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For thermal sensors and devices, porous silicon is a comparably novel alternative to standard materials such as thin glass substrates or silicon nitride membranes. These materials are primarily characterized by their thermal conductivity and heat capacity, as well as temperature stability and mechanical fragility. In this paper, we present details of the porous silicon technology for full wafer porosification as well as static and dynamic device and material characterization. The reduction of thermal conductivity is estimated with the dynamic 3ω technique and compared with pure silicon and silica glass wafers. Thin film microheaters have been deposited on the samples as proof of concept for the characterization and comparison of thermal insulation, heat capacity, as well as thermal and mechanical stability. [ABSTRACT FROM PUBLISHER]

This paper reports the optimization of electrode geometry on macroporous silicon biosensor for improved bacteria detection. Macroporous silicon of 8 μm thickness and 55% porosity has been fabricated on a 10-20 Ω-cm wafer using hydrofluoric acid and dimethyl sulfoxide. The electrode configurations selected for optimization are coplanar rectangular types with simple two-electrode, interdigitated comb finger-like electrode, and a combination of simple electrode and comb-shaped electrode designs. With these configurations, 14 different patterns have been generated with varying lengths and spacing within the technological constraints of low cost screen printing method of electrode fabrication with a wide range of effective exposed area from 0.04 to 0.6 mm2 by techniques from design of experiments. The sensitivity toward bacteria detection for all the patterns has been estimated by computation of an area utilization factor (AUF) which has been defined as the ratio of the effective area occupied by the captured bacteria to the effective exposed area available for capture. From the AUF values, eight patterns have been selected which are above a lower cut-off threshold for experimentation with E.coli O157 in the range of 102CFU/ml to 106 CFU/ml. It has been observed that the pattern with the highest AUF is capable of detecting 102CFU/ml E.coli O157 with a sensitivity of 11% without any pre-concentration which was not possible in the earlier reports of macroporous silicon. [ABSTRACT FROM PUBLISHER]

A conductivity-based catechol biosensor was developed using porous silicon as an immobilization matrix for enzyme tyrosinase. The enzyme was extracted from plant source Amorphophallus companulatus and immobilized in an electrochemically etched surface of p-type silicon. The presence of enzyme in a porous structure and the retention of enzyme activity were confirmed by scanning electron microscopy and spectrophotometric studies, respectively. The principle of the sensor is based on the change in the conductivity of the tyrosinase-entrapped porous silicon matrix. When the entrapped tyrosinase interacted with catechol, the change in the current voltage (I-V) characteristics was obtained, which was proportional to analyte concentration. The analytical characteristics of the sensor including response time, linearity range, lower detection limit, reusability, and storage stability were studied. [ABSTRACT FROM PUBLISHER]

A biosensor for the detection of food-borne pathogens (Salmonella Enteritidis) was fabricated based on nanoporous silicon (NPS). P-type silicon wafers (100, 0.01 ) were anodized electrochemically in an electrochemical Teflon cell, containing ethanoic hydrofluoric acid solution to produce the porous layer on the silicon surface. The porous silicon surface was functionalized with DNA probes specific to the insertion element (Iel) gene of Salmonella Enteritidis. A biotin-streptavidin system was utilized to characterize the availability of the nanopores and the specificity of the DNA probe. Based on the electrical property of DNA, redox indicators and cyclic voltammetry were used for the characterization of the biosensor. Results showed that the DNA probe was specific to the target DNA, and the porous silicon-based biosensor had more active surface area and higher sensitivity (1 ng/mL) than the planar silicon-based biosensor. This simple, label-free porous silicon-based biosensor has potential applications in high-throughput detection of pathogens. [ABSTRACT FROM PUBLISHER]

