Pressure Sensor Lecture ECE544 Prepared by D.J. Tegtmeier

Presentation Overview Pressure Sensor Theory Overview Pressure Sensor Process Overview and Fabrication Theory Review Express Safety Concerns for Particular Processes

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

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

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

Pressure Sensor Theory Vout =Iin*∆R Why use a Constant Source Bridge? Produces Linear Output Neglects Lead Resistance R + ∆R R - ∆R

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?

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 part) @ 70 C to remove metallic and ionic contaminants.

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)

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)

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

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”

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

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)

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

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 < 0.05 mm/hr SiO2 <0.01 mm/hr < 0.20 mm/hr Ref: Crain, Mark. “Powerpoint Thesis Defense”

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”

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

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?

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

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

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.

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

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

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