News Article

Testing Times

Keithley Instruments recently launched an electrical characterization tool that caters for the needs of producers of wide bandgap power electronics and high-brightness LEDs. David Ridsdale quizzes the company’s marketing director, Mark Cejer, about the capability of this new product.

Q Keithley has introduced a new tester that is suitable for scrutinizing the performance of LEDs and high-power semiconductors. Test is clearly an important part of the manufacturing process, but many manufacturers see it as an annoying additive to the process. Should they view it in that manner?

A It’s long past time for device manufacturers to start thinking about test as much more than ‘a necessary evil’. The growing demand for higher efficiency semiconductors is driven in part by the push for more energy-efficient devices. One of the goals for end products that employ lots of power semiconductors, such as power supplies for servers, must be reducing their energy consumption. When the power supply is in standby/off mode, leaky semiconductors in the inputs will waste a lot of electricity, especially when that leakage is multiplied by the number of power supplies in a big server farm.

To improve energy efficiency, IC manufacturers are constantly exploring ways to create more efficient silicon devices, as well as those based on compound semiconductors like SiC and GaN, which are inherently more efficient than silicon. All of that means greater testing challenges: ‘more efficient’ means materials and devices that are less leaky, ‘less leaky’ means IC makers need to be able to characterise ever-lower leakage currents, which is especially challenging in production.

Older instrumentation designed for characterizing relatively leaky silicon is typically not up to the challenge. The ability to characterise power semiconductors with pulsed measurements is also critical to ensuring accuracy because pulsed measurements let you test using high current levels without creating the self-heating problems that would occur if you were sourcing high DC currents.

High-brightness (HB) LEDs also present some critical testing challenges. For the types of applications that these devices serve, the colour of the light they output must be highly consistent from device to device because they’re typically packaged with multiple LEDs in a module, and multiple modules in a single end product. Any significant colour variation is immediately obvious and would be unacceptable to the consumer. Ensuring high colour consistency requires the ability to test these devices with extreme accuracy.

Here, too, high-throughput pulse testing is essential because HBLEDs are very susceptible to self heating, which will affect the colour of the light they output. And, of course, in production, test throughput is equally critical. And those are the big issues the Model 2651A High Power System SourceMeter instrument was designed to address.

Q Many companies highlight the generic capacities of their tools over a number of areas but with this tool Keithley have deliberately focused on the specialities the tester provides. What motivates such a decision and what process advantages does it provide the manufacturer?

A For some of our customers, all we have to say is ‘We’ve got a terrific new 50A SMU’ and they’ll know exactly how to use it in their applications. But for the rest, we feel that test vendors have a big responsibility to their customers to help them choose and use their products effectively. You’re right, this product is designed to address a specific set of applications, but it’s undoubtedly the fastest growing area of the semiconductor industry. Power semiconductors are used throughout more industries every year: in the auto industry for hybrid and electric vehicles, electric grid applications, solar and wind power generation, power supplies for PCs and consumer electronics, and many more. Just about every segment of the electronics industry and their downstream customers are using power semiconductors and HB LEDs in some way. And we want to serve all of them.

Q Test has become an ever increasing part of the manufacturing process but also an ever increasing part of the cost. In industries where margins are so tight how does this new tool help manufacturers with cost? And when they see specific tool requirements, should they assume that the cost will also be higher?

A Until recently, manufacturers of power semiconductors had to rely on what we call ‘big-iron’ ATE functional testers. And those systems were pretty expensive – typically hundreds of thousands of dollars each. Even more important, those systems aren’t really optimised to characterise modern power semiconductor materials and devices with their lower leakage currents and higher power levels.

In contrast, the Model 2651A is designed for exactly those characterisation challenges – and it costs about one-tenth as much. From a production test perspective, it not only dramatically lowers the cost of ownership, but provides higher-accuracy, better-quality measurements without a loss of throughput. That’s because the combination of the Test Script Processor (TSP) embedded in the Model 2651A, and the TSP-Link virtual backplane that system integrators can use to link multiple instruments together, makes it easy to scale a system as large as they need while ensuring high throughput. TSP makes embedded scripting and execution of commands possible, in contrast with line-by-line execution of commands over GPIB as in traditional instrumentation.

