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Artificial Nose based on Smell & Gas Sensors

Submitted By:

Sachin Verma 2008EE10361

Yashdeep Singh 2008EE10372

Under the guidance of Prof. S.M.K. Rahman

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Electrical Engineering Department

Indian Institute of Technology, Delhi

May 2011

Acknowledgement

We wish to express my immense gratitude to Prof. S.M.K. Rahman for his able guidance, constant encouragement and supervision provided throughout the semester for this study.

He provided valuable comments and suggestions, along with direction-pointers during the study.

Sachin Verma

Yashdeep Singh

Contents

• Motivation

• Introduction

• Sensory Mechanism in Humans

• Electronic Nose

• Smell and Gas Sensors

• Sensing Materials

• Sensing Technologies

• Metal-oxide Sensors

• MOSFET Sensors

• Quartz-crystal based Sensors

• Silicon Microfabrication Techniques

• Micromachined Gas Sensors

• Conclusion

• References

Motivation

Human dependence upon his sensory mechanisms in his quest to understand and know the world around him is well established. These sensory mechanisms work through various organs present in the human body.

The replication of these sensory organs artificially with increased sensitivities has always been a major challenge for the scientists and technologists. The utility associated with such developments is unparalleled. For instance, drug smuggling is a major concern for most countries in the world and hence drug detection becomes essential. Gas and smell sensors of enhanced sensitivities can come in quite useful herein. Similarly, such sensors find immense application for asthma patients, pollution control, et cetera.

Realizing this importance, it becomes quite sensible to take up a full-fledged study upon the same. This independent study as such, draws its motivation from above.

Introduction

A sensor is a device which measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards.

Sensors are used in everyday objects such as touch-sensitive elevator buttons and lamps which dim or brighten by touching the base. There are also innumerable applications for sensors of which most people are never aware. Applications include automobiles, machines, aerospace, medicine, industry, and robotics.

The topic of our concern is human sense based sensors (more specifically smell and gas sensors). This however also implies that the sensors' designs being intended need to be fairly sensitive to the entity required to be detected/measured. This is termed as the sensitivity of a sensor.

A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1cm when the temperature changes by 1°, the sensitivity is 1cm/1°. Sensors that measure very small changes must have very high sensitivities.

Technological progress is now allowing more and more sensors to be manufactured on a microscopic scale as micro-sensors using MEMS technology. In most cases, a micro-sensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. (MEMS – Micro-Electro-Mechanical Systems)8

Before we come to study the specific processes and technologies being used in the industry for creation of these sensors, we need to first form a formal background as to how the human sensory systems work. For this purpose, we shall concentrate on taste sensory mechanism followed by the more enhanced (and better correlated to this study) smell and gas sensory mechanism present in the human body.

Sensory mechanism in Humans

Taste

Taste (or, more formally, gustation) is a form of direct chemoreception and is one of the traditional five senses, viz. taste, smell, sight, hearing and touch. It refers to the ability to detect the flavor of substances such as food and poisons. In humans and many other vertebrate animals the sense of taste partners with the less direct sense of smell, in the brain's perception of flavor. Classical taste sensations include sweet, salty, sour, and bitter. More recently, psychophysicists and neuroscientists have suggested other taste categories (umami and fatty acid taste most prominently.)

Taste is a sensory function of the central nervous system. The receptor cells for taste in humans are found on the surface of the tongue, along the soft palate, and in the epithelium of the pharynx and epiglottis.

A diagrammatic structure of human tongue is shown as follows.

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Figure i. A Human Tongue (the colour coded areas represent taste sensitivities; green –

bitter, orange – sweet, blue – sour, red – salty)

Human tongue primarily functions by detecting a taste and then sending a corresponding signal to the human brain through the nervous system. The brain then resolves the signal and recognizes the sensation. In lay-man terms, this is nearly how the human tongue works.

