Christian A. Zorman

Nanoelectromechanical systems: Nanodevice motion at microwave frequencies

It has been almost forgotten that the first computers envisaged by Charles Babbage in the early 1800s were mechanical and not electronic, but the development of high-frequency nanoelectromechanical systems is now promising a range of new applications, including sensitive mechanical charge detectors and mechanical devices for high-frequency signal processing, biological imaging and quantum measurement. Here we describe the construction of nanodevices that will operate with fundamental frequencies in the previously inaccessible microwave range (greater than 1 gigahertz). This achievement represents a significant advance in the quest for extremely high-frequency nanoelectromechanical systems.

Evaluation of MEMS materials of construction for implantable medical devices

Medical devices based on microelectro-mechanical systems (MEMS) platforms are currently being proposed for a wide variety of implantable applications. However, biocompatibility data for typical MEMS materials of construction and processing, obtained from standard tests currently recognized by regulatory agencies, has not been published. Likewise, the effects of common sterilization techniques on MEMS material properties have not been reported. Medical device regulatory requirements dictate that materials that are biocompatibility tested be processed and sterilized in a manner equivalent to the final production device. Material, processing, and sterilization method can impact the final result. Six candidate materials for implantable MEMS devices, and one encapsulating material, were fabricated using typical MEMS processing techniques and sterilized. All seven materials were evaluated using a baseline battery of ISO 10993 physicochemical and biocompatibility tests. In addition, samples of these materials were evaluated using a scanning electron microscope (SEM) pre- and post-sterilization. While not addressing all facets of ISO 10993 testing, the biocompatibility and SEM data indicate few concerns about use of these materials in implant applications.

Silicon carbide MEMS for harsh environments

Silicon carbide (SiC) is a promising material for the development of high-temperature solid-state electronics and transducers, owing to its excellent electrical, mechanical, and chemical properties. This paper is a review of silicon carbide for microelectromechanical systems (SiC MEMS). Current efforts in developing SiC MEMS to extend the silicon-based MEMS technology to applications in harsh environments are discussed. A summary is presented of the material properties that make SiC an attractive material for use in such environments. Challenges faced in the development of processing techniques are also outlined. Last, a review of the current stare of SiC MEMS devices and issues facing future progress are presented.

SiC MEMS: opportunities and challenges for applications in harsh environments

Abstract Many measurement and control applications requiring MEMS technology are in the presence of harsh environments, e.g. high temperatures, intense shock/vibrations, erosive flows, and corrosive media. Unlike Si, SiC as a semiconductor material is exceptionally well suited for addressing such application opportunities. However, many challenges must be met in order to develop a mature SiC MEMS fabrication technology. These challenges are primarily technical in nature and relate to material and processing aspects. This paper presents a review of recent advancements in areas that are critical to the establishment of a SiC MEMS technology.

Monocrystalline silicon carbide nanoelectromechanical systems

SiC is an extremely promising material for nanoelectromechanical systems given its large Youngs modulus and robust surface properties. We have patterned nanometer scale electromechanical resonators from single-crystal 3C-SiC layers grown epitaxially upon Si substrates. A surface nanomachining process is described that involves electron beam lithography followed by dry anisotropic and selective electron cyclotron resonance plasma etching steps. Measurements on a representative family of the resulting devices demonstrate that, for a given geometry, nanometer-scale SiC resonators are capable of yielding substantially higher frequencies than GaAs and Si resonators.

Epitaxial growth of 3C–SiC films on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition

Silicon carbide (SiC) films have been grown on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition, using propane, silane, and hydrogen. X‐ray photoelectron spectroscopy data confirm that the films are stoichiometric SiC, with no major impurities. X‐ray diffraction and transmission electron microscopy (TEM) data indicate that the films are single‐crystalline cubic polytype (3C) across the 4 in. substrates. With the exception of slip lines near the edge of the wafers, the films appear featureless when observed optically. The nitrogen concentration, as determined by secondary ion mass spectroscopy, is 4×1018 cm3. Cross‐sectional TEM images show a fairly rough, void‐free interface.

