Winter 2005

This section is for practical, everyday advice about chambers, chamber design, and general vacuum practices. If you'd like to submit a tip, please e-mail it and we'll include it in future updates of this page.

Be sure to specify the material grade on your prints and e-files. The standard material used for high and ultra-high vacuum chambers is 304 or 304L. If you just indicate 300 series stainless steel, you could get 303. This grade of stainless is easy to machine but will discolor badly when it is electropolished.

Use dovetail O-ring grooves (full or half) on any O-rings that will be compressed and released on a regular basis. In high heat applications, the O-rings will stick to the mating material and can be pulled out of the groove, causing a leak and possibly damaging the O-ring.

It is sufficient for most vacuum chambers operating in the 10-8 or 10-9 range to specify a #4 polish (<32 RA finish measured against the grain) on the ID of the chamber. The OD of the chamber can be specified as necessary for a pleasing finish. Adding the #4 polish can add anywhere from 30 to 60 cents-per-pound to the cost of the plate so be sure you need it before you specify it.

You should note on your prints that the machining vendor use only water soluble sulfur-free, UHV compatible cutting oils when machining your chamber or "in-vacuum" components. Most simple machine shops use sulfer based cutting oils in their machines and will not change them out just to make a few parts. Consequently the sulfur can become embedded in the metal and outgas in to the chamber and in your process/product as well. Email and we will send you the specifications and MSDS for the oils we use in all our machines.

Watch out for default tolerances on your CAD or computer design programs. Many larger chambers do not need .010 tolerances on many of the features of the chamber and by not reviewing the drawings you can cause great swings in the quoted prices of chambers (some vendors quote exactly to the print and some don't). Unnecessary tolerances can also cause a higher price than is warranted. Most vendors will try to give you what you ask for so use caution about what you put on your drawings.

Be sure to specify that you want the welding on your chamber to be done using the GTAW method (TiG welding). TiG welding is the preferred welding method for high vacuum chambers. If you want the chamber to be welded using only the base materials of the two parts that are being joined together, then specify fusion welding. Some companies will MiG weld because it is what they are used to and it is a fast process. MiG welding is fine for some outside tacks, structural components or for low vacuum applications, but it is inherently a cold, porous weld and can cause outgassing problems in high and ultra-high applications

If you don't specify the orientation of the leak check slot on the conflat flanges, the industry standard is to straddle the vertical centerline of the flange.

Your high vacuum pump port should be as large and as short as possible so to increase the potential of the gas molecules in the chamber to come in contact with the pump. A 10" port that is 10" long causes a 22% reduction in the effective pumping speed of a 2200 L/S pump.

Most non-semiconductor process chambers are affected by organic contaminants only. A 100 AMU RGA is sufficient to see a major portion of the contamination in your chamber.