PDMS Processing at SNFVersion 1.00 (08/03/2015)

Outline

• PDMS raw materials

• Overview of PDMS processing capabilities at SNF

• Fabrication of the mold

• PDMS processing protocols

• Controlled thickness through spin-coating and dilution

• Bonding PDMS to the substrate

PDMS Raw MaterialsWhat you will need

• Dow Corning Sylgard 182/184 Silicone Elastomer Kit• Includes base polymer and curing

agent • Sylgard 182 cures slower at room

temperature and thus allows longer working time. It’s also better for thinner films.

• See datasheets below for more information in order to decide which one will work best for you.

Where to buy

• Can be purchased from online retailers like Sigma Aldrich, Ellsworth and even Amazon.

Base polymer Curing agent

PDMS Processing Tools at SNF

nSIL Lab• Most of the PDMS processing will take place in

the nSIL lab. Contact Michelle/Carsen for safety training and room access. (https://snf.stanford.edu/SNF/equipment/nSiL)

• Tools available for PDMS processing in nSIL:

• Glass/Plasticware

• Weighing balance

• Thinky mixer

• Environmental plasma etcher

• Curing oven

• Vacuum degassing chamber

In the fab

• Drytek 2/4 and MRC etchers can be used for surface activation of PDMS prior to bonding in addition to the environmental plasma tool in nSIL.

• Wet benches (solvent, RCA, piranha) can be used to clean your master mold and/or final substrate prior to PDMS pour or bonding.

• The DUV flood exposure tool located in the wafer saw room can also be used for surface activation.

Fabrication of the master mold

Photomask fabrication• https://snf.stanford.edu/SNF/p

rocesses/process-modules/photolithography/maskmaking

• Layout design (decide minimum feature, contact aligner or stepper, etc.)

• Software for layout design

• Submit to mask shop

Patterning the master • 4” Si/SIO2/quartz/SOI wafers

can all be used. Pattern will be etched into the wafer.

• SU-8 can also be used, in which case the pattern will be defined by litho only, with no etch required.

• Alternative substrates can also be used (machined Al, polycarbonate, Teflon, etc.) keeping in mind they likely cannot be further processed on SNF wafer tools.

Alternatives

• You may not need a master with direct patterning of the PDMS film—through lithography, CNC, hand-cutting, etc.

Mixing the PDMS

Step 2

• Combine base and curing agent at 10:1 ratio (by weight) using the balance and pouring each component directly into the Thinkycompatible plastic cup.

• Alternatively, a syringe can be used to dispense each component instead of pouring. Use a separate syringe for each (available in 10ml and 1ml sizes in the SNF stockroom).

Step 1

• Thoroughly mix using the Thinky which will also remove bubbles simultaneously.

• Alternatively, if you mix with a stirring rod you will then need to place into the vacuum chamber for a few minutes to remove the bubbles.

Preparing the master mold for easier PDMS release

Mold preparation

• Depending on the mold material, you may have difficulty peeling off the PDMS film once cured.

• There are two strategies to make this easier.

1. Treat mold surface

• Rendering the mold surface hydrophobic can help to peel off cured PDMS.• Use YES oven to deposit

HMDS on the mold prior to pouring PDMS.

• Use the Drytek2/4 or MRC etcher to coat the mold with fluorinated polymer prior to pouring PDMS.

2. Use a lift off layer• Coat mold with photoresist

(eg 3612) prior to pouring PDMS.

• Once PDMS is poured and cured, dip into acetone which will dissolve the photoresist and lift off the PDMS film.

• This technique is useful for PDMS films <100um thick, which may be hard to manipulate by hand without damaging the film.

PDMS pouring, degassing and curing

Pouring the PDMS

• Place mold into a wide-mouthed beaker or similar glass or plasticware in order to contain the excess PDMS which will flow off your mold.

• Additionally, you may consider sequestering the PDMS on the mold by physical isolation (see illustration below).

