Adhesion of thin films

The magnitude of adhesive forces that occur as the thin film is applied to the substrate and during drying or firing will depend on the nature of the film and the substrate surface. These adhesive forces can be generally classified as either primary interatomic bonds (ionic and covalent bonds) or secondary bonding (van der Waals bonding) (William and Callister, 1994 Kendall, 2001). 

Primary interatomic bonds provide much higher adhesion than do the secondary bonds because the latter are based on much weaker physical forces characterized by hydrogen bonds or dispersion forces. Hydrogen bonds typically arise on polar materials surfaces, whereas all surfaces give rise to interfacial dispersion forces. 

Chemical bonding is expected to be the most durable and the strongest, and it is often possible to form covalent bonds across the interface of the substrate and coating. However, a requirement for this type of bonding is the presence of mutually reactive chemical groups bounded firmly to the coating and the substrate surface. 

Under appropriate conditions, the formation of chemical bonds with the substrate material is also possible for some surfaces that contain various chemical functional groups, such as previously coated surfaces, composites, and some plastics (Lee, 1991). 

Depending on the curing conditions and material properties, atoms will diffuse across the interface to varying extents when two phases of coating and a substrate achieve molecular contact by wetting. The phenomenon is a two-stage process Wetting is followed by interdiffusion of an element across the interface to establish a chemical bond (Lee, 1991 Kendall, 2001). 

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It should be noted that approximately 1% of the APS used in the last experiment appears to be APS monomers before dilution. Could this low monomer concentration be responsible for adhesion It should be noted that the residual monomers present in this highly-oligomerized solution correspond to a 0.001 vol % solution if one were to remove the oligomerized APS. As a result of this observation, the dependence of the adhesion of thin films to native-oxide silicon wafers as a function of the concentration of the APS under conditions of T H stress was investigated. Adhesion studies were performed using APS solutions with concentrations that varied from the industry standard of 0.1 vol % down to 0.00001 vol %. The test wafers were prepared and exposed to T(200) and T(500) conditions as discussed above. Adhesion was measured by 90° peel test, as discussed above. The results of this study are presented in Fig. 13. The Lx-axis is APS concentration which decreases from left to right. The y-axis is the adhesion in the units of g mm"The three curves are the results at T(0), T(200) andT(500). 

The selected material must adhere to the device surface in such a manner that it moves synchronously with the AW, and must maintain this adhesion in the presence of expected analytes and interferants. The adhesion of thin films to many types of surfaces, including those that are chemically very dissimilar to the coating material, is a much-studied topic outside the sensor field. Often, adhesion-promoting interlayers have been developed for general classes of problems, such as securing a highly nonpolar polymer film to a very polar substrate. Anyone attempting to construct a reliable sensor would do well to examine the relevant literature [12]. 

Mehregany, M., Allen, M. G., and Senturia, S. D., "Use of Micromachined Structures for the Measurement of Mechanical Properties and Adhesion of Thin Films", IEEE 1986 Workshop on Solid-State Sensors, Hilton Head NC, June 1986. 

Benjamin, P. and Weaver, C. (1960) Measurement of adhesion of thin films. Proc. R. 

The adhesion of thin films is influenced by a large number of parameters. Some of these are defined by the choice of materials for the coating and the substrate. The others are influenced by the preparation of the substrate, the coating process and the handling of the film-substrate combination after the coating process is completed. 

More than 350 different methods for the determination of adhesion between film and substrate and between the individual films themselves are known [126] ranging from basic to highly sophisticated. There are two important aspects of the adhesion of thin films. From the academic view point adhesion is an interesting phenomenon in itself. The nature and strength of the forces acting across the interface which actually effect this adhesion (basic adhesion) are of special interest. 

B.N. Chapman, The adhesion of thin films, Ph. D. Thesis, Department of Electrical Engineering. Imperial College, London, 1969. 

Scratch tests have been used to evaluate the adhesion of thin films to a ceramic substrate. 

L. Adams, M. J., Application of Colloidal Probe Atomic Force Spectroscopy to the Adhesion of Thin Films of Viscous and Viscoelastic Silicone Fluids. Langmuir 2011, 27,11489-11500. 

Nevertheless, preparation of the multilayer substrate surface is more complex and is usually carried out by the users instead of manufacturers of ceramic substrates. Simply applying a polymer layer onto the as-fired ceramic surface cannot lead to successful thin-film layers. The solvent trapped in microporosities in the conductor for vias and/or at the interface between conductor and ceramics may outgas during the reflow soldering when populating components on the finished thin-film substrate, resulting in poor adhesion of thin-film metal and dielectric at the position of vias. 

Multiphase composites formed of polymer substrates coated with thin metal films show special properties and are in great demand for various applications. The metal-polymer substrate interaction and the morphological structure at the interface influence the final properties of the composites of thin metal-polymer substrates. Thus, modifying the properties of the polymer substrate by wet (acid, alkali), dry (plasma), and radiation treatments (ultraviolet radiation and laser) appears as a significant step for increasing adhesion of thin film onto polymeric substrates. 

Successful commercialization of low cost, high efficiency solar cell fabrication is highly dependent on fabrication methods that employ continuous processing techniques. One major issue encountered in solar cell construction is the adhesion of thin film solar cells on polyimide substrates. Another involves the adhesion between polymer nanocomposite solar cell structures. The examination of the adhesion promotion potential of variable chemistry atmospheric plasma surface modifications against wet primer chemistry in solar cell construction has shown that APT is a viable continuous and environmentally friendly processing alternative to batch plasma and surfactant-based surface modification protocols. 

Ion Plating Processes The beneficial aspects of ion bombardment on the growth and adhesion of thin films are also realized in the Ion Plating... 

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