Q The area of industry Keithley is targeting crosses some intense research fields as well as the growing production needs. Do you have a strategy that enables cost of ownership at the research level and the ability to transfer to production stage with minimal disruption or added cost?

A Absolutely. The Model 2651A, and in fact, the entire Series 2600A System SourceMeter family, incorporates this strategy. For example, researchers often need to characterise a device very quickly by just taking a few measurements. Series 2600A instruments have an embedded TSP Express software tool that allows researchers to perform common I-V tests quickly and easily without programming or installing software. TSP Express, which is LXI compatible, runs from the instrument and is controlled via a web browser running on a PC connected to the instrument via an Ethernet cable. It has an intuitive user interface that resides on the instrument’s built-in web page. A user can just connect a laptop to the instrument with an Ethernet cable, open up a web browser on the laptop, type the instrument’s IP address into the browser, and up comes the test application that’s embedded in the instrument. And from there, the user can quickly point, click, run any of a number of tests, and download the resulting data to a .csv file or view it in graphical or tabular formats. TSP Express supports basic and advanced tests, including nested step/sweeps, pulse sweeps, and custom sweeps for device characterisation applications.

As useful as this software tool is, this is obviously not the approach for production test applications. For system-level applications, our TSP architecture is designed to simplify building high speed, multi channel IV test systems of multiple instruments. The on-board microprocessor allows each Series 2600A instrument in the system to run its own test scripts, which can contain any sequence of routines that are executable by conventional programming languages. That means the instrument can manage an entire test without sending readings back to a PC for decision-making, eliminating delays caused by GPIB traffic congestion and greatly improving overall test times.

The TSP-Link bus allows system builders to connect multiple Series 2600A and other TSP instruments in a master-slave configuration so they behave as one integrated system. TSP-Link supports up to 32 units or 64 SMU channels per GPIB or IP address, so it’s pretty easy to scale a system to match the requirements of an application. We also have built-in 500 ns trigger controllers to ensure precise timing and tight channel synchronization of multi-instrument systems.

Series 2600A instruments also provide a parallel testing capability that allows each instrument in the system to run its own complete test sequence, creating a fully multi-threaded test environment. That means you can run as many tests in parallel as you have Series 2600A instruments in the system, which can boost throughput dramatically. And when test requirements change, it’s pretty simple to reconfigure a Series 2600A-based system via software without rewiring. Obviously, one of the big advantages of using the same instrument in the lab and on the production floor is measurement correlation. If manufacturing engineers discover a problem on the production floor, they can work with design engineers to track down the source of the problem much faster, because they can scratch data correlation concerns off their list of “unknowns."



In April 2011 Keithley Instruments launched its Model 2651A High Power System SourceMeter, an instrument specifically designed for characterizing high power electronics


Q What are the key interconnect issues that this testing platform addresses?

A With the ability to source and measure currents as high as 50A and the ability to resolve leakage currents as low as a picoamp, the Model 2651A offers the widest dynamic range of any SMU currently on the market. To make this possible, we put a lot of effort into developing  specialized low resistance cabling and connectors to ensure our customers could make low noise measurements on any range. That specialized cabling is included with the product, so users don’t have to worry if their measurements are being compromised by noisy connections.

Q Temperature control at interconnect junctions is of concern to manufacturers of high end products. How does the new platform address the industry needs?

A To minimize the unwanted effects of device selfheating during testing, the Model 2651A supports pulsed measurements. A single Model 2651A can pulse up to 50 A; two units can be combined using the TSPLink bus to pulse up to 100 A. It can capture transient behavior such as changing thermal effects with onemicrosecond per point (1 MHz) sampling. The width of a sourced pulse can be programmed from 100 μs to DC and duty cycles from 1 percent to 100 percent are also programmable.

