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Letter 26049
Thin Electroless Nickel low stress films
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I am plating with Boron Electroless nickel on semiconductor
wafers. The problem is two fold:
1) To much stress, Question How do I get the lowest possible
stress, an additive to the bath?
2) Plating too fast, Question How to lower the plating rate to 500
Ang/min. (I only want to plate 1500 Ang +/- 150 Ang, this means that
I need a 3 to 5 min process for controllability.)? I suspect that the
answer to #1 will also answer #2.
Thank You for your help.
Hector E. Celis
Semiconductor Engineer. Thin films - Lee's Summit, MO, USA
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The internal stress of Ni-B deposits is
naturally quite high, Mallory reported stress values of 69K psi for
coatings containing 0.23%B and 44K psi for 1.2%B. Slowing the bath
down (by lowering the temperature and pH to manufacturer's minimums)
will allow better control of thickness and will also directionally
reduce stress. However, it will probably also increase the coating's
boron content and reduce solderability.
I have not personally tried this, but
several years ago I was told that the addition of several ppm of
thiourea would reduce stress and hardness of DMAB reduced
coatings.
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Ron Duncan
Palm
International, Inc. -
LaVergne, TN
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Ed. note:
It is our sad duty to alert the readers to the passing
of Ron Duncan on Dec. 15, 2006. For those who would like to
know more about him, a brief obituary opened
Update No. 13 of our
Metal Finishing Industry Update Podcast.
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I agree with Ron Duncan. However, be careful adding thiourea
because at low bath loading thiourea will poison the reaction and
stop plating. If you add it use 1 ppm to start, maximize the bath
loading (sq ft/gallons of solution 0,25 sq.ft/gal is minimum) Lower
the pH to the low limit recommended in the suppliers data sheet. The
stress will be slightly less and the plating rate will be slower. (in
addition to lowering the temperature)
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