In technological world, the sensor used parallel to a tongue is the electronic tongue, quite aptly named so too. The electronic tongue uses taste sensors to receive information from chemicals on the tongue and send it to a pattern recognition system.

The result is the detection of the tastes that compose the human palate. The types of taste that is generated is divided into five categories sourness, saltiness, bitterness, sweetness, and umami (deliciousness). Sourness, which includes HCl, acetic acid, and citric acid is created by hydrogen ions. Saltiness is registered as NaCl, sweetness by sugars, bitterness, which includes chemicals such as quinine and caffeine is detected through MgCl2, and umami by monosodium glumate from seaweed, disodium in meat/fish/mushrooms.

Smell

Olfaction (also known as olfactics) refers to the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates. For air-breathing animals, the olfactory system detects volatile or, in the case of the accessory olfactory system, fluid-phase chemicals. For water-dwelling organisms, e.g., fish or crustaceans, the chemicals are present in the surrounding aqueous medium. Olfaction, along with taste, is a form of chemoreception. The chemicals themselves which activate the olfactory system, generally at very low concentrations, are called odors.

In vertebrates smells are sensed by olfactory sensory neurons in the olfactory epithelium. The proportion of olfactory epithelium compared to respiratory epithelium (not innervated) gives an indication of the animal's olfactory sensitivity. Humans have about 16 cm² of olfactory epithelium, whereas some dogs have 150 cm2. A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter.

Molecules of odorants passing through the superior nasal concha of the nasal passages dissolve in the mucus lining the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons. This may occur by diffusion or by the binding of the odorant to odorant binding proteins. The mucus overlying the epithelium contains mucopolysaccharides, salts, enzymes, and antibodies (these are highly important, as the olfactory neurons provide a direct passage for infection to pass to the brain).

The concept of electronic noses, or more generally, smell and gas sensors, is based upon this sense in human beings. These have been taken up in the coming chapters. A diagrammatic representation of human smell sensory system is shown, followed by a combined diagram depicting the working of the nose-tongue combined.

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Figure ii. A Human Nose

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Figure iii. Human Tongue – Nose combined

It becomes very important to study smell as against taste because, whereas the human tongue can distinguish only among five distinct qualities of taste, the nose can distinguish among hundreds of substances, even in minute quantities.

Electronic Nose

The electronic nose was developed in order to mimic human olfaction that functions as a non-separative mechanism: i.e. an odor / flavor is perceived as a global fingerprint. The basic working mechanism of an electronic nose is given as follows.

Electronic Noses include three major parts: a sample delivery system, a detection system, a computing system.

1.The sample delivery system enables the generation of the headspace (volatile compounds) of a sample, which is the fraction analyzed. The system then injects this headspace into the detection system of the electronic nose. The sample delivery system is essential to guarantee constant operating conditions.

2.The detection system, which consists of a sensor set, is the “reactive” part of the instrument. When in contact with volatile compounds, the sensors react, which means they experience a change of electrical properties. Each sensor is sensitive to all volatile molecules but each in their specific way. Most electronic noses use sensor-arrays that react to volatile compounds on contact: the adsorption of volatile compounds on the sensor surface causes a physical change of the sensor. A specific response is recorded by the electronic interface transforming the signal into a digital value. Recorded data are then computed based on statistical models. The more commonly used sensors include metal oxide semiconductors (MOS), conducting polymers (CP), quartz crystal microbalance, surface acoustic wave (SAW), and field effect transistors MOSFET). In recent years, other types of electronic noses have been developed that utilize

mass spectrometry or ultra fast gas chromatography as a detection system.

3.The computing system works to combine the responses of all of the sensors, which represents the input for the data treatment. This part of the instrument performs global fingerprint analysis and provides results and representations that can be easily interpreted. Moreover, the electronic nose results can be correlated to those obtained from other techniques (sensory panel, GC, GC/MS).

As a first step, an electronic nose need to be trained with qualified samples so as to build a database of reference. Then the instrument can recognize new samples by comparing volatile compounds fingerprint to those contained in its database. Thus they can perform qualitative or quantitative analysis.

