NOTES ON DEPOSITION PROCESSES AND APPLICATIONS
Deposition process technology continually evolves and adapts to address specific optical coating applications and production volumes. The deposition of thin solid films of desired composition, morphology, optical or electrical and mechanical properties involves complex physio-chemical processes. Film layers condensed and grown from the vapor state, while having the same chemical composition as the source material, often possess physical properties that differ from their source material. Their nano-structural morphology and electronic states differ from the bulk due to disorder such as poly-crystalline, single crystal and amorphous states and impurity or vacant states. Multiple deposition processes have been developed with the goal of reproducing the desirable bulk properties in film layers having thicknesses as small as a few nanometers.
While the same ultimate thin film compound can be fabricated by multiple deposition processes, no single technique serves all compounds optimally. All processes share the common goals of high reproducibility with high production yield and high throughput rate consistent with efficient and economical materials usage. As technology advances, more processes are duplexing techniques beyond the conventional isolated metal mirror, window, filter and reflector foundation of optics houses, and have begun to resemble cluster tools more typical of Integrated Circuit platforms. To best address new wafer level support, we present a basic overview of deposition processes and applications, and perhaps suggest an approach to best align resources to meet the challenges of tomorrow. As we have discussed in previous articles, there may be important differences in the metal or compound supply chain which are crucial in the most extreme usage cases. The reader is encouraged to refer to previous CMNs where these topics, materials and deposition technologies, are discussed in greater detail .
Film Growth Dynamics
Before discussing techniques for depositing thin optical film layers, we’ll present a brief outline of the growth process itself. Materials transformed to the vapor state during evaporation or sputtering of atoms of the source material condense on other surfaces. Rapid solidification leads to non-equilibrium and disordered semi-amorphous nano-structures. Consequently, the growing film exhibits optical and physical properties that differ significantly from the bulk source material. To counter the disorder associated with low-energy the processes thermal evaporation (EB and resistance-heated), additional momentum is supplied to mobilize the arriving atoms so they can grow with a compact denser and more ordered nano-structure.Dense amorphous nano-structure is required to achieve environmental stability of optical, mechanical, and laser-resistant properties.
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