Control of Mechanical Stress as Related to Deposition Process




The transformation of materials from bulk solid to vapor and back to thin solid films involves processes that invariably change mechanical and optical properties. In the thin-film state, materials can exhibit characteristics very different from the bulk state; some are desirable, some are not. Advances in materials and deposition process technologies have been directed toward controlling the physical properties of condensed thin-film forms and optimizing the starting materials for a given process to perform the task at different economies of scale, sometimes at the expense of durability. While it is typical to emphasize the optical properties, the path one takes and the tradeoffs with mechanical properties, equally important are intrinsic stress, hardness, density, strength and application durability. The goal for many applications is to reproduce the bulk-like neutral physical state as closely as possible; for some applications however, it is necessary to engineer the optical and mechanical properties to achieve specific goals. An example of the latter is to deliberately replace normal incidence amorphous growth morphology with tilted, structured growth features that exhibit birefringent properties resulting from the anisotropic growth structure.

Influence of Deposition Parameters on Stress

Deposition process development has been directed toward increasing the mechanical strength, wear resistance (hardness), and the spectral, thermal and chemical stability of thin-film coatings. Specific environments and applications require different emphases. Production of the desirable coating properties is often accompanied by problematic mechanical stress that can manifest as substrate bowing, crazing and increased scatter, and catastrophic tensile cracking or film separation from the substrate. Improvements are aimed at producing thin-film coating layers with dense low intrinsic stress microstructures that ideally are amorphous [1]. Intrinsic stress is differentiated from external strain forces imposed by, for example, thermal coefficient differences between the coating and its substrate material.

Many parameters are involved in the transformation from bulk materials to thin film, each playing a role in promoting or suppressing specific film properties. The dominant parameter in achieving bulk-like film growth kinetics is the energy of deposition, a parameter determined by the deposition technique. The energy of deposition refers to the supplied kinetic energy that promotes mobility of the adatoms on the substrate surface and powers chemical reactions such as oxidation and nitriding.

The evolution from initial nucleation sites to solid film layer growth is complex in that it involves several morphological stages [2]. Isolated islands grow into clusters from which grains and columns emerge that constitute an open low-density structure. Densification proceeds with the addition of external energy, and the stresses transition from tensile to compressive.

Low-energy processes grow coarse film microstructures that are characterized by columnar features with large void volume; the result of the lower mobility energy of low-energy processes. This class of microstructure is produced by thermal evaporation from resistance or by e-beam heating with adatom energies in the range ~1-3 eV. Figure 1 is the Structure Zone Model of Dr. Anders that relates film microstructure to the energy of deposition [3]. Anders emphasizes that a multi-dimensional diagram is required to truly describe the complex growth dynamics and reactivity of thin layers. The large grains and grain boundaries associated with the porous structure of Zone 1 are the result of low energy conditions and generally possess tensile stress. The large internal surface areas and empty spaces also lend susceptibility to water vapor absorption and desorption. In these under-dense structures, the exchange of the water accompanying atmospheric humidity changes cause changes in optical and in mechanical properties, especially stress.

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