V02_Resist Cross-Linking and Exposure Doses Quiz

Post-Exposure Bake (PEB) and Development

• 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.
• Immerse the exposed SU-8 coated substrate in a developer solution, typically an organic solvent like propylene glycol monomethyl ether acetate (PGMEA), to selectively dissolve the unexposed SU-8.
• The developer will reveal the pattern defined by the photomask.

Process Control

• Once the SU-8 microstructures are defined, perform various additional processes as needed, such as etching, metallization, or bonding, to create complex microdevices or structures.
• Rinse the substrate with a solvent like isopropanol to remove any remaining developer and debris.
• Dry the substrate carefully.

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.

Photo Resist 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.

Spin Coating a Thick Layer of Photo Resist (Gyrest)

Preparation

• Ensure the substrate is clean and free of contaminants.
• Thoroughly clean the substrate using a suitable cleaning process and dry it.

Prepare Photoresist

• Mix or dilute the photoresist solution according to the manufacturer's specifications to achieve the desired thickness or viscosity.
• Ensure the photoresist is free of bubbles.

Set up Spin Coater

• Place the clean substrate on the chuck of the spin coater.
• Ensure it is securely held in place and properly centered.

Dispense Photoresist

• Dispense the photoresist solution onto the center of the substrate.
• The amount of photoresist dispensed 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 the 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.
• 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, 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 to 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, including solubility in developer, stability in alkaline solutions, stability in electroplating, temperature stability, and resistance to etching.
• Photo Acid Generator (PAG): Defines photochemical properties, including wavelength of exposure light, absorbing capacity, and resolution.
• Solvent: Defines solvent power of RM and PAG in solvent, including solubility.

SU-8

• Consists of a glycidylether-derivate of bisphenol A with 8 epoxy groups in one molecule.
• Photo Acid Generator (PAG): Triaryl-sulfonium hexafluorantimonate (around 10 wt%).
• During exposure, the activator PAG is generated in negative photo resists.
• Activator is responsible for cross-linking of resist in PEB step.
• Cross-linking is proportional to exposure doses of light.
• Activator has influence on chemical and thermal stability of resist, shape of resist side walls, and more.

SU-8 Advantages

• 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.

SU-8 Drawbacks

• 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.

Properties of PDMS

• Colorless.
• Transparent.
• Non-Toxic.
• Chemically inert.
• Known in cosmetic products.

Process Technologies for Microfluidic Chips

• Lithography using thick photoresists: SU-8.
• Molding technology using soft silicone: PDMS.
• Etching of glass (HF solution, laser): SU-8.

SU-8 Processing

• Lithography process with SU-8.
• Substrate cleaning.
• Prebake: Start with a clean and flat substrate, often made of silicon, glass, or another suitable material.
• Coating: Apply a thin layer of SU-8 photoresist onto the substrate using a spincoating technique.
• Softbake (Hotplate): After spin-coating, perform a pre-bake to remove excess solvent and ensure that the photoresist layer is uniform.
• Alignment: Place a photomask, which contains the desired pattern or design, in close proximity to the SU-8 coated substrate.
• 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.