Comparison of Thermo-Resistive and Thermo-Electronic Transduction Methods in Thermal Flow Sensors

This paper investigates the differences between thermo-resistive and thermo-electronic flow sensors across 5 SOI CMOS designs. The two transduction principles (hot wire and thermo-diode) are both embedded within the same sensor, allowing for the first time, a fair and complete comparison. Results show that with the reduction in sensor size, the sensitivity and accuracy of thermo-resistive methods becomes worse. The thermo-electronic sensitivity is less significantly affected and the accuracy is unaffected by sensor and heater size. The length of the diode has been found to affect the non-linearity of the electro-thermal properties due to relative significance of current leakage. However, a shorter diode showed 50 % higher accuracy due to it measuring an average higher temperature. This paper can be used as a design tool for those choosing transduction methods within their sensors

2D and 3D thermal flow sensor modelling-A comparative analysis

In this paper, 2-Dimensional and 3-Dimensional numerical models of a thermal flow sensor are discussed and compared against one another whilst being validated with experimental results. The models involve and couple three physics domains: Electric, computational fluid dynamics and heat transfer. A comparative analysis has been performed focusing on the relative merits. Despite being less accurate, it is shown that 2D simulations can be used to portray device behavior whilst minimizing required computational resources whereas 3D models are needed to attain more accurate quantitative data. This ultimately provides the knowledge for objective driven modelling decisions

Differential Thermal Conductivity CO2 Sensor

In this paper we present a novel MEMS technology for CO detection, based on differential thermal conductivity. Holes have been positioned across the thin-film dielectric membrane to induce an asymmetric temperature profile in conjunction with a centrally heated tungsten resistor, leading to distinct magnitudes of gas interaction across the sensor, all of which is substantiated by a numerical model. Differential readings are taken from four surrounding resistors and shown to correlate to CO concentration. The best resistor combination is investigated and is shown to provide a sensitivity of 0.05 mV/%CO. Using separate heating and sensing elements greatly lowers the noise and a detection resolution of < 100 ppm is calculated. This proof-of-concept article introduces a device that could lead to low-cost, low-power, high-sensitivity, scalable gas detection

Simultaneous Flow and Thermal Conductivity Sensing on a Single Chip Using Artificial Neural Networks

A thin-film CMOS MEMS thermal sensor has been designed, fabricated and tested with the addition of through-membrane isolating holes. These holes have been shown to enhance the discrimination towards gases with differing thermal conductivity in the presence of flow. Using three on-membrane resistors as inputs, linear statistical methods alongside Artificial Neural Network pattern recognition techniques have been investigated for decoupling the two parameters of thermal conductivity and flow rate using a single sensor. In addition to this, the addition of the membrane holes increases the sensitivity towards flow rate by 10 times and the sensitivity towards thermal conductivity by 2 times. This sensor design coupled with well-known post-processing techniques will enable a new generation of multi-parameter sensing solutions

Thermal Conductivity Sensor with Isolating Membrane Holes

In this paper we present a thermal conductivity gas sensor with improved sensitivity by adding holes in the thin-film membrane. A numerical model is created and validated against the reference CMOS MEMS thermal conductivity sensor. The numerical model is used to investigate the advantages of having isolating holes through the membrane, located on either side of the heating resistor. These holes increase robustness and minimise catastrophic failure caused by pressure difference whilst simultaneously increasing sensitivity by enhancing convective interaction. The electro-thermal efficiency is shown to increase by 16% whilst the sensitivity to measuring percentage CO2 increases by 39.2%. It is also shown that increasing the width of the holes does not have significant effect on these sensitivities; thus, small holes can be incorporated, leaving room for multiple resistors across the membrane for different measuring techniques. This paper serves as proof that membrane holes can be used to optimise thermal conductivity sensors and will serve as a reference when these designs are fabricated and tested, leading to a new low-power, high-sensitivity gas sensor

MEMS Thermal Flow Sensors - An Accuracy Investigation

The geometric structure of a low-power thermal micro-hot film sensor has been investigated and optimized, using both computational and experimental methods. The response and accuracy of eight CMOS designs with different heater and membrane sizes were studied and found to vary considerably with geometry. It is found that reducing the heater length causes an improved electro-thermal efficiency and that a large reduction in accuracy was seen when reducing the membrane size. Our simulations suggest that this effect is due to higher temperature gradients causing localized stronger natural convective flows above the measuring resistor. However, the reduced accuracy disappears as flow rate increases due to a higher proportion of forced convection compared with natural convection. We believe that this paper will help in the design of a new generation of high accuracy MEMS thermal flow sensors for low-cost, low-power application

Thin-film mosfet-based pressure sensor

This article proposes a silicon-on-insulator complementary metal-oxide semiconductor (CMOS) micro-electromechanical system (MEMS) thin-film pressure sensor in which the sensing elements are based on stress-sensitive MOSFETs, and the carrier mobility and channel resistance vary with applied pressure. Four MOSFETs are embedded within a silicon dioxide membrane to form a Wheatstone bridge. The sensors are fabricated in a commercial foundry with p-channel and n-channel designs both investigated. The fabricated pMOSFET design gave a pressure sensitivity of 5.21 mV/kPa, whereas the nMOSFET gave about half the sensitivity at 2.40 mV/kPa. This shows a highly sensitive pressure sensor, with improved sensitivity on traditional piezoresistors, as well as significantly higher sensitivity than current MOSFET based pressure sensors. Moreover, the maximum dc power consumption was only 190 and 390 μW for the pMOSFET and nMOSFET, respectively. This low-cost, low-power, high-sensitivity CMOS MEMS technology with on-chip electronics could be used toward the implementation of MOSFET-based pressure transduction in a multitude of industrial and other applications

Diode-based CMOS MEMS thermal flow sensors

© 2017 IEEE. This paper reports on the intrinsic advantages of thermoelectronic flow sensors in comparison to their thermoresistive and thermoelectric counterparts. Hereafter, we will numerically and experimentally show that thermoelectronic flow sensors (i.e. thermal flow sensors employing p-n junction based devices as temperature sensors) benefit from the possibility of having the temperature sensor located in the hottest area of the heating element for enhanced convective effects and thus improved sensor sensitivity (Average Sensitivity +42%). Further improvements can be achieved by putting more diodes in series (Average Sensitivity +380%). A multidirectional thermoelectronic flow sensor is also reported