Soft Lithography

About

Soft lithography is the transfer of patterned microstructures with molded elastomer. The most common elastomer used in this technique is PDMS (polydimethylsiloxane), which is soft, low cost and easy to mold. It is also bio-compatible with high thermal stability and high chemical stability, is chemically inert, insulating, transparent to UV and visible light, mechanically flexible and durable, allowing many applications. The most common devices fabricated with this technique are microfluidics.
NOTE: You might hear soft lithography equated to "microfluidics fabrication." However, it is important to remember that not all microfluidic devices are fabricated by soft lithography and not all soft lithography is for microfluidic applications!

Process Flow

Standard QNF Soft Lithography Protocol, Updated May 2025.
Images from "Microfluidics at QNF" Process on Fabublox

• The master used can be fabricated out of photoresist or etched silicon. The most commonly used materials are epoxy-based negative photoresist (SU8 or HARE SQ). For a negative photoresist, the parts exposed to UV become cross-linked, while the remainder of the film remains soluble and can be washed away during development. The height of this structure is determined by the thickness of the resist, which we control through our application method. The length and width of features are patterned through the UV exposure, either by mask or direct write tools.
  
• For molding success, there are two important components to think about -- master durability and PDMS removal. Etched silicon molds will be more durable than patterned photoresist. However, an O2 plasma treatment under vacuum before applying resist has shown more than sufficient adhesion for patterned HARE SQ in our own tests. To enable easy release of the PDMS from the mold, it can be beneficial to do a silanization treatment before applying the PDMS.
  
• An O2 plasma step is included if applying the PDMS to glass or another piece of PDMS to make a device.

QNF Standard Protocols

• Making a Master: HARE SQ, MA-03 - visual overview
• Making a Master: HARE SQ, MA-03 - detailed instructions
• Making a Device: PDMS to PDMS bonding

Videos

• Spin Coating Thick Resist
• Negative Resist First Mask Alignment Mark
• Developing SU8 Resist
• Degassing PDMS

Equipment at QNF

Soft Materials

• DE-07: Anatech SCE-106 Barrel Asher
• MA-03: ABM Mask Aligner
• PVD-08: Silanization Dessicator
• SPN-08: CEE Apogee Spinner - Negative Epoxy
• SPN-09: CEE Apogee Spinner - PDMS
• LW-02: Nanoscribe Photonic Professional GT

Main Cleanroom

• LW-01: Heidelberg DWL 66+ Laser Writer
• LW-03: DMO MicroWriter ML3 Pro
• MA-01: SUSS MicroTec MA6 Gen3 Mask Aligner
• DE-03: SPTS Si DRIE

Want to Know More?

Related Wiki Pages

• Photolithography
• Resists at QNF
• How to Make a Mask

Internal Reports & Presentations

• Establishing HARE SQ Protocols at QNF -- Delamination Testing of Soft Litho Masters
• T-Topping Study Report
• QNF Soft Lithography Workshop Process Flow - SU8 3050
• Presentation for Workshop - Biological Researchers (PDF Format)
• Presentation for ESE 536 Course Module (PDF Format)

External Resources

• HARE SQ Dispensing and Handling Techniques - KemLab
• PDMS Bonding - PIE Scientific
• Microfluidics and Nanofluidics Handbook: SU-8 Photolithography and Its Impact on Microfluidics by Rodrigo Martinez-Duarte and Marc J. Madou