Download Chapter 29 MEMS

Chapters 28 and 29
April 10, 2006
Group 5
Michael Linley
Matt Sallee
Bryan Estvanko
Patrick Pace
Clean Rooms
Typical integrated circuits
are a few millimeters in
length and transistor on
the circuit can be only a
few nanometers.
Clean Room - a room
where contaminants
(dust, smoke, perfume,
and bacteria) are reduced
to a very low level by
special procedures so
that operations can be
performed effectively.
This is an enlarged image of
a grain of salt on a piece of a
microprocessor.
Clean Rooms
There are many different level or classes of clean
rooms. The system of classification refers to the
number of .5 micrometers (10^-6) or larger particles
in within a cubic foot of air.
The controlled atmosphere in a clean room is done
with ventilation which passes all the air through
high-performance particulate air (HEPA) filters.
Most clean rooms for microelectronics
manufacturing are from class 10 to class 1.
Clean Rooms
Clean Rooms
The greatest source of contaminants in
clean rooms, comes from the people who
work in them.
Skin particles, hair, makeup, clothing and
bacteria are given off by humans
constantly, due to this fact special covering
are required such as lab coats, gloves,
hair nets and avoidance of perfume and
makeup.
Clean Rooms
The more stringent clean rooms require
people to wear bunny suits.
Semiconductors and Silicon
Semiconductors are the foundation for
electronic devices because their properties
can be control by adding impurity atoms.
Silicon has become the industry standard
in the fabrication of these devices because
it’s the second most abundant atom and it
can operate at higher temperatures than
the alternative germanium.
Silicon is a diamond –type fcc structure. Miller indices are
used to identify the planes and directions within a unit cell.
Crystal Growing and Wafer Preparation
Single-crystal material is needed for
semiconductor device production.
Single-crystal silicon is usually obtained
through the Czochralski or CZ process.
Crystal Growing and Wafer Preparation
A seed crystal is dipped
into melted silicon, then
is pulled out slowly
while being rotated .
Controlled amounts of
impurities can be added
to obtain a uniformly
doped crystal.
Usually grown larger
then needed and
ground down to a
precise diameter
Film Deposition
Films are used widely in microelectronicdevice fabrication, especially insulating
and conducting films.
Epitaxy - is the growth of a vapor deposit,
epitaxy or electrodeposit happens when
the crystal orientation of the deposit is
related directly to the crystal orientation in
the crystalline substrate.
Film Deposition
Advantages of processing on the
deposited films instead of on the wafer are
fewer impurities, improved performance
and the tailoring of material properties.
Major functions of the films are masking
and protecting the semiconductor surface.