A Si thermal accelerometer without solid proof mass, based on porous silicon (PS) technology has been developed and characterized. The device is compatible with silicon technology and it consists of a polysilicon heater and two thermopiles, situated symmetrically on each side of a heater. A thick PS layer provides thermal isolation from the Si substrate. The operation principle is based on the movement induced thermal convection variations between the heater and the hot thermopile contacts, which are caused by the movement of the hot fluid medium on top of the heater with respect to the sensor die. A detailed simulation by the FEA package Ansys was carried out in order to find the optimum geometrical parameters and the theoretical device behavior. The porous silicon thermal accelerometer (PSTA) was tested in a specially designed vibration system for various frequencies and accelerations. Different packaging configurations were also evaluated for both air and oil surrounding environments. The dependence of the PSTA response on applied power was studied for each surrounding environment. In each case, the response was compared with a commercial reference accelerometer and the corresponding sensitivities were extracted. [ABSTRACT FROM PUBLISHER]

Porous-silicon (PS)-based potassium ion-selective microelectrode (K+ISME) was fabricated by using a microelectronic planar process and an electrochemical anodization etching technique. The apparent sensing area of the K+ISME is 100times100 mum 2. The response time t95% is 20 and 31 s when the concentration change is from low to high and vice versa, respectively. The potentials are constant at pH 2-8. The calibration curve for the K +ISME is linear within a wide range of pK=2.0~6.0. Its average slope during six months is 56.5 mV per decade, which is close to the Nernst response. The detection limit was found to be on the order of 5times10-7 M. The potentiometric selectivity coefficients (Ki,j pot) of the K+ ISE were 1.8 for NH 4 +, 3.6 for Li+, 4.1 for Na+, 4.5 for Mg2+, and 4.8 for Ca2+, respectively. Good performances of the K+ISME are attributed to large specific surface area and excellent adhesion between sensing membrane and the surface of PS [ABSTRACT FROM PUBLISHER]

This paper investigates the suitability of porous GaAs as a semiconductor material for sensing humidity. The authors have developed two types of sensors based on Pd/porous GaAs and Pd/GaAs Schottky contacts for humidity measurements. It was found that the porosity on GaAs wafer promoted the sensing properties of the contact used as highly sensitive humidity sensor toward different amounts of relative humidity operated at room temperature. On the contrary, the Pd/GaAs sample operated at room temperature exhibited negligible sensitivity to relative humidity. The advantages of using porous GaAs for Schottky humidity sensor are the following: high sensitivity, low response time, and insignificant dependence on temperature. Current-voltage (I-V) characteristics of the Pd/porous GaAs Schottky humidity sensor exhibited a saturation current value of 8.5times10-10 A under dry condition (5% relative humidity). This was increased to 7.0times10-9 A when submitted to a relative humidity of 25%. The saturation current was further increased considerably to 3.0times10-7 A as the relative humidity was increased to 95%. This is more than two orders of magnitude increase in saturation current compared to dry condition. A parameter called humidity sensitivity was defined using the current value at a fixed forward voltage of 0.2 V to present the sensitivity of the sensor. Response times are reported to discuss the adsorption and desorption characteristics of the device. Pd/porous GaAs sensor operated at room temperature showed a fast response time of 2 s and a sensitivity value of 93.5% in the presence of 25% relative humidity. Furthermore, the influence of increase in relative humidity as well as heating effects on the responsivity of the sensor is described. Scanning electron microscopy analysis of the Pd/porous GaAs sample exhibited highly porous structures [ABSTRACT FROM PUBLISHER]

Porous silicon is a structurally complex material, in which effects of the pore topology on its physical properties are even controversial. We use the Born potential and the Green's function, both applied to a supercell model, in order to analyze the Raman response and the phonon band structure of porous silicon. In this model, the pores are simulated by empty columns of atoms, in direction [001], produced in a crystalline silicon structure. An advantage of this model is the interconnection between silicon nanocrystals, then, all the states are extended. The results show that the Raman spectra are sensitive to the pore topology. Moreover, a shift of the main Raman peak towards lower frequencies is found, in agreement with experimental data [ABSTRACT FROM PUBLISHER]

