A viewport is the joint most likely to leak on a finished chamber, because it is a glass-to-metal seal sitting on a machined seat that has to be flat, concentric with the flange, and free of the tool marks that give a leak a path.
The two construction routes
Bonded viewports use an epoxy or a glass-to-metal seal and are simple but limited in temperature. Knife-edge viewports use a copper gasket against a polished glass face, the same principle as a ConFlat joint, and can be baked to UHV temperatures. Which one is right depends on whether the viewport has to survive bake-out.
What has to be machined accurately
The glass seating face flatness, its concentricity with the flange bore, the surface finish on the seat, and the flange bolt pattern. A seat that is flat but not concentric loads one side of the glass and cracks it during torque-down. A seat with a circumferential tool mark gives gas a path across the sealing land.
Optical grade glass
Standard viewports use borosilicate, which passes visible light and survives thermal cycling. Fused silica is specified where UV transmission matters, and sapphire where the window must resist abrasion or transmit into the infrared. Each has a different thermal expansion, and the seat geometry should follow the glass rather than be standardised.
Sizing and placement
Viewport size is normally chosen for the field of view required rather than for the flange size, and on a large chamber the positions have to be laid out before the shell is welded because a port cannot be added afterwards without re-machining the whole chamber. This is one of the reasons we machine our own viewport bodies rather than buying them in.
What we control in-house
- 304 / 316L stainless steel and 6061 / 5083 aluminium fabrication
- Clean-environment TIG welding with full penetration and no micro-porosity
- Helium mass-spectrometer leak testing to 1 × 10⁻⁹ mbar·l/s
- In-house electropolishing and passivation — not subcontracted
- Chamber bake-out to 150–400 °C for UHV outgassing control
- Precision-machined ISO-KF, ISO-K and CF (ConFlat) flanges
- Zeiss CMM verification to ±0.02 mm across large work envelopes
- 12 m gantry CNC machining — 12 m × 4 m × 2 m envelope
- SEMI-referenced documentation and inspection protocols
Frequently asked questions
What is a vacuum viewport?
A vacuum viewport is a flanged window that lets you see or shine light into a vacuum chamber while keeping the vacuum. It is a glass or crystal disc seated on a machined flange body, sealed either by an epoxy bond or by a copper gasket against a knife edge.
Can a viewport be baked?
Only a knife-edge, all-metal construction can be baked to UHV temperatures. Bonded viewports are limited by the epoxy, typically well below 200 °C. If the chamber has to be baked, the viewport has to be specified for it from the start.
Which glass should I choose?
Borosilicate for general visible-light viewing, fused silica where UV transmission or thermal shock resistance matters, and sapphire for infrared transmission or abrasion resistance. The seat geometry should be matched to the glass type.
Why do viewports leak?
Almost always because of the seat rather than the glass — a face that is not flat, not concentric, or that carries a circumferential tool mark. That is why we machine the seat in the same setup as the flange bore.
Can you make non-standard viewport sizes?
Yes. We machine viewport bodies in standard CF and KF sizes and in custom geometries, including angled and offset ports where the chamber layout requires them.
Related capabilities
Need this made to spec?
Send the drawing or the specification and we will come back with a DDP price, lead time and inspection plan.