Microelectromechanical-system-based Scanning Tunneling Microscopy



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Scanning Tunneling Microscope (STM) is one of the most versatile tools in nanotechnology. STM has made it possible to obtain three-dimensional topographic images from the surface of conductive samples with sub-atomic resolution. With the help of STM, researchers are able to study electronic properties of surfaces at atomic scale. Moreover, STM tip can be used to manipulate atoms and molecules or even used as an electron-beam (e-beam) source for patterning with atomic resolution and precision, putting atomically-precise fabrication into practice. This superiority in precision and resolution presents STM-based lithography as a viable alternative to conventional e-beam lithography for manufacturing the next generation of nano-electronic devices with atomic precision. In order for STM to have a widespread use as either an imaging or nanolithography tool, however, its apparatus needs to be improved. The STM severely suffers from a limited throughput due to its slow scan speed and single-tip structure. A number of STM components contribute to this shortcoming, with the nanopositioner being the main contributor. The nanopostioners used in STMs, i.e. piezotubes, are bulky components with limited bandwidth in all three axes. Their bandwidth is usually limited to about 1 kHz along Z axis which is in direct interaction with sample surface and ultimately determines the scan speed. Besides, the single-tip scheme in almost all STM systems severely inhibits the throughput. These limitations imposed by the piezotube hinder the widespread use of STM despite all of its unique atomic-scale applications. To tackle this issue, we take advantage of micromachining to develop miniaturized STM nanopositioners. In this approach, we propose one-Degree-of-Freedom (1-DOF) Microelectro- mechanical-System (MEMS) based nanopositioners to replace the Z axis of STM piezotubes. Thanks to micromachining, the proposed devices have less mass, and therefore, offer a higher bandwidth compared to the piezotube, at least up to 10 kHz in this work, while keeping the same range of motion along Z axis, i.e. 2 μm. Besides, due to a smaller footprint, the miniaturized STMs open up the way for parallelism where an array of 1-DOF MEMS STM nanopositioners can be placed closely to each other and engage with a sample surface simultaneously, to increase the throughput multiple times. To achieve these goals, we design MEMS devices to fit the tip holder of a commercial Ultra- High-Vacuum (UHV) STM system. As a result, they can be used in place of the STM’s regular tip, resulting a hybrid STM in which the motions in XY plane are carried out by the piezotube while the Z-axis motion is delivered by the MEMS device, improving the Z- axis positioning capabilities. Throughout the chapters in this dissertation, we describe the background work, necessary design criteria, and steps for integrating the MEMS devices into the UHV STM system. We fabricate the MEMS devices using silicon-on-insulator technologies and standard cleanroom processes, to make them suitable for batch fabrication. After integrating the devices into the commercial UHV STM system, we then demonstrate their capabilities for STM imaging and lithography on a hydrogen-passivated silicon sample, proving that MEMS-based STM systems are conducive to high-throughput scanning tunneling microscopy.



Engineering, Mechanical