High-temperature giant piezoresistivity of microstructured SiOC-based strain gauges

The foundation of this work is laid out based on the efficiency of silicon oxycarbide (SiOC) as a functional material for piezoresistive device applications. The realization of a cost-efficient strain gauge which can operate at elevated temperature serves as the foremost objective of this work. This goal is fathomed based on prevailing knowledge regarding the high piezoresistivity of SiOC at the range of 10 - 102 coupled with commendable properties such as electrical conductivity, good thermal resistance, and an excellent coating material for hostile environment. An optimized process of spin coating is used to deposit ~500 nm SiOC film onto the 100-mm diameter silicon substrate with a silica layer of 500 nm. The deposition process is screened with a Taguchi design of experiment resulting into a replicable and controlled process with a crack-free and homogenous coating. An in-house piezoresistivity test setup was fabricated with considerations of minimizing the electrical contact resistances, capability to perform mechanical cyclic loads, and the ability to operate at elevated temperature until 700 °C. After the annealing process, the SiOC film manifested round-shaped segregations which were identified as carbon-rich and oxygen-depleted, evenly dispersed in an oxygen-rich matrix. Deeper investigation of the segregated area revealed 2-level hierarchical microstructure of sp2-hybridized carbon, Si3N4 and SiC. On the other hand, Raman analysis confirmed presence of sp2-hybridized carbon not just on the segregated area but also on the matrix distinctive by the difference of crystal sizes. Larger domains of carbon including tortuosity (Leq) are present on the segregation than on the matrix of the film. Kinetics study showed that the segregations area results of free carbon diffusion through the silica layer with an activation energy equal to 3.05 eV. Platinum electrodes are printed on the surface of the SiOC film via photolithography for the PZR tests. The fabricated strain gauge prototypes have high sensitivity with gauge factors (GF) in the range of 2000 – 5000 tested at 25 – 400 °C. At 500 – 700 °C, the behavior of the material shifted from semiconducting to conducting decreasing its resistance to 11 Ω, and GF of 200. This GF is still comparably larger than commercial metal- and silicon-based strain gauges. The difference of mechanical cyclic loads applied on the prototypes influenced the degree of response’ hysteresis and the linearity of the strain range. In both cases, tests under compressive load showed superiority over tensile tests. Through these results, this study provides a working strain gauge prototype based on SiOC thin film with high sensitivity, reproducibility, and robustness. The giant piezoresistivity of the fabricated strain gauge at an elevated temperature, until 700 °C, surpasses the known application of the current commercial strain gauges. Furthermore, the perceived shift on electrical behavior of the material at 460 °C broadens its applications to current-limiting devices and temperature sensors

Effect of Native Oxide on Stress in Silicon Nanowires : Implications for Nanoelectromechanical Systems

Understanding the origins of intrinsic stress in Si nanowires (NWs) is crucial for their successful utilization as transducer building blocks in next-generation, miniaturized sensors based on nanoelectromechanical systems (NEMS). With their small size leading to ultrahigh-resonance frequencies and extreme surface-to-volume ratios, silicon NWs raise new opportunities regarding sensitivity, precision, and speed in both physical and biochemical sensing. With silicon optoelectromechanical properties strongly dependent on the level of NW intrinsic stress, various studies have been devoted to the measurement of such stresses generated, for example, as a result of harsh fabrication processes. However, due to enormous NW surface area, even the native oxide that is conventionally considered as a benign surface condition can cause significant stresses. To address this issue, a combination of nanomechanical characterization and atomistic simulation approaches is developed. Relying only on low-temperature processes, the fabrication approach yields monolithic NWs with optimum boundary conditions, where NWs and support architecture are etched within the same silicon crystal. Resulting NWs are characterized by transmission electron microscopy and micro-Raman spectroscopy. The interpretation of results is carried out through molecular dynamics simulations with ReaxFF potential facilitating the incorporation of humidity and temperature, thereby providing a close replica of the actual oxidation environment in contrast to previous dry oxidation or self-limiting thermal oxidation studies. As a result, consensus on significant intrinsic tensile stresses on the order of 100 MPa to 1 GPa was achieved as a function of NW critical dimension and aspect ratio. The understanding developed herein regarding the role of native oxide played in the generation of NW intrinsic stresses is important for the design and development of silicon-based NEMS

