Paddy J. French

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.

Reliable Inkjet-Printed Interconnections on Foil-Type Li-Ion Batteries

Shapeable rechargeable Li-ion batteries are a good option for the power source of system-in-package devices; nevertheless, their size and temperature limitations are a constraint during the fabrication process. Inkjet-printed interconnections on top of the battery are proposed in order to reduce the size and costs of wireless sensor network devices that require the use of Li-ion batteries. The reliability of such interconnections under high-humidity and elevated-temperature conditions is characterized in terms of electrical and adhesion properties; the micro- and macrostructures of the ink are observed in detail. Two silver inks are used to print the interconnections. The resistivity values of printed structures are in the range of 8.6-47.6 μΩ·cm, and all of them pass the reliability tests. The adhesion characteristics are good for Ink A; however, Ink B presents failures under high-humidity conditions. For a good adhesion, a plasma treatment should be performed prior to printing. The electrical performance of the interconnections is not affected by high-humidity and high-temperature conditions. Furthermore, there is no indication of silver migration. It is recommended that the curing temperature of the ink is kept low (<; 155°C) in order to avoid cracks in the ink structure and damages to the batterys packaging foil. The interconnections should be printed before filling the battery to avoid the decomposition of the electrolyte which happens at 80 °C.

The impact of morphology on light transport in cancellous bone

In recent years, optical techniques based on diffusion approximation have demonstrated their ability to gain rich spectral information about bone. However, these methods normally assume homogeneity, while cancellous bone and marrow form a highly heterogeneous two-phase medium. This paper studies the limitations of this assumption, and quantifies the role of microstructure on long-range transport properties. The propagation of light pulses through trabecular bone is calculated by Monte Carlo simulation of the scattering and absorption in reconstructions of bone samples obtained from x-ray micro tomographic scans. The time-resolved responses are then fitted with the analytical response of a homogeneous material to obtain the apparent transport properties. These properties are used to test different homogenization equations that have been postulated in the past for heterogeneous tissues and to check their accuracy. The results show that nonlinearity and crosstalk between absorption and scattering are statistically significant, although their impact is relatively small. More importantly, we found that the weight of the components is not only affected by their volume fractions, but need to be corrected by other morphologic measures like trabecular spacing or connectivity density. These deviations from the homogeneous assumption are stronger for scattering than for absorption. In conclusion, the average optical properties of cancellous bone are strongly determined by its microstructure, meaning that optical techniques are a valid method for tissue evaluation, but careful consideration of structure-related perturbation sources is required.

Surface contamination-induced resonance frequency shift of cantilevers

Nanoelectromechanical cantilevers have achieved unprecedented sensitivity in the detection of displacement, mass, force and charge. Although resonating cantilevers are demonstrated to be excellent mass sensors, environmental effects like humidity, ambient gases and contamination result in uncertainty in the calculation of either adsorbed mass or mechanical properties due to shifts in resonance frequencies. In this work the resonance frequency shift due to surface contamination has been studied. Single crystalline silicon (SCS) cantilevers of various dimensions (1025, 340, and 93 nm-thick) were fabricated and their resonance frequencies due to thermal noise were measured in air and in low vacuum. A resonance frequency shift was seen in air after keeping the samples in vacuum. Our measurement shows that this shift comes from surface contamination. The thinner cantilever showed more sensitive behavior to the air conditions. These results can be used to decrease the errors in the calculation of adsorbed mass and mechanical properties of nanostructures. The calculated equivalent mass-induced resonance frequency shifts of our experiments were measured to be 183×10−15 gram (183 femtogram) and a mass sensitivity of about 6.5×10−18 g/Hz (6.5 ag/Hz) was obtained. Our results and analysis indicate that mass sensing of individual molecules will be realizable when taking into account the surface contamination

Magnetic microheaters for cell separation, manipulation, and lysing

Precise heating is important for biological culturing, biological characterization, and thermal lysis, while cellular manipulation has been an area of significant interest and has been explored by a variety of methods. In this work, we present a preliminary study of the use of metallic thermal probes. The probes were used for magnetophoresis and micromanipulation of magnetically labeled HeLa cells. The probes were also used for rapid thermal lysis of magnetically labeled cells in liquid. The ultimate goal of this work is to combine thermal probes with magnetic tweezers for sample enrichment, magnetophoresis, manipulation, and thermal applications.

AN INVESTIGATION on ALD THIN FILM EVANESCENT WAVEGUIDE SENSOR for BIOMEDICAL APPLICATION

In this paper, we investigate the use of Al2O3 and TiO2 deposited by Atomic Layer Deposition (ALD) as evanescent waveguide sensors. These sensors will be employed to detect bacteria concentration in drain fluid in post anastomosis surgery. Surface roughness, conformality, and homogeneity of the sensor material are very important factors to obtain high sensitive sensor. ALD fulfill these requirements. Surface roughness before and after fabrication are investigated using AFM. As we aim at freestanding structure the buckling of freestanding ALD films is studied. Finally we build an optical characterization set up and measured the propagation loss of Al2O3 and TiO2 waveguides at 1.3 um. The results show that ALD thin films can be used as waveguide material to obtain very high sensitive sensors.

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

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.

Effect of laser deflection on resonant 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.