In Reply to: Re: Using diffusion pump systems for ARcoatings posted by Hemang Jesrani / Astro Optics Pvt. Ltd. on 08/28/02 at 7:33 AM:
I can understand that financial considerations often dictate what equipment can be used for a given project. So, should you decide to go with the diffusion pumped system, please consider these points.
1. Choose only a system with a liquid Nitrogen cold trap. Other cooling schemes promise that they are the same, but they are not. Also in your planning, be sure to account for the expense of liquid Nitrogen usage.
2. Make sure that your proposed diffusion pumped system has an adequate roughing/backing pump and that it is properly trapped. This trap will have to be regularly serviced and maintained to prevent backstreaming.
3. Provide for good monitoring of the foreline pressure of the diffusion pump and prevent it from rising too high at any time.
Also consider that the fluids used in diffusion pumps are rather expensive and are easily comtaminated and have a limited lifetime. (Using cheap, inferior fluids is asking for trouble).
If your budget allows, consider the purchase of a Residual Gas Analyser. This could be used on many systems and the cost spread out, but it could tell you a great deal about the contaminates in your systems.
Good luck with your decision.