Copper electromigration

FIGURE 17.39 Ta erosion after copper CMP. The Ta in the field area is not completely eroded. Inside the area of fine copper line, the Ta is locally completely eroded. The arrays of very fine lines have an uneven coloration with stained appearance. This type of Ta erosion can be confused with copper residues. It is possible to have both Ta erosion and copper residues simultaneously as shown in the bottom picture. In arrays of very fine copper lines, SEM and AFM are needed to remove the doubt. 

FIGURE 17.41 A large scratch in copper. The left picture shows a rather large scratch that has also marked the Ta between the copper lines. Both pictures show that a scratch marks the copper deeper than the surrounding Ta. 

On patterned copper wafers, after CMP, the surfaces are covered mainly by dielectric and copper features. The large scratches on the dielectric such as TEOS oxide will have similar shatter mark characteristics as described in Section 17.2. The scratches on the copper lines or features, however, have a very different signature. As the copper is a soft material with large plastic deformation area, it is very easy to scratch copper (Fig. 17.41). The scratches on copper usually show well-defined continuous lines. A copper scratch can be very shallow and very narrow (Fig. 17.42). It is worthwhile to point out that the extent of damage by scratch is also a function of the underlying dielectric. As a low-fe dielectric is usually much more fragile than silicon dioxide, the damage on copper lines with low-fc dielectric may be more severe (Fig. 17.43). 

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To mitigate the problem, a diffusion barrier is incorporated between the aluminum and the silicon (see Sec. 5 below). It is also possible to replace aluminum by alloys of aluminum and copper or aluminum and silicon, which have less tendency to electromigration. These alloys are usually deposited by bias sputtering. However, they offer only a temporary solution as electromigration will still occur as greater densities of circuit elements are introduced. It was recently determined that improvements in the deposition of aluminum by MOCVD at low temperature with a dimethyl aluminum hydride precursor may reduce the problem.bl... 

The electromigration problem has led to the investigation of other electrical-conductor materials, such as tungsten (presently used in contact and via fills) and more recently copper. 

A production process has recently been implemented by IBM. The aim was to reduce the electrical resistance of the interconnects in their chip to about one-third of the values attainable using aluminum and at the same time increasing the resistance against electromigration. This was made possible by employing electrodeposition of copper in a Damascene method. The manufacturing sequence is presented in Figure 17.11. 

Copper additions to aluminum films enhance electromigration resistance. However, copper does not form volatile chlorides or other halides, and therefore its removal during aluminum plasma etching is difficult. Two methods can be used to promote copper chloride desorption increase the... 

The final metallization of the standard single-layer metal conductor circuits has been provided by sputtered aluminum. As required, the sputtered Al can be doped with Si to minimize spiking of Al into the Si that it must contact. It can also be doped with copper to minimize electromigration effects. 

Again, in the previous chapter we saw that deposition of thermal CVD aluminum films is possible. Clearly, they improve the step coverage problem, since they are reasonably conformal. However, they are not completely satisfactory because of the inability to readily dope them with copper to limit electromigration, and they exhibit grain sizes that are too large. 

The first generation of the interconnect material is aluminum with a resistivity of p = 2.66 pQ cm. One approach to reduce RC delay is to switch to an interconnect material with lower resistivity as indicated by Eq. (1.1). A wide range of metals was considered as a potential candidate in the early 1990s. Gold has excellent resistance to corrosion and electromigration but its conductivity is similar to that of aluminum. Silver has the lowest resistivity (p = 1.59 pQ cm) but poor resistance to corrosion and electromigration. Hence, copper that has a resistivity of 1.67 pO cm and excellent resistance to electromigration was selected. Compared to aluminum, copper has one drawback. It cannot be deposited by RIE. Therefore, a copper interconnect is typically formed via a damascene process in which a pattern is first etched into the dielectric and overfilled with copper. The excess copper above the... 

The low-resistivity and high-electromigration properties have made copper the material of choice for the fabrication of interconnects in present-day IC... 

Tao J, Cheung NW, Hu C. Electromigration characteristics of copper intrercon-nects. IEEE Electr Dev Lett I993 I4(5) 249-251. 

Aluminum metallization 1961 Copper metallization 2001 Lower interconnect resistance electromigration... 

Deposition of copper alloys such as copper/aluminum can be carried out starting from copper(l) and copper(Il) enolates. Copper alloys may be of technological and scientific interest as they can help avoiding problems caused by electromigration, corrosion and poor adhesion to dielectric substrates. ... 

The competitor to YBaCuO in this application is copper, which has a resistivity at 77 K of 0.24 /x 2cm, compared to a value at 300 K of 1.7 p2cm< In fact at room temperature, copper alloys with even higher resisitivity are used to reduce electromigration, which is practically eliminated at the lower temperatures. Thus copper offers a reduction in resistance of more than a factor of 6 at 77 K. 

The dominant metal layer used in conventional silicon microelectronics technology is aluminum (Al), usually doped with silicon to reduce spiking and with copper to increase resistance to electromigration. Aluminum is used for electrical connections, for contacting the sensing elements, and as the standard material for bonding pads, which are needed to assure electrical connection to the outside world . 

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