White Paper: Nanoscale impedance and permittivity properties at microwave frequencies using SMM
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Nanoscale impedance and permittivity properties at microwave frequencies using SMM Ferry Kienberger, Keysight Technologies Austria GmbH Georg Gramse, Johannes Kepler University, Biophysics Institute
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canning microwave microscopy (SMM) is a nanoscale characterization technique used to extract accurate materials properties, including electrical, magnetic, and dielectric properties, at microwave frequencies (1–20 GHz). In comparison to scanning capacitance microscopy, the SMM works at higher broadband frequencies measuring absolute impedance values, including calibrated capacitance and conductance of the sample. For a given capacitance value, the higher the measurement frequency, the higher the complex impedance signal. Accordingly, sensitive measurements can be completed with SMM, and small impedances can be measured, which is useful to study small semiconductor devices, thin materials layers (e.g., 2D materials), and tiny biological cell compartments (e.g., membranes and viruses). SMM interfaces two well-known measurement tools: the atomic force microscope (AFM) for nanoscale imaging and the vector network analyzer (VNA) for high-frequency signal measurements and materials analyses.1 The AFM allows for nanometer lateral resolution imaging, and the VNA provides high precision impedance and admittance measurements. The conductive tip acts both as a nanometer scale AFM probe and as a GHz emitter-receiver antenna. The VNA reflection scattering signal is measured and converted to meaningful physical quantities, such as complex impedance and admittance, by means of a SMM calibration procedure. This article reviews the calibration and application Keysight Technologies The MRS Corporate Partner Program supports the Materials Research Society Foundation.
of SMM to determine various electric, magnetic, and dielectric materials properties, including resistivity, conductance, dielectric constant, complex permittivity, and magnetic permeability. The SMM merges the nanoscale imaging capabilities of AFM with the high-frequency broadband impedance measurement accuracy of VNA (Figure 1a). SMM allows for nanoscale impedance imaging and doping profiling of semiconductor materials using a microwave signal. Typically the SMM is operated in reflection mode, whereby the ratio of the reflected and incident
a Calibration plane
Transmission Line
electromagnetic waves, the so-called scattering parameter S11, is measured by the VNA at each pixel of the AFM tip-sample contact point. A microwave image is generated pixel by pixel simultaneously with the topographical imaging of the sample. There are two main imaging modes in SMM. The first is quantitative dopant profiling by means of differential capacitance (also called dC/dV), which is a widely used technique for semiconductor failure analysis and detecting leakages in solid-state devices with nanometer resolution.2 The dC/dV mode relies on a lowfrequency (kHz) modulation of the GHz
VNA
c
Capacitance
Port 1 DPMM
EFM + SMM approach curve
z
b
Cstray(
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