Ivan Elmi

Discontinuously Operated Metal Oxide Gas Sensors for Flexible Tag Microlab Applications

Micromachined silicon substrates have significantly reduced the heating power consumption of metal oxide (MOX) gas sensors. Specific applications, however, require further reductions far beyond the present state-of-the-art. In this paper, we report on discontinuously operated MOX gas sensors on micromachined heater platforms and show that such sensors allow power consumption levels to be reached which are consistent with Flexible Tag Microlab (FTM) operation. Such FTMs allow gas concentrations to be measured and recorded to reveal the transport history of goods along the logistics chain for later interrogation by a wireless reader.

Development of Ultra Low Power Consumption Hotplates for Gas Sensing Applications

This paper deals with the development of state-of-the-art ultra low power (ULP) consumption hotplates to be used as metal oxide gas sensor substrates. Several types of ULP devices, differing in shape and size, have been fabricated with the same front-side bulk silicon micromachining technology. Details on the device design and on the fabrication processes are provided. The ULP hotplates functional behavior was thoroughly investigated. Typical results on measurements of the hotplate temperature versus applied power are reported. A very satisfactory value of 8.9 mW at 400degC can be highlighted. Transient temperature responses and evaluation of the hotplate thermal time constant were also carried out.

A Programmable Interface Circuit for an Ultralow Power Gas Sensor

Abstract-An electronic system based on a microcontroller architecture, devoted to interfacing a three-terminal, ultralow power (ULP) Metal OXide (MOX) gas sensor is presented. The sensor features a novel three-terminal configuration where the microheater is not galvanically isolated with respect to the MOX sensor. The system provides both control of the operating temperature and management of the acquired data. A Pulse Width Modulation (PWM) signal with variable duty cycle is used to provide power to the heating resistor in order to set the desired operating temperature. The heating resistance value is measured in the range (100-300) Ω with a relative error of less than 1%. The circuit devoted to measuring the gas concentration is based on a logarithmic amplifier which measures the current flowing in the sensing layer of the sensor. The measurand range is 30 nA to 60 mA and the relative error of the measured current is less than 0.6%. The data acquisition system was successfully tested by acquiring data of a three-terminal ULP gas sensor located in an automatically controlled environmental chamber under benzene and NO2 flow.

Ultra Low Power MOX Sensors with ppb-Level VOC Detection Capabilities

This paper deals with the development of state-of-the-art metal oxide semiconductor (MOX) gas sensors based on ultra-low power (ULP) consumption micro-machined hot plates. Several gas sensors have been fabricated by means of a wafer level technological process, and deeply functionally and morphologically characterized. Details on device design and on the fabrication processes are provided. Results of functional characterizations towards different gases at different working conditions will be reported. The capability of detecting volatile organic compounds (VOC) down to few parts per billion (ppb), will be shown. Outcomes of morphological sensing layer characterization will show the well controlled nano-structured thin film of tin oxide.

A supramolecular approach to sub-ppb aromatic VOC detection in air

Micromachining technology is coupled to a selective pre-concentration material for the development of a portable sub-ppb level monitoring system for aromatic volatile organic compounds (VOC); the high sensitivity of Metal Oxide (MOX) gas sensors is combined with a supramolecular concentration unit to increase selectivity and reduce the detection limits.

Adaptive K-NN for the detection of air pollutants with a sensor array

The field of air-quality monitoring is gaining increasing interest, with regard to both indoor environment and air-pollution control in open space. This work introduces a pattern recognition technique based on adaptive K-nn applied to a multisensor system, optimized for the recognition of some relevant tracers for air pollution in outdoor environment, namely benzene, toluene, and xylene (BTX), NO/sub 2/, and CO. The pattern-recognition technique employed aims at recognizing the target gases within an air sample of unknown composition and at estimating their concentrations. It is based on PCA and K-nn classification with an adaptive vote technique based on the gas concentrations of the training samples associated to the K-neighbors. The system is tested in a controlled environment composed of synthetic air with a fixed humidity rate (30%) at concentrations in the ppm range for BTX and NO/sub 2/, in the range of 10 ppm for CO. The pattern recognition technique is experimented on a knowledge base composed of a limited number of samples (130), with the adoption of a leave-one-out procedure in order to estimate the classification probability. In these conditions, the system demonstrates the capability to recognize the presence of the target gases in controlled conditions with a high hit-rate. Moreover, the concentrations of the individual components of the test samples are successfully estimated for BTX and NO/sub 2/ in more than 80% of the considered cases, while a lower hit-rate (69%) is reached for CO.

