Lecture 23
MEMS Packaging
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MEMS Packaging
A simplified process flow for MEMS Packaging
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MEMS Packaging
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Key Design and Packaging consideration
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Wafer or wafer-stack thickness
Wafer Dicing concerns
Thermal Management
Stress isolation
Protective coatings and media isolation
Hermetic packaging
Calibration and compensation
Die-attach process
Wiring and Interconnects
– Electrical interconnects
– Microfluidic interconnects
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Types of Packaging
– Ceramic packaging
– Metal packaging
– Molded plastic packaging
A MEMS accelerometer chipset is used for
automotive airbag applications. Motorola
Inc. researchers are applying wafer-level
techniques and using a glass paste to achieve
a hermetic seal.
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Key Design and Packaging Consideration
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Wafer or wafer-stack thickness
– 4-in. wafer 525 µm thick
– 6-in. wafer 650 µm thick
– Current packaging facilities require the total
thickness of the MEMS devices be less than
1 mm
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Wafer dicing concerns
– Batch fabrication
– Diamond or carbide saw blades, 75- to 250
µm wide
– Each MEMS design merits its own
distinctive approach on how to Minimize the
adverse effects of dicing
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Thermal management
– Cooling of heat-dissipating devices.
– Thermal management at two levels:
» the die level and the package level
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Key Design and Packaging Consideration
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Thermal management (con.)
– Thermal actuators can dissipates significant power. With efficiencies
typically lower than 0.1%, most of the power is dissipated as heat.
– With high thermal conductivity, ceramics and metals make excellent
candidate materials for the packaging housing
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Stress isolation
– Stress isolation becomes critical for MEMS devices using piezoelectricity
and piezoresistivity.
– Long-term drift resulting from slow creep in the adhesive or epoxy that
attaches the silicon die to the package housing
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Protective coating and media isolation
– Environmental effects
– Coating materials:
» Parylene, usually deposited by CVD near room temp.
» Silicon carbide, deposited by PE-CVD
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Key Design and Packaging Consideration
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Protective coating and media
isolation (con.)
– Gas sensors: direct exposure to
surrounding media
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Hermetic packaging
– Theoretically defined as package that
prevents the diffusion of helium
– For small volume packages, leakage
rate of helium must be lower than 5 x
10-8 cm-3/s
– Prevention of moisture, water
diffusion
– Made of ceramic, metal, and glass
– can significantly increase the long
term reliability of devices
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Die-Attach Processes
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After dicing the substrate, each
individual die, is mounted inside
a package and attached onto a
platform made of metal or
ceramic
Metal alloys or organic or
inorganic adhesives are used as
the intermediate layers
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solders
epoxy
silicones
inorganic adhesives are usually
glass metrices embedded with
silver and resin
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Die-Attach Processes
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Silicon and glass cannot be directly
soldered to, and thus must be coated with a
thin metal film to wet the surface
– Pt, Pd, or Au
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Organic adhesives are inexpensive, easy to
automate and can cure at lower
temperature
– epoxies are thermosetting plastic with
cure temperature between 50 and 175oC
– filled with silver or gold, they can
become thermally and electrically
conductive
– Room-temperature vulcanizing (RTV)
silicones can also be used for automated
manufacturing
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Wiring and Interconnects
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Electrical interconnects: wire bonding and flip chip
Wire bonding
– thermosonic gold bonding
» The gold wire forms a ball bond
to the aluminum bond pad
» a stitch bond to the package lead
– gold wire less than 50 µm (2 mils)
– Aluminum bond wire
up to 560 µm
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Wiring and Interconnects
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Flip chip bonding
– Bonding the die, top-face-down, on the package substrate.
– Electrical contact are are made by means of plated solder bumps
between bond pads on the die and metal pads on the substrate
– High density of input/output (I/O)
connections than wire bonding
– The effective inductance is
miniscule because of short
height of the solder bumps
– Can closely package a number
of distinct dice on a single
package substrate
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Wiring and Interconnects
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Processing
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Sputtering of a Ti layer over bond pads, and sputtering of Cu
Patterning and etching of the Ti and Cu
Electroplating of thick Cu layer
Solder bump(a Sn-Pb alloy) is electroplated over the copper
– In a separate preparation process, solder paste is screen-printed on the package
substrate in patterns corresponding to the landing sites of the solder bumps
– Automated pick-and place machines position the die, and align the bond pads to
the solder-paste patterns on the package substrate.
– Heating in an oven or under IR radiation melts the solder into a columnar, smooth
and shiny bump
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Flip chip may not be compatible with packaging of MEMS that includes
microstructures exposed to the open environment.
– May lead to thin diaphragm damages.
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Wiring and Interconnects
Y.C. Lee, University of Colorado, Boulder
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Wiring and Interconnects
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Microfluidic Interconnects
– Micropumps, microvalves
– Via holes
– Fluidic sealing
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Most microfluidic interconnects
schemes remain at the level of
manually inserting a capillary into
Silicon cavity or via-hole, and sealing
the assembly with silicone or epoxy
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Types of Packaging
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Ceramic packaging
Metal packaging
Molded plastic packaging
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Types of Packaging
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Ceramic packaging
Process flow for the fabrication of a co-fired, laminated ceramic
package with electrical pins and access ports
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Types of Packaging
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The process fills via holes with metal (W). Via left unfilled can
be used as fluid or pressure access port
Pins or leads are usually brazed to the package, typically made of
ASTM F-15 alloy (Fe52Ni29Co18), known as Kovar
Ceramic package consists of a base or a header onto which dice
are attached by adhesives or solders
Substrate materials:
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Al2O3, AlN, Borosilicate, BeO
thermal conductivity
thermal expansion
electrical resistivity
dielectric permittivity
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Types of Packaging
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Ceramic package example
– DMDTM type-A ceramic package for SVGA displays
– Ceramic package for optical MEMS can be complex and costly
– 114-pins Al2O3 ceramic header
– Wire bonding establish electrical
connectivity between the die and the
metal traces on the ceramic header
– A Cu-Al brazed Kovar® seal ring
– Transparent window consists of a
polished Corning 7056 glass fused to a
stamped gold-nickel-plated Kovar®
frame
– Resistance seam welding of the the
seal ring on the ceramic base to the
Kovar® glass frame
– Zeolite getters to ensure long-term
desiccation
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Types of Packaging
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Metal package
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Metal package is robust and easy to assemble
Standard TO-type (transistor outline)
Can only accommodate fewer than 10 pins.
TO-type package remain in use in a few
applications
Metal packages are attractive to MEMS,
especially for microfluidic devices
A metal hermetic package is often made of ASTM
F-15 (Kovar® ), stainless steel is also common
Holes are punched, either through the bottom for
plug-in packages, or sides for flat packages
Metal leads are placed through the holes and
beads of borosilicate glass (Corning 7052).
The metal packaging of media
isolated Pressure sensor
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Types of Packaging
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Molded plastic package
– Not hermetic
– Two general approaches
» postmolding
» premolding
– In postmolding, the plastic housing is
molded after the die is attached top a
“lead frame” (a supporting metal
sheet). The process subjects the die and
the wire bonds to the harsh molding
environment.
– In premolding, the die is attached to a
lead frame over which plastic was
previously molded. It is attractive in
situations where the risk of damaging
the die is high, or openings through the
plastic are necessary. More expensive.
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Types of Packaging
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Molded plastic package
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