Posted by Dan (66.122.81.186) on November 27, 2006 at 13:00:07:
In Reply to: resisitivity limits posted by Andrei on November 22, 2006 at 14:34:38:
: Thank you for adding this bulletin board to your web site. I wonder if SRP has limitations for measuring high resistivity substrates. I am particularly interested in accurate measurements of resistivity of float zone silicon with resistivity over 1000 Ohm-cm (which corresponds to the doping levels below 1e13 cm-3). Will the SRP technique work at all on such wafers, can one expect higher error margins? Can you estimate the error - 1%? 10%? 100%?
: Thanks!
Thank you for your questions. Our limitations revolve more around determination of doping type for resistivities higher than 5000 ohm-cm than they do in actually measuring the resistivity. So Yes, SRA works on high resistivity material, but there are issues.
Our silicon calibration standards only go up to a little over 200 ohm-cm. So there is additional error but we don’t think it’s very much in most cases. And the additional error would vary on a case by case basis. Our calibration curves are more linear at the top end so extrapolating to higher values is pretty straightforward. Once we get over 7 or 8 Kohm-cm, (lower in thin layers,) the doping type becomes more uncertain – especially when there are no other lower resistivity layers that get profiled. Our main type determination is done with the hot probe, which senses the majority carrier. Unfortunately, N-type has 3 times the mobility in high resistivity so our signal will often indicate "N" even in P-type material. And extrapolating off the end of the wrong curve could then create a larger error than any error from not having calibration standards at the high resistivity levels. This is why knowing the type becomes so important.
We have found cases of high resistivity material (~10-20 Kohm-cm) actually being both types in small pockets close to each other. This type of material is especially difficult to characterize.