85 Metal Semiconductor Reactions

8.5.1. Introduction to Contacts

All semiconductor devices and integrated circuits require contacts to connect them to other devices and components. When a metal contacts a semiconductor surface, two types of electrical behavior can be distinguished in response to an applied voltage. In the first type, the contact behaves like a P-N junction and rectifies current. The ohmic contact, on the other hand, passes current equally as a function of voltage polarity. In Section 10.4 the electrical properties of metal-semiconductor contacts will be treated in more detail.

Contact technology has dramatically evolved since the first practical semiconductor device, the point-contact rectifier, which employed a metal whisker that was physically pressed into the semiconductor surface. Today, deposited thin films of metals and metal compounds are used, and the choice is dictated by complex considerations; not the least of these is the problem of contact

Figure 8-18. Schematic diagrams of silicide contacts in (a) bipolar and (b) MOS field effect transistor configurations. (Reprinted with permission from Ref. 17, © 1985 Annual Reviews Inc.).

instability during processing caused by mass-transport effects. For this reason, elaborate film structures are required to fulfill the electrical specifications and simultaneously defend against contact degradation. The extent of the problem can be appreciated with reference to Fig. 8-18, where both bipolar and MOS field effect transistors are schematically depicted. The operation of these devices need not concern us. What is of interest are the reasons for the Cr and metal silicide films that serve to electrically connect the Si below to the Al-Cu metal interconnections above. These bilayer structures have replaced the more obvious direct Al-Si contact, which, however, continues to be used in other applications. Contact reactions between A1 and Si are interesting metallurgi-cally and provide a good pedagogical vehicle for applying previously developed concepts of mass transport. A discussion of this follows. Means of minimizing Al-Si reactions through intervening metal silicide and diffusion-barrier films will then be reviewed.

Nature has endowed us with two remarkable elements: Al and Si. Together with oxygen, they are the most abundant elements on earth. It was their destiny to be brought together in the minutest of quantities to make the computer age possible. Individually, each element is uniquely suited to perform its intended function in a device, but together they combine to form unstable contacts. In addition to creating either a rectifying barrier or ohmic contact, they form a diffusion couple where the extent of reaction is determined by the phase diagram and mass-transport kinetics. The processing of deposited Al films for contacts typically includes a 400 °C heat treatment. This enables the Al to reduce the very thin native insulating Si02 film and "sinter" to Si, thereby lowering the contact resistance. Reference to the Al-Si phase diagram (Fig. 1-13) shows that at this temperature Si dissolves in Al to the extent of about 0.3 wt%. During sintering, Si from the substrate diffuses into the Al via GB paths in order to satisfy the solubility requirement. Simultaneously, Al migrates into the Si by diffusion in the opposite direction. As shown by the sequence of events in Fig. 8-19, local diffusion couples are first activated at several sites within the contact area. When enough Al penetrates at one point, the underlying P-N junction is shorted by a conducting metal filament, and junction "spiking" or "spearing" is said to occur.

Junction Spiking
Figure 8-19. Schematic sequence of Al-Si interdiffusion reactions leading to junction spiking.
Junction Spiking

because of the potential reaction to form a silicide as well as oxide; an example is 3Ti + 2Si02 -♦ TiSi2 + 2Ti02. For reliable device performance, the foregoing considerations have led to the adoption of poly-Si films as the gate electrode. Although there is now no driving force promoting reaction between Si and Si02, the chronic problem of Si-Al interdiffiision has re-emerged. The A1 interconnections must still make contact to the gate electrode. To make matters worse, reaction of A1 with poly Si is even more rapid than with single-crystal Si because of the presence of GBs. The dramatic alteration in the structure and composition in the Al-poly-Si-layered films following thermal treatment is shown schematically in Fig. 8-20. Reactions similar to those previously described for the Al-Si contact occur, and resultant changes are sensitive to the ratio of film thicknesses. It is easy to see why electrical properties would also be affected. Therefore, intervening silicide films and diffusion barriers must once again be relied on to separate A1 from Si.


8.6.1. Metal Silicides

In the course of developing silicides for use in contact applications, a great deal of fundamental research has been conducted on the reactions between thin metal films and single-crystal Si. Among the issues and questions addressed by these investigations are the following:

1. Which silicide compounds form?

2. What is the time and temperature dependence of metal silicide formation?

3. What atomic mass-transport mechanisms are operative during silicide formation? Which of the two diffusing species migrates more rapidly?

4. When the phase diagram indicates a number of different stable silicide compounds, which form preferentially and in what reaction sequence?

Virtually all thin-film characterization and measurement tools have been employed at one time or another in studying these aspects of silicide formation. In particular, RBS methods have probably played the major role in shaping our understanding of metal-silicon reactions by revealing compound stoichiometrics, layer thicknesses, and the moving specie. Examples of the spectra obtained and their interpretation have been discussed previously. (See Section 6.4.7).

A summary of kinetic data obtained in silicide compounds formed with near-noble, transition, and refractory metals is contained in Table 8-2. This

Table 8-2. Silicide Formation
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