An advanced impedance calibration method for nanoscale microwave imaging

We present a new calibration method for nanoscale complex impedance imaging by scanning microwave microscope (SMM), which does not require any calibration samples. Instead, the vector network analyser (VNA) and the corresponding electronically switched calibration (ECal) capabilities in combination with time domain gating and microwave network modelling are used to de-embed the full system. Based on this workflow the measured complex SMM-S11 signal can be directly split into the reactive and lossy sample behavior. Nano Schottky diodes on a semiconductor substrate were measured to demonstrate that the sample conductance and susceptance correspond directly to S11 amplitude and phase, respectively. In the final step of the workflow we apply a calibration based on SMM tip approach curves resulting in quantitative impedance values. This method can have various applications starting from semiconductor failure analysis to novel 2D materials and biological cells in liquid environment.

I. Introduction

Scanning Microwave Microscopy (SMM) is a method combining Atomic Force Microscopy and microwave network analysis thereby enabling electrical impedance measurements at the nanoscale. Applications are in the area of high and low loss material characterization [1], semiconductor dopant concentration measurements, failure analysis [2], [3] and measurements on biological samples [4]. Subsurface measurement capablities have been shown [5], [6] and a good general overview is given in [7]. In contrast to Scanning Capacitance Microscopy (SCM), SMM is operating at significantly higher frequencies and additionally to capacitance allows to measure resistances.