Development of Nonlinear Scanning Microwave Microscopy for 3D Tomographic Imaging at the Nanoscale

Although the potential of scanning microwave microscopy (SMM) to image subsurface-structures has been demonstrated, a 3D tomographic imaging technique with SMM has not been achieved to date. A modified STM that utilizes the harmonics from the nonlinear interactions between the probe and the sample has been proposed. In the nonlinear scanning microwave microscope, two microwave sources are introduced. One with frequency f₁, is excited at the scanning tip, and the other with frequency f₂, is generated by an antenna below the sample stage which is simultaneously modulated at its mechanical resonant frequency Ω. The frequencies of the sources are chosen such that Ω= |f₁ - f₂|. Intermodulation of the two excitation signals is expected in the presence of nonlinearities in the probe-sample interface. The heterodyne detection of the low-frequency harmonics of the beat signal Ω in the reflection spectrum of STM can be easily attained on a multi-frequency lock-in amplifier with improved S/N. The limited spatial extents of the higher harmonics of Ω from intermodulation ensure the desired depth resolution. Lastly, simulations to relate the detected parameters with the material properties to achieve quantitative imaging are also reported.