Anodic Bonding for MEMS

1
Anodic Bonding for MEMS

• by
• Henrik Jakobsen, SensoNor ASA and Vestfold
  University College
• Adriana Lapadatu, SensoNor ASA
• Gjermund Kittilsland, SensoNor ASA

2
Early work

• Patents and publications
• 1968 Anodic bonding Pomerantz, Mallory Co
  (US 3,397,278)
• 1968 Bonding electrically conductive metals to
  insulators Pomerantz et al., Mallory Co (US
  3,417,459)
• 1971 Bonding an Insulator to an Insulator
  Pomerantz et al., Mallory Co (CA 894986)
• 1972 Low-temperature Electrostatic
  Silicon-to-Silicon Seals using Sputtered
  Borosilicate Glass Brooks and Donovan, Research
  Triangle Institute and Hardesty, NASA
• 1977 Application of field-assisted bonding to
  the mass production of silicon type pressure
  transducer G. Wallis, P. R. Mallory Co (US
  4,121,334) (first silicon diaphragm on glass)
• 1979 Semiconductor pressure sensor having
  plural pressure sensitive diaphragms and method,
  Suzuki et al., Hitachi (US 4,322,980) (first
  etched silicon diaphragm sensor on glass)
• Previous review work
• E. Obermeier Anodic Wafer Bonding (1995)

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The mechanism of the anodic bonding
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Bonding current and the Sodium depleted layer
- Thermal activation energy for the sodium ions
0,97 /- 0,14 eV
Bonding at 1000 V
Bond temp 400 OC
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The bonding pressure for bulk bonding
Bonding pressure 20000 atm with bonding on bare
silicon at 800V
Bonding at 1000 V and 400 degC
Bonding at 400 degC
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Requirements for successful wafer bonding

• High surface quality (roughness, bow, warp)
• High surface cleanliness (particles, film
  contamination)
• Minimum temperature to mobilize ions
• Temperature uniformity
• Minimum voltage
• Voltage uniformity
• Bond front control to avoid trapping of gas
  (electrodes)

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Anodic bonding glass

• Key requirements
• Good matching of TCE to silicon
• Mobile ions at bonding temperature (Li, Na, K
  ....) below 450 degC
• High quality polished surface
• Availability/low cost
• Main glasses (alkali-borosilicate glasses)
• Corning Pyrex 7740
• Schott Borofloat
• Hoya SD-2
• Other glasses and ceramics
• Corning 7070 (for bonding on thick oxide, nitride
  etc...)
• Hoya PS100 (experimental material for low
  temperature, high etch rate...)
• Ceramics (Example AREMCOLOX 502-1100)

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Building of structures on bonding glass

• Holes and cavities
• Ultrasonic drilling
• Powder blasting
• Laser ablation
• Wet etch
• Electrochemical etching
• Dry etch

• Metal interconnections
• Surface interconnects and electrodes
• Via metallization (difficult)

Shallow cavities
Deep cavities
Via holes
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Bond quality, failure modes and characterization

• Bond quality
• Mechanical strength
• 10 - 200 MPa (typ. 20 - 50)
• depend on material, area, test method etc
• glass and silicon usually breaks before bond
  interface
• Residual gas (pressure)
• 0,1 - 50 kPa
• O2 released from the process
• dependent on process, cavity geometry etc
• Hermetic seal (leakage)
• Limited by gas diffusion through the materials
  (glass)

• Failure modes
• Voids due to particles
• Voids due to trapped gas
• Non-bonded areas due to wafer bow and warp
• Alignment errors
• Characterization methods
• Monitoring process parameters (Temp, Pressure,
  Voltage, Current)
• Visual inspection
• Pull/shear test (destructive)
• Bow measurements
• Q-factor of resonators for test of residual gas
  pressure
• Accelerated testing of leakage by bombing in
  He/H2/air at elevated temp etc...

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Typical defects seen by visual inspection
Larger void caused by particle (leaky)
Small void around thin residual oxide (no leak)
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Bonding of wafers with flexible structures
Non-bonding overload protection

• Electrostatic collapse
• Bonding for structure

Non-bonding overload protection by use of shield
electrodes
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Anodic bonding and sticking
Design for no touching during bonding if
possible!

• Type of effects Solution
• During anodic bonding
• Anodic bonding Silicon and glass must not touch
• Micro-welding and other Avoid materials that can
  react at inter-metallic reactions bonding
  temperature
• Compression bonding Avoid use of soft materials
• Contamination Cleaning procedures
• In use
• Electrostatic force Avoid insulators at touch
  points, (charges in/on insulators ) design
  of small touch areas
• Molecular bindings (H, OH -...) Surface
  treatment / small area
• Clamping Design

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Electrical Effects from Bonding
The Na contamination problem by purpose

Bonding on Silicon over pn-junctions
Bonding on Silicon near surface pn-junctions
These effects must be taken into account for when
bonding to electrical active surfaces
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Electrical feed-through methods

• Thin metallic crossings
• low resistivity
• difficult to assure hermetic seal
• oxide in the bonding area
• hazard of defective lines after
• bonding
• Diffused surface conductors
• hermeticity preserved
• bonding-related electrical effects
• high resistivity
• polarity limitations (pn-junction)

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Electrical feed-through methods (cont.)

• Diffused conductors/thin film
• hermeticity preserved
• flexibility
• bonding-related electrical effects
• high resistivity
• polarity limitation (pn-junction)
• Diffused buried conductors
• hermeticity preserved
• robustness against charging/ions
• high resistivity
• polarity limitations (pn-junction)

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Anodic bonding and process integration
Micro-packaging on the wafer level allow
low-cost 2-chip solutions
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Standard process for piezoresistive devices
Before bonding
After bonding
Final device with pressure sensor and
accelerometer on the same chip
Final device after sawing of top glass
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Key features in the design of the process
Anodic bonding against silicon
Buried resistors
High doping concentration in epitaxial layer
Buried conductors
Etch-stop against pn-junction
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Typical surface roughness in the bonding areas
340 micron
Measurement area
Typical values RMS 2nm PP 10nm
4nm
- 4nm
900 micron
Result from optical measurement method (Veeco
WYKO)
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Typical results from die share testing
Die
Typical breakage Top glass Bottom
glass Glass 22 80
Glass/Silicon 42 20 Silicon
40 0

• Top glass
• average 40E6 N/m2

• Bottom glass
• average 35E6 N/m2

Die size 1.85 x 1.85 mm - Top seal 0.11 mm2 -
Bottom seal 0.21 mm2

• Acceptance criteria 1
• gt 10E6 N/m2
• Acceptance criteria 2
• gt 20E6 N/m2
• (if breakage in seal)

min 2
min 1
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Alternative designs for piezoresistive pressure
sensors
Relative type (bottom inlets)
Absolute type (bottom inlet)
Relative type (inlets from both sides)
Relative type (top inlets)
Absolute type (top inlet)
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Summary

• Anodic bonding of bulk glass to silicon is a
  mature technology already used in a large variety
  of industrial products
• Anodic bonding for MEMS, including triple-stack
  bonding is available from industrial foundries as
  well as university type labs
• SensoNor is the first company to develop anodic
  bonding on electrically active surface

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Some future directions
Some areas where more basic work should be done

• Basics of the bonding mechanisms
• Thin-film bonding
• Reduction and control of residual gas pressure in
  cavities (below 0,1 kPa)
• Characterization of sticking forces and methods
  for reduction of sticking
• Flexible methods for electrical interconnects for
  sealed cavities
• Robust designs to limit ionic contamination/charge
  effects when bonding on/near electrical active
  devices