JNTU B.Tech IV MEMS DESIGN Previous Question Paper (July - 2021 - R16)

The MEMS industry faces several critical challenges that hinder rapid commercialization and mass production:

• Packaging and Integration: Unlike standard ICs, MEMS devices require interaction with the environment, making packaging complex. Protecting the mechanical structure while allowing sensing access is difficult and often accounts for a large portion of total costs.
• Reliability and Testing: MEMS devices involve moving parts subject to wear, stiction, and fatigue. Developing standardized testing procedures for these mechanical components is challenging compared to purely electrical testing.
• Design Complexity: MEMS design requires multi-physics modeling (coupling mechanical, electrical, thermal, and fluidic domains), which lacks the mature CAD tool support found in traditional CMOS design.
• Manufacturing Variability: Small variations in fabrication processes (e.g., etching rates, thin-film stress) significantly impact the performance of mechanical structures, leading to lower yields compared to standard microelectronics.
• Cost of Prototyping: High initial investment in cleanroom facilities and the lack of standardized fabrication processes make small-batch prototyping expensive.

Surface micromachining builds mechanical structures layer-by-layer on top of a substrate using sacrificial layers. The process typically follows these steps:

1. Substrate Preparation: A silicon wafer is cleaned and typically coated with an insulating layer (e.g., silicon dioxide or silicon nitride) to isolate the device from the substrate.
2. Sacrificial Layer Deposition: A sacrificial material, such as phosphosilicate glass (PSG), is deposited via Chemical Vapor Deposition (CVD). This layer will eventually be removed to create air gaps.
3. Patterning: The sacrificial layer is patterned using photolithography and etching to define the anchor points where the structure will attach to the substrate.
4. Structural Layer Deposition: The structural material, typically polysilicon, is deposited over the sacrificial layer and into the anchor holes.
5. Structural Patterning: The structural layer is patterned using photolithography and reactive ion etching (RIE) to define the mechanical components (e.g., beams, gears).
6. Sacrificial Etch (Release): The wafer is immersed in a chemical etchant (e.g., hydrofluoric acid) that selectively removes the sacrificial layer without damaging the structural layer. This leaves the mechanical structure suspended or free to move.
7. Drying: The device is carefully dried to prevent stiction, where surface tension forces cause the released structure to stick permanently to the substrate.