Gary M. Atkinson’s research while affiliated with Virginia Commonwealth University and other places

Cryogenic deep reactive ion etching (Cryo DRIE) of silicon has become an enticing but challenging process utilized in front-end fabrication for the semiconductor industry. This method, compared to the Bosch process, yields vertical etch profiles with smoother sidewalls not subjected to scalloping, which are desired for many microelectromechanical systems (MEMS) applications. Smoother sidewalls enhance electrical contact by ensuring more conformal and uniform sidewall coverage, thereby increasing the effective contact area without altering contact dimensions. The versatility of the Cryo DRIE process allows for customization of the etch profiles by adjusting key process parameters such as table temperature, O2 percentage of the total gas flow rate (O2 + SF6), RF bias power and process pressure. In this work, we undertake a comprehensive study of the effects of Cryo DRIE process parameters on the trench profiles in the structures used to define cantilevers in MEMS devices. Experiments were performed with an Oxford PlasmaPro 100 Estrelas ICP-RIE system using positive photoresist SPR-955 as a mask material. Our findings demonstrate significant influences on the sidewall angle, etch rate and trench shape due to these parameter modifications. Varying the table temperature between −80 °C and −120 °C under a constant process pressure of 10 mTorr changes the etch rate from 3 to 4 μm min⁻¹, while sidewall angle changes by ∼2°, from positive (<90° relative to the Si surface) to negative (>90° relative to the Si surface) tapering. Altering the O2 flow rate with constant SF6 flow results in a notable 10° shift in sidewall tapering. Furthermore, SPR-955 photoresist masks provide selectivity of 46:1 with respect to Si and facilitates the fabrication of MEMS devices with precise dimension control ranging from 1 to 100 μm for etching depths up to 42 μm using Cryo DRIE. Understanding the influence of each parameter is crucial for optimizing MEMS device fabrication.

We measure the stiction force using in-plane electrostatically actuated Si nanoelectromechanical cantilever relays with Pt contacts. The average current-dependent values of the stiction force, ranging from 60 nN to 265 nN, were extracted using the I DS vs V GS hysteresis curves, the cantilever displacement information from finite element method (Comsol Multiphysics) simulations, and the force distribution determined using an analytical model. It is shown that the stiction force is inversely and directly proportional to the contact resistance (R c) and drain-source current (I DS), respectively. Using the dependence of the stiction force on the contact current, we demonstrate the tuning of the voltage hysteresis for the same relay from 8 V to 36 V (equivalent to a stiction force of 70 nN to 260 nN, respectively). We attribute the stiction force primarily to the metallic bonding force, which shows a strong dependence on the contact current.

The current electromechanical and solid-state relays employed in the Nuclear PowerInstrumentation and Control (I&C) are outdated and subjected to maintenance and reliabilitychallenges primarily related to degradation caused by ageing, high voltage/current operations,electromagnetic interference ( EMI), and non -ionizing and ionizing radiation. To address theseconcerns, we assert that significant performance and cost advantages may be gained by shifting tomicro- and nano-scale electromechanical relays for I&C applications in nuclear power plants(NPPs). Single-crystal silicon is well known to be extremely resistive to material fatigue and creepdeformation, providing superior reliability of electromechanical actuation. Each micro-scaled relaycan be situated within a 2 mm x 2 mm area and nano-scaled relays may be implemented to have a 200 μm x 200 μm footprint. A single Si wafer (e.g., 100 mm in diameter) can then incorporate alarge number of relays produced in a single fabrication run and each relay, based on a cantilever that moves laterally under electrostatic attraction, can be uniquely designed and fabricated to operate ata specific turn-on (actuation) voltage. Moreover, these relays on the same wafer can be interconnected as desired to perform a variety of logic operations. In this work, we present the novelfabrication methods and test results for both micro- and nano-scale relays. Although micro- and nano-scale devices involve lithographic processes of different scales, the resultant operation of the fabricated relays is tested in the same manner. As an initial viability test, micro- and nano-scaled cantilever modeling simulations with actuation voltages (Vpi) of 10 – 45V are compared against analytical models showing a 3 - 4 % difference between them. Furthermore, simulated testing to ascertain the stress response of cantilevers suggest a potential fatigue lifetime of > 10 billion cycles. Additionally, preliminary DC and AC ON/OFF switching characteristics of fabricated nano-scalerelays were found to have actuation voltages (Vpi) near ~43V.

