In modern microfabrication, few materials are as widely used as SU-8 for creating microscale structures for devices like lab-on-a-chip systems and biosensors. SU-8 plays a central role in soft lithography workflows, particularly in the fabrication of master molds used to replicate microfluidic channels and other nanoscale/microscale features with high pattern fidelity in materials such as PDMS.
Its ability to produce precise, durable, and high-aspect-ratio structures has made SU-8 a preferred photoresist for a wide range of microfabrication applications.
This article explores the properties, fabrication processes, and applications of SU-8 photoresist, with a particular focus on its use in microfluidic device fabrication and master mold production for soft lithography.
What Is SU-8 Photoresist?
SU-8 is an epoxy-based negative photoresist widely used in photolithography to create thick and highly durable microscale structures. The term negative refers to the way it behaves under UV exposure: the areas exposed to light become solid and chemically crosslinked, while the unexposed regions remain soluble and can be washed away during development. The 8 in SU-8 refers to the average number of epoxy groups present in its molecular structure, which contribute to the extensive crosslinking that gives the material its strength and stability after processing.
SU-8 is available in several epoxy resin formulations that support the fabrication of structures with different thicknesses and aspect ratios. This versatility has contributed to its widespread adoption in prototyping environments.
Why IS SU-8 So Widely Used in Microfabrication?
SU-8 is widely used in microfabrication because it enables the fabrication of thick, high-aspect-ratio microstructures with excellent dimensional accuracy and pattern fidelity. It is fully compatible with standard photolithography processes and is available in multiple formulations to accommodate a wide range of film thicknesses.
Once cured, SU-8 offers high mechanical strength, chemical resistance, and long-term stability, making it the preferred photoresist for applications such as microfluidic master molds, MEMS, biosensors, micro-optics, and lab-on-a-chip devices.
The Role of SU-8 in Soft Lithography
In soft lithography, SU-8 is primarily used to fabricate the master mold from which microfluidic devices are replicated. Its ability to produce precise, high-aspect-ratio microstructures with excellent dimensional stability makes it an ideal mold material for PDMS casting. Because a single SU-8 master can be reused multiple times, it enables rapid prototyping, high reproducibility, and cost-effective fabrication of microfluidic devices.
SU-8 Lithography: The Core Process
The fabrication of SU-8 microstructures follows a standard photolithography workflow in which each processing step influences the final accuracy, quality, and reproducibility of the fabricated microstructures.
Although process parameters vary depending on the desired feature size and resist thickness, the overall sequence remains mostly the same: wafer preparation, spin coating, soft bake, UV exposure through a photomask, post-exposure bake, development, and, when required, a final hard bake to further improve the mechanical and chemical stability of the SU-8 structures.
The following sections describe the purpose and key considerations of each step.
Wafer Preparation
A proper wafer preparation provides a clean, stable surface for uniform SU-8 deposition and improves the quality and reproducibility of the final microstructures.
Therefore, before SU-8 is applied, the substrate—typically a semiconductor silicon wafer—must be thoroughly cleaned to remove particles, organic residues, and moisture that could affect photoresist adhesion and coating uniformity. In many fabrication workflows, the wafer is also dehydrated by heating to improve adhesion and minimize defects during subsequent photolithography steps.
Spin Coating: Creating a Uniform Film
Spin coating is the first processing step after substrate preparation and the most widely used method for depositing SU-8 onto a silicon wafer. During this step, liquid SU-8 is dispensed at the center of the substrate, which is then rotated at high speed (typically several thousand rpm) to spread the photoresist into a uniform thin film by centrifugal force.
The final film thickness is primarily determined by the spin speed, resist viscosity, and coating parameters. Uniform coating is essential, as variations in thickness, microbubbles, or surface contaminants can affect subsequent baking, UV exposure, and pattern quality.
💡 Spin Coating Tip: After dispensing the SU-8 onto the wafer, allow it to rest for a few minutes before spinning. This gives trapped microbubbles time to rise to the surface and burst, improving coating uniformity.
Soft Bake: Removing Solvent and Stabilizing the Film
After spin coating, the SU-8 layer still contains residual solvent (typically around 5–20%) that must be removed before UV exposure. The soft bake removes this solvent, stabilizes the photoresist, improves adhesion, and prepares the film for accurate pattern transfer.
