PACKAGING.pdf

Lecture on Microsystems Design and Manufacture
Chapter 11
Assembly, Packaging, and Testing (APT)
of Microsystems
● Like ICs, no MEMS or microsystem is made by only one single component.
They are almost all made of multi-components that need to be assembled
and packaged to make the microdevices
• Thus, packaging of microsystems involves: assembly, joining, interconnecting,
encapsulation of minute parts and components into a microsystem product
● Packaging also includes performance and reliability testing of the finished
products
● Packaging is the most critical factor of successful commercialization of micro-
scale products. Packaging cost can be as high as 95% of the overall cost of
the production. On average, packaging cost is about 30% of the total
production cost. Cost-effective and reliable packaging technique is thus the
key to the competitiveness of the microsystem product in the marketplace

Content
Overview of Assembly, Packaging and Testing (APT)
of MEMS and Microsystems
Part 1: Microassembly
Part 2: Packaging of Microsystems
Part 3: Reliability and Testing of Microsystems

High Cost in APT of MEMS and Microsystems
￭ MEMS and microsystems involve complex structural geometry and
a variety materials
￭ They are expected to perform multi-functions involving biological, chemical,
electrical, mechanical, and optical performances
￭ There is no standard in materials and process to follow in APT:
￭ Every microdevice requires special design, component fabrication and APT
1990 1995 2000 2005 2010 2015 2020
Materials
Equipment
Design & Modeling; Testing
Interconnect
Process
Packaging
Standards Timeline
Categories
High development cost No sharing in technical information
Every new device development requires APT from fresh start
High overall costs in APT

APT of a Microdevice Component

Top View
Elevation
(Cross-Section)
Silicon Die
Pyrex Glass
Constraint Base
Silicon Wafer
Pyrex Glass Wafer
A Micro Pressure Sensor Die

A Flow Chart for Integrated Assembly, Packaging and Testing
for mass production of micro pressure sensors
Wafers
Incoming wafer
inspection
Wafer bonding
Microfabrication
on wafers
Wafer dicing
Lift-off
Surface
coating
Part
sorting
(Parts)
Sub-group
assemblies:
Die attach and/or
Bonding
Surface
bonding
Wire
bonding
Die or parts
passivation
Electrical
inspection
Curing of
passivation
materials
Electrical
inspection
System
assembly
System
encapsulation
(Sealing)
Testing for
sealing
Testing for
electrical and
performance
functions
Product packaging
Shipping
(Packaged
sub-groups)
(1)
(2)
(3)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
(17)
Assembly: Steps (6) and (12)
Packaging: Steps (3), (7), (9) and (16)
Testing: Steps (2), (8), (11), (14) and (15)
µ-fabrication: Steps (4), (5), (10), (13)

Part 1
Microassembly
Microassembly = the assembly of objects with microscale and/or
mesoscale features under microscale tolerances.

The High Costs in Microassembly
￭ We have defined microcomponents of MEMS and microsystems to be in the dimensions
ranging from 1 µm to 1 mm
￭ Thus, most of them cannot be seen by naked human eyes
￭ Almost all assembly of microcomponents have to be performed under microscopes
￭ There are huge number of microcomponents to be assembled by MEMS industry:
MST
Products
1996 Units
(millions)
1996 Revenue
($millions)
2002 Units
(millions)
2002
Revenue
($millions
)
2006 Units
(millions)
2006
Revenue
($million)
Established
Products
1595 13033 6807 34290 10282 48461
Emerging
Products
33 107 1045 4205 1720 6937
Total 1628 13140 7852 38495 12002 55398
Source: NEXUS , hhtp://www.wtec.org/loyala/mcc/mes.eu/pages/chapter-6.html

The Needs for Cost-Effective Assembly of Microsystems
● By reliable estimate, there are 12 billion units worth $50 billion of microscaled
products to be in the marketplace in 2006.
● Among them, there are 2+ billion units are “read-write heads” for hard disk drives,
“inkjet printer heads” and “inertia sensors” that are automatically assembled.
● The remaining 10 billion units would be assembled with some degree of
automation, or entirely assembled by human effort.
● Manual assembly of MST products is prohibitively expensive, tiresome and
time consuming. Often, the products would not meet the extremely stringent
requirements in precision and thus the necessary quality and reliability of
the finished products.
● Awkward assembly and packaging techniques used by the MST industry are
the major stumbling blocks to successful marketing, and thus capitalizing the
enormous full potential benefits of microsystems technology.

Main reasons for lack of automated microassembly technology:
●There is lack of standard procedures and rules for such assemblies:
Products are assembled according to the specific procedures based either on
individual customer requirements, or on the personal experience of the design
engineer
● There is lack of effective tools for micro assemblies:
Tools such as micro grippers, manipulators and robots are still being developed
● Micro assemblies require reliable visual and alignment equipment:
Such as stereo electron microscopes, electron-beam, UV stimulated beam or
ion beam imaging systems specially design for microsystems assembly
● Lack of established methodology in setting proper tolerances:
The strategies for setting tolerances in parts feeder, grasp surface to mating surface,
fixtured surface to mating surface, etc. have not been established for micro assembly
● Micro assemblies are physical-chemical processes related with
strong material-dependence:
Traditional assembly techniques are not suitable for micro devices because of the
minute size of the components and the close tolerances in the orders of sub-microns.
Moreover, chemical and electrostatic forces dominate in micro assembly, whereas
gravity and physics are primary consideration in macro assemblies. There is little
theory or methodology developed to deal with these problems in micro assemblies.

