Elisabete Galeazzo

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.

Use of plasma polymerized highly hydrophobic hexamethyldissilazane (HMDS) films for sensor development

Hexamethyldissilazane (HMDS) plasma polymerized films can be used in a wide variety of applications due to the facility on plasma polymerization of HMDS molecule and the high resistance to acid or basic corrosion of the formed film. Nonetheless, the hydrophobic character, as well as the possibility of adsorption of organic molecules, did not receive attention yet. In this work HMDS plasma polymerized films, presenting high organic character, were obtained in order to analyze their properties as sensors for organic compounds in N2 and aqueous solutions. The films showed to be not only highly resistant to corrosion in a wide scan of pH, but also presented high water contact angle (approximately 908). The organic character of the films leads to a high adsorption characteristic for polar and non-polar compounds. The water contact angle measurements vary for 2-propanol aqueous solutions from 908 (0% of 2-propanol) to about 08 (50% of 2-propanol). When HMDS films were deposited on two different substrates, dielectric and porous silicon (PS), two main issues were found. The films suffered degradation by heating and porous silicon was planarized by the plasma deposition, as shown by infrared spectroscopy, optical and Raman microscopy. Deposition on piezoelectric quartz crystal (PQC) showed adsorption of polar and non-polar organic compounds carried by N2 and chloroform aqueous solutions. HMDS films deposited on silicon were electrically characterized showing quickly and reversible response for 2-propanol carried by N2 .I n saturated environment, the electrical current increases about five times when a range of � 5 to 0 V was applied. For n-hexane drops, no reversible electrical characteristics were found. Although some issues were revealed, HMDS highly hydrophobic films showed promising characteristics for sensor development. # 2003 Elsevier Science B.V. All rights reserved.

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.

Micromechanical structure development for chemical analysis: study of porous silicon as an adsorbent

The aim of this work is to investigate the use of microchannels to concentrate pollutants present in the air. Devices with a length of 30 cm, a width of 100 micrometers and a depth of 30 micrometers , sealed by anodically bonded glass, were manufactured. Tests of adsorption characteristics were made using n-hexane. To reliably insert N2 contaminated with 1000 ppm of n-hexane in the microstructure, a simple setup was manufactured. This setup allows to insert the reactant, remove the amount of reactant adsorbed and to detect it. An amount of 20 mg was inserted in the microstructure. In order to improve the adsorption characteristics, PS layers were manufactured in the microchannels using silicon nitride as mask. The sealing of the microstructure with anodic bonded glass showed to be feasible either if the surface present PS or silicon nitride. Samples of PS layers covered by a plasma polymerized film, produced using HMDS, were analyzed by Raman microscopy. It was noticed that the nanocrystals are completely fulfilled by the deposited material, indicating the high reactivity of the PS layer.

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.