D. Barbier

Measurement of porous silicon thermal conductivity by micro-Raman scattering

We present a noncontact and nondestructive method to measure thermal conductivity in layered materials using micro-Raman scattering. This method was successfully applied to monocrystalline silicon whose thermal conductivity was found to be 63 W/m K at about 550 °C and then applied to porous silicon layers. For a 50 μm thick layer with 50% porosity, we found a thermal conductivity of 1 W/m K confirming the thermal insulating properties of this material.

Thermal conductivity of thick meso-porous silicon layers by micro-Raman scattering

We report here a theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized meso-porous silicon layers. The model is in good agreement with experimental measurements performed by micro-Raman scattering on the layers surface. For the first time, thermal conductivity inhomogeneity along the porous layer thickness of 100 μm is studied. Direct correlation between the thermal conductivity and morphology variations along the layer thickness is brought to the fore. A new approach to estimate local porosity of the porous layers based on thermal conductivity and Si nanocrystallite size measurements is also proposed.

Covalent enzyme immobilization by poly(ethylene glycol) diglycidyl ether (PEGDE) for microelectrode biosensor preparation.

Poly(ethylene glycol) diglycidyl ether (PEGDE) is widely used as an additive for cross-linking polymers bearing amine, hydroxyl, or carboxyl groups. However, the idea of using PEGDE alone for immobilizing proteins on biosensors has never been thoroughly explored. We report the successful fabrication of microelectrode biosensors based on glucose oxidase, d-amino acid oxidase, and glutamate oxidase immobilized using PEGDE. We found that biosensors made with PEGDE exhibited high sensitivity and a response time on the order of seconds, which is sufficient for observing biological processes in vivo. The enzymatic activity on these biosensors was highly stable over several months when they were stored at 4 °C, and over at least 3d at 37 °C. Glucose microelectrode biosensors implanted in the central nervous system of anesthetized rats reliably monitored changes in brain glucose levels induced by sequential administration of insulin and glucose. PEGDE provides a simple, low cost, non-toxic alternative for the preparation of in vivo microelectrode biosensors.

On mechanical properties of nanostructured meso-porous silicon

Mechanical properties of meso-porous silicon are studied using topographic measurements and finite element simulations. Our approach is based on an original analysis of the strain at the free surface of porous silicon tub embedded in bulk Si regions allowing the determination of the Young’s modulus of the porous layers. In particular, the internal stress in the porous Si region is evaluated from the corresponding deformation of the monocrystalline Si adjacent region which mechanical parameters are well known. Moreover, a mechanical anisotropy of the columnar nanostructured porous Si is brought to the fore from the characteristic shape of the strained porous layer profile. Moderately oxidized, 70% in porosity, porous silicon patterns were investigated. Correlation of our measurements with x-ray data reported early in literature shows the macroscopic strain being close to the silicon lattice relative increase revealing an elastic deformation regime. The porous layers exhibit an unexpected low and strongly a...

Thick oxidised porous silicon layers for the design of a biomedical thermal conductivity microsensor

Abstract Porous silicon (PS) offers new possibilities to be applied as thermal insulating material for microsensor design due to its low thermal conductivity (TC) value compared with TC of SiO2. A biomedical TC microsensor based on differential thermoelectric measurements has been designed using a PS substrate. In order to ensure an efficient thermal isolation in the microsensor, main thermal and geometrical characteristics of the PS layers as well as of the whole microsensor have been numerically simulated. PS layers with low TC have to be thick and mechanically stable under further processing. To form thick (50–200 μm) and stable PS layers, a new approach based on progressive changing of anodisation current density (from 100 to 25 mA/cm2) during PS formation has been elaborated. To find a suitable compromise between low TC and mechanical stability of thick PS layers, an adapted thermal oxidation recipe at moderate temperatures (500–600°C) in dry oxygen atmosphere has been applied. It leads to 20–50% oxidation fraction in PS layers (measured by Energy Dispersive Spectroscopy) corresponding to SiO2 TC value. A test device has been realised and characterised. A Seebeck coefficient of 400 μV/°C per junction has been measured for a Poly-Si/Al thermopile deposited on the PS layer.

