Extending Microwave Impedance Microscopy to the Nonlinear Domain

Microwave impedance microscopy (MIM) is a scanning probe technique based on a self-homodyne reflectometer that has been widely used to study nanoscale variation of electronic properties. The current state of the art in MIM modalities, AC-MIM is a lock-in based technique where the resonant mechanical oscillations of the probe body are used to modulate the MIM signal. Taken together, self-homodyne microwave detection and baseband lock-in readout make AC-MIM remarkably robust, but thier rejection of harmonic content renders it largely oblivious to the rich information carried by nonlinear effects that is essential for quantitative and accurate material characterization. I will present an approach to analyze and measure two nonlinearities that arise in AC-MIM: (i) intrinsic material nonlinearity and (ii) the nonlinearity in the distance dependence of the tip–sample admittance. First, I will describe a circuit level theory for dealing with material nonlinearity and discuss its implications for instrument design and practical viability. Next I will present a new MIM modality that uses modern high-speed computing to directly capture the full nonlinear admittance-distance curve while retaining all the advantages of AC-MIM and show some applications of this technique to quantitative nanoscale characterization. Finally, I will outline future opportunities enabled by combining these two approaches.