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Thin Electroless Nickel low stress films
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. CelisSemiconductor Engineer. Thin films - Lee's Summit, Missouri, 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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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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Don Baudrand Don Baudrand, Consultant  Poulsbo, Washington |
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