Henrique E. M. Peres

Gas sensitive porous silicon devices: responses to organic vapors

Geometrically scaled PS-based structures were fabricated in order to develop gas sensing devices by exploring porous silicon (PS) electrical characteristics. The electrical behavior of PS devices respond to polar organic vapors (as acetone and ethanol) reversibly in a reproducible way. Devices were fabricated with three different perimeters, maintaining a constant area (5.76 mm 2 ) and constant PS porosity (60%) throughout samples, in order to evaluate their electrical impedance depending on the area/perimeter ratio. Electrical impedance was measured from 10 kHz to 10 MHz in acetone, ethanol and vacuum (as reference) environments. The results obtained show the general aspect for impedance variation as expected for disordered materials such as amorphous semiconductors or polymers. Measured impedance is fitted proportionally to (2pf) s , where f is the excitation frequency. The exponential factor ‘‘s’’ was found to be around � 0.55 for ethanol and � 0.45 for acetone, whereas in vacuum s equals � 0.97, thus providing a method for identifying polar molecules. The parameter ‘‘s’’ for the tested environments is independent of device geometry. # 2003 Elsevier Science B.V. All rights reserved.

Electrical Impedance Sensor for Real-Time Detection of Bovine Milk Adulteration

This paper presents a developed methodology for the detection of bovine milk adulteration by applying electrical impedance measurements. This parameter allows characterizing samples of raw and ultrahigh temperature milk, adulterated with different proportions of drinking water, deionized water, hydrogen peroxide (H2O2), sodium hydroxide (NaOH), and formaldehyde (CH2O). The samples were electrically analyzed by applying the electrical impedance spectroscopy measurements and a dedicated microcontroller system, developed for this application. In both cases, the measures allowed classifying the milk quantitatively, enabling the development of real-time monitoring systems for fraud detection in milk composition. A classification of the results is proposed through a k-nearest neighbors algorithm that allows to quantitatively qualify the samples of pure and adulterated milk.

Silicon Field-Emission Devices Fabricated Using the Hydrogen Implantation–Porous Silicon (HI–PS) Micromachining Technique

This paper presents a relatively simple method to fabricate field-emitter arrays from silicon substrates. These devices are obtained from silicon micromachining by means of the HI-PS technique - a combination of hydrogen ion implantation and porous silicon used as sacrificial layer. Also, a new process sequence is proposed and implemented to fabricate self-aligned integrated field-emission devices based on this technique. Electrical characteristics of the microtips obtained show good agreement with the Fowler-Nordheim theory, which are suitable for the proposed application.

Silicon micromechanical structures fabricated by electrochemical process

Silicon (Si) micromechanical structures were fabricated by means of sacrificial layers defined with porous silicon (PS) and masked by hydrogen ion implantation with adequate thermal annealing. The fabrication process to remove the PS layers with diluted KOH at room temperature does not cause damage in remaining Si microstructures which have less than 1 /spl mu/m thickness controlled by the anodization time.

Ethanol fuel analysis by Time-Domain Reflectometry

The technique of Time Domain Reflectometry (TDR) is applied for qualifying ethanol adulterated with water and / or methanol. We used the commercial TDR model VG400, which was originally developed for determining soil moisture, making this study an original approach for qualifying fuels. Several samples of alcohol with the addition of its main contaminants (water and methanol) were prepared and measured with the TDR sensor. The results indicate good response linearity, showing the TDR technique is a promising technique for fuel qualification.

ANALYSIS OF DIFFERENT PROBE GEOMETRIES FOR ETHANOL FUEL QUALIFICATION USING TIME-DOMAIN REFLECTOMETRY

This paper proposes sensors based on the time-domain reflectometry (TDR) technique for qualifying ethanol fuel. Four different probe geometries were proposed: bifilar, microstrip, coaxial, and helical. All probes allowed qualification of ethanol adulterated with water. Helical probe showed the best response. Thus, this proposal contributes to the development of electronic tongues.

P1.4.9 Improvement of Si Field Emission Sensors Fabrication Technology by Carbon Nanotubes

Field Emission (FE) devices were successfully improved by the association of silicon (Si) microtips and Multi-Walled Carbon Nanotubes (MWCNTs). MWCNTs were deposited over Si microtips by Electrophoretic Deposition (EPD), and improvements up to 80% in electron emission characteristics were reported, being suitable for the development of gas ionization sensors based on FE devices.

Optical properties modulation of porous silicon layers for optoelectronics applications

Refractive index of porous silicon (PS) were modulated throughout the chemical oxidation, thermal oxidation and controlling current density during PS formation. The oxidized PS layers showed a good gradient of refractive index to application in waveguide devices, with higher refractive index in sub layer near de surface and less refractive index close to PS/Substrate interface. In order to fabricate wave guide devices, Silicon wall were formed of 10 to 20 micrometer wide, 10 micrometer high and 1000 micrometer to 10000 micrometer length. Wall formation were achieved throughout anisotropic etching process in 7M KOH aqueous solution at 75 degrees Celsius. The properties of PS layer were analyzed by Raman spectroscopy, Infrared spectroscopy (FTIR), optical and scanning electron microscopy and reflectance spectroscopy. PS multilayers on the walls, as well as on the substrate, are formed by means of a chemical anodization process with various current densities. The structures are characterized using scanning electron microscopy, optical imaging, and Raman spectroscopy. Raman analysis shows a multilayer structure on the walls, suggesting different refractive indices for each layer. These results indicate the possibility of obtention of visible light waveguides formed by porous silicon multilayers made on silicon vertical walls.