5. NEMS Switch Fabrication
(a) Silicon chip with 500 nm of thermally grown oxide, 20 nm of tungsten, and PMMA. (b) Electron beam
lithography was used to define features in the PMMA layer. An ICP etch was used to pattern the tungsten and
etch down into the oxide. (c) A Cr/Au layer was evaporated and lifted off by removing the tungsten. (d) DEP was
performed to assemble a small bundle of nanotubes traversing the trench between the two side electrodes.
8. Accelerometers
Types of Accelerometers
How Surface Micromachined Capacitive
Accelerometers Work
Tilt Sensing with Accelerometers
9. Accelerometer
• An accelerometer measures proper acceleration, which is the
acceleration it experiences relative to freefall and is the
acceleration felt by people and objects.
• Conceptually, an accelerometer behaves as a damped mass on
a spring. When the accelerometer experiences an acceleration,
the mass is displaced to the point that the spring is able to
accelerate the mass at the same rate as the casing. The
displacement is then measured to give the acceleration.
• In commercial devices, piezoelectric, piezoresistive
and capacitive components are commonly used to convert the
mechanical motion into an electrical signal.
10. Types of Accelerometers
Peizo-film – used in
AC applications to
sense motion,
sound, temp. and
pressure
Surface
Micromachined
Capacitive – used
in DC applications to
measure tilt,
vibration, and inertial
11. Types of Accelerometers
Bulk Micromachined
Capacitive – used in
DC applications to
measure tilt, vibration,
and inertial
Electromechanical
Servo – DC
applications to
measure tilt and inertial
Piezo-electric – AC
applications to
measure vibration and
shock
12. How It Works – Surface
Micromachined Capacitive
Suspended Beam held in
place by tethers
Acceleration in either
direction caused movement
in the beam
With movement of beam,
capacitance between plates
changes
The difference is measured
and the direction and force
are determined
13. Tilt Sensing
Accelerometer
measures the static
gravity field
Acceleration of
9.8m/s = 1g
Changing the tilt
(along the sensitive
axis) changes
acceleration vector
14. ADXL202AE
Operating Temperature
– -40 to 85°C
Voltage – 3.0 to 5.25V
Current – Typical
0.6mA Max 1.0mA
Output – Digital – Duty
Cycle Modulated or
Analog
Weight – less than 1
gram
15. Accelerometers are used to measure the motion
and vibration of a structure that is exposed to
dynamic loads
Human activities – walking, running, dancing or skipping
Working machines – inside a building or in the surrounding area
Construction work – driving piles, demolition, drilling and excavating
Moving loads on bridges
Vehicle collisions
Impact loads – falling debris
Concussion loads – internal and external explosions
Collapse of structural elements
Wind loads and wind gusts
Air blast pressure
Loss of support because of ground failure
Earthquakes and aftershocks
18. 18
Physical principles of pressure
sensing Static pressure: defined as the force exerted perpendicularly on a unit area
Volume pressure: defined as mass per unit volume
Liquid pressure: e.g a person lies inside then bathtub
19. 19
Typical characteristics of
piezoelectric transducers
Transfer function
Sensitivity: the change in output resulting from a change in input
-33pC/bar
Operating temperature range:
-20 to 350 degrees
Measuring range:
0 to 300 bar (non-SI unit, like Pa)
20. Pressure Sensors
•Absolute – A Sensor That Measures Input Pressure in
Relation to a Zero Pressure.
•Differential – A Sensor That Is Designed to Accept
Simultaneously Two Independent Pressure Sources. The
Output Is Proportional to the Difference Between the Two
Sources.
21. Pressure Sensor Theory
Two Main Types of Pressure Sensors
Capacitive Sensors
• Work based on measurement of
capacitance from two parallel
plates.
• C = εA/d , A = area of plates d =
distance between.
• This implies that the response of a
capacitive sensor is inherently
non-linear. Worsened by
diaphragm deflection.
• Must use external processor to
compensate for non-linearity
22. Pressure Sensor Theory
Piezoresistive Sensors
Work based on the
piezoresistive properties of
silicon and other materials.