In masking The film effectively must inhibit the
passage of dopants and display the ability to
be etched into patterns of high resolution.
Film Deposition
Methods of film deposition
Evaporation, Metal is heated in a vacuum
to its vaporization point. Upon evaporation
the metal forms in thin layers on the
substrate surface.
Sputtering, bombarding a target with high
energy ions in a vacuum. As ions impinge
on the target, atoms are knocked off and
deposited on the wafers.
Film Deposition
Chemical vapor deposition (CVD), film
depositing is achieved by way of reaction or
decomposition of gaseous compounds.
Oxidation
Growth of an oxide layer as a result of the
reaction of oxygen with the substrates
material.
Can also be formed using deposition
techniques
Thermally grown oxides (dry, wet, and
selective) have higher level of purity than
deposition
Dry Oxidation
Simple Process
Elevated Temperatures (usually
around 750º to 1100º C (1300º to
2200º F))
Ratio of oxide thickness to the
amount of silicon consumed is
1:0.44
Wet Oxidation
Utilizes water vapor atmosphere as
an agent.
Higher growth rate than dry oxidation.
Lower oxide density (therefore lower
dielectric strength)
Dry-Wet-Dry
Selective Oxidation
Oxidizes only certain portions of
the surface
Uses silicon nitride.
Lithography
Process by which the geometric patterns
that define devices are transferred to the
substrate surface.
Types: Ultraviolet (Photolithography), Xray, and Electron beam.
Ultraviolet (Photolithography)
Most widely used
Uses reticle (photomask or mask)
Reduction lithography
Photoresist
Prebaking/Postbaking
Stripping
Ashing
Line Width
Extreme Ultraviolet
Light diffraction
Wavelength of 13 nm
Features in range of 30 to 100 nm
Highly reflective molybdenum –silicon
mirrors.
X-ray Lithography
Superior to photolithography
Aspect ratio of more than 100
LIGA Process
Electron-beam Lithography
Superior to Photography
Resolution limited to 10 nm
Very expensive
Slower in Scanning time
SCALPEL
Scattering with angular limitation
projection electron-beam
lithography
Small sized marks
High potential
Lithography Techniques
Method
Wavelength (nm)
Finest Feature Size (nm)
Ultraviolet (Photolithography)
365
350
Deep UV
248
250
Extreme UV
10 to 20
30 to 100
X-ray
0.01 to 1
20 to 100
-----
80
Electron beam
Etching
Process by which entire films or
particular sections of film are
removed.
Selectivity
Wet etching and dry etching
Wet Etching
Immersing the wafers in a liquid
solution, usually acidic.
Isotropic
Effective etching requires certain
conditions.
Isotropic Etchants
Many different uses
Strong acids
Size of the features
Anisotropic Etching
Takes place when etching is strongly
dependent on compostional and structural
variations in the material.
Orientation-dependent etching and Vertical
etching
Anisotropy ratio (AR = E1/E2)
Etch stop (Boron etch stop)
Back etching
Dry Etching
Involves the use of chemical reactants in a
low-pressure system.
High degree of directionality, resulting in
highly anisotropic-etching profiles
Requires small amounts of the reactant
gases.
Usually involves a plasma or discharge in
areas of high-electric and magnetic fields.
Sputter Etching
Removes material by hitting it
with noble gases (usually argon).
Concerns with sputter etching.
Reactive Plasma Etching
Also called dry chemical etching
Involves chlorine or fluorine ions and other
molecular species that diffuse to and
chemically react with substrate, forming a
volatile compound which is removed by
the vacuum system.
The Process
Physical-Chemical Etching
Two types: Reactive ion-beam etching (RIBE)
and Chemically assisted ion-based etching
(CAIBE)
Combines physical and chemical etching
Ion bombardment helps assist dry chemical
etching
Ion bombardment is directional, ionbombardment energy is low and does not
contribute much to mask removal
Very large aspect ratios
Cryogenic Etching
Deep features with vertical walls
Low temperatures
The Process
Diffusion and Ion Implantation
Diffusion
800º to 1200º C (1500º to 2200º F)
Drive-in diffusion
Ion Implantation
400º to 800º C (750º to 1500º F)
28.10 Metallization and Testing
Interconnections- are made with metals that exhibit low electrical
resistance and good adhesion to dielectric insulator surfaces.

Aluminum and Aluminum-copper alloys are most commonly used
for interconnections.
Electromigration- process where aluminum atoms are moved
physically by the impact of drifting atoms under high-current
conditions.

Solutions

a) addition of sandwiched metal layers such as tungsten or
titanium.

b) use pure copper which has better properties to resist
electromigration.
Connection levels
Level 0:
transistor within an IC
Level 1:
ICs, other discrete components
Level 2:
IC packages
Level 3:
printed circuit boards
Level 4:
chassis or box
Level 5:
system or computer
Interconnection patterns are produced by etching or lithographic
processes.
•Modern ICs normally have one to six layers of metallization and
each metal layer is insulated by a dielectric.
•Planarization- process of producing a planar surface.
– Planarization is critical for the dielectric layers to reduce
metal shorts and linewidth variations of the interconnections.
–Done by chemical-mechanical polishing (CMP)
–Accurate to 1.2 X 10-6 in of being perfectly flat.
•The layers of metals are then connected by vias.
•Access to these devices is made possible through contacts.
•The wafer is complete once a layer of silicon nitride is added to
provide scratch resistance.
Testing of the dies
Next, all the individual
currents on the chip are
tested by a small probe in
one of two ways:
1.
Test patterns or
structures: the probe
measures test
structures on scribe
lines that record various
bits of information.
2.
Direct probe: uses
100% testing on the
bond pads of each die.
After Testing
If a chip fails a test it is marked with a drop of ink.
Back grinding may take place to remove some of the
original substrate.
After back grinding, diamond sawing is used to cut each
die out of the wafer.
The dice are then sorted by functional and non
functional.
The non functional dice are discarded while the
functional dice are sent for packaging.
28.11 Wire Bonding and Packaging
Working dice must be attached to a more secure
foundation.
The die is then attached to the packaging with
epoxy cement or by an eutectic bond.
After being attached, the chip must be
electrically connected to the substrate.
Wire bonding: is the process of attaching very
thin gold wires from the packaging lead to the
bonding pads located on the chip.
Final Packaging
The chip is now ready for final packaging.
There are many ways the chip can be packaged
and the type of packaging largely impacts the
cost of the chip.
Package considerations: chip size, number of
external leads, operating environment, heat
dissipation, and power requirements.
Packages can be produced from polymers,
metals, or ceramics.
Metal Packages
Produced from alloys like Kovar.
Provide a hermetic seal and good thermal
conductivity.
Limited number of leads that can be used.
Plastic Packages
Very inexpensive and have high lead counts.
Have high thermal resistance but not
hermetically sealed.
Ceramic Packages
More expensive
Usually produced from Al2O3.
Have hermetic seal and have good
thermal conductivity.
Higher lead counts than metal and lower
lead counts than plastic.
Kinds of Packages
Dual-in-line Package (DIP)

