15 Thin film gas sensing application

What is Strain gauge??

Strain gauge is a device that is used to measure strain on an object. It was invented by Arthur C. Ruge and Edward E. Simmons in 1938.

Since 1938, metal foil Gauges have been used extensively for measuring strain. these metal foil gauges have gauge factor in the range 2-4 also these gauges possess low sensitivity to strain. These gauges have to be bonded on some support structure by the means of adhesives. Transducer industry have been looking for a better and cheap alternative to foil gauges for last two decades. Thin film deposited by vacuum deposition technique exhibits strong piezo-resistive (semiconductor strain gauges used for measuring small strain are called piezo-resistors) properties. Amorphous oxide materials exhibit strain sensitivity 5-50 times higher than metal foil gauges

For the last many decades thin film sensor techniques are being utilized in the field of sensors and has been the area of interest for scientists. Aerospace industry is still the main user of this technology. The main technological application in the field of strain study are the turbine blade temperature measurement, the study of compressor blade strain and study of bound layer states. Since thin film sensor have very less thermal mass and thus have fast response rate.

Resistance strain gauze thin film sensor can be made by adherent depositing thin metal films over non-conducting layer which are in turn deposited on some base material. A typical dynamic strain gauze sensor consists of 10μm of Aluminum Oxide (Al2O3) deposited by physical method onto a polished blade surface followed by Nickel(Ni) – 20% chromium(Cr) gauze matrix and lead-out track having thickness of 1μm. To reduce the unwanted resistance the lead-out track is overcoated with 1μm of gold (Au). The lead-out track and sensor grid are overcoated with 1-2μm of aluminum oxide (Al2O3), which act as a mechanical protective coating. The patterning of sensor grid and lead-out track is done by standard photolithographic process. Thin film strain gauzes have maximum overall thickness of 12μm which is 25 times thinner than conventional ceramic cement gauzes. This greater thickness of the conventional gauges is sufficient to disturb the air flow over the component making it less sensitive as compared to thin film strain gauze sensor. In the technology area where the gradients of both stress and temperature would be present and require minimum aerodynamic effects, always thin film strain gauge sensor is preferred.

Recently NASA Glenn Research Centre has developed thin film strain gauzes which can work at higher temperature also. picture of their final product has been shown below

These thin-film gauges provide the extra advantage of minimally intrusive surface strain measurements (which not seems viable in conventional method) and give highly repeatable readings of strain measurements with low drift at temperatures from room temperature to 1100 °C. This product is a 300°C advance in operating temperature over the PdCr wire gauge and 500 °C advance over the commercially available gauges made of other materials.

Thin film gas sensors

There are number of applications of thin film in the field of technology and devices beneficial for nature and mankind. Among those devices and technology gas sensors are one of the most advantageous technology which humans are using and also monitoring environment and health at very large scale. Research interest on making gas sensor can be seen from figure 1 (reported previously) which shows number of publication issued in the interest of fabricating gas sensor.

Now a days thin film gas sensors are used which is basically a class of chemiresistive sensor. A chemiresistive sensor is the one which changes its electrical resistance when there is a change in the nearby chemical environment. Chemiresistive sensors are class of chemical sensor that rely on the direct chemical interaction between analyte and the sensing material. This chemical interaction between the analyte and the sensing material can be by hydrogen bonding, covalent bonding or molecular recognition. There are several different materials which shows chemiresistive properties: Semiconducting Metal oxides, conductive polymers, and nanomaterials like graphene, carbon nanotubes, some nanoparticles. Transition metal dichalcogenides (TMD’s).

Basic schematic of chemiresistive sensor has been shown below (source Wikipedia).

A basic schematic of a single gap chemiresistive sensor.

