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September 1999

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From:
Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 27 Sep 1999 21:31:29 -0500
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Christian,

The hybrid microelectronics industry has routinely used plasma cleaning
techniques for many years.

We use both oxygen and argon/hydrogen plasmas for surface prep.

As you might expect, the oxygen plasma is often used to ash (oxidize/burn)
organic contaminates.  Can it be used to ash adhesive bleed-out?  Yes, but
it can take a while - many hours at the power & pressure levels we use!  A
better solution is to remove the cause of the bleed-out.  Whether this means
a switch in adhesives, cure conditions, substrate surface characteristics,
or substrate storage conditions is all dependant on the product and the
specific application.
I believe that it is fairly well established that aluminum oxide formation
is essentially self-limiting, so one should not expect to see great changes
in device bondability, but a wee bit of common sense is called for.

We use a argon/hydrogen mixture for cleaning prior to wire bonding for
Military, and implantable medical products.  Commercial products may not
warrant the (slight) additional cost - again on a specific product basis. In
some respects the argon/hydrogen plasma is like shooting microscopic marbles
at the surface - mechanically ablading the surface with the large argon ion.
Theoretically, the hydrogen ion should be aggressively chemically reducing
surface oxides, but at the power levels we operate at, it does not seem to
be all that effective at chemical reduction.

There are many types of plasma cleaners out there, and you can operate them
at many different power levels, pressure levels and times.  You really need
to perform qualifications on your specific products.

One additional item that I would like to point out is that we are NOT
operating our units at power and pressure levels capable of removing surface
films, even microns thick - certainly not mils thick.  If the surface is not
basically clean in the first place, we really are not totally cleaning the
surface!  I hope that makes sense to you.  We use it to assist in tightening
the distribution in wire pull strengths - kind of a fine tuning step.  I
prefer not to use it for gross cleaning, but under the right conditions you
certainly could.  Maybe another hybrid manufacturer is on-line, and wishes
to comment.

Since the plasma is essentially a charge-neutral environment, it should be
safe for devices.  There have been many studies in ISHM (now IMAPS) papers
over the last 10-15 years, so it seems to be a dead issue.

We do not use plasma for cleaning of optical devices such as LED's or PIN
diodes.   Depending on the AR (anti-reflective) coatings used, we can alter
the optical characteristics of the device.  Many times, these devices, and
lasers, have gold metallization anyway - so plasma cleaning should not be
required!

We have used plasma for cleaning of various MEMS devices including pressure
sensors, accelerometers, etc. with no known problem (the chip manufacturer's
conclusions)- for automotive applications.

We have used plasma for cleaning of GaAs devices with air bridges, with no
known negative effects.  Many times these devices are gold metallized as well.

I would not expect plasma cleaning of SAW (Surface Acoustic Wave) devices to
be a good idea.

Notes on COB (Chip On Board) applications - Plasma cleaning of solder coated
PWB's, or PWB's containing soldered devices can be a problem.  If set up
incorrectly, the plasma cleaner can re-deposit Sn and Pb coatings on wire
bond sites - making life all that much worse.

Yes, you can use the oxygen plasma to remove stray epoxy solder mask from
bonding pads, but again it takes a while - longer than we feel comfortable
with - and you need to make sure you do not re-deposit as much undesirable
material on the bond pads as you remove.  Again, the best way to remove
stray solder mask is to improve the solder mask deposit in the first place.

Hope this helps ....

Steven Creswick - CTS Microelectronics

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