NanoFab’s General Overview of
PDMS Micromolding
The use of PDMS in microfabrication, especially microfluidics and SAMs, is becoming increasingly popular. PDMS has many desirable qualities such as its low cost, ease of use, and robust nature. In the following SOP, the creation of a PDMS silicon master, the silanization of the master, the preparation and the curing of the PDMS, and the bonding of PDMS will be discussed.
PDMS Silicon Master
The first step in PDMS micromolding is developing a process and a process flow chart. Once the process flow is created, a mask must be designed and created. This mask will then be used to create PDMS masters using either an SU-8 or ICPRIE protocol.
SU-8
•SU-8 photoresist is spun onto a clean wafer, exposed, and developed in a typical optical lithography process.
•Due to the viscosity of SU-8, thick photoresist layers are produced.
•SU-8 is a chemically amplified negative photoresist which must undergo a post-exposure bake
o When SU-8 is exposed, strong acids are formed in the exposed areas. The post-exposure bake activates the areas that the
strong acids initialized, resulting in epoxy cross-linking.
SU-8 Features
SU-8 PDMS Master
Fig 1. Side view of a SU-8 PDMS master. Because of the difficulty
removing PDMS, the photoresist remain on the master and becomesreactive to
the features. Note: drawing not to scale.
•SU-8 is extremely difficult to remove due to the extreme epoxy cross-linking, so on PDMS masters, the SU-8 remains and becomes features as shown in Fig 1.
•More info at www.microchem/products/su_eight.htm
ICPRIE
•  A silicon dioxide layer is patterned on a silicon wafer using typical optical lithography procedures.
•The patterned silicon dioxide is a masking layer. When the patterned wafer is exposed to the plasma, the silicon is etched away while the silicon dioxide is etch resistant.
•The ICPRIE is used to create etch depths greater than 10 um with sidewalls that are vertical.
•The ICPRIE does has two modes of operation for deep silicon etching ~ the Bosch and Cryo-chuck process.
SiO2Masking Layer
ICPRIE PDMS Master
Figure 2. Side profile of an ICPRIE PDMS master. The silicon dioxide
layer resists the plasma etch while the exposed silicon does not.
Silanizing the PDMS Masters
Silanizing the PDMS master is important as many typical silicon surfaces result in PDMS adhering to the master, making peeling and preserving the PDMS more difficult. A drop of trichloro(1,1,2,2-perfluoocytl)silane is placed in a vial and placed in a desiccator with the PDMS master. The desiccator is placed under a vacuum causing the silanizing agent to evaporate and form a monolayer on the surface of the master that prevents the PDMS from adhering to the master.
Preparing and Curing the PDMS
Preparing the PDMS is very straightforward. Sylgard 184 silicone base and curing agent are mixed in a 10:1 ratio by weight. The mixture is then degassed to remove any bubbles and poured over the PDMS master. The PDMS is then baked under vacuum, causing it to cure and ensuring all entrapped gasses are evacuated. After cooling, the PDMS is easily peeled and cut.
PDMS Bonding
PDMS can be successfully bonded to glass, silicon, and itself using a standard
oxygen plasma in a reactive ion etcher (RIE). PDMS is quite hydrophobic with a low energy surface that is overall non reactive, making it difficult for it to bond
with other surfaces. By exposing PDMS to an oxygen plasma, the PDMS surface becomes hydrophilic and reactive, resulting in irreversible bonding when it comes into contact with glass, silicon, or another PDMS piece that was exposed to the same plasma. This contact should be made immediately because the PDMS surface will return to its hydrophobic state after time.
NanoFab’s Standard Operating Procedure for PDMS Micromolding
The following standard operating procedure outlines the PDMS micromolding process. The process includes creating a PDMS master, silanizing a PDMS master, and preparing and curing PDMS. Note: Training by the NanoFab is needed for each step in the PDMS micromolding process.
Creating the PDMS Master
Two types of PDMS masters can be made, the ICPRIE (deep silicon etch) or the SU-8 master. Please read the PDMS Micromolding General Overview for short descriptions of each and consult the NanoFab about the two options.
ICPRIE – Please contact the NanoFab for training and protocols.
Protocol
SU-8
1)Piranha clean a wafer and dehydration bake it on a contact hot
plate for 5 minutes at 200o C.
2)Turn the vacuum on the Headway Resist Spinner found in
Fumehood 1. Use the SU-8 spinner containers.
3)In Program 3, change rpm1 to 500, rmp1 to 5, and time1 to
10s. Turn rpm2 to 4000, rmp2 to 15s and time2 to 30s. (This
gives roughly 30 – 40um)
4)Pour the SU-8 2050 directly onto the wafer. Note: because of
the viscosity no beaker is used so that waste is reduced.
5)Put on cover and press the start button.
6)After it has spun, let the wafer sit for 5 minutes so that
entrapped air has a chance to escape.
7)Soft bake the substrate for 3 minutes at 65o C and 9 minutes at
95o C on the contact hot plate so that all solvents are driven off.
8)Let the wafer cool (1-2 hours).
9)Expose the design on the wafer for 75s under the mask aligners
(wavelength of 356 and 405 nm).
10)P ost exposure bake the resist for 1 minute at 65o C and 7
minutes at 95o C.
11)D evelop in SU-8 developer for 3 – 5 minutes.
12)R inse with IPA and Dry. If a white film occurs during after
rinsing, the substrate has been underdeveloped.
Note: This is just a guideline and should be optimized for each process.
The bake and exposure times increase with the thickness of the SU-8
layer.
Silanizing SU-8 and ICPRIE Masters
NOTE: Trichloro(1,1,2,2-perfluoocytl)silane is corrosive, reacts violently with water, and combustible (with a flashpoint of 87o C). Keep away from water and heat sources.
1)Masters are silanized to prevent PDMS adhering to it after curing.
2)Using a plastic dropper, place a drop of trichloro(1,1,2,2-
perfluorooctyl)silane in a glass vial.
3)Place the holder in the glass dessicator. Put the master and the vial
containing the silanizing agent in the holder.
4)Put the dessicator under a vacuum for 1 – 2 hours. During this time the
silane will evaporate and form a monolayer on the master.
5)Vent the dessicator slowly.
6)Remove and store the master.
7)Put the plastic eyedropper and the vial in a plastic bag and place it in the
Organic Waste bin underneath Fumehood 1. Note: The glass and the
eyedropper are both contaminated by the silanizing agent.  Preparation and Curing of PDMS
1)Measure about 25 g of the PDMS base and the curing agent in a 10:1
ratio by weight for one master in the plastic petri dishes found in the
PDMS box.
2)Mix the PDMS smoothly and well. Smooth mixing will minimize the
formation of bubbles.
3)Place the mixture in the desiccator under vacuum at about 22 in Hg until
there are no bubbles in the mixture. This should take 10-20 minutes.
4)Place the master in the PDMS Holder.
5)Pour the degassed PDMS on the master slowly to avoid trapping air.
6)Place the mold in the PDMS vacuum oven and cure at 80o C for 2 hours
under a vacuum of 5 in Hg. This vacuum will aid in evacuating entrapped
air, enabling finer features to be produced.
7)Carefully peel off the PDMS, blue tape features to prevent contamination,
and cut the PDMS as seen fit.
8)Place all the PDMS waste in the Organic Waste Bin underneath Fumehood
NOTE: The curing time may vary with thickness. The weight of the PDMS used should decrease for thinner layers.

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