1. Pressure Sensor Lecture
ECE544
Prepared by D.J. Tegtmeier

2. Presentation Overview
Pressure Sensor Theory Overview
Pressure Sensor Process Overview and Fabrication Theory Review
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:

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

7. Pressure Sensor Process Overview
Initial Wafer is 525 µm thick, n-type, <100> double-side polished (DSP).
Why use a <100> DSP wafer?
Why use an n-type substrate?

8. Pressure Sensor Process Overview – Step 1
What should ALWAYS be step 1?
Wafer Cleaning (RCA Clean)
Steps
TCE (Tetrachloroethylene) Immersion, Acetone, Methanol
Base Clean - H2O/H2O2/NH4OH (5 parts,1 part,1 part) @ 70 C to Remove Organic Contaminants
Dilute HF Immersion (2.5%) Why?
Acid Clean - H2O/H2O2/HCl (4 parts, 1 part, 1 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
Why do Wet Oxidation followed by Dry?
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:

12. Pressure Sensor Process Overview – Step 4 - Diffusion
Creates Resistors in Substrate
Three Methods
Solid Evap. (Tetramethyl Borate, Boron Nitride) - Rare
Gaseous – Diborane (B2H6) – Dangerous!! 160 ppm for 15 min life threatening
Liquid – Our Type PBF-6MK – Borosilicate polymer in ethanol. Creates borosilicate glass, boron oxide, and unused boron.
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:

16. Pressure Sensor Process Overview – Step 6 – Backside Etch
Need 20 µm Thick Diaphragm, therefore must etch approx. 500 µm.
Why use TMAH instead of KOH if KOH is faster?
TMAH/IPA
KOH
25% wt. TMAH
45% wt KOH
17% vol IPA
70oC
75oC
{100}
12 mm/hr
21 mm/hr
{111}
0.7 mm/hr
< mm/hr
SiO2
<0.01 mm/hr
< 0.20 mm/hr
Ref: Crain, Mark. “Powerpoint Thesis Defense”

17. Pressure Sensor Process Overview – Step 7 – Pholith
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?

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
Contacts Equivalent: Rs=0.65 squares
Resistive Element: 6.3 squares
R=rlw/xj
R=(squares)r/xj
Rs=r/xj

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
% Solutions May Produce No Immediate Symptoms
- 2.5% Produce Hypocalcemia
-Fatal Accidents Below 10%

25. Process Safety Protect Your Hands!!! Avoid Placing Hands Near Wafer
Be Aware of Those Working Near You
Communicate