Smell and Gas sensors

A smell/gas sensor is a chemical sensor that is operated in the gas phase. It converts chemical information, which is determined by different concentrations of gaseous chemical species, into an electrical signal. Thus, a chemical sensor gives a signal that in some way is related to the chemical environment it is exposed to. The response, x, of a sensor to a single gas can be described as: [pic]

where fgas is a function (usually non-linear) and cgas the concentration of the gas. The response is in most cases defined as the difference or ratio between the steady-state sensor response when exposed to the sample gas and the sensor response when exposed to a reference atmosphere (no sample gas).This is shown in Figure 1a.

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Figure 1(a) Typical gas sensor response curve; (b) Possible parameters to extract from a gas response curve

The concentration-response relationship for most gas sensors approximately exhibits either saturated linear behaviour, i.e. linear for low concentrations and saturated for higher concentrations, or logarithmic behaviour. Other values containing information about the kinetics of the reactions can also be extracted from the sensor response, such as the derivatives and integrals over certain time intervals (Figure1b). Three important parameters when describing the response of a sensor are the sensitivity, selectivity and stability.

The sensitivity, gas, of the sensor towards a specific gas is then defined as: [pic]

In general, the sensitivity is a non-linear function of concentration. The selectivity, , of a single sensor is usually defined as the ratio of the sensitivity related to the gas concentration to be monitored in the linear region and the maximal sensitivity tall other interfering components: [pic]

The stability of the sensor response is defined as the reproducibility of the sensitivity and selectivity as a function of time. Most of the drawbacks of the commonly used gas sensing technologies come from of their lack of stability.

There are other demands to be met when producing gas sensors, such as short response time, good reversibility, low cost, small size and low power consumption. The work presented here concentrated on optimization of the last three points. The gas-sensing properties were evaluated in collaboration with our partners.

In order to be able to reach these requirements, it is important to have a clear view of how a gas sensor is made. It usually consists of two parts: the sensing material and the transducer. The sensing material interacts with the analyte, e.g. by adsorption/desorption or chemical reactions on the surface and/or the bulk of the material. The interaction changes some physical property or properties of the sensing material, such as the electrical conductivity or the mass, which can be detected using a transducer. The latter converts the variation of the physical properties, containing the chemical information, into an electrical signal. Different transducer principles can be used in chemical sensors, such as changes in conductivity as detected by the voltage drop over a series resistor, or changes in mass as detected by the shift in frequency of a resonator. A schematic description of the working principles of solid-state gas sensors is depicted in Figure2.

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Figure 2. Principle of solid-state gas sensors

In the following sections, different aspects of sensing materials and transducer principles are described, together with the description of some of the most common gas sensors.

Sensing materials

A large number of different materials have their physical properties modified after interaction with a chemical environment. Properties of the analytes, such as molecular size, polarity, polarizability, and affinity, along with the matching characteristics of the sensing material, govern the interaction.

Two main types of interaction between the analyte and the sensing material can be distinguished. One type is chemical sensing with inorganic materials. Reactions at the surface and/or in the sensing material may lead to chemisorption or catalytic reactions between the molecules present. Thus, the charge distribution, or the carrier concentration or mobility in the sensing material might change, which can be detected by several transducer principles.

This type of gas-sensitive material is often unspecific. Instead, different sensitivities for broad groups of molecules are achieved. The sensitivities of these materials can be tuned by addition of dopants or operation at different temperatures for example. Examples of this type of sensing material are semiconducting metal oxide and catalytic metals. Another type of chemical sensing materials is based on lock-and-keytype interaction, which usually consists of organic materials. They can be arranged either as a monolayer of the recognition molecules or as specific recognition sites in a polymeric matrix. Typical materials employed are cage-like molecules, such as calixarenes. The recognition may be both geometric, depending on the size and shape of the material, and affinity-based between the key and lock molecules, via specific recognition sites in the sensing material.

