The source measure unit (SMU) is invaluable for test engineers and design engineers alike, whether they are performing an I-V characterization of a metal-oxide semiconductor, field-effect transistor (MOSFET) or analyzing the performance of a high-brightness LED. Since its beginnings in the semiconductor industry decades ago, the SMU has evolved to serve a broader range of applications, becoming a core piece of many automated test systems today.
As application requirements evolve, so do the demands placed on the performance characteristics of SMUs. For example, to capture detailed transient characteristics of a device, many test engineers now demand that the function be provided by an SMU rather than an external oscilloscope. This brings to the surface the disparity in performance between new-generation SMUs and legacy models. An increasing number of new test systems are deployed with modular SMUs that are faster and have a higher channel density—meeting not only the measurement performance requirements of test engineers, but also the minimum test system footprint requirements of facilities managers.
With this evolution, have you considered how the recent trend toward modular instruments can impact your next-generation test system? Ask yourself the following questions:
- How many channels do you need in your test system (today and over the life of the system) and how much space is required to reach the desired channel count? Modular SMUs save bench and rack space for multichannel test systems, providing 17 to 68 SMU channels in a 4U, 19-inch rack space.
- Can your traditional SMU protect your device under test (DUT) when sourcing highly capacitive or inductive loads? The latest modular SMUs feature programmable digital control-loop technology so you can custom tune the SMU response to any given load to avoid oscillations while protecting the DUT.
- Does your application require features that are beyond your traditional DC performance? The latest modular SMUs can sample fast enough to act as a high-voltage digitizer, or perhaps even source k-Hz range arbitrary waveform, or as a high-power pulse generator.
Below, explore the benefits of a modular SMU compared to a traditional SMU with fixed functionality.
Design a Multichannel Test System to Meet Cost of Test Goals
As semiconductor device performance improves and per-transistor fabrication costs continue to decline, meeting cost of test goals has become more challenging. One way to meet this goal is to test multiple devices in parallel, with common system configurations being two to eight parallel test sites. With this approach comes the challenge to design these multisite test systems to handle all the immediate and future test requirements while meeting rack and floor space constraints, which are significant given that per-square foot cost of semiconductor facilities could range in hundreds of thousands of dollars.
If you are facing this type of challenge, think about the options you have for your next test system design. Using a traditional SMU to meet channel count requirements in a parallel test configuration may require dozens of box instruments in a rack, resulting in a significant increase in the test system footprint. You are likely to encounter challenges with controlling or synchronizing multiple instruments over GPIB or Ethernet buses, causing latency and time-outs that add considerable test time and debugging to the process. You could even face a new cooling requirement to keep the system at an appropriate temperature, affecting calibration and accuracy.
Each traditional box instrument typically comes with a display, power circuitry, as well as a mechanical enclosure in addition to the actual test instrument. Take a critical view of a multichannel system with multiple boxes: all the display, power circuitry, and enclosures repeated at each and every box are extras that you don’t need. In fact, the extras are detrimental to your overall system efficiency. They waste space and power, not to mention the actual cost needed to pay for them. You shouldn’t have to pay extra for components that don’t directly contribute to your test goals.
With modular systems, however, each component is optimized to just what you need. For example, modular SMUs deliver high power, precision, and speed in a card-modular format without the unnecessary extras of a display and self-enclosure. Off-the-shelf PC technology gives you all the programmatic control and synchronization you need with room to add multiple modular SMUs to adapt to your channel count needs or to add other types of instruments without any wasted space. By taking advantage of the internal capabilities of a modular system, you can simplify the programming and system wiring of multiple SMUs required to create these parallel test systems, resulting in overall space savings and a streamlined instrument control infrastructure.
Use the Latest SMU Technology to Test a Range of Real-World DUTs While Ensuring DUT Safety
Today, SMUs are used to test the current-voltage (I-V) characteristics of a variety of devices at various stages from wafer-level test to package-level and board-level test. Most test engineers know that there would be invariably some amount of capacitive or inductive elements involved in testing just about any kind of device, whether it is a by-pass capacitor on an RFIC or the cable capacitance of the test infrastructure. This is an important issue as the potential outcome can range from system oscillations to damage to the DUT or SMU itself. Do you expect your SMU to handle inductive or capacitive loads without sacrificing performance (such as slowing down the test) or going to extreme measures (such as adding external circuitry)?
