V02 - Process Technologies for Microfluidic chips.docx

V02-Process Technologies for Microfluidic Chips Introduction Process Technologies for Microfluidic chips: Lithography using thick photoresists : SU-8 Molding technology using soft silicone: PDMS (Poly-dimethylsilocane) Etching of glass(HF solution, laser): SU-8 Processing: Lithography Process with S...

V02-Process Technologies for Microfluidic Chips Introduction Process Technologies for Microfluidic chips: Lithography using thick photoresists : SU-8 Molding technology using soft silicone: PDMS (Poly-dimethylsilocane) Etching of glass(HF solution, laser): SU-8 Processing: Lithography Process with SU-8 Substrate Cleaning Prebake : Start with a clean and flat substrate, often made of silicon, glass of another suitable material. Clean the substrate thoroughly to remove any contaminants or particles that may interfere with the adhesion of the SU-8 photoresist. Coating : Apply a thin layer of SU-8 photoresist onto the substrate using a spincoating technique. The rotation speed and time determine the thickness of the coated layer. The SU-8 should spread evenly across the substrate. Softbake (Hotplate): After spin-coating, perform a pre-bake to remove excess solvent and ensure that the photoresist layer is uniform. This step typically involves heating the substrate on a hotplate at a low temperature. Alignment: Place a photomask, which contains the desired pattern or design, in close proximity to the SU-8 coated substrate. Proper Alignment is critical to ensure that the pattern is transferred accurately. Exposure: Expose the SU-8 photoresist to UV light through the photomask. The areas exposed to UV light will undergo a chemical change, which the masked areas remain unchanged. Post Exposure Bake PEB (Hotplate): After exposure, perform a post-exposure bake (PEB) at a higher temperature to further cure the exposed SU-8, making it more resistant to subsequent processing steps. Development: Immerse the exposed SU-8 coated substrate in a developer solution (typically an organic solvent like propylene glycol monomethyl ether acetate, PGMEA). The developer will selectively dissolve the unexposed SU-8, revealing the pattern defined by the photomask. Postbake(Hotplate): Rinse the substrate with a solvent like isopropanol to remove any remaining developer and debris. Afterward, dry the substrate carefully. Process control: Once the SU-8 microstructures are defined, you can perform various additional processes as needed, such as etching, metallization, or bonding, to create complex microdevices or structures. Photo Resist Negative (e.g SU-8) Exposed areas are cross-linked in PEB. In the development step the non-exposed areas will be dissolved. Positive (e.g AZ-Novolack) Polymer structures in exposed areas will be cracked under UV exposure. No PEB is needed. In the development step the exposed areas will be dissolved High resolution How to spin Coat a Thick Layer of Photo Resist – Principle of Gyrest The process of spin coating a thick layer of photoresist, often referred to as Gyrest, involves using a spin coater to distribute and spin a photoresist solution onto a substrate. This technique is commonly used in semiconductor fabrication and microfabrication processes. Here are general steps to spin coat a thick layer of photoresist using the Gyrest principle: Preparation: Start by ensuring that your substrate is clean and free of contaminants. It should be thoroughly cleaned using a suitable cleaning process and dried. Prepare Photoresist: Mix or dilute your photoresist solution according to the manufacturer’s specifications to achieve the desired thickness or viscosity. Make sure the photoresist is free of bubbles. Set up spin coater: Place the clean substrate on the chuck of the spin coater. Ensure that it is surely held in place and properly centered. Dispense Photoresist: Dispense the photoresist solution onto the center of the substrate. The amount of photoresist you dispense will depend on the desired thickness and the specific photoresist used. Spin coating parameters: Configure the spin coater’s parameters, including rotation speed (RPM), acceleration, and time, based on the manufacturer's recommendations and your desired photoresist thickness. Spin coating process: Initiate the spin coater, which will start spinning the substrate. The centrifugal force will cause the photoresist to spread evenly across the substrate surface, forming a thin layer. Ramp Down and settle: After the desired spinning time, gradually reduce the rotational speed (ramp down) to allow the photoresist to settle uniformly. Post-spin coating: Allow the coated substrate to sit undisturbed for a short period to allow any remaining bubbles to escape and for the photoresist to further settle. Bake: Depending on the specific photoresist used, you may need to perform a soft bake, or a pre-exposure bake to remove any residual solvent and improve adhesion. Exposure and Development: Follow the standard photolithography process by exposing the coated substrate the UV light through a photomask and developing the pattern in the photoresist. Final Bake: Depending on the photoresist, perform a post-exposure bake (PEB) to complete the curing process. Inspect and Analyze: Inspect the development pattern to ensure it meets the desired specifications. Basic components of a negative photo resist: Resist Matrix: Defines physical properties. solubility in developer stability in alkaline solutions stability in electroplating temperature stability resistance to etching. viscosity layer formation Photo acid Generator (PAG): Defines photochemical properties. Wavelength of exposure light absorbing capacity resolution Solvent: Defines solvent power of RM and PAG in solvent. solubility Resist Matrix RM: EPON SU-8 Epoxy resin (Shell chemical) Consists of a glycidylether-derivate of bisphenol A 8 epoxy groups in one molecule (therefore the 8 at SU-8) Photo Acid Generator PAG: During exposure, the activator PAG is generated in negative photo resists. Activator is responsible for cross-linking of resist in PEB step. Cross-linking proportional to exposure doses of light. Activator has influence on : Chemical and thermal stability of resist Shape of resist side walls. PAG in SU-8 Triaryl-sulfonium hexafluorantimonate (around 10 wt) UV exposure at 365 nm resists in release of fluor antimony acid. HSbF acts as catalyzer. Provokes cross-linking of epoxy groups during PEB. PAG in SU-8: During cross-linking : reorientation of epoxy groups. Generation of 3D network Shrinking of film Mechanical tensile stress Cross-linking: Above glass temperature in PEB step. Tg = 55 Celsius Aspect Ratio = Height / width Standard Photo resists: In former times, resist layer thickness above 10 µm were not possible (They were only optimized for high-resolution semiconductor structures) Advantages of SU-8: Processing with standard UV-lithography Layer Thickness above 500 µm possible Aspect ratio above 100 is possible. Transparent Biocompatible High Thermal stability Good mechanical properties High resistance to chemicals Drawbacks of SU-8: Resist removal Mechanical stress Autofluorescence Soft-Lithography with PDMS: Definition: Transfer of micro and nano structures by printing using soft stamps General principle: Soft-Lithography is manufacturing of elastic PDMS stamps + the transfer of stamp structures to surfaces by printing. What are properties of PDMS? Colorless Transparent Non-Toxic Chemically inert Known in cosmetic products. Biocompatible Bonding Technologies: PDMS – PDMS or PDMS-Glass: Gluing Heating over glass Temperature Tg Thermal Compression Ultrasonic Laser welding Plasma (surface modification): Reversible PDMS-Glass Bonding: PDMS layer adhere to SiO2 surfaces up to a pressure of 0.35 bar without glue. Irreversible: Oxygen plasma (mostly used) : reactive OH-groups are formed at the surface. Concentration gradient (only for PDMS-PDMS bonding): Variation of components A and B. Cross-linking at interface through temperature treatment. Substrate Bonder Feedthroughs