Browsing by Subject "MEMS"

The primary goals of this study were to demonstrate and fully characterize a microscale ionization source (i.e. micro-ion source) and to determine the validity of impact ionization theory for microscale devices and pressures up to 100 mTorr. The field emission properties of carbon nanotubes (CNTs) along with Micro-Electro-Mechanical Systems (MEMS) design processes were used to achieve these goals. Microwave Plasma-enhanced CVD was used to grow vertically aligned Multi-Walled Carbon Nanotubes (MWNTs) on the microscale devices. A 4-dimensional parametric study focusing on CNT growth parameters confirmed that Fe catalyst thickness had a strong effect on MWNT diameter. The MWNT growth rate was seen to be a strong function of the methane-to-ammonia gas ratio during MWNT growth. A high methane-to-ammonia gas ratio was selected for MWNT growth on the MEMS devices in order to minimize growth time and ensure that the thermal budget of those devices was met.

A CNT-enabled microtriode device was fabricated and characterized. A new aspect of this device was the inclusion of a 10 micron-thick silicon dioxide electrical isolation layer. This thick oxide layer enabled anode current saturation and performance improvements such as an increase in dc amplification factor from 27 to 600. The same 3-panel device was also used as an ionization source. Ion currents were measured in the 3-panel micro-ion source for helium, argon, nitrogen and xenon in the 0.1 to 100 mTorr pressure range. A linear increase in ion current was observed for an increase in pressure. However, simulations indicated that the 3-panel design could be modified to improve performance as well as better understand device behavior. Thus, simulations and literature reports on electron impact ionization sources were used to design a new 4-panel micro-ion source. The 4-panel micro-ion source showed an approximate 10-fold performance improvement compared to the 3-panel ion source device. The improvement was attributed to the increased electron current and improved ion collection efficiency of the 4-panel device. Further, the same device was also operated in a 3-panel mode and showed superior performance compared to the original 3-panel device, mainly because of increased ion collection efficiency.

The effect of voltages applied to the different electrodes in the 4-panel micro-ion source on ion source performance was studied to better understand device behavior. The validity of the ion current equation (which was developed for macroscale ion sources operating at low pressures) in the 4-panel micro-ion source was studied. Experimental ion currents were measured for helium, argon and xenon in the 3 to 100 mTorr pressure range. For comparison, theoretical ion currents were calculated using the ion current equation for the 4-panel micro-ion source utilizing values calculated from SIMION simulations and measured electron currents. The measured ion current values in the 3 to 20 mTorr pressure range followed the calculated ion currents quite closely. A significant deviation was observed in the 20-100 mTorr pressure range. The experimental ion current values were used to develop a corrected empirical model for the 4-panel micro-ion source in this high pressure range (i.e., 3 to 100 mTorr). The role of secondary electrons and electron path lengths at higher pressures is discussed.

Biosensing is a critical area of research that involves detecting and measuring biological molecules. Among the various types of biosensors, acoustic biosensors are attractive for their simplicity, robustness, and low cost, particularly in point-of-care (POC) applications. However, the quality factor (Q-factor) of acoustic biosensors is often low, limiting their sensitivity and accuracy in terms of in-liquid gravimetric detection for biosensing applications. In this dissertation, we present a novel approach that eliminates nearly all dissipation and damping from sample liquids, rendering a significant improvement in Q-factor for in-liquid gravimetric detection. We constructed rigid microfluidic channels to confine liquids and the associated acoustic energy, thereby eliminating acoustic radiation damping. We also used the channels' side walls to create pressure waves, confining the liquids within and suppressing acoustic damping due to the viscous layer. The quartz crystal microbalance (QCM) was selected as the model system for implementing the new paradigm due to its widespread usage in various applications, simplicity, cost-effectiveness, and relevance of its principles to other types of acoustic biosensors. We hypothesized that the ratio of the wavelength of the pressure wave to the width of the channels is a crucial determining factor for optimal performance. We then tested the hypothesis by building the microfluidic QCM (the µ-QCM) to improve the Q-factor of conventional QCM. The combination of experiments, simulations, and theoretical studies demonstrated a 10-fold improvement in the Q-factor. The new system offers many other advantages, including direct data interpretation, minimized sample volume requirement, and easier temperature control for in-liquid gravimetric detection. Additionally, the same principles can be applied to other acoustic biosensors, benefiting the entire field.