Four Reasons for High Cost of Microassembly
￭ No standard procedure for microassembly
￭ Lack of effective tools for:
● Microgripping
● Manipulating
● Reliable visual and alignment
● Stereo imaging
￭ Lack of established methodologies in setting tolerances in:
● Insertion and assembly
￭ Lack of understanding in the influences of non-conventional forces,
e.g., the interface electrostatic and atomic/chemical forces during
microassembly

Microassembly Processes
￭ Parts feeding
￭ Part grasping by microgrippers, manipulators and robots
￭ Part mating by specially designed tools
￭ Part bonding and fastening
● Wire bonding
● Special surface bonding
￭ Encapsulation and passivation
● Mechanical and physical/chemical encapsulation
● Vacuum packaging
￭ Sensing and verification
● Visual inspection for structural integrity
● Performance testing

Major Technical Problems in Microassembly
- setting proper tolerances
￭ Dimensional tolerances inherited from microfabrication:
0.3
0.2 µm/10x10 or larger
Ni, PMMA, Au,
ceramics
LIGA process
0.1
Sub- µm-wafer size
Si crystal
Silicon-on-insulators
0.5
Sub- µm-wafer size
Poly-Si, Al, Ti
Poly-silicon
surfacemicromachining
0.1
Sub- µm-wafer size
Si, GaAs, quartz,
SiC, InP
Dry etching
1.0
Few µm – wafer size
Si, GaAs, quartz,
SiC, InP
Wet anisotropic
etching
Dimensional
Tolerance (µm)
Minimum/Maximum
Sizes
Materials
Fabrication Processes

Major Technical Problems in Microassembly - setting proper tolerances (Cont’d)
￭ Geometric Tolerances:
● Relating to the discrepancy of the geometry of microcomponents produced by
microfabrication processes and the intended application of the microsystem.
● Improperly setting of this tolerance may cause serious misfit in assembly:
V
Fixed
Electrodes Moving
Electrodes
w d
L
L = length
w = width
d = gap
with “fingers”
W=2 µm
L
=
40
µ
m
2o
4.8 µm
W=2 µm
2o
4.8 µm
36
µm
40
µm
dt = 1.74 µm
db = 3 µm
(a) Resonator actuated by comb-drive (b) An electrode finger with 2o tapered edges
(c) Variation in gaps
Figure 11.5

Major Technical Problems in Microassembly - setting proper tolerances (Cont’d)
￭ Alignment Tolerances:
● Proper setting is necessary in “inserting” and “placing” of parts
● This tolerance relates to specific applications, e.g., bioMEMS and OptoMEMS
● In most cases, these tolerances < 1 µm
￭ Other Tolerances:
● Part feeders
● Grasping surface to mating parts
● Fixture surface to mating surfaces

Tools and Fixtures for Microassembly
Major problem in microassembly is the minute size of microcomponents to be assembled
Many components can only be viewed under microscopes
with magnification at 300X - 500X
For typical optical microscope, the working distance
(the gap between the objective lens and the microcomponent) d
is inversely proportional to the magnification:
d
y
Y
X
1
∝
=
Y
y
Working
Distance
d
Ocular Lens
(Eyepiece)
Objective Lens
The small d for large X means very small working space for
microassembly tools, e.g., microgrippers, or other fixtures
→ tools and fixtures with very low aspect ratios
(aspect ratio = dimension in height to length)
Tools with high aspect ratios may not have sufficient rigidity
to provide high precision pick-n-place operations →
difficult in precision control without feedback tactile sensor feedback
Microgripper with LONG arms

Contact Problems in Microassembly Tools
● “Pick-n-Place” by microgrippers or micro robotic end-effectors are common
and necessary practice in microasdsembly
● This practice requires the tool surface to be in contact with that of the micro-
components to be “picked” from one location and “placed” at another location.
● Adhesive forces on the contacting surfaces of minute pieces develop, e.g.,
in the effort to peel of thin light-weight piece of paper from a transparency
● These adhesive forces likely developed between the surface of the tools
and that of the microcomponents – due to electrostatic and chemical
(atomic or van der Waals) forces.
● In the Pick-n-Place operations in microassembly, there is no problem in “Pick”
but often cannot “release” for the “Place” part of the operation because of
these adhesive forces compounded by insignificant gravity (weight) effect

d
Flat
Gripper
Arms
d
δ
Gripping
Force
Gravitation
(insignificant)
Adhesive
Force
Gravitation
Grasping: Releasing:
● Gravitation of minute objects is insignificant in pick-n-place operations.
As such, induced adhesive forces may dominate in this operation.
● These adhesive forces cause great difficulty in releasing the object at the
end of the operation:
Adhesive Forces in Micrograsping

Adhesive Forces in Micrograsping
● There are two principal contributing sources for the adhesive
forces:
● Van der Waals force, and
● Electrostatic force.
● In wet assemble, or assembly in humid environment, the
“surface tension” of the fluid between the contacting surfaces
become the 3rd adhesive force component.
● Exact quantification of these forces is not possible.
● Use a case involving Pick-n-Placing a sphere by a pair of
flat plate gripping arms for assessing the adhesive forces:

2
12 δ
η
d
A
Fv =
Adhesive forces in Pick-n-Placing of a sphere by a pair of flat
plate gripping arms:
(1) Van der Waals force (d < 100 nm, or 0.1 µm):
where A = Hamaker constant = 10-20 to 10-19 J
δ = atomic separation between the contacting surfaces
typically at 4 to 10
η = correction factor for rough surfaces (≈ 0.01)
(2) Electrostatic force (10 µm < d < 1 mm):
Induced in picking portion of the process due to
charge-generation, or charge- transfer during the contact.
2
2
4 d
q
Fe
ε
π
=
where q = electrostatic charge, ≈ 1.6x10-6 C/m2 in microgripping
ε = permittivity of the dielectric, = 8.85x10-12 C2/N-m2
d = diameter of the sphere (10 µm to 1 mm)
(3) Surface Tension:
d
δ
Adhesive
Force, F
Gravitation
(insignificant)
Flat
gripping
arm
o
A
Total adhesive force in assembly: F = Fv + Fe + Fs
γ
s
Fs =
where s = perimeter of microvoid in contacting area
γ = coefficient of surface tension

