A couple of questions/concerns come up when we speak with our existing and potential new customers:
1) What vacuum levels can you achieve with an aluminum casting?
2) Is it possible to use this method of manufacturing to make vacuum chambers?
A few years back we partnered with someone who was doing a Master’s Thesis on Vacuum performance under various scenarios. The Chart below highlights the vacuum performance of a chamber cast at Palmer Foundry. The internal surface finish of the chamber was “as cast” with an approximate surface finish of 158Ra with Gold Iridite plating. The graph shows subsequent pump down trials with the initial pump down trail done after a week of humid weather. Each subsequent trial shows a more rapid pump down with the final trial showing the chamber achieving 10-7 Torr vacuum level in under 2 hours.
This next Chart shows the initial pump down performance of various surface finishes:
1) 158 Ra Electroless Ni (Scratch–was made with 0.06″ diameter end mill 0.7″ in length and 0.010″ in depth)
2) 64 Ra Vibratory Debur (Gold Iridite)
3) 32 Ra Hand Polish (Gold Iridite)
4) 158 Ra Electroless Ni
What is interesting about this chart is the great performance of an “as cast” surface (158Ra) versus a polished surface (32 Ra) and the effect of one scratch on the initial pump down performance. When we were approached by the Mass Spec world to make vacuum chambers we worked hard to convince them that the “as cast” surface can achieve the same vacuum levels as a polished surface. This data supports this hypothesis. In the Semi world where wafer crashes can occur the “clean up requirements” most often push it over into using a polished surface inside the chamber.