Chung-Kai Yang

Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability

This letter presents the application of electrostatic pull-in instability to study the size-dependent effective Young’s Modulus ? ( ~170–70?GPa) of [110] silicon nanocantilevers (thickness ~1019–40?nm). The presented approach shows substantial advantages over the previous methods used for characterization of nanoelectromechanical systems behaviors. The ? is retrieved from the pull-in voltage of the structure via the electromechanical coupled equation, with a typical error of ? 12%, much less than previous work in the field. Measurement results show a strong size-dependence of ?. The approach is simple and reproducible for various dimensions and can be extended to the characterization of nanobeams and nanowires.

Effects of size and defects on the elasticity of silicon nanocantilevers

The size-dependent elastic behavior of silicon nanocantilevers and nanowires, specifically the effective Youngs modulus, has been determined by experimental measurements and theoretical investigations. The size dependence becomes more significant as the devices scale down from micro- to nano-dimensions, which has mainly been attributed to surface effects. However, discrepancies between experimental measurements and computational investigations show that there could be other influences besides surface effects. In this paper, we try to determine to what extent the surface effects, such as surface stress, surface elasticity, surface contamination and native oxide layers, influence the effective Youngs modulus of silicon nanocantilevers. For this purpose, silicon cantilevers were fabricated in the top device layer of silicon on insulator (SOI) wafers, which were thinned down to 14 nm. The effective Youngs modulus was extracted with the electrostatic pull-in instability method, recently developed by the authors (H Sadeghian et al 2009 Appl. Phys. Lett. 94 221903). In this work, the drop in the effective Youngs modulus was measured to be significant at around 150 nm thick cantilevers. The comparison between theoretical models and experimental measurements demonstrates that, although the surface effects influence the effective Youngs modulus of silicon to some extent, they alone are insufficient to explain why the effective Youngs modulus decreases prematurely. It was observed that the fabrication-induced defects abruptly increased when the device layer was thinned to below 100 nm. These defects became visible as pinholes during HF-etching. It is speculated that they could be the origin of the reduced effective Youngs modulus experimentally observed in ultra-thin silicon cantilevers.

Experimental Characterization of Roughness Induced Scattering Losses in PECVD SiC Waveguides

An atomic force microscope is used to directly measure the sidewall roughness of silicon carbide waveguides. In order to make the vertical walls accessible, the chip containing the rib waveguides was fixed on a 15 steel wedge and loaded onto an AFM scanner stage; this fitting ensures enough probe contact area on one of the sidewalls. The data was processed using a fully automated algorithm to extract the roughness in the direction of light propagation. This technique allows the investigation of devices at chip level without damaging the structures. The method was calibrated using a well-known smoothing process based on thermal oxidation of silicon waveguides to achieve low transmission loss and applied to PECVD silicon carbide waveguides. A very low loss behavior at 1.3 m ( dB/cm) is reported.

Some considerations of effects-induced errors in resonant cantilevers with the laser deflection method

Micro/nano resonant cantilevers with a laser deflection readout have been very popular in sensing applications over the past years. Despite the popularity, however, most of the research has been devoted to increasing the sensitivity, and very little attention has been focused on effects-induced errors. Among these effects, the surface effects and the so-called readout back-action are the two most influential causes of errors. In this paper, we investigate (1) the influence of the surface effects such as water adsorption, gas adsorption, and generally surface contaminations, and (2) the effect of the laser deflection detection, including power and positions of the laser, on the resonance frequency of silicon cantilevers. Our results show that both the surface contaminations and the laser back-action effects can significantly change the resonant response of the cantilevers. We conclude that the effects have to be taken into account, particularly in the case of ultra high sensitivity cantilevers.

Demonstration of PECVD SiC–SiO

We report on the first demonstration of guiding light in horizontal slot-waveguides using a plasma-enhanced chemical vapor deposition silicon carbide (SiC)-silicon oxide (SiO2)-SiC structure. Propagation losses of 23.9 ±1.2 dB/cm have been measured for the quasi-transverse-magnetic mode of the fabricated slot waveguides at 1.3 ¿m .

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Field emission provides an alternative sensing solution in scaled electromechanical systems and devices, when typical displacement detection techniques fail in submicron and nanodimenions. Apart from its independency from device dimension, it has also a high response, integration and high compatibility benefits. In this work, we propose using two modes of detection (fixed current and fixed bias) on two sensing methods: static sensing and dynamic resonance sensing. We measured the characteristic of the two modes and proved that field emission is a viable cantilever displacement detection technique. Customized tip on a fixed substrate has been fabricated and loaded to a UHV atomic force microscopy scanning tunneling microscopy system providing us a field emission environment with precise distance controls without the effects of cantilever bending. Thus, we are able to measure and determine the relationship of emission electric field to the electrode distance, as well as the relationship of the emission current to the electrode distance. The sensitivity obtained in our work for the static mode is 0.5 V/nm. In dynamic mode, we successfully measured a resonance of a piezoactuated cantilever at 162.2 kHz. Characterizing these relations enabled us to propose the possibility of using field emission as a cantilever displacement sensing technique.

–SiC Horizontal Slot Waveguides

Response of nanomechanical resonant mass sensors to adsorption does not only depend on the mass loading, but also on the adsorbate stiffness, adsorption induced surface stresses and location. It is therefore clear that with just resonance frequency measurement, decoupling the stiffness and the mass effects is difficult. A recent theory proposed using the electrostatic pull-in instability (EPI), in combination with the resonance frequency to decouple the aforementioned opposing effects, is presented hereof. In this paper, by performing experiments of adsorption on cantilevers, we 1) performed preliminary experiments on EPI stiffness measurement and show the stiffness effects of typical mass deposition, 2) show that EPI can be used in combination with the frequency measurements, to quantitatively analyze both the stiffness and the mass of the adsorbed material.

Field emission for cantilever sensors

Laser beam deflection is a well known method commonly used in detecting resonance frequencies in atomic force microscopes and in mass/force sensing. The method focuses a laser spot on the surface of cantilevers to be measured, which might change the mechanical properties of the cantilevers and affect the measurement accuracy. In this work we showed that the joule heating of the laser, besides other extrinsic effects such as surface contamination, can cause a significant amount of shift in the resonator. The longer and softer the cantilever is, the more significant the effect. We suggest that the laser effects on the resonant response of sensors have to be taken into account.

Quantitative analysis and decoupling of mass and stiffness effects in cantilever mass sensors

In the past decades, there is a considerable interest in the sensor community to move from micron to nano-devices, typically scaling of resonators such as cantilever beams. The scaled beams give advantages in faster response and higher sensitivity; however the detection of their resonance becomes challenging as dimensions scale down. In our work, we demonstrate the use of field emission characteristics as a detection method for scaled resonators. The advantages of using field emission are several: it is geometrically scalable without loss of signal, it has a high bandwidth and it can be integrated using standard fabrication processes.

Effect of laser deflection on resonant cantilever sensors

An atomic force microscope was used to directly measure the sidewall roughness of silicon on insulator (SOI) rib and silicon carbide waveguides. In order to make the vertical walls accessible, the chip containing the ribs was fixed on a 15° steel wedge and loaded onto an AFM scanner stage; this fitting ensures enough probe contact area on one of the sidewalls. The data was processed using a fully automated algorithm to extract the roughness in the direction of light propagation. The technique used allows the investigation of sensor devices at chip level without damaging the structures. The well-known smoothing process based on thermal oxidation of silicon waveguides to achieve low transmission loss was studied to demonstrate the possibilities of the method. Additionally, results on silicon carbide waveguides are presented.