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2007 | Buch

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CMOS Hotplate Chemical Microsensors

verfasst von: Dr. Markus Graf, Dr. Diego Barrettino, Prof. Dr. Henry P. Baltes, Prof. Dr. Andreas Hierlemann

Verlag: Springer Berlin Heidelberg

Buchreihe : Microtechnology and Mems

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract

In recent years, there has been increasing interest and development efforts in miniaturizing gas sensors and systems. Particularly strong efforts have been made to monitor environmentally relevant gases like carbon-monoxide (CO), methane (CH₄) and ozone (O₃). Commonly used chemically sensitive materials for these target gases are wide-bandgap semiconducting oxides such as tin oxide, tungsten oxide or indium oxide, which are operated at elevated temperatures of 200–400 °C [1–3]. At those high temperatures, these oxides show considerable resistance changes upon exposure to a multitude of inorganic gases and volatile organics. The most prominent example is tin oxide (SnO₂), which shows large electrical resistance changes upon exposure to the above-mentioned gases at operating temperatures between 250 °C–350 °C and has been engineered to provide sufficient long-term stability [4–6]. The miniaturization efforts in the field of metal-oxide-based gas sensors follow several major trends:

(a)

the development of micromachined sensor platforms [7–9],

(b)

the micro- and nanotechnological fabrication of the sensing materials [10, 11], and

(c)

the design and co-integration of application-specific circuits with the transducer leading to smart sensor systems [8, 12–14].

2. Miniaturized Metal-Oxide Sensors

Abstract

As already mentioned in the introduction, a so-called microhotplate (µHP) for metal-oxide-based gas sensing consists of a thermally isolated stage fabricated using microtechnological processes. The integrated heating element provides the typical operation temperature on the order of several 100 °C, and the temperature sensor measures the microhotplate temperature. Two or more electrodes are used to perform resistance or impedance measurements of the sensing material. The microhotplate becomes a chemical sensor through the deposition of a sensitive layer. Several reviews on micromachined metal-oxide sensors are available [7,9]. The scheme in Fig. 2.1 gives an overview on the design and development considerations for monolithic sensor systems.

3. Thermal Modelling of CMOS Microhotplates

Abstract

The main goals of improving microhotplate designs include (a) reducing its power consumption and (b) increasing the hotplate temperature homogeneity, i.e., an optimization towards a minimum temperature gradient in the sensitive area. To reach these goals, novel device and hotplate designs usually undergo an extensive simulation process such as thermal modelling using finite-element simulations. Modelling the transducer and hotplate behavior is an important step towards establishing a compact sensor model for monolithic system realizations [91]. The parameters of interest include the hotplate thermal resistance and the thermal time constant, a prediction of which facilitates and accelerates the design of monolithic systems for a given microhotplate design.

4. Microhotplates in CMOS Technology

Abstract

The following chapter includes the description of different types of microhotplates that feature resistor and transistor heating elements. Three of them were specifically designed to be monolithically integrated with circuitry, and one was a testing device that was used for the assessment of temperature distributions on the microhotplates.

5. Monolithic Gas Sensor Systems

Abstract

This chapter includes two different sensor system architectures for monolithic gas sensing systems. Section 5.1 describes a mixed-signal architecture. This is an improved version of the first analog implementation [81, 91], which was used to develop a first sensor array (see Sect. 6.1). Based on the experience with these analog devices, a complete sensor system with advanced control, readout and interface circuit was devised. This system includes the circular microhotplate that has been described and characterized in Sect. 4.1. Additionally to the fabrication process, a prototype packaging concept was developed that will be presented in Sect. 5.1.6. A microhotplate with a Pt-temperature sensor requires a different system architecture as will be described in Sect. 5.2. A fully differential analog architecture will be presented, which enables operating temperatures up to 500 °C.

6. Microsensor Arrays

Abstract

A well-known problem of tin dioxide is its lack of selectivity (Chap. 2). This situation is usually dealt with by using an array of sensors in combination with multi-component or pattern recognition algorithms such as principal component regression (PCR), multi-way analysis or artificial neural networks (ANN) [142, 143]. Doping of the tin dioxide also changes its selectivity characteristics to different gases [68]. Another parameter that can be varied is the operation temperature. The use of an array of microhotplates with individually controlled temperatures, the hotplates of which are covered with different sensitive materials, increases the overall information that can be extracted from metal-oxide-based gas sensing systems.

7. Conclusion and Outlook

Abstract

The central topic of the book was the integration of microhotplate-based metal-oxide gas sensors with the associated circuitry to arrive at single-chip systems. Innovative microhotplate designs, dedicated post-CMOS micromachining steps, and novel system architectures have been developed to reach this goal. The book includes a multitude of building blocks for an application-specific sensor system design based on a modular approach.

Backmatter

Titel: CMOS Hotplate Chemical Microsensors
verfasst von: Dr. Markus Graf
Dr. Diego Barrettino
Prof. Dr. Henry P. Baltes
Prof. Dr. Andreas Hierlemann
Verlag: Springer Berlin Heidelberg
Electronic ISBN: 978-3-540-69562-2
Print ISBN: 978-3-540-69561-5
DOI: https://doi.org/10.1007/978-3-540-69562-2

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