not occur.

  • The parts of my mask that are supposed to be opaque are very transparent.

or: I get visible drops of ink on my transparency that make my solid black regions very inhomogeneous in opacity.

Make sure you are at 1440 dpi, and that your “Transparency Rendering” setting is at “Complete Area.” If that doesn’t work, try setting the “Transparency Rendering” to Smallest Area and Color Mode: CMYK. Keep on tweaking those settings until you get a good test pattern.

  • My arcs/curves/turns are very jagged and pixilated.

I don’t know why this happens, but you can try copying your mask objects to a new file, or you may have to re-process your mask from the DXF file.

  • When I look at my transparency under the microscope, I see these zig-zag or tractor tracks in the transparent portions of my mask. Will this affect my lithography?

No, these are just byproducts of the transparency and printer feed, and you can’t avoid them. If you expose for long enough, you shouldn’t have a problem with these tracks. In general, you need to expose transparency masks for a slightly longer time to compensate for their less-than-ideal transparency and reflection. You can expose up to 1.5 times the recommended dose (120 mJ/cm^2) and still get a fine pattern. In fact, over-exposing a transparency mask is a great way to smooth out some of the jagged edges that are inherent to your mask.

  • My mask channels look smooth to the naked eye, but when I look at my transparency mask under a microscope, the edges appear jagged. What’s up with that?

It’s called ghetto-fab, foo’. If you want nice edges, go get Marilyn to make you a real mask. And then wait for two weeks…or three.

Chrome/Emulsion Masks

Submitting a mask

Your mask will have a number based on the Mask Tracking Table, which is a record of all of the masks made by people in the Mathies Lab. The mask numbers are sequential for each year, so the tenth mask made in 2009 will be mask number "09-10". Check the Mask Tracking Table for the next available number and then write down on the Tracking Table your mask title, the number of layers, and your name.

  • Once you've completed your mask design in AutoCAD, convert the file from .dwg to .dxf in AutoCAD, then from .dxf to .gds (GDSII) with LinkCAD.

o If you have a problem fracturing your mask properly, it may help to try doing the .dxf to .gds conversion with a different version of LinkCAD. Some of the computers have v.2, some have v.4, and some have both.

  • Move the .gds file to your account on one of the Microlab servers, then use the fracturing script gds2tap to convert the .gds file to .tix and .tap. These files are used by the Microlab staff to pattern your mask. (Detailed explanation of this process here.)

o To review your fractured mask pattern once the .tix and .tap files have been generated, type pgview (filename).tap at your xterm prompt.

o In the pgview window, use the + and - keys to zoom in and out.

o Select Fill or Outline at the top right of the window to toggle your view between the two.

o Clicking on a point on the image will give you the coordinates of that point; moving the cursor to a point on the image will recenter the zoom at that point.

o Make sure your pattern is scaled properly and check that all of your features have fractured properly.

  • Submit your mask request to the Microlab using this form:
     http://microlab.berkeley.edu/maskrequest/

Once you have submitted your mask, compress your .dwg, .dxf, .gds, .tix, and .tap files for all layers of the mask into an appropriately labeled .zip file and submit it to the mask archive. Samantha Cronier (cronier@berkeley.edu) manages the mask archive.

Types of masks

We use two types of masks: chrome and emulsion. Both types are square glass plates with a light-blocking layer on one side - either chromium with a protective coating or an emulsion of silver salts in a gelatin or colloidal.


  • Chrome masks are used where the area to be exposed is fairly small. These are used for patterning the channel, fluidic bus, or manifold layers of our chips.
  • We mainly use emulsion masks to pattern heaters, RTDs, or electrodes on the metal layers of the chips. On these layers, the photoresist must be patterned to protect (rather than expose) the desired features, and thus the area to be exposed is a large percentage of the total chip area.
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