● An integrated micropositioner:
● Linear movement with step sizes: 0.3 µm in X-Y and 0.07 µm in the Z-axis
● Rotation about both X & Y axes at 0.0028o/step.
● Resolution in linear movements: 40 nm.
● Microscope optics and imaging unit.
Require long working distance to 30 mm with 1 µm resolution.
● Micromanipulator unit:
● Microgripper with special end-effector, or micro tweezers.
● Provide proper gripping forces, and be able to overcome the induced
adhesive forces in releasing the object.
● A high precision transfer tool.
For transporting dies in wafers, or trays with discrete parts.
● A real-time computer with vision for precision alignment:
● Controls movements of transfer tools, micro positioner and micro manipulator.
● Implement assembly strategy, process monitoring, diagnosis and error recovery.
● A portable class 100 clean room.
Essential Elements of an Automatic Microassembly Work Cell
Microassembly Work Cells

Integrated
Micro-positioner
with Micro Servo
Actuator:
(Linear in X & Y
+
Rotations about x-Y)
Microtweezers
or manipulator
Stereo Microscope
& camera
Stereo Microscope
&
Camera
Vertical Microscope
& Camera
Portable Clean Room (class 100)
PC
Micro-controller
cards + operation
software
Optics
A Typical Automatic Microassembly Work Cell

An Experimental Microassembly Work Cell at Sandia National Laboratory
Grasping a ring by a
micro tweezers

An Experimental Automatic Microassembly Work Cell
at University of New Mexico

An Experimental Automatic Microassembly Work Cell
at University of Minnesota
Micro positioner
Micro manipulator

Part 2
Packaging of Microsystems

Overview of Mechanical Packaging of Microelectronics
● To provide support and protection to the IC, the associate wire bonds and
the printed circuit board from mechanical or environmentally induced
damages.
•To dissipate excessive heat generated by electric heating of the IC.
Objectives of mechanical packaging of microelectronics:
Chip (L0)
Module (L1)
Card (L2)
Board (L3)
Gate (L4)
Level 1
Level 2
Level 3
Level 4
The 4 levels of microelectronics packaging:
Level 1: Silicon chip into a module.
Level 2: Card level.
Level 3: Cards to boards
Level 4: Boards to system
Level 1 and 2 are of primary
interest to mechanical engineers.

Overview of Mechanical Packaging of Microelectronics – Cont’d
Level 1 & 2 packaging:
Wire bond
Si die
Die pad
Interconnect
J-Lead
Interconnect
Gull-wing Lead
Solder joint Solder joint
Printed Circuit Board (or Wirebound) Board
Die attach
Silicon die
with IC
Epoxy
encapsulant
Die pad and
die attach Wire bond
Interconnects
Solder joints
Printed circuit board
Printed circuit
Principal components in a chip: Plastic encapsulated chip:
Reliability issues:
● Die and passivation cracking.
● Delamination between the die, die attach, die pad and plastic passivation.
● The fatigue failure of interconnects.
● Fatigue-fracture of solder joints.
● The warping of printed circuit board.
Failure mechanisms:
● Mismatch of coefficients of thermal expansion between the attached materials.
● Fatigue-fracture of materials due to thermal cycling and mechanical vibration.
● Deterioration of material strength due to environmental effects such as moisture.
● Intrinsic stresses and strains from fabrication processes as described in Chapter 8.

There is no standards in packaging materials and methodologies adopted by
the industry at the present time.
MEMS and Microsystems Packaging
• Most MEMS and microsystems packaging have been carried out on the
basis of specific applications by the industry.
• Little has been reported in the public domain on the strategies,
methodologies, and materials used in packaging of MEMS and
microsystem products.
Current state:
1990 1995 2000 2005 2010 2015 2020
Materials
Equipment
Design & modeling;
Testing
Interconnect
Process
Packaging
Categories
Standards Timeline

Objectives of microsystems packaging:
• To provide support and protection to the delicate core elements (e.g. dies),
the associate wire bonds and transduction units from mechanical or
environmentally induced damages (e.g. heat and humidity).
• Most of these elements requiring protection are required to interface with
working media, which may be environmentally hostile to these elements.
• Interface is thus a major concern in microsystems packaging.
Diverse signal transduction in mcirosystems:
Yes
Yes
Optical
Yes
Yes
Mechanical
Yes
Magnetic
Yes
Yes
Fluid/hydraulic
Yes
Yes
Electrical
Yes
Yes
Chemical
Output
Input
Signals
MEMS and Microsystems Packaging – Cont’d

General considerations:
● The required costs in manufacturing, assemblies and packaging of the components.
● The expected environmental effects, such as temperature, humidity, chemical
toxicity, etc. that the product is designed for.
● Adequate over capacity in the packaging design for mishandling and accidents.
● Proper choice of materials for the reliability of the package.
● Achieving minimum electrical feed-through and bonds in order to minimize the
probability of wire breakage and malfunctioning.
The scope of this chapter:
● On silicon-based microsystems only.
● Packaging of microsystems produced by LIGA processes are not covered in
this chapter.

Level 1: The “die level”,
Level 2: The “device level”, and
Level 3: The “system level”.
Sensing
Element
Actuating
Element
Die Packaging:
Signal Mapping
& Transduction
Signal
Conditioning &
Processing
Power
Supply
Device packaging:
System Packaging:
Output
Motion
Input
Action
Output
Signals
The 3 levels of microsystems packaging

Die-level packaging:
Dies in most microsystems are the most delicate components, which require
adequate protection. Thus the objectives of this level packaging are:
● To protect the die or other core elements from plastic deformation and cracking,
● To protect the active circuitry for signal transduction of the system,
● To provide necessary mechanical isolation of these elements, and
● To ensure the system functioning at both normal operating and over-load
conditions.
Die-level packaging often involves wire bonding:
Silicon
Diaphragm
Pyrex Glass
Constraining
Base
Metal
Casing
Passage for
Pressurized
Medium
Silicon gel
Wire bond
Metal film
Dielectric layer
Piezoresistor
Die
Attach
Interconnect
Si die
Die attach
Wire bond
(Si gel)
Plastic encapsulant
Metal cover
Interconnect
Pressurized
medium inlet
Pressure sensor with metal casing:
Pressure sensor with plastic encapsulation:

Device-level packaging:
Sensing
element
Actuating
element
Signal mapping
& transduction
Signal
conditioning
& processing
Input
action
Output
motion
Output
signals
Power
supply
● electric bridges
● signal conditioning circuits
● Proper regulation
of input power
Major technical problems:
● The interfaces of delicate dies and core elements with other parts of the
packaged products at radically different sizes, and
● The interfaces of these delicate elements with environmental factors, such as
temperature, pressure and toxicity of the working and the contacting media.

