Keithley SMU Modules Aim to Aid in Low-current, High-capacitance Testing

October 30, 2019 by Gary Elinoff

Source measurement unit modules augment the effectiveness of Keithley's 4200A-SCS Parameter Analyzer.

Keithley, a unit of Tektronix, has unveiled its 4201-SMU and 4211-SMU (source measurement unit) modules, designed to work in conjunction with the Keithley 4200A-SCS Parameter Analyzer


Keithley 4200A-SCS Parameter Analyzer

The Keithley 4200A-SCS Pattern Analyzer. Image from Tektronix

Working together, this powerful combination can measure very low currents, even when high-load capacitance is part of the picture.


High-load Capacitances in Testing and Manufacturing

High-load capacitance is often a critical issue in testing because of the long cables and complex test setups that are often necessary. This difficult issue is often a factor in LCD display manufacturing and also frequently manifests itself in nanoFET device testing on a chuck. 

To combat these challenges, the 4201-SMU and 4211-SMU have been specifically designed for test setups involving long cables, gate contacts to the chuck, and switch matrices. These kinds of test setups are often required in certain low-current measurement applications. 



The 4200A-SCS offers capacitance-voltage (C-V), synchronizing current-voltage (I-V), and ultra-fast pulsed I-V measurements. Image from Tektronix

The problem lies in the fact that the long cables themselves can introduce far more capacitance to the setup than exists in the device itself being tested. The result is that instability is introduced to the process of making the required measurements of very low currents.


Addressing Traditional SMU Challenges

The 4201-SMU and 4211-SMU can source voltage and measure current in the presence of more capacitance better than traditional SMUs. These new devices are designed to save the time, effort, and cost that would be otherwise spent in troubleshooting and perhaps even rebuilding test engineers' and researchers' test setups. 

As described by Peter Griffiths, general manager of systems and software at the Keithley division of Tektronix, "High-load capacitance resulting from elaborate test setups is a growing problem as current levels are reduced to save energy, as is the case with testing the large LCD panels that ultimately end up in smartphones or tablet computers." He goes on to state that "our new modules excel at making stable low-current measurements and will immediately benefit many of our existing and future customers."


Device Specifics

The 4201-SMU is a medium-power device with a range of +/- 100mA at up to 200 volts.

The 4211-SMU is a high-power device with a range of +/- 100 amps at up to 200 volts.

The 4200A-SCS with SMUs installed in a FET testing application.

The 4200A-SCS with SMUs installed in a FET testing application. Image from Tektronix


Both devices can source voltage and measure current or, alternately, can source current and measure current voltage. Both the 4201-SMU and 4211-SMU can make stable measurements in the 1 to 100 picoamp range, even in the presence of as much as 1 microfarad of load capacitance. In contrast, competing units can only tolerate about 1 nanofarad (1,000 times less) before instability sets in. There is also an optional preamp, the 4200-PA, that can extend the range down to the femtoamp level.


Maximum Tolerable Capacitances

The 4201-SMU and the 4211-SMU can work within the following capacitances: 


Maximum tolerable capacitances for 4201-SMU and 4211-SMU


The SMUs are mounted internally within the 4200A-SCS Parameter Analyzer. The unit can be ordered from the factory with up to nine SMUs preinstalled or they can be easily field retrofitted.



What's your current method for measuring currents when you're faced with high-load capacitance? Share your ideas in the comments below.