mems and microsystem fabrication materials

1. UNIT – 3
Microsystems Design and
Packaging

3. • MEMS involves delicate components
with sizes in the order of micrometers
• These components are vulnerable to
malfunction and or structural damage
if they are not properly packaged
• Purpose of IC packaging is to protect
the silicon chip and the associated wire
bonds from environmental effects
• Not just protecting but also probe the
environment at the same time…
• Eg. Sensors are required to sense the
pressure and composition

4. Purpose of Packaging
• Mechanical Support
• Electrical Connections
• Protection of the delicate integrated
circuits from all possible attacks by
mechanical and environmental sources
• Removal of heat generated by
integrated circuits
• Power delivery to constituents
• Signal mapping from outside to within
and between the constituents

7. https://youtu.be/g5cpWSkQbsI
https://youtu.be/Hgv_DGTkrEE

14. Encapsulation protect device packages
by
Isolating the active devices from
environmental pollutants
(chemical protection),
and
at the same time offering mechanical
protection

15. Encapsulation- Two Types
• Ceramic
– Hermetic Sealing( Air tight or vaccum)
– High resistance to environmental effects
– Greater durability
– However they are costly to produce
• Plastic
– Less expensive
– Vulnerable to degradation due to moisture and
temperature effects
– Currently Plastic encapsulated microsystems PEMs make
up to 95% of microelectronic chips in the global market

18. • Major Challenge
• Core elements ---microsensors,
actuators…. Usually involve delicate
complex three dimensional geometry
made of layers of dissimilar materials
• Often required to interface with
environmentally unfriendly working
media such as hot pressurized fluids or
toxic chemicals----
• Yet are expected to generate a variety
of signals ----mechanical, optical,
biological, chemical…..

20. • Proper packaging of MEMS and
microsystems products is a critical
factor in the overall product
development cycle…
• Principal design requirements that
should be analysed are

22. Die is mounted and fixed to
the package or support structure

24. Die Level Packaging
• Involves the assembly and protection
of many delicate components in
microdevices
• Cantiliver beam of an actuator
• Sesimic mass assembly in the
accelerometer
• Electrodes in a micromotor
• Microvalves in Microfluidic
components
• Micropumps
• Microchannels

26. • Wire Bonds for Electronic signal
transmission and transduction
• Such as the embedded piezoresistors in
a pressure sensor die
• And
• The circuit that connects them

27. Similarities b/w packaging of MEMS
and Microsystems
• Both use Silicon Die
• Die attaches are involved
• Wirebonds between die and
interconnect
• Hence Microelectronic packaging in
these areas can be used for the MEMS
packaging.
• Encapsulation of dies and wire bonds
in microsystems packing can vary
from plastic to stainless steel casings

28. Device Level Packaging
• Requires proper signal conditioning and processing
• Most cases involves ……
• Electric bridges
• Signal conditioning circuitry for sensors and
actuators
• Proper regulation of input electric power is always
necessary

34. Mechanical Interfaces
• Design issue with moving parts in
MEMS
• These parts need to be interfaced with
their driving mechanisms which may
be….
• Thermal, Fluidic, Magnetic Such as
microvalves and pumps.
• Improper handling of the interfaces
may cause serious malfunctions or
damage to these device components

35. Essential Packaging Technologies
• Die Preparation
• Surface Bonding ---Adhesives,
Soldering, Anodic Bonding, Silicon
Fusion Bonding, Silicon on Insulator,
Low Temperature surface bonding with
lift-off Processes
• Wire Bonding—Thermocompression
Wire bonding, Wedge-wedge Ultrasonic
Bonding, Thermosonic bonding
• Sealing---- Sealing by microshells,
Reactive sealing technique

36. Die Preparation
• It’s a rare practise to use an entire
Silicon wafer to produce just one die of
a MEMS or microsystems
• Or
• Use one wafer to produce one device
• In reality….
• Hundreads of tiny dies required for
various parts of microdevices are
produced from a single wafer
• May have same type and size—Batch
production
• Have different sizes and shapes