MEMS and Microsystems Packaging – Ends
System-level packaging:
● This level packaging involves the packaging of primary signal circuitry with
the package of the die or core element unit.
● Major tasks involve proper mechanical and thermal isolation as well as
electromagnetic shielding of the circuitry.
● Metal housings usually give excellent protection for mechanical and
electromagnetic influences.
● MEMS devices or microsystems at the end of this packaging level are ready
to be “plug-in” to the existing engineering systems:
Packaged inertia
Sensor for airbag
Deployment system

The packaged systems need to be biologically compatible with human
systems and they are expected to function for a specified lifetime.
Every micro biosystem must be built to satisfy the following requirements
that are related to interface:
• It is inert to chemical attack during the useful lifetime of the unit.
• It follows mixing with biological materials in a well-controlled manner
if it is used as biosensors.
• It causes no damage or harm to the surrounding biological cells in the
cases of instrumented catheters such as pace makers.
• It causes no unwanted chemical reactions such as corrosion between the
packaged device and the contacting human body fluids, tissue and cells.
All biomedical devices and systems are subject to FDA regulations.
Interfaces in Microsystems Packaging
● Various parts, in particular, the delicate dies of microsystems are expected to be in
contact with various working media, e.g. chemicals, optical, corrosive gases, etc.
● Interface between these parts with working media becomes a major design issue in
packaging.
Biomedical interfaces

Optical interfaces
There are two principal types of optical MEMS:
• The devices that direct lights, e.g. micro switches involving
mirrors and reflectors.
• Optical sensors.
Optical MEMS require:
• Proper passages for light beams to be received and reflected.
Fiber-optics are common light conduits in optical MEMS.
• Proper surface coating for receiving and reflect lights.
• The quality of the coating must be enduring during the lifetime of the device.
• The surfaces must be free of contamination of foreign substance.
• The enclosure must be free of moisture. The presence of moisture may
cause stiction of the enclosed components.
Interfaces in Microsystems Packaging – Cont’d

Electromechanical interface
Electrical insulation, grounding and shielding are typical problems
to be dealt with in MEMS and microsystems packaging.
Interfaces in microfluidics
• Precise fluid delivery.
• Thermal and environmental isolation and mixing.
• Material compatibility between the fluid and the containing walls.
• Interface of the fluid and containment wall, e.g. corrosion, friction, etc.
• Another major interface problem is in sealing
Interfaces in Microsystems Packaging – Ends

Die preparation
• Dies, or substrates in MEMS, are normally cut
(sliced) from single wafers using thin diamond
saw blades.
Enabling Packaging Technologies
Spacing between dies: ≈ 50 µm with saw blade thickness of 20 µm.
Cutting wheel: 75 – 100 mm diameter
Cutting speed: 30,000 – 40,000 rpm.

Surface bonding
There are four (4) techniques available for surface bonding in MEMS
and microsystems:
(1) Adhesives
(2) Eutectic soldering
(3) Anodic bonding
(4) Silicon fusion bonding (SFB)
Enabling Packaging Technologies – Cont’d
Bonding by adhesives:
● Epoxy resin and silicone rubbers are two
commonly used adhesives.
● Good bonding by epoxy resin rely on
surface treatments and curing process
control. Avoid glass transition temperature
at 150-175oC.
● Soft silicone rubbers are used for bonding
parts require “flexibility.” It is vulnerable
to chemicals and air. A typical micro dispenser of
epoxy resins (Courtesy of
Asymtek Co., Carlsbad, CA

Surface bonding – Cont’d
Eutectic bonding:
● Eutectic bonding involves the diffusion of atoms of eutectic alloys into the
atomic structures of the materials to be bonded together.
● Must first select a candidate material that will form a eutectic alloy with the
materials to be bonded.
● A common material to form eutectic alloy
with silicon is thin films made of gold
or alloys that involve gold.
● Gold-tin (80% Au+20% Sn) films around 25 µm
thick is commonly used.
● Bonding takes place at about 300oC.
● Offers much solid bonding than adhesives.
Si Substrate
Doped Si
Weight
Heat
Au/Sn Film

Anodic bonding:
● Bonding wafers of different materials.
● Also called “electrostatic bonding” or “Field-assisted thermal bonding.”
● It is popular because of simple set-up and inexpensive equipment.
● Bonding temperature is relatively low in the range: 180-500oC.
● Possible to bond wafers of:
● Glass-to-glass
● Glass-to-silicon
● Glass-to-silicon compounds
● Glass-to-metals
● Silicon-to-silicon
● Most common application is for Glass-to-silicon wafer bonding.

Anodic bonding-Ends
The working principle of Glass-to-silicon wafer bonding:
Weight for contacting pressure (Cathode)
Glass wafer
Silicon wafer
Heated mechanical support (Anode)
Applied DC voltage:
200-1000 volts
Silicon
Si+
Si+
Si+
Si+
Si+
Na+←
Na+←
Na+←
Na+←
Na+←
O2
-
O2
-
O2
-
O2
-
O2
-
SiO
2
layer
Glass
Hot Plate (Anode)
Weight for contact
Pressure (Cathode) ≈ 20 nm
Bonding interface
Na Depletion
Layer ≈ 1 µm

Silicon Fusion bonding
● Silicon fusion bonding is like “welding” a silicon wafer to another silicon wafer.
● It is relatively simple and inexpensive bonding method.
● Silicon fusion bonding (SFB) has been used to bond:
● Silicon-to-silicon
● Silicon with oxide-to-silicon
● Silicon with oxide-to silicon with oxide
● GaAs-to-silicon
● Quartz-to-silicon
● Silicon-to-glass
● It is the induced chemical forces that bond the pieces together.
● Wafer surfaces need to be extremely flat (at 4 nm) to be bonded.
● Bonding strength between silicon wafers can be as high as 20 MPa.
● The SFB process begins with thorough cleaning of the bonding surfaces.
These surfaces must be polished, then make them hydrophilic by exposing
them in boiling nitric acid.
● These two surfaces are naturally bonded even at room temperature.
● Strong bonding occurs at high temperature in the neighborhood of
1100oC to 1400oC.

