Connecting two metal layers without breaks
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Suppose you need thick metal wires for bonding or probes, connected to a thin metal layer connecting to a device. The thick metal might be 100 nm of gold, used for both alignment marks and for wire bonding. The second layer of thin metal (for narrow lines) might be 10 nm thick aluminum or gold. If you connect the two layers with a simple overlap, then there is a good chance that the thin metal will break over the edge of the thick layer. Perhaps it will not break everywhere, but you can never get the evaporation angle to be exactly perpendicular to the surface, and so some of those connections will break.
One solution is simply to put the thin metal on the bottom. Unfortunately, the thin metal cannot be used for e-beam alignment marks, since the backscatter signal would be too weak. You would need to fabricate a third layer just for the alignment marks. To avoid this extra work, we have a simple trick: create a connection as shown here - a pattern called a "keyhole" or "lollipop". Then the second, thin layer will always have a place where the angle is good. The pattern can be a lollipop or an inverted lollipop, or really any pattern that includes all angles for the evaporation.
I don't know who first invented this clever trick. If you do, then please let me know so I can give them credit here.
If the first metal is aluminum, then you will encounter the additional problem that the aluminum will oxidize upon removal from the evaporator. To get a reliable connection to the second metal, you can start with an oxygen-loving metal like chromium or titanium, thereby breaking up the thin aluminum oxide. Alternatively, you might use in-situ ion milling just before evaporating the second metal. That would be rather tricky. Also: the first aluminum layer would have to be very thick to make a good alignment mark (about 1 um) and so it was a poor choice anyway. If your lab is infected with gold-phobia, you could use niobium instead. Niobium will have the same oxidation problem as aluminum, which can be solved the same way - by using titanium or chromium under the top metal.
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