Development of Probe-based Devices and Methods for Applications in Micro/Nano Characterization and Fabrication

The ability to investigate mechanical and material properties of micro- and nano-sized devices and surfaces is of significance in many fields of science and technology. Scanning tunneling microscope (STM), atomic force microscope (AFM), and MEMS force sensors are in high demand for this purpose, whereas their functionality is limited in many cases. This dissertation focuses on novel scanning methods, MEMS devices, and control design methodologies that facilitate micro/nano characterization and fabrication of devices and surfaces. High-speed and high resolution scanning tunneling spectroscopy (STS), as well as atomically precise fabrication of silicon quantum electronic devices are still among challenging topics in the STM. It is due to the limitation of conventional methods that are used for these purposes. Besides the STM hardware specifications, control methods contribute significantly to achieving the ultimate goal of ultrafast STS and atomically precise manufacturing (APM). The STM control system needs to be improved to harness the STS full potential, as well as to provide adequate precision and robustness during lithography. In part I of this dissertation, we detail our research on the STM-based probe microscopy for applications in surface characterization and atomically precise manufacturing. Modulation technique is conventionally used to obtain dnI/dV n STS images of a surface. However, images that are obtained from this method suffer from noise, which is mainly due to the small amplitude of the modulation voltage. The modulation technique is a powerful tool which enables us to utilize the unused frequency band of the STM for other purposes without disturbing its normal operation. We exploit this technique in different STS and APM methods throughout the part I of this dissertation. First, we propose a modified STM feedback loop to improve the SNR of STS images. Then, a novel STM imaging mode is introduced, which is based on keeping dI/dV constant. We also propose an ultrafast STS method that can provide an I–V curve for every pixel of the image simultaneously with the topography image. This method significantly reduces the spectroscopy time. Finally, we introduce a method for hydrogen depassivation lithography (HDL) with STM. Unlike the conventional approach, where a positive bias voltage is applied to the sample, we have developed an automated scheme to perform HDL at negative bias voltages. We show that our proposed lithography method significantly decreases chance of a tip-sample crash and can potentially increase the lithography precision. Probe-based force measurement systems are also widely utilized to investigate characteristics of micro- and nano-sized devices and surfaces. These systems work based on measuring the interaction forces between a known probe and an unknown surface. In part II of this dissertation, we proceed by proposing novel design and control methods for two well-known probe-based force measurement systems: AFM and MEMS force sensor. AFM plays a crucial role in a myriad of applications in science and technology. It is the most widely used tool for imaging and manipulating matter at the nanoscale. The AFM utilizes a microcantilever with a very sharp tip that interacts with a sample surface. The use of this technology alone does not guarantee its efficient functionality. It is of significant importance to harness the full functionality of an AFM by employing efficient control methods. We implement a positive position feedback controller on a previously designed active microcantilever (Coskun et al., 2017) to achieve a faster cantilever response. Then, we exploit MEMS technology to realize an on-chip MEMS AFM. The tracking performance of the device is enhanced by implementing different control methods. Finally, we propose a MEMS-based force sensor. On the contrary to the AFM that mainly relies on its microcantilever for force measurements, a variety of mechanisms can be incorporated in a MEMS force sensor. This makes it possible to readily adjust the force sensor parameters. Our force sensor features built-in electrostatic actuation, piezoresistive displacement sensor, and stiffness adjustment mechanism. It works in the closed-loop, which mitigates the adverse effect of flexural nonlinearities on the precision of the force measurement.