Flex those displays
An exclusive interview with a test engineer
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2006
Ed Bawolek is a test engineer at Arizona State University's Flexible Display Center (Tempe, AZ, flexdisplay.asu.edu), where engineers develop MOSFET arrays on plastic substrates that drive a display's pixels. Member companies such as General Dynamics and Honeywell integrate the displays with drive electronics and cases to form wearable displays. Initial applications are being designed for the US Army. Bawolek recently described his test responsibilities to senior technical editor Martin Rowe.
Q: How is a flexible display made?
A: We use a deposition process to build MOSFETs onto the plastic substrate. The substrate sits on a 150-mm silicon wafer that's used as a carrier. We deposit five to six layers of metal on the substrate to build the MOSFET array. We must run the process at a low temperature, typically 180°C, to avoid melting the plastic.
Q: What's your role at the Display Center?
A: I handle all the test responsibilities for the flexible displays. I make performance measurements on fabricated MOSFETS and provide feedback to the process engineers.
I provide data on drive current, threshold voltage, and channel mobility by measuring I DS vs. V DS and by performing I DS vs. V GS sweeps. The measurements are difficult because of the wide range of drive current—from microamps to femtoamps. That's nine orders of magnitude. (Click here to view a plot of I DS vs. V DS for a MOSFET.)
I also measure the ratio of a MOSFET's I DS on current to its off current. Finally, I measure how the MOSFET responds to the polarity of V GS. These devices have a hysteresis when the gate voltage swings from negative to positive and back to negative.
Q: How do you make the measurements?
A: I assembled an automated test station consisting of a semiconductor parameter analyzer and an old wafer probe station, built in 1986. Before I could use the probe station, I had to repair it and modify it to handle 150-mm wafers. It was designed for 100-mm wafers. The upgrade included replacing the original black-and-white cameras with color cameras. The system can photograph probe marks after testing a display to prove that it made good electrical contacts. I use the photographs as a process control for the test station.
A colleague wrote a new motion-control algorithm for the stages so I can control them from a PC. We also developed an alignment and pattern-recognition algorithm so the probes could land in the proper locations.
Next, I defined the tests and determined what data I needed to capture. We wrote code to capture data from the component analyzer and export the data to Excel for analysis and plotting.
The project took about three months to complete. I spent half the time on hardware, and a colleague spent six weeks writing the code. I've since made modifications to the code that adds functionality.
Q: What were you able to accomplish with the automated test station?
A: I spent the first 18 months of the project taking data that process engineers used to refine the fabrication process. We now have four stations in operation: two for production, one for electrical stress tests, and one for test development. The wafer probers range in age from 5 to 20 years.
We plan to move from 150-mm silicon wafers as carriers to 370-mm x 470-mm glass sheets, because the sheets let us build larger displays or test more displays at once. Now, we can produce transistor arrays on substrates for member companies.
