Copper technology

Starting at feature sizes below 250 nm, aluminum, even with copper additions, can barely meet the requirements needed for use in integrated circuits any longer. The resistivity of copper, at 1.7 μΩ·cm, is about a third lower than that of aluminum at 2.7 μΩ·cm. For the same cross-section, less voltage therefore drops across a copper interconnect, it heats up less, and signals travel faster. Stress and electromigration are also considerably more pronounced in aluminum: copper has the higher melting point and more strongly bound atoms, so it tolerates significantly higher current densities. Given the continuous miniaturization, a switch to copper was therefore unavoidable.

Copper, however, has the negative property of contaminating almost everything it comes into contact with. In silicon it is extraordinarily mobile and creates defects there that trap charge carriers and render devices unusable. The areas and equipment in manufacturing where copper is processed must therefore be strictly separated from the others, and every copper interconnect is enclosed by a diffusion barrier. In addition, copper corrodes very easily and, like aluminum, must therefore be sealed with a passivation layer.

While in aluminum technology the vias between layers are made with tungsten, in copper technology the metal itself is used for this purpose (only the very first layer, at the contact to the doped silicon regions, requires separation with tungsten). This not only eliminates the negative thermoelectric effects that arise at the junctions between different metal layers, but also the additional process steps needed to deposit multiple layers. Unlike aluminum, however, copper cannot be patterned by dry etching, because its halogen compounds are not volatile at process temperature.

The "ordinary," subtractive process for patterning a layer, as is also used for aluminum, essentially proceeds through the following steps:

• deposit the layer to be patterned
• apply photoresist, expose, and develop it
• transfer the resist mask into the underlying layer in an etch step
• remove the resist
• deposit the passivation layer

For copper, an additive process is used instead: the so-called damascene process.