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Steve Silverman, President
Bartlett Bay Consulting
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Coming
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Contamination
Control Questions:
What are the biggest contamination control
problems in the semiconductor industry? |
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As I see it, the biggest problem
is with ignorance of the specifications: not only
of
their existence and where to find them, but also,
their meaning. Take a very simple case: “PWP” (Particles/wafer/pass)
referring to the number of particles added to the
wafer by a particular operation or process tool.
The question is exactly which operations are involved:
the robot? The LoadPort? The Pre-Aligner? The process
tool itself? The list goes on, but whenever one looks
at a PWP spec, it is not clear how/where it is measured
(Particles added to the backside is in an even worse
situation). SEMI E14-93 was some help, but that has
been withdrawn from the SEMI Standards. The good
news is that there are resources that can offer help,
but generally, they are not focused in any one place.
It has been my hope, incidentally, that this newsletter
will provide some of this focus, and become such
a resource.
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Will
the contamination control specs continue to decrease?
Is there a “limit”? |
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Yes, specs will continue to decrease
because chip design/lithographic parameters must
keep in step with Moore’s Law (doubling chip
speed/capacity every 18 months). Consequently, as
each new chip generation gets smaller and more sensitive,
their sensitivity to defects gets more acute. This
has been the basis for all contamination roadmaps.
So if we believe that Moore’s Law will continue,
and scientists think it will be valid for at least
another 5-6 years, then the contamination control
specs will also continue to track these changes for
this time period. The limits, however, are quite
another story: not only do we have to be concerned
that both airborne and PWP (particles/wafer/pass)
demands change by about an order of magnitude (!)
every 4-5 years making achieving these limits a Herculean
task, but we also have to be concerned about whether
the measurement instruments will be able to measure
these small particles. Right now, for example particle
measurements and specifications in the air and on
wafers are in the range of 80-100nm, and laser measurement
tools are available for that, but in the near future,
particle sizes will be in the 40-50nm range, and
this increasing sensitivity places considerable pressure
on the instrumentation manufacturers and the testers.
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How does one best characterize/measure a minienvironment?
By airborne particles? By PWP? Or other factors? |
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Actually, there are a number of factors
that should be considered. While the ultimate test
is the PWP, this test is so expensive and time consuming
that a contamination control engineer would not want
to perform the test unless there is a high probability
it would pass; consequently, airborne measurements,
and others, are performed first. Such measurements
include differential pressure, and air velocity.
Each is an indicator of the overall design of the
minienvironment. In addition, a good contamination
control engineer will be able to assess other important,
but non-quantifiable items, such as design inadequacies,
which could contribute to high PWP values.
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| Can I use a ceiling HEPA rather than an dedicated
Filter Fan Unit (FFU) for my minienvironment? |
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Yes. BUT there are potential problems.
Factors which should be considered are:
1. Does the
minienvironment have a good seal to the ceiling:
Too large a gap between the top of the minienvironment
and the filter can allow leaks at this gap resulting
in “dirty” air entering into the minienvironment.
Minienvironments with built-in FFUs are designed/sealed
to specifically prevent this problem
2. Does the Filter provide enough air:
Air velocity at the FFU filter face in a minienvironment
usually is set at 0.4-0.5m/sec such that the
velocity at the wafer level is in the 0.2-0.3m/sec
range; this may be hard to do with some ceiling
heights.
3. Will the velocity at the filter change because
of changes in room design/load:
This is a very common problem: new cleanrooms
with adequate air volume, for example, often change,
as increasing demands on air handlers in the ceiling
can result in the filters providing less air volume.
Net: If I had a choice between using existing
filters or a dedicated one, I would go for the
dedicated one as there is much more control and
a larger margin of error.
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| Is my cleanroom really clean or are there areas
which could be problematic and how can I find them. |
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If the cleanroom has been certified
by a qualified company/organization, then it is
probably clean, and any trouble spots would have
been found,
identified, and probably fixed during the certification.
However, cleanrooms change (this is one of the
strengths of minienvironments: that it is a fixed
environment)
due to new tools or tools being moved around, exhaust
changes, etc. If your cleanroom hasn’t been
measured/certified for 3-4 years, it is probably
time to do it again.
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