As is the case with many sourcing and measurement challenges, when attempting to measure extreme values of most anything, factors that you can be blissfully unaware of, because they normally have an inconsequential impact on results, can become a dominant error to deal with. One example of this is when trying to make good low level leakage current measurements on devices and components and “phantom” leakages exceed that of the device you are attempting to test.
When measuring leakage currents of around a µA and lower, it is important to pay attention to your test set up as it is fairly easy to have leakage currents paths in the set up itself that range from adding error to totally obscuring the leakage current of the DUT itself you are trying to test. These leakage current paths can be modeled as a high value resistor in parallel to the DUT, as shown in Figure 1.
Figure 1: Leakage current path in DUT test fixture
- Many things can cause leakage currents on the fixture contributing to leakage current measurement error of the DUT:
- Is the PC fixture board made from appropriate high impedance material?
- Is the PC board truly clean?
- Was de-ionized water used to clean the PC board?
- If already in service for quite some time, have contaminants slowly built up over time?
- Any components associated with the connection path to the DUT are, or have become, unexpectedly leaky?
- Any standoffs and insulators associated with the connection path to the DUT are, or have become, unexpectedly leaky?
Even with all the above items in check there are still times when more needs to be done to further reduce leakage current inherent in the test set up. To help in this regard a guard amplifier is often added on high performance source-measure units (SMUs) to mitigate errors introduced from leakage current paths in the test set up. The Agilent N678xA and the B2900 series are examples of SMUs that include guard amplifiers. Application of a guard amplifier is illustrated in Figure 2.
Figure 2: Guard amplifier in a leakage current test set up