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Module 3: Fabrication Techniques

  1. Q1(b). SU-8 is an example of: (i) Positive Photoresist (ii) Negative Photoresist (iii) Electron Beam Resist (iv) Inorganic Resist2025?m

    Module 3: Fabrication Techniques

    SU-8 is an example of:

    (i) Positive Photoresist
    (ii) Negative Photoresist
    (iii) Electron Beam Resist
    (iv) Inorganic Resist

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    Worked Solution

    Answer: (ii) Negative Photoresist

    SU-8 is an epoxy-based negative photoresist. The exposed regions polymerize and become resistant to the developer, allowing high-aspect-ratio microstructures to be formed.

  2. Q1(d). The LIGA process utilizes which type of radiation for lithography? (i) Ultraviolet (UV) (ii) X-ray (iii) Infrared (iv) Visible Light2025?m

    Module 3: Fabrication Techniques

    The LIGA process utilizes which type of radiation for lithography?

    (i) Ultraviolet (UV)
    (ii) X-ray
    (iii) Infrared
    (iv) Visible Light

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    Answer: (ii) X-ray

    LIGA uses deep X-ray lithography to create high-aspect-ratio microstructures. The X-rays expose a thick resist, traditionally PMMA, through a mask.

  3. Q5(a). Explain the fabrication steps for high-aspect-ratio structures using SU-8 photoresist and Ultraviolet (UV) lithography.20257m

    Module 3: Fabrication Techniques

    Explain the fabrication steps for high-aspect-ratio structures using SU-8 photoresist and Ultraviolet (UV) lithography.

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    Worked Solution

    SU-8 High-Aspect-Ratio Structures Using UV Lithography

    SU-8 is a negative epoxy-based photoresist capable of forming thick, high-aspect-ratio structures.

    Fabrication steps

    1. Substrate preparation: Clean and dehydrate the silicon/glass substrate.
    2. SU-8 coating: Dispense SU-8 and spin coat it to the required thickness. For very thick layers, multiple coating steps may be used.
    3. Soft bake: Heat the coated wafer to remove solvent.
    4. Mask alignment: Place the patterned photomask over the wafer.
    5. UV exposure: Expose the SU-8 through the mask. Exposed regions undergo cross-linking.
    6. Post-exposure bake: Further cross-link the exposed regions.
    7. Development: Developer removes the unexposed SU-8, leaving the patterned high-aspect-ratio structures.
    8. Rinse and dry: Remove developer residue.
    9. Hard bake, if required: Improves stability and mechanical robustness.

    Why SU-8 is suitable

    It provides thick films, high aspect ratios, good chemical resistance, good mechanical strength and relatively high resolution, making it useful for microchannels, molds and microstructures.

  4. Q5(b). Write a detailed note on X-ray lithography and the LIGA process. What are its advantages?20257m

    Module 3: Fabrication Techniques

    Write a detailed note on X-ray lithography and the LIGA process. What are its advantages?

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    X-ray Lithography and LIGA

    X-ray lithography uses short-wavelength, high-energy X-rays to transfer a mask pattern into a thick resist. The short wavelength provides low diffraction and enables high-resolution, high-aspect-ratio structures.

    LIGA

    LIGA is a German acronym derived from Lithographie, Galvanoformung, Abformung (lithography, electroplating and molding).

    Process

    1. Coat a conductive substrate with a thick X-ray-sensitive resist such as PMMA.
    2. Align a patterned X-ray mask.
    3. Expose the resist using deep X-rays.
    4. Develop the resist to form deep cavities/columns.
    5. Electroplate metal into the resist mold.
    6. Remove the resist to obtain a metal microstructure.
    7. The metal structure can also serve as a mold for replication.

    Advantages

    • Very high aspect ratio.
    • Nearly vertical sidewalls.
    • High dimensional accuracy.
    • Excellent replication capability.
    • Suitable for relatively thick microstructures.
    • Useful for microgears, nozzles, molds and other precision MEMS structures.

    Limitation

    The process requires specialized X-ray sources and masks, making it expensive and less accessible than conventional UV lithography.

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