Elsevier

Sensors and Actuators A: Physical

Volume 249, 1 October 2016, Pages 186-198
Sensors and Actuators A: Physical

Micro thermal conductivity detector with flow compensation using a dual MEMS device

https://doi.org/10.1016/j.sna.2016.08.019Get rights and content

Highlights

  • Generic method to reduce the in-line flow dependence of thermal gas sensors is presented.

  • A high speed and high sensitivity MEMS TCD is presented.

  • Effect of flow on the response of the sensor can be reduced by a factor 4–15.

Abstract

A generic method to reduce the in-line flow dependence of thermal conductivity detectors (TCDs) is presented. The principle is based on a dual-MEMS device configuration. Two thin-film sensors on membranes in parallel in the gas stream on the same chip are differentially operated. Both micro-TCDs are designed to be identical in terms of contact with the main gas flow, however a different depth of the detection chamber results in a different response to the thermal conductivity of the sample gas. Static and dynamic simulations have been performed to characterize the design of the fabricated structures. Devices have been fabricated in a MEMS process using a combined surface- and bulk micromachining process. The devices have been characterized statically and dynamically. Measurements on prototypes show that depending on the range of gases, device size and flow range device the effect of flow on the thermal conductivity can be reduced by a factor 4–15.

Introduction

Gas sensors are increasingly used in the growing markets of the automotive industry and in environmental monitoring. The most common gas sensors are based on chemical interaction [1], [2]. These sensors have a high sensitivity and can be low cost. Gas sensors based on physical principles, such as optical absorption, density and thermal conductivity detectors (TCDs) have also found many areas of applications, mainly because of their superior long-term stability. Gas detection based on thermal conductivity is widely used in process control for helium and hydrogen and in gas chromatography [3], [4], [5]. The operation of a TCD is based on the gas-specific thermal conductivity, which is measured by means of the temperature reduction in a heated element in the gas stream, due to the heat loss through the gas. The heat loss can be either by conduction or convection and in most devices both phenomena are present. If the heat loss is mainly by convection a TCD can also be used as a thermal flow sensor, however if the heat loss is mainly by conduction through the gas the device operates a gas sensor. Therefore the TCD and the thermal flowmeter [6], [7] are closely related devices.

The most common and simplest TCDs are the so-called hot-wire devices [8], [9] where the hot-wire acts as a heater and temperature sensor at the same time. An electric current through the wire produces Joule heating and, depending on the surrounding gas, the temperature of the wire changes and can be detected as a change in resistance. Instead of a wire, other heater geometries, such as thin metal sheets, are also been utilized [10], and based on these basic robust devices thermal conductivity detectors have grown into mature, commercially available devices [11]. When used for gas pressure measurements these sensors are also referred to as Pirani gauges or thermal conductivity gauges [12], [13]. Recent advances in MEMS and integrated circuit (IC) technology enable the fabrication of many types of small, IC-compatible, high sensitivity and low cost thermal sensors and heaters [14]. Nowadays, TCDs can be miniaturized enabling low sample volumes and low cost [13]. MEMS TCD’s have already replaced the traditional hot-wire and hot-strip sensors in many applications [14], [15], [16], [17], [18]. Micro-strip TCD’s based on thin-film platinum wires have been published by several authors [12], [19], [20] and are also commercially available [10], [13]. Most of these devices use bulk micromachining of the substrate for thermal isolation of the sensor. The basic TCD design as presented here is based on a combination of bulk- and surface-micromachining technology as has been used in our previous work on a sensor platform for natural gas composition measurement [21]. In these systems the fabrication of a TCD [22], [23] and thermopile detector arrays [24] for infrared absorption spectroscopy on a single chip has been realized. In these TCD devices the separation of the heater and the temperature sensor enabled a more flexible design, a very small sample chamber, a short response time and a high sensitivity [23], [25].

