David:
You gave a great paper at the PCMI, and now you have asked one of the more
fascinating, and difficult to answer questions to appear on Technet, and since
we like to think of ourselves as experts in exactly this field, I am going to
reply, with more detail than you likely want.
First, an easy part of of the question, about rinse water.
It turns out that resist strippers pickup Copper during stripping, and in
most, but not all, cases, it tops out at around 250 PPM. With a stripper
containing 250 PPM of Copper, the rinse waters are going to have more than an
allowable level of Copper, thus they must be waste treated, and this is
expensive, especially since the strippers are usually chelated.
Some stripper chemistry vendors are getting around this matter by using feed
and bleed, and feeding enough stripper chemistry through the system to keep
the Copper levels down, thus keeping the rinse waters legal to dump. This is
simply a keep-the- --stripper-chemistry-rich concept, but there are some folks
who buy into it.
It is possible, by using the proper inhibitors in the stripper chemistry, to
keep the Copper levels down to less than 50 PPM, no matter how deeply you go
into the chemistry, but of course, I am too modest to tell you those suppliers
that can supply this kind of chemistry.
Now about actual stripping costs.
First, understand that because of overplating issues, the outerlayer strippers
have to be real high performance, compared to innerlayer strippers, and this
high performance can cost a whole lot, or not so much more, depending on the
chemistry in the stripper.
To be specific, the cost of stripping can vary from a low of around US$0.006
per square foot (and this is for a fully formulated proprietary stripper) to
as high as US$0.12 per square foot, for folks using really badly formulated
strippers in a feed and bleed mode.
Some facts to remember about costs of stripping:
1.) The faster you want to strip at, the higher the cost of stripping, this
means that the longer the strip chemistry is in contact with the resist, the
deeper into the chemistry you can go. Thus the longer the strip chamber, the
lower the chemistry costs, because the longer the chamber, the longer the time
the chemistry is in contact with the resist, for a given throughput.
2.) The higher the temperature you strip at, the faster the stripping, thus,
this really means that the higher the temperature you strip at, the lower the
cost, as long as the stripper does not tarnish the Copper because it is being
used so hot.
3.) The issue of tarnishing the Copper is crucial, especially in the
innerlayer line, as in the US, most innerlayers go to AOI after stripping, and
for most AOI, any tarnish slows the AOI dramatically. This is so important,
that some people throw out stripper chemistry when it starts to tarnish. And
the propensity to tarnish increases as the Copper level in the stripper
increases, unless you have a properly designed anti-tarnish system in the
stripper chemistry.
4.) Feed and Bleed generally increases the cost of stripping by a factor of
2+, because good chemistry is thrown out with the bad.
I think that I have given you more than you asked for, but if you have further
questions, get back in touch.
Regards,
Rudy Sedlak
RD Chemical Company
Mountain View CA
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