Refractory metals such as tantalum and titanium have been used as diffusion barriers, adhesion layers or X-ray lithography masks in electronic applications. Unfortunately, these materials oxidize easily when annealed causing reliability problems due to the increased resistivity and stress. Other refractory metals such as Nb, V, Zr, Hf, Cr, Mo and W also oxidize easily in annealing ambients containing minimal amounts of oxygen and at relatively low temperatures. It is an expensive and difficult process to remove all oxygen from an annealing environment and some processes actually require an oxidizing ambient. Small amounts of oxygen incorporated interstitially in the refractory metals can cause large increases in compressive stress. For instance, in a Ta film that has 10% oxygen incorporation, an increase of 4 to 5 GPa in compressive stress and an increase of 63 .mu..OMEGA.-cm in resistivity have been observed. When used in electronic applications, such increases in resistivity and stress can cause major reliability problems such as poor contact resistance, peeling and cracking.
Other researchers have utilized refractory metals in electronic applications as diffusion barriers, i.e., Ta diffusion barrier between Si and Cu was used by Holloway, et al., J. Appl. Phys., 71 (11), 5433 (1992). The use of Ta diffusion barrier between a high dielectric material and Si in the fabrication of a high dielectric oxide material has been shown in Grill, et al., J. Mater. Res., 7 (12), 3260 (1992). In the latter work, the fabrication of high dielectric constant oxide materials requires high temperature processing (&gt;650.degree. C.) in oxygen ambients which generally cause the underlying Si to oxidize, and thus creating an additional in-series low dielectric capacitor.
Other diffusion barrier materials such as TiN, WN and TaN have been used in VLSI applications, i.e., in contact holes for separating conductors from underlying silicon. However, these materials are not suitable oxygen diffusion barriers because they cannot withstand oxidation anneal cycles that the devices may be subjected to. In addition, other researchers have investigated elemental metals such as Pt, Au and Ru for the prevention of diffusion of oxygen to the underlying layer of silicon and its subsequent oxidation. It was found that none of the pure metals prevented the diffusion and the resulting SiO.sub.2 formation. A break in the electrical conduction path to the silicon substrate occurred as a result.
In certain semiconductor processes, high dielectric constant Perovskite compounds (such as PZT, PLZT or Ba.sub.x Sr.sub.1-x TiO.sub.3) are deposited onto a substrate. These materials require high temperature (&gt;650.degree. C.) oxygen anneal in order to crystallize. It also requires a Pt seed layer for crystallization into a Perovskite phase which has the highest dielectric constant, i.e., 380 vs. about 40. For simple process integration, it is desirable to use Si as the lower electrode. A problem thus incurred during an oxygen anneal of the Perovskite compound/Pt/Si structure is the formation of an SiO.sub.2 layer at the Pt/Si interface which reduces the effective dielectric constant. A diffusion barrier layer is therefore needed for preventing oxygen diffusion down to the Si layer, and also for preventing the Si diffusion up to the Pt layer.
It is therefore an object of the present invention to provide a diffusion barrier for use in a semiconductor device that does not have the shortcomings of the prior art diffusion barriers.
It is another object of the present invention to provide a diffusion barrier that can be fabricated in a simple manufacturing process.
It is a further object of the present invention to provide a diffusion barrier that is capable of sustaining the high processing temperatures encountered in semiconductor processing steps.
It is another further object of the present invention to provide an oxygen diffusion barrier for devices incorporating high dielectric constant oxide layers.
It is yet another object of the present invention to provide an oxygen diffusion barrier of a multi-layer aluminum/refractory metal material such that the refractory metal layer does not oxidize (remains electrically conducting) in a subsequent annealing step.
It is still another object of the present invention to provide an oxygen diffusion barrier that is a multi-layer thin film of aluminum/refractory metal such that when the film is exposed to atmosphere a thin layer of Al.sub.2 O.sub.3 forms on the surface of aluminum.