27 Feb Proper Cycle Purge and Other Best Practices for Maintaining Clean High Purity Gas Lines
Moisture, oxygen, or any molecular contamination are enemies to specialty gases, such as Silane (SiH4), Hydrogen Bromide (HBr), etc. Keeping your gases clean will not only preserve your gas delivery system, but will also help maintain maximum yields on your process tool. A well maintained gas cabinet will last for many years — conversely, a contaminated gas line can ruin a system instantly, and be a costly mistake.
Most contamination occurs during cylinder change-outs. Here are the critical pieces to maintaining a clean system when changing out your gas bottles:
1. Proper Cycle Purge
Make sure to adequately cycle purge your gas cabinet when changing bottles. Always use the automated cycle purge function on your gas cabinet and set it to a minimum of 75 cycle purges. What this does is effectively remove the corrosive gas before you open the connection and allow atmosphere to enter the tubing, and then removes the atmosphere effectively after you install the bottle, before you reintroduce the corrosive gas. A higher quality gas cabinets will pull a vacuum on the pigtail to the primary shutoff valve, before you open the gas cylinder, so you don’t mix spec gas with N2.
Also, make sure that you have the “trickle purge” option for the process purge enabled, and that you quickly change cylinders. The trickle purge allows a small amount of nitrogen to fill the tube during the cylinder change so atmosphere can’t migrate up the pigtail, while the quick cylinder change minimizes the time that the tubing is open to atmosphere.
2. Use DISS Gaskets
Always use DISS fittings for both the spec gas AND purge gas. No CGA fittings! With DISS fittings you get a high purity connection as opposed to the friction seal of a CGA. DISS gaskets also provide a higher purity purge gas which lowers the moisture content of the nitrogen (N2).
3. Use UHP Gases
Make sure you get UHP grade spec gas and nitrogen, as the purity is higher therefore the moisture is lower.
4. Cylinder Storage
Keep your cylinders, as well as your gas cabinets, in a climate controlled room. If you house your cylinders in a cold room and hook them up to a warm system, you’re eventually going to have an issue with moisture.
5. Dedicated Purge Cylinders
Make sure you have a dedicated purge cylinder per gas cabinet. Don’t connect all the cabinets to a house purge or 1 purge bottle. The reason is the potential of contaminating your N2 if you lose a check valve and back stream spec gas into your N2 supply.
As we all know, gas delivery systems are expensive. Fortunately, the above steps will maximize the lifetime of your investment.