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

3. FILM DEPOSITION
The first step in creating the combined mask is to create the binary patterns on a chrome photomask by a
conventional commercially available process. The mask was planned to contain a range of structures down to 1 µm
mask size, and up to large clear areas, for a complete test of the chrome/grayscale mask. Once the binary patterns are
defined the masks were shipped in sealed containers to Simon Fraser University for the grayscale bimetallic layer to be
deposited on top of the chrome photomask. Typically the bimetallic layer is composed of either Bi/In of Sn/In. Because
Bi/In has the advantages of smoother surfaces, lower laser conversion power for grayscale levels and is able to produce
more gray levels, Bi/In was chosen as the top grayscale layer for the chrome/grayscale photomask.
To create the Bi/In coated chrome photomask, bismuth and indium film of approximately 40 nm in thickness are
sequentially DC-sputtered using a Corona sputtering system in Simon Fraser University’s Microfabrication Laboratory.
Both metal layers are sputtered onto the chrome photomask without a vacuum break. The vacuum chamber is normally
pumped down to approximately 6 x 10 -7 torr and during deposition argon gas is introduced into the chamber at 10 sccm,
bringing the sputterer chamber pressure to about 4 mTorr. Typical sputter rates for Bi and In are listed in Table 1. Note
that the sputter deposition tool was not designed for 6” masks. Film thickness control was good within the central 4”
circle but was much poorer at the mask edges. Fortunately the outer edges of the mask were not used in this design.
Table 1: Bi and In Sputter Rate
Metal argon Pressure DC bias Current Watt⋅Min Deposit rate Å/W⋅min
Bi 4mTorr 470V 0.23A 2500 12
In 3mTorr 450V 0.23A 2500 4
Once Bi/In deposition is completed, the Bi/In coated chrome mask is ready for grayscale patterning step. Next we
discuss the mask creation process for creating grayscale patterns in this Bi/In coated chrome photomask.
4. GRAYSCALE MASK WRITING PROCESS
Having added the bimetallic film layer on top of the chrome masks, the next step is grayscale patterning the
bimetallic layer. As shown in Figure 1, the combined bimetallic-chrome mask is placed on an X-Y-Z table and
positioned such that the grayscale pattern to be made would be aligned with clear alignment structures of the chrome.
For alignment, the blocking and non-blocking chrome have a slight difference in height which can be seen visually on
the mask through the Bi/In film. The ability to see the underlying chrome layer simplifies the process of aligning the
chrome layer and the bimetallic layer to just orientating the mask correctly to match the orientation of the X-Y-Z table.
A bitmap file containing the grayscale pattern is supplied and converted by a function generator into voltages that
are used to control an electro-optic shutter. The shutter is used to modulate the laser beam from a 488/514 nm CW argon
laser as the X-Y-Z table moves the photomask in a raster-scan motion. As a result of this motion combined with the
laser beam’s modulation, the bitmap file is patterned line-by-line onto the bimetallic film.
Figure 1. Grayscale mask patterning: A bitmap file is read line-by-line (1) and converted into a voltage waveform (2) that is used to
modulate the writing laser beam via an electro-optic shutter (3). The photomask moves in a raster-scan pattern as the focussed beam
patterns the mask (4).