G.C. Cardinali

Gas sensors for air quality monitoring: realisation and characterisation of undoped and noble metal-doped SnO2 thin sensing films deposited by the pulsed laser ablation

Abstract Undoped and noble metal-doped very thin SnO2 sensing layers deposited by the pulsed laser ablation (PLA) technique upon micromachined Si substrate heater elements have been extensively characterised. The main objective behind the effort carried out in this work has been the challenging perspective to define a sensing layer technology and a sensor operating mode (continuous (DC) vs. pulsed temperature (PT) mode) which allows the long term measuring and the easy discrimination of very low concentration of benzene by an interfering species like carbon monoxide, both present in the outdoor air. The experimental results reported in this work have shown that combining the sensor technology with a suitable catalytic element (in the present case, gold) and a proper sensor operating mode, very low concentration of benzene can be revealed with a sufficient selectivity towards CO. One of the major problems encountered during the long term test of the sensors has been the loss of the sensitivity to benzene with the time. An attempt has been made to explain the reason of this experimental result. We firmly believe that many factors contribute to it. Among them, the annealing of point defects associated with the oxygen vacancies as well as the loss of the catalytic efficiency by the added Au layer as effect of a possible coalescence of the metal clusters at the sensor working temperature (T≅400°C). To partially reduce the cluster mobility and to preserve the benzene sensitivity, the last assumption would suggest to operate the sensors, no matter if in DC or in PT mode, at a temperature lower than that used in this work.

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.

Improved electrical characterization of Al–Ti ohmic contacts on p-type ion implanted 6H-SiC

This paper deals with the electrical characterization of low resistance Al–Ti 72/28 wt% ohmic contacts to a p-type ion implanted 6H-SiC layer. Transmission line model (TLM) structures were realized on the top of MESA islands defined in this ion implanted layer. A metal scheme composed of Al-1%Si(350 nm)/Ti(80 nm) was deposited by sputtering, photolithography defined and annealed at 1000 °C in Ar for 2 min. TLM structures were measured as a function of the temperature in the range 25–290 °C. The TLM data were mainly analysed by a two-dimensional finite difference simulation tool that takes into account the current crowding effect at the contact periphery. Extracted contact resistivity values fall in the low range of data from the literature. The sheet resistance values computed from the TLM data agreed with those measured using Van der Pauw devices realized next to the TLM structures.

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 simple interface circuit for micromachined gas sensors

Abstract This paper describes a microcontroller-based interface circuit for chemoresistive metal-oxide gas sensor dedicated to environmental pollution detection. The circuit controls the sensor operating temperature, measures the sensing layer resistance, offering also a user-friendly interface. The main system features are: wide dynamic measurement range, self-calibration capability, built-in RS-232 interface and two different methodologies of data acquisition, namely constant and pulsed temperature modes.

Comparison between Different Schottky Diode Edge Termination Structures: Simulations and Experimental Results

Four different Schottky diode edge terminations have been fabricated on 6H SiC. The metal contact was Ni2Si in all the structures and the epitaxial layer has a carrier concentrations of 3x10 cm and a thickness of 4 microns. With these characteristics of the epitaxial layer, the ideal breakdown voltages should be 800 V. In the best structure an edge efficiency of about the 95% has been reached. The comparison between the experimental and the simulated results shows that, when the breakdown is not influenced by defects present in the substrate, a good agreement with the simulation can be reached.

Contact Resistivity and Barrier Height of Al/Ti Ohmic Contacts on p-Type Ion Implanted 4H- and 6H-SiC

Recently, the Al-Ti alloy with the 70 wt% Al was found to give the most reproducible contact resistivity ρc on epitaxial and implanted p-type SiC after vacuum annealing at 1000°C for 2 min. In this work contact resistivity values of this Al-Ti alloy are analyzed aiming at extracting the Schottky barrier height with particular attention to p-type ion implanted 4Hand 6H-SiC specimens. The ρc values extracted from Transmission Line Method measurements were always lower than 2×10 -4 Ωcm 2 for a medium/high doping concentration of the implanted layers, i.e. 4×10 19 cm -3 . Moreover, a significant portion of values below the minimum resolution of 1×10 -6 Ωcm 2 was found. The barrier height φB has been extracted from the contact resistivity as a function of the temperature, according to the thermionic-field emission model, for all the dies featuring a ρc greater than the minimum resolution. For these dies the φB is (0.53±0.05) eV for the 6H-SiC epilayer, (0.82±0.08) eV for the implanted 4H-SiC and (0.95±0.08) eV for the implanted 6H-SiC. It was found that the dies which have a ρc minor than the minimum resolution could feature a barrier height similar to that found on the epilayer.

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.

A Comparative Study of High-Temperature Aluminum Post-Implantation Annealing in 6H- and 4H-SiC, Non-Uniform Temperature Effects

4H-and 6H-SiC small samples were implanted by keV Al + ions at room temperature and annealed in an induction heating furnace, at the center of the susceptor, for different temperatures and times in the range 1600-1800°C and 5-60 min, respectively. The implanted layers were amorphous but the SiC crystalline structures were recovered after annealing, as measured by Rutherford Back-Scattering analyses in Channeling geometry. Al + electrical activation determined by sheet resistance and Hall effect measurements increases with the annealing temperature or time, on both polytypes. When whole SiC wafers were annealed in the same induction heating furnace, sheet resistance mapping systematically presented a radial gradient from the center to the periphery of the wafer. The measured linear dependence between sheet resistance and temperature allowed us to rebuild the radial temperature gradient at the crucible-susceptor furnace during the annealing process. Introduction Silicon carbide (SiC) is envisaged as a promising semiconductor material for a wide variety of high-temperature, high-power and high-frequency electronic applications. Ion implantation, an indispensable technique to locally dope silicon carbide still presents many problems in particular for p-type zone creation. High ionization energy of dopants imposes to raise the implanted dose above the amorphization threshold for room temperature implantations. Structure recrystallization and electrical activation of dopants, i.e. their incorporation in active SiC atomic sites, require high temperature annealing, about 1700°C in special configuration, with an overpressure of silicon and carbide. In this work p-type 6H and 4H-SiC layers created by Aluminum (Al) ion implantations followed by high temperature annealings are studied in order to realize efficient p +-n junctions for bipolar power diodes. Dopant electrical activation dependence on the post-implantation annealing conditions is discussed considering the non-uniform temperature at the SiC sample surface during this process.

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.