EEL5225: Principles of MEMS Transducers (Fall 2003)1

EEL5225: Principles of MEMS Transducers (Fall 2003)

Fabrication Technology, Part II

Agenda:Process Examples

– TI Micromirror fabrication process– SCREAM– CMOS-MEMS processes

Wafer BondingLIGA

Reading: Senturia, pp. 79-98.

Lecture 9 by H.K. Xie 9/15/2003

EEL5225: Principles of MEMS Transducers (Fall 2003)2

Process ExamplesSurface micromachining: TI Digital Micromirror Devices (DMDs)

Invented by L.J. Hornbeck

Ref. Maluf, Introduction to MEMS Engineering, p.145.

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Process Examples

Bulk-MicromachinedPressure Sensor

Thermal oxideBoron implantationBoron drive-inLPCVD Si3N4

Backside KOH etch– Electrochemical etch stop

Metallization

Detailed process steps refer to Senturia p.93

Ref. Senturia, Microsystem Design, p.97

EEL5225: Principles of MEMS Transducers (Fall 2003)4

Process Examples

Single Crystal Reactive Etching and Metallization (SCREAM) Process

First demonstrated by MacDonald’s group at Cornell UniversitySingle crystal silicon (SCS) microstructuresLarge forceLarge displacementPost-CMOS process for electronics integration

Ref. Maluf, Introduction to MEMS Engineering, p.82.

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CMOS-MEMS

Why CMOS-MEMS?“Smart” on-chip CMOS circuitryMulti-vendor accessibilityScalabilityCompact sizeMore functions Low cost

MEMS structures can be madeBefore CMOS processes (“pre-CMOS”)In-between CMOS processes (“intermediate-CMOS”)After CMOS processes (“post-CMOS”)

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Pre-CMOS MEMS Process

www.sandia.gov

Pre-etched trench to house MEMS structuresCMP to planarize the wafer for regular CMOS processingWet etch to release MEMS structuresNeed a dedicated production line

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Intermediate-CMOS MEMS

Form transistors on bare wafers firstThen deposit and anneal MEMS structural materialsNo CMP neededOnly one interconnect metal layerWet etch to release MEMS structuresNeed a dedicated production line

Thox

Nwell

BPSG

Sensor Poly

MetPassivations

NPN NMOS Sensor Area

EmitterBase NSD Courtesy of Mr. John Geen

of Analog Devices, Inc.

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Thin-Film Post CMOS-MEMS

(a) After standard CMOS processes

CMOS

metallization layers

movable anchored

silicon substratedielectric layers

metal-3

metal-2

metal-1

gate polysilicon

statormicrostructure

regionmicrostructural

region

Proof mass

Spring beams

Sensing comb fingers

(b) Pattern microstructure• Metal as etching mask• Anisotropic etch • CHF3 + O2

(c) Release microstructure• Metal as etching mask• Undercut Si substrate• Isotropic etch• SF6 or XeF2 G. Fedder et al., Sensors & Actuators A,

v.57, no.2, 1996H. Xie et al., Thin-film z-axis accelerometer

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DRIE CMOS-MEMS Process

CMOS-region(a) Backside etch

Single-crystal Si (SCS) membrane

metal-2metal-3

STS: 12-sec etching, 130-sccm SF6, 13-sccm O2, 23 mT, 600 W coil power, 12 W platen power; 8-sec passivation 85-sccm C4F8, 12 mT, 600 W coil power, 0 platen power.

metal-1

oxide

poly-Si

(b) Oxide etchPlasmaTherm-790: 22.5-sccm CHF3, 16-sccm O2, 100 W, 125 mT for 125 minutes and then 100 mT for 10 minutes.

CMOS layer(c) Deep Si etch

STS: same as Step (a).

H. Xie et al, Journal of Microelectromechanical Systems, April 2002

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Flat structure

(d) Si undercutSCS layer (20~100µm)

Thin-film structure

STS: 130-sccm SF6, 13-sccm O2, 23 mT, 600 W coil power, and 0 platen power.

Proof mass

Spring beams

Sensing comb fingers

bimorph actuator

mirror

Xie et al, 1-D Scanning MicromirrorXie et al, DRIE z-axis accelerometer

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Wafer Bonding

Wafer bondingAddresses need to obtain greater vertical dimensions and vacuum packaging, and to seal channels

MethodsEpoxy bonding (low temperature 100°C)Metal eutectic bonding (low-moderate temperature 100-400°C)Glass frit bonding (low-moderate temperature 450°C)Anodic bonding (moderate temperature 450-500°C)Silicon fusion bonding (high temperature 1000-1100°C)

Ref. Kovacs, Micromachined Transducers Sourcebook, p. 139.

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Wafer Bonding

Anodic bondingModerate temperature 450-500°CSodium-rich glass plate (7740 Pyrex)500-1000V at 500°C to diffuse ions and to form electrostatic bond

Bond chamber

EVG501 Wafer Bonding System Ref. Kovacs, Micromachined Transducers Sourcebook, p. 120.

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LIGALIGA

IssuesHigh energy radiation source

– AlternativesThick UV sensitive resist (‘poor man’s LIGA’)

AssemblyEnables fabrication of microsizemetal partsRef. Maluf, Introduction to MEMS Engineering, p.76.

LIGA: German acronym for X-ray Lithographie, electrodeposition(Galvanoformung), and molding (Abformung)

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