1. Prototyping Techniques: Soft Lithography
MAE 195-MAE 165
Spring 2009, Dr. Marc Madou
Class 5
4/23/2017

2. What is Soft Lithography?
Molding of a polymer (usually PDMS) using a photoresist master mold
Fast, cheap prototyping method
Device features limited only by the complexity of master mold. Down to 10 nm feature resolution.
4/23/2017

3. PDMS (Polydimethylsiloxane)
Silicon-based elastomer
Mix the two components for various amounts of cross-linking
Hydrophobic surface
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4. Properties/Benefits Property Characteristics Consequence Optical
Transparent
Optical detection from 240 to 1100 nm
Electrical
Insulating, breakdown voltage,
2 x 10 E7 V/m
Allows embeded surfaces,
Intentional breakdown to open connections
Mechanical
Elastomeric, high youngs module ~70 kPa
Conforms to surface when not polymerized, reversible deformation
Thermal
Insulating, thermal conductivity of 0.2 W/(m.k)
can be used to insulate heated solutions

5. Properties/Benefits Property Characteristics Consequence Interfacial
low surface free energy
20 erg/ cm2 (0.02 J/m2)
replicas release easily from molds; can be reversibly sealed to materials
Permeability
impermeable to liquid water; permeable to gases and nonpolar organic solvents
contains aqueous solutions in channels
Allows gas transport
Incompatible with many organic solvents
Toxicity
Non toxic
Can be implanted in vivo
Reactivity
Inert, can be oxidized by exposure to plasma
Unreactive toward most reagents; can be modified to hydrophilic and reactive towards silanes

6. PDMS device Applications
Cell patterning through surface modifications
Valving via electrical breakdown of PDMS
Configurable Gradient Generator
Generation of Gradients Having Complex Shapes Using Microfluidic NetworksStephan K. W. Dertinger,, Daniel T. Chiu,, Noo Li Jeon, and, George M. WhitesidesAnalytical Chemistry (6),
Fabrication of a Configurable, Single-Use Microfluidic Device, McDonald, J. C.; Metallo, S. J.; Whitesides, G. M. Anal. Chem. 2001,73,

7. SolidWorks Design
A system of channels and reservoirs is designed in a CAD software.

8. SolidWorks Design
Deep and shallow features are separated

9. SolidWorks Design

10. SolidWorks Design
The design is saved in a DXF file format and sent to the printing company

11. Photoresist Mask
A commercial printer uses the CAD file to produce a high-resolution transparency

12. Fabrication of the mold (Photolithography)
Silicon
photoresist
mask
Clean the silicon wafer and dry
Spincoat a thin layer of SU-8 photoresist
Soft bake to evaporate the solvent.
Align the mask
Flood exposure to UV light
Hard bake to cross-link the exposed portions of he photoresist film
Develop to wash away the non-exposed area of the photoresist
Rinse with Isopropenol and dry with nitrogen gun

13. Rapid Prototyping A system of channels is designed in a CAD program.
A commercial printer uses the CAD
file to produce a high-resolution transparency
This transparency is used as a photomask in contact photolithography to produce a master.
A master consists of a positive relief of
photoresist on a silicon wafer and serves as a mold for PDMS.
Liquid PDMS pre-polymer is poured over the master and cured for
1 h at 60 °C. (C) The PDMS replica is peeled from the master
(D) the replica is sealed to a flat surface to enclose the channels.

14. Soft Lithography Techniques
Soft lithography includes several techniques such as:
Replica Molding (REM)
Micro-contact printing (µCP)
Micro-transfer molding (µTM)
Micro-molding in capillaries (MIMIC)

15. Replica Molding (REM)
Master mold is replicated in PDMS by casting and curing PDMS pre-polymer.
PDMS mold is oxidized in oxygen plasma for 1 minute and exposed to fluorinated silane for 2 hours to make a surface with low adhesion to PDMS.
PDMS is cast against this negative replica, cured and peeled off.
This will make the positive replica of the original master.

16. Micro-Contact Printing (µCP)
It uses a PDMS stamp to form patterns of self-assembled monolayers (SAMs) on the surfaces of substrates.
PDMS stamp is coated with an ink of the molecules and pressed onto the solid surface
Inking creates a Self Assembled Monolayer on the solid surface
the use of elastomers allows the micropatterned surface to come into conformal contact with the surfaces over large areas

17. Micro-Transfer Molding (µTM)
PDMS stamp is filled with pre polymer
Excess prepolymer is removed
Press the rubber stamp against the surface
Cure the polymer
Peel off the stamp

18. Micromolding in Capillaries (MIMIC)
Push the PDMS stamp against the substrate.
prepolymer is applied to access holes in the mold (vacuum assisted).
Prepolymer fills the channels using capillary forces.
Cure the polymer
Peel off the PDMS mold