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