Jean-Luc Seguin

NiTi shape memory alloy thin films: composition control using optical emission spectroscopy

One of the major barriers in the development of applications using NiTi shape memory alloy films is the difficult control over their chemical composition. In fact, the transformation temperatures of NiTi are very sensitive to the alloy composition. In this paper we show that optical emission spectroscopy can be used to monitor the composition of NiTi films during sputter deposition. The parameter controlling the composition of the films is the product of sputtering gas pressure and target-to-substrate distance. The influence of this parameter is discussed in terms of thermalisation of the sputtered atoms. We find a linear relationship between the ratio of optical emission intensity of Ni and Ti atoms and the Ni concentration in the films.

NiTi thin films as a gate of M.O.S. capacity sensors

Abstract In this study, a NiTi-shape memory alloy thin film is used as a gate in a M.O.S.-type structure. The shape memory effect involves a thermally induced phase transformation between a low temperature ductile phase and a high temperature high strength phase. The NiTi gate is deposited on a SiO 2 /Si substrate using RF sputtering deposition method. We have studied the evolution of C ( V ) characteristics as a function of temperature in order to provide evidence of phase transition within the NiTi layer, and also to investigate the effect of such a transition on the electrical properties of the structure. A rapid change in the C ( V , T ) curves which is a direct consequence of the modification in the NiTi structure is observed and discussed. The possibility of realizing a sensor of current to be integrated in microelectronic process and power integrated circuits is envisaged.

Composition study of high temperature sputtered amorphous GaxAs1−x films

Abstract We present a systematic study of the affects of the sputtering parameters on the chemical composition of amorphous GaAs (a-GaAs) sputtered films. This study shows that the composition of sputtered a-GaAs films is controlled by the self-bias voltage, the temperature, and the properties of the substrate. a-GaAs films are stoichiometric when deposited at 300 ∘ C on glass substrate, at 400 ∘ C on molybdenum substrate, and at 500 ∘ C on indium tin oxide coated glass. We suggest that these observations show that the composition of the films is only governed by the As re-evaporation process at the surface of the growing film, which is mainly increased by increasing the substrate temperature. The affect of the thermal conductivity of the substrate is discussed.

High-temperature sputtered amorphous GaAs

Abstract Amorphous gallium arsenide films have been deposited on polycrystalline molybdenum and amorphous glass substrates heated at temperatures as high as 600°C using rf sputtering. This has been achieved by operating at lower self-bias voltages when the substrate temperature is increased. The effect of the self-bias voltage on the crystallographic structure of the films is discussed in terms of the kinetic energy of the particles bombarding the growing film. Measurements of the chemical composition of the films using electron microprobe analysis show that films deposited on molybdenum substrates are stoichiometric at a substrate temperature of 400°C.

A temperature compensated CMOS ring oscillator for wireless sensing applications

This paper presents a CMOS voltage controlled ring oscillator (VCO) with temperature compensation circuit suitable for low-cost and low-power wireless sensing applications. To operate at low frequency, a control voltage generated by a CMOS bandgap reference (BGR) is described and the measurement results of the fabricated chips are presented. The output voltage of the reference is set by resistive subdivision. In order to achieve small area and low power consumption, n-well resistors are used. This design features a reference voltage of 1V. The chip is fabricated in AMS 0.35 µm CMOS process with an area of 0.032mm2. Operating at 1.25V, the output frequency is within 200±l0kHz over the temperature range of −25°C to 80°C with power consumption of 810µW.

Physical-Based Characterization of Noise Responses in Metal-Oxide Gas Sensors

The noise level in the gas microsensors is a tool for characterizing the electrical conduction under various gases and a means to improve selectivity. Metal-oxide gas microsensors with WO3 sensitive thin film are characterized using a low-frequency noise technique. The spectral form of the noise responses measured using our specific systems is similar for tested gases (ozone and nitrogen dioxide). We observe a clear Lorentzian behavior according to adsorption-desorption (A-D) noise theory. To identify the detected gas, a physical-based characterization model of A-D noise source is proposed and compared with the empirical flicker noise model. We show that the excess noise is due to the A-D processes on the surface of the sensors sensitive film. The Lorentzian parameters depend on the nature of the gases and the noise-level dependence with gas concentration is clearly demonstrated. This confirms the interest on noise spectroscopy to improve the selectivity of gas sensors.

A low power consumption CMOS differential-ring VCO for a Wireless Sensor

This paper describes a three-stage Voltage Controlled ring Oscillator (VCO) based on 0.35 μm CMOS standard technology. The VCO was designed for a transmitter operating in the 863–870 MHz European band for Wireless Sensor applications. A voltage VCTRL controls the oscillation frequency of the VCO. Simulation results of the fully differential VCO with positive feedback show that the estimated power consumption, at desired oscillation frequency and under a supply voltage of 3.3 V, is only 7.48 mW. The proposed VCO exhibits a phase noise lower than −126 dBc/Hz at 10 MHz offset frequency.

Noise spectroscopy measurements in metallic oxide gas microsensors

Noise spectroscopy is one of the solutions to enhance the selectivity of metal oxide sensors. This experimental technique is based on analysis of the power spectral density of noise fluctuations measured at the terminals of sensors in the presence of one or more gas. A specific noise measurement system has been developed to characterize the low frequency noise behavior of SnO2 gas microsensors under various gases. The noise voltage spectral density can be interpreted in terms of number fluctuations of the adsorbed molecules called adsorption-desorption noise. Our experimental results confirmed that noise spectroscopy could be used as a mean of improving gas sensors selectivity.

New type of Schottky barriers using NiTi shape memory alloy films

Abstract With the aim of studying the feasibility of sensors using NiTi shape memory alloy films, we have realised NiTi/Si(n)/Si(n+) structures by sputter depositing NiTi contacts on Si substrates. The NiTi sputtering was performed at low substrate temperature (423 K) to avoid the formation of a silicide interfacial layer. Current–voltage (I(V)) measurements on these structures show a rectifying behaviour. The barrier height and ideality factor were determined from these measurements. The influence of the martensitic phase transformation in the NiTi electrode on the electrical properties of the diodes was investigated by recording the I(V) characteristics at various temperatures. This study shows an unusual current decrease with increasing temperature above 353 K. The mechanisms responsible for this phenomenon are discussed.

Combiners based on CMOS inverters and application in RF transmitter for wireless sensors

A new Combiner architecture for direct conversion transmitter based on CMOS inverters only and operating in transconductance mode is presented in this paper. Typical applications of an adder and a subtractor of two small amplitude signals are proposed to illustrate the circuit capabilities. The proposed circuit operation has been acted from measurements with the HCC/HCF4069UB Hex Inverter from the SGS Thomson Microelectronics [1].