Hello , your concern is right that aluminium is less conductive but it has been resolved in past from 45nm onward most fab uses copper as metal option , the only issue with gold it its melting point that can limit it from using it once we can find a solution  which can remove this issue hen gold can be one option from technical point .

Now coming to financial perspective , as we are using copper  other good conductor then would replacing it with gold can give good performance over cost ????

The usage of gold is generally avoided in semiconductor technology, since it acts as a deep-level trap and recombination center, i.e. charge carriers of opposite sign do recombine at Au defects in Si and get lost to the current. Gold is also effectively diffusing from a deposited thin film into the underlying substrates even at moderate temperatures. This makes gold rather poisonous to semiconductors. More information can be found in textbooks on semiconductors or on Wikipedia “Carrier recombination”. However, gold layers are often applied to immobilize organic molecules and biomolecules on solid supports via thiol bonds. If semiconductors are used as substrates for bioelectronics applications, for instance, special precautions have to be taken to block the Au diffusion.

One other important point to favor aluminum is that its adhesion to silicon is  strong because of the formation of a common very thin oxide layer. On an annealing for forming more strong contact, it forms micro alloy with heavily doped p type layer acting as an ohmic contact.

Gold is very inert and donot form strong contact except when it forms alloys. It is also fast diffuser and build recombination centers reducing the minority carrier lifetime.

In the semiconductor industry gold is used a life time killer for the fast recovery diode.

It is also expensive.

The high conductivity is required in the current carrying fine interconnects and not in  metal lines where the signals currents is small. Cupper is superior than gold as it forms strong contacts because its affinity to oxygen.

However gold wires are used to interconnect the chips to the metal frame legs in the frst level assembly of integrated circuits. The main property is that it builds a bond ball when it is subjected to the hydrogen flame. Aluminum wires can be used for thermal compression bonding interconnection too, but they normally form wedges not balls.This link may be useful:http://www.lbma.org.uk/assets/3c_c_itabashi_lbmaconf2004.pdf

Best wihes

Dear colleagues,

I saw it useful to complement the subject of metals used in integrated circuit with copper. A copper has greater conductivity than aluminum it use will bring enhancement to the integrated circuits especially when the interconnect lines gets smaller because of calling the dimensions of the transistors. It i found that copper requires special patterning and shaping process. They succeeded by the so called 

Damascene process. Please for more interesting information on copper metallization,

please see the white paper in the link: http://www.advanced-energy.com/upload/File/White_Papers/SL-ELECTROPLATING-270-01.pdf

I hope you found it interesting.

Best wishes