Metallization of silicon substrates with aluminum or alloys thereof, i.e. Al-Si or Al-Si-Cu, is commonplace in the manufacture of integrated circuits. The appearance of large hillocks in the surface of layers of aluminum metallization during metal deposition and/or patterning is relatively unusual. This is generally true for conventional deposition techniques, such as e-beam evaporation and magnetron sputtering.
It is conventional to heat silicon wafers after deposition of aluminum metallization to alloy the aluminum to the silicon substrate. Typically, the substrates are heated in a furnace to a temperature in excess of 400.degree. C. for about thirty minutes. The alloying procedure substantially lowers the contact resistance between the aluminum metallization and the substrate and is necessary for the former to function, e.g., as interconnects in complex IC devices such as very-large-scale-integrated circuits, (VLSI).
Among the detrimental effects of conventional furnace alloying are loss of diode integrity and hillock formation. The former is a particular manifestation of the phenomenon known as "spiking". Spiking is caused by the silicon in the substrate being dissolved into the overlying aluminum film. This does not occur uniformly across the contact area, but at numerous discrete sites and results in downward projections of metal having, generally, the configeration of inverted pyramids. When one or more of these aluminum "spikes" reaches a subsurface active or passive device, such as a shallow p.sup.+ n junction, the diode effect is lost. Spiking can result from either extended heating or heating to too high a temperature.
Hillocks are the result of the aluminum metallization being in compression during conventional alloying due to its high coefficient of thermal expansion, combined with being softened as a result of being heated to a temperature approaching its melting point of 660.degree. C. This combination of stress and softening acts over residence time in the furnace to cause stress relief which is manifested as hillocks. The hillocks are typically about one micrometer in height and generally occur in densities of about 10.sup.5 /cm.sup.2. Hillocks create both circuit yield and reliability problems resulting from an undermining of overlying passivation or interlevel-dielectric layers.
The rapid alloying of aluminum metallization in only a few seconds with pulsed, high intensity visible light has been reported in the literature. This is an example of a heating means capable of supplying a pulse of energy to heat the substrate to alloying temperature within a few seconds. Another example of such heating means is graphite strip heating wherein a strip of graphite is heated to a high temperature, shielded and brought into close proximetry to the structure to be heated. The shield is removed for a few seconds, thus rapidly heating the substrate. Such rapid alloying is reported to improve contact resistance and virtually eliminate spiking. Although not reported in the literature, such rapid alloying also eliminates hillock formation resulting in consistently smooth surfaces. In accordance with this invention, a method has been found to utilize the rapid heating technology to produce a further, unexpected result.