Microstructure control in semiconductor metallization

The microstructure of semiconductor metallization is becoming increasingly important as linewidths decrease below 0.5 μm. At these dimensions, reliability and performance are greatly influenced by specific microstructural features rather than only by the average material properties. In this article, we address the prospects for controlling the microstructure of thin film interconnection metals as linewidths are predicted to decrease below 0.1 μm by the year 2010. First, we evaluate the sources of energy available to drive microstructure changes in thin films, both during and after deposition. The internal energy sources considered are grain boundaries, interfaces, surfaces, strain, solidification, crystallization, solute precipitation, and phase transformations, with energy densities ranging from less than 1 meV/atom to greater than 100 meV/atom. The external energy sources considered are particle bombardment during deposition, mechanical deformation, and radiation damage, which may deliver energies greater than 100 eV/atom. Second, we review examples of microstructure changes in terms of these energy sources. These examples include the dependence of Al–Cu and Ti fiber texture on the roughness of SiO2, orientation change and abnormal Cu grain growth coupled to the precipitation of Co in Cu–Co alloys, and in-plane orientation selection during phase transformation of TiSi2 in very narrow lines. A substantial degree of microstructure control is also achieved in films deposited with off-normal incidence energetic particle bombardment, which has been used to produce both in-plane and out-of-plane crystallographic orientations in metals (Mo, Nb), nitrides (AlN), and oxides (ZrO2). Drawing on these examples, we discuss the prospects for microstructure control in future semiconductor metallization with respect to the list of energy sources, the decreasing dimensions, and the changing fabrication processes. One mechanism in particular, discontinuous precipitation of supersaturated solute atoms, is highlighted as having a substantial amount of stored energy available to drive microstructure evolution, and may provide a means to more fully control the microstructure of semiconductor metallization.