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

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

PDMS (Polydimethylsiloxane) Silicon-based elastomer Mix the two components for various amounts of cross-linking Hydrophobic surface 4/23/2017

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

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

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 2001 73 (6), 1240-1246 Fabrication of a Configurable, Single-Use Microfluidic Device, McDonald, J. C.; Metallo, S. J.; Whitesides, G. M. Anal. Chem. 2001,73, 5645-5650

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) 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

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