PDMS processing & devices

2nd master

PDMS

1st master

PDMS

control channel

active channel

PDMS

3rd substrate

2nd master

PDMS

1. Substrate: anything smooth and flat silicon is the easy choice

Surface preparation:-bake adsorbed water vapor out-clean: remove particles -adhesion promotion: HMDS

Adhesion promotion

HMDS More hydrophobic surface,Better adhesion with resist (which is organic polymer)

Resist processing

Resist dispensing Acceleration Final spinning 5000 rpm

(a few milliliters) (resist expelled) (evaporation/partial drying)

Resist processing

SpinBake(Align)Expose(Post exposure bake)Development Hard bake

PDMS processing

Pre-polymer and curing agent

10:1 ratio

Mixing bubble free

Casting

Curing, e.g. 65oC, 2 h; or 40oC 10 h

PDMS propertiesTg Tdegr CTE Ther UV-

transparencyconduc

0C oC ppm/oC W/K.m

PMMA 100 200 70 0.2 opaque

PC 150 230 65 0.2 >350 nmPDMS -130 400 300 0.15 >240 nmSU-8 240 340 100 0.2 >350 nmPolyimid400 620 3-50 0.2 opaqueParylene 150 290 35 0.1

>300 nmTeflon 130 330 100 0.1 opaque

PDMS Young’s modulus 10 MPa very elastic, peels off easily

Release

Release criteria:

-no retrograde angles-smooth surfaces-low surface energy-no interfacial reactions

Masters

•photolithography of SU-8•photolithography and silicon etching•photolithography and metal electroplating•discharge machining•laser machining•mechanical milling•natural objects

Masters (2)

1.Why not silicon master ?

2.Why not Teflon release layer ?

1st master

PDMS

Tg = glass transition temperature

Above Tg polymer flows

hemispherical shape

PDMS

3rd substrate

Thin PDMS

What is thin ?

10-50 µmSpin coating

Applicable to viscous polymers

Typical thickness: 1-100 µm

Casting: 100 µm to mm’s

PDMS

3rd substrate

PDMS bonding

Self-adhesive bonding ?

Need strong bond for a valve application.

Oxygen plasma activation

Reactive OH-groups on surface permanent bonding

Valve closing

Valve operation

Flow switch

Valve array

Todd Thorsen, Sebastian J. Maerkl, Stephen R. Quake, Science 2002, Vol 298