Carbon nanotubes (CNTs) have recently emerged as promising candidates for electron field emission (FE) cathodes in integrated FE devices. These nanostructured carbon materials possess exceptional properties and their synthesis can be thoroughly controlled. Their integration into advanced electronic devices, including not only FE cathodes, but sensors, energy storage devices, and circuit components, has seen rapid growth in recent years. The results of the studies presented here demonstrate that the CNT field emitter is an excellent candidate for next generation vacuum microelectronics and related electron emission devices in several advanced applications.

The work presented in this study addresses determining factors that currently confine the performance and application of CNT-FE devices. Characterization studies and improvements to the FE properties of CNTs, along with Micro-Electro-Mechanical Systems (MEMS) design and fabrication, were utilized in achieving these goals. Important performance limiting parameters, including emitter lifetime and failure from poor substrate adhesion, are examined. The compatibility and integration of CNT emitters with the governing MEMS substrate (i.e., polycrystalline silicon), and its impact on these performance limiting parameters, are reported. CNT growth mechanisms and kinetics were investigated and compared to silicon (100) to improve the design of CNT emitter integrated MEMS based electronic devices, specifically in vacuum microelectronic device (VMD) applications.

Improved growth allowed for design and development of novel cold-cathode FE devices utilizing CNT field emitters. A chemical ionization (CI) source based on a CNT-FE electron source was developed and evaluated in a commercial desktop mass spectrometer for explosives trace detection. This work demonstrated the first reported use of a CNT-based ion source capable of collecting CI mass spectra. The CNT-FE source demonstrated low power requirements, pulsing capabilities, and average lifetimes of over 320 hours when operated in constant emission mode under elevated pressures, without sacrificing performance. Additionally, a novel packaged ion source for miniature mass spectrometer applications using CNT emitters, a MEMS based Nier-type geometry, and a Low Temperature Cofired Ceramic (LTCC) 3D scaffold with integrated ion optics were developed and characterized. While previous research has shown other devices capable of collecting ion currents on chip, this LTCC packaged MEMS micro-ion source demonstrated improvements in energy and angular dispersion as well as the ability to direct the ions out of the packaged source and towards a mass analyzer. Simulations and experimental design, fabrication, and characterization were used to make these improvements.

Finally, novel CNT-FE devices were developed to investigate their potential to perform as active circuit elements in VMD circuits. Difficulty integrating devices at micron-scales has hindered the use of vacuum electronic devices in integrated circuits, despite the unique advantages they offer in select applications. Using a combination of particle trajectory simulation and experimental characterization, device performance in an integrated platform was investigated. Solutions to the difficulties in operating multiple devices in close proximity and enhancing electron transmission (i.e., reducing grid loss) are explored in detail. A systematic and iterative process was used to develop isolation structures that reduced crosstalk between neighboring devices from 15% on average, to nearly zero. Innovative geometries and a new operational mode reduced grid loss by nearly threefold, thereby improving transmission of the emitted cathode current to the anode from 25% in initial designs to 70% on average. These performance enhancements are important enablers for larger scale integration and for the realization of complex vacuum microelectronic circuits.

The observation and use of thermal radiation has a long history. Significant advance was made in 1879 when Josef Stefan found that “the total radiated power per unit surface area of a black body across all wavelengths is directly proportional to the fourth power of its temperature”, which was later named the Stefan–Boltzmann law. The Stefan–Boltzmann law sets a limit for the thermal radiation from most of natural materials, since their total radiated energy is proportional to the fourth power of their temperature. Thus, use of natural materials for the control and manipulation of thermal emission is hindered from further development. Metamaterials are artificial materials consisting of sub-wavelength unit cells, and good candidates to break these limitations, since the optical properties of metamaterials originates from their geometrical designs, as opposed to their chemical composition. Here we propose and demonstrate the idea of metamaterial based on microelectromechanical system capable of dynamically tailoring the energy emitted from a surface, with its emission performance going beyond the Stefan–Boltzmann law. Our dynamic metamaterial emitters have great application prospects in energy harvesting, space exploration, sensing and detecting, and many other areas. In addition, our results are not limited to the thermal infrared band, demonstrate here, but may be scaled to nearly any sub-optical range of the electromagnetic spectrum, and verify the potential of MEMS metamaterials to operate as reconfigurable multifunctional devices with unprecedented energy control capabilities.

