A photomask (oftenly shortened to just mask) is a plate with dark (opaque) and clear (transparent) patterns. The clear regions of the mask allow light to pass through to expose a light sensitive film underneath. The patterns are drawn using KLayout and sent to a supplier to make the mask.
Layout
A layout contains pattern data (polygons) that is used to produce clear or dark regions on a mask. A layout should be drawn using a layout editor such as KLayout and saved in the GDSII Stream format (*.gds file). This format is an industry standard and it ensures that the mask supplier can produce an accurate mask. Layout editors such as KLayout is designed specifically for this application providing many useful tools to efficiently produce a layout. Other CAD software is not recommended for preparing layouts.
Photomask Quality
The quality of photomask is defined by patterning requirements and cost. The table below compares some specifications and costs of 3 types of photomasks. The plastic photomask costs the least and it is recommended for new users. Besides the device patterns, a photomask should contain other patterns such as title, alignment marks, windows, calibration patterns, et cetera. The need for these other patterns are typically discovered after some experience. The plastic mask is flimsy and more difficult to use, but will produce patterns down to 10 [μm]. For regular use, the sodalime glass mask is easier to handle and can be cleaned. A quartz mask is required to produce patterns down to 300 [nm] using a Deep UV stepper available in select academic facilities.
| Plastic Mask | Sodalime Mask | Quartz Mask | |
|---|---|---|---|
| Mask Size | 5"x5" | 5"x5" | 5"x5" |
| Critical Dimension [μm] | 8 | 5 | 2 |
| CD Tolerance [μm] | ±2 | ±0.5 | ±0.25 |
| Cost [$] | 70 | 300 | 600 |
Polarity
The polarity specifies whether the polygons in the layout will become dark or clear on the photomask. Express the polarity as “make polygons clear” or “make polygons dark”. The supplier often provide a proof for verification.
While performing photolithography, a sample is placed underneath a mask. When the mask is mostly clear, the sample underneath the mask is visible. This makes it easy to position the mask pattern on the sample. However, when the mask is mostly dark, the sample can not be seen, and it becomes difficult to position the mask pattern on the sample. This problem can be avoided by adding clear features to the mask that serves as windows so see the sample underneath.
Parity
The parity defines whether the pattern is read right or read wrong when chrome side is chrome up or chrome down. Express the parity as “read right chrome down”. This means that the photomask pattern will look like the layout pattern when the chrome side of the photomask is facing down. This polarity designation is not standard and may need verification with the supplier. If printing a mirrored version of the layout is not a problem, then parity does not matter.
Photomasks are always installed into a photolithography tool with the ink side or chrome side facing down. This is required to accurately replicate the photomask pattern in the photoresist. For glass masks, there is no mistaking the chrome side from the glass side. However, it is very difficult to tell the ink side from the plastic side of a plastic mask. This problem can be avoided by adding text patterns to the mask and expressing the parity as “read right chrome down”. This will produce a mask where the text reads normally when the ink side is facing down. The text patterns should contain characters that are not symmetric such as “Project Drifting Cloud: Mask 001”.