V02_Resist Cross-Linking and Exposure Doses Quiz

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Questions and Answers

What is the purpose of allowing the coated substrate to sit undisturbed after spin coating in photolithography?

To remove any residual solvent

To improve the adhesion of the photoresist

To allow any remaining bubbles to escape (correct)

To speed up the exposure process

Why is a post-exposure bake (PEB) performed after the exposure and development steps in photolithography?

To remove any residual solvent

To develop the pattern in the photoresist

To improve the stability of alkaline solutions

To complete the curing process (correct)

Which component of a negative photoresist is responsible for defining the physical properties such as solubility and stability?

Resist Matrix (correct)

Solvent

Photo Acid Generator

Activator

During which step is the pattern developed in the photoresist in the photolithography process?

Exposure and Development Signup and view all the answers

Why is it important to perform a soft bake or pre-exposure bake after spin coating in photolithography?

To remove any residual solvent Signup and view all the answers

What role does the activator play in a negative photoresist during exposure?

Cross-linking of resist Signup and view all the answers

Which step in photolithography involves exposing the coated substrate to UV light through a photomask?

Exposure and Development Signup and view all the answers

What does the resist matrix of a negative photoresist define?

Solubility in developer and stability in alkaline solutions Signup and view all the answers

What is the purpose of the activator in SU-8 resist?

Enhancing chemical and thermal stability Signup and view all the answers

Which step is performed after the exposure step in the SU-8 photolithography process?

Post exposure bake (PEB) Signup and view all the answers

During the development step in SU-8 photolithography, which areas of the SU-8 layer are dissolved?

The unexposed areas Signup and view all the answers

What is the role of PAG (Photo Acid Generator) in SU-8 during UV exposure at 365 nm?

Promoting cross-linking of epoxy groups Signup and view all the answers

What is the significance of HSbF (Triaryl-sulfonium hexafluorantimonate) in SU-8 processing?

It acts as a catalyzer for cross-linking of epoxy groups during PEB Signup and view all the answers

What is the purpose of the post-bake step in the SU-8 photolithography process?

To remove any remaining developer solution Signup and view all the answers

What occurs during cross-linking in SU-8 resist?

Reorientation of epoxy groups Signup and view all the answers

In a positive photoresist process (e.g., AZ-Novolack), which areas of the photoresist are dissolved during the development step?

The exposed areas Signup and view all the answers

What is the principle behind the Gyrest process for spin coating a thick layer of photoresist?

Utilizing a spin coater to distribute and spin a photoresist solution onto a substrate Signup and view all the answers

What is the significance of Tg (glass transition temperature) = 55 Celsius mentioned in the text?

Cross-linking occurs above this temperature during PEB step Signup and view all the answers

Which parameter is typically adjusted to achieve a thicker photoresist layer during spin coating?

All of the above Signup and view all the answers

Why were standard photoresists not optimized for resist layer thickness above 10 µm in former times?

They were meant only for high-resolution semiconductor structures Signup and view all the answers

In the ramp down and settle step of the spin coating process, what is the primary purpose?

To allow the photoresist to settle and achieve a uniform thickness Signup and view all the answers

Which step in the spin coating process is responsible for removing excess photoresist from the substrate?

Spin Signup and view all the answers

What is the primary purpose of the 'ramp down' step in the spin coating process?

To allow the photoresist to settle uniformly after spinning Signup and view all the answers

Which parameter is typically adjusted to control the thickness of the photoresist layer during spin coating?

Rotational speed (RPM) Signup and view all the answers

What is the primary purpose of the 'exposure' step after spin coating and before development?

To create a pattern in the photoresist layer Signup and view all the answers

Which step is typically performed before spin coating to ensure optimal adhesion and uniformity of the photoresist layer?

Substrate cleaning Signup and view all the answers

During the spin coating process, what is the primary force responsible for spreading the photoresist across the substrate surface?

Centrifugal force Signup and view all the answers

What is the purpose of the 'development' step after exposure in photolithography?

To remove unexposed areas of the photoresist Signup and view all the answers

Which step is typically performed after spin coating to improve the uniformity and adhesion of the photoresist layer?

Soft bake Signup and view all the answers

What is the purpose of the 'acceleration' parameter in the spin coating process?

To ensure uniform distribution of the photoresist Signup and view all the answers

What is the purpose of the softbake step after spin-coating SU-8 photoresist?

To remove excess solvent and ensure a uniform photoresist layer Signup and view all the answers

During the spin-coating process, what determines the thickness of the SU-8 photoresist layer?

The rotation speed and time of the spin-coater Signup and view all the answers

What is the purpose of the alignment step in the SU-8 lithography process?

To position the photomask correctly relative to the substrate Signup and view all the answers

What is the significance of the ramp-down and settle step in the spin-coating process?

To allow the photoresist to gradually slow down and settle evenly Signup and view all the answers

During the exposure step, what happens to the areas of the SU-8 photoresist exposed to UV light?

They undergo a chemical change and become cross-linked Signup and view all the answers

What is the purpose of the development step in the SU-8 lithography process?

To selectively remove the unexposed areas of the photoresist Signup and view all the answers

What is the purpose of the substrate cleaning step before spin-coating SU-8 photoresist?

To remove any contaminants or particles that may interfere with photoresist adhesion Signup and view all the answers

Which of the following parameters is not directly related to the spin-coating process?

Exposure time to UV light Signup and view all the answers

Study Notes

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.

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Description

Test your knowledge on resist cross-linking in photolithography processes. Learn about the influence of activators, PAGs, and catalyzers on the chemical and thermal stability of the resist, as well as the shape of resist side walls.