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Electroless Copper Plating for Micro-probing Transistors

October 16, 2008

I work in the semiconductor industry and I frequently micro-probe SRAM transistors in the 45nm technology scale. Individual transistors can be probed in the cell at the tungsten contact level but when I need to probe a 6 transistor SRAM bitcell as a whole it must be done at the metal 1 level which is copper. The copper is very soft and tends to smear and short to adjacent nodes of the bitcell when I land the micro-probes on it. A 45nm bitcell is (~.5 X 1um) in area and I have to land 8 - 50nm radius tungsten probes in that area to acquire the electrical data from the cell. I'm looking for a way to conductively coat the copper to make it harder and keep it from smearing. The coating must be selective to copper and not adhere to the surrounding Si02 insulator. I'm looking for a way to do this that doesn't require excessive temperatures.
Any help would be greatly appreciated.

Wayland Seifert
Engineer - Austin, Texas

October 22, 2008

If it is possible to touch the copper using a small steel or stainless steel wire, then it would be possible to use electroless nickel. Electroless nickel boron at less than 1% B(typically 0.3-0.5% B)the deposit is very hard and with a specific resistivity of about 10-12 micro-ohm-cm.
Alternatively at lower cost is electroless nickel phosphorus. Using a low %P at 2-4%P (resistivity of about 30-60 micro-ohm-cm.) Each isolated area to be plated must be touched with the wire while the plating solution is present. The plating will start immediately onto the copper. Remove the wire (S) immediately at the first sign of bubbles. {Plating will continue until the plating solution is removed. There will be no plating on non- conducting portions of the device nor will it plate silicon if it has an oxide layer.(sometimes extraneous plating can occur on silicon or other conductive materials starting as dissimilar metals together (battery effect) If there are multiple isolated copper areas touch each individually.

don baudrand
Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [affil link to the book on: Amazon or AbeBooks ])

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