The Model 2651A provides a digitizing measurement mode that uses 18-bit A/Ds for characterising transient behavior precisely. A separate integrating measurement mode, based on 22-bit A/Ds, provides the maximum measurement accuracy and repeatability.

For applications like studying the thermal impedance of power diodes and LEDs, characterising the slope of the measured voltage at the top of the pulse is important. This capability is also useful for characterizing pulse amplitude flatness. The Model 2651A’s high speed A/Ds simplify digitizing the top of the pulse accurately when the measurements are made synchronously with the source.

Q What are the key areas of power semiconductors that this testing platform addresses?

A Perhaps the most significant area is the enhanced efficiency of new materials and the testing challenges that come along with that greater efficiency. ‘More efficient’ means that when the semiconductor is ‘on’, it’s really on and when it’s ‘off,’ it’s really off. Because it is designed to source and measure pulses of up to 50 A and measure voltages down to a microvolt, the Model 2651A offers the developers of new materials the ability to characterize the resistance from drain to source when the device is on (RDSon) with high accuracy. At the same time, manufacturers of these new materials are striving to minimize leakage current from drain to source when the device is ‘off’(IDSoff); with its one-picoamp current measurement resolution, the Model 2651A makes it possible to characterise this parameter with high confidence.

Q What are the key areas of LED brightness that this testing platform addresses?

A One of the methods HBLED manufacturers use to control the brightness of the devices they produce is known as pulse width modulation. In this technique, the current through the HBLED is pulsed at a constant frequency with a constant pulse level, but the width of the pulse is varied. This changes the amount of time the device is in the ‘on’ state, as well as the perceived level of brightness. In this drive scheme, the HBLED is actually flashing, but the frequency of the flashing is so high that the human eye can’t distinguish it from a constant light level.

Although it’s possible to control the brightness of a HBLED simply by lowering the forward drive current, the pulse width modulation technique is preferable for several reasons, the most important of which is to maintain the consistency of the colour of the light as the device’s brightness is reduced. In a HBLED, the colour of the light it emits is related to the forward voltage at which it operates. Although the forward voltage will remain relatively constant as the forward current is changed, it actually does vary by as much as tens to even hundreds of millivolts. This occurs especially at lower current levels. This slight variation in forward voltage equates to a slight variation in light colour, which is undesirable for the end user. If heating effects are ignored, in the pulse width modulation technique,  the LED is pulsed using exactly the same current level on every pulse, so the forward voltage is the same for every pulse; therefore, the colour of the light emitted won’t vary.

Fortunately for HBLED device developers, the Model 2651A is capable of outputting a pulse width modulated waveform with up to 100 percent duty cycle from 020 A, 50 percent duty cycle from 20 30 A, and 35 percent duty cycle from 30–50 A. Its advanced trigger model allows for precision pulse widths and duty cycles and tight synchronization with other instruments. These synchronization features can be used to combine two Model 2651As to achieve a pulsed width modulation waveform with pulse current levels twice as high as a single Model 2651A allows with the same duty cycle.

Q Keithley has a long history of test and measurement in the semiconductor and related industries. What are the key issues that Keithley sees facing the industry over the next five to ten years as more advanced and multiple device requirements are needed to meet roadmap intentions?

A Obviously, the demand for higher efficiency devices won’t be going away. That means that not only will current manufacturers be experimenting with new materials – new companies will also enter this segment of the market. Typically, when that happens, to meet the new manpower demands, less experienced people are going to be chasing more complicated technologies. That obliges Keithley and other test vendors to keep producing products that are as simple as possible to use, so someone doesn’t have be a test expert to start using them effectively. It also means we have to stay on top of providing applications support to get these new users up to speed quickly so they can find the products they need to do their jobs more efficiently. High accuracy products alone aren’t enough—we have to continue making those products easy to use.

© 2011 Angel Business Communications. Permission required.

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