37. • Wafer Dicing----Sawing the wafer along
the dotted lines will produce individual
dies
• Dicing--- Mount the wafer on a sticky
tape
• Saw Blade--- made of a diamond/resin
----diamond/nickel composite
material
• Method adopted--- Cutting between the
dies imprinted on the silicon wafer
• Dies are spaced according to the thickness
of the saw blades

39. Surface Bonding (Difficulties)
• MEMS and Microsystems are of three
dimensional geometry
• Made up of layers of dissimilar materials
• Many contain fluids or Environmentally
hostile substances
• Hermetic Sealing(Air Tight) of these
media is required for many of the
bonding surfaces
Expectation of a MEMS product
• Bonded dissimilar materials are expected
to simultaneously achieve hermetic
sealing and provide flexibility at the
sealed surface for die isolation

41. Adhesives
• Primarily used for attaching dies onto the
supporting constraint bases
• Example--- Silicon die in a pressure sensor
attached to the glass constraint base.
• Two common adherents-----
• Epoxy resins and silicon rubbers
• Epoxy resins---flexibility for the bonded dies.
• ---Good Sealing
• Vulnerable to thermal
environments( Bonding should be
kept below the glass transition
temperature---150’C-175’C)
• Good bonding depends on proper surface
treatment and control of the curing process

42. • If flexibility of bonding surfaces is a
primary requirement-----Soft adhesives are
used
• Room temperature vulcanising Silicone
Rubber(RTV)….This material cures at
room temperature.
• Soft nature of this adherent makes it the
most flexible for die bonding and thus
provides the best die isolation
• Unfortunately the chemical resistance of
this type of material is not as good ..
• Peeling and flaking develop when it gets
in contact with air…
• Not suitable for high-pressure
applications

43. Soldering
• Bonding of a Silicon die to the base uses
Eutectic Solders…
• Mixture of substances in specific proportion
Melts or Solidifies at a single temperature
• Lower than the melting points of separate
constituents
• Super lattice formation(Periodic structure
of Layers)
• Chemically inert

44. • Provides stable and hermetic seals
• Good bond requires- intimate contact of
the bonding surfaces (closely aquantined)
• Thin films of gold ---fractional
micrometers thick---are plated on the
surfaces of both the bonded parts by a
sputtering process
• Normal melting temperature of the gold-
silicon system is 370’C

45. • Solder alloy-60Sn-40Pb(Tin-Lead Solder)
• Nearly eutectic, with a solidus
temperature of 183’C
• Major shortcoming- Vulnerability to
creep at elevated temperature.
• (Time dependent deformation of a
material that is subjected to a constant
load, and occurs after the initial elastic
deformation produced by the initial
loading. ---Adhesive suffers creeping
not the adherends)

46. • Creeping takes place at a temperature
above half of the homologous melting
temperature
• Temperature of a material as Fraction
of its melting point temperature using
the kelvin scale…. Lead at 25’C-----
0.50(298K/601K)

47. • Silicon-Gold solder bond systems, the
corresponding homologous melting
temperature-370’C or 643K
• Half melting temperature for Solder
alloy - 322K or 72’C
• A soldered silicon bond operating at
72’C or above will be subjected to creep
deformation

50. Anodic Bonding
• Process is reliable and effective for
attaching silicon wafers to thin glass or
quartz substrates
• It provides a hermetic seal and is an
inexpensive method of die bonding
• Silicon wafer is placed on the top of a
thin glass constraint base

51. • A dc voltage source about 1000V is
applied across the set at a temperature
between 450 and 900’C.
• Major drawback-low aspect ratio of the
bonded compounds
• A Silicon wafer 500 micrometer was
successfully bonded to 750 to 3000
micrometer thick Pyrex glass substrate

53. Silicon Fusion Bonding( SFB)
• Is an effective and reliable technique
for bonding two silicon wafers or
substrates without the use of
intermediate adhesives
• 1960-concept of joining two silicon
wafers was first developed for bonding
discrete transistor chips
• 1988- Its application in MEMS was
first reported

