Mar 14, 2014
Calibrating the scanning microwave microscope in situ
The scanning microwave microscope (SMM) operates at a high frequency in the gigahertz (GHz) range and can detect local changes in material properties. This is in contrast to most electrical characterization techniques with nanoscale resolution. Now, researchers at the Johannes Kepler University and Agilent Labs in Linz, together with researchers from IBEC Barcelona propose an effective and simple way to calibrate SMMs in situ without a specific calibration sample.
New nanoscale high-speed materials and devices require metrology tools that enable quantitative electrical characterization, at both the nanoscale and at high frequency, in the microwave range. Combining atomic force microscopy (AFM) with vector network analysers can extend the GHz frequency characterization to the nanoscale and gives access to local electrical information. However, to obtain reproducible and quantitative electrical measurements from the vector network analyser, an electrical calibration of the system including the AFM-tip is necessary. The use of calibration standards for this purpose is not practical and can lead to limitations in any following measurements.
In situ calibration
Reporting in Nanotechnology, the authors propose to overcome this issue by calibrating directly on the substrate that contains the sample under study, but without any calibration sample. Therefore just a single tip sample approach curve on a clean spot of the substrate is performed. This method allows for the quantitative extraction from the SMM measurements of the sample impedance, as well as intrinsic material properties such as the conductance and dielectric contrast.
Demonstrating the framework
The workflow is demonstrated on 20–60 nm thin silicon oxide films from which the dielectric constant is successfully determined at 20 GHz. Silicon samples with different dopant densities show as expected a falling bulk resistance with increasing dopant density. They also change the depletion layer capacitance when applying a varying bias voltage at the tip.
The work summarizes a basic framework for quantitative SMM measurements on various kinds of nanoscale specimen. It is easy to apply on most SMMs and does not require any modification of the instrument or a calibration sample.
More information can be found in the journal Nanotechnology 25 145703.
About the author
Georg Gramse is currently working as a postdoctoral researcher in the University of Linz Biophysics department in collaboration with Agilent Technologies to bring the scanning microwave microscopy closer to the field of biophysics. His scientific interest and background is in nanotechnology, biophysics and electrical characterization.