PDMS protocol (SOP #5)
Craig Story 1/9/07 Modified from: http://microsystems.stanford.edu/merl/merl/OperatingProcedures/PDMS.doc Accessed 1/4/07
Safety: Polydimethylsiloxane (PDMS) is not toxic, although care should be taken not to ingest or swallow the product. Trimethlylchlorosilane (TMCS) which is used to treat the surface of the silicon wafer/mold is extremely toxic and is considered hazardous waste. Care should be taken not to expose the vapors outside of the fume hood. The PDMS oven is typically at a temperature of 60C and care should be taken to avoid burns.
Introduction: This operating procedure is for casting PDMS into microfabricated molds. PDMS we use is commercially available as Sylgard 184 in two parts – silicone elastomer base and curing agent. It can be obtained as a kit from Dow Corning or K. R. Anderson. We purchase from Essex Brownell, Cat # DC-184-1.1 (1-800-774-4643) PDMS is used to replicate features on wafer molds that are commonly fabricated by one of many various techniques.
The following procedure involves five steps: mixing, degassing, pouring, baking, removal.
1. Handling PDMS could potentially be a messy process due to its viscous and slippery nature, so you should always be prepared for the possibility of a spill. We typically use a clear plastic PETE 16 oz “Solo” cup and a plastic knife as the stirrer. Use dust-free gloves throughout procedure.
2. Lay down a couple of clean wipes on the weighing scale before starting.
3. Tare the scale and carefully pour the required amount of PDMS elastomer base into it. A mass of 70-100 grams is a good amount for 5 or so pours. Wipe the edges of the container with Kimwipe before closing.
4. While the beaker is still on the scale, re-tare and add curing agent such that the ratio of elastomer base to curing agent is 10:1 by weight. For example if you weigh out 100g, weigh 10g of curing agent.
5. Remove the beaker from the scale and mix well with your stirrer. The mixing generates a lot of bubbles and a homogeneous distribution of bubbles is an indicator of good mixing. Poor mixing may lead to clumps in the PDMS texture while curing.
6. Normally we degas the PDMS in the cup before pouring, although it is possible to degas after pouring. Use the plastic vacuum chamber labeled “PDMS only” for degassing. Place the cup inside the vacuum chamber.
7. Close the lid and turn its valve to connect the vacuum to the chamber. After several minutes you may want to break the vacuum to facilitate popping of bubbles. Overflow is not usually an issue with the cups and volumes we typically use.
8. Allow the PDMS to degas for 15-30 minutes or until bubbles are gone.
9. Open the red valve on the dessicator to allow air to flow in and remove the PDMS.
10. Note: For high aspect ratio, negative features it is beneficial to pre-treat the surface of the mold with TCMS vapor. This only needs to be done one time when the wafers are new. The procedure for this is listed below in steps 11-12 below. Otherwise go to step 13.
11. Perform the following in the fume hood. Place a glass scintillation vial along with uncovered wafers to be coated in plastic chamber labeled “silanes.” Use one drop from the end of a glass Pasteur pipet to transfer ~50 µl TCMS into the vial. Remember that TCMS is an extremely toxic gas that should not be opened outside the fume hood.
12. Place the wafer in the box and close the lid. The wafer is typically allowed to sit in the vapor (TCMS is volatile) for 10 min. This might be done while the PDMS is degassing or before you start with PDMS.
13. Pour PDMS from the cup to achieve the required thickness, try to avoid making bubbles. Some small bubbles will float up and can be popped or moved to the edge with a pipet tip before baking.
14. Make sure the oven is set at 60C (a mark at the dial). Place the Petri dish in the oven on the top shelf on the black metal “leveling tray.” Plastic Petri dishes have a glass transition temperature of around 75C, so place your dish at the top of the oven where it is further away from the heating coils at the bottom.
15. The recommended bake time is 2hrs at 60C, although this will depend upon numerous factors. It is acceptable to leave overnight.
16. Remove the Petri dish from the oven. Take care while handling the hot contents.
E. Removing cured PDMS from dishes.
17. Silicon wafers are brittle and can shatter if too much force is used. In all cases avoid running over features with the blade tip. Cut around the perimeter of the grid holding the scalpel vertically and pulling it smoothly through the PDMS.
18. Pry up the PDMS with the tip of blade and grab the corner and pull up slowly, releasing the grid from the surface of the wafer.
19. Apply Scotch ™ tape to the feature surface of the PDMS for storage.
See SOP #7 for Plasma Cleaning of PDMS.