In contrast to electronics, film growth in optics is rather undefined and poorly understood. Despite the fact that optical coatings are used with an ever increasing leverage effect on optics, we are only just beginning to understand film growth in detail. Film growth is an unique low-cost nanofabrication process. It has the property of self-assembling material over large areas into periodic structures that exhibit photonic bandgap properties. For controlling the optical properties of films nowadays it is of utmost importance to have a thorough knowledge of the real structure on the atomic scale.
The properties of thin films of a given material depend on their real structure. Real structure is the link between thin film deposition parameters and thin film properties. With the aim to engineer properties by way of real structure, thin film formation is a process starting with nucleation followed by coalescence and subsequent thickness growth, all stages of which can be influenced by deposition parameters. The focus in is on dielectric and metallic films and their optical properties. In contrast to optoelectronics, we are just beginning to use all these film growth possibilities for the engineering of novel optical films with extraordinary properties.
Factors controlling the properties of thin films are:
- deposition process (temperature, rate, energy, residual gas)
- substrate surface structure
- real structure (grain size, orientation, defect density properties (complex refractive index, stress, hardness, …)
- application (antireflection, high reflection, filtering)
Fig. 1 Brightfield (left) and darkfield (right) TEM image of a MoSi multilayer system on Si substrate. Si layers appear gray and Mo layers appear black in the BF image. The DF image shows that Mo layers are polycrystalline which can be seen by the typical white diffraction contrast. MoSi multilayers act as Bragg mirror and are developed for the next generation of optical lithography that works with extreme ultra violet (EUV) light (13.5nm wavelength) instead of UV light (193 nm wavelength).
O. Stenzel, S. Wilbrandt, S. Yulin, N. Kaiser, M. Held, A. Tünnermann, J. Biskupek and U. Kaiser
Plasma ion assisted deposition of hafnium dioxide using argon and xenon as process gases
Optical Materials Express, Vol. 1, 2, 278-292 (2011)
O. Stenzel, S. Wilbrandt, N. Kaiser, M. Vinnichenko, F. Munnik, A.Kolitsch, A. Chuvilin, U. Kaiser, J. Ebert, S. Jakobs, A. Kaless, S. Wüthrich, O. Treichel, B. Wunderlich, M. Bitzer and M. Grössl
The correlation between mechanical stress, thermal shift and refractive index in HfO2, Nb2O5, Ta2O5 and SiO2 layers and its relation to the layer porosity
Thin Solid Films, 517, 6058 - 6068 (2009)
M. Bischoff, D. Gäbler, N. Kaiser, A. Chuvilin, U. Kaiser and A. Tünnermann
Optical and structural properties of LaF3 thin films
Applied Optics, Vol. 47, Issue 13, 157-161 (2008)
T. Pilvi, M. Ritala, M. Leskelä, M. Bischoff, U. Kaiser and N. Kaiser
Atomic layer deposition process with TiF4 as a precursor for depositing metal fluoride thin films
Applied Optics, Vol. 47, Issue 13, 271-274 (2008)
T. Pilvi, T. Hatanp, E. Puukilainen, K. Arstila, M. Bischoff, U. Kaiser, N. Kaiser, M. Leskela and N. Ritala
Study of a novel ALD process for depositing MgF2 thin films
J. Mater. Chem., 17, 5077-5083 (2007)