In situ electrical testing across nano‐scale contact interfaces in the transmission electron microscope
Authors: Alsem Daan Hein , Sood Siddharth , Salmon Norman , Jacobs Tevis 
Published: December 2016

Understanding the electrical properties of nanoscale contacts is paramount in small‐scale devices, including probe‐based microscopies [1], nanomanufacturing techniques [2] and micro/nano‐electromechanical systems (M/NEMS) [3]. In many cases, the electrical transport properties of the contact determines the device's functionality, and yet the behavior of the contact conductance is multi‐faceted and not easily characterized. There has been extensive characterization of the electrical properties of ultra‐small contacts using mechanically controllable break junctions and scanning probe techniques [4]. However, in these techniques the shape, size, and atomic structure of the contacting bodies and the contact itself are typically unknown. Thus, confounding factors such as the presence of oxide films and contaminants; the evolving shapes of the bodies due to inelastic deformation; and inaccurate estimation of contact sizes causes uncertainty in experimental measurements based on contact properties.
In situ
transmission electron microscopy (TEM) measurements of electrical contacts can overcome these limitations. While investigations have been performed using
in situ
electrical measurements inside a TEM before – including on single‐atom‐width nanocontacts in gold [5] – these methods typically require specially prepared contacts and are limited to a range of materials and geometries. In this study we show initial results obtained with a new
in situ
TEM electrical characterization tool that contains a movable probe, which allows to make site‐specific electrical contact measurements to study device‐related nanoscale electrical contacts (Fig. 1). The flexibility of the present
in situ
tool rests in its unique removable sample cartridge that enables simple, repeatable and accurate probe positioning, high‐resolution imaging, and accommodates a wide range of nanoscale contact samples.

Two contact configurations that are common to conductive scanning probe microscopy were recreated
in situ
in the TEM. Namely, a W substrate was contacted by a sharp nanoscale tip that is composed either of Pt/Ir or of doped Si. We demonstrate that current‐voltage sweeps can be performed while real‐time images of the nanoscale contact are acquired. As shown in Fig. 2(a), the metal/metal contact is ohmic (resistance 730 ohms). By contrast, the metal/semiconductor contact of Fig. 2(b) has a highly asymmetrical IV curve, displaying Schottky‐type behavior – as commonly seen in conductive probe microscopy with doped‐silicon tips [6].

As an example of the benefits of
in situ
imaging we compute the contact resistivity of the metal/metal contact. From the images of the contact we estimate a contact radius of 9.8 nm. The resistivity can be calculated using the classical (Maxwell), ballistic (Sharvin), or intermediate (Knudsen) limits [7]. The mean free path for W (estimated from the Fermi velocity and the bulk conductivity [8]) is close to 15 nm. Because this value is on the order of the contact radius, the intermediate resistivity limit is appropriate, leading to a value of r
Knudsen
= 620 mW‐cm. By having a direct measure of the contact area – obviating the reliance on continuum contact models – we can compute the contact's resistivity directly. It should be noted that this value is much larger than the bulk resistivity of W which is 4.82 mW‐cm [9]. This is attributable to the presence of insulating surface films (such as oxide or contamination).