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.
Highlights
Metal films with thicknesses of ~10 nm and below are ubiquitous in many modern life technologies, including microelectronics, displays, sensors, and energy storage/saving/conversion devices [1,2,3,4,5,6,7,8]
It is said that film formation proceeds far from thermodynamic equilibrium and the resulting film morphology and microstructure are determined by the occurrence rates of atomic-scale structure-forming mechanisms [19,20,21,22,23,24,25]
In the present review article, we demonstrate the strength of combining laboratory-scale electrical and optical in situ and real-time diagnostic tools for shedding light onto morphological evolution, structure formation, and growth dynamics in a wide gamut of film/substrate systems, whereby films are grown by physical vapor deposition techniques
Summary
Metal films with thicknesses of ~10 nm and below are ubiquitous in many modern life technologies, including microelectronics, displays, sensors, and energy storage/saving/conversion devices [1,2,3,4,5,6,7,8]. Evolution of electrical properties (e.g., film resistivity) can be measured using four-point probe techniques [44,45,46,47], while typical optical diagnostics include reflectance spectroscopy [48,49,50,51] and spectroscopic ellipsometry [52,53,54,55,56,57,58,59,60] These techniques can characterize all relevant film-growth stages up to the formation of a continuous layer and beyond, while they provide morphological information over mesoscopic length scales.
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