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My platinum coated titanium rings shrink after 700 °C anneal...why?




2003

I am a process eng. for a company that builds equipment for semiconductor manufacturers. One of my tasks is diffusion processing, i.e high temp oxidation of silicon, Low Pressure Chemical Vapor Deposition (LPCVD), etc. I also do a bit of annealing.

Lately I've been annealing titanium contact rings (they hold a Si wafer by the edge for wet chem. process), AFTER they have been coated with platinum. These rings range in diameter size from 140mm up to 240mm, thickness is .016, Pt coating is approx. 200 nanometers(nm). We machine our parts from extruded roles of Ti that has already been annealed(not sure what temp yet) I anneal the rings at 700C for 1 hour in N2 and they are shrinking in diameter approx. .009 of an inch. I annealed one titanium ring before Pt coating and it only shrank .001. Any ideas?

Mark Herron
equipment supplier to semiconductor industry - Kalispell, MT, USA


Hi, our machine shop has had an increasingly hard time with titanium shrinkage and warpage on precision parts, like your platinum plated one. We have overcome ALL of these difficulties by doing a full anneal on the pre-machined roughed out blanks. We use all of the common grades of Ti, and this anneal works well on all. The recipe is: 700C for one hour in vacuum, followed by a furnace cool. You could probably do it in nitrogen, but I wouldn't recommend it in air because of hydrogen embrittlement. Follow this recipe, and your parts will not shrink. (Incidentally, your parts are shrinking due to a non-uniform distribution of residual cold work in the material - yes, EVEN in mill annealed material there is residual cold work - after all, it is MILL annealed. Oh, and by the way, if your parts shrink in one direction, they are probably GROWING in another direction - you know, conservation of mass and all that). OK, so have fun!

Nelson Settles
- East Wenatchee, Washington, USA
2004




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