Low cost ease of handling

Modular design
Flat Package



Flat, low profile, leads on same plane
Permanente, not Modular
Used in multiple-level circuit boards
Surface Mount Package

Chip is slightly above board and soldered to it
Pin-Grid Arrays (PGAs)
Tightly packed pins that connect by way of
through-holes onto printed circuit boards.
Susceptible to plastic deformation
Solution: Ball-Grid Arrays (BGAs)
Many tiny little balls connected on the
underside of the package that are evenly
spaced.
The package is reflow soldered to the
circuit board, adding rigidity.
More Testing
After the chip is sealed in its
package it undergoes final
testing.
The tests make sure the
chip is sealed off from the
environment.
The testing tests for: heat,
humidity, mechanical shock,
corrosion, and vibration.
Destructive tests are also
performed to investigate the
effectiveness of sealing.
28.13 Printed Circuit Boards
A printed circuit board is the substrate for the
final interconnections among all of the
completed chips.
Circuit boards can contain: resistors, capacitors,
inductors, high-performance, transistors, large
capacitors, precision resistors, and crystals.
A printed circuit board is a plastic material
containing several layers of copper foil.
There are single sided boards, double sided
boards, and multi-layer boards.
Building the Boards
The boards are sheared to a desired size and
locating holes are drilled or punched in them.
Holes for vias or connections are drilled by a
CNC. Multiple boards can be drilled at one time
due to the precision of the CNC.
The conductive patterns are defined by
lithography (originally screen printed).
Methods of Laying Conductive Paths
Subtractive method: copper foil bonded to the
board. Desired pattern is defined with a positive
mask by photolithography and the rest is
removed by wet etching.
Additive method: a negative mask is placed on
the board to define the desired shape.
Electroless plating and electroplating of copper
serve to define the connections, tracks, and
lands on the circuit board.
Design Considerations for PCBs
1. Wave soldering should be used only on
one side of the board.
2. IC packages should be laid out carefully
on the printed circuit board.
3. Allow room between ICs to provide
sufficient cooling requirements.
4. Leave enough room around ICs to allow
for reworking and repairing without
causing more damage.
Chapter 29
MEMS
microelectromechanical
devices and systems
What are they?
They are devices the size of a pinpoint that combine
electrical and mechanical elements such as:
SMALL ELECTRIC TWEEZERS
SMALL SENSORS
(( Micrometer = 1x10^-6 m ))
MANUFACTURING PROCESS
Bulk Micro-Machining
 Older method but still prevalent.
 Simpler Shapes
Surface Micro-Machining
 This method has more steps, and can create
more complex shapes.
The processes in CH28 are semiConductor based, MEMS do not have the
Same restriction.
Bulk Machining
Uses orientation dependant etching, and
single crystal silicon.

Some enchants will eat along the planes of a
crystal.
Diffuse with p-type silicon
Place a mask – silicon nitride
Etch the mask
And silicon base
Block to leave
cantilever
Surface Micromachining
Deposit phosphosilicate glass by chemical
Vapor deposition, can be removed by HF acid
Put on a mask, etch the PSG, then
Remove the mask.
Deposit Polysilicon, which is the finished
Shape.
Remove the unwanted PSG to finish the
Part.
Surface Micromachining produces more complex parts.
Applications
Hinges can be used for such applications
As micromirrors for use in High Definition
TV’s.
Applications
Micro-grabbers and tweezers
Movie of MEMS in action