Scanning microwave microscopy (SMM) is a scanning probe technique, that measures the interactions of a microwave from a sharp tip with the sample. The microwave reflection coefficient (S11 parameter), which is the ratio between the microwave power sent to the tip and the one received back after being reflected at the tip-sample contact, is used to deduce the local tip-sample microwave impedance. The microwave impedance yields information about the local capacitance from which one can deduce the dielectric constant and the dopant density. In terms of the dopant density measurement, SMM is similar to scanning capacitance microscopy (SCM), but offers wider range of measurements. SMM can be used for measurements of not only semiconductors, but also dielectric materials and metals, since it is not solely relying of the modulation of the depletion capacitance in the sample.
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SMM at a glance
- Electrical characterization of materials – dielectrics, semiconductors and metals
- High sensitivity to changes of dopant density in semiconductors
- High sensitivity to DC capacitance
- Minimal sample preparation
- Imaging of buried structures
Understanding the S11 Parameter
The S11 parameter is defined as a function of the complex impedances of reference impedance (Z0) and load (ZL) – Eq.1. Impedance is a complex value and can be represented as a sum of real and imaginary parts – Eq.2. The real part of Z is resistance (R) and the imaginary part can be considered as capacitance (C) -Eq.3. Depending on the sample, resistance and capacitance are functions of conductivity, dielectric constant and carrier density.
Powerful digital signal processing is analysing signal at 1 MHz, far away in frequency from 1/f noise. The signal is digitized early at the analog front-end and such typical issues of analog systems as DC-offset re-adjustment are absent in this configuration. FPGA software has sideband detection capabilities, and dS/dV (dC/dV) measurement does not require any additional hardware, e.g. lock-in amplifiers. Both S11 and dS/dV signals are digital. High working frequency around 5 GHz means better sensitivity to capacitance.
Webinar: Scanning Microwave Microscopy
In this webinar, Dr. Denis Vasyukov shows how Scanning Microwave Microscopy works theoretically, and how this mode of measurement performs on Nanosurf instrumentation. The webinar is aimed at novice and advanced AFM users.
Watch on-demand webinar
Application notes and detailed technical explanation
SMM enables the precise measurement of local capacitances with exceptional sensitivity, providing crucial material and device parameters essential for the thorough characterization of semiconductor materials and electronic structures. This selection of application notes offers a detailed description of the measurement process and technology, along with numerous examples of its applications.
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