Project 13

Optimization and Fabrication of MEMS Bistable Oxide-Polyimide Diaphragms Using DRIE

(Dr. Dilan Ratnayake, KY Multiscale)

This undergraduate research project is part of a larger initiative focused on developing no-electrical-power (NEP) sensing devices based on bistable MEMS structures. The student will focus on optimizing the deep reactive ion etching (DRIE) process for releasing oxide-polyimide diaphragm structures with high precision and reproducibility. Achieving vertical, smooth sidewalls is critical to ensure the patterned diaphragm diameter on the back side of the wafer precisely matches the design specifications defined through lithography. After optimization, the student will apply the developed process to fabricate functional oxide-polyimide bistable diaphragms.

Project Objectives:

1. Characterize and optimize a DRIE process recipe for etching through silicon to create vertical, smooth, and dimensionally accurate openings that align with lithographic patterns.

2. Apply the optimized DRIE process to fabricate oxide-polyimide bistable diaphragms for use in NEP sensing devices.

Research Tasks:

Task 1: DRIE Process Characterization

• Familiarize with the DRIE system, including key process parameters such as etch/passivation cycle time, gas flow rates, RF power, and chamber pressure.

• Perform initial etching trials on silicon wafers with lithographically patterned features.

• Characterize etched profiles using Scanning Electron Microscopy (SEM) and surface profilometry to assess etch depth, sidewall angle, and uniformity.

• Evaluate etch quality, focusing on sidewall undercutting, and selectivity to the masking material.

Task 2: DRIE Recipe Optimization

• Systematically, DRIE process parameters vary to achieve smooth, vertical sidewalls with high pattern fidelity.

• Use SEM and profilometry measurements after each iteration to guide and document improvements.

• Finalize an optimized recipe for high-aspect-ratio silicon etching that meets diaphragm fabrication requirements.

Task 3: Diaphragm Fabrication Process

• Perform backside lithography on oxide-polyimide coated silicon wafers to define diaphragm geometries.

• Apply the optimized DRIE recipe to etch through the silicon substrate.

• Execute release steps to free the diaphragm structures and clean wafers for post-fabrication inspection.

Task 4: Final Characterization

• Use SEM and profilometer to verify diaphragm dimensions, etch precision, symmetry, and successful release.

• If time permits, conduct basic mechanical testing to observe bistable buckling behavior and assess diaphragm performance.

Extra Notes:

Figure 01 shows the overall fabrication process that will be used to fabricate oxide-polyimide diaphragms, with the student focusing primarily on characterizing Section E as described in the tasks above.

Figure 02 presents an optical image of a DRIE-released polyimide/oxide buckled diaphragm, along with Dektak and Zygo profiles showing its reversible 'buckled-up' and 'buckled-down' states.