● The three (3) wire bonding techniques
used in IC industry are adopted for
MEMS and microsystems:
● Thermocompression wire bonding
● Wedge-wedge ultrasonic bonding
● Thermosonic bonding.
● Common wire materials are Au, Ag, Al,
Cu and Pt with diameters at 20-80 µm.
● Wire bonding is fully automatic.
Silicon
Diaphragm
Pyrex Glass
Constraining
Base
Metal
Casing
Passage for
Pressurized
Medium
Silicon gel
Wire bond
Metal film
Dielectric layer
Piezoresistor
Die
Attach
Interconnect
Si die
Die attach
Wire bond
(Si gel)
Plastic encapsulant
Metal cover
Interconnect
Pressurized
medium inlet
Wire Bonding
● Wire bonding techniques developed for microelectronics are applicable for
bonding electric lead wires in MEMS and microsystems.

Wire Bonding – Cont’d
Thermocompression wire bonding
● Wire bonding is accomplished with mechanical compression at elevated
temperatures at about 400oC.
● The bonding process is illustrated as:
Capillary
Tool
Metal Wire
(HEAT)
Substrate
Metal Pad
Substrate
Substrate
● Heat the wire to form a bead
● Feed the bead to the pad by
pulling down the capillary tool:
● Compress the bead to
pad mechanically:
● Retract the capillary tool
after the bead is bonded
to the pad:

Wire Bonding – Ends
Wedge-wedge ultrasonic bonding
● This bonding process takes place at room temperature.
● The energy supply to the bonding is from ultrasonic vibration of the tool
at 20 – 60 kHz.
● The process is illustrated as:
Wedge
Bonding
Tool
Wire
Tool Direction
Metal Pads
Metal pad
Substrate
Wedge
Bond
Wire
Wedge
Bonding
Tool
Metal Pads
Thermosonic bonding
● This process uses ultrasonic energy with thermocompression.
● As such, wire bonding can take place at 100-150oC.
● Joints can be in either ball-wedge or wedge-wedge form.
With mechanical
compression

Sealing
• Sealing is a key requirement in MEMS and microsystems packaging.
• Hermetic sealing is essential in devices or systems such as: microfluidic,
optoMEMS, bioMEMS, pressure sensors, etc.
• There are generally 3 sealing techniques available for MEMS and microsystems:
(1) Mechanical sealing technique:
• Epoxy for microfluidics. It is flexible but ages with time.
● Eutectic soldering for hermetic seals.
(2) Sealing by microfabrication processes - Sealing by micro shells:
Doped silicon PSG sacrificial
layer
Die
Constraint base Constraint base
Doped silicon
micro shell
(a) With sacrificial layer (b) After the removal of sacrificial layer

Sealing
(3) Sealing by chemical reactions:
• Sealing is accomplished by “growing” the sealant using chemical reactions.
• Example is the production of SiO2 as the sealant for sealing a delicate die
with a silicon shell.
• The growth of SiO2 from the silicon encapsulant to the constraint base
provides reliable and hermetic seal for the die.
Si Constraint base
Silicon shell
Si Constraint base
SiO2
seals
SiO2
seals
Die Die
SiO2 film
(a) Unsealed encapsulant (b) Sealed encapsulation by oxide
grown from silicon shell
Sealing – Ends

3-Dimensional Packaging
● 3-dimensional packaging is a popular R&D topic in microelectronics
packaging.
● Principal reasons for 3-D packaging are:
● Provides high volumetric efficiency,
● Provides high-capacity layer-to-layer signal transport.
● Has the ability to accommodate wide range of variation of layer types.
● Has the ability to isolate and access a fundamental stackable
element for repair, maintenance or upgrading.
● Has the ability to accommodate multiple modalities, e.g. analog,
digital, RF power, etc.
● Provides adequate heat removal among the package layers.
● Allows for high pin-count delivery to the next level of packaging with
high electrical efficiency.
3-D microelectronics packaging

3-Dimensional Packaging - Ends
3-D MEMS and microsystems packaging
● Package MEMS and microsystems with distinct functions stacked up
with signal processing units in compact configurations
● Shielding of electromagnetic and thermal effect, and hermetic sealing of moving
fluids are the critical issues in 3-D packaging.
x
y
Accelerometer
for x-direction
Accelerometer
for y-direction
Signal conditioning
and processing
Acceleration
in x-direction
Acceleration
in y-direction
Signal conditioning
and processing
x
y
Planar (2-D) packaging
3-D packaging

Vacuum Sealing and Encapsulation
Many MEMS and microsystems can perform better, or can only perform
in vacuum.
It is a very important requirement for many MEMS and microsystems.
Examples such as microgyroscopes and micromirrors in micro fiber optical switches
require vacuum to provide free air-resistance and a moisture-free environment.
High vacuum in the MEMS devices must be maintained while the system
is packaged.
Hermetic and enduring sealing is required to maintain vacuum in the system.
Two vacuum sealing techniques will be introduced here:
(1) Vacuum sealing by RTP bonding process, and
(2) Vacuum sealing by localized CVD process.

Cap wafer
Al-to-nitride bond
Device wafer
Heating Elements
Quartz Tube
To vacuum pump
Example of Sealing by RTP bonding
RTP = Rapid Thermal Processing, a process that is commonly used in IC packaging
Vacuum Sealing – Cont’d
Two wafers:
Cap wafer = the wafer with cavity for passivation of the device
Device wafer= the wafer with microcompenents
Both the device and cap wafers are pre-baked in vacuum at 300oC for 4 hours in a vacuum
quartz tube to drive out and entrapped gas from microfabrication processes.
The two wafers are assembled and loaded into a sample holder and placed in the vacuum
heating tube again.
The set is then placed inside of the RTP equipment and the base pressure was pumped
down to about 1 mTorr.
The vacuum was held steady for about 4 hours to drive out entrapped gas inside the cavity.
The sealing is completed by RTP heating in 10 s at 750oC.

