Masks for generating the molds for soft lithography are commonly
designed using AutoCad and printed on transparencies in a laser
printer with 20,000dpi resolution. We recommend using CAD/Art
Services Inc. for printing the transparencies.
The typical mask design workflow for printing at CAD/Art Services
is as follows (please contact CAD/Art Services for up to date instructions
on how to submit a design for printing):
a) Each PDMS layer (i.e. control and flow) is drawn in
a separate AutoCad layer. In designs where a single PDMS layer
needs a mold with different photoresists (i.e. shallow channels
made with thin positive resist and deep channels made with thick
SU8 negative resist), each photoresist pattern needs a separate
mask and is drawn in a separate AutoCad layer.
b) All patterns
should be composed of closed polylines.
c) All polylines
that form a single pattern should overlap to avoid any gap
between them in the printed mask.
d) All layers should
have text labels identifying the device name, version number,
designer name, date, and layer name. All these labels should
be within the perimeter of each device so that they appear
on the masks. The height of the characters in the labels should
preferably be such that the letter "L" is at least 650µm high, if space
is available. Any text created by the standard AutoCad text
tool is not composed of closed polylines, so it will not be
printed correctly. Polyline-based text can be created by copying
and pasting (and scaling appropriately) individual characters
from either one of these files: alphabet.dwg (AutoCad 2004
format), or alphabet.dxf (AutoCad R12 format). You can also
use the linetext AutoLisp application to draw text (instructions
on how to use the application are in the linetext-readme.txt
file included with the application).
e) As is indicated
in the Basic Design Rules, each layer should have lines indicating
where the PDMS should be cut around the perimeter for the
devices.
a) Decide how many devices you will have per mold. Try
to fit as many devices as possible on a single wafer. We can
currently handle only 3" molds (3" diameter silicon wafers) and the mask
aligner can only expose a square area that is approximately 5.2cm
x 5.2cm (2" x 2"). Your whole design should fit into this square
area.
b) Choose
a page size for printing. Printing as many masks as possible
on a simple page saves you money, but pages bigger than 8.5"x11" can
easily be damaged during shipping.
c) Create
a new AutoCad file for the mask layout.
d) Create
a layer in the layout file (layout layer) and draw on it
an outline of the page at the chosen size and lines that will
help you place the devices on the right spots.
e) Create a new layer for each one of the pages that
you want printed.
f) Copy each layer of the design from the original AutoCad
file and paste it on the appropriate mask layer, using the layout
layer for guidance.
g) Any patter corresponding to a thick layer of PDMS
that is peeled off the mold for subsequent alignment and bonding
onto a patterned thin layer should be scaled up by 1.5% (a factor
of 1.015) to compensate for the shrinkage of the PDMS after peeling.
h) Masks for negative photoresist (SU8) must be printed
with black background and transparent patterns, while the masks
for positive photoresist (SPR-220, AZ 5740, AZ 100) have a transparent
background and black patterns. If masks for positive and negative
photoresist are printed on the same page, you should decide on
the overall tone of the page (positive or negative, depending
on the tone of the largest number of masks on the page) and place
squares around each mask that should be printed on a tone opposite
to that of the page. Make sure you tell CAD/Art Services about
these squares and that you want the tone reversed inside them.
i) Save the mask file in AutoCad 2000 format (.dwg, not
.dxf) for sending to the printer.
a) All printed pages should be "emulsion down." This
means that the pattern is printed on the bottom side of the page
(when looking at the page with the patterns in the right orientation).
b) The size of each page to be printed.
c) The
tone of each page, and whether you have squares around masks
that have a tone opposite to that of the page.