We report experimental manipulation of the magnetic states of elliptical cobalt magnetostrictive nanomagnets (of nominal dimensions ~ 340 nm × 270 nm × 12 nm) delineated on bulk 128° Y-cut lithium niobate with acoustic waves (AWs) launched from interdigitated electrodes. Isolated nanomagnets (no dipole interaction with any other nanomagnet) that are initially magnetized with a magnetic field to a single domain state with the magnetization aligned along the major axis of the ellipse are driven into a vortex state by acoustic waves that modulate the stress anisotropy of these nanomagnets. The nanomagnets remain in the vortex state until they are reset by a strong magnetic field to the initial single domain state, making the vortex state non-volatile. This phenomenon is modeled and explained using a micromagnetic framework and could lead to the development of extremely energy efficient magnetization switching methodologies for low power computing applications.

We report nanomagnetic switching with Acoustic Waves (AW) launched from interdigitated electrodes that modulate the stress anisotropy of elliptical cobalt nanoscale magnetostrictive magnets (340 nm x 270 nm x 12 nm) delineated on 128 degree Y-cut lithium niobate. The dipole-coupled nanomagnet pairs are in a single-domain state and are initially magnetized along the major axis of the ellipse, with their magnetizations parallel to each other. The magnetizations of nanomagnets having lower shape anisotropy are reversed upon acoustic wave propagation. Thereafter, the magnetization of these nanomagnets remains in the reversed state and demonstrate non-volatility. This executes a 'NOT' operation. This proof of acoustic wave induced magnetic state reversal in dipole-coupled nanomagnets implementing a 'NOT' gate operation could potentially lead to the development of extremely energy-efficient nanomagnetic logic. Furthermore, fabrication complexity is reduced immensely due to the absence of individual contacts to the nanomagnets, leading to lower energy dissipation

We report nanomagnetic switching with Acoustic Waves (AW) launched from interdigitated electrodes that modulate the stress anisotropy of elliptical cobalt nanoscale magnetostrictive magnets (340 nm x 270 nm x 12 nm) delineated on 128 degree Y-cut lithium niobate. The dipole-coupled nanomagnet pairs are in a single-domain state and are initially magnetized along the major axis of the ellipse, with their magnetizations parallel to each other. The magnetizations of nanomagnets having lower shape anisotropy are reversed upon acoustic wave propagation. Thereafter, the magnetization of these nanomagnets remains in the reversed state and demonstrate non-volatility. This executes a 'NOT' operation. This proof of acoustic wave induced magnetic state reversal in dipole-coupled nanomagnets implementing a 'NOT' gate operation could potentially lead to the development of extremely energy-efficient nanomagnetic logic. Furthermore, fabrication complexity is reduced immensely due to the absence of individual contacts to the nanomagnets, leading to lower energy dissipation