Technology and micro-Raman characterization of thick meso-porous silicon layers for thermal effect microsystems

Thermal effect microsystems (TEMS) need a highly thermally insulated substrate. Porous silicon (PS) offers promising applications for insulation of thermal transducers from silicon wafers as its thermal conductivity is close to that of silicon oxide. A thorough investigation of PS thermal conductivity has been carried out regarding its technological parameters, i.e., porosity, thickness and oxidation temperature, by means of micro-Raman spectroscopy which yielded thermal conductivity values less than 2 W/m K as predicted by theoretical considerations. For TEMS, a 100-μm-thick meso-PS layer with a porosity of about 50% and oxidized at a moderate temperature (300°C) presents the best attributes to ensure both an efficient thermal insulation, as its thermal conductivity value was found to be 0.6 W/m K, and a good mechanical strength.

Magnetic properties of sputtered barium ferrite thick films

The development of devices that combine a magnetic material with a semiconductor chip is a major focus of current research. Barium hexaferrite (BaFe12O19 or BaM) thick films are deposited here using a rf sputtering system. The films are amorphous and nonmagnetic after deposition. Postdeposition thermal annealings are employed to obtain magnetic properties. The effects of the substrate, thermal annealing and thickness of BaM on the magnetic properties are studied using a vibrating sample magnetometer. The initial results show good magnetic properties for the two subtrates studied and after thermal annealing above 800 °C. The magnetic properties of the thick films are close to the bulk (BaM) ones.

Defect-state generation in Czochralski-grown (100) silicon rapidly annealed with incoherent light

Defect‐state generation in Czochralski‐grown (100) silicon after rapid thermal annealing has been studied. Deep‐level transient spectroscopy experiments have been carried out using Schottky barriers made on n‐ or p‐type as‐grown wafers after irradiation with a commercially available incoherent light annealing device. Neither electrical degradation nor electron‐trap generation appeared in the case of n‐type silicon wafers annealed for 5 s. On the other hand, the junction degradation together with the generation of three hole‐trap levels H1(0.45 eV), H2(0.29 eV), and H3(0.3 eV) have been observed in boron‐doped silicon using a short duration (5 s) plateau temperature between 850 and 1050 °C. Peak concentrations ranging from 1013 to 1014 cm−3 were measured after annealing at 1000 °C for the three hole traps. By increasing the plateau duration up to 20 s hole traps were no longer detected in boron‐doped Czochralski‐grown silicon. Moreover H2(0.29 eV) is stable at room temperature whereas both H1(0.45 eV) and ...

Immobilization method to preserve enzyme specificity in biosensors: consequences for brain glutamate detection.

Microelectrode biosensors are a promising technique to probe the brain interstitial fluid and estimate the extracellular concentration of neurotransmitters like glutamate. Their selectivity is largely based on maintaining high substrate specificity for the enzymes immobilized on microelectrodes. However, the effect of enzyme immobilization on substrate specificity is poorly understood. Furthermore, the accuracy of biosensor measurements for brain biological extracts has not been reliably established in comparison with conventional analytical techniques. In this study, microelectrode biosensors were prepared using different enzyme immobilization methods, including glutaraldehyde, a conventional cross-linker, and poly(ethylene glycol) diglycidyl ether (PEGDE), a milder immobilization reagent. Glutaraldehyde, but not PEGDE, significantly decreased the apparent substrate specificity of glutamate and glucose oxidase. For glutaraldehyde prepared biosensors, detection of secondary substrates by glutamate oxidase increased, resulting in a significant overestimate of glutamate levels. This effect was not observed with PEGDE-based biosensors, and when brain microdialysates were analyzed, the levels of glutamate detected by biosensors were consistent with those detected by capillary electrophoresis. In addition, basal concentrations of glutamate detected in vivo were approximately 10-fold lower than the levels detected with glutaraldehyde-based biosensors (e.g., 1.2 μM vs 16 μM, respectively). Overall, enzyme immobilization can significantly impact substrate specificity, and PEGDE is well-suited for the preparation of stable and selective biosensors. This development questions some of the previous biosensor studies aimed at detecting glutamate in the brain and opens new possibilities for specific neurotransmitter detection.

Straining of monocrystalline silicon thin films with the use of porous silicon as stress generating nanomaterial

A simple and low cost technological approach for the straining of thin crystalline silicon (Si) films using porous silicon (PS) as stress generating nanomaterial is reported. Structural analysis of the PS∕Si structures is performed by transmission electron microscopy. Raman scattering spectroscopy is used for the evaluation of stress and strain values in the strained thin Si films. Depending on the thickness of the strained Si films, the maximum strain values are found to be in a range from 1% to 1.4%. Various modifications of electronic properties of the strained Si films are observed by photoluminescence spectroscopy. For example, strain induced redshift of the Si energy band gap and splitting of the valence band are detected.