Scanning probe microscopy for advanced nanoelectronics
Scanning probe microscopy (SPM) refers to a family of techniques that can examine several mechanical, electronic, physical and chemical phenomena at the nanoscale, as well as perform local modification of materials. However, their potential for nanoelectronics research could be further extended. In this perspective article we foresee future SPM-based developments that could lead to unique nano-fabrication and nano-characterization experiments.
The first SPM was invented in 1981 at IBM laboratories in Zurich by Gerd Binnig and Heinrich Rohrer, a development that granted them the Nobel Prize in Physics in 1986. That technique was called scanning tunneling microscopy (STM), and consisted on using an ultra-sharp and conductive tip to scan the surface of a conductive sample when the distance between them was few nanometers. The tip-sample distance could be controlled by applying a potential difference between them and measuring the current — which can be converted into distance. Using this technique the shape of molecules and even the position of single atoms could be clearly mapped.
Just one year later, the inventors of the STM had the idea of placing the ultra-sharp tip at the end of a cantilever, which allowed them detecting the tip-sample distance by monitoring its bending using an optical system — both tip and sample do not need to be conductive. This technique, called atomic force microscopy (AFM), represented a complete revolution in different fields of nanoscience because it allowed using the ultra-sharp tip to monitor other different magnitudes during the scan (simultaneously to the tip-sample distance). Consequently, 1993 Sean O’Shea and co-workers at University of Cambridge used an AFM provided with a conductive tip and a source-meter to monitor the topography and the conductivity of a SiO2 sample. This technique was called conductive AFM, and after it many other similar SPM-based techniques were developed in order to measure capacitance, contact potential difference, piezoelectricity, flexoelectricity, and photoelectricity (among many others). Furthermore, the tip of the SPM has been also used to carry out local material modification, patterning and nanolithography, allowing nanofabrication of different structures and devices.
Now, the moment of combining all these techniques to carry out advanced experiments has arrived. Recently, different manufacturers have started to build prototype SPM systems provided with more than one tip in order to measure different magnitudes simultaneously. However, these machines still present many technical complications that need to be solved. For example, finding with one tip the exact location previously analyzed with another tip is extremely challenging, and avoiding collisions between the tips when placing them very near to each other is very laborious. Based on our experiences during the past 14 years, in this perspectives article we foresee future setups, experiments and capabilities of SPM systems, and discuss their potential impact in the future of nanoelectronics research. We propose the development of an SPM system combining multiple tips working simultaneously under a vacuum environment for advanced in situ nano-fabrication and nano-characterization (see image below). We also describe several limitations of individual techniques and propose different ways to improve their capabilities.
This manuscript is the result of hundreds of hours in front of more than 20 different SPM systems all around the world, searching for connections between different parameters and optimizing the measurements. We got confused when we found signal instabilities, felt frustrated when expensive tips broke accidentally, and got tired when the SPM session expanded until late night. But overall, we got excited when we were able to observe unique nanoelectronic phenomena, and enjoyed technical discussions with colleagues like Guenther Benstetter and Werner Frammelsberger (DIT, Germany), Oliver Krause (NanoWorld), Terry Yang (Park Systems), Louis Pacheco (CSInstruments), and David Lewis (Nanonics) among many many others, who share the passion for this amazing machine. During the past five years we have been working with different manufacturers in order to optimize their SPM systems, and we felt that sharing our vision in this perspective article could be of interest of the readership of Nature Electronics. We hope you enjoy reading it as much as we enjoyed writing it.