STS2: Funnel Installation

Jim McVittie mcvittie at snf.stanford.edu
Mon Nov 9 20:45:01 PST 2009


Hi,

Over the last few months we have done a lot of work on STS2 to get it 
back to proper operation. Much of the work was done by Elmer. The whole 
inside of the chamber was cleaned, all the o-rings and seals where 
changed and problems with the cooling, heating and matching were all 
corrected. I think we have all the problems solved. After a lot of work 
by Rostam Dinyari on tuning his SOI etch process, we are seeing great 
etch results.

Back in May, STS suggested that we would get better results if we added 
the so called funnel to our system. As I see it, there has been a 
chamber design problem with the STS2 went we purchased the tool. The 
tool was designed to maximize the transport of the plasma from the ICP 
zone to the wafer.  Normally, one wants lots of ion bombardment at the 
wafer to enhance etching in a plasma etch tool.  However, the Bosch 
process, which relies on F chemical etching of the silicon, does not 
need such a high ion flux. In fact, the high ion flux is a problem in 
that it causes resist etching, which lower resist selectivity. The 
original tool design tried to overcome this problem by adding the 
electromagnet to enhance electron loss to the chamber walls. Since ions 
have to follow the electron, the ions loss was also enhanced.  This 
solution was not enough since we have users complaining about low 
selectivity on STS2 compared to STS1. To correct the problem STS has 
added a can with open ends or what they call a funnel between the ICP 
zone and the wafer. As I understand the funnel, it adds an electron/ion 
recombination surface, which attenuates the plasma and thus lowers the 
ion flux to the wafer. Although one might expect reactive radicals to 
also be lost by recombination on this surface, STS says that the etch 
rate will go up indicating that the funnel is actually directing more of 
the radicals to the wafer than without the funnel.

The Bosch process relies on a delicate flux balance of ions and 
radicals.  I expect the funnel will alter this balance so all the 
processes will have to adjusted or tuned after the addition of the 
funnel. Hopefully, the adjustment will be minor and all the processes 
will need the same type of adjustment.

The plan is to install the funnel next Monday (Nov 16). I will do some 
characterization before and after the installation to get a sense of 
what has to be changed on the processes. Starting Tuesday, I will work 
Rostam to tune his SOI process. Once we have his process working with 
the funnel, I will let everyone know what results we are seeing. 
Hopefully, the funnel will meet expectations and we will not have to 
remove it.

    Thanks, Jim






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