Paper EM+NS-FrM9
Compositional Dependence of the Dielectric Function and Optical Conductivity of NiPt Alloy Thin Films

Friday, November 2, 2012, 11:00 am, Room 14

Optical properties of metals are less well known than those of insulators and semiconductors, because it is hard to achieve similar purity and crystallinity in metals. Many metals are reactive and easily form oxides, or they exhibit significant surface roughness. We report the dielectric function and optical conductivity of Ni-Pt alloys as a function of composition (10 to 25 atomic % Pt) from 0.8 to 6.5 eV. Our films are 10 nm thick and were prepared by physical vapor deposition (co-sputtering from pure Ni and Pt targets). To avoid reaction between Si and the metal alloy, films were deposited on thick thermal oxides (220 nm). Some films were annealed at 500°C for 30 s. Similar Ni-Pt alloys are used as Ohmic contacts in CMOS device processing, to achieve highly stable low-resistance contacts between copper back-end metallization and front-end silicon transistors. Our results will enable in-line process control of Ni-Pt alloy deposition using spectroscopic ellipsometry.

Since our metal thickness is below the penetration depth, the interference from the thick SiO₂ layers creates artifacts when extracting the optical conductivity. We minimize this issue by acquiring the ellipsometric angles over a broad range of incidence angles (20° to 80°), which varies the optical path length and thus shifts the interference problems to different energies. Our resulting dielectric functions are similar to those tabulated by Palik for pure Ni. The data are dominated by a Drude divergence due to free carriers at low photon energies. We can remove this divergence by multiplying with the photon energy. We find several trends: (1) The optical conductivity of the annealed films is always greater than that of the as-deposited films, due to improvements in crystallinity and reduced grain boundary scattering after annealing. (2) All four alloys show conductivity peaks near 1.5 and 4 eV due to transitions from the d-like valence bands to the s-like conduction bands. (3) These peaks are significantly broader and weaker than those in pure Ni, but at the same energy. The broadenings increase with increasing Pt content. However, the amplitude of the 4 eV conductivity peak remains constant near 3500/Ωcm, independent of Pt content.

From electronic structure calculations for pure Ni and Pt and a Ni₃Pt ordered compound, we find that Ni, Pt, and Ni-Pt d-bands have similar energy, which explains why the 4 eV peak in the conductivity does not shift with Pt addition. Furthermore, the bandwidth of the Ni 3d bands is smaller than that of the Pt 5d bands, consistent with the increase in the broadening of the optical transitions.

This work was supported by NSF (DMR-11104934).