54. • Process begins---
• Thorough cleaning of the bonding surfaces
• Surfaces must be polished, then made
hydrophilic(mixeddissolvedwetted by
water) by exposure to boiling nitric acid
• Two surfaces are naturally bonded even
at room temperature.
• Strong bonding occurs at high
temperature in the neighborhood of 1100
to 1400’C…
• Its considered a chemical bonding process
and surface treatment is critical to the
success of such bonding

56. Silicon on Insulator
Silicon is a Semiconductor- It has the
ability to conduct electricity when it
is subjected to high electric potentials
or at elevated temperature above
125’C
This limits Silicon sense elements to
being effective in elevated
temperature applications

57. • Solve the issue---Involves bonding the
silicon with an amorphous material
such as SiO2 ---epitaxial crystal growth
• An amorphous ( without a clearly
defined shape or form) material does
not have long-range order or crystal
structure.

58. Process
• Uses two silicon substrates
• one with one of its surfaces heavily doped
with boron atoms to produce a layer of p-
silicon
• Other with a thin silicon oxide film on one of
its faces
• The two substrates are then mounted one on
the top of the other.

60. • Process of silicon fusion bonding joins the
two substrates together
• Bonded substrates are then exposed to
etching
– to etch the exposed surfaces of the bonded
substrates.
• Heavily p-doped region can act as an etch
stop.
• Consequently one may obtain either
• A p-silicon layer on the SiO2 insulator or
• Sandwiched silicon substrates with SiO2
insulator in between
• Bonding-and-etchback---------
– Technique which involves bonding of
substrates followed by etching the bonded
substrates

61. Silicon Fusion Bonding
Or
Direct Bonding

62. • Instead of doping with boron
– to form a p-silicon layer in the so-called donor
wafer,
• one may implant H2 ions into the
contacting surface.
• More Active hydrogen ions
– allow the donor wafer to be bonded to the
receptor wafer with SiO2 grown on its
surface at a lower temperature.
– There is no loss of materials in etching
• compared with the bonding-and-etch
back process

63. How to introduce a SiO2 insulation layer
over a microchannel.
• The substrate is called the receptor
wafer
– contains an embedded microchannel of 1
micrometer depth
• A 60nm thick oxide film
– thermally grown over the surface of the
donor wafer.
– It is then implanted with H+ ions
– with a dose of 8X1016/cm2 at 40eV of
energy

65. • Two wafers are then bonded face to
face at room temperature
– after thorough cleaning of the surfaces.
• Bonded pair is heated to 470’C .
• Temperature is sufficient
– to cause a crack at the peak of the
implanted hydrogen region.
• Cracking at this face
– results in the bonded insulator adhering to
the receptor wafer after being separated
from the donor wafer.
• SiO2 film insulator is produced to cover the
Silicon Substrate, creating an embedded
channel

66. Low Temperature Liftoff Process
• Silicon fusion bonding and SOI
techniques…..
– Both involve processes that require high
temperatures(500˙C)
– Results in adverse effects
– Hence the need of bonding techniques that
can be carried out low or moderate
temperatures
– Epitaxially grown thin films are bonded to
the receptor substrate by applying
mechanical pressure
– Lift off process is used to separate the thin
film from the donor substrate by passing a
UV laser beam

67. How to bond Gallilium Nitride to Silicon
wafer at 200˙C
• Commonly used in micro optical devices
• GaN-3µm thick--- deposited on a Sapphire
Alumina(Al2o3) donor substrate
• Thin Palladium (Pd)—100nm is deposited
on GaN---followed by indium 1µm thick
• Pd adheres well to most semiconductors
• In has a low melting point 100-200˙C
• Hence can be readily bonded to most
semiconductors and polymers
• PdIn3 alloy melting point-664-710˙C

69. • Receptor wafer-Silicon ---deposited with
Pd film
• Receptor ----Donor-----placed one on top
of the other----applied pressure 3MPa---
200˙C
• Bonding-30 minutes
• Bonding ready for liftoff process
– using low power excimer KrF pulse laser
– 600mJ/cm2
– Krypton fluoride laser
– 248nm wavelength
– 38ns pulse width
– deep ultra violet laser

70. • Laser radiation results in fracture at
the interface of the GaN and Al2O3
sapphire
• This successfully bonds GAN to a
Silicon wafer
• Bonding agent--- PdIn3---Palladium
Indium
• -----Ag??? Al???? Au??? Cu??? Pt???