This step is typically performed in two temperature stages: an initial bake at a lower temperature (around 65°C) to gradually evaporate the solvent, followed by a higher-temperature bake (around 95°C) for a duration determined by the resist thickness, to further solidify the film. The wafer should then be allowed to cool gradually to room temperature.
Heating too quickly can trap solvent beneath a hardened surface layer, creating internal stress that may lead to poor pattern quality or cracking. Insufficient baking, on the other hand, often results in incomplete solvent removal that can negatively affect subsequent UV exposure and development.
💡 Soft Bake Tip: Use a programmable hot plate whenever possible instead of a convection oven. It provides more uniform temperature distribution, allowing better solvent removal and reducing stress, particularly in thick SU-8 layers.
UV Exposure and Mask Patterning
Once the soft bake is complete, the SU-8 layer is aligned with a photomask containing the desired pattern and exposed to near-UV light (typically around 365 nm).
Depending on the application, either positive (transparent patterns on an opaque background) or negative (opaque patterns on a transparent background) photomasks can be used. For microfluidic devices, however, negative photomasks are the most common, allowing the desired channel structures to remain after development.
During exposure, the transparent regions of the photomask allow UV light to activate the exposed SU-8, initiating the chemical reactions that will later produce the crosslinked microstructures (during the post-exposure bake).
💡 UV Exposure Tip: The geometry and quality of the final features largely depend on accurate mask alignment and proper exposure dose. Excessive UV exposure can reduce feature resolution and distort sidewalls, while insufficient exposure may result in incomplete crosslinking and poor pattern definition.
Post-Exposure Bake (PEB): Completing Crosslinking
Following the UV lithography step, the wafer undergoes a post-exposure bake (PEB), which completes the crosslinking reactions initiated during exposure. Similar to the soft bake, the wafer is gradually heated to approximately 95°C, activating the photo-generated acid and allowing the epoxy groups in SU-8 to fully polymerize into a mechanically strong and chemically stable polymer structure.
Because the degree of crosslinking directly affects pattern fidelity and structural integrity, PEB is one of the most critical steps in the SU-8 fabrication process.
💡 Post-Exposure Bake Tip: Use a slow, controlled heating and cooling profile during PEB, especially for thick SU-8 layers. Rapid temperature changes can introduce internal stress, leading to pattern deformation or cracking.
Development: Revealing the Final Structure
During development, the wafer is immersed in a developer solution (typically PGMEA or a dedicated SU-8 developer) that dissolves the unexposed, uncrosslinked portions of the photoresist, revealing the final patterned SU-8 microstructures.
Gentle agitation is often used to improve development efficiency. Development time must be carefully controlled to ensure complete removal of the unexposed photoresist without damaging the patterned features.
💡 Development Tip: After development, rinse the wafer with fresh PGMEA or isopropyl alcohol (IPA). If a white or cloudy residue appears during the IPA rinse, the SU-8 is likely underdeveloped due to residual uncrosslinked resist. Return the wafer to the developer for additional time, then rinse with IPA again. If the wafer remains clear during the IPA rinse, development is likely complete.
Hard Bake: Strengthening the Final Mold
After development, an optional hard bake may be performed to further strengthen the SU-8 structures. This is particularly useful when the SU-8 master will be repeatedly used for PDMS casting and subsequent device bonding.
Typically carried out at temperatures between 140°C and 200°C for around 20–30 minutes, this additional thermal treatment improves mechanical hardness, chemical resistance, and long-term durability while reducing residual solvent.
Common Challenges When Working with SU-8
Despite its many advantages, SU-8 processing requires careful control at every stage of photolithography. The most common challenges include:
- Poor adhesion to the substrate
- Air bubble formation during spin coating
- Edge bead formation
- Cracking caused by internal thermal stress
- Over- or underexposure during UV lithography
- Incomplete development
- Residual resist (scumming)
- Pattern deformation in thick SU-8 layers
Most of these defects can be minimized through proper substrate preparation, optimized baking conditions, accurate exposure doses, and controlled development.
