Improving the reliability of conductive atomic force microscopy-based electrical contact resistance measurements

Current-Limited Conductive Atomic Force Microscopy

Conductive atomic force microscopy (CAFM) has become the preferred tool of many companies and academics to analyze the electronic properties of materials and devices at the nanoscale. This technique scans the surface of a sample using an ultrasharp conductive nanoprobe so that the contact area between them is very small (<100 nm2) and it can measure the properties of the sample with a very high lateral resolution. However, measuring relatively low currents (∼1 nA) in such small areas produces high current densities (∼1000 A/cm2), which almost always results in fast nanoprobe degradation. That is not only expensive but also endangers the reliability of the data collected because detecting which data sets are affected by tip degradation can be complex. Here, we show an inexpensive long-sought solution for this problem by using a current limitation system. We test its performance by measuring the tunneling current across a reference ultrathin dielectric when applying ramped voltage stresses at hundreds of randomly selected locations of its surface, and we conclude that the use of a current limitation system increases the lifetime of the tips by a factor of ∼50. Our work contributes to significantly enhance the reliability of one of the most important characterization techniques in the field of nanoelectronics.

A multi-resistance wide-range calibration sample for conductive probe atomic force microscopy measurements

Measuring resistances at the nanoscale has attracted recent attention for developing microelectronic components, memory devices, molecular electronics, and two-dimensional materials. Despite the decisive contribution of scanning probe microscopy in imaging resistance and current variations, measurements have remained restricted to qualitative comparisons. Reference resistance calibration samples are key to advancing the research-to-manufacturing process of nanoscale devices and materials through calibrated, reliable, and comparable measurements. No such calibration reference samples have been proposed so far. In this work, we demonstrate the development of a multi-resistance reference sample for calibrating resistance measurements in conductive probe atomic force microscopy (C-AFM) covering the range from 100 Ω to 100 GΩ. We present a comprehensive protocol for in situ calibration of the whole measurement circuit encompassing the tip, the current sensing device, and the system controller. Furthermore, we show that our developed resistance reference enables the calibration of C-AFM with a combined relative uncertainty (given at one standard deviation) lower than 2.5% over an extended range from 10 kΩ to 100 GΩ and lower than 1% for a reduced range from 1 MΩ to 50 GΩ. Our findings break through the long-standing bottleneck in C-AFM measurements, providing a universal means for adopting calibrated resistance measurements at the nanoscale in the industrial and academic research and development sectors.

Hydrocarbons in the Meniscus: Effects on Conductive Atomic Force Microscopy

It is commonly accepted that during conductive atomic force microscopy (CAFM) measurement in ambient, a liquid meniscus can form between the tip and the sample. Such a liquid bridge, normally assumed to be composed of water, is a major factor in analyzing and understanding CAFM results. Here, we show that the adsorption of adventitious hydrocarbons from the air to a surface can greatly affect CAFM data both in imaging mode and in local spectroscopy (current-voltage or I-V curves). We propose a model to explain the phenomena whereby hydrocarbon contaminates contribute to the composition of the liquid bridge between the tip and the sample.

True Atomic-Resolution Surface Imaging and Manipulation under Ambient Conditions via Conductive Atomic Force Microscopy

A great number of chemical and mechanical phenomena, ranging from catalysis to friction, are dictated by the atomic-scale structure and properties of material surfaces. Yet, the principal tools utilized to characterize surfaces at the atomic level rely on strict environmental conditions such as ultrahigh vacuum and low temperature. Results obtained under such well-controlled, pristine conditions bear little relevance to the great majority of processes and applications that often occur under ambient conditions. Here, we report true atomic-resolution surface imaging via conductive atomic force microscopy (C-AFM) under ambient conditions, performed at high scanning speeds. Our approach delivers atomic-resolution maps on a variety of material surfaces that comprise defects including single atomic vacancies. We hypothesize that atomic resolution can be enabled by either a confined, electrically conductive pathway or an individual, atomically sharp asperity at the tip-sample contact. Using our method, we report the capability of in situ charge state manipulation of defects on MoS₂ and the observation of an exotic electronic effect: room-temperature charge ordering in a thin transition metal carbide (TMC) crystal (i.e., an MXene), α-Mo₂C. Our findings demonstrate that C-AFM can be utilized as a powerful tool for atomic-resolution imaging and manipulation of surface structure and electronics under ambient conditions, with wide-ranging applicability.