Atomic-scale diffusion rates during growth of thin metal films on weakly-interacting substrates

A Jamnig,K Sarakinos,D G Sangiovanni,G Abadias

doi:10.1038/s41598-019-43107-8

Abstract

We use a combined experimental and theoretical approach to study the rates of surface diffusion processes that govern early stages of thin Ag and Cu film morphological evolution on weakly-interacting amorphous carbon substrates. Films are deposited by magnetron sputtering, at temperatures TS between 298 and 413 K, and vapor arrival rates F in the range 0.08 to 5.38 monolayers/s. By employing in situ and real-time sheet-resistance and wafer-curvature measurements, we determine the nominal film thickness Θ at percolation (Θperc) and continuous film formation (Θcont) transition. Subsequently, we use the scaling behavior of Θperc and Θcont as a function of F and Ts, to estimate, experimentally, the temperature-dependent diffusivity on the substrate surface, from which we calculate Ag and Cu surface migration energy barriers {{boldsymbol{E}}}_{{boldsymbol{D}}}^{{bf{exp }}} and attempt frequencies {{boldsymbol{nu }}}_{{bf{0}}}^{{bf{exp }}}. By critically comparing {{boldsymbol{E}}}_{{boldsymbol{D}}}^{{bf{exp }}} and {{boldsymbol{nu }}}_{{bf{0}}}^{{bf{exp }}} with literature data, as well as with results from our ab initio molecular dynamics simulations for single Ag and Cu adatom diffusion on graphite surfaces, we suggest that: (i) {{boldsymbol{E}}}_{{boldsymbol{D}}}^{{bf{exp }}} and {{boldsymbol{nu }}}_{{bf{0}}}^{{bf{exp }}} correspond to diffusion of multiatomic clusters, rather than to diffusion of monomers; and (ii) the mean size of mobile clusters during Ag growth is larger compared to that of Cu. The overall results of this work pave the way for studying growth dynamics in a wide range of technologically-relevant weakly-interacting film/substrate systems—including metals on 2D materials and oxides—which are building blocks in next-generation nanoelectronic, optoelectronic, and catalytic devices.

Highlights

TS = 300, 400, 600, 800 and 1000 K with total simulation times of ~0.77 ns for Ag and ~0.66 ns for Cu
Γ-point sampling of the Brillouin-zone and 300 eV cutoff energy for the planewave basis set were used for all simulations
Such approaches may, for example, be relevant for directed growth of metals on 2D-material (e.g., graphene and MoS2) and oxide (e.g., TiO2 and ZnO) substrates, and thereby fabricate high-performance nanoelectronic, catalytic, and optical devices[3,15]

Summary

Methods

Ag and Cu films were deposited by direct current magnetron sputtering in a high-vacuum chamber (base pressure ~8 × 10−6 Pa). Ar gas (purity 99.999%) at a pressure of 0.25 Pa was used to generate plasma and sputter magnetron sources were equipped with Ag (diameter 7.62 cm, purity 99.99%) and Cu (diameter 7.62 cm, purity 99.999%) targets. Films were grown on Si (100) substrates covered by a 6.5 nm thick a-C layer grown in situ, prior to Ag and Cu deposition, by sputtering a graphite target (7.62 cm, purity 99.995%), with a power of 150 W, at an Ar pressure of 0.25 Pa. Ag and Cu films were deposited with growth rates in the respective ranges 0.11 to 5.38 ML/s and 0.08 to 2.5 ML/s, set by changing the power applied to the two targets from 5 to 300 W. XRR measurements verified that changing TS had only minor effects on F for a given target power (

Results

Discussion

Conclusion