Part V:
Fabrication of Microelectronic Devices
and Micromanufacturing
Group 5
Josh Agenbroad
Joy Best
Iris Gallegos
Keith Griego
November 21, 2005
Fabrication of Microelectronic Devices
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The invention of the Transistor in 1947 was the catalyst for
Microelectronics
Microelectronics are now more abundant in our lives since
Integrated Circuit (IC) technology became the foundation for
calculators, robotics, space travel, weaponry, wrist watches,
and automotive controls.
ICs are very small, and low cost so they have allowed for
manufacturers to make smaller and smaller products.
The semiconducting material that the IC is made on is called a
Chip, which typically hold from 10 million devices to 100 million
devices, a huge improvement from 100 devices.
Clean Rooms
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ICs are typically a few
millimeters in length, and the
smallest feature maybe as
small as a few nanometers.
Clean rooms are need to
keep dust and other
contaminates from coming
into contact with the IC.
Clean rooms are classified
from 1 to 10. Class 10 has
10 or fewer particles per
cubic foot. Hospitals= 10K
particles per cubic foot.
Semiconductors and Silicon
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Silicon has surpassed Germanium as the industry standard for
several reasons:
There is an abundance of silicon
Silicon has a larger energy gap than germanium,
which allows silicon to operate at high
temperatures (270F vs. 180F)
Silicon dioxide is a great electrical insulator, which
is the bases for Metal-Oxide-Semiconductors
used for memory devices, processors, the largest
semiconductor material produced worldwide.
Silicon has a diamond type crystallographic structure
Crystal Growing and Wafer Preparation
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Take Silica and corbon and
heat together in a furnace =
95-98% pure Polycrystalline
silicon.
Convert this into
trichlorosilane, which is
purified making electronicgrade silicon (EGS)
EGS goes thru the CZ
process, which produces a
crystalline cylinder ingot
This is then sliced into
wafers
Wafers
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The process is as
follows to the right:
Film Deposition
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Films of many different types are used extensively in microelectronicdevice processing, particularly insulating and conducting films.
Common deposited films include polysilicon, silicon nitride, silicon
dioxide, tungsten, titanium, and aluminum.
Epitaxy is defined as the growth of a vapor deposit, occurring when the
crystal orientation of the deposit is related directly to the crystal
orientation in the underlying crystalline substrate.
Techniques for film deposits include evaporation, sputtering, chemical
vapor deposition, lower chemical vapor deposition, vapor-phase
epitaxy, liquid phase epitaxy, molecular beam epitaxy.
Oxidation
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Like film deposition,
Oxidation is a process of a
growth of a layer of oxide
layer. These layer will have
less impurities in them than
the deposition layers.
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Dry oxidation
Wet oxidation
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Lithography
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Lithography is the process by which the geometric patterns
that define devices are transferred to the substance.
Most common technique would be photolithography, however
Electron beam and X-ray lithography are the most useful for
miniaturization of ICs.
Lithography
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Photolithography is limited
to the wavelength of light
being used.
X-ray is superior to
Photolithography due to the
shorter wavelength, which
allow for finer patterns.
Electron beam and Ion
beam are even better
allowing for finer detail to the
pattern.
Etching
Is the process
by which entire
films or
particular
sections of
films are
removed, and it
plays an
important role in
the fabrication
sequence.
Wet Etching
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Wet etching is the removal of material by immersing the wafer
in a liquid bath of chemical etchant. There are two kinds of wet
etching etchants, isotropic etchants and anisotropic etchants:
- Wet etching works very well for etching thin films on substrates,
and can also be used to etch the substrate itself.
I
Effective etching requires the following conditions:
1) Etchant transport to the surface.
2) A chemical reaction.
3) Transport of reaction products away from the surface.
4) Ability to stop the etching process rapidly in order to obtain
superior pattern transfer(etch stop).
Isotropic and Anisotropic Etching
Anisotropic etchants are used for:
Isotrophic etchants are used for:
• Terminating crystal planes with little
undercut
• Removal of damaged surfaces
• Vertical etching
• Rounding of sharp etched corners to
avoid stress concentrations
• Creating structures in single-crystals
• Evaluating defects.
Dry Etching
• What is dry etching?
- Materials removed from reactions that occur in the gas
phase. The dry etching process requires only small
amounts of the reactant gases, whereas the solutions used
in the wet-etching process have to be refreshed periodically.
Types of dry etching
- Sputter etching
- Plasma based dry etching
Dry Etching Advantages
• Eliminates handling of dangerous acids and solvents
• Uses small amounts of chemicals
• Isotropic or anisotropic etch profiles
• Directional etching without using the crystal orientation of Si
• Faithfully transfer lithographically defined photo resist patterns
into underlying layers
• High resolution and cleanliness
• Less undercutting
• No unintentional prolongation of etching
• Better process control
• Ease of automation (e.g., cassette loading)
Chemical vs. chemical/physical
etching
Purely chemical etching
(using only reactive
neutral species)
Isotropic etching
- Makes the surface more
reactive.
- Clears the surface of reaction
products and allows the
chemically reactive species to
access the cleared areas.
- Also provides the energy to
drive surface chemical
reactions, however the neutral
species do most the etching.
