1. Field of the Invention
The present invention relates to an interconnect for semiconductor devices.
2. Description of the Related Art
The trend to achieve higher packaging density and smaller chip size with increased circuit complexity requires the use of minute holes to connect circuit elements. This has necessitated the development of a new technology for embedding a material in the minute hole for interconnection. One way of achieving this object is by high-temperature sputtering of aluminum or aluminum alloy (both collectively referred to as aluminum hereinafter). This sputtering is designed to form an aluminum film while heating the semiconductor substrate at a temperature close to the melting point of aluminum, thereby keeping aluminum fluid. The molten aluminum flows into the hole for interconnection, forming a film with a smooth surface. This method offers the advantage of forming the interconnect film and embedded plug simultaneously and making use of the fact that aluminum has a low resistance. (According to the conventional method, the interconnect is formed after the tungsten plug has been formed by the selective growth of tungsten.)
Unfortunately, the above-mentioned high-temperature sputtering is not necessarily satisfactory, because it gives rise to an underlying film (2) which does not orient exactly perpendicular to the surface (1a) of the substrate (1), as shown in FIG. 1. (The direction of orientation is indicated by arrows.) Since the underlying film (2) affects the growth of the aluminum film (3) to be formed thereon, the direction of orientation of the aluminum film (3) is not perpendicular to the surface (1a) of the substrate (1). This leads to the undesirable surface morphology of the aluminum film (3). In other words, the aluminum film (3) has an irregular surface. (Hatching to signify a section is omitted from FIG. 1 for legibility.) When the aluminum film on the substrate (11) is used as an interconnect (12), the irregular surface remains on the interconnect (12), as shown in FIG. 2. This irregular surface reproduces another irregular surface on a first interlayer insulating film (13) covering the interconnect (12). The second irregular surface presents difficulties in forming on the film (13) a second layer for the interconnect (14). A second interlayer insulating film (15) covering the interconnect (14) has a more irregular surface than the first one (13). The larger the number of layers placed on top of another, the more it is difficult to form the interconnect.
The irregular surface also poses a problem in the photolithography process as illustrated in FIG. 3. There are shown a substrate (21), an aluminum film (22) having an irregular surface, and a photoresist film (23). The exposing radiation (31) penetrating the photoresist film (23) is irregularly reflected by the irregular surface of the aluminum film (22). The irregular reflection brings about halation in the cross-hatched regions. After development, halation gives rise to a greatly thinned resist pattern (not shown). (Hatching to signify the section of the photoresist film (23) is omitted from FIG. 3.)
The irregular surface poses another problem, as shown in FIG. 4. An aluminum film formed on a metal (41) has an irregular surface and hence the interconnect (42) fabricated from it fluctuates in thickness. Thus, current flowing through the interconnect (42) fluctuates, generating heat at a thin part (42a) where the current density is high. The increased current density accelerates the electromigration and eventually breaks the interconnect.