Instability in thin films

D. J. Srolowitz, S. A. Safran. Capillary instabilities in thin films. J Appl Phys 60 247-260, 1986. 

J. S. Sharp, K. R. Thomas, and M. P. Weir, mechanically driven wrinkling instability in thin film polymer bilayers. Phys. Rev. E 75, 011601 (2007). 

E. Jiran and C. V. Thompson, Capillary instabilities in thin films,/. Electr. Mater., 11,1153 (1990]. 

In general, surface morphological instabilities driven by stresses are an important subject to investigate in connection with microelectronic applications. In particular, the degree of surface waviness in thin films as a consequence of surface and volume diffusion is a matter of pivotal importance. This topic has attracted considerable attention in the last two or so decades (11). Although surface diffusion is an important kinetic process, other kinetic processes may affect the evolution of stressed surfaces. Indeed, a possibility at high temperatures is the diffusion of atoms through the bulk. 

In contrast to swollen homopolymer films, only a limited number of studies on thin films of block copolymers have been reported in which the degree of the film swelling has been directly accessed. In situ SE has been used to evaluate the polymer-solvent interaction parameters [144], to construct phase diagrams of surface structures [49, 51], and to control the mechanism of lamella reorientation in thick swollen films [118, 163, 164], Spectroscopic reflectometry combined with real-time GISAXS has been used to follow structural instabilities in swollen lamella films [165], Recently, it was demonstrated that swelling of diblock copolymer films in organic selective and non-selective solvents follows the same physical behavior as in thin films of homopolymers [119]. 

The formation and replication of patterns into polymer films using instabilities is a new contribution in the field of soft lithography, which typically requires the mechanical contact between a patterned master and the resist. Two classes of instabilities were discussed. The demixing of two incompatible polymers leads to a well known spinodal pattern. In thin films, this structure formation process can be guided by a pattern in surface energy. 

Formation of various patterns like the honeycomb structure mentioned above can originate from gradients in surface tension or density due to temperature variations caused by solvent evaporation. Both effects can produce convection instability, namely Marangoni [295] and Rayleigh [296] convection. In thin films Marangoni convection predominates. 

Another source for a destabilizing force in thin films is electrostatic pressure. It has been found in substrate-fluid-fluid systems that if the central fluid layer is a dielectric and the substrate and top fluid layer are both conducting (with differing work functions), then an electric field across the dielectric can drive instabilities [21, 22]. In other words, the presence of the two different work functions (of the... 

J. R. Dutcher, K. Dalnoki-Veress, B. G. Nickel, and C. B. Roth, Instabilities in thin polymer films From pattern formation to rupture. Macromol. Symp. 159, 143 (2000). 

The details of thin-film formation by PVD or CVD on the atomic and molecular scale are unknown in most cases, but such knowledge would be very helpful to design new processes and to tailor film properties. Information is lacking due to the high reactivities, short lifetimes, and low concentrations of the relevant transient gas-phase species. Furthermore, many of the thin-film properties in statu nascendi are unknown due to the experimental difficulties of thin-film characterization during deposition, particularly with non-crystalline films and if established methods such as RHEED and FEED cannot be applied. Among the film properties, mechanical stress in thin films can lead to unwanted (uncontrolled) instabilities and peel-off phenomena. Therefore, in situ diagnostic methods have been developed to... 

Thomas Russell is Silvio O. Conte Distinguished Professor, Polymer Science and Engineering Department Director, Energy Frontier Research Center (EFRC), Polymer-Based Materials for Harvesting Solar Energy. His research interests are polymer-based nanoscopic structures, polymer-based nanoparticle assemblies, electrohydrodynamic instabilities in thin polymer films, surface and interfacial properties of polymers, polymer morphology kinetics of phase transitions, and supercritical fluid/polymer interactions. 

Lyngaae-Jorgensen, J. (1996) Experimental methods for in situ studies of morphology development during flow The case of instability of thin films studied by light scattering. J. Macromol. Sci. Phys., B35, 357. 

G. Reiter, R. Khanna, and A. Sharma, Enhanced instability in thin liquid films by improved compatibility, Phys. Rev. Lett., 85,1432 (2000). 

The different mechanisms that may lead to instabilities in thin polymer films can be broadly subdivided into two major categories instability resulting from the inherent dynamics of the system and instability triggered by an external field. 

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