CARNEGIE MELLON / MEMS DESIGN / 2025

Theoretical Design and Fabrication of a Cavity-Based MEMS Faraday Cup

I designed a two-wafer Faraday-cup concept with a 22 µm aperture, balancing charge retention, signal collection, and fabrication constraints.

My role Device geometry, fabrication-process design, and structural analysis · four-person team.

MEMS design
Microfabrication
Structural FEA
Analytical modeling

CAD concept of a nine-cup MEMS array with small apertures and contact pads. — Array concept · theoretical design and proposed fabrication

22 µmAperture diameter · final sizing study

350 µmCavity depth · final sizing study

2 wafersProposed bonded structure

01 / ENGINEERING PROBLEM

Balance charge collection and fabrication

The goal was a microscale Faraday cup for measuring ion-beam charge. A covered cavity limits secondary-particle escape, but a smaller opening also reduces incoming signal and complicates fabrication. I worked on the geometry, process sequence, and aperture-plate strength to make that tradeoff practical.

02 / METHODS & ANALYSIS

Geometry, process, and structure

Design schematics and simulation figures
Click images to enlarge

Final sizing schematic: 22 µm aperture, 44 µm cavity diameter, and 350 µm depth; not to scale. — Smaller apertures suppress escaped charge but also reduce incoming signal. I used the report’s beam assumptions and a 1 fA readout noise floor to size the aperture, then selected a cavity diameter that relaxed the internal coating aspect ratio to approximately 8:1.

Develop a two-wafer fabrication process

Proposed cavity wafer after deep reactive ion etching and growth of insulating oxide. — 01 Etch the cavity and grow oxide

Copper liner and routing trace after conformal deposition and polishing. — 02 Deposit copper and polish the surface

Aperture wafer bonded above the copper-lined cavity. — 03 Bond the aperture wafer at 200–250 °C

Finished process concept with an aperture, suppressor electrodes, and a top-side contact via. — 04 Open the aperture and form contacts

I proposed separating the cavity and aperture wafers so the deep cavity could be etched and metallized before sealing its narrow opening. Low-temperature oxide bonding limits copper diffusion; subsequent patterning forms suppressor electrodes and accessible contacts.

Preliminary aperture-plate FEA showing a peak von Mises stress of 3.975e4 Pa under the assumed photoresist loading. — I compared plate-bending calculations with FEA to screen the aperture plate under assumed photoresist loading. This preliminary case used an earlier geometry; residual stress, bonding, and pressure loads still need evaluation for the final design.

03 / RESULTS & OUTCOME

A defined design and fabrication route

ANALYTICAL + SIMULATED · NOT FABRICATED

≈99.9%Idealized geometric collection estimate
22 µm aperture · 350 µm depth

The geometry study and two-wafer process define a candidate design, not measured collection performance. Next steps are bonding and stress checks on test structures, followed by leakage, capacitance, and charge-collection measurements on fabricated devices.

Team electrical-model cross-check

Cup-to-ground capacitance

792 fFAnalytical model

816.4 fFCOMSOL

The team’s electrical model differed by approximately 3.1% from its analytical estimate. This cross-check used a separate 63.5 µm-diameter, 500 µm-deep cavity with 5 µm oxide.