8.3. Key Processes in MEMS Fabrication

8.3.1 Photolithography

Used to define patterns on wafers through exposure to light.

• Steps:
  1. Apply photoresist
  2. Align and expose through a mask
  3. Develop the exposed resist
  4. Transfer pattern to the material beneath
• Types:
  • Positive photoresist: Exposed areas dissolve
  • Negative photoresist: Unexposed areas dissolve
• Importance: Enables high-resolution patterning for microfeatures.

Photolithography is a key technique in MEMS fabrication, primarily used to create intricate patterns on semiconductor wafers. The process starts by applying a light-sensitive material called photoresist onto the wafer surface. Then, a mask— which contains the desired pattern— is aligned over the wafer. When exposed to ultraviolet light, the photoresist undergoes a chemical change. After exposure, the next step is to develop the photoresist, which removes either the exposed or unexposed areas depending on whether a positive or negative photoresist is used. This technique allows for the precise transfer of patterns onto the underlying material, essential for building microfeatures in MEMS devices.

Think of photolithography like creating a stencil for spray painting. You place a stencil on a surface and spray paint over it. When you remove the stencil, a clear pattern of the desired shape remains. In MEMS fabrication, the photoresist serves as that stencil, allowing engineers to create tiny but complex designs on a semiconductor surface.

8.3.2 Deposition Techniques

Used to add thin material layers on the wafer surface.

• a) Physical Vapor Deposition (PVD)
  • Evaporation or sputtering to deposit metals
  • Suitable for conductive layers and interconnects
• b) Chemical Vapor Deposition (CVD)
  • Uses chemical reactions to deposit materials like polysilicon, SiO₂, or Si₃N₄
  • Low-Pressure CVD (LPCVD) and Plasma-Enhanced CVD (PECVD) are common
• c) Spin Coating
  • Used for uniform photoresist or polymer films (e.g., SU-8)
• d) Electroplating
  • Deposits metals into patterned molds
  • Common in LIGA processes.

Deposition techniques are crucial in MEMS fabrication for adding various thin layers of materials onto the wafer. Physical Vapor Deposition (PVD) is one method that achieves this by evaporating or sputtering metals onto the wafer, forming conductive layers necessary for electrical connections. Chemical Vapor Deposition (CVD) is another method that relies on chemical reactions to deposit materials, allowing for controlled growth of thin films like polysilicon or silicon dioxide. Spin coating is employed to create uniform layers of photoresist or polymers by spinning the wafer at high speeds. Lastly, electroplating is a specialized method that involves plating metals into patterns, often used in high-precision applications like LIGA processes.

Imagine building a cake with multiple layers. Each layer must be carefully added and can be of different materials like frosting, fruit, or sponge. In the same way, MEMS fabrication uses various deposition techniques to layer materials on the wafer, each serving a specific function, much like adding different flavors to a cake to achieve a delicious outcome.

8.3.3 Etching Techniques

Used to selectively remove material and define 3D structures.

• a) Wet Etching
  • Uses chemical solutions (e.g., KOH, HF)
  • Can be isotropic or anisotropic based on crystal orientation
• b) Dry Etching
  • Uses plasma or reactive gases (e.g., RIE, DRIE)
  • Offers high aspect ratio and vertical sidewalls
• c) Sacrificial Layer Etching
  • Removes underlying layer to release movable structures in surface micromachining.

Etching techniques are employed in MEMS fabrication to remove specific material layers and create three-dimensional structures. Wet etching involves applying chemical solutions that dissolve specific materials; this can affect the etching precision based on whether the etching is isotropic (uniform in all directions) or anisotropic (directionally controlled). Dry etching, on the other hand, involves the use of plasma or reactive gases to achieve more precise etching with features like high aspect ratios and vertical sidewalls. Sacrificial layer etching is particularly important in applications where moving parts are needed. This process removes a layer that acts as a spacer, thus allowing the fabrication of movable components in the final MEMS device.

Consider sculpting a figure out of a block of marble. By chiseling away certain areas of the block, the sculptor reveals the desired shape. In MEMS, etching is like this chiseling process—removing excess material to reveal the functional components of the device, shaping them into precise microscale structures.

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