Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

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Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

Microfabrication has greatly matured and proliferated in use amongst many disciplines. There has been great interest in micromachined flow sensors due to the benefits of miniaturization: low cost, small device footprint, low power consumption, greater sensitivity, integration with on-chip circuitry, etc. This paper reviews the theory of thermal flow sensing and the different configurations and operation modes available. Material properties relevant to micromachined thermal flow sensing and selection criteria are also presented. Finally, recent applications of micromachined thermal flow sensors are presented. Detailed tables of the reviewed devices are included.

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Micromachined Thermal Flow Sensors—A Review

In the present brief review modern views on the reasons of time instability of gas sensors parameters, as well as approaches, which could be used for the improvement of this important sensor's parameter, are summarized. In particular, the influence of factors such as structure transformation, phase transformation, poisoning, degradation of contacts and heaters, bulk diffusion, errors in design, change of humidity, fluctuations of temperature in the surrounding atmosphere, and interference effect was analyzed. It was shown that while designing devices such as solid-state gas sensors, there are no secondary issues or tasks—all are important. Sensors work in extreme temperatures in the presence of active gases, and therefore every element of the sensor could be responsible for its long-term stability. The conclusions, regarding the efficiency of approaches such as optimization of technological processes and optimization of exploitation processes used for improvement of stability of conductometric metal oxide gas sensors, were made as well.

Instability of metal oxide-based conductometric gas sensors and approaches to stability improvement (short survey)

This work describes noncontact, local temperature measurements using wavelength shifts of CdSe quantum dots (QDs). Individual QDs are demonstrated to be capable of sensing temperature variations and reporting temperature changes remotely through optical readout. Temperature profiles of a microheater under different input voltages are evaluated based on the spectral shift of QDs on the heater, and results are consistent with a one-dimensional electrothermal model. The theoretical resolution of this technique could go down to the size of a single quantum dot using far-field optics for temperature characterizations of micro/nanostructures.

Single quantum dots as local temperature markers.

The characterization of temperature and thermal properties is of particular importance in micro- and nanotechnology. Considering the highly increased density of structures and the increased power dissipation per unit area associated with miniaturization, good thermal design is of great importance for device reliability and performance. Locating hot spots, for example, on a microelectronic circuit, can be of great value in evaluating a design, optimizing the performance, and performing failure analysis. [1,2] Apart from the industrial applications of micro- and nanoscale thermometry, fundamental questions of the thermal behavior, for example, thermal transfer at a scale comparable to the phonon wavelength, [3] could be more effectively addressed with improved characterization tools. The common approach for mapping temperature on the microscale is based on infrared microscopy, which relies on the analysis of the thermal radiation that is emitted from any material. IR microscopy is a well-established technique and can be used with relative ease for temperature mapping on large scales. However, the technique suffers from a diffractionlimited resolution, giving it an optimal spatial resolution of around 5 mm. [2,4,5] Nanoscale scientists typically use scanning thermal microscopy (SThM) for high-resolution measurements. Since the invention of the scanning probe microscope at the beginning of the 1980s, [6] several scanning probes for thermal characterization have been developed. The thermal probes used are generally based on either thermocouple or thermistor elements. [7–11] Other approaches have proposed bimaterial cantilevers or fluorescent particles as temperaturesensing probes. [12–14] The highest spatial resolution obtained

High‐Spatial‐Resolution Surface‐Temperature Mapping Using Fluorescent Thermometry

Based on an extensive review of research results on the material, process, device and circuit properties of thin-film fully depleted SOI CMOS, our work demonstrates that such a process with channel lengths of about 1 mum may emerge as a most promising and mature contender for integrated microsystems which must operate under low-voltage low-power conditions, at microwave frequencies and/or in the temperature range 200-350 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.

Fully-depleted SOI CMOS technology for heterogeneous micropower, high-temperature or RF microsystems

Co-integration of sensors with their associated electronics on a single silicon chip may provide many significant benefits regarding performance, reliability, miniaturization and process simplicity without significantly increasing the total cost. Micromachined Thin-Film Sensors for SOI-CMOS Co-integration covers the challenges and interests and demonstrates the successful co-integration of gas-flow sensors on dielectric membrane, with their associated electronics, in CMOS-SOI technology. We firstly investigate the extraction of residual stress in thin layers and in their stacking and the release, in post-processing, of a 1 µm-thick robust and flat dielectric multilayered membrane using Tetramethyl Ammonium Hydroxide (TMAH) silicon micromachining solution. The optimization of its selectivity towards aluminum is largely demonstrated. The second part focuses on sensors design and characteristics. A novel loop-shape polysilicon microheater is designed and built in a CMOS-SOI standard process. High thermal uniformity, low power consumption and high working temperature are confirmed by extensive measurements. The additional gas flow sensing layers are judiciously chosen and implemented. Measurements in the presence of a nitrogen flow and gas reveal fair sensitivity on a large flow velocity range as well as good response to many gases. Finally, MOS transistors suspended on released dielectric membranes are presented and fully characterized as a concluding demonstrator of the co-integration in SOI technology.

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Micromachined thin-film sensors for SOI-CMOS co-integration

Thin films stress extraction using micromachined structures and wafer curvature measurements

In this paper, an original design of a polysilicon loop-shaped microheater on a 1-/spl mu/m thin-stacked dielectric membrane is presented. This design ensures high thermal uniformity and insulation and very low power consumption (20 mW for heating at 400/spl deg/C). Moreover, the use of completely CMOS compatible tetramethyl ammonium hydroxide-based bulk-micromachining techniques allows an easy, smart gas sensor integration in SOI-CMOS technology.

SOI CMOS compatible low-power microheater optimization for the fabrication of smart gas sensors

This paper investigates novel isolation and patterning schemes to increase the sensitivity of a capacitive humidity sensor based on a polyimide sensitive layer. We obtained an optimum sensitivity of 23% using aluminum interdigitated electrodes with fingers of 1 nm width separated by 1 pm, deposited on a first insulating polyimide layer and covered by two more polyimide layers whose upper one features a regular array of holes to increase the active surface area. A mathematical model was developed to optimize the sensitivity regarding water absorption factor, electrodes design and ratio between active and parasitic capacitances. The process was optimized to be fully compatible with usual CMOS-IC processes in order to finally be able to fabricate a humidity smart sensor.

J. Laconte

Papers

Fully-depleted SOI CMOS technology for heterogeneous micropower, high-temperature or RF microsystems

Micromachined thin-film sensors for SOI-CMOS co-integration

Thin films stress extraction using micromachined structures and wafer curvature measurements

SOI CMOS compatible low-power microheater optimization for the fabrication of smart gas sensors

High-sensitivity capacitive humidity sensor using 3-layer patterned polyimide sensing film