When it comes to control loop technology of SMUs, which governs the SMU behavior in working with various loads, there is a significant technology gap in the industry. As closed-loop instruments, SMUs rely on “feedback control” to ensure the programmed source value (set point) is correctly applied to the load (or device) under test. Conventional box SMUs use analog control loop technology physically made up of sophisticated circuitry involving op-amps, resistors, and capacitors. These SMUs are specified for a range of loads but the actual range of ideal loads where the SMU provides perfect response is extremely narrow and typically involves little or no reactive ranges (capacitive or inductive). Once the capacitive or inductive loads are involved, the SMU response tends to become less than ideal, which typically involves some amount of oscillations, overshoots, or slower rise and fall times. The reality is once you have a reactive load (which unfortunately is very common), it is extremely difficult to correct the SMU behavior. Sometimes you may be able to slow down the rise time, other times you may need to add an external circuitry that effectively augments the SMU control loop to compensate for working with a particular load. The bottom line is none of these are good solutions and, unfortunately, the portfolio of traditional SMUs offered on the market today relies on analog control loop technology.
Instead of the traditional analog control loop, the latest NI modular SMUs are powered by digital control loop technology known as NI SourceAdapt Technology. SourceAdapt technology fundamentally addresses the issue of working with capacitive or inductive loads by giving you, the test engineer, the ability to custom tune the SMU response to a given load. Because the control loop resides in the digital domain, this NI technology gives you the programmatic control of critical control loop parameters, which in turn gives you the ability to “train” the SMU to behave in a certain way to a specific load. You would typically find the ideal control loop settings for given types of loads or DUTs during the system development process. Once the ideal settings are identified and stored in the control program, all you need is to know what DUT to test. By applying the appropriate settings for that particular DUT, you get the SMU to provide a perfect response without overshoots (which is the primary cause of DUT damage) or oscillations and without slowing down the SMU response (optimal rise/fall times).
This ability to tune the SMU response to your DUT ultimately protects your investment in the test system by effectively guaranteeing that you don’t have system oscillation or device damage issues due to a mismatch between the DUT load characteristics and your SMU’s ability to handle it.
Capitalize on the Versatility of the Latest Modular SMUs
Most test engineers are accustomed to relying on an SMU to provide a precision source and measure functionality—typically for DC measurements—while relying on other instruments such as oscilloscopes for waveform capture or arbitrary waveform generators for waveform generation. The fact is most test requirements include both DC and higher frequency needs. The latest modular SMUs can provide you with both functionalities, potentially saving you test capital as well as simplifying your test.
The latest modular SMUs from NI can sample as fast as 1.8 MS/s, which can potentially save you from needing an external oscilloscope. Additionally, these SMUs have floating inputs with high voltage and current ranges, which are convenient for applications that traditionally require an oscilloscope with an external attenuator or current probe.
With the prevailing trend toward generic test systems for testing a variety of devices, future upgrade ability or reconfigurability is another important concern. Consider the system you have today. Can it accommodate changing requirements or the new devices of tomorrow? With traditional SMUs, adding capabilities—such as adding different types of instruments from different vendors—to the system could mean a complete re-architecture of the system because control and triggering across instruments may be different. This approach doesn’t give you the flexibility you need to reduce the cost of test. Modular SMUs take advantage of the latest capabilities by only upgrading the instrument and not the entire system, which means you don’t have to worry about a drastic architecture change.
With a modular approach, you can create a generic tester that can test a wide range of devices. You can configure multiple SMU instruments in a single system or choose from an extensive portfolio of modular instruments with built-in triggering and control mechanisms. For example, pair a modular SMU with a modular RF transceiver to create a highly integrated RFIC test system, all with built-in triggering and handshaking between different instruments. With this flexibility, you now have the control to create a system that spans from RF test to precision DC test for a broad range of applications.
With the modular approach, the software you write instructs how the instrument works, providing you the benefit of having the most efficient instrument for your task. With this approach, you could see an improvement of up to 100X in measurement speed compared to traditional SMUs. Take the control you want for your test system and use the flexibility to make the measurements for your device.
Make the Switch to Modular SMUs
As the industry is evolving and test needs are expanding, traditional SMU users are making the switch to a modular approach. Modular SMUs can perform as well or better than stand-alone SMU instruments while offering an integrated platform to support modern technology with inherent flexibility to meet changing needs.
