14' Packaging

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14. Packaging
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Introduction

• Three major steps for electronic and mechanical
  micromachine fabrication
• 1. Fabrication with additive and subtractive
  processes
• 2. Packaging such as bonding, wafer scribing,
  lead attachment and encapsulation
• 3. Testing (leak test, electrical integrity
  and sensor functionality)

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Packaging in IC vs. in Mechanical Micromachines

• IC
• Packing provides 4 functions
• 1. Signal redistribution
• 2. Mechanical support
• 3. Power distribution
• 4. Thermal management

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Packaging in MEMS

• Protecting from the environment and interaction
  with environment.
• Packaging problem is the least severe for a
  physical sensor and the most severe
• for chemical and biological sensor.
• Hybrid MEMS in which MEMS and electronics are
  fabricated in separate processes
• and put together afterward, is recommended.

Dicing

• Final step in fabrication of a 3D
  microstructure, and the first in packaging.
• Cutting processes involve surfactants,
  cleanliness, and blade/depth ratio applied to
• MEMS wafers
• If dices are less than 0.5mm on a side, edge
  definition such as anisotropically
• etched V-grooves and separation of individual
  devices are recommended.
• Most mechanical sensors and actuators are
  equipped with a bonded cap or cover
• protecting them during dicing, followed by a
  final sacrificial release.

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Cavity Sealing and Bonding
Cavity sealing
Sealing of Polysilicon and Silicon Nitride
Cavities
Reactive sealing
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• Sealant films, such as oxides and nitrides, can
  be deposited over small etchant holes.

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• Packaging shells takes long time due to the long
  
• etch process through the etch holes at the
  perimeter
• of the shell.
• Permeable polysilicon windows are used to
• remove the underlying sacrificial PSG using
• concentrated HF in 120s.

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Epitaxial Cavity Sealing

• Cavity is formed by selective etching of p
  epitaxial Si over more heavily doped Si p
  layers.

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HEXSIL Cavity Sealing

• Reactive sealing requires 1000oC and thick SiN
  sealing requires 850oC.
• Some sealant gas deposits on the encapsulated
  micro devises.
• Tethered cap structures are sealed down to the
  substrate employing a low-temperatures
• Au-Si eutectic bond at 363oC by HESIL process.

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Bonding
Field-Assisted Thermal Bonding (anodic bonding or
electrostatic bonding)

• Glass to Silicon (L1 wafer-scale die bonding)
• Sodium-rich glass and metal make bond.
• Coring 7070, soda lime 0080, and potash soda
  lead 0120, and aluminosilicate
• 1720 are suitable besides pyrex.
• Si-Pyrex bonding occures between 180 and 500oC.
• Depending on the thickness of the glass and the
  temperature, voltages between
• 200 and 1000V.

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• The operating temperatures are near the glass-
• softening point but well below its melting
  point, as
• well as below the sintering temperature of
  standard
• AlSi metallization.
• Advantage of a low-temperature process with a
• lower residual stress and less stringent
  requirements
• for the surface quality of the wafers.

• Surface roughness Ra lt 1 ?
• Dust free
• Native or thermal oxide layer on Si must
• be thinner than 200 nm.

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• Anodic bonding mechanism is not clean. At
  elevated temperatures, the glass
• becomes a conductive solid electrolyte and the
  bonding results through the
• migration of sodium toward the cathode.
• Sodium migration leaves a space charge (bound
  negative charges) in the region of
• the glass/silicon interface. The high electric
  field between the glass and Si results
• in an electrostatic force that pulls the glass
  and Si together.
• Problems
• 1. Difficult technology
• 2. Mismatch in the thermal coefficient between
  the glass and Si.
• 3. The viscous behavior of the glass results in
  degraded long-term stability
• of the components.

Field-assisted Thermal Bonding Modification

• NES used a Ti mesh bias electrode (400 oC, 600 V
  less than 5 min bonding).
• Protecting Si (Glass-Al-SiO2-Si or Glass-poly Si
  SiO2 Si).
• Two Si wafers between 4 7 ? thick borosilicate
  glass.