Piezoresistivity is a response to
stress.
Some piezoresistive materials
are Si, Ge, metals.
In semiconductors,
piezoresistivity is caused by 2
factors: geometry deformation
and resistivity changes.
Reference: http://en.wikipedia.org/wiki/Piezoresistance_Effect
23. Pressure Sensor Theory
Our Sensor is a
Piezoresistive
Sensor based on a
Wheatstone Bridge
Configuration.
Resistors are made
with Boron Diffusion.
24. Pressure Sensor Theory
Vout =Iin*∆R
Why use a Constant
Source Bridge?
Produces Linear
Output
Neglects Lead
Resistance
R + ∆R R - ∆R
R - ∆R R + ∆R
26. Pressure Sensor Process
Overview – Step 1
What should ALWAYS be step 1?
Wafer Cleaning (RCA Clean)
Steps
1. TCE (Tetrachloroethylene) Immersion, Acetone, Methanol
2. Base Clean - H2O/H2O2/NH4OH (5 parts,1 part,1 part) @
70 C to Remove Organic Contaminants
3. Dilute HF Immersion (2.5%) Why?
4. Acid Clean - H2O/H2O2/HCl (4 parts, 1 part, 1 part) @ 70 C
to remove metallic and ionic contaminants.
27. Pressure Sensor Process
Overview – Step 2
Any guesses?
Thermal Oxidation
Wet Oxidation
Followed by Dry
Oxidation
Si + O2 → SiO2 (Dry Oxidation)
Si + 2H2O2 → SiO2 (Wet Oxidation)
28. Pressure Sensor Process
Overview – Step 3
Photolithography for
Piezoresistive Elements
Contact Lithography
Use Shipley 1813
Positive Resist
What happens to areas
exposed to UV light in
Positive Resist?
Si + O2 → SiO2 (Dry Oxidation)
Si + 2H2O2 → SiO2 (Wet Oxidation)
29. Pressure Sensor Process
Overview – Step 3 Cont.
DNQ Method using
Mercury Lamp
Diazonap. Changes to
carboxylic acid via Wolf
re-arrangement
Carboxylic Acid is more
soluble in a base than
Novolak. So exposed
areas dissolve.
Use TMAH (a base)
mixture to develop
Ref: http://chem.chem.rochester.edu/~chem421/polymod2.htm
30. Pressure Sensor Process
Overview – Step 4 - Diffusion
Creates Resistors in
Substrate
Three Methods
1. Solid Evap. (Tetramethyl
Borate, Boron Nitride) - Rare
2. Gaseous – Diborane (B2H6) –
Dangerous!! 160 ppm for 15
min life threatening
3. Liquid – Our Type PBF-6MK –
Borosilicate polymer in
ethanol. Creates borosilicate
glass, boron oxide, and
unused boron.
5 Squares
1 Square
3Squares
Ref: Jaeger, Richard. “Introduction to Microelectronic Fabrication”
31. Pressure Sensor Process Overview –
Step 5 – Backside Photlithography
Windows Must Be
Opened in New Oxide
For Backside Etch.
Use Front to Backside
Alignment
Etch Silicon Dioxide
w/BOE (HF 6:1)
Finished when wafer is
hydrophobic (water rolls
off)
33. Pressure Sensor Process Overview –
Step 8 Metal Deposition and Pattern
Several Methods, we
use Sputtering
2 Types (Magnetron)
-RF Sputter
-DC Sputter
http://en.wikipedia.org/wiki/Sputtering
34. Pressure Sensor Process Overview –
Photolithography and Aluminum Etch
First Photoresist is
deposited on metal
and patterned for
desired traces
Uses Aluminum
Etch, 85-95%
Phosphoric Acid, 2-
8% Nitric Acid, and
Water.
35.
36. Pressure Sensor Process Overview –
Wire Bonding and Packaging
Several Types – Ball, Wedge, etc.
Heated gold wire is pressed onto surface, melted,
and then cooled.
37. 37
Applications of pressure sensors
Touch screen devices
Automotive industry
Biomedical
Aviation
Marine industry