A basic chemiresistive sensor consists of a sensing material and metal electrodes where sensing element bridges the gap between interdigitated electrodes. In these circuits sensing element is in the form of thin film deposited by various physical and chemical route. Interdigitated electrodes are also deposited thin film of metals such as Titanium /Platinum and gold in general. Metallic heater made up of thin films are also integrated in the circuit whenever required. The resistance between the electrodes can be easily measured with the help of multimeter. The sensing material has some inherent resistance that is being modulated by the absence or presence of the analyte. During exposure of target analyte for which sensing element has been grown, target analytes interact with the sensing material. These interactions cause changes in the resistance value which is being monitored continuously using multimeter having precise accuracy.

These sensors are based on semiconducting metal oxide thin films which are gas sensitive material. Number of metal-oxides are being used for this purpose depending on the planned application and required temperature for adsorption of planned gas to measure the significant response. Semiconducting metal oxides thin film sensors are one of the most widely researched and studied group of chemiresistive gas sensor.

Metal oxide chemiresistive sensors were first commercialized in early 70’s in carbon monoxide detector that uses powdered SnO2. However, there are number of other metal oxides that have chemiresistive properties. Metal oxide sensors are primarily used as Gas sensor since they can sense oxidizing gases and reducing gases both, this remarkable property make them ideal to be used as gas sensors. Metal oxide sensors require high temperatures to operate because an activation energy must be overcome, in order for resistivity to change after the interaction with the analyte. Given below is the table describing favorable metal oxide for detecting selective gas vapors.

Gas sensor systems also known as electronic noses that are able to measure signal and process signal generated by some specific reproducible interaction processes with target gas molecules, in one or more built in fabricated sensitive layer. The development and advancement of these kind of sensing devices has only been possible by quality production and characterization of newly developed sensing materials, the availability of sensitive and fast electronic measuring system and fast growing technical knowledge in information theory to compute and analyze complex multidimensional data. Sensing layer made up of thin film, thin film based heater, thin film based electronic circuitry, when these three things are integrated in a single chip then the devices can be made very compact and cost of production significantly lowered.

The critical part of any sensor system is sensing element and research is always on to design superior sensor as compared to the one which is already in market in terms of selectivity, sensitivity etc. In order to fabricate most sensitive sensing layer, a great deal of research and development is required and here come the biggest advantages of using thin film for fabricating sensing layer because the physical and chemical processes for growing thin film offers many parameters to play for depositing high quality thin film which will give best response for target gas molecule. In order to make sensor system cheaper, portable and usable for monitoring target gas molecule: should require low power for its operation, it should be compatible with sampling system, it should not require any external carrier or reactant gases, it should be made as small as possible and if possible it should be compatible with microelectronic technology. keeping above listed requirement in mind currently SnO2 based sensor are being utilized for gas sensor.

Gas Sensing Mechanism

It is necessary to explain the gas sensing mechanism of metal oxide based gas sensor which is very helpful in designing and fabricating novel gas sensing devices with excellent performances. Although the exact mechanism for gas sensing is still very controversial, it is essentially responsible for some change in resistance that trapping of electrons at adsorbed molecule and band bending induced by these charged molecules. Sensing mechanism of n- type SnO2 with target gas molecule in air is as follows. Generally, oxygen gas molecules are adsorbed on surface of sensing layer of SnO2 in air. These adsorbed oxygen species can capture electron from inside the SnO2 thin film. The negative charges trapped by oxygen species causes the formation of depletion region and so conductivity is reduced. when this sensing layer is exposed to reducing gases, the electrons trapped by oxygen adsorbate will give it back to the SnO2 film, which leads to -the decrease in potential barrier height and thus increase in its conductivity. There are 3 different kind of oxygen species including molecular (O2-) and atomic (O2-, O-) ions on the surface depending on working temperature. Generally, upto 150 °C molecular form dominates while above 150 °C atomic oxygen species dominates.

The overall surface chemistry has very decisive influence on the conductivity of surface for the metal oxides. Oxygen vacancies acts as electron donors, increasing the surface conductivity, on the other hand adsorbed oxygen ions act as a surface acceptor, binding electrons and decreasing the surface conductivity. Reaction which takes place on the surface of SnO2 thin film as the temperature increases is

O2^– _ads + e⁻  = 2O^– _ads

The desorption temperatures from the SnO2 film surface is around 550˚C for O-ads ions and around 150˚C for O2-ads ions. When the oxygen coverage is constant on thin film, the transition causes increase in surface charge density with corresponding variations of surface conductivity and band bending. It is concluded from the conductance measurements that the transitions take place slowly. Therefore, rapid change in temperature on the sensors usually followed by continuous and gradual change in the conductance.