Freestanding microstructures are essential elements in thermal and mechanical microsensors. In this paper, microbridges were fabricated by silicon surface micromachining using porous silicon as sacrificial layer. Two different approaches were considered. In first approach, n-Si was used as anodization masking material and n-Si/SiO2 as microstructure material. In the second approach, silicon nitride and SiO 2 /Si3 N4 bilayer has constituted masking and microstructure materials, respectively. In order to characterize their thermal behavior, platinum heating elements were defined on developed microbridges. Microstructures fabrication process was described, insisting specially on silicon anodization step. The process parameters (HF-electrolyte concentration, current density, and process duration) were established for both approaches. Thermal behavior of developed microbridges was studied in relation to anodization masking materials and freestanding microstructure materials. Microbridge temperature versus applied power to heating element was analyzed. Additionally, entire sample thermal behavior and microbridge dynamic thermal behavior was characterized. The obtained results suggest developing a third approach where n-Si will be used as masking material and SiO2 / Si3 N4 bilayer as freestanding microstructure material [ABSTRACT FROM PUBLISHER]

Different ways to reduce hysteresis in a capacitive-type thermally carbonized porous silicon (TC-PS) humidity sensor are studied and compared. Modification of the contact angle of the dielectric surface, enlargement of the pore size of dielectric, and operating the sensor at elevated temperature proved all to be possible ways to reduce hysteresis in a TC-PS humidity sensor. By variation of the carbonization temperature, we produced TC-PS surfaces of different contact angles. Although the hydrophobic surface prevents hysteresis, it also decreases considerably the sensitivity of the sensor. Enlargement of the pore size reduces and tunes the hysteresis loop into the higher relative humidity (RH) values. Also operation of the sensor only few degrees above room temperature was found to be a workable method to prevent hysteresis. However, a constant temperature is crucial for exact humidity measurement using a TC-PS sensor [ABSTRACT FROM PUBLISHER]

Gas-sensing ability of n-type PtSi/porous Si Schottky junctions is investigated at room temperature. These junctions exhibit a breakdown-type current-voltage (I-V) curve at low reverse bias voltages (5-15 V) due to very large electric fringing fields (105-106 V/cm) developed at the sharp edges and the bottom of the pores. Experimental results for selected gases are presented. Gases with inherent dipole moments tend to decrease the breakdown voltage. Gases without dipole moments do not affect the I-V curve directly, but they can replace gas content inside the pores and decrease the dipole moment of the ambient gas, which results in an increase in breakdown voltage. Based on these experiments, the possible application of this structure as a gas sensor at room temperature is discussed. [ABSTRACT FROM PUBLISHER]

Adsorption porous silicon FET (APSFET) is a porous silicon (PS)-based device constituted of a FET structure with a porous adsorbing layer between drain and source. Adsorbed gas molecules in the porous layer induce an inverted channel in the crystalline silicon under the PS itself. The mobile charge per unit area in the channel depends on the molecular gas concentrations in the sensing layer so that adsorbed gas molecules play a role similar to the charge on the gate of a FET. In this work, NO2 detection by using the APSFET is demonstrated for the first time. NO2 concentration as low as 100 ppb was detected. Devices with both as-grown and oxidized PS layers were fabricated and compared in order to investigate the effect of a low-temperature thermal oxidation on the electrical performances of the sensor. Nonoxidized sensors show a high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery time, and interference with the relative humidity level, keeping the high sensitivity to nitrogen dioxide. [ABSTRACT FROM PUBLISHER]

The sensing of chemicals and biochemical molecules using several porous silicon optical microsensors, based both on single-layer interferometers and resonant-cavity-enhanced microstructures, is reported. The operation of both families of sensors is based on the variation of the average refractive index of the porous silicon region, due to the interaction with chemical substances either in vapor or liquid state, which results in marked shifts of the device reflectivity spectra. The well established single-layer configuration has been used to test a new chemical approach based on Si-C bonds for covalent immobilization of biological molecules, as probe, in a stable way on the porous silicon surface. Preliminary results on complementary oligonucleotide recognition, based on this technique, are also presented and discussed. Porous silicon optical microcavities, based on multilayered resonating structures, have been used to detect chemical substances and, in particular, flammable and toxic organic solvents, and some hydrocarbons. The results put in evidence the high sensitivity, the reusability, and the low response time of the resonant-cavity-enhanced sensing technique. The possibility of operating at room temperature, of remote interrogation, and the absence of electrical contacts are further advantages characterizing the sensing technique. [ABSTRACT FROM PUBLISHER]

