Aluminum for Integrated Circuit Interconnects

Aluminum for Integrated Circuit Interconnects:
From Materials Science and Engineering By William D. Callister, Jr. (Very good Book)

The heart of all computers and other electronic devices is the integrated circuit (or IC).Each integrated circuit chip is a thin square wafer having dimensions on the order of 6 mm by 6 mm by 0.4 mm; furthermore, literally millions of interconnected electronic components and circuits are embedded in one of the chip faces. The base material for ICs is silicon, to which has been added very specific and extremely minute and controlled concentrations of impurities that are confined to very small and localized regions. For some ICs, the impurities are added using high-temperature diffusion heat treatments.

One important step in the IC fabrication process is the deposition of very thin and narrow conducting circuit paths to facilitate the passage of current from one device to another; these paths are called “interconnects,” and several are shown in Figure 1 (left), a scanning electron micrograph of an IC chip. Of course the material to be used for interconnects must have a high electrical conductivity— a metal, since, of all materials, metals have the highest conductivities. Table 1 cites values for silver, copper, gold, and aluminum, the most conductive metals.

Metal (Symbol)	Electrical Conductivity (Ω-mt)-1(@ room Temp)
Silver(Ag)	6.8 X  107
Copper (Cu)	6.0 X  107
Gold (Au)	4.3 X  107
Aluminum (Al)	3.8 X  107

On the basis of these conductivities, and discounting material cost, Ag is the metal of choice, followed by Cu, Au, and Al. Once these interconnects have been deposited, it is still necessary to subject the IC chip to other heat treatments, which may run as high as 500 ^oC. If, during these treatments, there is significant diffusion of the interconnect metal into the silicon, the electrical functionality of the IC will be destroyed. Thus, since the extent of diffusion is dependent on the magnitude of the diffusion coefficient, it is necessary to select an interconnect metal that has a small value of D in silicon. 

Figure 2. plots the logarithm of D versus 1/T for the diffusion, into silicon, of copper, gold, silver, and aluminum. Also, a dashed vertical line has been constructed at 500 ^oC, from which values of D, for the four metals are noted at this temperature. Here it may be seen that the diffusion coefficient for aluminum in silicon (2.5 X 10²¹ m²/s) is at least four orders of magnitude (i.e., a factor of 10⁴) lower than the values for the other three metals. Aluminum is indeed used for interconnects in some integrated circuits; even though its electrical conductivity is slightly lower than the values for silver, copper, and gold, its extremely low diffusion coefficient makes it the material of choice for this application. An aluminum-copper-silicon alloy (Al-4 wt% Cu-1.5 wt% Si) is sometimes also used for interconnects; it not only bonds easily to the surface of the chip, but is also more corrosion resistant than pure aluminum. More recently, copper interconnects have also been used. However, it is first necessary to deposit a very thin layer of tantalum or tantalum nitride beneath the copper, which acts as a barrier to deter diffusion of Cu into the silicon.