The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (> or =2,000 degrees C). Solid silicon has recently been generated directly from silica at much lower temperatures ( 500 m(2) g(-1)), and contained a significant population of micropores (< or =20 A). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications.

Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas

The design, fabrication and characterisation of a novel humidity sensor are presented. The device consists of a capacitor with a porous silicon dielectric, two thermoresistors and a refresh resistor. The porous silicon is formed with a back-end process underneath a meshed metal electrode, which is fabricated in the same layer as the thermo- and refresh resistors. Due to this concept, very thin porous silicon could be formed with reproducible and stable metal contacts. At the same time, both the response time and the overall fabrication yield of the devices could be improved. The properties of the sensor are modelled and demonstrated with several experimental results.

http://www.dfx.nl/userfiles/file/pdf/Rittersma%20ZM%20-%20A%20novel%20surface-micromachined%20capacitive%20porous%20silicon%20humidity%20sensor.pdf

A novel surface-micromachined capacitive porous silicon humidity sensor

In case of typical chemical gas sensors reacted by gas adsorption on surface of an active layer, it is difficult to detect some gases which have low chemical adsorption energy like inert gases. In this paper, we report a gas sensor using carbon nanotube(CNT) array as electron emitters for the purpose of detecting these gases. Specifically, sensors were fabricated with applications of glass patterning by a sand-blast process and of anodic bonding between glass and silicon to improve the compactness of the structure and the reliability in process. The proposed sensor, based on an electrical discharge theory known as Paschen's law, worked by figuring the changes of dark discharge current and initial breakdown voltage depending on the concentration and the identity of gases. In this work, air and Ar gases were examined and discussed.

/pdf/cnt-sensors-for-detecting-gases-with-low-adsorption-energy-1x7ngnoio7.pdf

CNT Sensors for Detecting Gases with Low Adsorption Energy by Ionization.

In this paper, a new integrated sensing device, that is a crystalline silicon resistor provided with an adsorbing porous silicon layer on its top, is proposed. The sensing mechanism is based on a gas-induced modulation effect of the resistor cross-section due to the adsorbed molecules in the porous film. The sensitivity of the device can be tuned by properly changing the polarization of a gate electrode. The working principle, the fabrication process and the electrical characterization of the device in presence of isopropanol vapors are presented and discussed.

/pdf/a-silicon-crystalline-resistor-with-an-adsorbing-porous-3myx5cs0hr.pdf

A silicon crystalline resistor with an adsorbing porous layer as gas sensor

In this work, the temperature behavior of the adsorption porous silicon FET (APSFET) in the presence of different isopropanol vapors concentrations is investigated. Sensor current at different temperatures and concentrations of the sensed species is measured. Experimental data is analyzed it terms of possible mechanisms involved in the modulation of the sensor current. The temperature dependence of the sensitivity suggests that the mechanism involved in the sensing is some kind of physical adsorption of the sensed species by the porous layer. The sensitivity of the device does not seem to be significantly affected by aging effects.

/pdf/temperature-behavior-of-the-apsfet-a-porous-silicon-based-3s8rys1jm6.pdf

Temperature behavior of the APSFET— a porous silicon-based FET gas sensor

Organic vapor sensitive device using anodized porous silicon has been developed. The device consists of two pn junctions surrounded with the porous silicon layer as a vapor sensing element. The devices show an increase of current for exposure to thousands ppm of organic vapor at room temperature. A high sensitivity is observed for ethanol vapor. It is discussed that the adsorption of polar molecules induces a "soft" breakdown in the reverse biased pn junction.

T. Okada

Papers

Organic Vapor Sensitivity In A Porous Silicon Device