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Measuring dielectric properties with a VNA below 2GHz

Question asked by DavidFabijan on Apr 2, 2011
Latest reply on Apr 4, 2011 by DavidFabijan

I have come across an issue that doesn't really pose a major problem for me, but I would still like to find an explanation for it since I'm pretty new to the whole field of MW measurements. I'm trying to measure the capacitance of an on wafer capacitor with a capacitance of approx. 3pF. To achieve this I'm using an Agilent E8361C VNA and a Cascade probe station with a GSG probe. I'm measuring the S11 parameter and extracting the impedance from it. As of now I'm interested in the range up to 10GHz and the results from 2-10GHz behave as well as can be expected. But bellow 2GHz I get some strange results. On the Smith chart this look like loops as thought there would be some sort of a resonance there, but I don't know what could cause it. A s1p file of such a measurement is attached. As far as I can see, this anomaly is not a result of a bad calibration, the 1 port on wafer SOL calibration I'm using should be OK. Also I'm pretty certain it's not caused by a faulty probe or cable. It could be a problem with the design of the capacitor, because it's repeatable across different samples on different wafers (all with the same design though). Or, and this is more likely, I'm missing something fundamental about the VNA's ability to measure in this range. Any hints to a solution would be most welcome.