The Role of SMUs in Automated Testing

Source-measure units (SMUs) are more than the next generation of power supplies. They are fast-response read-back voltage and current sources with high-accuracy measurement capabilities, all tightly integrated in a single enclosure. They are designed for circuit and device evaluation where a DC signal must be applied to a device under test (DUT) and the response to that signal measured.

Many SMUs are capable of four-quadrant operation, acting like a positive or negative DC source or a sink (load). They also provide highly repeatable measurements, typically with 6½-digit resolution. Figure 1 illustrates a typical SMU circuit.

Traditionally, a combination of benchtop instruments, such as voltage or current sources and digital multimeters or picoammeters, has been used for material testing and component characterization. A common type of data-collection application is generation of current-voltage (I-V) curves to describe component or material behavior in a circuit. Using separate instruments to perform this task requires a significant amount of work to program each individual instrument, resolve timing issues, and properly connect signal and triggering cables.

Using an integrated source-measure instrument can reduce the effort required to acquire data, generate I-V curves, and otherwise characterize device performance. Because source and measurement functions are designed simultaneously for tightly coupled operation, they have features that simplify setup and operation.

For example, this tight integration allows easy programming of source and measurement functions, such as speed and noise-rejection trade-offs. Most SMUs also have built-in voltage and current-sweep features for automatically collecting I-V data and a compliance limit function that assures the safety of a DUT and test personnel.

As a rule, SMU capabilities exceed those of almost any combination of similar individual instruments, particularly when it comes to throughput and accuracy. The knowledge of both source and measurement circuitry during design and feedback between those circuits during operation accommodates compensation techniques that produce excellent instrument characteristics. The input and output impedances can be dynamically adjusted for specific operating conditions. Such tight integration allows fast source-measurement cycles with high resolution.

Applications

These advantages are more apparent in semiconductor measurements conducted on work-in-process wafers as well as on finished products. In addition to measuring I-V characteristics, these applications often require calculation of resistance and derivation of other parameters based on the measurements. Many of these applications need instruments with outputs and measurement sensitivity covering a very wide dynamic range. SMUs can simultaneously apply and measure current and voltage from microvolts and femtoamps up to more than 1,000 V or 10 A.

When low-leakage devices must be tested, SMUs are available with noise floors as low as 0.4-fA p-p, and their guard circuits can alleviate measurement errors due to stray leakage in cables and fixtures. To reduce these leakage currents, the SMU guard buffer (upper right area of Figure 1) creates a low-impedance point in the circuit that is nearly the same potential as the high-impedance point to be guarded. The guard-sense lead is used to detect the potential at that point.

At the low end of the current range, SMUs can be used to source voltage and measure femptoamp levels of gate or drain (IDDQ) leakage current as part of a wafer-level test-monitoring program. In testing finished diode products, SMUs can interface with switching systems for automatic parts-binning operations.

In these operations, the diodes are connected through the switching system between In/Out HI and In/Out LO (Figure 1). A sweep of voltage is done in the reverse-bias region of a diode to determine leakage current at a known voltage, then a sweep of current is done in the forward-bias region to control power in the device. The effect of incident light or temperature on the diode can be examined by generating a family of I-V curves for different levels of these variables.

For production applications, SMUs have program memory that can rapidly execute 100 or so such tests without tying up an external data bus and a remote computer to control the measurements. Each test sequence can have totally different test conditions, measurements, embedded math functions, pass/fail or binning criteria, and conditional branching.

Usually, SMUs designed for production applications also have a digital I/O interface that lets you link the unit directly to a component handler. I/O signal capabilities include start of test, end of test, category bits, and a 5-V relay control output.

A hard-wired trigger link between the SMU, the component handler, and the switching system will further reduce external bus traffic, speed up testing, and help assure adequate settling time between source-application and DUT response measurements. With these features and the data buffers in most SMUs, they can be programmed for source-measure sweep cycles at rates up to 2,000 readings/s.

When high power pulses are required in applications such as varistor and power MOSFET testing, some source-measure instruments can supply up to 1-kV amplitudes. In these and many other test setups, there can be significant voltage drops in test leads due to the high currents. In this case, you can use an SMU’s remote test connections, where voltage is measured or controlled between the Sense HI and Sense LO terminals to ensure accurate readings.

For example, in the case of a power MOSFET, the drain current is controlled by the gate-source voltage. But as drain current increases, the source voltage tends to increase (be driven above ground potential) due to test-lead resistance. If you are driving 1 A through the MOSFET and there is 0.5 W in the test lead, the source can be off ground by 0.5 V. The remote-sense feature of an SMU can detect this and automatically adjust the output by -0.5 V to hold the source at ground to allow more accurate measurements.

While most SMUs are built for general use, some are designed with a particular application focus. For example, in pulsed testing and other time-sensitive applications, the source must have a fast response to load changes.

While general-purpose power supplies typically have large output capacitances to maintain a stable voltage, SMUs use an entirely different design that minimizes output capacitance and allows high slew rates. This is essential in pulsed testing of laser-diode chips, and for this application, there are SMUs with current pulse rise and fall times as short as 60 ns with pulse widths as narrow as 500 ns.

For laser-diode testing, the SMU also has multiple measurement channels. One channel can measure voltage across the laser diode, and two voltage-biased current channels can be used for simultaneous measurement of front and back photodiode detector outputs that are needed to characterize edge-emitting lasers.

The wide array of capabilities and functions in SMUs can reduce costs such as instrument purchases, system integration costs, and testing time. Many models provide a wide range of features, from simple low-cost units to specialized instruments for highly demanding applications. These instruments offer a fast, easy, and cost-effective method for characterizing a collection of devices and materials.

About the Author

Lee Stauffer is a senior marketer at Keithley Instruments. His formal education in electrical engineering and semiconductor device physics complements 17 years experience in semiconductor process and product engineering, device characterization, and instrumentation design. Keithley Instruments, 28775 Aurora Rd., Cleveland, OH 44139, 440-248-0400, e-mail: [email protected].

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Published by EE-Evaluation Engineering
All contents © 2001 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

September 2001

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