Silicon contact for area reduction of integrated circuits

In the design of integrated circuits, the contacts are made smaller than the diffused semiconductor area. The reason for the smaller contact area is to avoid the contact of the metal to areas of opposite conductivity type to that of diffused area when misalignment occurs. Because aluminum is universally used as the interconnecting metal and the aluminum work function is comparable to that of silicon, an alloyed aluminum contact makes good ohmic connection with p-type silicon, but a leaky reverse junction with the n-type silicon. For n-channel MOS transistors, the situation is even worse because the alloyed aluminum in contact with p-type substrate would cause a short circuit.