Mechanical Properties of Low Dielectric-Constant Organic-Inorganic Hybrids
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ABSTRACT Spin-on glasses, generated by the condensation of an organic-inorganic hybrid silsesquioxane (SSQ), have great potential as low dielectric-constant semiconductor interconnection materials. After curing and condensation SSQ materials consist of an amorphous, inorganic, -Si-O-Sibridging network with organic, non-bridging -Si-R side groups. Relative dielectric constants in the range 2.5-3.3 are obtained for SSQ materials, depending on the curing conditions, and compare with 4.0 for conventionally-used fused silica. The non-bridging side groups significantly disrupt the SSQ network-occupying more than 25% of the Si bonds-and lead to materials that are considerably less stiff, hard and tough than fused silica. Perhaps more importantly, SSQ materials have thermal expansion coefficients greater than that of the intended Si substrate and therefore finish curing in a state of residual tension, leading to a susceptibility to stress-corrosion cracking. In this paper the development of thermomechanical properties during curing of SSQ spin-on glasses is considered and related to the driving force for film cracking deriving from the residual tension. Various crack suppression schemes involving mechanisms both intrinsic and extrinsic to the base SSQ are discussed. INTRODUCTION The performance of ultra-large scale integrated circuits depends on both the speed with which information can be processed through the active logic elements fabricated in the semiconductor substrate and the speed at which information is transferred between the elements by the interconnection structure constructed above the substrate. The interconnection structure consists of a three-dimensional network of fine (sub-jLm) conductive lines (usually Al or Cu) supported by an insulating dielectric (usually Si0 2 ). As well as providing support and constraint for the conductive lines during mechanical, thermal and electrical loadings generated in processing and in use, the insulating "scaffold" also provides a chemical barrier for diffusion of reactive species between the lines and from the external environment. A major additional requirement for interconnection structures with improved performance at decreased dimensions is a reduction in the dielectric constant of the insulator [1, 2]. Such a reduction (i) minimizes the capacitance between lines and ground planes, thereby minimizing the "RC'-time constant characterizing the delay of signals travelling along lines; (ii) minimizes the power requirements of the circuitry; and, perhaps most importantly, (iii) minimizes the extent of the fringing fields extending laterally from lines, thereby minimizing the capacitive "cross-talk" between lines. Low dielectric-constant candidates to replace the conventionally-used chemical vapor deposited (CVD) Si0 2 interconnection insulator are silsesquioxane spin-on glasses (SSQ SOG). These materials have many advantages as advanced interconnection dielectrics. First, they can be processed to yield thin, planar films with low dielectric constant in the range k = 2.5-3.3,
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