520.495/580.495 Microfabrication Laboratory

Laboratory Assignment #1

Objectives: To learn how to do mask design.

    Production grade photolithography masks are made of quartz  plates (transparent to UV light) that are coated with chrome (opaque to UV light). A CAD tool such as LEDIT is used to design the mask pattern and then lithography is done on the chrome plate to define the pattern; this process costs between $400 to $1,000 per mask. We have several sets of quartz/chrome  masks in the lab; if you would like to see one ask the T.A.

 In this course we will take a different approach and we will make masks at a fraction of the cost compared to quartz/chrome masks. Our masks will be made out of plastic film and will be done locally by printing the pattern on a high resolution printer available in one of the local art design shops. This shop has the capability of printing designs at 3386 dpi (dots per inch).  To create masks of such high resolution, we must design the masks in a very specific format to be able to transfer the design on the film with  no loss of information.  This stringent requirement constrains the type of program that is used to create the design.  We will use Adobe Photoshop to do the art work.

I. Prelab Work:

1. If the printer resolution is 3386 dots per inch, what is the size (in microns) of each individual dot on the film.

II. Lab Work:

1. Sketch out your design on paper, including all of the dimensions (for your own benefit). Plan to make a 5 x 5 array of your pattern and the size of each element in the array should be 0.5 inch x 0.5 inch. The pattern area should include approximately 150 microns  of empty area around your design. This will be the scribe line area; i.e. the area that will be used to dice the wafer into individual chips. Your design will involve essentially two patterns (masks). We will use positive photoresist for the lithography.

The first pattern (mask) will define the metal pattern on the chip i.e. the active DNA Test Sites and the fan-out Wires to the periphery of each chip as well as the Contact pads. Make the Contact pads at least 300 micron each side. Make the Wires about 100 microns wide and the active DNA Test Sites about 200 micron each side separated by at least 200 microns. (See page 5 of Introduction to DNA Chips handout). The patterns on this mask will be black (opaque). You are otherwise free to do your individual design of the pattern; and don't forget to include your name and the year!

The second pattern (mask) will define the cuts on the overglass insulation over the DNA Test Sites and the Contact pads. The patterns on this mask will be white (transparent) on an otherwise black background. Make the overglass cuts  25 micron smaller on each side compared to the metal patterns. We do this so that the metal areas are well secured under the glass overglass insulation; this alleviates any problems with the metal pads pealing off the chips.

2. Open Adobe Photoshop and create a new file.

3. In the “New” window, enter the following:

            Your file name

Width and height: 2.5 inches in either direction.

            Resolution: 3386 pixels/inch

            Mode: Bitmap

            Contents: White

4. Create your black and white design.

5. Save the file as a TIF with the IBM PC byte order and with LZW compression .

Tips on Photoshop

Adobe Photoshop is not the friendliest environment for this type of design, so it is important to find shortcuts to create your design.  The following are just a few hints to help you design your mask.

Plan your design well before you begin using Photoshop.  It may also be helpful to figure out the “coordinates” of each shape in your design.  (Use the first quadrant of the Cartesian coordinate system.)

The zoom options are found under View.  You will need to zoom in to the actual pixel level and out to create a symmetric design.

The ruler and gridlines are toggled on and off under View.  Turning these options on is highly recommended.

The resolution of the ruler is set under File >>Preferences >>Units & Rulers . Under ruler, set the resolution to pixel.  This will allow you to use the ruler as a coordinate system for your design. 

The resolution of the gridlines is set under File>>Preferences>>Guides & Grid . Set the grid to “gridline every 1 pixel”.

To make drawing less tedious you may want to create a custom brush.  This is a “stamp” that you can use to repeat a pattern that you define.  The instructions for this can be found in the help file under “creating a custom brush shape”.  Go to Help>>Contents>>Index and enter brushes, then click on brushes: choosing>>creating>>to create a custom brush shape  (use the paintbrush tool to create the original pattern, then use the rectangular marquee tool to select it).  You will probably want at least a couple different square sizes to speed up the design process in addition to any shapes you find useful.

Do not use any filtering techniques or other options that create pixels that are not completely white or black.  No color or grayscale can be used in your design.

You may want to explore the help files if you are not familiar with Photoshop.

III. Postlab Work:

1. Do a search on the web and get quotations for a 6 x 6 inch quartz/chrome masks with feature size of 1 micron.

2. Explain why we design the patterns as dark areas on the first mask and as white (transparent) areas on the second mask.

3. Given the resolution of the printing process do you think you have enough dots in your patterns to achieve reliable processing? Explain.

Original lab  procedure prepared by J. Blain, Fall 2000.