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3 MEMS FABRICATION

•Ken Gilleo PhD•ET-Trends LLC

24%

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MEMS Material Classes

MEMS Material Classes

Silicon

Aluminum

Si3N4 (Nitride)

GaAs

SiC

Polymers

SiO2

Copper

Aluminum

Polymers

SiGe

Silicon

Structures Sacrificial

Etchant or process will determine if material is structural or sacrificial

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Movable StructuresMovable

Structures Pivots Bearings Hinges Beams (bend/twist) Gears Rack Wheels Latch Switches Valves Diaphragms Turbines Springs

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Stationary StructuresStationary Structures Base/platform

Reference mass

Chambers

Channels

Back plate (microphone)

Fittings/ports

Needles/blades

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MEMS ManufacturingMEMS Manufacturing Use SEMICONDUCTOR processes

Silicon Machining; (1) surface, and (2) bulk:

• Define mechanical parts by lithography

• Form sacrificial SiO2 (or other) in “removal” areas

• Etch away SiO2 to free mechanical parts

Many other micro-machining processes and

variations are available

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Sandia SUMMiT Process

Sandia SUMMiT Process

Substrate

oxidenitride

sacrificial oxide 1Poly 1

Poly 2sacrificial oxide 3

Poly 3sacrificial oxide 4

Poly 4

Poly 0

Sacrificial oxide 2Sandia

Sandia

(Surface Machining Process)

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MEMS Bulk FabricationMEMS Bulk Fabrication

Start with single crystal silicon

Apply etch resist (mask)

Can selectively implant stop etch into Si

Etch unwanted silicon

• Wet/chemical (anisotropic; etches alone planes)

• Dry plasma (high resolution)

Simpler shapes, lowest cost (?)

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Wet or Dry Bulk Process

Wet or Dry Bulk Process

Wet Etch

Mask: SiO2, Si3N4

Etchant: KOH, org. base

Etch Retarder: Boron (B)

Dry Etch

Deep Reactive Ion Etching (DRIE)

SF6/O2 Gas

30:1 Aspect Ratio

SiSi

membrane

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Bosch Processbest dry etch methodBosch Process

best dry etch method

1. Plasma etch

2. Deposit resist

3. Plasma etch

4. Repeat 2, 3.Very high

aspect ratios

Also being adopted for TSV (vias)

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LIGA ProcessLIGA Process LIGA mask design

Mask fabrication

Substrate preparation

X-ray exposure

X-ray resist development

Electroplate

Panelization

Replication

Lithographie Galvanoformung Abformung

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MEMS ReleaseMEMS Release

Remove sacrificial layer

Oxide 2

Si Etc

h &

Rel

ease

More fragile

Final stepbefore packaging

A critical very step

Remove holding structure• Wet Chem Etch; dissolve silica ; SiO2

• Dry Plasma Etch

Packaging foundry may run this process

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Wafer BondingWafer Bonding Fabricate MEMS devices• Pumps, pressure sensors• Values, controllers• Chambers; analyzers, reactors

Integrated MEMS• Electronics; logic, memory, sensors• Optics, photonics

Pre-packaging; capping wafers

Complete Wafer-Level Package (WLP)

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Wafer Bonding Methods

Wafer Bonding Methods

PROCESS Temp Hermeticity Reliability Availability Cost

Metallurgical mod-high

high excel. high mod.

Fusion high high excel. mod mod.

Anodic mod high excel. mod mod.

Inorg. adhes. mod good-high excel. high mod.

Brazing high high excel. high mod.Laser-assisted localized ~ ~ limited ~Polymer, thermoset

low Non-hermetic good high low

Polymer, thermoplastic

mod. Near-hermetic good limited low

Polymer, UV low Non-hermetic mod limited low

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Summary of FabricationSummary of Fabrication

MEMS can use existing semiconductor mfg

Major boost with existing infrastructure

Parallel process; productivity

Older foundries 6” moving to 8” (don’t need 12”)

Combine CMOS logic & MEMS

MEMS can build almost any macrostructure

Cavities, electronics may require wafer bonding