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• Field-assisted bonding between two Si wafers
  containing thermally grown oxide film (1 ? thick)
  is succesful (850 950 oC, 30 V for 45 min)

Silicon Fusion Bonding

• Based on a chemical reaction between OH group at
  the surface of oxide layer of
• wafer
• Roughness must be smaller than 4 nm.
• Oxidize Si/Si, oxidized Si/ oxidized Si, Si/Si,
  SiN layered Si/Si, SiN layered Si/Si-
• N layered Si, GaAs/Si, Si/glass.
• Before fusion bonding, the oxidized Si surface
  must be hydrated by soaking
• wafers in H2O2-H2SO4, diluted H2SO4, or
  boiling HNO3.
• Oxygen-plasma treatment increases the number of
  OH groups.
• Then, the wafers are rinsed in deionized water
  and dried.
• Self bonding occurs
• Polymerization of the silanol group is believed
  to be the main bonding reaction.

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Thermal bonding with intermediate layers

• LPCVD PSG (1-2 ? thick) layer with two Si wafers
  at 1100 oC for 30 min shows good bonding as long
  as the wafers are clean and reasonably flat..
• Low-temperature sealing glasses (glass frits
  75xx) with sealing temperatures from 415 to 650
  oC.
• RF sputtering of corning 7593 glass frit to
  obtain 8000 Å thick glass film.
• APCVD boron oxide hygroscopic problem
• B-doped SiO2 (softening T 450 oC) crack
  problem
• SOG and sodium silicate layer
• To bond the unpolished back side of a Si die to
  another Si part, an aqua-gel based on a
  hydrophilic pyrogenic silica powder and PVA as a
  binder may be used. (15 min at RT 1 Mpa bond
  strength)

Eutectic Bonding

• Au-Si eutectic bonding at 363 oC.
• Difficult to obtain complete bonding over large
  areas.

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Photopatterned bonding

• Native oxide prevent the bonding to take place
• Great mounting stress, causing long-term drift
  due
• to the relaxation of the built-in stress.

Bonding with organic photopatternable layers

• Lithographic patterning of thick resist layers
• AZ-4000 ans SU-8 photoresist, Liga resist PMMA.
• Low bonding temp, high bond strength, no metal
  ions,
• reduced stress.
• L1 and L2 packaging are possible
• Impossible hermetic seals, high vapor pressures,
  poor
• mechanical properties.

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Bonding of plastic to plastic

• Using adhesives, tapes, plastic welding (hot
  plate and ultrasonic welding), and selected
  solvents by partially dissolving the bonding
  surfaces.
• Development of low-cost, high-speed, and
  reliable bonding techniques for microfluidic
  devices is challenging.

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Alignment during Bonding

• Guiding holes -gt 50 ? accuracy
• Bonding machine equipped with an in situ optical
  alignment set up (- 2.5 ?
• accuracy)

Alignment during Bonding

• Guiding holes -gt 50 ? accuracy
• Bonding machine equipped with an in situ
• optical alignment set up (- 2.5 ? accuracy)

Imaging and Bond Strength and Package Hermeticity
Tests

• Imaging a bonded pair of Si wafers IR
• transmission, ultrasound and X-ray topography
• Mechanical test

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• Hermeticity test was carried out by He leak
  detection. (5 X 10-11 5 X 10-10 Torr
• l/s leak rate)
• FTIR measurement of N2O inside sealed Si cavity.
• To control cavity pressure for critical damping
  of packaged micromechanical
• devices, non-evaporable getters (Ni/Cr ribbon
  covered with a mixture of porous Ti
• and Zr-V-Fe alloy that absorbs gases after
  ativation at 400 oC)

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Higher Levels of Packaging L2 to L5
Sensor Die attach and Wire Bond in a TO-8 Header

• After dicing a sensor die, die was attached to a
  TO-8 header.

