Cornel Cobianu

SnO2 sol–gel derived thin films for integrated gas sensors

Abstract In this paper, we present for the first time the compatibility of sol–gel method for SnO2 thin film preparation with the silicon technology for integrated gas sensor microfabrication. An integrated circuit (IC) compatible test structure of medium power consumption equipped with boron-doped silicon heater and Au/W metallization is developed. The acid composition of the (liquid) sol phase, the thermal budget of sensing layer structuring, selective wet etching of SnO2 sensing film, thickness uniformity and step coverage of SnO2 sol–gel films are fitted with the requirement of above test structure where metal layer is deposited before SnO2 film. Nanometric grain sizes of undoped and antimony doped polycrystalline SnO2 films are obtained, as revealed by XRD investigations. The AFM measurements of SnO2 thin films deposited on existing Au/W metallization shown the excellent step coverage and morphology of SnO2 films used for gas sensing applications. Low temperature gas sensing properties of our SnO2 sol–gel derived thin films in reducing (CH4, CH3COOH) and oxidizing (NO2) are preliminary reported by using our integrated test structure.

Surface Acoustic Wave devices and their sensing capabilities

Since mid 60s, Surface Acoustic Wave (SAW) devices have been using extensively in electronics for telecommunication. However, their potential use in the sensors field is only a matter of recent interest. This paper presents an overview of the sensing mechanisms that allow detection of temperature, strain (for pressure & torque), mass, conductivity (e.g. gas detection) and viscosity by means of SAW based devices. Design and technological challenges are presented, as well as solutions to overcome them in order to obtain reliable devices. The possibility of using such devices as passive wireless sensors is also discussed.

Surface acoustic wave CO 2 sensing with polymer-amino carbon nanotube composites

The synthesis of two new types of nanocomposite matrices, the first based on polyallylamine (PAA) and aminocarbon nanotubes, the second on polyethyleneimine (PEI) and aminocarbon nanotubes, are reported. The surface acoustic wave (SAW) sensors, coated with the two selected nanocomposites, showed good sensitivities when varying the CO2 concentrations in the range (500-5000) ppm. The sensor sensitivity is larger when using polyethyleneimine aminocarbon nanotubes than in the case when only a pure polyethyleneimine layer is considered for coating.

Lead-free galvanic oxygen sensors — A conceptual approach

Within this paper we present a thermodynamic methodology for the selection of non-toxic metals which could be used as lead-free consumable anodes in electrochemical galvanic oxygen sensors. Starting from thermodynamic nobility theory, metals like copper, bismuth or antimony are proposed to replace lead in future galvanic O2 sensors. The thermodynamic theory provides voltage windows which increase from copper (0.7 V), to bismuth (0.857 V) and antimony (1.076 V). The experimental voltage windows are smaller than the theoretical ones, but these experimental values increase in the same order from Cu, to Bi and Sb, as predicted by our methodology.

Selection of gas sensing materials using the Hard Soft Acid Base theory; application to Surface Acoustic Wave CO 2 detection

The Hard Soft Acid Base (HSAB) theory is introduced as a new tool to select or design sensitive materials for carbon dioxide detection with SAW-BAW (Surface Acoustic Waves - Bulk Acoustic Waves) devices. According to HSAB, CO2 is hard acid, thus small organic or inorganic molecules, or polymers which can act as hard bases could be suitable candidates as sensing layers for carbon dioxide detection. As a consequence of this theory, we propose the following polymers as potential candidates for CO2 sensing: simple polyallylamine, N-substituted polyallylamine, polydiallylamine and polyvinylamine, and mixtures of these polymers. The SAW device coated with one of the selected polymers, polyallyamine, shows good sensitivity for CO2 concentration (in the range 500–5000 ppm), long term stability and repeatability.

