K. Brueckner

Two-dimensional electron gas based actuation of piezoelectric AlGaN/GaN microelectromechanical resonators

Free-standing piezoelectric AlGaN/GaN beam resonators have been prepared on silicon substrates. The two-dimensional electron gas at the interface of the III/V heterostructure has been employed to act as back electrode for the piezoelectric active layer. The fundamental mode as well as higher order resonant modes of flexural vibration has been excited piezoelectrically and analyzed using optical laser–Doppler vibrometry. The experimental investigations were carried out under normal ambient conditions. The specific piezoelectric actuation scheme is described and the dependence of the measured resonant frequencies between 0.2 and 8.1 MHz on geometry and material parameters is investigated.

Strain- and pressure-dependent RF response of microelectromechanical resonators for sensing applications*

MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. A semiconductor fabrication process has been applied to prepare resonant AlN and SiC beams operating at frequencies between 0.1 and 2.1 MHz. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. For systematic studies of the resonant frequencies and quality factors, the induced voltage was measured using time domain and frequency domain techniques. Resonator geometry, material and ambient pressure were varied to attain a generalized understanding of the RF performance. The dependence of the resonant frequency on tensile axial strain has been derived analytically and extended to include highly strained beams. Based on these formulas, accurate detection of the residual layer strain after fabrication is presented. To describe the quality factor a chain of beads model has been applied successfully. The influences of the beam width and the pressure-dependent viscosity on the model parameters are analyzed.

Pulsed mode operation of strained microelectromechanical resonators in air

A pulsed mode magnetomotive operation of micro- and nanoelectromechanical devices in air is demonstrated, where viscous damping determines the quality factor of the device. An enhancement of the quality factor by increasing the resonant frequency using strained resonator structures is proposed. Internal strain is the result of the thermal mismatch between heteroepitaxial SiC or AlN layers and the silicon substrates. Comparing unstrained and strained resonators, an increase of the quality factor by one order of magnitude from about 30 to 300 was achieved. This increase will improve the sensing performance of such resonant structures for an operation in ambient environment.

Electromechanical resonances of SiC and AlN beams under ambient conditions

MEMS resonators offer great potential for RF sensor and filter applications. A semiconductor process has been used to prepare SiC and AlN beam resonators. The metallised beams are excited by an RF current in a permanent magnetic field. The resonant response is detected by the induced voltage. Despite the weakness of the signal, accurate detection has been achieved in the time domain, under ambient conditions in a magnetic field of about 0.5 T. The resonant response bears valuable information on the structural quality of the material and identifies potential for further improvement. The time domain technique presents an elegant approach to sensing applications.

Resonant Piezoelectric ALGaN/GaN MEMS Sensors in Longitudinal Mode Operation

Free-standing piezoelectric AlGaN/GaN beam resonators have been prepared on silicon substrates using a semiconductor fabrication process. To realize the back electrode for the piezoelectric active layer, the two-dimensional electron gas at the interface of the III/V heterostructure was employed. Longitudinal acoustic resonances have been excited and detected electrically. The fundamental and higher order vibration modes were analyzed in the frequency domain. The dependences of the measured resonant frequencies between 3.8 and 63.0 MHz are related to geometrical and material parameters. The sensitivity of the resonant response to environmental parameters is demonstrated exemplarily by investigating its dependence on ambient pressure.

Performance Modification of SiC MEMS

The adjustment of the properties of 3C-SiC based MEMS devices, i.e. the quality factor and resonant frequency, was achieved by changing the residual stress and the 3C-SiC material quality of the SiC-layers grown on Si(111) by manipulating the nucleation conditions and growth conditions with Ge deposition prior to the carbonization and epitaxial growth. Previous Raman analysis of the SiC-layers and measured resonant frequencies and quality factors of the processed MEMS show a dependence on the Ge amount at the interface of the Si/SiC heterostructure, which allows to adjust the MEMS properties to the requirements needed for certain applications.

FTIR Ellipsometry Analysis of the Internal Stress in SiC/Si MEMS

The resonant frequencies and quality factors of MEMS and NEMS depend critically on the layer quality and the residual stress in the SiC/Si heterostructure. It is demonstrated, that FTIRellipsometry is a suitable technique for monitoring the inhomogeneous residual stress inside SiC/Si heterostructures containing thin layers and their variation with during processing.

Micromachining of novel SiC on Si structures for device and sensor applications

In this paper the multifariousness of SiC/Si heterostructures for device and sensor applications will be demonstrated. 3C-SiC based microelectromechanical resonator beams (MEMS) with different geometries actuated by the magnetomotive effect operating under ambient conditions were fabricated. The resonant frequency reaches values up to 2 MHz. The applications of these resonators are the measurement of the viscosity of liquids or mass detection. Furthermore, photonic devices in the form of SiC/Si infrared gratings for wavelength and polarization filters in infrared spectra are processed. SiC wear protection for a dosing system with the possibility to dose nano- or picoliter droplets of water based liquids as well as SiC nanomasking for catalytic agent nanostructures are demonstrated.

AlGaN/GaN based heterostructures for MEMS and NEMS applications

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are receiving more attention. The outstanding properties of group III-nitrides offer many more possibilities for the implementation of new functionalities and a variety of technologies are available to realize group III-nitride based MEMS. In this work we demonstrate the application of these techniques for the fabrication of full-nitride MEMS. It includes a novel actuation and sensing principle based on the piezoelectric effect and employing a two-dimensional electron gas confined in AlGaN/GaN heterostructures as integrated back electrode. Furthermore, the actuation of flexural and longitudinal vibration modes in resonator bridges are demonstrated as well as their sensing properties.

Property Modification of 3C-SiC MEMS on Ge-Modified Si(100) Substrates

The manipulation of nucleation and growth conditions with Ge deposition prior to the carbonization and epitaxial growth changes the residual stress and the material quality of 3C-SiC(100)-layers grown on Si(100). This enables the modification of quality factor and resonant frequency of microelectromechanical systems (MEMS) based on 3C-SiC-layers. Measured resonant frequencies and quality factors of the magnetomotively actuated MEMS exhibit a dependence on the Ge amount at the interface of the Si/SiC heterostructure. This offers a degree of freedom to adjust the MEMS properties within a certain range to the requirements necessary for specific applications. The observed dependencies of the Young’s modulus are in good agreement with the trends of residual stress and Young’s modulus, which were determined on as grown 3C-SiC(100):Ge samples by fourier transform infrared (FTIR) spectroscopy and nanoindentation.