Experiment 10 - Vacuum Testing
Purpose
The purpose of this lab is to test your micro-fabricated devices under vacuum
conditions to see if they function as pressure sensors as designed.
Introduction
In this experiment, a pressure differential will be placed across the micromachined
diaphragm of your MEMS pressure sensor and its response will be evaluated. To do
so, a special test jig has been designed which exposes the backside of your device
to vacuum. The top side of the diaphragms will remain at atmospheric pressure.
The small vacuum pump used for this experiment is capable of producing
approximately -23 inches of Hg of vacuum (absolute vacuum is equal to -29.92
inches of Hg).
Equipment and Tools
• Probe Station
• Curvetracer
• Voltage Source Power Supply
• Digital Multimeter (DMM)
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Keithley 2400 Source Meter (?)
High Power Optical Microscope
Vacuum Pump
Vacuum Regulator
Materials and Supplies:
• Device Wafers
• Wafer Tweezers
SOPs
• None
PRE_LAB ASSIGNMENT AND QUESTIONS
(answer questions in your Lab Notebook and then submit a photo-copy to
the TA at the start of this Lab)
1. None
LAB PROCEDURES
(make comments and observations in your Lab Notebook)
PART 1 – EVALUATION OF THE PIEZORESISTORS TO VACUUM
1. Align one of your MEMS pressure sensor diaphragms over the vacuum port of the
test jig.
2. Place probes on one of the PZRs (piezoresistors).
3. Check response on the Curvetracer for good ohmic behavior.
4. Read its resistance value on the DMM.
5. Turn on the Vacuum Pump and expose it to full vacuum. Record the vacuum
level from the pressure gauge. Record the PZR’s new resistance value. Determine
∆R (sign and magnitude).
6. Repeat the above procedure for the remaining 3 PZRs.
7. If you are not pleased with this die, proceed to another.
8. Once you have found an acceptable sensor, generate a Response Graph for one
of the Longitudinal PZRs (PZR Resistance vs Vacuum) by slowly increasing the
vacuum using the vacuum regulator (perhaps every 5 inches of Hg). Sweep both
up and down.
9. Repeat for one of the Transverse PZRs.
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PART 2 - EVALUATION OF THE WHEATSTONE BRIDGE TO VACUUM
1. Turn off the vacuum and then hook up your device as a Fully Active Wheatstone
Bridge using a DC Voltage Power Supply and DMM.
2. Apply 1 volt DC.
3. Record the Bridge Offset Output Voltage.
4. Slowly increase vacuum and generate an Output Response Graph (Output
Voltage vs Vacuum). Sweep both up and down.
POST-LAB ASIGNMENT AND QUESTIONS
(answer questions in your Lab Notebook and then submit a photo-copy to
the TA at the start of the next Lab)
1. Using your measured PZR values, calculate your theoretical Offset Voltage using
circuit analysis techniques. Compare to your experimental value.
2. Referring to one of your previous homework problems in ECE543, calculate the
expected % resistance change and Vout for our Pump's full vacuum and your
design. Assume a 20 micron diaphragm unless told otherwise bt the TA. Compare
your experimental result with your theory.
3. Curve fit a straight line to each of your 3 graphs to determine pressure
sensitivity of each PZR element (ohms per inches of Hg) and your Bridge (mV per
inches of Hg).
4. I hope you learned alot and found this lab experience enjoyable. Congrats on
fabricating successful sensors!!!!!!!!!!!!
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