Investigation and Integration of Piezoresistive Silicon Nanowires for MEMS applications

Ph.DDOCTOR OF PHILOSOPH

High Frequency Thermally Actuated Single Crystalline Silicon Micromechanical Resonators with Piezoresistive Readout

Over the past decades there has been a great deal of research on developing high frequency micromechanical resonators. As the two most common and conventional MEMS resonators, piezoelectric and electrostatic resonators have been at the center of attention despite having some drawbacks. Piezoelectric resonators provide low impedances that make them compatible with other low impedance electronic components, however they have low quality factors and complicated fabrication processes. In case of electrostatic resonators, they have higher quality factors but the need for smaller transductions gaps complicates their fabrication process and causes squeezed film damping in Air. In addition, the operation of both these resonators deteriorates at higher frequencies. In this presented research, thermally actuated resonators with piezoresistive readout have been developed. It has been shown that not only do such resonators require a simple fabrication process, but also their performance improves at higher frequencies by scaling down all the dimensions of the structure. In addition, due to the internal thermo-electro-mechanical interactions, these active resonators can turn some of the consumed electronic power back into the mechanical structure and compensate for the mechanical losses. Therefore, such resonators can provide self-Q-enhancement and self-sustained-oscillation without the need for any electronic circuitry. In this research these facts have been shown both experimentally and theoretically. In addition, in order to further simplify the fabrication process of such structures, a new controlled batch fabrication method for fabricating silicon nanowires has been developed. This unique fabrication process has been utilized to fabricate high frequency, low power thermal-piezoresistive resonators. Finally, a new thermal-piezoresistive resonant structure has been developed that can operate inside liquid. This resonant structure can be utilized as an ultra sensitive biomedical mass sensor

Development Of Polymer Derived Sialcn Ceramic And Its Applications For High-temperature Sensors

Polymer-derived ceramic (PDC) is the name for a class of materials synthesized by thermal decomposition of polymeric precursors which excellent thermomechanical properties, such as high thermal stability, high oxidation/corrosion resistance and high temperature multifunctionalities. Direct polymer-to-ceramic processing routes of PDCs allow easier fabrication into various components/devices with complex shapes/structures. Due to these unique properties, PDCs are considered as promising candidates for making high-temperature sensors for harsh environment applications, including high temperatures, high stress, corrosive species and/or radiation. The SiAlCN ceramics were synthesized using the liquid precursor of polysilazane (HTT1800) and aluminum-sec-tri-butoxide (ASB) as starting materials and dicumyl peroxide (DP) as thermal initiator. The as-received SiAlCN ceramics have very good thermal-mechanical properties and no detectable weight loss and large scale crystallization. Solid-state NMR indicates that SiAlCN ceramics have the SiN4, SiO4, SiCN3, and AlN5/AlN6 units. Raman spectra reveals that SiAlCN ceramics contain “free carbon” phase with two specific Raman peaks of “D” band and “G” band at 1350 cm1 and 1600 cm1 , respectively. The “free carbon” becomes more and more ordered with increasing the pyrolysis temperature. EPR results show that the defects in SiAlCN ceramics are carbon-related with a g-factor of 2.0016±0.0006. Meanwhile, the defect concentration decreases with increasing sintered temperature, which is consistent with the results obtained from Raman spectra. iv Electric and dielectric properties of SiAlCN ceramics were characterized. The D.C. conductivity of SiAlCN ceramics increases with increasing sintered temperature and the activation energy is about 5.1 eV which higher than that of SiCN ceramics due to the presence of oxygen. The temperature dependent conductivity indicates that the conducting mechanism is a semiconducting band-gap model and follows the Arrhenius equation with two different sections of activation energy of 0.57 eVand 0.23 eV, respectively. The temperature dependent conductivity makes SiAlCN ceramics suit able for high temperature sensor applications. The dielectric properties were carried out by the Agilent 4298A LRC meter. The results reveal an increase in both dielectric constant and loss with increasing temperature (both pyrolysis and tested). Dielectric loss is dominated by the increasing of conductivity of SiAlCN ceramics at high sintered temperatures. SiAlCN ceramic sensors were fabricated by using the micro-machining method. High temperature wire bonding issues were solved by the integrity embedded method (IEM). It’s found that the micro-machining method is a promising and cost-effective way to fabricate PDC high temperature sensors. Moreover IEM is a good method to solve the high temperature wire bonding problems with clear bonding interface between the SiAlCN sensor head and Pt wires. The Wheatstone bridge circuit is well designed by considering the resistance relationship between the matching resistor and the SiAlCN sensor resistor. It was found that the maximum sensitivity can be achieved when the resistance of matching resistor is equal to that of the SiAlCN v sensor. The as-received SiAlCN ceramic sensor was tested up to 600 C with the relative output voltage changing from -3.932 V to 1.153 V. The results indicate that the relationship between output voltage and test temperature is nonlinear. The tested sensor output voltage agrees well with the simulated results. The durability test was carried out at 510 C for more than two hours. It was found that the output voltage remained constant for the first 30 min and then decreased gradually afterward by 0.02, 0.04 and 0.07 V for 1, 1.5 and 2 hours