Sensing technologies

There are a multiple number of technologies that are used for smell and gas sensing. For the purpose of this study, the main ones that I shall be concentrating upon are the metal-oxide type, the MOSFET type and the quartz crystal microbalance type.The first two specifically deal with gas sensing, the more basic and primal form of sensing, whereas the latter deals with smell sensing.

These technologies are explained hereunder.

Metal-oxide sensors

Sensing mechanism

These sensors are based on the gas-sensitive properties of a semiconducting metal-oxide layer which is usually polycrystalline, and whose conductivity is modulated by the oxygen adsorbed at the surface and at grain boundaries. These metal oxides change their conductivity in the presence of reducing or oxidizing gases, such as O2, H2, CO, NOx, C2H5OH and hydrocarbons. The sensitivity and the selectivity of these sensors can be modified by changing the oxide microstructure and/or by using catalytic metals (dopants) as Pt, Pd, Au or Ag.

A schematic diagram is as shown in Figure 3.

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Figure 3. Principle of metal-oxide semiconductor operation

The detection principle of the n-type SnO2 sensor is closely related to the number of oxygen ions adsorbed on the SnO2 grains. In air, oxygen is adsorbed on the surface of the grains, depleting the surface and the grain boundaries of electrons, which subsequently leads to a decreased conductivity of the device. Depending on the operating conditions, the nature of the oxygen ions formed can be O2-, O-, or O2-. Hydroxyl groups (OH) may also be present. In the presence of reducing molecules, the number of adsorbed oxygen ions decreases, increasing the concentration of electrons in the material.

The reverse occurs in the presence of oxidizing molecules. The role of dopants is to promote the reaction between the reducing and oxidizing gases with the sensor surface and grain boundaries. At low temperatures, physical adsorption can dominate the chemical sensing, while chemisorption becomes more influential at somewhat higher temperatures. For higher temperatures, catalysis and surface defects, and finally bulk effects, start to dominate the sensing mechanism. Since the chemical reactions are strongly dependent on temperature, the sensitivity and selectivity of the device can be tuned by the variation of the operating temperature from 200 to 450°C.

Technology description

Metal-oxide sensors are fabricated as sintered powders or thin films with variable thickness. The sintered powder is usually screen printed on top of an alumina substrate with previously integrated electrodes and heater on the front and on backside, respectively (Figure 4). This thick film technology is considered as a relatively high power technology and does not allow a rapid variation of the operating temperature.

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Figure 4. Thick film metal-oxide gas sensor structure

The thin film technology is mostly utilized in combination with the microhotplate concept (Figure 5).

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Figure 5. Thin film metal-oxide gas sensor structure: schematic cross-sectional

view

The devices are based on a micromachined hotplate on a silicon substrate. The gas-sensitive thin film material is deposited on top of the remaining thermally isolated membrane. The sensors show a low-power consumption compared to the thick sensors previously mentioned. Moreover their small thermal mass enables fast temperature variations. As noted earlier, the sensitivity and selectivity of a metal-oxide sensor is highly temperature dependent. Thus, a large amount of information can be obtained by modulating the temperature and conductance of the metal-oxide film. However, one of the major drawback of this technology is the poor stability (drift) of the gas-sensitive thin metal-oxide film.

Alternatively, we can use the technology based on the merging of thick film sensing technology with the micro-hotplate concept. The thick film sensing material provides better gas sensing characteristics in term of stability and the hotplate substrate makes this technology suitable for markets where low-power consumption, low-cost and reliable devices are needed, such as in portable instrumentation and the automotive industry.

MOSFET based sensors

Sensor structure

Metal-Oxide Semiconductor devices can be built as Schottky diodes, capacitors (MOSCAP) or transistors (MOSFET). The semiconductor is normally silicon and the insulator, silicon dioxide. This study concentrates upon the gas-sensitive field effect devices. It has been observed that palladium gate metal-oxide semiconductor structures are highly sensitive to hydrogen. This essentially becomes the basis for this study.

