Wafer bonding is a process that joins two wafers or substrates permanently or temporarily using suitable techniques. There are several wafer bonding technologies, each with their own operational conditions and advantages/disadvantages. Direct bonding joins wafers without adhesive but requires high temperature and flatness. Anodic bonding also uses high temperature and voltage but produces strong, hermetic bonds. Adhesive bonding occurs at lower temperature but does not produce hermetic seals. The document discusses these various bonding techniques and their applications in areas like SOI wafer fabrication, sensor packaging, and 3D IC construction.

1. WAFER BONDING
TECHNOLOGY
Toya Amechi
Dept. of Electrical and Computer Engineering
Georgia Institute of Technology, Atlanta, GA.

2. INTRODUCTION
Wafer bonding is a process that joins, temporarily or
permanently two wafers or substrates using a suitable
technology.
Importance
• To make silicon-on-insulator (SOI) wafers
• To create complex 3D structures
• Used in packaging to create closed environments
• Integration technology for joining two chips fabricated
separately (CMOS+MEMS)
Paper Objective
To discuss the different technologies used in wafer bonding,
the possible areas of application, operational conditions and
pros and cons.

3. CLASSIFICATION

4. TECHNOLOGIES
• Direct bonding is the joining of mirror-polished
semiconductor wafers without the use of an
adhesive.
• Anodic bonding involves bonding of
semiconductor wafers at slightly elevated
temperature using the assistance of a strong
electrostatic field.
• Thermocompression bonding involves the use
of metals like Au, Cu and Al as intermediate
layers in the bonding process, depending on
inter-diffusion of the metallic atoms at
elevated temperature with applied force.
• Eutectic bonding is a bonding technique
using an intermediate metal layer that can
produce a eutectic system.
• Glass frit bonding involves the use of glass as
an intermediate layer in the bonding process.
The glass is heated until it completely wets the
surfaces to be bonded, creating a sealing
bond.
• Adhesive bonding makes use of an
intermediate polymer layer to create a bond
between two surfaces.

5. APPLICATIONS AREAS
Technology Application
Area
Direct
Bonding
SOI wafer
fabrication
Anodic
bonding
Sensor
packaging
Thermocompr
ession
bonding
Wire-bonding
and flipchip
bonding
Eutectic
bonding
Hermetic
packaging,
bump and
flipchip
Glass frit
bonding
Sensor
packaging
Adhesive
bonding
MEMS,
sensors, 3D IC
packaging and
temporary
bonds.

6. COMPARISON
Technology Typical Bonding conditions Advantages and Disadvantages
Direct bonding 600-1200C
Small to no applied pressure
+strong bond, hermetic, resistant
to high temperature.
-high surface flatness required,
high bond temperature, bad for
electronics.
Anodic bonding 150-500C
200-1500V
No bonding pressure
+strong bond, hermetic, resistant
to high temperature.
-high temp. with high voltage,
not good for electronics.
Thermocompression bonding 350-600C
100-800MPa (high bond pressure)
+hermetic, compatible with
electronic wafers.
-very high force and high flatness
required.
Eutectic bonding 200-400C
Low to moderate bond pressure
+strong bond, hermetic,
compatible with electronics.
-sensitive to surface oxide.
Glass frit bonding 400-1100C
Low to moderate bond pressure
+strong bond, hermetic.
-high temperature not good for
electronic wafers.
Adhesive bonding Room Temp.-400C
Low to moderate bond pressure
+strong bond, low temp. works
with any substrate material.
-not hermetic, limited
temperature stability.

7. WAFER BONDING AND DEVICES

8. CONCLUSION
The different wafer bonding technologies find their
uses in different areas of microelectronics fabrication
and packaging, depending on the requirements and
materials to be bonded.
While the technologies are functional, further research
is being carried out on various ways to improve on the
already existing technologies.