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Die Protection

• Vapor-deposited organics for mildly aggressive
  environments
• (2 3 ? poly(p-xylene)).
• Silicone oil over the die.
• Coating of the die surface with soft substances.
• SiC coating for harsh environments.
• Plastic or ceramic cap for particle and handling
  protectioin.
• Welded-on Ni cap with pressure pore.

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Stress Isolation and Thermal Management

• Sensor elements should not be subject to
  undesirable mechanical stresses originating from
  their packaging structure.

Multichip Packaging

• Micromachined chips can be packed laterally as
  in multichip modules (MCM).

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Connections between Layers (Vias)

• Wet etching (aspect ratio lt1)
• Dry etching (aspect ratio 30)
• Through-wafer electrical interconnect
  fabrication compatible with standard
• CMOS processing (High-density SF6 plasma Bosch
  Process)
• Laser Drilling (aspect ratio 50)
• Ultrasonic Drilling
• Temperature Zone Melting (TZM)
• Via formation and metal deposition are one and
  the same process
• Interconnects between plastic layers

Al
SiAl eutectics
Radiant heat (1000 1200 oC)
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Partitioning

• Partitioning is one of the major challenges in
  MEMS
• How far can we push integration of electronics
  with the MEMS sensing function?
• What can we include with the MEMS disposable ?
• What can we put into the fixed reader instrument
  ?
• On board or off-board fluidics ?
• Battery or main power ?

Monolithic vs. Hybrid MEMS

• Hybrid integration means combining thin film Ics
  with thick film technology
• Hybrid sensor keeps the electronics separate
  from the sensor
• Tow pieces of Si on the same substrate connected
  by a short wire bridge.
• Si sensor mounted in a header plugged into an
  electronics board.
• In monolithic MEMS, electronics and MEMS
  elements are cofabricated within
• one single sequential silicon process, in
  which yield is low.

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Partitioning in a Microfluidic Instrument

• Nozzle, pump, channel, reservoir, column, mixer,
  oscillator, diode, amplifier, valves

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Fluid Propulsion Methods and MEMS Integration

• Mechanical pumps Piezoelectric,
  electro-osmotic, electrowetting,
  electrohydrodynamic pumping.
• Acoustic streaming Constant fluid motor induced
  by an oscillating sound field at a solid/fluid
  boundary. Mixing is possible
• Electrophoresis / Electro-osmosis
• Centrifugal pumping
• Vacuum Pressure Reservoir

Heating and Cooling and MEMS Integration

• Electrical current passing through resistors
  integrated on thin membrane.
• External heating and cooling system
• Heating fluid within the micro instrument using
  radiation (ir, rf or microwave)

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Sample Introduction

• Flow injection analysis (rotary of sliding
  valves)
• Creating wells into which the sample is dropped.
  Then the sample is then wicked into an internal
  chamber by capillary action.

Micro and Nano Assembly

• By humans with tweezers and microscopes or
  pick-and-place robots.

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Scaling of the Assembly Process

• Surface forces dominate over volume forces.
• To avoid some of these problems, the smallest
  components are often manipulated in a liquid
  medium.

Micro Assembly Examples
Serial micro assembly

• Optical tweezer light has momentum and can be
  used to catch and manipulate objects in a size
  range from nanometers to micrometers.
• 0.7 1.06 ? wave length
• 25 500 mW in a focal spot between 0.5 and 1.0
  mm in diameter
• Laser scalpel cutting biological objects
  inside cells.
• STM to manipulate individual atoms

Parallel micro assembly

• HELIX flip-chip process
• Microgripper arrays

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Stochastic approaches

• Assemble magnetically coated semiconductor parts
  employing an array of magnetic sites.
• Solvent-surface force micro assembly

• Self-assembly using electrostatic levitation

• At nanogen, electric fields are used to
  transport and the control the placement of
  proteins, RNA and DNA.

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DNA-Meditated Assembly