Gilles Scheen

Fabrication of a miniaturized ionization gas sensor with polyimide spacer

Gas sensing can be achieved by fingerprinting the ionization characteristics of distinct species. In this study, the fabrication of a miniaturized gas ionization sensor using polyimide as sacrificial layer is reported. The sensor consists of two planar metallic electrodes with a gap spacing obtained by the polyimide under-etching. This known sacrificial layer has the advantage besides a high planarization factor, to be CMOS compatible. Furthermore, its chemical resistance up to high temperatures, high resistance to radiation from both electrons and neutrons, and low outgassing are of primary importance to avoid interferences with the ionization gas sensing. A suspended micro-bridge with dimensions 20 μm width and 220 μm length has been developed and released by using etching holes in the membrane. The ionization characteristics of air at controlled temperature, humidity and pressure (21°C, 40% humidity and 1 atm) have been obtained during non-destructive electrical characterizations, with a breakdown voltage of 350 V for a 6 μm gap. The growth of metallic nanowires templated in ion track-etched polyimide on the electrode is envisioned in order to enhance the ionization field and to reduce the required measurement power of the sensor.

Metal electrode integration on macroporous silicon: pore distribution and morphology.

In this work, a new approach for the one-step integration of interdigitated electrodes on macroporous silicon substrates is presented. Titanium/gold interdigitated electrodes are used to pattern p-type silicon substrates prior the anodization in an organic electrolyte. The electrolyte characteristics, conductivity, and pH have been found to affect the adherence of the metal layer on the silicon surface during the electrochemical etching. The impact of the metal pattern on size distribution and morphology of the resulting macroporous silicon layer is analyzed. A formation mechanism supported by finite element simulation is proposed.

Development of a Miniaturized Gas Ionization Sensor for Harsh Environments by Using Polyimide Spacer

Gas sensing can be performed by fingerprinting their field ionization characteristics. This paper presents the development of a miniaturized ionization sensor using ion-track etched polyimide as structural layer and template for Ni nanowires synthesis. The device consists in two parallel plate electrodes with gaps varying from 5 to 12 μm. The nanowires impact on breakdown voltage has been analyzed during first electrical characterizations and I-V curves measurements. For a 5.5 μm-gap, breakdown voltage is reduced from 320 to 80 V with a corresponding current at least three order of magnitude lower. Using the sensor in harsh environments such as space applications is also discussed. Miniaturized ionization sensors are powerful candidates as integrated universal gas sensor based on pattern recognition for environmental monitoring. Such a system should be easily integrated in picosatellites such as CubeSats dedicated to the physical analysis of low thermosphere composition.

Palladium nanoparticle deposition via precipitation: a new method to functionalize macroporous silicon

Abstract We present an original two-step method for the deposition via precipitation of Pd nanoparticles into macroporous silicon. The method consists in immersing a macroporous silicon sample in a PdCl2/DMSO solution and then in annealing the sample at a high temperature. The impact of composition and concentration of the solution and annealing time on the nanoparticle characteristics is investigated. This method is compared to electroless plating, which is a standard method for the deposition of Pd nanoparticles. Scanning electron microscopy and computerized image processing are used to evaluate size, shape, surface density and deposition homogeneity of the Pd nanoparticles on the pore walls. Energy-dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS) analyses are used to evaluate the composition of the deposited nanoparticles. In contrast to electroless plating, the proposed method leads to homogeneously distributed Pd nanoparticles along the macropores depth with a surface density that increases proportionally with the PdCl2 concentration. Moreover EDX and XPS analysis showed that the nanoparticles are composed of Pd in its metallic state, while nanoparticles deposited by electroless plating are composed of both metallic Pd and PdOx.

Synthesis of patterned freestanding nickel nanowires by using ion track-etched polyimide

Nowadays, a lot of applications including nanoelectronics, spintronics or miniaturized sensors are using nanowires. Unfortunately, current techniques used for local synthesis of nanowires are still not fully compatible with common microfabrication techniques. In this study, we focus on the synthesis of patterned metallic nanowires by electrodeposition within nanoporous polyimide membranes integrated on 3 inch Si bulk wafers. Known to have a high planarization factor, a good resistance to most non-oxidizing acids and bases and to be CMOS compatible, polyimide is increasingly used in microsystems. Furthermore, like polycarbonate or polyester, nanoporous polyimide can be obtained by ion track-etching process. This polymer shows then a great interest to be used as a mold for nanowires growth. Patterned freestanding Nickel nanowires have been synthesized over a 100 nm thickness gold layer evaporated onto a SiO2/Si substrate, with diameters of 20 and 60 nm, and length between 2 and 2.5 μm, depending on the electrodeposition time. Such fabrication process is promising to achieve more complex microelectromechanical systems incorporating nanostructures.

Ultra Low Power 3-D Flow Meter in Monolithic SOI Technology

Silicon-on-Insulator technology, with unique properties such as harsh environment resistance and lower power consumption, is presented here as a platform for CMOS and MEMS co-integration. An original CMOS-compatible process has been developed for the design and the co-fabrication of out-of-plane movable cantilevers and ring oscillators circuits on the same chip. The measured transducer, by deflection of the out-of-plane MEMS component, shows until 10% variation of the frequency under different flow rates.

RF and non-linearity characterization of porous silicon layer for RF-ICs

Nanostructured porous silicon is very promising for RF applications by overcoming the high-frequency losses originating from the bulk silicon substrate. RF performance and non-linearity analysis of different silicon substrates including, porous (PSi), trap-rich (TR) high resistivity (HR) types are explored experimentally. The investigation is done by means of coplanar transmission lines (CPW) fabricated on these substrates. RF measurements of transmission lines demonstrate the successful reduction of the permittivity and increase of the resistivity of the PSi substrate. It also demonstrated that the insertion losses and linearity are efficiently enhanced.