The authors present a new low-temperature nanowirefabrication process that allows high-aspect ratio nanowires to be readily integrated with microelectronic devices for sensor applications. This process relies on a new method of forming a close-packed array of self-assembled high-aspect-ratio nanopores in an anodized aluminum oxide (AAO) template in a thin (2.5 μm) aluminum film deposited on a silicon substrate. This technique is in sharp contrast to the traditional free-standing thick film methods, and the use of an integrated thin aluminum film greatly enhances the utility of such methods. The authors have demonstrated the method by integrating ZnOnanowires onto the metal gate of a metal-oxide-semiconductor (MOS) transistor to form an integrated chemical field-effect transistor (ChemFET) sensor structure. The novel thin film AAO process uses a novel multistage aluminum anodization, alumina barrier layer removal, ZnOatomic layer deposition(ALD), and pH controlled wet release etching. This new process selectively forms the ZnOnanowires on the aluminum gate of the transistor while maintaining the remainder of the aluminum film intact for other integrated device components and interconnects. This self-assembled high-density AAO template was selectively formed in an ultrasmooth 2.5 μm thick aluminum layer deposited through e-beam evaporation without the electropolishing required in AAO template formation in traditional 100 μm thick free standing films. The resulting nanopore AAO template consists of nanopores of 90 nm in diameter and 1 μm in height at an aerial density of 1.3 × 1010 nanopores/cm2. This thin film AAO template was then filled with ZnO using ALD at 200 °C, forming polycrystalline ZnOnanowires inside the pores. The alumina template was then removed with a buffered NaOH solution, leaving free standing ZnOnanowires of 1 μm height and 90 nm diameter, offering an increase in 38× the surface area over a standard flat ZnO film for sensing applications. The aluminum film remains intact (unanodized) in nonselected regions of the device as well as underlying the ZnOnanowires, acting as the gate of the MOS transistor. The ZnOnanowires were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy to verify stoichiometry and crystal structure. Additionally, the response of a ZnOnanowire ChemFET was measured using ammonia as a target gas. I-V characterization and transient response to ammonia in the range of 25–200 ppm were examined. The ammonia response to the threshold limit value concentration of ammonia (25 ppm) shows a 56 mV shift in threshold voltage, an overall sensitivity of 14%, an 8 min response time, and a 27 min recovery period. The ZnOnanowirefabrication sequence that the authors present is accomplished at low-temperature (<200 °C) and can be accomplished selectively, making it readily amenable to integration with standard metal-oxide-semiconductor field-effect transistor processing as well as other microelectronic sensors such as surface acoustic wave devices. This new process has initially been demonstrated using ZnO, but is also adaptable to a variety of nanowire materials using appropriate deposition methods as well as selective nanowire release methods. This allows the potential to conveniently fabricate a variety of high-aspect ratio nanowire based microelectronic sensors for a range of applications.

We report manipulation of the magnetic states of elliptical cobalt magnetostrictive nanomagnets (of nominal dimensions ~ 340 nm x 270 nm x 12 nm) delineated on bulk 128{\deg} Y-cut lithium niobate with Surface Acoustic Waves (SAWs) launched from interdigitated electrodes. Isolated nanomagnets that are initially magnetized to a single domain state with magnetization pointing along the major axis of the ellipse are driven into a vortex state by surface acoustic waves that modulate the stress anisotropy of these nanomagnets. The nanomagnets remain in the vortex state until they are reset by a strong magnetic field to the initial single domain state, making the vortex state non-volatile. This phenomenon is modeled and explained using a micromagnetic framework and could lead to the development of extremely energy efficient magnetization switching methodologies.

NASA aeronautical programs require rigorous ground and flight testing. Many of the testing environments can be extremely harsh. These environments include cryogenic temperatures and high temperatures (>1500 °C). Temperature, pressure, vibration, ionizing radiation, and chemical exposure may all be a part of the harsh environment found in testing. This paper presents a survey of research opportunities for universities and industry to develop new wireless sensors that address anticipated structural health monitoring and testing needs for aeronautical vehicles. Potential applications of passive wireless sensors for ground testing and high-altitude aircraft operations are presented. Some of the challenges and issues of the technology are also presented.

Surface acoustic wave sensors on langasite substrates are being investigated for aerospace applications. Characterization of langasite material properties must be performed before sensors can be flight tested in research vehicles. Analysis and empirical data have been used to determine the coefficients of velocity for both strain and temperature. This work presents temperature compensated strain results that are based upon the empirical values for the strain and temperature coefficients of velocity.