Example of Sealing by localized CVD process
Vacuum Sealing – Cont’d
Glass cap
Microheater
Vent
Silicon substrate
Microstructure
CVD
Deposition
Vacuum Cavity
The set is put into a vacuum chamber at about 250 mTorr with the flow of silane gas.
The microcomponent is assembled to the silicon substrate.
The silicon substrate with assembled microcomponent is anodically bonded to a glass cap.
A vent hole is created in the assembly.
There is a small heater at the vent hole made by electrically conducting polysilicon
The intense heat release by the microheater decomposes silane for localized polysilicon
deposition to seal the venting hole as shown in the right of the Figure.
The CVD deposition process provide the necessary seal for the microdevice

Selection of Packaging Materials
● There are a broad range of materials used in packaging MEMS and microsystems.
● Commonly used materials for various parts of MEMS and microsystems are:
Refer to Chapter 7 and
Section 10.2.2 for selection.
Materials are in order of
Increasing quality and cost.
Pyrex and alumina are more
commonly used materials
Solder for better seal, silicone
rubber for better die isolation.
Gold and aluminum are
popular choices.
Silicon, polycrystalline silicon, GaAs,
ceramics, quartz, polymers
SiO2, Si3N4, quartz, polymers
Glass (Pyrex), quartz, alumina, silicon
carbide
Solder alloys, epoxy resins, silicone
rubber
Gold, silver, copper, aluminum and
tungsten
Copper and aluminum
Plastic, aluminum and stainless steel
Die
Insulators
Constraint base
Die bonding
Wire bonds
Interconnect pins
Headers and casings
Remarks
Available Materials
Microsystem Components

Selection of Packaging Materials – Cont’d
Summary of die packaging material properties:
2.33
6.0-7.0 (25-300oC)
26
63 below 126oC
140 above 126oC
370
0.29
0.27
0.44
0.49
190,000
344,830-408,990 (20oC)
344,830-395,010 (500oC)
31,000
4,100
1.2
Silicon
Alumina
Solder (60Sn40Pb)
Epoxy
(Ablebond 789-3)
Silicone rubber
(Dow Corning 730)
Thermal Expansion
Coefficient ppm/oK)
Poisson’s ratio
Young’s Modulus (MPa)
Materials

Selection of Packaging Materials – Ends
Temperature-dependent properties of epoxy resin (Ablebond 789-3):
55
60
1.5
Same as above
Same as above
Same as above
0.42
0.42
0.49
Same as above
Same as above
Same as above
at –40oC: 7,990
at 25oC: 5,930
at 125oC: 200
at –40oC: 4,680
at 25oC : 4,360
at 125oC: 110
at –40oC: 3,830
at 25oC: 3,620
at 125oC: 60
at –40oC: 3,610
at 25oC: 2,650
at 125oC: 40
at 25oC: 1,790
at 125oC: 30
0 – 500
500 – 2,000
2,000 –10,000
10,000 – 20,000
20,000 – 30,000
Fracture strength
(MPa)
Poisson’s ratio
Young’s modulus (MPa)
Strain range (10-6)

Signal Mapping:
Develop and establish strategies in selecting both the type and positions
of the transducers for the MEMS device of microsystem.
Transducers Electric signals Input or Output Typical applications
Piezoresistors
Piezoelectric
Capacitors
Electro-resistant
heating/Shape
memory alloys
Resistance, R
Voltage, V
Capacitance, C
Current, i
Output
Input or Output
Input or Output
Input
Pressure sensors
Actuators,
accelerometers
Actuators by
electrostatic forces,
Pressure sensors
Actuators
Common Transducers for MEMS and Microsystems
Signal Mapping and Transduction

Signal Mapping and Transduction – Cont’d
Signal mapping for a micro pressure sensor:
Piezoresistors are used to sense the change of electrical resistance relating to the
Induced stresses at the location.
Three locations are chosen for these piezoresistors in the following 3 cases:
Outline of diaphragm
Piezoresistors
Outline of silicon die
45o
Case 1: Square die/square diaphragm:
Case 2: Rectangular die/rectangular diaphragm
Case 3: For shear deformation in square diaphragm

Vo
Vin
R1=Rg
R2
R3
R4
+
-
a
b
Signal transduction by Wheatstone bridge:
● 4 gages involved in the bridge.
● R1= Rg – the variable resistance
R2, R3 and R4 have fixed resistance.
■ For static conditions:
The voltage Vo is adjusted to zero:
R
R
R
Rg
2
4
3
= (11.6)
■ For dynamic conditions:
The voltage Vo changes with time, and the changes
are recorded.
The change of the measured resistance is:
1
1
3
2
3
2
3
1
4
1 3
−
+
−
∆
−
⎟
⎠
⎞
⎜
⎝
⎛
+
+
∆
=
∆
R
R
R
V
V
R
R
R
V
V
R
R
R
R
in
o
in
o
g
(11.8)
where R1 = the original value of Rg

Signal transduction bridge for capacitance measurements:
Vo Vin
C C
C
Variable
capacitor
● 4 capacitors are involved in the bridge.
● There are 3 identical capacitors with
capacitance C.
● The 4th capacitor with varying capacitance,
e.g. with gap change between two plate
electrodes.
● The bridge is subjected to a constant input
voltage, Vin.
● The variation of capacitance, ∆C in this
capacitor may be obtained from the
measured output voltage, Vo:
V
V
V
C
C
o
in
o
2
4
−
=
∆ (11.9)

Design case: Packaging of Micro Pressure Sensor Dies
Primary packaging considerations
● The die in a pressure sensor is to support the thin diaphragm that senses
the medium pressure by the induced stresses.
● For accurate sensing the medium pressure, the stresses that the diaphragm
has sensed should be those stresses induced by the medium pressure ONLY.
● Unfortunately, there could be stresses induced in the diaphragm by sources
other than the medium pressure – the “parasite stresses”.
● A major source of parasite stress is from the thermal stresses induced by
significantly different CTE of various components attached to the diaphragm:
Constraint base (Pyrex):
α: 7 ppm/oC
Die attach
(60Sn40Pbsolder):
α: 26 ppm/oC
Silicon die:
α: 2.33 ppm/oC
Silicon diaphragm
Dielectric film
● How to ISOLATE the die/diaphragm from these sources of parasite stresses
become a primary consideration in the packaging design.