Low Power Resistive Oxygen Sensor Based on Sonochemical SrTi0.6Fe0.4O2.8 (STFO40)

The current paper reports on a sonochemical synthesis method for manufacturing nanostructured (typical grain size of 50 nm) SrTi0.6Fe0.4O2.8 (Sono-STFO40) powder. This powder is characterized using X ray-diffraction (XRD), Mössbauer spectroscopy and Scanning Electron Microscopy (SEM), and results are compared with commercially available SrTi0.4Fe0.6O2.8 (STFO60) powder. In order to manufacture resistive oxygen sensors, both Sono-STFO40 and STFO60 are deposited, by dip-pen nanolithography (DPN) method, on an SOI (Silicon-on-Insulator) micro-hotplate, employing a tungsten heater embedded within a dielectric membrane. Oxygen detection tests are performed in both dry (RH = 0%) and humid (RH = 60%) nitrogen atmosphere, varying oxygen concentrations between 1% and 16% (v/v), at a constant heater temperature of 650 °C. The oxygen sensor, based on the Sono-STFO40 sensing layer, shows good sensitivity, low power consumption (80 mW), and short response time (25 s). These performance are comparable to those exhibited by state-of-the-art O2 sensors based on STFO60, thus proving Sono-STFO40 to be a material suitable for oxygen detection in harsh environments.

Nano-scale resonant sensors for gas and bio detection: Expectations and challenges

It is the purpose of this paper to show expectations, challenges and initial steps concerning the realization of resonant chemical NEMS sensors able to meet the needs of future applications. Here, we focus on the functionalization principles of the sensing Self-Assembled Monolayer (SAM), modeling and simulation of CMOS-SOI resonant NEMS sensor with electrostatic actuation and MOSFET detection, first CMOS-SOI experimental results for making Si nano wire for piezoresistive detection schemes, noise limits of the resonant nano-sensors, challenges for the design of the on-chip readout circuitry, and the specific reliability issues of resonant NEMS. Some of the simulated sensitivity results of about 5 Hz/zg at 433 MHz and MOSFET detection are close to the best state- of- art experimental data from literature of 0.7 Hz/zg at 127 MHz. It is our challenge to pursuit at experimental level with our nanosensor concepts for making reliable nanodevices addressing the needs of integrated sensing.

Amino groups-based polymers for CO 2 detection; A comparison between two sensing mechanism models

Two CO2 sensing mechanisms, based on the Hard Soft Acids Bases (HSAB) and Bronsted-Lowry theories, are discussed and compared. They are evaluated by selecting amino groups-based coating layers, which are deposited on Surface Acoustic Wave (SAW) devices for CO2 detection. Experimentally measured CO2 sensitivities of different coating layers, such as polyallylamine (PAA), polyethyleneimine (PEI), nanocomposite matrix based on PAA-aminocarbon nanotubes and PEI-aminocarbon nanotubes, emeraldine, 4-sulfocalix[4]arene-doped polyaniline, matrix based emeraldine and carbonic anhydrase (PACA) are compared and evaluated according to their corresponding sensing mechanism.

Design and Analysis of an In-Plane Resonant Nano-Electro-Mechanical Sensor for Sub-Attogram-Level Molecular Mass-Detection

This paper presents the In-Plane Resonant NEM (IP R-NEM) sensor based on the mass-detection principle. The proposed sensor features a mass-detection limit of sub atto gram and the sensitivity of the order of zepto gram/Hz, more than eight orders smaller than that of recent QCM-based sensors. The sensor is designed and analyzed using the three-dimensional FEM simulation. Self-assembled linker molecules and adsorbed target molecules on the beam surface are modelled by adding an extra surface coating layer. We study two different surface detection schemes, top-and-bottom and all-around surface detection. The IP R-NEM sensor is integrated with a MOSFET for electrical detection, and the transient response is studied on the system-level using a hybrid electro-mechanical circuit simulation.

A novel concept for low drift chemical sensing at micro and nano-scale

It is the purpose of this paper to present a novel generic concept for low drift chemical sensing which is applicable at micro and nanometer scale, based on a new, all-differential approach. At micrometer level, our principle is explained by means of surface acoustic wave (SAW) chemical sensing, while at nano level, we are using the resonant sensing principle to develop our genuine differential concept. Unlike the traditional differential approaches based on functionalized sensing layer in the sensing loop, and on a uncoated surface in the reference loop, our all differential concept provides a better response subtraction between the two paths, as the sensing loop consists of a functionalized sensing layer, as before, but, the reference loop consists of a functionalized non-sensing layer, with the same ageing and humidity behavior as the sensing layer. Twinned electronic reading is used for both loops, and thus all the common mode signals are subtracted in the differential reading, assuring the minimum base line drift of the sensor. Preliminary results of all differential sensor response to humidity and temperature variations are shown for the SAW sensors, with the sensor signal kept independent of their changes.