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Bio MEMS

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Module 1: MEMS Overview

  1. Q1(h). Which etching process is isotropic in nature? (i) Wet Chemical Etching (ii) Deep Reactive Ion Etching (DRIE) (iii) Plasma Etching (iv) Ion Beam Milling2025?m

    Module 1: MEMS Overview

    Which etching process is isotropic in nature?

    (i) Wet Chemical Etching
    (ii) Deep Reactive Ion Etching (DRIE)
    (iii) Plasma Etching
    (iv) Ion Beam Milling

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    Answer: (i) Wet Chemical Etching

    Wet chemical etching is generally isotropic, meaning material is removed at approximately similar rates in different directions. This can cause lateral undercutting beneath a mask. Some wet etchants can be crystal-orientation dependent, but in the standard classification used in this question, wet etching is treated as isotropic.

  2. Q1(i). The dominant force scaling advantage in MEMS at the micro-scale is: (i) Gravity (ii) Inertia (iii) Surface Tension (iv) Magnetic Force2025?m

    Module 1: MEMS Overview

    The dominant force scaling advantage in MEMS at the micro-scale is:

    (i) Gravity
    (ii) Inertia
    (iii) Surface Tension
    (iv) Magnetic Force

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    Answer: (iii) Surface Tension

    At the microscale, surface-area-to-volume ratio becomes very large. As a result, surface-related forces such as surface tension and adhesion become much more significant compared with body forces such as gravity.

  3. Q2(a). Define MEMS. Discuss the history of MEMS development and highlight the salient features of MEMS technology.20257m

    Module 1: MEMS Overview

    Define MEMS. Discuss the history of MEMS development and highlight the salient features of MEMS technology.

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    MEMS: Definition, History and Salient Features

    MEMS (Micro-Electro-Mechanical Systems) are miniaturized systems that integrate mechanical structures, sensors, actuators and electronic circuits, generally fabricated using microfabrication techniques on a small substrate such as silicon.

    Brief history

    • 1950s–60s: Development of semiconductor fabrication and silicon technology provided the foundation for miniaturized devices.
    • 1960s: Early silicon pressure sensors and micromachined structures demonstrated practical MEMS concepts.
    • 1970s–80s: Surface and bulk micromachining developed rapidly; accelerometers and other sensors became feasible.
    • 1990s: MEMS accelerometers entered automotive airbag systems, while micromachined optical and fluidic devices expanded applications.
    • 2000s onward: MEMS became widespread in smartphones, medical devices, inertial sensors, microphones, microfluidics and biomedical systems.

    Salient features

    1. Very small size and low mass.
    2. Low power consumption.
    3. Batch fabrication enables high-volume production.
    4. Integration of mechanical and electronic functions.
    5. High sensitivity and fast response.
    6. Low material consumption.
    7. Ability to integrate sensors, actuators and signal processing.
    8. Applications across automotive, aerospace, healthcare, consumer electronics and industrial systems.

    Conclusion: MEMS combines mechanical and electronic engineering at the microscale to create compact, sensitive and multifunctional devices.

  4. Q2(b). Explain the standard Photolithography process flow with the help of neat diagrams.20257m

    Module 1: MEMS Overview

    Explain the standard Photolithography process flow with the help of neat diagrams.

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    Standard Photolithography Process Flow

    Photolithography transfers a geometric pattern from a mask onto a photoresist-coated substrate.

    Steps

    1. Wafer cleaning: Remove particles, organic contamination and moisture from the substrate.
    2. Dehydration/bake: Heat the wafer to remove residual moisture.
    3. Photoresist coating: Spin-coat a uniform layer of photoresist.
    4. Soft bake: Remove solvent and stabilize the resist film.
    5. Mask alignment: Align the photomask with existing wafer features.
    6. UV exposure: Expose selected resist regions through the mask.
    7. Development: Developer removes the soluble portion of the resist. In a positive resist the exposed area is removed; in a negative resist the exposed area remains.
    8. Rinse and dry: Remove developer residues.
    9. Hard bake: Improve adhesion and mechanical stability where required.
    10. Etching/deposition: Transfer the resist pattern into the underlying material or use it as a mask for subsequent fabrication.
    11. Resist stripping: Remove the remaining resist after pattern transfer.

    Simplified flow

    Clean wafer → Coat resist → Soft bake → Align mask → UV exposure → Develop → Rinse → Etch/deposit → Strip resist

    Photolithography provides precise pattern definition and is fundamental to MEMS and microelectronic fabrication.

  5. Q3(a). Compare and contrast Wet Etching and Dry Etching. Explain the significance of Anisotropic Etching in MEMS fabrication.20257m

    Module 1: MEMS Overview

    Compare and contrast Wet Etching and Dry Etching. Explain the significance of Anisotropic Etching in MEMS fabrication.

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    Wet Etching vs Dry Etching and Importance of Anisotropy

    Parameter Wet Etching Dry Etching
    Medium Liquid chemical etchant Plasma/reactive gas or ions
    Nature Often isotropic; some crystal-dependent etchants are anisotropic Can be controlled for anisotropic profiles
    Equipment Relatively simple More complex vacuum/plasma equipment
    Selectivity Often high Can be engineered through plasma chemistry
    Feature control Lateral undercut may occur Better control of vertical sidewalls is possible
    Waste/safety Chemical waste and handling issues Gas/plasma safety requirements

    Anisotropic etching

    Anisotropic etching removes material preferentially in one direction, producing steep or well-defined sidewalls rather than equal lateral and vertical removal.

    It is important in MEMS because it enables:

    • high-aspect-ratio structures,
    • accurate microchannels and cavities,
    • vertical sidewalls,
    • precise dimensions and better device performance.

    Example: DRIE can produce deep silicon structures with nearly vertical sidewalls.

Next moduleModule 2: Fundamentals of Microfluidic Systems