1. Requirements on metallization
In metallization, contacts are made to the doped regions of semiconductor devices, and these are connected by interconnects. From there, the connections are routed to the edge of the individual chips in order to establish the connection to the package or for testing the circuit with probes during manufacturing.
What requirements must metallization layers meet in order to be used in semiconductor technology?
- good adhesion to silicon dioxide
- high current-carrying capacity, low electrical resistance
- low contact resistance between metal and semiconductor
- a process for depositing the conductive layer that is as simple as possible
- low susceptibility to corrosion for a long chip lifetime
- good contactability
- the possibility of multilayer wiring to save chip area
- high purity of the material
- resistance to electromigration at high current densities
- compatibility with chemical mechanical polishing, which is used to planarize every layer
- no diffusion into the surrounding insulation layers or into the silicon
The last three points have only become important with small structures, and it is precisely these that have driven the change of material. Aluminum meets the classic requirements well and was the standard material for decades. However, as interconnect cross-sections shrink, current densities increase, and aluminum migrates under this stress. Since the late 1990s, copper has therefore been the material of choice for wiring in high-performance circuits. This does not mean aluminum has disappeared: bond pads, power and analog devices, as well as non-critical layers, continue to be made with it.