Review Article: Stress in thin films and coatings: Current status, challenges, and prospects

Abstract

The issue of stress in thin films and functional coatings is a persistent problem in materials science and technology that has congregated many efforts, both from experimental and fundamental points of view, to get a better understanding on how to deal with, how to tailor, and how to manage stress in many areas of applications. With the miniaturization of device components, the quest for increasingly complex film architectures and multiphase systems and the continuous demands for enhanced performance, there is a need toward the reliable assessment of stress on a submicron scale from spatially resolved techniques. Also, the stress evolution during film and coating synthesis using physical vapor deposition (PVD), chemical vapor deposition, plasma enhanced chemical vapor deposition (PECVD), and related processes is the result of many interrelated factors and competing stress sources so that the task to provide a unified picture and a comprehensive model from the vast amount of stress data remains very challenging. This article summarizes the recent advances, challenges, and prospects of both fundamental and applied aspects of stress in thin films and engineering coatings and systems, based on recent achievements presented during the 2016 Stress Workshop entitled “Stress Evolution in Thin Films and Coatings: from Fundamental Understanding to Control.” Evaluation methods, implying wafer curvature, x-ray diffraction, or focused ion beam removal techniques, are reviewed. Selected examples of stress evolution in elemental and alloyed systems, graded layers, and multilayer-stacks as well as amorphous films deposited using a variety of PVD and PECVD techniques are highlighted. Based on mechanisms uncovered by in situ and real-time diagnostics, a kinetic model is outlined that is capable of reproducing the dependence of intrinsic (growth) stress on the grain size, growth rate, and deposited energy. The problems and solutions related to stress in the context of optical coatings, inorganic coatings on plastic substrates, and tribological coatings for aerospace applications are critically examined. This review also suggests strategies to mitigate excessive stress levels from novel coating synthesis perspectives to microstructural design approaches, including the ability to empower crack-based fabrication processes, pathways leading to stress relaxation and compensation, as well as management of the film and coating growth conditions with respect to energetic ion bombardment. Future opportunities and challenges for stress engineering and stress modeling are considered and outlined.

Structural Properties and Oxidation Resistance of ZrN/SiNx, CrN/SiNx and AlN/SiNx Multilayered Films Deposited by Magnetron Sputtering Technique

“…For multilayers, the deposition process always started with the MeN sublayer being deposited first. The evolution of intrinsic stress developed during growth was monitored in situ and in real-time using the wafer curvature technique [33]. A multiple-beam optical stress sensor (MOSS) designed by kSpace Associates (kSA, Dexter, MI, USA) was implemented in the deposition chamber.…”

Crystallization kinetics and role of stress in Al induced layer exchange crystallization process of amorphous SiGe thin film on glass

“…This is because the value of α for Al (~ 21 to 25 10 -6 K -1 ) is significantly higher than α for Si, Ge and SiGe (~ 2.9 to 6.910 -6 K -1 ) for temperatures ranging from RT to 350 °C [47,48]. The evolutionary trend of thermal stress during annealing and subsequent cooling in thin films can be predictably understood if α of the materials are known for the range of temperatures [49,50]. So when the Al-SiGe system is heated at high rate ~ 21 °C/min, there will be large and rapid expansion of the Al lattice.…”

Strain engineered domain structure and their relaxation in perpendicularly magnetized Co/Pt deposited on flexible polyimide

“…There are various possible ways to generate strain on a thin film deposited on rigid substrate e.g. by lattice mismatch [16,17], voltage application via transducers [18] etc., whereas for a film deposited on flexible substrate bending, stretching, peeling or twisting mechanism also can generate adequate stress [19]. Such mechanical methods transfer almost uniform stress from substrate to the film, which gives rise to device flexibility.…”