Development of a Thin Film Gas Microsensor of Low Power Consumption for Monitoring of Environmental Pollutants
Authors: Rodriguez Daniel Fabian , Bonaparte Juan , Boggio Norberto , Vorobioff Juan , Berlin Guido , Fascizcewski Alejandro 
Published: May 2020

In this work the development and microfabrication of a gas sensor is described. The microsensor consists of a thin film of SnO2 deposited on a micro-machined silicon substrate [1] . This film is heated by a microheater integrated in the microsensor to obtain a temperature between 100 and 400 °C (microhot plates). With the aim to analyze the proposed design, simulations of the mechanical (structural rigidity, maximum and minimum sizes of the structures) and thermal behaviors (distribution of temperatures and electrical powers involved) were performed [2].
The manufacturing process carried out in a clean-room area involved a series of steps that include: the design and mastering of masks, photolithography operations, sputtering metal deposition, chemical attacks with dry (RIE) and wet etching (with KOH), dicing of the silicon wafer in individual sensors of 3mm x 3mm and finally, the encapsulation of the sensors by means of wire bonding (see Fig.1)
It is important to note that the silicon substrate is micromachined in order to reduce its thermal inertia and the power necessary to heat the sensor film. Such a film is deposited on a silicon nitride insulating membrane that is only 250 nm thick, which significantly reduces the consumption in terms of electrical power, with respect to the known commercial sensors.
The SnO2 thin films were fabricated using the technique Rheotaxial Growth and Thermal Oxidation technique (RGTO) on a microhot plate[3]. RGTO is a technique composed of two steps: deposition
of a Sn thin film on the substrate maintained at the temperature slightly above the Sn melting point and oxidation at a temperature up to 400 oC. In the first step of the RGTO technique tin was deposited by sputtering DC on the central part of the membrane by means of a shadow mask. During the deposition, substrates were heated at 310 oC . Thickness of the deposited tin was 100 nm. The second step, involving the thermal oxidation of Sn thin layer, was performed in two sub-steps: 2 h at 250 oC and 60 h at 450 oC in wet air. The final result is a granular film with quasi-spherical grain shape. The microstructure was characterized by X-ray diffraction analysis and scanning electron microscopy (SEM).
The characterization of the sensors with gases at different concentrations was done. In the case of ammonia in air, a linear response between 50ppm (maximum permissible limit value) and 6ppm were obtained, the latter being its detection limit (Fig 2). Also H2S were tested (Fig. 3) . The measurements were made by applying a voltage of 5 V to the heater, obtaining dissipated power between 40 and 50 mW. The operating temperature of the sensor film under these conditions was approximately 180°C. On the other hand, the sensor film is connected to a potential of 1 V. The sensor response is then measured as the variation of the electrical resistance as a function of the response time.
Additionally, electronic control and signal processing was developed, suitable for incorporating sensors into portable instruments. [4]
The results of this development also allow us to have a platform for the microfabrication of gas sensors at the request of other public institutions or the private sector.

References

[1] Mirzaei, A., Leonardi, S.G., Neri, G. Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review. Ceramics International 42, 15119–15141(2016). http://dx.doi.org/10.1016/j.ceramint.2016.06.145
[2] G.Korotcenkov, B.K. Cho, Metal oxide composites in conductometric gas sensors: Achievements and challenges . Sensors and Actuators B, Vol. 244, 182–210 (2017). http://dx.doi.org/10.1016/j.snb.2016.12.117.
[3] G. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli , A. Camanz. A new technique for growing large surface area SnO2 thin film (RGTO technique) . Semicond Sci. Technol. 5:1231–1233 (1990) https://doi.org/10.1088/0268-1242/5/12/015

[4] J.Vorobioff , D. F. Rodriguez, N.G. Boggio , C.A. Rinaldi . Development of an Electronic Nose for Determining the Freshness of Fish by the Desorption Constants of Sensors. Sensors Letters, Vol. 11, N° 12, 2013, pp. 2215-2217. DOI: 10.1166/sl.2013.3075

Figure 1