Takao Iwaki

Design and simulation of resistive SOI CMOS micro-heaters for high temperature gas sensors

This paper describes the design of doped single crystal silicon (SCS) microhotplates for gas sensors. Resistive heaters are formed by an n+/p+ implantation into a Silicon-On-Insulator (SOI) wafer with a post-CMOS deep reactive ion etch to remove the silicon substrate. Hence they are fully compatible with CMOS technologies and allows for the integration of associated drive/detection circuitry. 2D electro-thermal models have been constructed and the results of numerical simulations using FEMLAB® are given. Simulations show these micro-hotplates can operate at temperatures of 500°C with a drive voltage of only 5 V and a power consumption of less than 100 mW.

Identification of Different Vapors Using a Single Temperature Modulated Polymer Sensor With a Novel Signal Processing Technique

This paper reports on a novel temperature modulation technique to improve the ability of a single carbon black/polymer resistive sensor to discriminate, identify and characterize environmental pollutants. Such a sensor can be low cost, low-power consumption, highly reliable, and thus ideal for indoor gas monitoring. Here, a carbon black/polyvinylpyrrolidone film was deposited onto an SOI-CMOS microhotplate. A novel temperature modulation technique is proposed that allows the detection of vapors using a single chemoresistor. Identification and quantification of water, methanol and ethanol vapors with different concentrations (water: 0-6000 ppm, methanol: 1360-5430 ppm and ethanol: 2270-9080 ppm) are shown.

SQI-CMOS based single crystal silicon micro-heaters for gas sensors

Here we report on novel high temperature gas sensors that have been fabricated using an SOI (silicon-on-insulator) -CMOS process and deep RIE back-etching. These sensors offer ultra-low power consumption, low unit cost, and excellent thermal stability. The highly-doped single crystal silicon (SCS) layer of a standard SOI-CMOS process, which is traditionally used to form the source and drain regions of a MOSFET, is used, for the first time, to form a resistive heater of a micro-hotplate in a high-temperature gas sensor. Our sensors have a power consumption of only 12-30 mW at a temperature of 500degC. We have observed that the drift in resistance of a SCS heater held at 500degC for 500 hours, without burn-in, was less than 1%. SCS micro-hotplates are not only suitable for chemoresistive sensors, as described here, but also calorimetric gas sensors that require these high operating temperatures. Tungsten oxide nanorods have been deposited onto our micro-hotplates by atmospheric chemical vapour deposition and have shown reasonable sensitivity to ethanol vapour in air.

Enhancement of effective electromechanical coupling factor by mass loading in layered SAW device structures

This paper describes drastic enhancement of K_e² by mass loading in layered SAW device structures such as the ScAlN film/Si substrate. It is shown that this phenomenon is obvious even when an amorphous SiO₂ film is deposited on the top surface for temperature compensation. This enhancement is caused by SAW energy confinement to the top surface of the ScAlN layer where the IDT is placed. This K_e² enhancement is also found when other electrode and/or substrate materials are employed.

Enhancement of effective electromechanical coupling factor by mass loading in layered surface acoustic wave device structures

This paper describes a drastic enhancement of the effective coupling factor by mass loading in layered surface acoustic wave (SAW) device structures such as the ScAlN film/Si substrate structure. This phenomenon occurs when the piezoelectric layer exhibits a high acoustic wave velocity. The mass loading decreases the SAW velocity and causes SAW energy confinement close to the top surface where an interdigital transducer is placed. It is shown that this phenomenon is obvious even when an amorphous SiO2 film is deposited on the top surface for temperature compensation. This enhancement was also found in various combinations of electrode, piezoelectric layer, and/or substrate materials. The existence of this phenomenon was verified experimentally using the ScAlN film/Si substrate structure.

Novel dual transient temperature modulation technique for multi-vapour detection

Here we present a novel signal processing technique for a square wave thermally-modulated carbon black/polymer composite chemoresistor. The technique consists of only two mathematical operations: summing the off-transient and on-transient conductance signals; and subtracting the steady-state conductance signal. A single carbon black/polyvinylpyrrolidone composite chemo -resistor was fabricated and used to demonstrate the validity of the technique. Classification of water, methanol and ethanol vapours was successfully performed using only the peak time of the resultant curves. Quantification of the different vapours was also possible using the height of the peaks, because it was linearly proportional to concentration. This technique does not require zero-gas calibration and thus is superior to previously reported methods.

High Temperature SQI CMOS Tungsten Micro-Heaters

Here we present for the first time, the design, fabrication and characterization of novel high temperature SOI micro-hotplates employing tungsten resistive heaters. Tungsten has a high operating temperature, good mechanical strength, and could be used for CMOS interconnects. These devices have been fabricated using a commercial SOI-CMOS process followed by a DR1E back-etch step, offering low cost and the option of circuit integration. Here we report on micro-hotplates with two different diameters (560mum and 300mum), 3D electro-thermal simulation in ANSYS and their electro-thermal characterization. Results show that these devices can operate at a high temperature (600degC), have ultra low DC power consumption (12mW at 600degC), fast transient time (as low as 2ms to 600degC), stability in time and temperature and, more importantly, a high reproducibility both within a wafer and from wafer to wafer. The SOI micro-hotplates could be used in fully integrated micro-calorimeters or resistive gas sensors.