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Contact Resistance
There is a small resistance between the two materials. This resistance is called the "contact resistance" Rc.
Rc = r c/A where r c specific contact resistance W - cm2. A is the area of the contact.
Electromigration
Electromigration is the movement of atoms in a metal film due to momentum transfer from the electrons carrying the current. Under high-current-density conditions, metal-atom movement causes voids in some regions and metal pileup or hillocks in other regions. Voids can become open circuits and pileups short circuits, resulting in contact failure.
where Ea activation energy (0.4 to 0.5 for Al)
Electromigration can be reduced by adding copper (Cu) to the metal system -- 95% Al 4% Cu 1% Si
Summary of Silicon technology interconnects
| Type | Advantages/Disadvantages |
| Diffusion of n+ layer | relatively high sheet resistance (short or local interconnections) |
| Polysilicon | relatively high sheet resistance (short or local interconnections) |
| Metal silicides or polysilicon or diffusion layers | reduce sheet resistance |
| Ohmic contacts between Al-Si | low contact resistance tungsten and titanium reduces spiking Cu (Copper) reduces electromigration |
| Multilevel metal process | more complex |
| Heterojunction contacts | used in III-V systems like GaAs; example Ge GaAs |