A gas sensor system based on ellipsometric readout is presented. It includes a gas chamber and a compact ellipsometer operated in off- mode. Small, low-cost optical components are used to demonstrate that this advanced methodology can be implemented in simplified instrumentation. The off- ellipsometric sensing principle and transducer mechanisms of the sensing layers, as well as the instrumentation, are described. The application of the sensor system is exampled with experimental results on low-concentration alcoholic gases (methanol, ethanol, and 2-propanol) using porous silicon as a sensing layer. Optimization of the optics of the sensor system, improvement of sensitivity or alteration of selectivity by modification of sensing layers, and multisensing by using several ellipsometric units in parallel are discussed. [ABSTRACT FROM PUBLISHER]

Silicon micromechanical structures were fabricated by means of sacrificial layers defined with porous silicon and masked by hydrogen ion implantation with adequate thermal annealing. The fabrication process to remove the porous silicon layers with diluted potassium hydroxide at room temperature does not cause damage to the remaining silicon microstructures, which are less than 1 μm in thickness controlled by process parameters. [ABSTRACT FROM PUBLISHER]

This paper presents quantitative analysis of electron emission from nanocrystalline Si dots, and discusses its mechanism based on the calculations of electronic and phononic states. Analysis of emission energy distribution measured from the vacuum level shows that the energy at the peak of the distribution increases linearly with increasing voltage applied across the nanocrystalline Si system. The slope of the linear law is unity, regardless of process conditions. Increasing voltage significantly changes the shape of the distribution at the energies smaller than the peak, while it has minimal impact at the energies larger than the peak. Both the conventional field emission model and the metal-oxide-semiconductor model fail to explain those behaviors. Calculations of electronic and phononic states in a chain of the nanocrystalline Si dots indicate a possibility of strong suppression of electron energy relaxation, which may be a possible mechanism of the high-energy electron emission phenomena. [ABSTRACT FROM PUBLISHER]

A novel hygrometer is presented, comprising a capacitive humidity sensor with a porous silicon (PS) dielectric and electronics. The adsorption of water vapor by the PS layer leading to change of its effective dielectric constant is modeled with an effective medium approximation (EMA). A simple, but precise, phase-sensitive electronic circuit has been developed. This detects any change of phase of a sinusoidal signal transmitted through the PS dielectric and correlates to ambient humidity. It is outlined how the nonlinear response of the sensor is compensated through piecewise linearization. The sensor is tested in combination with the phase detection circuitry. Excellent linearity over the entire range of relative humidity is achieved. Experimental results show a resolution better than 0.1% and an accuracy of 2% (near the transition region) and better than 0.1% (otherwise). The response time is less than 10 s with good stability. [ABSTRACT FROM PUBLISHER]

Presents the first experimental results on PtSi/porous Si Schottky detectors. Si pores have been filled by Pt through electrodeposition. Under proper temperature treatment, Pt reacts with Si creating a PtSi layer that uniformly covers the walls of the pores. The excess unreacted Pt inside the pores is etched away leaving empty spaces behind. The spectral response of such a detector is very wide, covering from 0.9 up to at least 7 μm of IR radiation in back illumination mode. Excellent responsivities, such as 60 A/W at 1 μm and 0.96 A/W at 4 μm of IR radiation is exhibited. Reverse bias current-voltage characteristics exhibit a breakdown type behavior with a breakdown voltage at about 10 V. The general shape of the reverse bias I-V curve, the wide spectral range, and high responsivity are explained through tunneling and avalanche multiplication. It is proposed that large fringing fields developed at sharp edges of the porous surface cause tunneling and avalanche multiplication. [ABSTRACT FROM PUBLISHER]

In this paper, the results of full characterization of a micromachined-silicon thermal gas flow sensor will be presented. The sensor is composed of two series of thermocouples on the right and left side of a polysilicon resistor, used as heater. The resistor and the hot contacts of the thermocouples lie on a thick porous silicon layer, which assures local thermal isolation, while the thermopile cold contacts lie on bulk silicon. Gas flow is parallel to the surface of the sensor and perpendicular to the resistor, which is heated at constant temperature. The power of the heater is stabilized by an external circuit, which provides a feedback current to compensate changes in the resistance of the heater under flow. Characterization of the sensor both under static conditions and under flow of different gases will be presented. The sensor shows high sensitivity [of the order of 175 × 10-3 mV/(m/s)12/ per thermocouple] and very rapid response, below 1 ms, which makes it appropriate for use both under laminar and under turbulent flows. [ABSTRACT FROM PUBLISHER]

