Physical Origins of Intrinsic Stresses in Volmer-Weber Thin Films
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Physical Origins of
Intrinsic Stresses in Volmer–Weber Thin Films
Jerrold A. Floro, Eric Chason, Robert C. Cammarata, and David J. Srolovitz Abstract As-deposited thin films grown by vapor deposition often exhibit large intrinsic stresses that can lead to film failure. While this is an “old” materials problem, our understanding has only recently begun to evolve in a more sophisticated fashion. Sensitive real-time measurements of stress evolution during thin-film deposition reveal a generic compressive–tensile–compressive behavior that correlates with island nucleation and growth, island coalescence, and postcoalescence film growth. In this article, we review the fundamental mechanisms that can generate stresses during the growth of Volmer–Weber thin films. Compressive stresses in both discontinuous and continuous films are generated by surface-stress effects. Tensile stresses are created during island coalescence and grain growth. Compressive stresses can also result from the flux-driven incorporation of excess atoms within grain boundaries. While significant progress has been made in this field recently, further modeling and experimentation are needed to quantitatively sort out the importance of the different mechanisms to the overall behavior. Keywords: intrinsic stress, physical vapor deposition, thin films.
Introduction Anyone with experience in the physical vapor deposition of thin solid films has had to deal with the problem (or opportunity) of film stress. The stress levels in asdeposited thin films commonly are well in excess of the typical yield stresses of the corresponding materials in their bulk form. This observation spans essentially all types of film materials and deposition techniques. To put these stress levels in perspective, we note that the pressure at the bottom of the deepest ocean trench on Earth is still an order of magnitude smaller than the stresses that are routinely observed in solid thin films. Such high stress levels can lead to severe problems, including buckling, spallation, fracture, and surface roughening. Understanding the mechanisms that dictate the formation and evolution of film stress and developing strategies to control these stresses represent some of the most important outstanding issues in the field.
MRS BULLETIN/JANUARY 2002
Early observations and measurements of film stress date back to the American Civil War era.1 While it has long been recognized that the physical origins of growth-induced stresses are intimately related to the far-from-equilibrium evolution of film microstructure during deposition, surprisingly slow progress has been made in developing quantitative models
for stress generation in nonepitaxial films. This is largely due to the complex microstructural evolution that occurs in films grown by means of the Volmer–Weber (V–W) growth mode. In the V–W mode (see Figure 1), films initially grow by the nucleation of discrete islands of different crystallographic orientations (or no crystallinity at all, in the case of amorphous solids). Upon additional dep
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