Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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178 • Chapter 6 / Diffusion

MATERIAL OF IMPORTANCE

Aluminum for Integrated Circuit Interconnects

T

he heart of all computers and other electronic

devices is theintegrated circuit(orIC).^4 Each

integrated circuit chip is a thin square wafer hav-

ing dimensions on the order of 6 mm by 6 mm by

0.4 mm; furthermore, literally millions of intercon-

nected electronic components and circuits are em-

bedded 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 con-

centrations of impurities that are confined to very

small and localized regions. For some ICs, the im-

purities are added using high-temperature diffu-

sion heat treatments.

One important step in the IC fabrication pro-

cess is the deposition of very thin and narrow con-

ducting circuit paths to facilitate the passage of cur-

rent from one device to another; these paths are

called “interconnects,” and several are shown in

Figure 6.9, a scanning electron micrograph of an IC

chip. Of course the material to be used for intercon-

nects must have a high electrical conductivity—a

metal, since, of all materials, metals have the high-

est conductivities. Table 6.3 cites values for silver,

copper, gold, and aluminum, the most conductive

metals. On the basis of these conductivities, and

Interconnects

Figure 6.9 Scanning electron micrograph of an inte-

grated circuit chip, on which is noted aluminum inter-

connect regions. Approximately 2000×. (Photograph

courtesy of National Semiconductor Corporation.)

Table 6.3 Room-Temperature Electrical

Conductivity Values for Silver,

Copper, Gold, and Aluminum (the

Four Most Conductive Metals)

Electrical Conductivity

Metal [(ohm-meters)−^1 ]

Silver 6.8× 107

Copper 6.0× 107

Gold 4.3× 107

Aluminum 3.8× 107

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◦C. If,

during these treatments, there is significant diffu-

sion 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 nec-

essary to select an interconnect metal that has a

small value ofDin silicon. Figure 6.10 plots the

10 –12

10 ^14

10 ^18

10 ^16

10 ^20

10 ^22

Diffusion coefficient (m

2 /s)

0.6 0.8 1.0 1.2 1.4

Reciprocal temperature (1000/K)

1200 1000 900 800 700 600 500 400

Temperature (°C)

Cu in Si

Au in Si

Ag in Si

Al in Si

2.5 × 10 ^15

4.2 × 10 ^17

2.5 × 10 ^21

4 × 10 ^13

Figure 6.10 Logarithm ofD-versus-1/T(K) curves

(lines) for the diffusion of copper, gold, silver, and

aluminum in silicon. Also noted areDvalues at 500◦C.

(^4) Integrated circuits, their components and materials, are discussed in Section 12.15 and Web Module E.