(596f) Fabrication of Nanostructured Thin Films By ALD for Spacecraft Applications

The deposition of thin films by atomic layer deposition is a natural technological fit for manufacturing spacecraft components where weight, conformality, processing temperature, and material selection are all at a premium. Applications include optical, thermal control, electric charge dissipation, and protective coatings for the surprisingly reactive environment of low Earth orbit.

Indium oxide (IO) and indium tin oxide (ITO) are widely used in optoelectronics applications as a high quality transparent conducting oxide layer [1]. These coatings also are useful for enhancing the electrical properties of spacecraft thermal radiator coatings, where dissipating built-up static charge is crucial. In this work, we present the thickness-dependent electrical, optical, and morphological properties of IO thin-films synthesized by ALD with the aim of finding the optimum condition for coating radiator pigments [2]. Trimethylindium and ozone were used as precursors for IO, while a tetrakis(dimethylamino)tin(IV) source was used for Sn doping to produce ITO. As-deposited IO films prepared at 140°C resulted in a growth per cycle of 0.46 Å/cycle and relatively low film resistivity.

For the case of ITO thin-films, an ALD process supercycle consisting of 1 Sn + 19 In cycles was shown to provide the optimum level of Sn doping corresponding to the 10 wt.% widely reported in the literature. These ITO films were deposited onto a nanoparticle pigment that was then combined with a binder to form the finished thermal control coating. Samples of our coating are currently on- board the International Space Station as part of the one-year MISSE-10 materials test mission.

Metal halide coatings provide protection from atmospheric oxidation of Al mirror components prior to flight. Their excellent transparency to short-wavelength light make them particularly suitable for orbiting UV astronomy platforms [3]. We have developed an ALD process for AlF3 films using a TiF4 and TMA precursor system. Film spatial nonuniformity, particle formation, and other challenges will be described in this talk along with our hypothesized reaction mechanism.

As a third spacecraft application, we will discuss the ALD of Ni and NiO films using a nickelocene and ozone precursor system [4]. Because of the grazing-incidence focusing mechanism of X-rays [5], these focusing elements consist of very high aspect-ratio microchannels. ALD is a perfect means of conformally coating these glass elements with a thin film of reflective (Ni) material. Reduction of NiO to Ni and the wetability of the glass substrate during the nucleation phase will be discussed, as well as the potential for the synthesis of Ni and NiO nanoparticles.

[1] T. Asikainen, M. Ritala, and M. Leskela, J. Electrochem. Soc. 142 (1995) 3538

[2] H. Salami, A. Uy, A. Vadapalli, C. Grob, V. Dwivedi, R. A. Adomaitis, J. Vac. Sci. Tech. A 37 (2019) 010905

[3] J. Hennessy, K. Balasubramanian, C. S. Moore, A. D. Jewell, S. Nikzad, K. France, M. Quijada, J. Astron. Telesc, Instrum. Syst. 2.4 (2016) 041206

[4] J. Bachmann, A. Zolotaryov, O. Albrecht, S. Goetze, A. Berger, D. Hesse, D. Novikov, K. Nielsch, Chem. Vap. Dep. 17.7-9 (2011) 177-180

[5] P. Gorenstein, X-ray Optics Instrum. 2010 (2010) 19