Design case: Packaging of Micro Pressure Sensor Dies – Cont’d
Die down
● It is a process to bond the die to the constraint base with “die attach”.
● Three commonly used bonding techniques:
● Anodic bonding
● Eutectic soldering
● Adhesive
Constraint base
Die attach
Silicon die
Silicon diaphragm
Height
H
Constraint base
Die attach
Silicon die
Silicon diaphragm
Height
H
Spacer
L
Normal die down Die down with “spacer” for die isolation:
The extension of the height by the spacer
increases the flexibility and thereby reduces
the parasite thermal stress.
Disadvantage: takes up extra space.

Die protection
● The delicate die in a pressure sensor needs to be protected from possible
damage by the contact pressurized medium.
● There are three (3) ways to do this:
(1) By vapor-deposited organic on the die surface:
The deposited organic coating will insulate the die surface from the contact
medium. Unfortunately the deposited organic also serve as a “reinforcement”
and make the diaphragm undesirably stiff.
Silicon die
Thin organic protective layer
Glass constraint base

Die protection –Cont’d
(2) By coating with silicone gel:
● Silicone gel containing one or two
parts of siloxanes has very low
Young’s modulus. So, it is very soft.
● Being soft, it would not add
unwanted stiffness to the diaphragm.
● A few mm thick coating gives
sufficient protection to the die.
● The only problem is aging and
become contaminated with
impurities from the contact medium.

Die protection –Cont’d
Diaphragm in contact with
pressurized medium
Ball seal
Oil fill
Header
TIG weld
Stainless
casing
Ceramic volume
compensator
(3) Indirect pressure transmission:
● This method is used in situation in
which the pressurized medium is so
environmentally hostile that direct
contact of the die and medium is
not possible.
● A special arrangement is made for
a special case that involved:
● P = 70 kPa – 350 MPa
● Impact force = 10-20,000 g
● T = 5,000oF in milliseconds
● Media contain high-velocity dusts
● Die and wirebonds are submerged in silicone oil.
● Pressure from the media was transmitted to the diaphragm through silicon oil.
● The stainless steel diaphragm has compliance is 100 times less than that of
silicon diaphragm.
● Minimum volume of silicone oil in order to mitigate thermal expansion.

Part 3
Reliability and Testing of Microsystems

Reliability Testing for ICs and Microelectronics
These routine tests are performed before the products are shipped to
the customers:
Thermal Shock Tests
-60oC
+100oC
∆t according to specification Time, t
Temperature,
T(t)
Thermal Cycling Tests
-60oC
+100oC
Time, t
Temperature,
T(t)
∆t1
∆t2
∆t3
Burn-in Tests Products are placed in autoclaves at specified temperatures
and humidity for hundreds of hours for endurance tests.

Reliability of MEMS and Microsystems
Reliability of ICs and microelectronics are more structure-related.
Reliability of MEMS and microsystems have all the issues as in ICs and
microelectronics +
Failure mechanisms for microsystems are much more complicated than those
in microelectronics for the following reasons:
● Microsystem components are designed to interact with various substances
(e.g., optical, chemical and biological fluids) at various environmental
conditions (temperatures and pressures).
● Microsystem components are hermatically sealed and are expected to
perform in immediate and long-terms.
● Example of the damage of stiction of delicate components in sealed plastic
package by slow release of moisture (de-gassing) of plastic encapsulating
materials – impossible to predict and prevent.
Unlike IC and microelectronics, NO standard is available for reliability testing for
MEMS & microsystems.
New testing procedures and criteria need to be developed for every new product.
many more issues related to their performances both upon
shipping and in the subsequent in-service in the designed
life span.

High
Temperature, humidity, dusts and toxic gas
Environmental effects
High
Improper bonding and sealing, poor die
protection and isolation
Packaging
High
Residual stresses and molecular forces
inherent from microfabrication.
Excessive intrinsic
stresses
Moderate
Aging and degassing of plastic and polymers.
Corrosion and erosion of materials
Deterioration of
materials
High
Collapse of electrodes due to excessive
deformation.
Electromechanical
break-down
Low
Moderate
Low in silicon,
moderate in plastic
Moderate to high
High
▪ Local stress concentration due to
surface roughness.
▪ Improper assembly tolerances
▪ Vibration-induced high cycle fatigue
failure.
▪ Delamination of thin layers.
▪ Thermal stresses by mismatch of CTE.
Mechanical
Probability
Causes
Failure Mode
Failure Mechanisms in MEMS and Microsystems

Testing for Reliability of MEMS and Microsystems
Following major tasks are involved:
● Design for testing:
● Set the testing strategy, e.g., identifying testing points.
● Parametric testing
● Testing during assembly
● Burn-in and final testing
● Self testing
● Testing during use
Reference: “MEMS Packaging,” ed. T.R. Hsu, IEE, United Kingdom, 2004.
Chapter 1 Fundamentals of MEMS Packaging by T.R. Hsu & J. Custer;
Chapter 6 Testing and Design for Test by A. Oliver & J. Custer.
Design for Test is an important responsibility of design engineers for
MEMS and microsystems
● Establish Range of Acceptable Device Performance:
● Proper PASS/FAIL limits for test results
● a proper balance between Quality (being too lenient)
and Waste (being too stringent)
● Performing tests:

Testing for Reliability of MEMS and Microsystems – Cont’d
Parametric testing
● For inspecting key components during and after the fabrication.
● Requires the definition of parameters, e.g., film resistances, surface stress/strain
for such testing.
● Requires proper selection of test points on the workpiece.
● Parametric test structures are attached to the workpiece for the testing.
Example 1: The van der Pauw sheet resistance test structure.
Pass current to Pad 2 & 3
Measure voltage across
Pad 1 & 4
The surface resistance
in the area is:
3
,
2
4
,
1
I
V
Rc =

Parametric testing - Cont’d
Example 2: Parametric test structure for measuring tensile strain.
Induced tension
Buckling of
thin beam
ε
π
3
2
2
t
Lc =
The compressive strain ε responsible for the buckling of the thin beam is:
where Lc = length of the beam, t = thickness

Example 3: Parametric test structure for measuring both tensile and
compressive strains.
Beam electrodes are connected and anchored on the workpiece at shallow angles.
Tension gap change in A & B
Compression Gap change in A & C
Associated tensile or compressive strains can be correlated to the measured
capacitances from these beam electrodes.

Example 4: Parametric test structure using resonator for monitoring
surface stresses.
Comb Drives
Springs
Vibrating
Mass
● Change of stiffness of springs
due to change of stresses in
attached workpiece leads to
change of resonant frequencies.
● Resonant frequency of the resonator
can be generated by electrical
stimulator.
● Shifting of resonant frequencies in
the resonator can be related to the
surface stresses in the workpiece.

Testing During Assembly
For two (2) purposes:
● To determine which device components are good enough for further
packaging into devices.
● To monitor the yield of the packaging process.
Example 1: Texas Instrument’s digital micromirror device with 0.5 to 1.5 million electro-
statically actuated mirrors at 16 µm x 16 µm
Micromirrors
Dies
● Mirrors offer 0 or 1 signals on its reflected
intensities.
● The open center in the array shows the CMOS
beneath that supplies voltage to rotate the
mirrors for reflecting lights.
● Mirrors are tested for reflecting lights at
increasing voltage supplies by the CMOS.
● Dies with mirror fails to perform are rejected.
● Further inspection on mirror functions after
dies are assembled.

Testing During Assembly - Cont’d
Example 2: infrared detectors by Dexter Research Center Inc. of Dexter, Michigan
The particular device is thermopile-based single element bulk micromachined infrared
detector for home security, tympanic thermometers, fire detection, and remote temperature
measurement.
● Final assembly of device only after passing all these tests.
Series of tests during assembly:
● Electric parametric tests on wafers on:
e.g., sheet and contact resistances.
● Same tests after bulk manufacturing by wet etching.
● Further Testing on:
● die with infrared black coating
● wafer dicing
● mounting
● wire bonding
● Partially packaged device exposed to calibrated
blackbody infrared source, with further testing on:
coating, wirebond, tilting & mounting.

Burn-in Tests for MEMS and Microsystems
● These are the tests conducted after all components are assembled into a device.
● Some microdevices can only be tested after the assembly.
● “Burn-in” tests are necessary b/c many microdevices can fail to perform due to
invasion of unwanted foreign substances, e.g., air to some packaged infrared
detectors, or dust particles and moisture to the packaged micromirrors.
● A typical failure rate history for a product – a “Bath-tub” curve:
Failure
Rate
Time (in logarithmic scale)
Useful Life
Infant
Mortality
Wear-out
● The GOAL of “Burn-in” tests is to have the “Infant mortality” failure of the
device occurs in the factory, but not in the field.

Burn-in Tests for MEMS and Microsystems - Cont’d
● Requirements for proper design of Burn-in tests:
● Identify possible failure modes of the particular device.
● Identify factors that can accelerate the failure rates of the device.
● Possible factors for accelerating failure rates:
● Mechanical and thermal loading.
● Humidity.
● Shifting of applied threshold voltage.
● Arrhenius model can be used to identify accelerating loading for Burn-in tests.
This model states: “device failure is dependent of the energy barrier surmounted
for failure to occur. “
● This model can relate failure rate of a device at one temperature to the failure
rate at another temperature.
● We may thus accelerate the failure of a device at a higher temperature using
this model.

Self Testing
● For MEMS and microsystems, it involves using electric stimuli that mimics
the real input loads.
● Self testing is important to many electronics devices and computers to ensure
proper functioning of various components in the device before actually using
the device.
Cavity Cavity
Silicon Die
with
Diaphragm
Constraint
Base
Measurand
Fluid Inlet
Measurand
Fluid Inlet
(a) Back side pressurized (b) Front side pressurized
● Self test device, e.g., a pair of electrodes can mimic mechanical load to
micro pressure sensors:

Self Testing - Cont’d
A self testing device for a thermopile-based infrared detector:

Self Testing - Cont’d
Stimuli other than electrical means:
Ambient air for micro pressure sensors.
Earth gravitation for microaccelerometers.
Oxygen content in air for gas sensors.
Testing During Use
It is used for calibrations of microsensors in the designed life span.
Input for these tests usually involve the natural loads as in self testing.
Testing during use ensures the proper functioning of devices for the intended
applications.

Summary on Testing and Reliability of Microsystems
● Failure mechanisms in ICs and microelectronics are primarily attrobuted to
mechanical means, e.g., over-stressing or over-heating.
● Failure mechanisms in MEMS and microsystems attribute to many more
causes:
● Improper interfacing of delicate core components and the working
media can also result in failure of MEMS and microsystems.
● Improper sealing and encapsulation in packaging may cause failure of
microsystems such as by undesired dusts and moisture to delicate core
components.
● Fabrication induced means such as intrinsic and residual stresses
and strains.
● Fault-proof design for reliability for MEMS and microsystems appear unrealistic.
Intelligent testing is a practical approach to insure reliability of these products.

1990 1995 2000 2005 2010 2015 2020
Materials
Equipment
Design & modeling;
Testing
Interconnect
Process
Packaging
Categories
Standards Timeline
Summary on Testing and Reliability of Microsystems – cont’d
● While testing is viewed to be a practical solution to reliability assurance
of MEMS and microsystems, cost for developing effective and reliable
testing remain high – mainly because of lack of standard to follow:
Likely extension

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