Curing

• Sylgard 184 can be cured at room temperature for 48h or more quickly at higher temperatures.• 35min at 100C• 20min at 125C• 10min at 150C

• Sylgard 182 will not cure appreciably at room temperature and will require the use of the oven.

Degassing

• After pouring, you may have trapped air pockets between the PDMS and mold especially with smaller feature sizes.

• Place the container with mold into the vacuum chamber and pump down for 15-30min. This will remove the trapped air pockets.

Spincoating PDMS for controlled thickness: Part 1

Thickness vs spin speed• Sylgard 184 thickness as a function of spin

speed, just after base and curing agent were mixed in a 10:1 ratio.

• Closed circles are measurements with a spin time of t=30s, open circles are data points from [Zhang et al. (2004)] with t=60s, and the solid line is the theoretical fit W=0.23 ω-1.14 (W in meters, ω in rpm).

• The inset shows the same plot on logarithmic scales.

From http://willem.engen.nl/uni/intern-mbx/material/Sylgard-184-spincoat.php

Spincoating PDMS for controlled thickness: Part 2

Thickness vs spin time• Thickness of the 100% PDMS film under two

different spin speeds as a function of spin time.

• Each data point is the average of the mean thickness of three slides. The error bar is the 95% confidence interval.

• Each curve is a plot of the following equation:

t = spin timew = angular velocityh = thicknessh0 = 180umc = 2.86 x10-10 RPM-2um-2s-1

Spincoating PDMS for controlled thickness: Part 3Thicknesses less than 5um with dilution• For thicknesses less than 5um, PDMS can also be

diluted first which will reduce the required spin speed and time. Koschwanez et al have used tert-butyl alcohol (TBA) with the following results (see right panel).

• Each data point is the average of the mean thickness of three slides. Each slide was spun for 5 min. The error bar is the 95% confidence interval.

Thicknesses less than 5um without dilution• Thicknesses down to 200nm can also be achieved

by using less curing agent (20:1 or 30:1) and high spin speeds (12k RPM) and spin times (>60s).

Bonding PDMS to the working substrate: Part 1

PDMS surface activation

• PDMS can form a van der Waals bond to glass, quartz and oxide surfaces, but will only withstand limited pressure and shear force. In order to bond permanently to the substrate, you will need to activate the surface (ie break bonds).

• After surface activation, PDMS will bond permanently to hydrophilic substrates. These include Si/SiO2/quartz wafers, glass slides, among others.

Surface activation options

• Environmental plasma etcher in nSILlab

• Oxygen plasma in the fab on Drytek2/4 and MRC

• https://snf.stanford.edu/SNF/equipment/dry-etching/drytek2-model-100-drytek2-semiclean

• https://snf.stanford.edu/SNF/equipment/dry-etching/drytek4-model-100-modified-drytek4

• https://snf.stanford.edu/SNF/equipment/dry-etching/mrc-model-55-rie-mrc-contaminated

• DUV flood exposure tool in wafer saw room

Bonding PDMS to the working substrate: Part 2

Substrate preparation

• For best bonding, it’s a good idea to ensure the substrate surface is clean of contaminants that can interfere with the bonding and/or cause voids or air pockets at the interface. This can be achieved by a piranha or solvent rinse.

• (Optional) The substrate may also be activated by plasma or UV/ozone in the same manner as PDMS to promote better bonding.

Bonding to substrate• After surface treatment, there is a ~30min window in

which to form a permanent bond between the PDMS and substrate. This window can extended by immersing the PDMS in water immediately after surface activation.

• Activated PDMS will bond immediately to the substrate on contact, which can make alignment difficult. A thin layer of methanol dispensed between the two prior to contact will allow adjustments to be made for a few minutes afterwards.

• After bonding, it’s a good idea to heat the assembled device for 15-30min at 60-80C to finalize the bonding and to quench any dangling bonds on remaining unbonded surfaces.