Schematic illustration of an n-type Pd-MOSFET sensor structure is shown in Figure 6 and Figure 7.

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Figure 6. Schematic Pd-MOSFET structure [I]

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Figure 7. Schematic Pd-MOSFET structure [II]

The sensor is composed basically of three layers: doped silicon as substrate, a typically 100 nm-thick oxide film, topped by a continuous catalytic metal film forming the transistor gate. With a negative gate voltage, majority carriers (holes) are drawn towards the semiconductor-insulator interface (Figure 6). Because of the rectifying properties of p-n junctions, there will be no drain current (ID) at a positive applied drain voltage (VD).

positive gate voltage, electrons accumulate at the interface (Figure 7). At high enough positive voltage, the electrons outnumber the negative acceptor ions in a thin layer just below the interface. This is called an inversion layer, which makes it possible for current to flow between the two n-doped areas. In a field-effect transistor, a small change in applied gate voltage can give rise to a relatively larger change in conductance in the inversion layer.

In a sensor configuration, the gate and drain are connected together (VG = VD). The MOSFET operates at constant current between the source and drain. The voltage at the gate and drain constitutes the sensor signal (VGD).

Sensing mechanism

When exposed to the catalytic metal, hydrogen gas molecules dissociate and adsorb on the palladium surface as hydrogen atoms. Some of the atoms diffuse rapidly through the metal layer to be adsorbed at the metal-oxide interface, resulting in its polarization. These atoms appear to be residing on the oxide side of the interface. They give rise to a dipole layer, which is in equilibrium with the outer layer of adsorbed hydrogen and the gas phase. The dipole layer induces an abrupt step in charge and hence potential distribution in the structure (Figure 8).

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Figure 8. Detection principle for a thick metal gate MOSFET sensor

The voltage drop, Vi that appears at the interface is added to the externally applied voltage (VGD) and a shift in the I-V curve towards lower voltages is obtained as an output signal (Figure 9).

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Figure 9. I-V characteristics for a MOSFET sensor with and without hydrogen exposure

The voltage drop ( Vi) is proportional to the number of hydrogen atoms absorbed per unit area at the metal-oxide interface, and is used to monitor the hydrogen concentration in the ambient environment. When the hydrogen gas is not anymore present in the ambient, the hydrogen atoms at the metal-air interface recombine into molecules (or water if oxygen is present), and the metal-oxide interface, which is in equilibrium with the outer interface, is emptied. This shift in the I-V curve is therefore reversible.

Hydrogen containing molecules can also be detected with a Pd-MOSFET sensor if they can be dehydrogenated on the palladium surface, such that hydrogen atoms are released and diffuse through the metal layer to the metal-oxide interface. This is valid for, e.g., alcohols, hydrogen sulphide and unsaturated hydrocarbons, but not ammonia and amines. By changing the temperature it is also possible to detect different molecules (tune the selectivity) with a single Pd-MOSFET sensor since they require different temperatures to start reacting on the catalytic metal. However, the temperature of operation is limited by the silicon technology to a value not higher than 200-225°C, due to leakage currents at p-n junctions increasing with temperature.

If the catalytic metal gate is made so thin that it is discontinuous with holes and cracks, but still useful as a gate electrode, a large sensitivity to e.g. ammonia is found. In this case, it is believed that the voltage shift is not only due to the electrical polarization phenomenon at the metal-oxide interface, Vi, but also from charges/dipoles on the insulator surface, Va, and possibly on the metal surface, Vs (Figure 10).

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Figure 10. Detection principle for a thin metal gate MOSFET sensor

Present work indicates that polarization phenomena at the insulator surface, when they occur, together with the hydrogen dipoles at the metal-insulator surface, might give the most significant contributions to the voltage shift. A detailed model for the generation of a voltage shift in thin metal films does not exist yet. Thin discontinuous metal gates can therefore detect all kinds of molecules that give rise to polarization phenomena in the thin metal film, including those detected by the thick film sensor, and some additional ones like ammonia and amides. In order to increase the selectivity of these sensors, other catalytic metals, such as Pt and Ir, that have different response characteristics towards different molecules, can be used.

