Chamber Optical Viewport

Chamber optical viewports and endpoint optical-window assemblies both let light cross the chamber wall — a viewport for general visual or diagnostic access, and an endpoint window specifically positioned and optically engineered to carry the signal an OES or interferometry endpoint detector depends on.

An endpoint window has to stay optically clean throughout the process in a way a general-purpose viewport doesn't need to, because any deposit buildup on it directly corrupts the endpoint signal the whole detection method depends on. Process byproducts can deposit on any chamber window over time, but a general viewport getting slightly cloudy mostly just makes visual inspection less clear; an endpoint window getting the same deposit buildup actually degrades the optical signal strength and quality the endpoint detector is reading, which can shift or corrupt the detected endpoint time itself — this is exactly why endpoint windows often get purge gas flow or heating specifically to resist deposit accumulation.

Endpoint window placement also matters, since the window has to sit exactly where it can collect the specific optical signal — plasma emission or a reflected interference pattern — the detector needs to read. A general viewport's exact position is largely a matter of convenience for an operator or camera; an endpoint window's position is a deliberate engineering choice tied to where the relevant optical signal is strongest and cleanest, making window placement itself part of the endpoint-detection system's design rather than an afterthought.

Chamber optical viewports and endpoint optical-window assemblies both carry light across the chamber wall, but only one of them is load-bearing for actual process control, which is exactly why endpoint windows receive the extra engineering — purge flow, heating, precise placement — that a general viewport doesn't need.