STEP 6 • ENCAPSULATION

Sealing it in

Encapsulation and overmolding protect a device from corrosion and mechanical damage. The challenge is bimetallic action — thermal stress that builds up when bonded materials have different coefficients of thermal expansion.

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BUILDING A SHELL ON-CHIP

An LPCVD encapsulation process

1. Base process Standard surface micromachining

2. Thick PSG Defines the encapsulation region

3. Thin PSG Defines the etch channels

4. Nitride shell Deposited, with etch holes defined

5. Release & seal Remove sacrificial PSG, supercritical CO2 dry, then global LPCVD seal

This builds a protective shell directly on the wafer — using the exact same deposition and release toolbox from earlier units.

STEP 7 • TESTING

Proving it works

Pressure sensors

A custom setup controlling voltage, temperature, and pressure

Accelerometers

A shaker table with an AC stimulus — expensive, and control samples are required

Microfluidics

Usually tested at the component level (valves, pumps) — full-system testing is too costly

Every device family needs its own custom way of proving the package didn't break what the fabrication built.

UNIT 32 STUDY COMPLETE

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