Technology

The MOSFET transducer is fabricated using standard microelectronic processes on silicon, such as thin film deposition and patterning, and ion implantation. As a kind of post-processing, the standard gate material is removed and replaced by a catalytic metal film. Multiple sensors can be fabricated simultaneously on a substrate, and batches processed. However, this technology is limited to operational temperatures of 200–225°C, and the power consumption is relatively high. The operation of the sensor in a modulated temperature mode and its application in hand held instruments are therefore practically not possible.

Another design based on the field-effect sensing mechanism is the suspended gate field-effect transistor structure (SGFET), with an insulating air gap between the gate metal and the insulator. In such devices, the response originates from the species adsorbed to the gate and insulator surface, but not from a metal-oxide interface as in the MOSFET sensor. The adsorbed molecules and occurring species give rise to a shift in the work function, which can be detected by a shift in the operating point of the MOSFET.

For instance, the gate metal could be Pd and the sensor would have then approximately the same hydrogen sensing mechanism as the thick Pd-MOSFET, except that the response is smaller. Other conducting materials such as conducting polymers can also be used as gate sensitive materials.

These polymer films can also replace the catalytic metal in a MOSFET sensor structure to obtain a gas sensor based also on the principle of the work function variation. The SGFET and the polymer gate FET (PolFET) technologies have the advantage that they manifest gas sensitive properties at room temperature and therefore can be considered as low-power devices.

In the case of temperature limitation, it has been shown that Schottky diodes and transistors made on silicon carbide substrates can be used at temperatures up to 1000°C, due to the larger bandgap of SiC. Using such structures, it is possible to detect e.g. saturated hydrocarbons, which is difficult with an ordinary MOSFET sensor. Another interesting feature is the short response time of the sensors, less than 10 ms at high temperature, which makes SiC devices useful for e.g. monitoring of the fuel-to-oxygen ratio in the exhausts from individual cylinders in car and truck engines. It is suggested that silicon-on-insulator technology (SOI) is a possible candidate to reduce the power consumption of MOSFET sensors and increase their temperature of operation up to 300– 350°C. SOI could fill the gap left in temperature between the standard silicon and the more expensive SiC technologies.

Quartz crystal based sensors

Sensor structure

The Quartz Crystal Microbalance (QCM) is an extremely sensitive mass sensor, capable of measuring mass changes in the nanogram range. These are very often used for various smell as well as gas sensing applications.

QCMs are piezoelectric devices fabricated of a thin plate of quartz with electrodes affixed to each side of the plate. In that regard, a QCM primarily consists of a thin quartz disc sandwiched between a pair of electrodes.

The sensor crystal can be coated with almost any material as long as it can be applied as a thin (nm) homogenous layer firmly attached to the underlying surface. The layer thickness can vary between nanometers and micrometers, depending on the viscoelastic properties of the applied material.Gold, Ti, SiO2, AlO3, stainless steel and polystyrene are used as coating materials mostly. Several other materials are also used though, for example most metals, metal oxides, spin-coated polymers et cetera.

The standard sensor crystal has a diameter of a few millimetres. Technologies developed by certain manufacturers use a diameter of 14mm as the standard. Proper temperature stabilization and functionality is usually obtained at temperatures between 18ºC (64ºF) and 40ºC (104ºF) in normal room temperature around 20 ºC.