Now how to improve gas sensing performances??

When the particle size of SnO2 is less than or close to double thickness of space charge layer, the sensitivity of sensor shows remarkable increase which is called “small size effect”, but small size of metal oxide nanoparticle will be sintered together compactly during the deposition process which is disadvantageous for gas sensing phenomenon. In order to overcome this disadvantage, nanostructures if various kinds of shapes such as porous nanospheres, porous nanotubes etc is now a days incorporated, because these nanostructures possesses large surface area, relatively mass reactive sites and form relatively loose film structures which provides advantage for gas diffusion consequently improves gas sensing performance. Besides this doping is also used which decreases particle size and hence gas sensing performance can be improved.

There are various techniques for depositing thin film as sensing layer such as sputtering, pulsed laser deposition (PLD), Sol-gel method, Molecular beam epitaxy etc. Depending on the interaction nature of target gas molecule’s electronic structure and electronic band structure of sensing layer, different thin film deposition techniques can be used for enhanced sensing performances.

There are different key parameters which need to be considered during thin film synthesis in various techniques which are listed in the table below.

 REFERENCES

1. “Amperometric Gas Sensor Response Times,” P.R. Warburton, M.P. Pagano, R. Hoover, M. Logman, and K. Crytzer, Yi.J. Warburton; Anal. Chem., 70 (5), 998 -1006, 1998.

2.  D. Pletcher, J. Evans, P.R. Warburton, T.K. Gibbs, US Patent 5,071,526, Dec. 10, 1991, “Acidic Gas Sensors and Method of Using the Same”.

3. B.T. Marquis et al., A semiconducting metal oxide sensor array for the detection of NOx and NH3, Sens. Actuators B 77 (2001) 100–110.

4. J. Brunet et al., An optimised gas sensor microsystem for accurate and real-time measurement of nitrogen dioxide at ppb level, Sens. Actuators B 134 (2008) 632–639.

5. G.H. Jain et al., Studies on gas sensing performance of pure and modified barium strontium titanate thick film resistors, Bull. Mater. Sci. 30 (2007) 9–17.

6. L. Kong, Y. Shen, Gas sensing property and mechanism of CaxLa1−xFe2O3 ceramics, Sens. Actuators B 30 (1996) 217–221.

7. W. Yan, L. Sun, M. Lui, W. Li, Study of sensing characteristics if rare earth pervoskite for alcohol, Acta Scientiarium Naturaliumu Universitaties Jilinesis 2 (1991) 52–56.

8. L. Chen , B.C. Luo, N.Y. Chan , J.Y. Dai, M. Hoffman, S. Li, D.Y. Wang, Enhancement of photovoltaic properties with Nb modified (Bi, Na)TiO3–BaTiO3 ferroelectric ceramics, J. Alloys and Compounds, 587 (2014) 339–343.

9. G. J. Reynolds, M. Kratzer, M. Dubs, H. Felzer, R. Mamazza, Sputtered Modified Barium Titanate for Thin-Film Capacitor Applications, Materials 5 (2012) 575-589.\

10. D. J. Kim, J. Y. Jo, Y. S. Kim, Y. J. Chang, J. S. Lee, J. G. Yoon, T. K. Song, T.W. Noh, Polarization Relaxation Induced by a Depolarization Field in Ultrathin Ferroelectric BaTiO3 Capacitors, PRL, 95 (2005) 237602.

11. P. W. Haayman, R. W. Van Dam, H.A. Klaasens, Method of preparation of semiconducting materials, German Patent 929350 (1950).

12. O. Sahuri, K. Wakin, Processing techniques and applications of positive temperature coefficient thermistors, IEEE Trans. Component 10 (1963) 53.