Fabrication and Physical Properties of Thin Films TiNx for Infrared Absorption

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 MoSi_x and TiN_x. 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 SiN_x, 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 TiN_x on SiN_xsubstrates.     

 

The thin films TiN_x 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 TiN_x growth. The relatively high substrate temperature (200 ^oC) is desirable for the TiN_x 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 (TiN_x/SiN_x/Al, SiN_x/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 (TiN_x/SiN_x/Al) shows higher IR absorption than that of the structure SiN_x/Al. The structure TiN_x/SiN_x/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 SiN_x can be indexed to Peak B and D. As compared with the pristine SiN_x, it can be seen that the absorption dramatically increases at the wavelength 8 um (peak C).

In summary, we report preparation of thin film TiN_x 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 SiN_x.

 

References

[1] T. Schimert, J. Brady, T. Fagan, M. Taylor, W. McCardel, R. Gooch, S. Ajmera, C. Hanson, A.J. Syllaios, (2008) Proc. SPIE, Vol. 6940, article id. 694023, 7 pp

[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.

[3] K.C. Liddiard, (1993), Infrared Physics 34 : 379-387.

[4] P. Ericsson, A.C. Fischer, F. Forsberg, N. Roxhed, B. Samel, S. Savage, G. Stemme, S. Wissmar, O. Öberg, F. Niklaus, (2011) Proc SPIE, Volume 8012, article id. 801216, 10 pp