By:- Nishant Malik
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Introduction to wafer bonding
Classification
Fusion Bonding
Anodic Bonding
Thermocompression Bonding
Eutectic Bonding
Glass Frit Bonding
Adhesive Bonding
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Comparison
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Wafer bonding is a process that temporarily
or permanently joins two wafers or substrates
using a suitable process.
Microtechnology involves various types of
bonding processes
◦ Wafer bonding
◦ Wire bonding
◦ Flip chip bonding
Used in IC packaging and printed circuit
board assembly. Will not be covered in this
lecture.
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To make Silicon on Insulator (SOI) wafers.
To create complex 3D structures and cavities
which create device functionality (µ-channel).
Packaging method to create closed environments
(vacuum packages for resonators, and for optical
and IR devices).
Integration technology for joining
fabricated separately (CMOS + MEMS).
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chips
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Wafer Bonding
No Intermediate layer
Fusion Bonding
Anodic Bonding
Intermediate layer
Metallic
Insulating
Thermocompression
Bonding
Si/Glass Frit Bonding
Eutectic Bonding
Adhesive Bonding
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Identical materials with ultra smooth (<10Å
roughness) are bonded without intermediale
layer.
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Method
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Mechanism
• Surface preparation: O2 plasma, HF dip, hydration.
• Wafers brought into contact to form weak bonds (van
der wall forces, hydrogen bonding).
• Annealing at 600-1200 oC to convert physical forces to
chemical bonds.
• Hydrophilic => Si – O – Si
• Hydrophobic => Si – Si
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(a) Hydrophilic bonding
(b) Hydrophobic bonding
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Vacuum level ~ 10-5 mbar
Temperature ~ 500oC
Force ~ 1000N
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Advantages:
• No problem of thermal expansion mismatch at the
interface.
• The bond strength is a function of annealing
temperature and can be as high as single crystal silicon.
• For certain applications such as micro-turbine
application, fusion bonding can be used at much higher
operating temperatures.
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Disadvantages:
• Stringent surface roughness requirements to the order
of a fraction of a nm, also surfaces must be ultra-clean.
• Annealing of fusion bonded wafers above 450ºC cannot
be performed if the substrate contains CMOS devices.
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Anodic bonding is also referred to as Electrostatic Bonding or
Field-Assisted Thermal Bonding. This bonding process is
assisted by electric field. In this method we bond a
conductive silicon substrate to a glass substrate which is rich
in sodium.
In 1969, Wallis and Pomerantz invented the concept of glassmetal seal bonding using assisted electric field.
Technique
• Voltage applied ~ 600-1000 V
• Elevated temperature ~ 200-500°C
• Positive ions in glass migrate, creating depletion layer near Si surface;
voltage drop creates large E-field pulling surfaces into contact.
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Silicon and glass wafers are aligned and maintained in intimate
contact.
Silicon substrate is the anode and the glass metallization is the
cathode.
The resultant applied electric field drive Na ions from the glass
silicon interface.
The disassociated oxygen ions react with the silicon to form an
irreversible silicon– oxide bond.
The primary variables in this process are time, temperature, and
voltage.
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Advantages:
• Bonding is achieved at lower temperatures, 200–450 °C, which in
turn gives more design flexibility.
• It facilitates hermetic sealing in vacuum.
• Very high strength permanent bond can be achieved with relative
ease.
• Parasitic capacitances are small since glass has low permeability
and is usually quite thick.
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Disadvantages:
• Bow or warp in the wafer, which may be associated with the
considerable volumetric changes undergone by the glass during
anodic bonding and thermal mismatch of the substrates.
• Anodic bonding generates outgassing of oxygen in particular.
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Thermocompression bonding is a technique
for wafer packaging, using metals such as Au,
Cu and Al as the bonding layer between two
silicon wafers.
Method:
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•
•
•
Metal deposited on substrates.
Bring them into intimate contact.
Apply temperature and pressure.
Diffusion of metal atoms between the surfaces, due
to atomic contact between both substrates.
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Aluminum (Al)
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Gold (Au)
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Copper (Cu)
• bonding temperature 400 to 500 °C.
• Applied force above 70 MPa
• Bonding temperature 260 and 450 °C
• Applied force ~ 20 MPa
• Bonding temperature 380 to 450 °C
• Applied force above 15-30MPa.
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SiO2 can be used a diffusion barrier layer.
Annealing can be done to increase the bond
strength after bonding for short duration.
Native oxide from the Cu and Al surface can be
removed by forming gas treatment.
Surface roughness of wafers can be reduced by
chemical mechanical planarization (CMP).
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Advantages:
• Good level of hermeticity can be achieved.
• Metal good conductor of heat.
• Allows electrical contact and mechanical support in
one step.
• Easy and clean process.
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Disadvantages:
• High pressure requirements.
• Flat surface requirements.
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Eutectic bonding is a wafer bonding technique with an intermediate metal
layer that can produce a eutectic system.
Those eutectic metals are alloys that transform directly from solid to liquid
state, or vice versa from liquid to solid state, at a specific composition and
temperature without passing a two-phase equilibrium, i.e. liquid and solid
state.
Liquid and two solid phases co-exist in equilibrium at the eutectic
composition CE and the eutectic temperature TE .
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The fact that the eutectic temperature can be much lower than the melting
temperature of the elements is of importance.