The efficiency of Pd-doped porous silicon (PS) as a catalytic material for hydrogen sensing is studied. Pd is deposited by an electroless process on the internal surface of porous silicon. The catalytic behavior of Pd-doped PS samples is estimated and the parameters that influence the kinetics of the chemical reaction are evaluated. The catalytic activity is examined through the kinetics of the chemical reaction, which occurs in low hydrogen content mixtures with air (up to 1% v/v in air), far below the mixture flammability limit. It was found that the catalytic activity of Pd-doped porous silicon at 160°C is significantly higher than that of a planar surface covered with Pd. The dependence of the catalytic activity on processing conditions was also evaluated. These results open important new possibilities for applications in gas sensors [ABSTRACT FROM PUBLISHER]

We present a detailed study of two-dimensional (2D) photonic crystals based on macroporous silicon technology, showing a broad range of wavelengths accessible for applications with this material. In this work, we have reached 1.55 μm, this certainly represents a decisive issue for this technology. First, the reflection performances of a hexagonal and a triangular lattice of air holes are compared. The triangular lattice reduces technological requirements because the complete photonic bandgap (PBG) results from the overlap of broader forbidden bands of lower order. Second, a combined experimental and theoretical study is presented of the reflection properties of a 2D photonic crystal in a three-dimensional (3D) optical environment. This reveals the critical parameters that can degrade the performances of such 2-D structures. Reflection coefficients up to 98% are obtained with optical quality interfaces. Finally, a complete PBG centered at 1.55 μm is demonstrated with a submicrometer period triangular lattice defined by holographic lithography. The influence of the air filling factor on the band position and the interface quality is analyzed by reflection measurements. The overall results show the high flexibility of the macroporous silicon technology and its applicability to integrated optics at telecommunication wavelengths [ABSTRACT FROM PUBLISHER]

The optical form birefringence in porous silicon films is measured by analyzing the transmitted interference intensity of a polarization interferometer. A novel form birefringence model called "boundary condition (BC) model" for porous materials is introduced and evaluated experimentally against samples of porous silicon films. The variation of optical indexes of refraction vs the porosity in silicon films agrees with the calculated values of no/ne within 1% error using the BC model, in contrast to the ∼15% error using effective medium approximation model. [ABSTRACT FROM PUBLISHER]

Using the silicon bulk micromachining and selectively oxidized porous silicon (SOPS) technology, a silicon dioxide microlens was implemented on thick oxidized porous silicon (OPS) membrane. Because the surface of microlens was the interface of OPS-Si, the surface roughness of lens was directly affected by the anodization current density, substrate resistivity, and high-frequency percent in ethanol-contained electrolyte. In the SOPS technology, the porous silicon layer (PSL) was formed under the mask layer with its horizontal-vertical ratio of about 0.8. During the formation of lens part, Si3N4 mask layer was also etched with constant etch rate and the PSL formed on the masked area was used as a membrane supporter. The initial open area and the thickness of the mask layer controlled the focal length of the OPS lens and its range was from 20 to 110 μm. By using these microlenses, a new silicon-based optoelectronic-multichip module configuration was proposed. [ABSTRACT FROM PUBLISHER]

A novel vapor sensor that uses polarization interferometry in a form birefringent porous-silicon film is introduced, analyzed, and experimentally characterized. Simulations and analysis of accuracy, versatility, stability, and control of dynamic range of the device are provided. The simulation accurately predicts the polarization interference signal, which is used to estimate the effective refractive indexes characterizing the form birefringence of a porous-silicon film with 0.001 accuracy. The device was tested for the detection of heptane concentration in a range of 342-20 000 ppm (=2.0%). [ABSTRACT FROM PUBLISHER]