Processing dependent properties of silica xerogels for interlayer dielectric applications

One of the current and near future research focus in microelectronics is to integrate copper with a new low dielectric constant (K) material. The traditional low K is dense SiO₂ (K = 4). Introducing porosity in materials with silica backbone is promising as processing and integration methods are well known. This thesis focuses on studying silica xerogel, also known as nanoporous silica. A new low-K material has to be tested for an array of electrical, mechanical, thermal, and chemical properties before it is deemed successful to replace dense SiO₂. These properties of silica xerogels are characterized using various analytical techniques and the effect of processing conditions is studied. The property data is explained by the models and mechanisms relating processing-structure-property behavior. The processing effects on thermal and mechanical properties are studied in great detail and the theories for generic porous low-K materials are developed.; The xerogel films are processed at ambient conditions and crack free, thick (0.5–4 μm), highly porous (∼25–90%) films are obtained. Two methods of porosity control were used. One is the traditional single solvent (ethanol) method and another is a binary solvent (mixture of ethanol and ethylene glycol) method. The films underwent aging and silylation procedures to make the backbone stiff and hydrophobic. Sintering of xerogel films eliminates defects and organics and additional condensation reactions make matrix more connected, dense and ordered. Films were characterized for their refractive index, thickness, porosity, pore size and surface roughness. Dielectric constant measurements at 1 MHz show that K varies linearly with porosity. Dielectric loss tangents are low and breakdown strength meets the standards. FTIR and XPS analysis show that films are stable chemically and remain hydrophobic even after boiling in water.; Mechanical and thermal properties of porous materials are dependent on the microstructure and various models are available to explain microstructural effects. The cellular models which represent foam type microstructure explain xerogels' elastic modulus variation with density. The modulus measured by nanoindentation increases as power law with density and is affected by the method of processing. By comparing xerogels that were sintered, templated or made with ethylene glycol or ethanol as solvents, we show that process history is at least as important as the chemistry of the solid matrix or the porosity. The modulus extrapolated to zero porosity for porous sintered films is same as those of the dense films made by CVD of SiO₂. This suggests that the solid matrix for sintered xerogel films is close to ideal and their modulus is better because of ordered arrangement of pores and fusion of particles making up the matrix. (Abstract shortened by UMI.)...