Showing papers by "Helmut Seidel published in 2013"

AlN micromechanical radial-contour disc resonator

Abstract: Micromechanical resonators that have high performance and are small in size are competitive candidates for radio frequency (RF) device minimization, paving the way for high performance monolithic transceivers. A piezoelectric aluminium nitride radial-contour mode disc resonator with a CMOS-compatible fabrication process is presented. The piezoelectric properties of the resonator disc, concerning the driving voltage and resonance mode deformation, are analysed, revealing the advantages introduced when using a sputter deposited AlN thin film. The radial-contour resonance mode of the disc resonator was investigated by finite element method simulation and theoretical evaluation. The resonance frequency was found to be thickness-independent, which is beneficial for fabrication and integration. In view of CMOS compatibility, a fabrication process with a low thermal budget and a tungsten titanium sacrificial layer was designed; devices were fabricated on 4 inch silicon wafers. The RF performance of the resonators with a diameter of 150 µm was measured using a HP4395A network analyzer, yielding a resonant frequency of 14.11 MHz with a Q value of 3125 and a return loss of 31.46 dB. These results indicate that this device is attractive for use in RF frequency-selecting and generation devices in high performance wireless communication systems.

Multimodal characterisation of high-Q piezoelectric micro-tuning forks

Abstract: This work presents an electrically actuated, aluminium nitride based, piezoelectric tuning fork designed at the micro-scale for selective modal actuation. This well known resonator, whose advantages have been widely studied and exploited in the milli-scale, has been implemented and studied in the micro-scale, showing promising results. A complete optical and electrical characterisation of the device has been carried out, in which various out-of-plane and in-plane vibration modes have been analysed. Its performance has been studied in vacuum, air and water. High-quality factors ( Q s) up to 72 in water and up to 5166 in air have been measured for the in-plane anti-phase mode. This Q -factor is higher than any other value published with the in-plane piezoelectric micro-cantilevers in air. Sensitivity as mass sensor and minimum detectable mass has also been estimated in air. Sensitivity values almost three orders of magnitude higher than millimetric commercial tuning forks have been achieved. Easy integration, simple and selective actuation and a Q make this kind of resonator an attractive alternative in a wide range of applications.

Periodic subwavelength ripples on lithium niobate surfaces generated by tightly focused sub-15 femtosecond sub-nanojoule pulsed near-infrared laser light

Abstract: On exposure to 85 MHz sub-15 fs pulsed 800 nm Ti:sapphire laser light in the focal spot of a high-numerical aperture oil immersion objective, characteristic periodic nanostructures emerged on the surface of z-cut congruent LiNbO3 crystals. Close to the ablation threshold shallow ripples oriented parallel to the laser polarization appeared on the surface at a periodicity Λ∥ ≈ 200 nm as well as ripples running perpendicular to the laser polarization at almost the same period (Λ⊥ ≈ 190 nm). Nanostructure formation involved melting and resolidification of LiNbO3 as evidenced by scanning electron microscopy images of structural peculiarities. Micro-Raman spectroscopy demonstrated that the resolidified material crystallized in the original structural phase. Ripple formation is attributed to plasma wave interference patterns that arise in the electron–hole plasma generated by multiphoton and avalanche collisional excitation.

Piezoresistivity and Electrical Conductivity of SiC Thin Films Deposited by High Temperature PECVD

Abstract: We introduce a novel high temperature PECVD process and use it for the deposition of silicon carbide thin films on oxidized silicon wafers at 900°C substrate temperature. A variation of the atomic composition over a wide range is achieved by altering the flow ratio of the precursors silane (SiH4) and acetylene (C2H2). XPS analysis is performed to verify the silicon to carbon ratio in the deposited layers. The resistivity of the obtained thin films shows a strong dependence on the Si/C-ratio. Four point measurements show the resistivity ranging between 5•10-3Ωcm for C-rich layers and >107Ωcm for near stoichiometric layers. We investigate the piezoresistivity of the SiC layers at room temperature under compressive and tensile strain using the four point bending method. The same method is used to analyze selected layers at elevated temperatures up to 600°C. Based on the results we evaluate the applicability of the obtained thin films for strain transducing in harsh environment MEMS sensors.

ITO nanowires for gas-sensor applications

Abstract: In this work we have realized ITO nanowires with typical dimensions of 700 nm width and 200 μm length. They were fabricated by using a novel approach of laser writing in a sputtered indium tin oxide (ITO) film by using a high-repetition rate near-infrared Ti:sapphire laser system based on a 85 MHz, sub-10 fs resonator. These nanowires were characterized electrically and tested as resistive gas sensors with self-heating capability. For this purpose they were exposed to NO2 concentrations in the ppm range within synthetic air, showing a clear increase of resistance. At ambient temperature the sensor exhibits an integrating behavior with relatively long relaxation times. It was shown that the relaxation times can be shortened by exploiting the self-heating capability of this sensor. The self heating effect was studied by FEM simulations.

Simulation and design optimization of transparent heaters for spectroscopic micro cells

Abstract: For several applications, micro cells with a uniform temperature profile and at least one optical port are required. One example for those cells is the physics package of a chip-scale-atomic-clock. It is necessary that the micro chambers are heated homogeneously to 353 K using a low energy consumption heater. In this work transparent heating structures are investigated to achieve this goal. First an analytical approach is used to describe the behavior of thermal energy dissipation of the heating structures. Then different approaches of possible heater structures are simulated to find the optimal basic configuration. Furthermore, this configuration is optimized to obtain a uniform temperature distribution in the whole cell.