Nanomaterial Characterization Using Actuated Microelectromechanical Testing Stages

In this work, microfabricated mechanical systems have been created in a variety of forms and operated to perform nanomaterials characterization tests. A simplified integrated test system was developed and used to collect data from a range of materials including gallium nitride nanowires. A new force estimation approach was developed which enables estimation of the forces provided by electrothermal microelectromechanical (MEMS) actuators, and with knowledge of a material specimen cross-section area, an estimation of the engineering stress within the nanomaterial specimen. In an expanded design, a MEMS micromanipulator probe interfaced with a removable specimen holder, also known as a test coupon, to apply strain to and acquire tensile data from carbon nanotubes grown directly on a test coupon. A novel approach for removably interfacing two microfabricated chips was created. This interface mechanism enables the test coupon to incorporate a selection of possible experiments. These test devices can be operated in vacuum or air environments, and serve as a proof-of-concept of a microsystem testbed for mechanical measurements that can be performed simultaneously with other types of measurements such as electron diffraction, piezoresistive measurement, scanning tunneling microscope observation, or optical measurements. The goal of this thesis was the demonstration of a microsystem capable of performing tensile characterization of nanowire or nanotube specimens that are not permanently interfaced to the actuators used to apply mechanical strain, with emphasis on the overall operation and characterization of this system