71. Guess their Automic Number
• Gold-Au-Aurum-79
• Silver-Ag-Argentum-47
• Aluminium-Al-13
• Cu-copper-29
• Pt-Platinium-78
• Wedge Shape---

73. 400C wire tip turns into the shape of a ball
40ms

74. Bonding tool travels in parallel to the bonding
pad, lowered to pads surface to make bonds
Ultrasonic energy is released to break down the
contact surfaces and achieve the desired surface
bonding

75. Difference Between Wedge and Ball Wire Bonding

76. For MEMS applications wire Bonding is often
combined with Ball-Wedge joints or Wedge –
Wedge Joints

79. Reactive Sealing Technique

80. Involves stacking of IC and multichip modules(MCMs) in compact configurations

81. Lab –on -a -chip

82. Assembly of MicroSystems
• Assembly of micro systems with
components
• who’s sizes range from 1µm to a
few millimeters
• presents major challenges to
engineers in terms of
• reliability and cost
• Some of the reasons for the high
cost associated with the
microassembly are:

83. Assembly of MicroSystems
• Lack of standard procedures
and rules for assemblies…
– Either based on customer
requirements
– Experience of the design engineer
• Lack of effective tools for micro
assembly
– Tools such as microgrippers,
manipulators and robots are still
being developed

84. Microassemblies require
– reliable visual and alignment
equipment
– stereo electron microscopes and electron-
beam
– UV simulated beam
– Ion beam imaging systems
– Which are specially designed for micro
system assemblies

85. • Lack of established methodology in
setting proper tolerances of parts in
insertion and assemblies
Many sets of tolerances are involved in assemblies
• Strategies for setting proper tolerances for
– Parts feeder
– Grasping surface to binding surfaces
– Fixtured surface to binding surface…..
– Have not been established for micro assembly at
the present time

86. • Microcomponents to be assembled are
…
– mostly made by physical-chemical
processes that have strong material
dependence
Traditional assembly techniques are not
suitable for microdevices because of
– Minute size of the components
– Close tolerances in the order of sub
micrometer
– Chemical and electrostatic forces dominate
at micro assemblies…
– Whereas…
– Gravity and physics are primary
consideration in macroassembly

87. • Lack of effective tools and assembly strategies
– has resulted in lengthy time required for microsystem
assembly
– Figure shows a hand assembled gear train…
– One may well imagine the length of time required
– for even highly skilled personnel
– to manually assemble these minute machine
components

88. Similarities between microassembly
and traditional assembly
• Part feeding- tape
bonded feeders
• https://youtu.be/qH6
V-7I6HBA

89. Similarities between microassembly
and traditional assembly
• Part grasping- Micro
grippers,
manipulators and
robots are desirable
tools ….
• https://youtu.be/d5u
wcmDrXWQ

90. Similarities between microassembly
and traditional assembly
– However these tools cannot function
properly for handling minute parts
without intelligent end effectors
– An intelligent end effector requires the
integration of gripping, positioning,
sensing and orientation for accurate
alignment in microscale of binding parts

91. • Part mating
– At the micrometer scale, electrostatic and
chemical forces dominate the interaction
between grippers
– And parts, as well as binding parts
– Special design of grippers
– that can discharge these forces for possible
stiction is necessary
– for easy releasing of parts from the gripper
and for the binding of small components

92. • Part bonding and fastening
• Sensing and verification
Three-dimensional machine vision systems
suchas stereo microscopy are effective for visual
identification of parts
and for part alignment.
Other types of microsensors, e.g., tactile and
thermal sensors, are also required in assembly
and inspection.
Near- and far-field infrared (IR) sensors can be
used for microthermal feedback for monitoring
welds or solder joints.