Electrical scanning probe microscopes (eSPMs) are a subset of scanning probe microscopes which measure some electrical parameter as well as surface topography. These include techniques such as scanning capacitance microscopy (SCM), scanning spreading resistance microscopy (SSRM), conductive atomic force microscopy (C-AFM), scanning Kelvin force microscopy (SKFM), and scanning microwave impedance microscopy (SMIM). These techniques can provide a high spatial resolution mapping of an electrical property across a material or within an electronic device. For example, SCM has been used to measure the local tip-to-sample differential capacitance of cross-sectioned transistors, which is then related to the local sample dopant density of the semiconductor. When applied to a source/drain region, the two-dimensional dopant profile of the active region of the device can be obtained with spatial resolution of 10 nm. Quantitative interpretation of eSPM images requires some level of modeling of the physics of the tip-sample interaction. The most accurate results require consideration of the details of the tip shape, the measurement conditions, and the physics of the tip-sample interaction.
Reference Materials – We are building and characterizing test chips containing prototype Electric Field Gradient Reference Materials and Tip Profiler Artefacts. SKFM is often calibrated by measuring the contact potential between the tip and a freshly deposited gold film. For our reference materials (Figure 1), the dimensions and applied potentials of the devices are known precisely, allowing the actual electric field at the surface to be calculated precisely. This will allow a known staircase in surface potential to be produced, producing a long-term stable artefact to calibrate both the accuracy and spatial resolution of a probe based electric field measurement. Such artefacts have never been produced before. The major difficulty is producing an easily useable structure, validating model results, and producing a validated calibration procedure. Other structures on the test chip include buried interdigitated electrodes and field plates surrounded by a ground electrode. These structures are proving useful to help understand how tip shape effects measured potential across an abrupt boundary and as a means of isolating samples from the effects of surrounding material.
Current Thrusts – SPMs are ideally suited for rapid evaluation of candidate two-dimensional materials for spintronic and quantum applications, high-band gap, photovoltaic and electro-optic materials; and for determining the spatial variation of electrical properties within nanostructured devices. We are pursuing multiple applications and seek additional collaborations in these areas.