Physical-chemical etching is
anisotropic. This tecnique allows
the generation or near vertical
walls with very large aspect
ratios. Since the ion
bombardment does not remove
material directly, masks can be
used. Chemical + physical etching
(using reactive neutral species
and ionic species)
Anisotropic etching
This simple device is known
as junction diode. This is
the fabrication sequence of
the conductor. These are
mostly semiconductor
devices.
Metallization and Testing
In the previous
sections we talked
about device
fabrication.
However
generating a
functional circuit the
devices must be
interconnected.
Here is the process
of metallization and
testing. This really
means the final
product.
Wire Bonding and Packaging
This is a
schematic
illustration of
thermosonic
welding of gold
wires from
package which
leads to
bonding pads.
Micromechanical Devices
Small scale like
microelectronic
 3D instead of 2D
 Scale of Micrometers
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MEMS
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Micro-Electo-Mechanical Systems
Include both mechanical features and electric circuits
A gyroscopic sensor for use in
automobiles. Sensors are a
common application of MEMS.
MEMS
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Manufactured using similar techniques to
microelectrical devices.
Do no have to be semiconductor based.
High wear/adhesion problematic with silicon.
MEMS
Microdevices
generated from
3-D CAD data
Bulk Micromachining
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Traditional and simple technique for
micromaching.
For simple shapes
3D
Bulk Micromachining
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Diffuse dopent to desired pattern
Deposit masking film
Etching leaves elevated cantilever
Micro Surface Machining
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More complicated designs
Uses a sacrificed spacer layer along with
bulk machining techniques
Accelerometers, pressure sensors, micro
pumps, micro motors, and actuators.
Micro Surface Machining
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Deposition of spacer layer
(phosphosiliate glass)
Etching spacer layer
Deposition of polysilicon
Etching of polysilicon
Selective wet etching
removes spacer layer
Bulk & Surface Micomachining
Example
Micro Lamp
Stiction
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Traped liquid meniscus after spacer layer is removed
due to surface tension at small scale.
Causes deformation or damage
Folding
Deployed folding mirror
Folding
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Deposition of spacer layer followed by polysilicon hinge piece
Deposition of second spacer layer
Selective etching of spacer layer
Deposition of polysilicon staple
Selective etching of spacer to allow rotation
Hinge can be set in folded position and will remain due to high
adhesion of silicon at small scale
SCREAM
single crystal silicon reactive etching and metalization
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1-3 Standard lithography & etching 10-50 micrometers
Vapor deposited silicon oxide
Anisotropic etching removes bottom
Dry plasma etching extends cavities
SIMPLE
Silicon Micromachining by Single Step Plasma-Etching
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Single step process to create deep cavities like
those created with SCREAM
Uses plasma etching with layers of various doping
Etching Combined with Diffusion
Bonding
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Very complex shapes
with large suspended
sections.
The LIGA Microfabrication
Process
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LIGA is a German acronym for the combined
process of x-ray lithography, electrodeposition, and
molding.
LIGA Microfabrication Process
LIGA Process
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Final product may consist of:
- A free-standing metal structure resulting
from the electrodeposition process.
- A plastic injection-molded structure.
- An investment-cast metal part using the
injection-molded structure as a blank.
- A slip-cast ceramic part produced with the
injection-molded parts as the molds.
Multilayer X-Ray Lithography
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Powerful technique in producing MEMS devices
with large aspect rations and reproducible
shapes.
HEXSIL
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This process combines hexagonal
honeycomb structures, silicon
micromachining, and thin-film deposition to
produce high aspect-ratio, free-standing
structures.
HEXSIL can produce tall structures with a
shape definition that rivals LIGA.
HEXSIL Process
Solid Free-Form Fabrication of Devices
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Another term for rapid prototyping.
Rapid prototyping automates the making of a
prototype.
It builds a prototype part from a 3-D CAD
drawing
Automotive
Medical
Entertainment
Methods
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Microstereolithography
- Similar to stereolithography but laser is more highly
focused (to a diameter as small as 1 µm), as
compared to 10-100 µm.
- Layer thicknesses are around 10 µm, which is an
order of magnitude smaller than in stereolithography.
- Support structures are not needed since the
smaller structures can be supported adequately by
the fluid.
Electrochemical Fabrication
(EFAB)
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Is the solid free-form fabrication of MEMS devices
using instant masking.
The Instant Mask consists of an insulator
patterned on an anode.
Instant Masking patterns a substrate by pressing
the mask against the substrate, electrodepositing
material through apertures in the insulator and
then removing the mask from the substrate.
EFAB Process
(a). A blanket of
material is rapidly
deposited
(b). The layer is
planarized
(c). The process is
repeated to create
multiple layers
(d). A selective chemical
etch removes
sacrificial material.
Electrochemical Fabrication
(EFAB)
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The result is a layer rapidly
deposited and patterned in a
single step.
The process is significantly
faster than photolithography
and makes it possible to
fabricate MEMS devices with
dozens of patterned layers in
a single day, compared with
several weeks for a
conventional 3-layer MEMS
device.
References
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http://www.devicelink.com/mem/archive/02/10/bang.html
Kalpakjian, Serope, and Schmid, Steven R. Manufacturing
Engineering and Technology. Prentice-Hall, Fifth Edition.
Lindbeck, John R. Product Design and Manufacturing.
Prentice-Hall, 1995
http://geae.com
http://boeing.com