Performing On-Wafer Testing with NI LabVIEW, NI TestStand, and PXI
"Using LabVIEW and NI TestStand, we developed the characterization system from scratch in approximately four man-months. "
- Sudhir Gopinath,
Proteus Biomedical Inc.
Developing a reliable on-wafer test system for microelectromechanical system (MEMS) devices used for cardiac resynchronization therapy (CRT).
Using the National Instruments PXI platform and NI TestStand and LabVIEW software to develop a highly reliable on-wafer test system to ensure our devices comply with all U.S. Food and Drug Administration (FDA) regulations.
Sudhir Gopinath - Proteus Biomedical Inc.
We at Proteus Biomedical Inc., headquartered in Redwood City, California, are pioneering the field of intelligent medicine by integrating our proprietary in-body computing and sensing platforms into existing pharmaceutical and device therapies.
The Proteus CRT
We are developing a novel pacing and sensor-enabled device in partnership with a world leader in cardiology for CRT. Fabricating approximately 4,000 of these devices on an 8 in. wafer, we build this device with MEMS manufacturing technology using techniques similar to those in integrated circuit manufacturing. To test these devices, we developed an on-wafer test system using National Instruments hardware and software.
CRT systems are implanted in and interact with the human heart. The degree of reliability required of a life-saving implanted medical device is much greater than that required of almost any other semiconductor or MEMS application. Thorough testing of the product evaluates not only its reliability but also its behavior during failure. Some tests, such as a corrosion test, are designed to run for years, and serve to qualify the product for the next level of tests, such as animal tests. FDA test standards are justifiably rigorous and require that every step of the test process be documented, reviewed, validated, and verified.
We perform the first set of tests directly on the wafer immediately after fabrication by using a prober, which is common in the semiconductor industry, to make electrical contact with the devices on the wafer. We perform the following three main types of on-wafer tests:
1.Functional tests – ensure the device functions as expected when supplied with inputs that fall within specified ranges
2.Characterization tests – observe the behavior of the device when the various inputs are simultaneously varied across a range of values within and outside the device’s specified operating ranges
3. Production tests – sort the devices in a production facility before they are diced and packaged
For the most part, the production tests and the functional tests are subsets of the characterization tests.
Developing the Characterization Test System
We perform characterization tests primarily to validate the design of the device.
We required a characterization test system possessing the ability to:
• Interface with a probe station
• Build complex digital and analog waveforms (with frequencies as high as 50 MHz) that can be modified during the test
• Synchronize signals and supply them to the device
• Capture waveforms and analyze them on the fly
• Perform various current, voltage, and impedance measurements
• Connect different points on the MEMS device selectively to the instruments that perform the above functions
• Log all test information, such as test details, input parameters, and results, to a database
Test Solution Using National Instruments PXI Hardware
We developed a test solution using multiple NI hardware components. First, we connected the PXI-1045 chassis to the PC using a MXI bus. Then we made all connections between the prober and the devices on the PXI chassis via the NI PXI-2530B switch matrix, which allowed us to selectively connect different devices on the PXI chassis to points on the device. We then synchronized the NI PXI-5412 arbitrary waveform generator, the NI PXI-6541digital waveform generator, and the NI PXI-5122 digitizer using the PXI trigger bus. We performed measurements at different points on the device using the NI PXI-4071 digital multimeter (DMM). We programmed some of these measurements so that the DMM and the switch matrix triggered each other, resulting in fast, hardware-triggered measurement cycles.
Developing Characterization Test Software Using LabVIEW and NI TestStand
The automated characterization test software has three main levels, with each level of code calling code from the level below it. At Level 1, we implement CRT device drivers as code modules that perform the basic functions required of the characterization system, such as generating waveforms and taking measurements. We write these drivers using LabVIEW. The Level 1 software also serves one other purpose. We can use a manual test interface, which we also write in LabVIEW, to run simple tests on a single device, either directly on the wafer or after the device has been diced and packaged.
Level 2 code consists of a number of tests. Each of these tests calls some of the CRT device drivers in sequence with varying input parameters. Each module at Level 2 represents a complete test that we can perform on a single device. We implemented Level 2 in NI TestStand, in which each test, called an NI TestStand sequence, performs a particular type of characterization test. We set up the system to automatically log the results of each step into a database.
Level 3 code is the automated user interface, which we implemented using LabVIEW. This level provides test operators with a user interface in which they may select and run tests and change test parameters. Because this level interfaces with the prober, it can run a selected test on all the devices on a wafer without any human intervention.
The biggest advantage of developing this system in-house was achieving a custom on-wafer test system, designed from the ground up, to specifically interface with our product. Using LabVIEW and NI TestStand, we developed the characterization system from scratch in approximately four man-months. Also, by modifying the software, we can use the same hardware to perform on-wafer characterization of a second product designed by Proteus, providing us with additional test cost savings.
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