Mechanical Properties of Low Dimensional Materials

Recent advances in low dimensional materials (LDMs) have paved the way for unprecedented technological advancements. The drive to reduce the dimensions of electronics has compelled researchers to devise newer techniques to not only synthesize novel materials, but also tailor their properties. Although micro and nanomaterials have shown phenomenal electronic properties, their mechanical robustness and a thorough understanding of their structure-property relationship are critical for their use in practical applications. However, the challenges in probing these mechanical properties dramatically increase as their dimensions shrink, rendering the commonly used techniques inadequate. This Dissertation focuses on developing techniques for accurate determination of elastic modulus of LDMs and their mechanical responses under tensile and shear stresses. Fibers with micron-sized diameters continuously undergo tensile and shear deformations through many phases of their processing and applications. Significant attention has been given to their tensile response and their structure-tensile properties relations are well understood, but the same cannot be said about their shear responses or the structure-shear properties. This is partly due to the lack of appropriate instruments that are capable of performing direct shear measurements. In an attempt to fill this void, this Dissertation describes the design of an inexpensive tabletop instrument, referred to as the twister, which can measure the shear modulus (G) and other longitudinal shear properties of micron-sized individual fibers. An automated system applies a pre-determined twist to the fiber sample and measures the resulting torque using a sensitive optical detector. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers. Two industrially important fibers, IM7 carbon fiber and Kevlar® 119, were found to have G = 17 and 2.4 GPa, respectively. In addition to measuring the shear properties directly on a single strand of fiber, the technique was automated to allow hysteresis, creep and fatigue studies. Zinc oxide (ZnO) semiconducting nanostructures are well known for their piezoelectric properties and are being integrated into several nanoelectro-mechanical (NEMS) devices. In spite of numerous studies on the mechanical response of ZnO nanostructures, there is not a consensus in its measured bending modulus (E). In this Dissertation, by employing an all-electrical Harmonic Detection of Resonance (HDR) technique on ZnO nanowhisker (NW) resonators, the underlying origin for electrically-induced mechanical oscillations in a ZnO NW was elucidated. Based on visual detection and electrical measurement of mechanical resonances under a scanning electron microscope (SEM), it was shown that the use of an electron beam as a resonance detection tool alters the intrinsic electrical character of the ZnO NW, and makes it difficult to identify the source of the charge necessary for the electrostatic actuation. A systematic study of the amplitude of electrically actuated as-grown and gold-coated ZnO NWs in the presence (absence) of an electron beam using an SEM (dark-field optical microscope) suggests that the oscillations seen in our ZnO NWs are due to intrinsic static charges. In experiments involving mechanical resonances of micro and nanostructured resonators, HDR is a tool for detecting transverse resonances and E of the cantilever material. To add to this HDR capability, a novel method of measuring the G using HDR is presented. We used a helically coiled carbon nanowire (HCNW) in singly-clamped cantilever configuration, and analyzed the complex (transverse and longitudinal) resonance behavior of the nonlinear geometry. Accordingly, a synergistic protocol was developed which (i) integrated analytical, numerical (i.e., finite element using COMSOL ®) and experimental (HDR) methods to obtain an empirically validated closed form expression for the G and resonance frequency of a singly-clamped HCNW, and (ii) provided an alternative for solving 12th order differential equations. A visual detection of resonances (using in situ SEM) combined with HDR revealed intriguing non-planar resonance modes at much lower driving forces relative to those needed for linear carbon nanotube cantilevers. Interestingly, despite the presence of mechanical and geometrical nonlinearities in the HCNW resonance behavior, the ratio of the first two transverse modes f2/f1 was found to be similar to the ratio predicted by the Euler-Bernoulli theorem for linear cantilevers

Flexible ecoflex®/graphene nanoplatelet foams for highly sensitive low-pressure sensors

The high demand for multifunctional devices for smart clothing applications, human motion detection, soft robotics, and artificial electronic skins has encouraged researchers to develop new high-performance flexible sensors. In this work, we fabricated and tested new 3D squeezable Ecoflex® open cell foams loaded with different concentrations of graphene nanoplatelets (GNPs) in order to obtain lightweight, soft, and cost-effective piezoresistive sensors with high sensitivity in a low-pressure regime. We analyzed the morphology of the produced materials and characterized both the mechanical and piezoresistive response of samples through quasi-static cyclic compression tests. Results indicated that sensors infiltrated with 1 mg of ethanol/GNP solution with a GNP concentration of 3 mg/mL were more sensitive and stable compared to those infiltrated with the same amount of ethanol/GNP solution but with a lower GNP concentration. The electromechanical response of the sensors showed a negative piezoresistive behavior up to ~10 kPa and an opposite trend for the 10–40 kPa range. The sensors were particularly sensitive at very low deformations, thus obtaining a maximum sensitivity of 0.28 kPa−1 for pressures lower than 10 kPa

DEVELOPMENT OF PIEZORESISTIVE SILICON NANOWIRES (SINWS) AND THE APPLICATIONS IN NEMS TECHNOLOGY.

Ph.DDOCTOR OF PHILOSOPH