Adding Grayscale Layer to Chrome Photomasks - Professor Glenn ...

underlying chrome patterns, and writing different shapes and patterns with grayscale. In the original chrome photomask
shown in Figure 3, there are various binary patterns used for grayscale and resolution tests. On the top part of the mask,
there are ten cleared squares of 5000 µm by 5000 µm, each will eventually be patterned with one of ten distinct
grayscale levels on the bimetallic layer. On the left-bottom part of the mask, there are several line test structures with
chrome lines of different sizes ranging from 1 µm to 10 µm wide, and with each line being 50,000 µm long. On the right
side, there are a series of small cleared squares with sizes from 1 – 50 µm 2 that are used for resolution tests. In the
middle of the mask, there is a large cleared area that is used for writing test grayscale patterns. Lastly, at the four
corners just outside the center cleared area are the alignment cross marks that will be used to test the alignment accuracy
of the grayscale patterns in the Bi/In bimetallic layer with respect to the chrome layer underneath.
In Figure 4, the final grayscale photomask design is shown for Bi/In layer DC-sputtered on top of the chrome mask
and the grayscale patterns written. In the top part of the grayscale mask, there are the ten 5000 µm by 5000 µm squares
with the bimetallic layer for each square being written with a different grayscale level. Ten grayscale levels from 0.1%
to 100% maximum transmission (with equal steps) were set with one level in each of the ten squares. Right below the
ten squares, but within the large cleared chrome area, we also created additional grayscale squares for calibration
purposes (as this was the first mask of this type). The top row was written with the same grayscale levels as the larger
ten grayscale squares along the top part of the mask. The middle grayscale squares were written with 75% of the argon
power used by the top row, while the bottom row with 50% of the maximum argon power of the top row. After writing
OD tests were done on these to confirm the desired gray levels. On the left and bottom part of the mask, the two line test
structures are scanned with a 1 cm wide by 5 cm long 64 grayscale level pattern. Therefore, the spaces in those two test
structures should appear as continuous, grayscales varying lines with 64 gray levels in each of the spaces. On the right
side of the mask, there is a 1 cm by 1 cm square at 100% argon power over the squares test structure making the
grayscale layer is completely transparent and fully revealing the original binary pattern of the underlying chrome layer.
Grayscale
Squares
Test Squares
Clearing Bi/In To
Show Patterns
Below
Grayscale
Patterns
Grayscale Lines
& Spaces
Figure 3. Original chrome photomask patterns. White
areas define the chrome blocking area, gray and black
colored areas define the clear areas of the chrome.
Figure 4. Chrome photomask with grayscale patterns.
Bi/In grayscale layer is DC-sputtered on top of the chrome
mask.
5.1 Chrome Pattern Alignment Tests
To test our system’s ability to align a grayscale pattern with the underlying chrome pattern, patterns originally
present on the chrome layer were traced over and replicated onto the Bi/In layer. In addition, to provide a better visual
measure of our alignment, the grayscale writing system in some cases also placed smaller patterns on the Bi/In layer
within alignment crosshairs present on the underlying chrome. Particularly for this mask, at the four corners of the
photomask there are two types of cross-alignment mark structures shown in Figure 5 and Figure 6. Each corner contains
one 60 µm by 60 µm and one 200 µm by 200 µm crosshair, with widths of 10 µm and 20 µm, respectively. All of the
60 µm by 60 µm crosshairs were laser scanned and the Bi/In converted to maximum transmission. The 200 µm by
200 µm crosshairs were also laser scanned except for a 10 µm by 10 µm square aligned to the center where the Bi/In was
opaque. The 10 µm square in the crosshair measures the accuracy and resolution of our Bi/In laser writing system.
Figure 7 and Figure 8 show the 200 µm by 200 µm and 60 µm by 60 µm crosshair at 20X, respectively.