October 3, 2009

Not really!. But all of us know, or think, that a slim healthy hysteresis loop lurks inside each measurement we make of a ferroelectric device, whether it is made with thin films or a hot-pressed ceramics. Excessive leakage is a common occurance when developing a new composition, using a new electrode, or modifying sintering conditions. As well, some materials, particularly the ferroic kinds, leak naturally. The leakage makes it difficult to identify the traditional device parameters such as the saturation polarization or the coercive voltages.

In the past, linear resistors were used in a bridge with the device under test to correct a leaky hysteresis loop to its natural shape. This technique is easy to implement mathematically with the modern ferroelectric tester, requiring only a single leakage measurement as an input to the math algorithm. This technique is flawed, however, because it quickly becomes apparent that the leakage in ferroelectric capacitors is not linear with either voltage or time so compensation with a linear resistor only changes a leaky loop into a less leaky loop that still does not represent the true hysteresis of the material being tested. That is why Radiant removed this type of compensation from its testers around 1995. Because the loop looks better does not mean that it is better. It might mislead our customers.

Meyer, et al. in APL 86 142907 (2005) describe an inspired technique for separating a true hysteresis loop from the measured loop by subtracting the currents measured at two different frequencies. This technique does not work with Radiant testers because they measure charge, not current. Radiant has developed another algorithm that not only compensates charge loops but also provides an exact solution. The only requirement is that the ferroelectric capacitor being tested not change the shape of its hysteresis loop between the two different test frequencies.

Radiant presented this technique at the latest ISIF meeting in Colorado Springs this week, showing proper correction of the deliberate distortion of a good sample. The link below will download a PDF file of the presentation:

Time Dependent Component Compensation

The "Time-Dependent Component Filter" will soon be available in the Vision Library. Please contact Radiant for more information.

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