Didac Vega

High-Density Capacitor Devices Based on Macroporous Silicon and Metal Electroplating

This paper presents a novel technique for the fabrication of ultrahigh capacitance structures based on macroporous silicon. Electrochemical etching is used to create a 3-D template in silicon. These structures reach high specific capacitances and can be incorporated into integrated circuits. Very low series resistance is attained using a metal electrode. The fabrication technology uses standard UV lithography for silicon patterning, and a low-temperature electroplating process for the electrode formation. Devices have been fabricated with several insulator thicknesses to demonstrate the technology. The fabricated devices have a pore diameter of 3 μm arranged in a square lattice of 4-μm pitch, achieving capacitance density up to 110 nF/mm2. Further gains in capacitance can be obtained by reducing pore size and increasing pore density, and also using alternate geometries for the macroporous silicon template. Moreover, to increase capacitance, the use of alternative dielectrics, like high- k materials, is discussed.

Macroporous silicon for spectroscopic CO2 detection

A carbon dioxide sensor based on a photonic crystal of macroporous silicon and measurements of its response are presented in this paper. Photonic crystals have opened new ways for the miniaturization of optical detector devices. In particular, macroporous silicon has proven to be a versatile material for many applications, and more specifically for the creation of photonic crystals in the MIR and NIR spectral regions. In this paper we present the design and fabrication of a MpSi photonic crystal with a photonic stopband in the lambda 4.2 μm region. This bandgap creates a spectral reflection peak centred on one of the main absorption bands for carbon dioxide which can be used to detect the presence of this gas. The device structure is a square lattice of modulated pores with a 700 nm pitch produced by electrochemical etching of silicon. No sensing materials such as polymers are used to perform detection, relying just on the spectral response of as-etched MpSi.

Macroporous silicon for high-capacitance devices using metal electrodes

In this paper, high-capacity energy storage devices based on macroporous silicon are demonstrated. Small footprint devices with large specific capacitances up to 100 nF/mm2, and an absolute capacitance above 15 μF, have been successfully fabricated using standard microelectronics and MEMS techniques. The fabricated devices are suitable for high-density system integration. The use of 3-D silicon structures allows achieving a large surface to volume ratio. The macroporous silicon structures are fabricated by electrochemical etching of silicon. This technique allows creating large structures of tubes with either straight or modulated radial profiles in depth. Furthermore, a very large aspect ratio is possible with this fabrication method. Macroporous silicon grown this way permits well-controlled structure definition with excellent repeatability and surface quality. Additionally, structure geometry can be accurately controlled to meet designer specifications. Macroporous silicon is used as one of the electrodes over which a silicon dioxide insulating layer is grown. Several insulator thicknesses have been tested. The second capacitor electrode is a solid nickel filling of the pores prepared by electroplating in a low-temperature industry standard process. The use of high-conductivity materials allows reaching small equivalent series resistance near 1 Ω. Thanks to these improvements, the presented devices are capable of operating up to 10 kHz.PACS84.32.Tt; 81.15.Pq; 81.05.Rm

Macroporous silicon microreactor for the preferential oxidation of CO

A macroporous silicon micromonolith containing ca. 40,000 regular channels of 3.3 μm in diameter per square millimeter has been successfully functionalized with an Au/TiO2 catalyst for CO preferential oxidation (CO-PrOx) in the presence of hydrogen. The functionalization of the silicon microchannels has been accomplished by growing a SiO2 layer on the channel walls, followed by exchange with a titanium alkoxyde precursor and decomposition into TiO2 and, finally, by anchoring carbosilanethiol dendron protected pre-formed Au nanoparticles. Catalytically active centers at the Au-TiO2 interface have been obtained by thermal activation. With this method, an excellent homogeneity and adherence of the catalytic layer over the microchannels of the macroporous silicon micromonolith has been obtained, which has been tested for CO-PrOx at 363-433 K and λ=2 under H2/CO=0-20 (molar). The macroporous silicon micromonolith converts ca. 3 NmL of CO per minute and mL of micro reactor at 433 K under H2/CO=20, suggesting that it could be particularly effective for hydrogen purification in low-temperature microfuel cells for portable applications.

