Andreas Steiger

Anomalous piezoresistance effect in ultrastrained silicon nanowires.

In this paper we demonstrate that under ultrahigh strain conditions p-type single crystal silicon nanowires possess an anomalous piezoresistance effect. The measurements were performed on vapor-liquid-solid (VLS) grown Si nanowires, monolithically integrated in a microelectro-mechanical loading module. The special setup enables the application of pure uniaxial tensile strain along the <111> growth direction of individual, 100 nm thick Si nanowires while simultaneously measuring the resistance of the nanowires. For low strain levels (nanowire elongation less than 0.8%), our measurements revealed the expected positive piezoresistance effect, whereas for ultrahigh strain levels a transition to anomalous negative piezoresistance was observed. For the maximum tensile strain of 3.5%, the resistance of the Si nanowires decreased by a factor of 10. Even at these high strain amplitudes, no fatigue failures are observed for several hundred loading cycles. The ability to fabricate single-crystal nanowires that are widely free of structural defects will it make possible to apply high strain without fracturing to other materials as well, therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value.

Full three-dimensional simulation of focused ion beam micro/nanofabrication

2D focused ion beam simulation is only capable of simulating the topography where the surface shape does not change along the third dimension, both in the final result and during processing. In this paper we show that a 3D topography forms under the beam even though the variation in the final result along the third direction is small. We present the code AMADEUS 3D (advanced modelling and design environment for sputter processes), which is capable of simulating the surface topography in 3D space including angle-dependent sputtering and redeposition. The surface is represented by a structured or unstructured grid, and the nodes are moved according to the calculated sputtering and redeposition fluxes. In addition, experiments have been performed on nanodot formation and box milling for a case where a 3D temporary topography forms. The excellent agreement validates the code and shows the completeness of the model.

Level set approach for the simulation of focused ion beam processing on the micro/nano scale

The level set method, introduced by Osher and Sethian (1988 J. Comput. Phys. 79 12-49), has recently become popular in the simulation of etching, deposition and photolithography processes in semiconductor manufacturing, as it is a highly robust and accurate computational technique for tracking moving interfaces. In this paper, the level set approach is applied to focused ion beam fabrication, allowing for the first time the simulation of targets with sub-regions that change their connectivity during processing. It is implemented in the code AMADEUS-level set (advanced modelling and design environment for sputter processes), which is capable of simulating surface topography changes in two dimensions taking re-deposition fluxes into account. We present two examples of comparisons between simulation and experiment that demonstrate the predictive capability of the code.

Simulation-based approach for the accurate fabrication of blazed grating structures by FIB.

Accurate direct fabrication of diffractive gratings is an important task in optical engineering. Several methods have been reported to realize optical diffractive gratings on a silicon substrate using focused ion beams. A method, however, is necessary to improve the overall shape and dimensional accuracy. In this paper a simulation-based technique is presented taking into account redeposition fluxes. First, the influence of the process parameters on the blazed grating structure is studied experimentally. Then the process parameters for a structure with a planar sidewall, a maximum depth of 200 nm, and an opening width of 350 nm are determined. The approach is finally verified by comparing the designed with the fabricated structure. The method may be readily extended to various micro/nano structures in optics.

Terahertz Laser Power Measurement Comparison

A comparison of terahertz (THz) laser power measurements was undertaken among three national metrology institutes. At two laser frequencies, 2.52 THz (119 μm) and 0.762 THz (394 μm), a power level of approximately 3 mW was compared at one place at one time by means of national standard THz detectors, which had been calibrated by each participant at their own metrology institute beforehand. The measurements took advantage of the power stability and Gaussian beam profile of the THz radiation source, consisting of a molecular gas laser pumped with a line-tunable CO2 laser at the THz detector calibration facility of Physikalisch-Technische Bundesanstalt. A reference value was determined as weighted average of the measurement results with a maximum weight obtained from the arithmetic mean of the uncertainties stated by the participants. All measurement results agreed with the reference value and to each other within the stated expanded uncertainties.