How to make electrical connection to silicon wafer for testing? I am trying to make an electrical connection to a silicon wafer using a wire (approx 32AWG).
I find that conventional lead/tin (60/40, no flux) solder cannot adhere well to the silicon wafer, since the solder will simply fall off the wafer after cooling (same to both polished and unpolished side of wafer).
I tried depositing a layer of aluminium (1µm thick, by DC sputtering) on the silicon with the guess that solder and silicon were incompatible. However, the adhesion between the sputtered aluminium layer and the solder was still very weak.
What can I do to the silicon wafer so that I can make an electrical connection to it?
 A: You can't reasonably solder to aluminum, since it almost instantly forms an aluminum oxide layer, and alumina is as close to inert as makes no never mind. The flux that removes oxides from materials like copper simply aren't active enough for aluminum.
There is nothing to keep you from depositing a different metal, though. Gold, silver, copper or nickel will all work. And I'd recommend something thicker than 1µm.
As I recall, gold with a nickel plating used to be used - the nickel keeps the gold from migrating into the solder and causing embrittlement.
Wikipedia, of course, has a basic treatment of the subject.
A: I used indium solder to do this. However, there is a trick to it. Silicon wafers typically have a layer of insulating oxide on them. You have to get the indium through the oxide layer into silicon. Put small blobs of indium on the silicon sample where you want to make the connections, and put the sample on a hot plate. Once indium melts, rub each blob in for several minutes with the tip of a sharp, pointed object like a tungsten carbide scribe. I think this process creates small scratches in the oxide layer and molten indium can diffuse into silicon through them. Note that silicon oxidizes even more quickly on the hot plate, so you don't want to keep it there for too long.
