measurement application
Key Considerations for Selecting an SMU Instrument
When evaluating a specific SMU instrument for a specific application, it’s essential to consider: System-level speed/throughput Source resolution vs. stability Measurement settling time, offset error, noise Cabling and connections
Let’s examine each of these characteristics. System-level speed or throughput. In other words, how quickly can you get a final measurement or set of measurements (such as a suite of current vs. voltage parameters) back to the PC controller? For example, let’s consider a typical diode or LED test, which will consist of three measurements—forward voltage, reverse voltage, and reverse current—each of which is typically compared to upper and lower limits. The part is considered “bad” if any one parameter fails. The objective is to test this part as quickly as possible without sacrificing accuracy in order to minimize the cost of test.
The challenge is that all the source and measure values are different. Although the readings/second spec is important, a range or function change must occur before a reading can be taken. This type of test isn’t about taking multiple readings of the same value repeatedly; it’s about taking single-point measurements at different source/measure levels. Therefore, the speed of the ADC (the NPLC spec) alone won’t be a good indication of how quickly the instrument can test this part. One should also consider a variety of other operating parameters, including trigger in time, range change time, function change time, source settling time, trigger out time, and command transfer, processing, and execution time.
Figure 7 shows a comparison of the actual test results from a Keithley Series 2600A System SourceMeter instrument with that of another brand of SMU instrument. The data shows the number of diodes tested per second, so the higher the number the higher the speed. This is a true measure of test throughput.
Recall that the larger the NPLC is, the more accurate the measurement will be (corresponding to lower speed). Note how reducing the NPLC setting to less than 0.1 NPLC does not make a significant difference in overall test time per part. In typical applications in which multiple parameters are being tested, the speed of other characteristics, such as range or function change time, triggering time, bus communication time, or program execution time, start to dominate. Even at 1 NPLC, these other
Issue 2 2012
www.siliconsemiconductor.net 27 Figure 7.Test results: parts per second
characteristics, if not optimized by the SMU instrument manufacturer, can have a big impact on overall test throughput. The Keithley Series 2600A System SourceMeter instrument in this example can test more than twice as many parts per second at 1 NPLC; therefore, it has more than 100% faster throughput than the other SMU instrument while maintaining optimum accuracy.
Although range and function change times are important, it’s also possible to obtain major breakthroughs in system throughput by embedding then executing the majority of the test program within the SMU instrument itself. This eliminates most of the communications bus traffic, speeds up triggering, and optimizes command processing time. Using this type of feature is a major reason an SMU instrument running at 0.1 NPLC can be as much as four times faster and much more accurate than an SMU running at 0.00048 NPLC in real-world applications.
Keithley’s Series 2600A System SourceMeter instruments employ a feature known as Test Script Processing, or TSP technology. TSP technology optimizes command transfer, command processing, and command execution times by embedding the actual test program (or script) into the instrument’s non-volatile memory. However, TSP technology
Figure
8.Programming resolution based on specification sheet
Figure
9.Actual output stability
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