In Situ and Real-Time Nanoscale Monitoring of Ultra-Thin Metal Film Growth Using Optical and Electrical Diagnostic Tools

Abstract

Continued downscaling of functional layers for key enabling devices has prompted the development of characterization tools to probe and dynamically control thin film formation stages and ensure the desired film morphology and functionalities in terms of, e.g., layer surface smoothness or electrical properties. In this work, we review the combined use of in situ and real-time optical (wafer curvature, spectroscopic ellipsometry) and electrical probes for gaining insights into the early growth stages of magnetron-sputter-deposited films. Data are reported for a large variety of metals characterized by different atomic mobilities and interface reactivities. For fcc noble-metal films (Ag, Cu, Pd) exhibiting a pronounced three-dimensional growth on weakly-interacting substrates (SiO2, amorphous carbon (a-C)), wafer curvature, spectroscopic ellipsometry, and resistivity techniques are shown to be complementary in studying the morphological evolution of discontinuous layers, and determining the percolation threshold and the onset of continuous film formation. The influence of growth kinetics (in terms of intrinsic atomic mobility, substrate temperature, deposition rate, deposition flux temporal profile) and the effect of deposited energy (through changes in working pressure or bias voltage) on the various morphological transition thicknesses is critically examined. For bcc transition metals, like Fe and Mo deposited on a-Si, in situ and real-time growth monitoring data exhibit transient features at a critical layer thickness of ~2 nm, which is a fingerprint of an interface-mediated crystalline-to-amorphous phase transition, while such behavior is not observed for Ta films that crystallize into their metastable tetragonal β-Ta allotropic phase. The potential of optical and electrical diagnostic tools is also explored to reveal complex interfacial reactions and their effect on growth of Pd films on a-Si or a-Ge interlayers. For all case studies presented in the article, in situ data are complemented with and benchmarked against ex situ structural and morphological analyses.

Manipulation of thin metal film morphology on weakly interacting substrates via selective deployment of alloying species

“…For all curves, the resistivity ρ exhibits an initial sharp drop after which it reaches a steady-state value ρ ss . We have previously shown that 53,54,57,61 : (i) the nominal thickness at which ρ ss is established (denoted as Θcont) signifies the formation of continuous layer; and (ii) the magnitude Θcont can be used as a proxy for assessing the character of growth, i.e., decrease of Θcont indicates that the film grows flatter and 2D morphology is promoted. The data show that all alloying elements used yield Θcont values that are smaller than that of pure Ag (Θcont = 81 ML; black squares), with the maximum decrease achieved for the case of Cu (Θcont = 47 ML; red circles).…”

“…This is achieved using spectroscopic ellipsometry, a nondestructive optical technique that determines the optical properties of materials by measuring changes in the polarization state of a light beam upon its interaction with the sample under investigation. 67 The methodology used for collecting and analyzing ellipsometric data has been explained in detail in our previous studies, [61][62][63][64] hence here we only summarize key aspects.…”

“…Using as foundation pioneering work from past decades, [36][37][38][39][40][41][42][43][44][45] we have studied growth in weakly-interacting film/substrate systems and identified key structure-forming processes that govern 3D morphological evolution during island nucleation, growth, and coalescence. 22,[53][54][55][56][57][58][59][60] Inspired by these studies and guided by in situ and real-time monitoring of film growth by spectroscopy ellipsometry, 61 we have recently developed a versatile concept for growth manipulation using magnetron-sputter deposited Ag on SiO2 as a model system. This concept entails the use of minority species-both gases (N2,O2) 62,63 and less-noble-metal alloying agents(Cu) 64 -which are deployed selectively to target specific film-formation stages and either promote 2D metal-layer morphology without compromising its electrically conductivity, or enhance 3D morphology at a given set of process parameters.…”

Atomistic Insights into Aluminum Doping Effect on Surface Roughness of Deposited Ultra-Thin Silver Films

“…Ultra-thin metal films are wildly applicated in metamaterials, plasmonic devices, nanophotonic, and light-emitting diodes [ 1 , 2 ]. Recently, continuous metallic films became a good candidate for the replacement of indium tin oxide (ITO) for flexible optoelectronic devices [ 3 , 4 , 5 ].…”

Editorial for Special Issue: Nanostructured Surfaces and Thin Films Synthesis by Physical Vapor Deposition

“…In ref. [ 13 ] the growth of metallic ultra-thin films by continuous dynamic monitoring is reviewed in depth, by combining in situ and real-time optical and electrical probes to analyze the first stages of growth in the MS deposition of a large number of metals with different crystalline structures. In ref.…”