10/7/2015 prototyping techniques: soft lithography mae 195-mae 165 spring 2009, dr. marc madou class...
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04/21/2304/21/23
Prototyping Techniques:
Soft LithographyMAE 195-MAE 165
Spring 2009, Dr. Marc MadouClass 5
04/21/23
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.
04/21/23
PDMS (Polydimethylsiloxane)
• Silicon-based elastomer
• Mix the two components for various amounts of cross-linking
• Hydrophobic surface
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
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
Properties/Benefits
PDMS device Applications
Configurable Gradient Generator
Valving via electrical breakdown of PDMS
Cell patterning through surface modifications
Fabrication of a Configurable, Single-Use Microfluidic Device, McDonald, J. C.; Metallo, S. J.; Whitesides, G. M. Anal. Chem. 2001,73, 5645-5650
Generation of Gradients Having Complex Shapes Using Microfluidic NetworksStephan K. W. Dertinger,, Daniel T. Chiu,, Noo Li Jeon, and, George M. WhitesidesAnalytical Chemistry 2001 73 (6), 1240-1246
SolidWorks Design
A system of channels and reservoirs is designed in a CAD software.
SolidWorks Design
Deep and shallow features are separated
SolidWorks Design
SolidWorks Design
The design is saved in a DXF file format and sent to the printing company
Photoresist Mask
A commercial printer uses the CAD file to produce a high-resolution transparency
Fabrication of the mold (Photolithography)
• 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
Silicon
photoresist
mask
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.
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)
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.
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
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
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
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