There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

http://iopscience.iop.org/article/10.1088/0022-3727/40/20/S19/pdf

Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications

Silicon carbide has attracted considerable attention in recent years as a potential material for sensor devices. This paper reviews the current status of SiC technology for a wide range of sensor applications. It is shown that SiC MEMs devices are well-established with operational devices demonstrated at high temperatures (up to 500 °C) for the sensing of motion, acceleration and gas flow. SiC sensors devices using electrical properties as the sensing mechanism have also been demonstrated principally for gas composition and radiation detection and have wide potential use in scientific, medical and combustion monitoring applications.

https://iopscience.iop.org/article/10.1088/0022-3727/40/20/S17/pdf

SiC sensors: a review

http://server.ihim.uran.ru/files/info/2008/sic rew.pdf

Prospects for SiC electronics and sensors

Single crystalline tungsten nanowires were prepared from directionally solidified NiAl-W alloys by a chemical release from the resulting binary phase material. Electron back scatter diffraction (EBSD) proves that they are single crystals having identical crystallographic orientation. Mechanical investigations such as bending tests, lateral force measurements, and mechanical resonance measurements were performed on 100-300 nm diameter wires. The wires could be either directly employed using micro tweezers, as a singly clamped nanowire or in a doubly clamped nanobridge. The mechanical tests exhibit a surprisingly high flexibility for such a brittle material resulting from the small dimensions. Force displacement measurements on singly clamped W nanowires by an AFM measurement allowed the determination of a Young's modulus of 332 GPa very close to the bulk value of 355 GPa. Doubly clamped W nanowires were employed as resonant oscillating nanowires in a magnetomotively driven resonator running at 117 kHz. The Young's modulus determined from this setup was found to be higher 450 GPa which is likely to be an artefact resulting from the shift of the resonance frequency by an additional mass loading.

https://downloads.hindawi.com/journals/jnm/2008/638947.pdf

Nanomechanics of single crystalline tungsten nanowires

Novel microelectromechanical resonators structures have been realized based on AlGaN/GaN heterostructures, which provide a basis for sophisticated sensor structures. There were grown on SiC substrates confining a two dimensional electron gas (2DEG). By means of the developed etching technology, freestanding resonators were patterned without degrading the sheet carrier concentration and electron mobility of the 2DEG inside the beams, which was confirmed by electrical measurements before and after the various process steps. As actuation and read out principle magnetomotive and piezoelectric effects were used, respectively. Due to the high sensitivity of the 2DEG and the chemical stability of the utilized materials these structures are suitable for chemical and biological sensor applications, where the sensitivity of the 2DEG on the surrounding environment acts as additional sensing signal, for example for simultaneous measurements of the viscosity and pH – value of a nanoliter droplet. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN/GaN-based MEMS with two-dimensional electron gas for novel sensor applications

Micro-electromechanical systems based on 3C-SiC/Si heterostructures

MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. Resonant AlN and SiC beams with operation frequencies between 0.01 and 3.4 MHz have been prepared using a semiconductor fabrication process. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. The resonant response was detected in the frequency domain. Resonator geometry and material were varied to attain a generalized understanding of the RF performance in dependence of the ambient pressure. In particular the quality factor shows a high sensitivity on pressure, allowing potential application as absolute pressure sensor. Theoretical models have been applied that match well to the measurement.

Gas Pressure Sensing Based on MEMS Resonators

In this paper, an approach towards the realization of a hybrid MEMS-CMOS RF oscillator module using the novel silicon-ceramic (SiCer) compound substrate technology is described. Piezoelectric aluminium-nitride MEMS resonators with quality factors Q up to 2,200 and resonant frequencies of 240, 400 and 600 MHz have been investigated as frequency-selective elements. For RF-compatible hybrid-integrated assembly and packaging, the SiCer compound substrate has been adapted, promising an efficient integration of both, microelectronic and micromechanical devices, on a single carrier substrate. Multiphysical circuit design and simulations using parametrized behavioural MEMS models have been carried out, indicating stable oscillator operation at the design frequency. As one prospective application, such an oscillator module could form part of a compact and power-efficient reconfigurable RF transceiver frontend in SiCer technology, e.g., for mobile communications.

Klemens Brückner

Papers

Nanomechanics of single crystalline tungsten nanowires

AlGaN/GaN-based MEMS with two-dimensional electron gas for novel sensor applications

Micro-electromechanical systems based on 3C-SiC/Si heterostructures

Gas Pressure Sensing Based on MEMS Resonators

Multi-technology design of an integrated MEMS-based RF oscillator using a novel silicon-ceramic compound substrate