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Glass-to-Metal Seals Technical Notes

Cautions when Using GM Seals
  • Where glass is the transition between two fixed metal points in a vacuum system, anneal the glass after all parts are clamped in position. The smallest misalignment may cause the glass to shatter under the additional stress of evacuation.
  • Do not use a metal bellows GM seal as a substitute for the annealing. Under vacuum, all bellows contract—which puts the glass and metal joint under considerable tension and frequently causes failure.
  • For safety, cover the line-of-sight between GM seals and people with a suitably thick, transparent rigid plastic (Lexan®) to stop glass particles that may be released if an implosion or explosion occurs.

Glass-to-metal (GM) seals are joints between metal and glass tubes. They are often used to observe a process or to provide electrical isolation.

Typically, the metal tube-end is terminated in a variety of standard vacuum flanges.

For a GM seal to work, the coefficients of thermal expansion (CTE) of the glass and metal must match, as in the Kovar-to-7052 glass combination. Where the required tube materials have non-matching CTEs, as in SSto-7740 Pyrex® or SS-to-quartz, the seal is called mismatched.

To form a mismatched seal, a graded glass seal is formed by fusing together many rings of glass, each with slightly different expansions. The graded seal's one end almost matches the Pyrex (or quartz) expansion coefficient, while the other end almost matches stainless steel. Both matched and mismatched seals have proven reliable in vacuum service if correctly used.

The metal tube GM seals are constructed in following forms:

  • glass tube—metal tube
  • glass dome end—metal tube
  • glass tube—metal bellows tube
  • glass tube—metal tube—glass tube
  • metal tube—glass tube—metal tube
  • metal bellow—glass tube—metal bellow


Viewports are windows installed on vacuum chambers to transmit electromagnetic radiation from ultraviolet, through visible, to infrared— depending on the window material used.

Positive Pressures

When discussing system pressures, a common question is: What is the maximum internal pressure?

Positive pressures are inherently dangerous and failure comes without warning. In a vacuum system, failure depends on various strength properties of bolts, clamps, chamber walls, welds, feedthroughs, valves, viewports, etc. The only safe answer is the internal absolute pressure cannot exceed the external absolute pressure.

Some of their applications are to:

  • Let the operator view a process
  • Initiate chemical or physical action using specific wavelengths
  • Make measurements of emissions occurring in a process
  • Monitor the effects of specific wavelengths, e.g., ellipsometry

Two basic designs:

Zero Length Viewports

  • Have a greater field-of-view than top-hat models
  • Less subject to accidental damage because the window does not protrude beyond the flange face

Top-Hat Viewports

  • Do protrude, but undergo less stress during bolt tightening and chamber heating

All viewports are fragile and should be handled and mounted with extreme caution. Always make small adjustments. For example, when securing a bolted viewport, finger tighten all bolts and then, following the normal pattern for the flange, tighten each bolt no more than a 1/16 turn using a wrench.

For CF flanges, make sure the gasket is a fully annealed type.

Never scratch the viewing area—a weakened viewport may implode (or explode under wrong conditions). Where the viewport is for looking at the process, cover it externally with a thick Lexan disk. Replace three flange bolts with threaded rods and mount the Lexan on those.

Make sure special material viewports are protected from conditions that affect them. For example, protect alkyl halide viewports from water vapor; AgCl from visible light; or MgF2 from high temperature.

Do not subject viewports to rapid temperature changes or gradients. Opinions vary about maximum heating rate, but there is no penalty for being cautious and using the lowest quoted heating rate of ~2°C/minute. If a viewport is heated at the flange rim, cover it with layers of aluminum foil before bakeout to reduce temperature gradients.

It is critical to observe the caution noted in the sidebar on positive pressures.

Any chamber equipped with a viewport must not be subject to a positive internal pressure.

Make sure the "glass" chosen has a reasonable transmission at the wavelengths of interest. And remember, what is not transmitted may be reflected or absorbed. That is, the viewport material may be heated by absorption.

In addition, where film deposition may obscure the viewport, use a shutter mechanism.

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