Although metamaterial may yield advanced thermal emission control, they are difficult to apply to some applications, such as in thermal imaging and energy harvesting with thermophotovoltaics. This is because they are typically fashioned with metallic materials and thus possess low melting points, high Ohmic loss, and high thermal conductivity. Here we present an all dielectric metamaterial absorber/emitter. By overlapping the electric and magnetic dipole resonances, a high absorptive / emissive state can be achieved. Due to its great thermal properties, such as heat localization and thermal stability, an all dielectric metamaterial absorber/ emitter can replace metal-based metamaterial in some application areas, and offers a new route for applications in thermophotovoltaics, imaging, and sensing.

This dissertation consists of seven chapters. The first chapter gives a brief introduction to thermal radiation, metamaterials, metamaterial absorbers, and all dielectric metamaterials. The second chapter discusses in detail thermochromic infrared metamaterials. The third chapter demonstrates a reconfigurable room temperature metamaterial infrared emitter. The fourth chapter shows a THz all dielectric metamaterial absorber. The fifth chapter gives another example of all dielectric metamaterial emitters that can be used in thermophotovoltaic systems. The sixth chapter is a summary. The seventh chapter is an executive summary of original contributions.

Piezoelectric micromachined ultrasound transducer (pMUT) two-dimensional (2D) arrays have been proposed as an alternative to conventional bulk-PZT thickness-mode transducers for high frequency, forward-looking, catheter-based ultrasound imaging of the cardiovascular system. The appeal of pMUTs is based on several key advantages over conventional transducer technologies, including high operational frequencies, small element size, and low cost due to their microelectromechanical system (MEMS) silicon-based fabrication. While previous studies have demonstrated acoustic performance characteristics suitable for ultrasound image formation, pulse-echo B-mode imaging of tissue and tissue-like phantoms using 2D pMUT arrays small enough for forward-looking catheter-based applications have been demonstrated only at Duke University by Dausch et al.

Having demonstrated the suitability of 2D pMUT arrays for tissue imaging, an important step is to demonstrate effective design control. The frequency of operation is a fundamental component of transducer design. Previous modeling efforts for pMUT vibration have used classical/Kirchoff thin plate theory (CPT) or Mindlin thick plate theory, however pMUTs with geometric dimensions similar to those explored here, have not been modeled with experimental comparison to physical devices.

It is hypothesized that the frequency of vibration of pMUTs can be predictively modeled based on experimental data from various pMUT configurations. Experimental frequency results were acquired and used to develop an empirical model based on a modified Mindlin thick plate theory. This dissertation presents the development of the frequency design theory culminating in a set of predictive design equations for the frequency of vibration of 2D pMUT arrays aimed at improving their use in high-frequency, forward-looking, catheter-based ultrasound imaging applications.

Among the various physical systems considered for scalable quantum information processing (QIP), individually trapped ions or neutral atoms have emerged as promising candidates. Recent experiments using these systems have demonstrated the basic building blocks required for a useful quantum computer. In many of these experiments, precisely tuned lasers control and manipulate the quantum bit (qubit) represented in the electronic energy levels of the ion or atom. Scaling these systems to the necessary number of qubits needed for meaningful calculations, requires the development of scalable optical technology capable of delivering laser resources across an array of ions or atoms. That scalable technology is currently not available.

In this dissertation, I will report on the development, design, characterization, and implementation of an optical beam steering system utilizing microelectromechanical systems (MEMS) technology. Highly optimized micromirrors enable fast reconfiguration of multiple laser beam paths which can accommodate a range of wavelengths. Employing micromirrors with a broadband metallic coating, our system has the flexibility to simultaneously control multiple beams covering a wide range of wavelengths.

The reconfiguration of two independent beams at different wavelengths (780 and 635 nm) across a common 5x5 array of target sites is reported along with micromirror switching times as fast as 4 us. The optical design of the system minimizes residual intensity at neighboring sites to less than 40 dB below the peak intensity. Integration of a similar system into a neutral atom QIP experiment is reported where 5 individually trapped atoms are selectively manipulated through single qubit rotations with a single laser source. This demonstration represents the first application of MEMS technology in scalable QIP laser addressing.