Material Properties Measurement and Numerical Simulation for Characterization of Ultra-Low-Power Consumption Hotplates

The results of a thorough thermoelectric characterization, performed both in a vacuum chamber and at atmospheric pressure, of ultra-low-power hotplates based on suspended structures with different layouts are presented in this work and compared with thermoelectric 2D and thermal 3D finite elements simulations. Electrical and thermal properties of the thin films used in the devices have been also measured, involving appropriate on-chip test structures, and their values were employed in both 2D and 3D model. Temperature vs. heating power experimental curves showed the great influence of conduction through air on power consumption and an excellent agreement with the simulated results.

In Search of the Ultimate Benzene Sensor: The EtQxBox Solution

In this work we report a comprehensive study leading to the fabrication of a prototype sensor for environmental benzene monitoring. The required high selectivity and ppb-level sensitivity are obtained by coupling a silicon-integrated concentration unit containing the specifically designed EtQxBox cavitand to a miniaturized PID detector. In the resulting stand-alone sensor, the EtQxBox receptor acts at the same time as highly sensitive preconcentrator for BTEX and GC-like separation phase, allowing for the selective desorption of benzene over TEX. The binding energies of the complexes between EtQxBox and BTX are calculated through molecular mechanics calculations. The examination of the corresponding crystal structures confirms the trend determined by computational studies, with the number of C-H···N and CH···π interactions increasing from 6 to 9 along the series from benzene to o-xylene. The analytical performances of EtQxBox are experimentally tested via SPME, using the cavitand as fiber coating for BTEX monitoring in air. The cavitand EFs are noticeably higher than those obtained by using the commercial CAR-DVB-PDMS. The LOD and LOQ are calculated in the ng/m3 range, outperforming the commercial available systems in BTEX adsorption. The desired selective desorption of benzene is achieved by applying a smart temperature program on the EtQxBox mesh, which starts releasing benzene at lower temperatures than TEX, as predicted by the calculated binding energies. The sensor performances are experimentally validated and ppbv level sensitivity toward the carcinogenic target aromatic benzene was demonstrated, as required for environmental benzene exposure monitoring in industrial applications and outdoor environment.

Thermal Transient Measurements of an Ultra-Low-Power MOX Sensor

This paper describes a system for the simultaneous dynamic control and thermal characterization of the heating of an Ultra Low Power (ULP) micromachined sensor. A Pulse Width Modulated (PWM) powering system has been realized using a microcontroller to characterize the thermal behavior of a device. Objectives of the research were to analyze the relation between the time period and duty cycle of the PWM signal and the operating temperature of such ULP micromachined systems, to observe the thermal time constants of the device during the heating phase and to measure the total thermal conductance. Constant target heater resistance experiments highlighted that an approximately constant heater temperature at regime can only be obtained if the time period of the heating signal is smaller than 50 𝜇s. Constant power experiments show quantitatively a thermal time constant 𝜏 that decreases during heating in a range from 2.3 ms to 2 ms as a function of an increasing temperature rise Δ𝑇 between the ambient and the operating temperature. Moreover, we calculated the total thermal conductance. Finally, repeatability of experimental results was assessed by guaranteeing the standard deviation of the controlled temperature which was within ±5.5∘C in worst case conditions.

Interface circuit for an ultra low power gas sensor

An electronic system based on a microcontroller architecture devoted to interfacing an Ultra Low Power (ULP) Metal OXide (MOX) gas sensor is presented. The circuit controls the operating temperature and manages the acquired data. A Pulse Width Modulation (PWM) signal with variable duty cycle is used to provide power to the heating resistor in order to set the desired operating temperature. The heating resistance value is measured in the range [100 Ω - 500 Ω] with a relative error less than 1%. The circuit devoted to measure the gas concentration is based on a logarithmic amplifier which measures the current flowing in the sensing layer of the sensor. The measurand range is [30 nA - 60 μA] and the relative error on the measured current is less than 0.6%.