PDMS Microfluidic Device Fabrication: SU-8 Soft Lithography Tutorial

Polydimethylsiloxane (PDMS) is a silicone-based polymer with a unique silicon-oxygen backbone instead of carbon-carbon bonds. As an elastomer, it has rubber-like flexibility with limited chemical stability. Unlike glass and silicon, PDMS is porous, allowing air to pass through its structure (though permeability is very low). It is transparent and has good thermal stability up to approximately 150-200°C. The main advantage of PDMS is its suitability for soft lithography prototyping, enabling rapid device replication outside cleanroom environments at much lower cost and faster turnaround times compared to silicon and glass fabrication.

Polydimethylsiloxane (PDMS) is an elastomer material that can sustain large deformation and recover its shape after force application. It has major applications in silicon-based elastomer family for sensors and actuators. Key properties include transparency, electrical insulation, mechanical elasticity, gas permeability, and biocompatibility. PDMS is widely used in microfluidics and sensing applications due to these characteristics.

Polydimethylsiloxane (PDMS) is a widely used material in microfluidic device fabrication due to its favorable properties. Key characteristics include optical transparency for visual monitoring, biocompatibility for cell culture applications, gas permeability that allows oxygen diffusion to support cell survival, chemical inertness toward many organic solvents, elastomeric properties enabling reversible deformation, and surface modifiability (can be made hydrophilic). These properties make PDMS ideal for creating microfluidic chips for biological experiments.

Silicones are inorganic polymers with silicon-oxygen backbone and pendant organic groups. The Si-O bond polarity and low rotation energy provide exceptional stability and flexibility. Properties range from fluids to elastomers to gels depending on molar mass, cross-linking degree, and pendant groups. PDMS has very low glass transition temperature and high free volume, providing excellent elastomeric behavior and gas permeability. However, hydrophobic methyl groups make PDMS prone to protein adsorption and fibrous capsule formation. Silicone elastomers are 3D covalently cross-linked networks with fillers for reinforcement. Silicone gels are lightly cross-linked networks between fluids and elastomers. Applications include extracorporeal equipment (dialysis, blood oxygenators), catheters, drains, shunts, and reconstructive surgery.

Cleanroom safety requires understanding particle contamination control (ISO Class 6/Class 1000 cleanrooms limit particles to 1,000 per cubic foot), proper gowning procedures (hairnet, face shield, beard bag, booties, smock, gloves, safety glasses), emergency protocols (evacuation to assembly points, calling UNM police for emergencies), chemical safety (SDS review, proper PPE including triple tria gloves and acid aprons, never submerging body parts in chemicals), and waste disposal (separate containers for corrosives, flammables, sharps, and regular waste).

This extensive section covers clean room operations essential for microfabrication. Topics include: (1) Wafer contamination sources—organic (skin oils, body fluids), metallic (sodium, potassium, heavy metals), native oxide (<10nm), and micro roughness requiring thorough cleaning; (2) Clean room classification by particle concentration: Class 1 (1 particle/cubic ft), Class 10, Class 100, etc., with HEPA filtration and controlled environmental parameters (temperature, humidity, sound, lighting); (3) Personnel protocols: gowning sequence (gloves → gown → hairnet → face mask), prohibited items (fur, jewelry), prohibited activities (eating, mobile phones), and communication restrictions; (4) Wet bench safety: acid-specific gloves, HF handling in Teflon containers, fume hood requirements, proper chemical mixing (acid to water), MSDS consultation; (5) Emergency response: spill procedures (flush skin immediately), disposal protocols, and first aid availability.

Clean rooms require comprehensive safety protocols including yellow lighting (filtered blue/green light to prevent wafer exposure), emergency information sheets at exits, Patron and Halon fire extinguishers for electrical fires, and CO2 systems for solvent hoods. Humans are the biggest contamination source, shedding thousands of particles per second. Clean rooms are classified by particle count standards: Class 1,000 allows up to 1,000 half-micron particles per cubic foot, while Class 10 allows only 10 particles. Semiconductor devices are extremely small: transistor gates are 2 microns or smaller, while gear teeth in devices are 8 microns (red blood cell size). A single particle landing on a transistor during processing can short out the device. Clean room rules prohibit makeup, hair gel, cologne, and smoking (20-minute waiting period required). PPE includes hair nets, shoe covers, face masks, smocks, and chemical gloves. Contact lenses are prohibited because chemical fumes can collect on or under lenses, potentially fusing to the cornea. Proper gowning order is: shoe covers, hair net, face cover, gloves, smock, and safety glasses.

