Ohmic Contact Formation

We desire to form "ohmic" contacts between the metal and semiconductor.  True ohmic contacts would exhibit a straight-line I-V characteristic with a low value of resistance.

I-V characteristics of contacts between integrated-circuit material. (a) Ideal ohmic contact; (b) rectifying contact;
(c) practical non-linear "ohmic" contact.

In silicon technology, aluminum is the main metal contacting system used.  There are three possible types of aluminum contacts to silicon.

Three possible types of aluminum contacts to silicon. (a)  Aluminum to p-type silicon forms an ohmic contact with an I-V characteristic; (b) Aluminum to n-type silicon can form a rectifying contact (Schottky barrier diode); (c) Aluminum to n⁺ silicon yields a contact.
 

The aluminum-silicon system forms a "eutectic" (easy melt solution) Al 88.7% Si 11.3% which melts at 577 C.  This process sequence of ohmic contacts using Al-Si alloging is after all high temperature processing.  The phase diagram of the aluminum-silicon system is shown below.