Download Lecture 9

EEL5225: Principles of MEMS
Transducers (Fall 2003)
Fabrication Technology, Part II
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Agenda:
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Process Examples
– TI Micromirror fabrication process
– SCREAM
– CMOS-MEMS processes
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Wafer Bonding
LIGA
Reading: Senturia, pp. 79-98.
EEL5225: Principles of MEMS Transducers (Fall 2003)
Lecture 9 by
H.K. Xie
9/15/2003
Process Examples
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Surface micromachining: TI Digital Micromirror Devices (DMDs)
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Ref. Maluf, Introduction to MEMS Engineering, p.145.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
Invented by L.J. Hornbeck
Process Examples
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Bulk-Micromachined
Pressure Sensor
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Thermal oxide
Boron implantation
Boron drive-in
LPCVD Si3N4
Backside KOH etch
– Electrochemical etch stop
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Metallization
Detailed process steps refer to
Senturia p.93
Ref. Senturia, Microsystem Design, p.97
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EEL5225: Principles of MEMS Transducers (Fall 2003)
Process Examples
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Single Crystal
Reactive Etching and
Metallization
(SCREAM) Process
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First demonstrated by
MacDonald’s group at
Cornell University
Single crystal silicon (SCS)
microstructures
Large force
Large displacement
Post-CMOS process for
electronics integration
Ref. Maluf, Introduction to MEMS Engineering, p.82.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
CMOS-MEMS
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Why CMOS-MEMS?
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MEMS structures can be made
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“Smart” on-chip CMOS circuitry
Multi-vendor accessibility
Scalability
Compact size
More functions
Low cost
Before CMOS processes (“pre-CMOS”)
In-between CMOS processes (“intermediate-CMOS”)
After CMOS processes (“post-CMOS”)
EEL5225: Principles of MEMS Transducers (Fall 2003)
Pre-CMOS MEMS Process
www.sandia.gov
ƒ Pre-etched trench to house MEMS structures
ƒ CMP to planarize the wafer for regular CMOS processing
ƒ Wet etch to release MEMS structures
ƒ Need a dedicated production line
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EEL5225: Principles of MEMS Transducers (Fall 2003)
Intermediate-CMOS MEMS
ƒ Form transistors on bare wafers first
ƒ Then deposit and anneal MEMS
structural materials
ƒ No CMP needed
ƒ Only one interconnect metal layer
ƒ Wet etch to release MEMS structures
ƒ Need a dedicated production line
NPN
NMOS
Sensor Area
Sensor Poly
Met
Passivations
BPSG
Thox
Nwell
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Emitter
Base
NSD
EEL5225: Principles of MEMS Transducers (Fall 2003)
Courtesy of Mr. John Geen
of Analog Devices, Inc.
Thin-Film Post CMOS-MEMS
(a) After standard
CMOS processes
CMOS
region
microstructural
region
metallization layers
dielectric layers
silicon substrate
gate polysilicon
(b) Pattern
microstructure
• Metal as etching
mask
• Anisotropic etch
• CHF3 + O2
metal-3
metal-2
metal-1
(c) Release
microstructure
• Metal as etching
mask
• Undercut Si
substrate
• Isotropic etch
• SF6 or XeF2
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Spring beams
movable
microstructure
anchored
stator
Sensing comb
fingers
Proof mass
G. Fedder et al., Sensors & Actuators A,
v.57, no.2, 1996
EEL5225: Principles of MEMS Transducers (Fall 2003)
H. Xie et al., Thin-film z-axis accelerometer
DRIE CMOS-MEMS Process
(a) Backside etch
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.
CMOS-region
Single-crystal Si
(SCS) membrane
metal-3
metal-2
metal-1
oxide
poly-Si
(b) Oxide etch
PlasmaTherm-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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EEL5225: Principles of MEMS Transducers (Fall 2003)
Flat structure
Thin-film
structure
(d) Si undercut
SCS layer
(20~100µm)
STS: 130-sccm SF6,
13-sccm O2, 23 mT,
600 W coil power,
and 0 platen power.
bimorph
actuator
mirror
Spring beams
Sensing comb
fingers
Proof mass
Xie et al, 1-D Scanning Micromirror
Xie et al, DRIE z-axis accelerometer
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EEL5225: Principles of MEMS Transducers (Fall 2003)
Wafer Bonding
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Wafer bonding
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Addresses need to obtain greater vertical dimensions
and vacuum packaging, and to seal channels
Methods
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Epoxy 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 10001100°C)
Ref. Kovacs, Micromachined Transducers Sourcebook, p. 139.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
Wafer Bonding
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Anodic bonding
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Moderate temperature 450-500°C
Sodium-rich glass plate (7740 Pyrex)
500-1000V at 500°C to diffuse ions
and to form electrostatic bond
Bond chamber
Ref. Kovacs, Micromachined Transducers
Sourcebook, p. 120.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
EVG501 Wafer Bonding System
LIGA
LIGA
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LIGA: German acronym for X-ray
Lithographie, electrodeposition
(Galvanoformung), and molding
(Abformung)
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Issues
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High energy radiation source
– Alternatives
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Assembly
Enables fabrication of microsize
metal parts
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Ref. Maluf, Introduction to MEMS Engineering, p.76.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
Thick UV sensitive resist (‘poor
man’s LIGA’)
Project Issues
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Teams
Schedule
EEL5225: Principles of MEMS Transducers (Fall 2003)