Fabrication and Physical Properties of Thin Films TiNx for Infrared Absorption
Authors: Jiang Bo , Dong Tao , He Yong , Yang Zhaochu , Su Yan , Wang Kaiying 
Published: February 2020

Infrared absorptivity of pixel arrays plays a pivotal role in bolometer performance. There are a series reports focusing on improving the absorptivity in last two decades [1, 2]. Many materials have been examined as suitable infrared light absorbers, such as thin films MoSix and TiNx. In general, the optimized sheet resistance is ~ 377Ω/square related to the absorption [3-4]. However, there are fewer reports on the investigation of synergetic effects from ultrathin substrate SiNx, which is a desirable material during microfabrication process of the bolometers. In this paper, we present the experimental and optical analysis for construction of the film TiNx on SiNxsubstrates.

The thin films TiNx were prepared by reactive magnetron sputtering technique under Ar/N2 atmosphere. The RF sputtering power is 350 W and the total gas flux is about 25 sccm. The amount (N2%) of nitrogen as a function of Ohm-square at the temperature (25, 200 oC) are summarized in fig 1. It shows that the sheet resistance is sensitive to the substrates temperature. The logarithmic value of sheet resistance is approximately proportional to N2 ratio at the substrate temperature from 25 oC to 200 oC. The reaction rate is rather slow at low temperature (25 oC) even with a high reactant gas concentration for TiNx growth. The relatively high substrate temperature (200 oC) is desirable for the TiNx growth. The resistivity with 13% N2 at 200 oC is close to the value 377Ω/square in literature [3-4].

The light absorptivity of multilayer structure (TiNx/SiNx/Al, SiNx/Al) has been recorded by Fourier Transform Infrared Spectroscopy, as shown in Fig. 2. It can be seen that the infrared absorptivity of the sandwich-type structure (TiNx/SiNx/Al) shows higher IR absorption than that of the structure SiNx/Al. The structure TiNx/SiNx/Al shows obviously enhanced resonant peak in 1/4 of 3/4 wavelength optical thickness due to the existence of Fabry-Perot resonator, which related to the Peak A and C of the recorded spectra. The intrinsic absorption of SiNx can be indexed to Peak B and D. As compared with the pristine SiNx, it can be seen that the absorption dramatically increases at the wavelength 8 um (peak C).
In summary, we report preparation of thin film TiNx through reactive magnetron sputtering technique. The optimized resistivity of the films is applied to build a resonant structure for enhancing infrared absorptivity. A drastically increases has been obtained as compared with the pristine SiNx.

References

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[2] F. Niklaus, J. Pejnefors, M. Dainese, M. Haggblad, P-E Hellstrom, U.J. Wallgren, G. Stemme 2004 Proc. SPIE 5406, Infrared Technology and Applications XXX.
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