SU-8 vs Hare SQ™
Both SU-8 and HARE SQ™ are epoxy-based negative photoresists designed for fabricating high-aspect-ratio microstructures. They share many characteristics, including excellent mechanical strength, chemical resistance, and compatibility with standard UV photolithography processes. Both materials are widely used for microfluidic master molds, MEMS, micro-optics, and other microscale applications.
The main differences lie in their processing characteristics and available formulations. SU-8 has been the industry standard for decades and offers an extensive range of viscosities, well-established processing protocols, and broad adoption in both research and industrial microfabrication. HARE SQ™ is a newer alternative that integrates easily into existing SU-8 fabrication workflows while providing competitive pricing, improved optical transparency, better cleanliness, and higher lot-to-lot consistency.
💡 Conclusion
SU-8 remains one of the most widely used photoresists in microfabrication due to its versatility, durability, and ability to produce high-quality microscale structures. Combined with standard photolithography, it continues to be the preferred material for fabricating microfluidic master molds and a wide range of MEMS and lab-on-a-chip devices.
Stay tuned for more insights on SU-8 photoresists, soft lithography, microfabrication, PDMS microfluidics, and other techniques driving innovation in microfluidic devices 🔬!
📧 Si vous avez des questions ou des commentaires, n'hésitez pas à nous contacter à l'adresse contact@darwin-microfluidics.com.
❓ FAQ: SU-8 Photoresist
What is SU-8 photoresist?
SU-8 is an epoxy-based negative photoresist, a light-sensitive material used in photolithography to create thick, high-aspect-ratio microscopic patterns on a substrate for microfabrication applications.
Why is SU-8 called a negative photoresist?
SU-8 is called a negative photoresist because the regions exposed to UV light become hard and remain on the substrate, while the unexposed areas remain soft and are washed away during the development process.
What is SU-8 used for?
SU-8 is commonly used to fabricate microfluidic master molds, MEMS devices, biosensors, micro-optical components, and lab-on-a-chip systems.
Why is SU-8 commonly used for microfluidic master molds?
SU-8 is commonly used for microfluidic master molds because it produces durable, high-resolution structures with smooth vertical sidewalls, allowing repeated and accurate replication of PDMS microfluidic devices.
What are the main steps of the SU-8 photolithography process?
A typical SU-8 process includes substrate preparation, spin coating, soft bake, UV exposure through a photomask, post-exposure bake, development, and an optional hard bake.
Why is a post-exposure bake (PEB) important for SU-8?
The post-exposure bake completes the crosslinking reactions initiated during UV exposure, giving the patterned SU-8 its mechanical strength, chemical resistance, and dimensional stability.
What are the most common challenges when processing SU-8?
Common issues include poor adhesion, air bubbles, cracking from thermal stress, over- or underexposure, incomplete development, and residual resist after development.
Why are cracks formed in SU-8 during fabrication?
Cracks are usually caused by excessive internal stress resulting from rapid heating or cooling, improper baking conditions, or thick SU-8 layers on substrates with different thermal expansion properties.
Why do air bubbles form during SU-8 spin coating?
Air bubbles can form during spin coating because of improper resist dispensing or trapped air. Allowing the SU-8 to rest briefly before spinning helps bubbles rise and disappear.
🔗 Références
- Mata, A., Fleischman, A. J., & Roy, S. (2006). Fabrication of multi-layer SU-8 microstructures. Journal of micromechanics and microengineering, 16(2), 276-284. https://doi.org/10.1088/0960-1317/16/2/012
- Yang, R., & Wang, W. (2006). UV Lithography of Ultrathick SU-8 for Microfabrication of High-Aspect-Ratio Microstructures and Applications in Microfluidic and Optical Components. Bio-MEMS: Technologies and Applications, 11.
- Keller, S., Blagoi, G., Lillemose, M., Haefliger, D., & Boisen, A. (2008). Processing of thin SU-8 films. Journal of micromechanics and microengineering, 18(12), 125020. https://doi.org/10.1088/0960-1317/18/12/125020
- Martinez-Duarte, R., & Madou, M. (2011). SU-8 photolithography and its impact on microfluidics. Microfluidics and nanofluidics handbook, 38.
- Lee, J. B., Choi, K.-H., & Yoo, K. (2015). Innovative SU-8 Lithography Techniques and Their Applications. Micromachines, 6(1), 1-18. https://doi.org/10.3390/mi6010001
