Understanding the Impact of Porosity and Pore Structure in Ultra Low Dielectric Constant Organosilicate Glasses

ABSTRACTRecently there have been a number of reports indicating concern relating to the effect of porosity, pore size distribution, and pore interconnectivity on the integration of highly porous ultra low-k organosilicate glasses (OSGs) as back-end-of-line (BEOL) interconnect dielectrics. In an effort to address these concerns a number of options to control the skeleton and pore structure of OSGs have been proposed, from adding alternative OSG precursors to alternative porogen precursors. In all these options there is a need to balance pore structure modification with critical film properties such as dielectric constant and mechanical strength. In this context, this paper examines porosity and its impact on film properties for highly porous ultra low dielectric constant films. A series of PDEMS® porous OSG films were deposited by plasma enchanced chemical vapor deposition (PECVD) from DEMS® precursor (diethoxymethylsilane) and porogen ATRP (alpha-terpenine). The percent porosity and pore interconnectivity of these films relative to the dielectric constant were measured by ellipsometric porosimetry (EP) and positron annihilation spectroscopy (PALS) respectively. Porosity and pore-size distribution for films deposited using several different species (structure former or porogen precursors) were examined using EP in an effort to understand the impact of the chemical nature of the precursor on pore morphology. Results from these depositions show that it is possible to deposit films with smaller pores using alternative structure formers (ASFs) with bulky organic groups, although there are tradeoffs with respect to other film characteristics. The addition of a separate porogen (ATRP) to the ASF lowered the dielectric constant and the addition of DEMS® precursor to the ASF/ATRP mix gave the films added structural integrity and mechanical strength. Such a fundamental understanding of structure-property relationships will help support successful integration of these porous OSG films.

Bottom-up Modeling of the Elastic Properties of Organosilicate Glasses and their Relation to Composition and Network Defects

ABSTRACTOrganosilicate glasses (OSG), also known as SiCOH or carbon-doped oxide are used as low-k inter-metal dielectrics for integrated circuits. The material must fulfill two conflicting requirements: It has to have low density to reduce the dielectric constant and be mechanically stable enough to withstand mechanical stress during subsequent production steps. Experimental advances in improving their mechanical and electrical properties have not yet been theoretically examined at the ab initio level, due to the relatively large model sizes necessary for amorphous materials. We employ the density-functional based tight-binding (DFTB) method to achieve an accurate description of OSG properties at different compositions. We analyze the influence of composition and topological defects on the density and bulk modulus of non-porous OSG. We find that the dependence of density and stiffness on chemical composition is of different nature. This difference is traced to a transition between different mechanisms of elastic deformation in silica glass and in silicon hydrocarbide, which is also the reason for different sensitivity to topological defects in the two materials.

Elastic Properties of Dense Organosilicate Glasses Dependent on the C/Si Ratio

Organosilicate glasses (OSG) are of great technological interest as low-k dielectrics. We use ab initio simulations to analyze the influence of the C/Si ratio in OSG on the density and elastic properties of pore-free material to gain a detailed understanding of the origins of these properties in this class of materials. We find that the addition of carbon to amorphous silica leads to a stiffening of the material by removing the bending degrees of freedom of Si-O-Si bonds. This transition in mechanisms of stiffness, leads to a distinctly nonlinear behavior of the bulk and elastic modulus over the C/Si ratio. Experimental results do not contradict our findings, however dominant pore effects impede conclusive comparison.