Spectroscopic Ellipsometry Characterization of Pd Thin Film Grown by Atomic Layer Deposition

Ultrathin noble metal films have attracted lots of attentions in fuel cell catalysts, hydrogen storage, emission control, etc. Among many synthesis methods, atomic layer deposition (ALD) is a powerful one to fabricate ultrathin films with great precision ^[1]. The measurement of such noble metal films is quite important to both understand the initial growth behavior and guide the film fabrication process. Spectroscopic ellipsometry (SE) is a fast and non-destructive thin film characterization technique with atomic sensitivity ^[2]. The initial growth of noble metals, however, is in island growth mode and bulk metal model could not correlate with such structures. Similar structures have been reported by the Maxwell-Garnett Effective medium approximation (MG-EMA) model to study the optical properties of Au, Ag thin films ^[3-4]. In this paper, we choose the MG-EMA model with General oscillator (GO) to study the size and volume fraction.

In this study, MG-EMA model has been chosen to study Pd ultrathin film grown on Si substrate, as well as its growth on self-assembled monolayers (SAMs) modified substrate respectively. The GO model with one Drude and two Lorentz oscillators is firstly applied to fix the optical constants of Pd. Compared with Pd bulk model, MG-EMA model with GO is more reliable to predict the film thickness verified by X-ray reflection test. The stable growth rate on Si substrate reveals our methods are feasible and is in accordance with the quartz crystal microbalance measurement. For Pd coverage, MG-EMA fitting result is similar with the statistical computation from scanning electron microscope when cycles of Pd ALD cycles are over 400, while large bias exists for cycles under 400, might be due to that air is not the proper filling medium between nanoparticles. Then we change the filling medium into SAMs as a comparison, better fitting performance is obtained. It is demonstrated that the filling medium between nanoparticles is important for the application of MG-EMA model.

References:

 

1. H. Feng, J. Lu, P. C. Stair, J. W. Elam. Catal. Lett, 141, 512-517(2011).
2. K. Vedam, Thin Solid Films, 313, 1-9(1998).
3. K. C. See, J. B. Spicer, J. Brupbacher, D. Zhang, T. G. Vargo, J. Phys. Chem. B, 109, 2693-2698(2005).
4. T. Ung, L. M. Liz-Marzán, P. Mulvaney, J. Phys. Chem. B, 105, 3441-3452(2001).