This review deals with gas sensors combining a metal oxide based sensing layer and a substrate realized by using micromachining. It starts by giving an overview of the design principles and technology involved in the fabrication of micromachined substrates examining thermal and mechanical aspects. Both kinds of micromachined substrates, closed-membrane-type and the suspended-membrane-type, are discussed. The deposition of the sensing layer is complicated by the mechanical fragility of the micromachined substrates. Different approaches used for the formation of the sensing layer such as thin film and thick film deposition techniques are reviewed. Finally, the gas sensing function of the sensitive layer is analyzed and various ways for extracting the information are presented with respect to the improvement of sensor performance brought by this new approach.

Micromachined metal oxide gas sensors: opportunities to improve sensor performance

Calorimetric biosensors with integrated microfluidic channels.

Microfabrication processes for chemical and biochemical sensors are reviewed. Standard processing steps originating from semiconductor technology are detailed, and specific micromachining steps to fabricate three-dimensional mechanical structures are described. Fundamental chemical sensor principles are briefly abstracted and corresponding state-of-the-art examples of microfabricated chemical sensors and biosensors are given. The advantages and disadvantages of either fabricating devices in IC fabrication technology with additional microfabrication steps, or of using custom-designed nonstandard microfabrication process flows are debated. Finally, monolithic integrated chemical and biological microsensor systems are presented, which include transducer structures and operation circuitry on a single chip.

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Microfabrication techniques for chemical/biosensors

This paper is concerned with the design, fabrication, and characterization of novel high-temperature silicon on insulator (SOI) microhotplates employing tungsten resistive heaters. Tungsten has a high operating temperature and good mechanical strength and is used as an interconnect in high temperature SOI-CMOS processes. These devices have been fabricated using a commercial SOI-CMOS process followed by a deep reactive ion etching (DRIE) back-etch step, offering low cost and circuit integration. In this paper, we report on the design of microhotplates with different diameters (560 and 300 mum) together with 3-D electrothermal simulation in ANSYS, electrothermal characterization, and analytical analysis. Results show that these devices can operate at high temperatures (600degC ) well beyond the typical junction temperatures of high temperature SOI ICs (225degC), have ultralow dc power consumption (12 mW at 600degC), fast transient time (as low as 2-ms rise time to 600degC), good thermal stability, and, more importantly, a high reproducibility both within a wafer and from wafer to wafer. We also report initial tests on the long-term stability of the tungsten heaters. We believe that this type of SOI microhotplate could be exploited commercially in fully integrated microcalorimetric or resistive gas sensors.

Tungsten-Based SOI Microhotplates for Smart Gas Sensors

This paper describes a new generation of integrated solid-state gas-sensors embedded in SOI micro-hotplates. The micro-hotplates lie on a SOI membrane and consist of MOSFET heaters that elevate the operating temperature, through self-heating, of a gas sensitive material. These sensors are fully compatible with SOI CMOS or BiCMOS technologies, offer ultra-low power consumption (under 100 mW), high sensitivity, low noise, low unit cost, reproducibility and reliability through the use of on-chip integration. In addition, the new integrated sensors offer a nearly uniform temperature distribution over the active area at its operating temperatures at up to about 300–350°C. This makes SOI-based gas-sensing devices particularly attractive for use in handheld battery-operated gas monitors.

This paper reports on the design of a chemo-resistive gas sensor and proposes for the first time an intelligent SOI membrane microcalorimeter using active micro-FET heaters and temperature sensors. A comprehensive set of numerical and analogue simulations is also presented including complex 2D and 3D electro-thermal numerical analyses.

Design and simulations of SOI CMOS micro-hotplate gas sensors

A differential catalytic microcalorimeter has been fabricated using silicon micromachining techniques. The steady-state device response to different combustible gases was investigated in the laboratory as a function of concentration of the combustible species, oxygen level, and gas flow. The device output was found to be proportional to the concentration of the different combustible gases, while the relative sensitivity to different reactants depends on the specific catalyst used. This paper discusses the design issues and the fabrication process for maximizing the detection limit of the device.

Fabrication and properties of a Si-based high-sensitivity microcalorimetric gas sensor

This investigation reports the effect of dilute additions of ozone in molecular oxygen on the limiting reactive gas pressures required for in situ deposition of YBa2Cu3O7−x thin films. Thermodynamic analysis indicates that the high oxidizing potential of ozone assures stability at equilibrium under virtually all experimental conditions of ozone pressure and substrate temperature. Thus the generation of a minor fraction of ozone in oxygen enables in situ deposition to be carried out without further enrichment at gas pressures well below those required in pure molecular oxygen. In situ superconductor deposition at various rates using coevaporated metals revealed a lower limit at an ozone‐to‐copper ratio of approximately unity. Superconducting films deposited at O3/Cu flux ratios above this threshold were highly (00l) oriented with critical temperatures of 87–90 K and critical currents of 2×106 A/cm2 that exhibited only small or ambiguous trends with increasing ozone above the threshold. Films deposited at O...

Reactive codeposition of in situ Y‐Ba‐Cu‐O superconducting films using dilute mixtures of ozone in oxygen

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FABRICATION AND PROPERTIES OF A Si-BASED HIGH SENSITIVITY MICROCALORIMETRIC GAS SENSOR

In the last three years, many different film deposition techniques have been successfully employed for the preparation of YBa2Cu3O7 films on a variety of substrates (1). Initially, as-grown films were poor superconductors and required a high-temperature post-growth anneal to attain good superconducting properties. Using this two-step (deposition and subsequent anneal) procedure, high quality superconducting films were obtained on many inert substrates such as SrTiO3, MgO, and ZrO2. However, on technologically important substrates, e.g. Si, SiO2, and sapphire, the quality of the films after annealing was poor due to a strong interaction between the films and the substrates. For sapphire substrates, for example, the highest Tc reported was in the 70–75 K range. To avoid or minimize this film/substrate interaction, several different approaches have been explored including the use of buffer layers (1–4) and the use of very rapid thermal annealing procedures (5). Simon et al (6) used a ZrO2 buffer layer to obtain films on sapphire with zero resistance at about 88 K. The most successful approach has been the in-situ growth process in which the temperature of the substrate during deposition is high enough to produce a film with good crystallinity, but sufficiently low to avoid excessive film/substrate interaction. Recently, in-situ growth techniques have produced YBCO films on sapphire with Tc of about 87 K (7) and on Si substrates with Tc of about 86 K (8). In this paper, we describe our results on the growth of YBCO films directly on sapphire by the two-step process. We find that films deposited on sapphire by rf triode sputtering and annealed in O2 at 860–890°C have a Tc as high as 85 K. Our work shows that it is indeed possible to grow good quality YBCO films directly on sapphire by a high temperature post-growth annealing.

D. W. Hoffman

Papers

Fabrication and properties of a Si-based high-sensitivity microcalorimetric gas sensor

Reactive codeposition of in situ Y‐Ba‐Cu‐O superconducting films using dilute mixtures of ozone in oxygen

FABRICATION AND PROPERTIES OF A Si-BASED HIGH SENSITIVITY MICROCALORIMETRIC GAS SENSOR

Deposition of YBa2Cu3O7 Films on Sapphire by Rf triode Sputtering