Sensing mechanism

Due to the piezoelectric properties of quartz, it is possible to excite the crystal to oscillation by applying an AC voltage across its electrodes. The resonance frequency (f) of the crystal depends on the total oscillating mass, including water coupled to the oscillation. When a thin film is attached to the sensor crystal, the frequency of oscillation decreases. If the film is thin and rigid the decrease in frequency is proportional to the mass of the film. In this way, the QCM operates as a very sensitive balance. The mass of the adhering layer is calculated by using the

Sauerbrey relation:

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The Sauerbrey relation describes the linear relation between frequency changes and changes in mass for thin films adsorbing to the sensor surface. It gives a good estimation of mass/ thickness, as long as the dissipation is relatively low.

The dissipation (damping) is the sum of all energy losses in the system per oscillation cycle. It is defined as 1/Q, i.e. the energy dissipated per oscillation, divided by the total energy stored in system. Essentially thus, a soft film attached to the quartz crystal is deformed during the oscillation, which gives a high dissipation while a rigid material gives a low dissipation. The frequency response of a quartz crystal represents the total oscillating mass. This oscillating mass always includes a certain amount of water. However, the amount of water may vary between 10 and 90% depending on the type of molecule and the way it adsorbs on the surface (an elongated protein that adsorbs flat to the surface gives low dissipation while the very same molecule standing up on the surface gives high dissipation). By measuring the dissipation it becomes possible to determine if a soft film (water rich) has formed on the surface or if the film is rigid (less water). Only when the film is fairly rigid, the Sauerbrey relation gives a good estimation of adsorbed mass.

Measuring of the dissipation means that it is possible to determine if the Sauerbrey relation is valid or not. When the dissipation value typically reaches above 1×10-6 per 10 Hz, the film is

too soft to function as a fully coupled oscillator- the upper parts, far away from the surface, do not couple to the oscillation of the sensor. This means that the normal relation to calculate the mass directly from the change in frequency, will underestimate the mass.

However, by measuring both dissipation and frequency at several harmonics it becomes possible to extract correct thickness estimations even in these cases and also calculate viscoelastic properties using viscoelastic models.

Micromachined Gas Sensors

There is a demand for gas sensing devices for many applications, including monitoring for combustible or toxic gases to ensure industrial safety, climate control of buildings and vehicles to improve comfort, and in process control and laboratory analytics. Accurate, high performance gas analysers based on gas chromatography or mass spectrometry are available, but tend to be relatively large and expensive. Their use in thus confined to analytical labs or process control. For other purposes, such as remote monitoring, cheaper, smaller, and user-friendly sensors are required. Hence, a lot of research and development has been done to design small, low-power, and inexpensive gas sensors that possess sufficient sensitivity, selectivity and stability for a given application. A large variety of sensors based on different sensing principles, such as semiconductor gas sensors, optical gas sensors, mass sensitive devices, catalytic sensors, dielectric sensors, and electrochemical sensors, have been reported.

On one hand, some types of gas sensors are just by their nature considered as low power gas sensors. As already mentioned, polymer-coated transducers, such as chemoresistors, MOSFETs, quartz microbalances (QMB) and surface acoustic waves (SAW) fall in this category. This type of sensor is usually operated at a temperature slightly higher than room temperature. On the other hand, chemo-resistors based on gas-sensitive metal oxide materials and MOSFETs coated with catalytic metals, with operating temperatures up to 450°C and 200°C, respectively, exhibit high power consumption.

Taguchi was the first to develop metal-oxide gas sensors to the level of an industrial product. These Taguchi-type sensors are still on the market. However, most of the commercially available sensors nowadays are manufactured using screen-printing techniques on small and thin ceramic substrates. Screen-printing has the advantage that thick films of gas-sensitive metal-oxide sensors are produced in batch processing. This technology is well established, and high performance has been achieved using screenprinted ceramic sensors in various field applications.

However, screen-printed ceramic gas sensors are still in need of improvement, especially with respect to power consumption, mounting technology and selectivity. The power consumption of these sensors is typically in the range of 200 mW to about 1 W, which is too much for applications where only battery-driven elements may be used. Moreover, the use of arrays of these sensors, which on the one hand is very promising with respect to improve discrimination between gases in a mixture, can lead on the other hand to the increased size of the sensor elements or combined sensor assemblies and thus, to increased power consumption.

