This lecture covers pressure sensor theory, fabrication processes, and safety concerns. It discusses two main types of pressure sensors - capacitive and piezoresistive - and describes the process used to create a piezoresistive pressure sensor using boron diffusion on silicon wafers. Key steps include cleaning, oxidation, photolithography, diffusion, backside etching, contact patterning, and anodic bonding. Safety hazards from chemicals like hydrofluoric acid are also addressed.

1. Pressure Sensor Lecture

2. Presentation Overview
1. Pressure Sensor Theory Overview
2. Pressure Sensor Process Overview
and Fabrication Theory Review
3. Express Safety Concerns for Particular
Processes

3. 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

4. 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

5. Pressure Sensor Theory
 Our Sensor is a
Piezoresistive
Sensor based on a
Wheatstone Bridge
Configuration.
 Resistors are made
with Boron Diffusion.

6. 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

7. Pressure Sensor Process
Overview
 Initial Wafer is 525
µm thick, n-type,
<100> double-side
polished (DSP).

8. 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.

9. 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)

10. 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?
 If we want piezoresistive
holes, do we use dark-
field or light-field mask?
Si + O2 → SiO2 (Dry Oxidation)
Si + 2H2O2 → SiO2 (Wet Oxidation)

11. 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

12. 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”

13. Pressure Sensor Process Overview –
Step 4 – Diffusion Continued
 What is a constant source diffusion?
 What is a limited source diffusion?
 What is drive-in?
 How can you get rid of boron oxide
and borosilicate glass?
 What additional step does this
create?
Surface
Concentration
Junction Depth

14. 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)

15. Pressure Sensor Process Overview –
Step 5 – Backside Photlith. Cont.
 Must Align Marks
from 1st
Mask on
Front of Wafer to
Those on Back
 What is Split Field
Alignment?
 What is a critical
dimension?
Photo:
http://www.ee.byu.edu/cleanroom/alignment.phtml

16. Pressure Sensor Process Overview –
Step 6 – Backside Etch
 Need 20 µm Thick
Diaphragm, therefore
must etch approx. 500
µm.
TMAH/IPA KOH
25% wt.
TMAH
45% wt KOH
17% vol IPA
70o
C 75o
C
{100} 12 µm/hr 21 µm/hr
{111} 0.7 µm/hr < 0.05 µm/hr
SiO2 <0.01 µm/hr < 0.20 µm/hr
Ref: Crain, Mark. “Powerpoint Thesis Defense”

17. Pressure Sensor Process Overview –
Step 7 – Pholith. For Contact
Windows
 Topside Alignment
 Use Shipley 1813
Postive Resist
Ref: Crain, Mark. “Powerpoint Thesis Defense”

18. Pressure Sensor Process Overview –
Step 8 Metal Deposition and Pattern
 Several Methods, we
use Sputtering
 2 Types (Magnetron)
-RF Sputter
-DC Sputter
 When do you use RF
sputter?
 What is a sputter etch?
 What is argon used? http://en.wikipedia.org/wiki/Sputtering

19. 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
 Why in the picture is there a
hole in the metal?
 What is the difference
between lift-off and metal
etch?
 Must Thermal Anneal After
Etch, Why?

20. Pressure Sensor Process
Overview – Wafer Testing
5 Squares
1 Square
3Squares
R=ρlw/xj
Rs=ρ/xj
R=(squares)ρ/xj
Contacts Equivalent: Rs=0.65 squares
Resistive Element: 6.3 squares

21. Pressure Sensor Process
Overview – Wafer Testing
 2 Testing Structures
 Van Der Pauw
-Contacts on Structure
Edge, Symmetrical
Rs = (π/ln 2)Vcd/Iab
 Kelvin Structures
-Used for Effective Line
Width with Rs
Weff = Iab*L*Rs/V
A B
D
C

22. Pressure Sensor Process
Overview – Anodic Bonding
 Negative Polarity
 Why?
 Positive Polarity is
faster.
 High Temperature, High
Voltage
 Na+ ions moved from
interface, leaving
Oxygen and forming
SiO2.

23. Pressure Sensor Process Overview –
Wire Bonding and Packaging
 Several Types – Ball,
Wedge, etc.
 Heated gold wire is
pressed onto surface,
melted, and then
cooled.

24. Process Safety
 Hydrofluoric Acid
- 20-50% Solutions
May Produce No
Immediate Symptoms
- 2.5% Produce
Hypocalcemia
-Fatal Accidents Below
10%
http://www-safety.deas.harvard.edu/advise/accident.html

25. Process Safety
 Protect Your
Hands!!!
 Avoid Placing Hands
Near Wafer
 Be Aware of Those
Working Near You
 Communicate
http://www.emedicine.com/emerg/topic804.htm