Photolithography Guide for Microfluidic Mold Fabrication

The Complete Guide to Photolithography for Microfluidic Mold Fabrication

January 21, 2025
Sakshi Shruti
3453 Views
Microfluidic chips, Microfluidic Mold Fabrication, Photolithography

Photolithography is a microfabrication technique used to create the molds later used in the production of microfluidic chips. It enables the precise patterning of microstructures by selectively exposing a photosensitive material to UV light. This blog will explore the key steps of photolithography, best practices, and the advanced mold fabrication solutions offered by Elveflow.

What is Photolithography?

Photolithography is a crucial microfabrication technique that transfers intricate patterns onto a substrate using a light-sensitive material known as photoresist resin. This process is widely used in microfabrication to achieve precise control over microchannel dimensions, ensuring accuracy and functionality in lab-on-a-chip devices. The process involves coating the substrate with a photoresist resin, exposing it to UV light through a photomask, and developing the exposed areas to create microscale features.

Types of Photoresists:

Positive photoresist: Exposed areas become soluble and are removed during development.
Negative photoresist (e.g., SU-8): Exposed areas harden and remain after development, forming the desired structure.

Photolithography is the backbone of microfluidic mold fabrication, enabling the creation of highly precise and reproducible microchannel patterns. By utilizing SU-8 photoresist, known for its excellent chemical resistance and high aspect ratio capabilities, photolithography ensures the production of durable and functional molds. These molds are critical to produce optimized microfluidic chips that can maintain uniform fluid flow and achieve reliable experimental outcomes in microfluidic applications.

Now, let’s dive into the key steps involved in mold fabrication, exploring each stage to ensure optimal results.

Steps Involved in Microfluidic Mold Fabrication

Creating microfluidic molds using photolithography involves several crucial steps, each contributing to the accuracy and reliability of the final device.

1. Deposition of SU-8 Photoresist

The first step involves spin-coating a uniform layer of SU-8 photoresist onto a silicon wafer. The coating thickness depends on the desired channel dimensions and spin speed settings. Achieving uniformity is critical for precise microstructures.

2. Soft Baking

Soft baking is performed on a hot plate at temperatures typically between 65- 95°C for a short period of time (1-5 mins) to remove solvents from the photoresist, improving adhesion and ensuring uniform exposure. Insufficient baking can lead to defects such as bubble formation during UV exposure. To achieve optimal results, gradual heating should be used to prevent shrinkage, ensure uniform heat distribution and avoid resin variations. It’s also crucial to avoid prolonged baking, as it can lead to premature hardening of the resin, affecting subsequent processing steps.

3. UV Exposure

During UV exposure, the coated wafer is illuminated through a photomask containing the desired pattern, with carefully controlled energy and duration to ensure proper cross-linking of the SU-8 resin. This step defines the features and dimensions of the final microfluidic mold. To achieve accurate pattern transfer, it is crucial to use a collimated UV light source to minimize diffraction effects and enhance resolution. Ensuring close, uniform contact between the photomask and the SU-8 resin-coated substrate further reduces edge distortions and improves accuracy.

4. Post-Exposure Bake (PEB)

After exposure, a secondary baking step helps finalize the cross-linking of the exposed photoresist. Typical temperatures range from 95-115°C. Proper PEB settings reduce stress and improve pattern resolution.

5. Development (Revelation)

In this stage, the unexposed photoresist is dissolved using a developer solution, revealing the micro-pattern. This step requires precise timing to prevent under- or over-development, which can impact the feature dimensions and structural integrity.

Quality Control & Final Inspection

After fabrication, the mold undergoes quality checks to ensure it meets design specifications. Inspections may include:

Optical microscopy: To check for defects like cracks, residues, or incomplete development.
Profilometry: To measure the resin thickness and uniformity.
Contact angle measurement: To evaluate surface hydrophobicity.

Ensuring these quality control steps helps in achieving reproducible and high-quality microfluidic molds.

Do’s and Don’ts of Photolithography

Do’s:

Ensure a controlled environment, as traditional microfabrication typically requires a cleanroom to minimize contamination. However, with Elveflow’s mold fabrication solutions, you can achieve high-quality results without the need for costly cleanroom facilities.
Control spin-coating parameters carefully to achieve uniform resin thickness and avoid edge bead formation.
Optimize exposure parameters such as time, intensity, and mask alignment for accurate pattern transfer.
Follow precise soft and post-exposure baking protocols to prevent structural defects and ensure proper resin curing.
Store photoresist correctly, away from light and heat, to maintain its chemical integrity and performance.

Don’ts:

Avoid overheating during baking, as it can lead to cracking and deformation of the resin.
Do not expose the photoresist to ambient light, which can lead to unintentional partial exposure and pattern distortion.
Refrain from under-developing, as it can leave residual resin and affect pattern definition.
Never use expired photoresist, as it may lead to poor adhesion, incomplete cross-linking, and reduced resolution.
Avoid skipping quality checks, which can help detect defects early and prevent costly errors later.

Elveflow’s Mold Fabrication Station – Your Complete Solution

To achieve the highest precision and reliability in microfluidic mold fabrication, Elveflow offers a state-of-the-art Mold Fabrication Station. This turnkey solution is designed to streamline the photolithography process by providing:

High-precision spin coaters for uniform resin deposition.
Pro-grade hot plates for consistent baking processes.
Optimized UV exposure systems with precise intensity control.
Very useful training for reproducible and efficient fabrication workflows.

Elveflow’s mold fabrication system ensures reproducibility and reliability in the production of high-resolution microfluidic molds, making it the perfect choice for researchers and manufacturers alike. Additionally, it can be tailored to your specific needs with the integration of the mask aligner, providing enhanced precision tailored to your fabrication needs.

Conclusion

Photolithography is an essential step in microfluidic device fabrication, enabling the creation of precise microscale patterns. By following best practices and utilizing advanced equipment like Elveflow’s Mold Fabrication Station, researchers can achieve high-quality, reproducible results. Whether you’re developing lab-on-a-chip devices or conducting biomedical research, optimizing your photolithography process is crucial to success.

For expert advice and state-of-the-art equipment, turn to Elveflow to enhance your microfabrication capabilities.

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