1. Field of the Invention
The present invention generally relates to micro-scale features formed of polycrystalline materials, such as metallizations for microelectronic applications. More particularly, this invention relates to a method of forming metal lines so that lateral grain growth that occurs during extended heating does not cause bridging and electrical shorting.
2. Description of the Prior Art
As represented in FIG. 1, backend of the line (BEOL) interconnect metallization 10 often includes an electrically-conductive layer 12 of a polycrystalline aluminum-copper alloy that is sandwiched between a pair of diffusion barrier layers 14. The diffusion barrier layers 14 reduce the solid state diffusion rate between the conductive layer 12 and metals from surrounding metal structures, so as to improve the reliability and sheet resistance of the metallization 10. However, it has been determined that, if the metallization 10 is heated for a sufficient period, as during annealing or other subsequent thermal processing, stresses occur within the metallization 10 as a result of the barrier layers 14 constraining the thermal expansion of the conductive layer 12. Under certain conditions, these stresses can induce grain growth in the conductive layer 12 parallel to the plane of the conductive layer 12 (hereinafter termed lateral grain growth), especially if the metallization 10 is sufficiently thin (e.g., 0.25 micrometer or less), which can result in metal shorting between adjacent metallization structures. As an example, FIG. 2 represents a metal pad 16 in which lateral grain growth (i.e., grain growth in the plane of the metal pad 16) has occurred to create a metal bridge 20 that protrudes from the metal pad 16 and contacts an adjacent metal line 18. The likelihood of the metal shorting represented in FIG. 2 can be a significant yield and reliability issue for the metallization 10. While greater spacing between the metal pad 16 and line 18 would avoid metal shorting, such an option may not be feasible or practical in view of the demand for greater miniaturization of microcircuits, termed xe2x80x9cdesign shrinkxe2x80x9d in the electronics industry.
Accordingly, it would be desirable to provide a method for inhibiting or preventing bridging between fine metallization features without necessitating greater feature spacing.
The present invention provides a method of preventing or at least significantly reducing the likelihood of bridging between adjacent polycrystalline materials, and particularly to reducing electrical shorting between adjacent metallizations of a microcircuit. The invention also encompasses metallizations formed in accordance with the method. A key aspect of the invention is the determination of the mechanism by which grain growth is more likely to occur, namely, the growth of metal grains that have been sectioned by patterning to have fewer than six grain boundaries, the most thermodynamically stable condition. If constrained, as is the case with a conductive layer between a pair of diffusion barrier layers, excessive grain growth is most likely to occur in grains with fewer than six boundaries, and is predominantly lateral (two-dimensional). It has also been determined that grain growth of this type is favored by wider metallization features, such as metal pads as compared to thin metal lines (as used herein, the former includes relatively wide metal lines). By inhibiting or blocking lateral grain growth from metallization features more susceptible to such growth, the incidence of metal bridging between the features is significantly reduced or even eliminated.
The method of this invention generally entails forming a metal layer on a substrate, which is then patterned to yield at least two metal structures. A first of the metal structures has a patterned edge that is spaced apart from a patterned edge of the second metal structure, so that the first and second metal structures are electrically insulated from each other. However, as explained above, certain thermal treatments can lead to lateral grain growth along that patterned edge of the first metal structure which, if the first and second metal structures are sufficiently close and the grain growth is excessive, will create a metal bridge between the metal structures. Therefore, the present invention further provides metallization features that will prevent electrical shorting between the first and second metal structures if excessive lateral grain growth were to subsequently occur along the patterned edge of the first metal structure. Metallization features contemplated by this invention include patterning the metal layer to form a dummy metal line between and spaced apart from the first and second metal structures, patterning holes near the patterned edge of the first metal structure, forming the patterned edge of the first metal structure to have teeth that project toward the second metal structure, and forming the patterned edge of the first metal structure to be stepped so that the first metal structure has corners at opposite ends of the patterned edge that are closer to the second metal structure than the remainder of the patterned edge.
According to the present invention, the presence of a dummy metal line between the first and second metal structures permits lateral grain growth from the first metal structure to occur without any detrimental effect, since any metal bridge that might occur between the first metal structure and dummy line does not create an electrical short because the dummy metal line is not electrically connected to any electrical component of the microcircuit. Importantly, and according to the invention, there is not a significant concern for metal bridging between the dummy line and second metal structure caused by lateral grain growth of the dummy and/or second metal structure as compared to metal bridging caused by lateral grain growth from the larger first metal structure. The remaining bridge-inhibiting metallization features of this inventionxe2x80x94the patterned holes, teeth, and steps formed along the patterned edge of the first metal structurexe2x80x94serve to accommodate inward grain growth from the patterned edge of the first metal structure, as opposed to outward grain growth toward the adjacent second metal structure. The effect of each approach contemplated by the present invention is the reduction or elimination of metal bridging between closely adjacent metallization features, even where lateral grain grown has occurred.
Other objects and advantages of this invention will be better appreciated from the following detailed description.