IAN LANG ELECTRONICS
On the left is another picture from Intel showing a testing machine. The individual probes are tooled to make contact with the terminal pads of an IC and a computer is used to apply a sequence of signals to the circuit and analyse the response. Any response that falls outside the pre-determined parameters causes that circuit to be marked with ink, so that it can be discarded later. Each circuit on the slice is tested and as may be imagined this takes some time if there are many small ones on the slice. In the manufacture of ICs a rate of rejection as high as 70% is not considered unusual- a reflection on the precision of engineering needed to fabricate them.
The wafer containing dozens of chips must now be split down into individual ones. This is done by a technique known as scribing and breaking and is carried out by another precision machine.
The wafer is first scratched by a very fine diamond tip scalpel along the lines between the circuits which causes a deep score. The wafer can then be placed on a yielding surface such as a rubber pad and gently compressed, whereupon it breaks into the individual circuits. Those marked with ink are known to be faulty and so are discarded.
The circuits that are not faulty go on to the next stage, for what we have made here is known technically as a die. To be any use in applications it needs to be mounted and enclosed. This is the next part of our study.