Mechanical Stress in VLSI Interconnections: Origins, Effects, Measurement, and Modeling
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detection. If a problem is not found until late in the product cycle, the economic consequences may be severe. To avoid costly late detection of problems and frantic scrambles for fixes, it is essential to have a clear understanding of the various origins of mechanical stress, the behavior of various materials and interfaces under stress, and the potential failure mechanisms. With this understanding and adequate modeling capability, mechanical stress effects can be included in the design process, and the likelihood of unpleasant surprises can be reduced.
Origins of Mechanical Stress Many factors contribute to mechanical stress in interconnection structures.1 The most obvious, but far from the only one, is the thermal-expansion difference between the silicon substrates and the various interconnection materials. The silicon acts as a Procrustean bed. Since it is two orders of magnitude thicker than any of the structures on the device, the structures must change dimensions to match the silicon when it is heated or cooled during processing or in service. Materials with thermal expansion larger than that of silicon will be forced into compression on heating and into tension on cooling. Those with thermal expansion less than that of silicon show the opposite effect. A second factor contributing to stress,
and one often larger than the thermalexpansion effect, is deposition under nonequilibrium conditions. This is a factor of increasing importance, as the reduced thermal budget requires deposition at ever lower temperatures and increased deposition rates. Whenever the conditions are such that a new layer of film is deposited before the underlying atoms have had time to diffuse into their equilibrium positions, an "intrinsic" stress will be produced in the film. This stress may be either tensile or compressive, depending on the details or the process. Chemical-vapor-deposition processes involving the escape of reaction products can result in tensile stress if the products escape but the remaining atoms lack the mobility to fill in the gaps properly. This is typically the case for CVD tungsten where the deposition conditions for good step coverage require a low deposition temperature and the tungsten atoms are relatively immobile. Chemical-vapor-deposition processes can also result in intrinsic compressive stress; for example, it occurs in plasma nitrides and oxynitrides as a consequence of hydrogen implanted during the deposition. Sputtered materials may also have high intrinsic stresses if the substrate temperature is low relative to the melting point of the material so that equilibrium structures are not attained. The stress in sputtered silica can be highly tensile; that in sputtered tungsten is normally highly compressive. Changes in chemical composition also result in changes in stress. Silica-based glasses, for example, can absorb relatively large amounts of water from moist air and can develop highly compressive stresses. The water can be driven off by heating, resulting in tensile stress. Excess hydrogen can
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