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Figure 11. Schematic of a micromachined sensor

In contrast, MOSFET gas sensors are small in size. In addition, microelectronicsbased processing allows the formation of arrays on one substrate. However, their mounting on metallic packages leads to power consumption in the range of 500 mW to 1 W and thus to the same problems as for metal-oxide sensors.

In the last few years, the above mentioned difficulties have led to new developments in substrate technology. The integration of gas-sensitive films in standard microelectronic processing has been achieved, and has led together with the use of micromachining to micromachined metal-oxide gas sensors like the one shown in Figure 13, above. This technology shows great promise for overcoming the difficulties of screen-printed ceramic sensors. The sensitive layer is deposited onto a thin dielectric membrane of low thermal conductivity, which provides good thermal isolation between the substrate and the gas-sensitive heated area on the membrane. In this way, the power consumption can be kept very low, with typical values on the order of 30–150mW, with the substrate remaining almost at ambient temperature. This type of thermally heated device is commonly called a micro-hotplate, and is used in different applications.

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Figure 12. Schematic of a closed-membrane-type hotplate: (a) top view, (b) side view

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Figure 13. Schematic of suspended-membrane-type hotplate: (a) top view, (b) side view

The mounting of membrane-based sensors is much easier than for an overall hot ceramic sensor element, and control and signal-processing electronics can be integrated on to the same substrate if desired. Moreover, sensor arrays, which are often needed to overcome the bad selectivity of single sensor elements, can easily be implemented in this technology. The small thermal mass of each micromachined element allows rapid thermal programming, a unique feature, which can be used to study the kinetics of surface processes and to achieve kinetically controlled selectivity.

Conclusion

Smell and gas sensors have come a long way in scope and application. However, the technological advancement has yet not quite matured and a lot of work needs to be done upon the same. Industrially feasible and compatible processes and technologies need to be conceived and implemented. In future, we should be expecting portable and efficient sensors being used around us.

References

1. I. Lundström, A. Spetz, F. Winquist, U. Ackelid, and H. Sundgren, “Catalytic metals and field-effect devices – a useful combination”, Sensors and Actuators, B1, pp. 15-20,1990.

2. A. D’Amico, C. Di Natale, A. Macagnano, F. Davide, A. Mantini, E. Tarizzo, R.Paolese, and T. Boschi, “Technologies and tools for mimicking olfaction: status of the Rome ‘Tor Vergata’ electronic nose”, Biosensors & Bioelectronics, 13, p. 711-721, 1998.

3. I. Simon, N. Barsan, M. Bauer, and U. Weimar, “Micromachined metal oxide gas sensors: Opportunities to improve sensor performance”, Sensors and Actuators, B73, pp. 1-26, 2001.

4. V. Demarne and A. Grisel, “An integrated low-power thin-film CO gas sensor on silicon”, Sensors and Actuators, 13, pp. 301-313, 1988.

5. D.R. Walt, T. Dockinson, T. White, J. Kauer, S. Johnson, H. Engelhardt, J. Sutter, and P. Jurs, “Optical sensor arrays for odor recognition”, Biosensors & Bioelectronics, 13, pp. 697-699.

6. M.Takano, Y.Fujiwara, I.Sugimoto, and S. Mitachi, "Real-time sensing of roses' aroma using an odor sensor of quartz crystal resonators", IEICE Electronics Express, Vol. 4, No. 1, 15-20.

7. Tzong-Zeng Wu, "A piezoelectric biosensor as an olfactory receptor for odour detection: electronic nose", Biosensors & Bioelectronics 14 (1999) 9–18.44

8. O. Postolache, M. Pereira, P. Girão, "Smart Sensor Network for Air Quality Monitoring Applications", IMTC 2005 – Instrumentation and Measurement Technology Conference, Ottawa, Canada, 17-19 May 2005

9.

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