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Method:
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•
•
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Substrate processing
Conditioning prior to bonding (e.g. oxide removal)
Bonding process (Temperature, Mechanical Pressure for a few minutes)
Cooling process
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Eutectic reaction – transition
from liquid to mixture of two
solid phases, α + β at eutectic
concentration CE.
At most two phases can be in
equilibrium. Three phases
(L,α, β) may be in equilibrium
only at a few points along the
eutectic
isotherm.
Single
phase regions are separated
by 2-phase regions.
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Eutectic alloy
Eutectic composition
Eutectic temperature
Au-In
0.6 / 99.4 wt-%
156 °C
Cu-Sn
5 / 95 wt-%
231 °C
Au-Sn
80 / 20 wt-%
280 °C
Au-Ge
28 / 72 wt-%
361 °C
Au-Si
97.15 / 2.85 wt-%
370 °C
Al-Ge
49 / 51 wt-%
419 °C
Al-Si
87.5 / 12.5 wt-%
580 °C
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Advantages:
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good reliability
large fabrication yield
low demands on surface topography and roughness
simple process technology
high bonding strength
relatively small seal ring geometries
small device feature size possible
Disadvantages:
• different CTE of intermediate layer and wafer material
• restrictions of widespread connections caused
mechanical stress
by
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Glass frit bonding, also referred to as glass soldering or seal glass bonding,
describes a wafer bonding technique with an intermediate glass layer.
Glass frit bonding can be used for many surface materials, e.g., silicon with
hydrophobic and hydrophilic surface, silicon dioxide, silicon nitride, aluminum,
titanium or glass, as long as the CTE are in the same range.
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The glass between the wafers to be bonded is heated, so that its viscosity
continually decreases until the so called wetting temperature is reached.
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The glass comes into contact with the surface to be bonded at the atomic level,
and flows into surface roughness and around surface steps.
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Method:
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•
•
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Deposition of glass paste
Thermal conditioning
Bonding process
Cooling down
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The glass frit is a paste consisting glass
organic binder, inorganic fillers and solvents.
powder,
The physics of glass frit bonding is related to the
glass chemistry .
When the glass wets the surface to be bonded at high
temperature, thin layers of the material to be bonded
are fused into the glass, so that finally after cooling, a
continuous material interaction occurs which forms a
strong bond.
Optimal process temperature & pressure(insufficient
spreading/overflow).
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Advantages:
• simple process technology
• connection of different materials (different melting
temperatures)
• connection of rough surfaces
• absence of voltage during bonding
• hermetic sealing
• high bonding yield
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Disadvantages:
• risk of mechanical stress caused by CTE mismatch
• risk of glass material flowing into structures
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Adhesive bonding (also referred to as gluing or glue bonding)
describes a wafer bonding technique with applying an
intermediate layer to connect substrates of different materials.
The commercially available adhesive can be organic or inorganic
and is deposited on one or both substrate surfaces.
Adhesives especially the wide established SU-8 and
Benzocyclobuten (BCB) are specialized for MEMS or electronic
component production.
Method:
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Cleaning and pre-treatment of substrates surfaces
Application of adhesive, solvent or other intermediate layers
Contacting substrates
Hardening intermediate layer
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Adhesion layer
Softbake
Hardening of the adhesives are possible:
•at room temperature
•through heating cycles
•using UV light
•by applying pressure
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 Adsorption theory,
wetting of a surface.
based
on
 Like most bonding techniques,
based on the fact that atoms and
molecules adhere to each other
when they are brought in
sufficiently close contact.
 For wetting to occur, the solid
surface must have a greater
surface energy than the liquid.
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Advantages:
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wide range of adhesives adjusted for MEMS and electronic components
feasibility at vacuum or different atmospheric gases
simple and low cost process
low bonding temperature ≤ 200 °C
absence of electric voltage and current
applicable to various wafer materials
compensation of surface non-uniformities and contamination
high transparency
Disadvantages:
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penetration of moisture
no hermetic seals with organic materials
limited long-term stability in harsh environments
limited temperature stability
relatively low bond strengths
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Andreas PloÈûl*, Gertrud KraÈuter, Wafer direct bonding: tailoring adhesion between
brittle materials, Materials Science and Engineering, R25 (1999) 1-88 .
Shawn J. Cunningham and Mario Kupnik, Chapter 11Wafer Bonding, MEMS Materials and
Processes Handbook.
G. Wallis, D.I. Pomerantz: Field-assisted glass-metal sealing, J. Appl. Phys. 40, 3946–
3949 (1969).
Ji Fan and Chuan Seng Tan, Chapter 4 Low temperature wafer-level metal
thermocompression bonding technology for 3D integration, Metallurgy-Advances in
Materials and Processes.
Sumant Sood, Shari Farrens, Ron Pinker, James Xie and Wilbur Cataby, Al-Ge Eutectic
Wafer Bonding and Bond Characterization for CMOS Compatible Wafer Packaging, ECS
Transactions, 33 (4) 93-101 (2010).
Peter Ramm, James Jian-Qiang Lu, Maaike M.V. Taklo, Handbook of Wafer Bonding.
F. Niklaus, G. Stemme, J. -Q. Lu, and R. J. Gutmann, Adhesive wafer bonding, J. Appl.
Phys., 99, 2006.
Roy Knechtel, Glass frit bonding: an universal technology for wafer level encapsulation
and packaging, Microsyst Technol (2005) 12: 63–68.
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