Structural devices
Structural devices

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Structural devices

7.2.4 Microstructure

There are many practical requirements for a film at the microscopic level.

Firstly, it must be firmly bonded to the surface on which it's deposited. A poorly adhered film can peel or flake away under the stresses of later processing steps, or may lead to a reliability issue that will plague the device throughout its lifetime. Similarly, issues can arise with wetting if a deposited material prefers to form droplets on the surface rather than spreading out into a uniform film. A thin adhesion layer or seed layer can work wonders here, but this must be compatible with later processing stages. For example, gluing down aluminium with a layer of copper can produce difficulties if there is a need to chemically etch through it later.

Deposition processes are likely sources of particulate contamination. In a process that can build up solid layers on a surface, what is to stop a particle growing around a nucleation centre above it? Material must form only on the wafer, and perhaps on the walls of the chamber if they are monitored, but it must never be allowed to solidify in free space. Generation of this ‘dust’ can be problematic if it reaches the wafer surface and affects deposition quality.

A dense structure is (almost) always required, without pores or cracks or voids, and this is easier to achieve with atoms deposited from a vacuum than with a solution or a chemical reaction, where there are other components involved that must move out of the way to let the film form. Sometimes, densification can be achieved by compaction (by ion bombardment) during deposition, or by annealing or reflowing (heating and allowing to flow) the film after the deposition is complete.

Dense structure may not be enough for an active device layer, which may need to be composed of crystalline grains, or even aligned into a single crystal. Heating the wafer during the deposition allows atoms to move around on the surface and find the most comfortable sites on which to sit, so deposition that requires good crystallinity is usually performed hot. Cold deposition is more likely to produce polycrystalline or even glassy or amorphous films. This crystal texture can sometimes be improved by annealing, but will never be perfect, so if it is really important we must achieve crystallinity in the original process.

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