Ingo Behrens

Piezoresistive cantilever as portable micro force calibration standard

Design, fabrication and test of cantilever-type sensors for active micro force calibration are described. The cantilever comprises a probing tip at its free end where a force can be applied which is measured using a piezoresistive strain gauge at the cantilever suspension. The stiffness of the cantilever is approximately given by the spring constant of a beam with a rectangular cross section as confirmed by finite element modelling. Prototypes with a stiffness of 0.66 N m?1 and 7.7 N m?1 are realized using standard silicon bulk micromachining technology. Testing is performed using a high-resolution micro force measuring set-up. In either case a highly linear relationship between the gauge output voltage and the probing force is revealed in the ?N range. Low scatter and drift corresponding to a standard deviation of ?0.22% was found for the resulting force sensitivity.

Micromachining of silicon carbide on silicon fabricated by low-pressure chemical vapour deposition

We describe the fabrication of silicon carbide layers for micromechanical applications using low-pressure metal–organic chemical vapour deposition at temperatures below 1000 °C. The layers can be structured by lift-off using silicon dioxide as a sacrificial layer. A large selectivity with respect to silicon can be exploited for bulk micromachining. Thin membranes are fabricated which exhibit high mechanical quality, as necessary for applications in harsh environments.

Hetero-Micromachining of Epitaxial III/V Compound Semiconductors

Abstract Hetero-micromachining, a novel technique for the fabrication of miniaturized sensors and actuators is described. It is based on III/V compound semiconductor layers epitaxially grown on (001) silicon. Cantilevers composed of single indium phosphide or gallium arsenide layers or a layer sequence of different III/V compound semiconductors were realized exploiting the etching selectivity of the layer against the silicon substrate in KOH solution. Both etching and fracture properties of InP cantilevers are dependent on the concentration of silicon impurities in the layer. For GaAs, a fracture limit in excess of 1.5 GPa was found. Thermally actuated micromirrors that were fabricated by hetero-micromachining of InP could be deflected to up to 0.07°/mW of electrical input power under quasistatic excitation conditions. Typically, an increase of this efficiency by an order magnitude was observed at resonance that was in the kHz range.

Elastic and anelastic properties of Fe-doped InP films on silicon cantilevers

The effect of Fe doping on the elastic and anelastic properties of heteroepitaxial InP films on microfabricated silicon cantilevers has been investigated by the vibrating-reed technique (typical frequencies 100 Hz to 10 kHz) with strain amplitudes in the range of 10−6 to 10−3 and in the temperature interval from 113 to 508 K. A matter of particular interest has been the effect of iron doping on the motion and multiplication of dislocations which are known to restrict the application of the material for instance in optoelectronic devices. For this purpose, the amplitude dependence of damping as well as internal friction as a function of temperature, thermal treatment, and frequency have been investigated and are discussed in terms of the present knowledge about twins and dislocations in InP. In addition, Young’s modulus as well as film stress have been measured as a function of temperature, which permits an estimate of the thermal expansion coefficient.

Influence of substrate preparation on fracture properties of InP cantilevers

In this study we investigate the influence of the substrate preparation on the mechanical properties of indium phosphide layers used for hetero-micromachining. These layers were grown on silicon by metalorganic vapour-phase epitaxy. Exploiting the etching selectivity of indium phosphide versus silicon we realized freestanding cantilevers oriented in <100> and <110> crystal directions. Fracture-stress measurements revealed that cantilevers fabricated from a layer grown on structured substrates show a lower value compared to those grown on unstructured silicon.