Novel Electronic Devices in Macroporous Silicon: Design of FET Transistors for Power Applications

In this paper, we study the application of macroporous silicon (MpSi) to the fabrication of transistors: Four different FET transistor structures are proposed using MpSi as the base material. These devices have been studied by simulation, and their characteristics are shown herein. The proposed structures include JFET, MOSFET, and trio de-like devices; in this study, we have considered both vertical and horizontal structures. For the vertical case, the proposed devices use the MpSi tubes to create the channel, filling them with a semiconductor and using the bulk silicon as a cylindrical gate all around. In contrast, for the horizontal transistors, the MpSi structure is used as the channel medium, while the conductor-filled pores serve as the controlling gate, thus obtaining a trio de-like device. The use of MpSi allows one to obtain large transistor arrays of extremely high density, thus obtaining a large amount of parallel devices in a moderate-to-small device area. Even more, pore engineering introduces a degree of freedom in the design of the transistor characteristics. We show that the proposed devices have a low threshold voltage and can support large currents. Finally, the behavior of these structures is studied for different geometries.

Macroporous silicon photonic crystals for gas sensing

In this paper we study a compact gas sensor based on a photonic crystal built from macroporous silicon. Its sensing mechanism is based in the absorption of infrared light by a gas. Photonic crystals are structured materials which can be engineered to have photonic bandgaps. They also can be tailored to create localized states inside the bandgaps. We exploit the possibility to confine light inside a cavity with very high-Q, which allows for long interaction time between the gas and light. Simulation of different 2-D and 3-D structures have been done to extract the appropriate dimensions for gas detection, and their optical behaviour. Resonant cavities were created by adding defects in the ordered geometrical structure, thus creating a single state and confining the trapped light in a crystal bandgap. The structures were tested by simulating the presence of ethanol inside the structures. Gas is to be detected by a noticeable change in the resonance peak both in amplitude and spread, caused by the gas detuning the cavity. Macroporous silicon samples of the investigated structures with defects were fabricated and measured by IR spectrography. Cavity resonances can be clearly seen in the samples, though we need to improve fabrication to adjust the theoretically calculated dimensions.

Development of gas sensors based in photonic crystal slabs

This work describes an innovative application of structures of photonic crystal slabs for gas sensors. Structures consisting in double ring have been studied by simulation. Their working mechanism is based in the effect of a given target gas in the pores of the slab. The effect pursued is to translate the variation of refraction index of the gas inside the pores into a shift in the position of peaks of transmission associated to resonances in the device.

Improved transmission and thermal emission in macroporous silicon photonic crystals with 700 nm pitch

In this paper we study the transmittance and the emission response of two different macroporous silicon structures with cavities. The aim is to evaluate the viability of these structures to be employed in a future gas sensor device.

Impact of the absorption in transmittance and reflectance on macroporous silicon photonic crystals

The characteristics of reflection and transmission peaks in the spectra of photonic crystals have been studied theoretically and the results compared to measurements performed in fabricated samples. The aim of this work is to investigate the relation between material losses and the effective Q factors that can be obtained in photonic crystals made with it. Photonic crystals have been designed with defects of periodicity to introduce states in the band gap that give place to reflectance and transmittance peaks at adjustable specific wavelengths. The fabricated structures are described together with their reflection and transmission spectra. The influence of losses in the material in these spectra is evaluated.

Compact device for CO 2 optical sensing using macroporous silicon photonic crystals

A photonic crystal based on a macroporous silicon structure has been fabricated and successfully used for the detection of carbon dioxide. In this paper, the device and the measurement results are presented. The sensing device here described uses an optical approach to the detection of the gas. The use of a photonic crystal allows creating a compact device working in the medium infrared spectral range. A 700 nm square lattice macroporous structure was fabricated by electrochemical etching, creating a photonic gap centered at the 4.2 μm, a CO2 absorption line. The obtained results rely only on the absorption spectra measurement.