Hamed Alemansour

The principle of scanning tunneling microscopy is based on the quantum mechanical phenomenon of “tunneling,” whereby the wavelike properties of electrons allow them to “tunnel” beyond the surface of a solid into regions of space that are forbidden under the rules of classical physics [1] . This phenomenon is used in the scanning tunneling microscope (STM), where electrons tunnel from the apex of a sharp tip to a conducting surface held at a different potential (see “Summary”). The probability of electrons tunneling decreases exponentially as the distance between the two surfaces increases [2] . Hence, electrons tunnel mostly from the very last atom on the tip apex to the surface. The STM makes use of this extreme sensitivity to distance. A positioner brings the sharp tip of a tungsten probe to a distance of a few angstroms from the sample surface. A bias voltage is applied between the probe tip and the surface, causing electrons to tunnel across the gap.

The biomechanics of peripheral nerves are determined by the blood-nerve barrier (BNB), together with the epineural barrier, extracellular matrix, and axonal composition, which maintain structural and functional stability. These elements are often ignored in the fabrication of penetrating devices, and the implant process is traumatic due to the mechanical distress, compromising the function of neuroprosthesis for sensory-motor restoration in amputees. Miniaturization of penetrating interfaces offers the unique opportunity of decoding individual nerve fibers associated to specific functions, however, a main issue for their implant is the lack of high-precision standardization of insertion forces. Current automatized electromechanical force sensors are available; however, their sensitivity and range amplitude are limited (i.e. mN), and have been tested only in-vitro . We previously developed a high-precision bi-directional micro-electromechanical force sensor, with a closed-loop mechanism (MEMS-CLFS), that while measuring with high-precision (−211.7μN to 211.5μN with a resolution of 4.74nN), can be used in alive animal. Our technology has an on-chip electrothermal displacement sensor with a shuttle beam displacement amplification mechanism, for large range and high-frequency resolution (dynamic range of 92.9 dB), which eliminates the adverse effect of flexural nonlinearity measurements, observed with other systems, and reduces the mechanical impact on delicate biological tissue. In this work, we use the MEMS-CLFS for in-vivo bidirectional measurement of biomechanics in somatic and autonomic nerves. Furthermore we define the mechanical implications of irrigation and collagen VI in the BNB, which is different for both autonomic and somatic nerves (∼ 8.5-8.6 fold density of collagen VI and vasculature CD31+ in the VN vs ScN). This study allowed us to create a mathematical approach to predict insertion forces. Our data highlights the necessity of nerve-customization forces to prevent injury when implanting interfaces, and describes a high precision MEMS technology and mathematical model for their measurements.

A scanning tunneling microscope (STM) combines unique capabilities in imaging and spectroscopy with atomic precision, and it can obtain energy-resolved spectroscopic data with atomic resolution. In this paper, we utilize a recently proposed modification to the STM feedback control loop to acquire high quality d2I/dV2 images. We have developed a constant differential conductance imaging method by closing the STM feedback loop with a high precision dI/dV measurement. In this mode, the tip’s vertical position is adjusted so as to keep the differential conductance constant during raster scanning of the surface. Based on this imaging mode, we propose a new technique to acquire fast and reliable scanning tunneling spectroscopy (STS) data simultaneously with the imaging.

The scanning tunneling microscope (STM) has enabled manipulation and interrogation of surfaces with atomic-scale resolution. Electronic information about a surface is obtained by combining the imaging capability of the STM with scanning tunneling spectroscopy, i.e., measurement of current-voltage (I/V) characteristics of the surface. We propose a change in the STM feedback loop that enables capturing a higher quality dI/dV image. A high frequency dither voltage is added to the bias voltage of the sample, and the fundamental frequency component of the resulting current is demodulated. The in-phase component of this signal is then plotted along with the X and Y position data, constructing the dI/dV image. We show that by incorporating notch filters in the STM feedback loop, we may utilize a high-amplitude dither voltage to significantly improve the quality of the obtained dI/dV image.

The controlled formation of dangling bond structures on a H-terminated silicon surface is the first step in an atomically precise method of fabrication of silicon quantum electronic devices. An ultrahigh vacuum scanning tunneling microscope (STM) tip is used to selectively desorb hydrogen atoms from a Si(100)-2×1:H surface by injecting electrons with the sample held at a positive bias voltage. The authors propose a lithography method that allows the STM to operate under negative bias imaging conditions and simultaneously desorb H atoms as required. A high frequency signal is added to the negative bias voltage to deliver the required energy for hydrogen removal. The resulting current at this frequency and its harmonics are filtered to minimize their effect on the operation of the STM’s feedback control loop. The authors show that the chance of tip-sample crash during the lithography process is reduced by employing this method. They also demonstrate that this approach offers a significant potential for controlled and precise removal of H atoms from a H-terminated silicon surface and thus may be used for the fabrication of practical silicon-based atomic-scale devices.

We present a microelectromechanical system (MEMS)-based closed-loop force sensor, capable of measuring bidirectional forces along one axis of motion. The device comprises an on-chip electrothermal displacement sensor and electrostatic actuators. It features a voltage-controlled stiffness-adjustment mechanism, providing the user with an additional means of tuning the sensor's characteristics. This silicon-on-insulator device is an improvement on our previous design, offering a better dynamic range and a less complex calibration process. The sensor is fabricated based on a standard SOI MEMS process. It is then fully characterized and instrumented with a feedback controller. The closed-loop characterization reveals a 1σ-resolution of 4.74 nN and a force sensing range of -211.7μN to 211.5μN, leading to a dynamic range of 92.9 dB. [2019-0248]

Hydrogen deppasivation lithography (HDL) carried out by a scanning tunneling microscope (STM) is used to make patterns of Si dimers on a hydrogen passivated silicon surface. In this paper we discuss a new STM mode of operation that is highly suitable for removing single H atoms from the surface. This is made possible by changes we have made to the STM feedback control system that allows the STM to remove hydrogen atoms without switching the operational mode from imaging to lithography. Employing this method can potentially reduce the chance of tip-sample crash and increase the lithography precision.

This paper presents the design and characterization of a microelectromechanical systems (MEMS)-based probe scanner proposed to function as an on-chip atomic force microscope (AFM). The device comprises an in-plane stage with electrostatic actuators and electrothermal displacement sensors. The stage is able to precisely position an AFM probe over a sample to perform tapping-mode AFM imaging. For implementation, a standard silicon-on-insulator (SOI) microfabrication process is used. In a previously reported design, the embedded AFM probe featured only one piezoelectric transducer for simultaneous actuation and sensing, making its use in imaging problematic. To address this issue, the new design features separate actuation and sensing AlN piezoelectric transducers. An extra electrode is also incorporated on the probe that enables canceling the electrical feedthrough from the actuation to sensing. To accommodate the extra signal routing paths, mechanical design of the probe scanner is modified. Device characterization reveals an in-plane displacement range of 8 µm × 7 µm with a bandwidth of up to 2.7 kHz. The frequency-domain behavior of the AFM probe is also studied and feedthrough cancellation is performed at the resonant frequency of 127.35 kHz.