Cleanrooms are controlled environments that minimize airborne particles to enable precision manufacturing and R&D activities, with Class 100 and Class 1,000 cleanrooms housing lithography and deposition equipment respectively; human personnel are the primary contamination source, releasing millions of particles daily, so strict protocols including pre-entry preparation (no food, cosmetics, jewelry, or personal items), proper gowning procedures (hairnet, coverall, booties, gloves), and adherence to safety guidelines (no eating/drinking, proper chemical handling, waste disposal protocols) are essential to maintain sterile conditions and prevent contamination of sensitive processes.

Clean rooms are precisely controlled environments where particle count, temperature, and humidity are strictly maintained to prevent contamination during semiconductor fabrication. The National Nanofabrication Centre (NNFC) at IISc Bangalore operates Class 100 and Class 1000 clean rooms, with Class 100 reserved for highly sensitive lithography areas. Contamination is categorized into three types: airborne/molecular (from outgassing, oils, and chemicals), surface contamination (fingerprints, oils, skin products), and particulate contamination (from clothing fibers, skin scales, and materials). Humans contribute approximately 75% of all particles, making proper gowning procedures essential—using face masks, hair nets, polyester bunny suits, booties, and gloves. Emergency protocols include using lined phones to contact Building Maintenance System (BMS) at 115, following buddy systems, and immediate evacuation during gas or fire alarms. Wet bench areas require additional PPE including chemical-resistant gloves (nitrile for general chemicals, epilon for HF), face shields, and aprons. Hazardous chemicals like HF and BOE require special handling due to their corrosive nature and potential to affect bone calcium and central nervous system.

Micrfluidics manipulates fluids at the microscale (microliter to picoliter range) using channels thinner than a human hair (10-500 micrometers). At this scale, surface forces dominate over gravity, and Reynolds numbers typically fall below 1, resulting in laminar flow where fluids move in parallel layers with minimal mixing. This enables precise control of fluid behavior that differs fundamentally from macroscopic fluid dynamics.

Microfluidics exploits size reduction effects: volume scales with L³, reducing reagent consumption; viscous/inertial forces decrease, promoting laminar flow; Laplace pressure increases, making surface tension critical; diffusion times decrease, requiring active mixing; electric fields strengthen, enhancing electrokinetic effects. Five pumping mechanisms exist: capillary-driven (hydrophilic/hydrophobic surfaces), pressure-driven (pumps), centrifugal (rotating structures), electrokinetic (electric fields), and droplet-driven (surface tension effects). Each mechanism offers advantages for specific applications, from medical diagnostics to chemical synthesis. Understanding these principles enables rational design of microfluidic devices for various analytical and synthetic purposes.

Microfluidics manipulates small fluid volumes in channels ranging from tens to hundreds of micrometers. The fundamental principle is that reducing system dimensions dramatically increases the surface-to-volume ratio, which directly improves heat and mass transfer between phases. At the microscale, surface phenomena (diffusion, surface tension, viscosity) become dominant over bulk phenomena (inertial effects). The Reynolds number (Re = ρvD/μ) determines flow regime, with microfluidic systems typically showing Re values of 0.001 to 0.5, indicating laminar flow where viscous forces dominate. In laminar flow, mixing occurs primarily through molecular diffusion rather than convective turbulence, with a parabolic velocity profile (v = Vmax(1 - r²/R²)). The pressure drop in laminar flow is directly proportional to flow rate (ΔP ∝ Q), enabling precise control over flow rates in microfluidic systems.

Microfluidics is the study of fluid flow through channels at micrometer scales (approximately the diameter of a hair, about 100 micrometers). At this scale, fluids behave differently than in larger systems—unlike rivers where turbulence creates mixing, microfluidic systems show laminar flow where different liquids flow side by side without mixing. Gravity becomes negligible at this scale, and the physics has unique characteristics that make it valuable for scientific applications. This field combines biology, chemistry, physics, and engineering, with applications extending to energy research and nanotechnology.