Paper TF-ThP18
Investigations of Temperature and Humidity Sensors Constructed by Oxide Thin Films

Thursday, November 2, 2017, 6:30 pm, Room Central Hall

Currently commercial temperature and humidity sensors have a low sensitivity for both sides of a high and a low humidity. On the other hand, because the main parts of these sensors are manufactured using polymer materials, there is the problem of a short life in environments such as a high temperature or a high humidity. Therefore, the next-generation type sensors are required in a longer life and a higher sensitivity.

As a material satisfying some of the above-mentioned functions, TiO₂ was adopted in our study. TiO₂ is an n-type oxide semiconductor and has the stable photo-excited state as the material without autolysis. In addition, TiO₂ shows the hydrophilicity under ultraviolet light irradiation. The next-generation type sensor with a higher sensitivity is promised by constructing some oxide layers such as a TiO₂ layer or a Cu₂O (p-type oxide semiconductor) layer.

In this study, basic investigations of TiO₂ and Cu₂O thin films prepared by reactive magnetron sputtering were carried out about those physical properties and the multi-layer thin films were constructed to improve the sensitivity of temperature and humidity on sensor characteristics. The TiO₂/Cu₂O thin film with each layer of 100 nm in a thickness and the TiO₂/Cu₂O/TiO₂/Cu₂O thin film with each layer of 50 nm in a thickness were prepared by reactive magnetron sputtering.

The electric resistance of each sample was measured by changing the voltage from 0 V to 10 V. The resistance changes for temperature and humidity were measured about a range of the temperature from 25 degrees to 60 degrees and about a range of the humidity from 30 % to 60 %. Furthermore the water contact angle was measured by the q/2 method to investigate the relationship to humidity. In these experiments, the photoreaction on a semiconductor characteristic was examined by irradiating the White-LED light or UV-LED light to the sample surface.

The photo-excitation reaction of the TiO₂/Cu₂O thin film was not confirmed by the light irradiation, because Cu atoms diffused to the TiO₂ layer. The temperature coefficient of resistance (TCR) of the TiO₂/Cu₂O thin film and the TiO₂/Cu₂O/TiO₂/Cu₂O thin film were -16820×10^-6 [/K] and -18400×10^-6 [/K], respectively. The resistance of the TiO₂/Cu₂O/TiO₂/Cu₂O thin film was higher than that of the TiO₂/Cu₂O thin film, because the number of an interface between a TiO₂ layer and a Cu₂O layer influenced the bulk resistance of the thin film.