Section snippets

Flow compensation

One of the major issues of TCD sensors is their flow dependence [3]. In many cases it is necessary to keep the flow constant or to calibrate the sensor depending on the flow rate for every gas in the gas stream. For instance in a gas chromatography system, a flow independent design could avoid the time-consuming re-calibration for each gas. In gas chromatography the effect of flow velocity can be partly compensated by adding a reference channel with a reference gas flow [4], [20] which results

Description of the device

The proposed dual-TCD is composed of two identical devices fabricated on the same substrate, as shown in Fig. 1. Each device consists of a resistive heating strips and thermopiles measuring the temperature difference between the membrane and the substrate at both sides of the heater. The heater/temperature sensor are integrated on a thin membrane. Each membrane covers a volume, which is the gas-filled detection chamber. Gas venting to the volume between the membrane and the substrate is

Fabrication process of the dual-TCD

Fabrication of the TCD sensor was done in the EKL facility of the TU-Delft, based on a MEMS process as described in [23]. The sensors were fabricated on a 525 μm silicon 〈100〉 wafer with a 2 μm layer of sacrificial silicon-dioxide. Stress compensation techniques as described in [27] have been applied to reduce the stress in the structures, thus enabling the fabrication of large devices with low heat losses in the suspension resulting in a high sensitivity. A layer of 0.6 μm of silicon-nitride

Conclusions

A generic concept of flow compensation using a dual thermal conductivity detector (TCD) configuration has been presented. Both TCDs basically consist of a heated membrane, a temperature sensor and a gas sample chamber operating at constant temperature. The difference between both TCDs is only the depth of the sample chamber. The shallow sample chamber device is very sensitive to thermal conductivity of the gas, while the deep device is used for compensation only. A theoretical framework is

Acknowledgements

This work has been supported by the Dutch technology foundation STW under grant DEL.11476 and partially by the Chilean Agency of Science and Technology CONICYT under grant 15316126-7. The devices have been fabricated at EKL Technology Centre of TU Delft. The authors gratefully acknowledge the help of Pieter Visser and Sander Gersen from DNV-GL in Groningen, The Netherlands with the flow measurements.

Accel Abarca Prouza was born on 25 December 1983 in Santiago, Chile. He received his B.Sc. and Diploma degree (highest distinction) in Electrical Engineering from the University of Chile, Chile, in 2011. After that he worked as researcher at the University of Chile and the Advanced Mining Technology Center (AMTC) in the area of sensors for meteorology and seismology. In 2013 he started his Master at TUDelft in Microelectronics thanks to a full scholarship granted by the Chilean government. He

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    Accel Abarca Prouza was born on 25 December 1983 in Santiago, Chile. He received his B.Sc. and Diploma degree (highest distinction) in Electrical Engineering from the University of Chile, Chile, in 2011. After that he worked as researcher at the University of Chile and the Advanced Mining Technology Center (AMTC) in the area of sensors for meteorology and seismology. In 2013 he started his Master at TUDelft in Microelectronics thanks to a full scholarship granted by the Chilean government. He graduated in 2015 with a Master thesis work in the area of flow independent thermal conductivity detectors for gas sensing. He is currently working as a researcher at the Electronic Instrumentation Laboratory of TUDelft in the area of integrated temperature sensors into CMOS image sensors.

    M. Ghaderi received the M.Sc. degree in 2011 from Shahid Beheshti University, Tehran, Iran, on optical MEMS devices. Currently, he is at the Delft University of Technology, Delft, The Netherlands, working towards the PhD in microelectronics. Since 2012, he is involved in a research on the optical microsystems and materials technology for optical gas sensing applications.

    Reinoud F. Wolffenbuttel is an associate professor at the Delft University of Technology and is involved in instrumentation and measurement in general and on-chip functional integration of microelectronic circuits and silicon sensor, fabrication compatibility issues and micromachining in silicon and microsystems in particular. He published over 500 papers at international conferences and peer-reviewed journals on these subjects. He was a visitor at the University of Michigan, Ann Arbor, USA in 1992, 1999 and 2001, Tohoku University, Sendai, Japan in 1995 and EPFL Lausanne, Switzerland in 1997. He is the recipient of a 1997 NWO pioneer award. He served as general chairman of the Dutch national sensor conference in 1996, Eurosensors in 1999 and MicroMechanics Europe in 2003.

    Ger de Graaf is a staff member at the Faculty of Electrical Engineering of Delft University of Technology. He received his Ph.D. Degree from Delft University for his work on MEMS infrared spectrometers in 2008. His main research interest is in MEMS, sensors and actuators and microfabrication in general. Currently he is mainly working on sensors based on optical, or other physical principles, for composition detection in gases, in bioprocesses and in fuel.

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