High-temperature single-crystal 3C-SiC capacitive pressure sensor

Single-crystal 3C-silicon carbide (SiC) capacitive pressure sensors are proposed for high-temperature sensing applications. The prototype device consists of an edge-clamped circular 3C-SiC diaphragm with a radius of 400 /spl mu/m and a thickness of 0.5 /spl mu/m suspended over a 2-/spl mu/m sealed cavity on a silicon substrate. The 3C-SiC film is grown epitaxially on a 100-mm diameter <100> silicon substrate by atmospheric pressure chemical vapor deposition. The fabricated sensor demonstrates a high-temperature sensing capability up to 400/spl deg/C, limited by the test setup. At 400/spl deg/C, the device achieves a linear characteristic response between 1100 and 1760 torr with a sensitivity of 7.7 fF/torr, a linearity of 2.1%, and a hysterisis of 3.7% with a sensing repeatability of 39 torr (52 mbar). A wide range of sensor specifications, such as linear ranges, sensitivities, and capacitance values, can be achieved by choosing the proper device geometrical parameters.

Silicon carbide for microelectromechanical systems

Abstract Silicon carbide (SiC) has recently attracted attention as a wide bandgap semiconductor with great potential for microelectromechanical systems (MEMS). SiC exhibits excellent electrical, mechanical, and chemical properties, making it well suited for harsh environment applications where traditional MEMS are constrained by the physical limitations of silicon (Si). This paper reviews the material properties, deposition techniques, micromachining processes, and other issues regarding the fabrication of SiC-based sensors and actuators. Special emphasis is placed on the properties that make SiC attractive for MEMS, and the Si-based processing techniques that have been adapted to realise SiC MEMS structures and devices. An introduction to micromachining is provided for readers not familiar with MEMS fabrication techniques.

Polytype control of spin qubits in silicon carbide.

Crystal defects can confine isolated electronic spins and are promising candidates for solid-state quantum information. Alongside research focusing on nitrogen-vacancy centres in diamond, an alternative strategy seeks to identify new spin systems with an expanded set of technological capabilities, a materials-driven approach that could ultimately lead to ‘designer’ spins with tailored properties. Here we show that the 4H, 6H and 3C polytypes of SiC all host coherent and optically addressable defect spin states, including states in all three with room-temperature quantum coherence. The prevalence of this spin coherence shows that crystal polymorphism can be a degree of freedom for engineering spin qubits. Long spin coherence times allow us to use double electron–electron resonance to measure magnetic dipole interactions between spin ensembles in inequivalent lattice sites of the same crystal. Together with the distinct optical and spin transition energies of such inequivalent states, these interactions provide a route to dipole-coupled networks of separately addressable spins.

VHF, UHF and microwave frequency nanomechanical resonators

Nanomechanical resonators with fundamental mode resonance frequencies in the very-high frequency (VHF), ultra-high frequency (UHF) and microwave L-band ranges are fabricated from monocystalline silicon carbide (SiC) thin film material, and measured by magnetomotive transduction, combined with a balanced-bridge readout circuit. For resonators made from the same film, we measured the frequency dependence (thus geometry dependence) of the quality factor. We have seen a steady decrease of quality factor as the frequency goes up. This indicates the importance of clamping losses in this regime. To study this source of dissipation, a free-free beam SiC nanomechanical resonator has been co-fabricated on the same chip with a doubly clamped beam resonator operating at similar frequencies. Device testing has been performed to directly compare their characteristics and performance. It is observed that a significant improvement in quality factor is attained from the free-free beam design. In addition, from studies of resonators made from different chips with varying surface roughness, we found a strong correlation between surface roughness of the SiC thin film material and the quality factor of the resonators made from it. Furthermore, we experimentally studied the eddy current damping effect in the context of magnetomotive transduction. A high-aspect ratio SiC nanowire resonator is fabricated and tested for this study. Understanding the dissipation mechanisms, and thus improving the quality factor of these resonators, is important for implementing applications promised by these devices.