kenney at slac.stanford.edu
Fri Dec 2 21:16:07 PST 2005
Thanks for your comments. I most certianly agree that the present
holder is not ideal. However, a significant fraction of the
work done in the etchers can't be done without it.
So, yes, I dislike the holder at least a much as anyone for contamination
and other reasons.
> 0. The present holder has a top Al ring exposed to the plasma during
Is there an alternative material that the top rign can be made from?
Al2O3? Quartz? Any other suggestions? What are the STS fingers
> 1. Some years ago when we did contamination tests for all the etchers, we
> ran wafers with and with the holder in STS1. We found a large Al
> contamination level on wafers etched with the holder.
How bad was the contamination? Was it possible to remove the Al?
> 2. Over the years many users have reported grass problems when using the
> holder. Grass or microtrenched is often associated with metal contamation,
> especially Al contamination in F based Si etching. As you well know, the
> holder does not always cause grass in the STS1. It appears to be an
> contributing factor to grass formation.
Yes, grass is a problem.
> 3. STS2 runs at about 3X the rf power level than STS1 run, This means the
> ion flux to the wafer is about 3X higher in STS2,
> 4. Sputter rates increase linearly with ion flux. Therefore, one expects 3X
> the Al sputter rate for the present holder design in STS2. Thus, grass is
> likely be a much worth problem with the holder in STS2.
Given that the silicon etch rate should also scale with ion flux
to first order, it is unclear that the grass problem would be
worse for a similar etch in the new etcher than in the old one.
It would seem to scale with etch depth in both cases.
The use of the magnet in the new etcher to turn-off the ion
flux for part of the cycle may reduce any grass caused by ion bombardment
of the aluminum ring.
Granted there could well be non-linear effects and it may be
very sensitive to the chamber environment, so it is unclear to
me in which etcher the grass problem would be worse in.
> 5. The holder increases the thermal resistance between the wafer and the
> chuck. With 3X the power heating is more of a problem in STS2.
> 6. I believe Elmer did try running the STS1 holder in STS2 while the STS2
> startup engineer was here last year. My understanding in that the wafer
I agree that thermal issues will be more important in the STS2.
A redesign of the holder with lower thermal impedance as
a goal might solve this.
Bottom line some sort of holder is very important to the full
utilization of both STS etchers and the current one is far from ideal.
Can we come up with a better one?
> 7. The Al dduttering problem can be reduced by coating the exposede Al
> a polymer such as resist but it does not last long.
> Chris Kenney wrote:
> > Request to use an aluminum holder similar to the one
> > used in the old stsetcher for the past five or more years.
> > Between 25 and 50 percent of the wafers etched in the old
> > machine use th holder.
> > Allowing a holder in the new machine would increase the number
> > of processes and people who could use it substantially.
> > 1) Contact info
> > Chris Kenney and Jasmine hasi
> > Coral: kenney or jasmine
> > Phone: 725-3661 or 3655
> > 2) Material
> > Aluminum or aluminum oxide
> > 3) Vendor
> > Machined at Stanford
> > 4) Reasons
> > Many processes in the STS DRIE systems involve multi-hour
> > etches.
> > Given the lower loading near the wafer edge, the etch tends
> > to proceed faster at and near the wafer's outer radius.
> > When coating a wafer with resist, the few millimeters near the
> > wafer edge develop a standing wave pattern with local variations
> > in the film thickness. In particular at a radial distance about
> > 2 mm from the edge there is a significant minimum in the resist
> > thickness.
> > During long etches the resist in this local minimum tends to
> > be etched away and the silicon etched to significant depths.
> > This is especially true when the features being intentionally etched are
> > small
> > as this allows for most of the wafer thickness to be etched through
> > if a large area (~100 microns) is exposed near the edge.
> > Also the very side of a wafer, which is typically curved to
> > reduce stress, is etched quite badly during long etches.
> > Any damage near the edge of a wafer greatly increases its likelihood
> > of breaking either within the STS process chamber or in another
> > machine later in the process flow.
> > A second reason for a holder is that many etches make the
> > wafer more fragile. Most users feel more comfortable using
> > the holder in such cases as a safety feature incase the wafer
> > breaks in the process chamber. I have had wafers
> > break in the STSETCH1, but be caught by the holder and
> > unload normally without affecting other users.
> > Potential objections involve thermal issues and sputtering
> > of the aluminum.
> > Thermal issues will depend on the exact recipe and how it
> > is used. I believe thermal issues are best resolved by trial
> > and error by users as this will basically affect individual users
> > by damaging their resist and wafers.
> > The ion energies in the old and new STSETCHERS are very similar
> > given the essentially identical process pressures. Since there
> > is now significant problem with this in the old STSETCH
> > it is unlikely to be a problem in the new one.
> > Again a straightforward experiment can resolve this issue by looking
> > for excess grass when using the holder.
> > 5) Process
> > Indentical use as in the original STSETCH.
> > 6) Amount
> > Potentially identical to the existing STSETCH holder.
> > Please ask if you have any questions about this.
> > Thanks for your time,
> > Chris and Jasmine
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