Ravi Saxena

Mechanism of Cu diffusion in porous low-κ dielectrics

This work is aimed at understanding the nature of the interactions between metal interconnects and nanoporous dielectrics in integrated circuits. Electrical testing of metal-insulator-semiconductor capacitors is used to assess Cu diffusion and charge injection in the dielectric in the presence of an electric field. We show that the effect of surface chemistry in Cu diffusion is stronger than the effect of porosity. The surface chemistry or the amount of organic content in a porous dielectric depends on the pore morphology and it is limited by steric hindrance during dielectric fabrication. Furthermore, we show that the reinclusion of organic groups in a damaged dielectric results in an increasing resistance to Cu diffusion. We propose that a combination of moisture-related species in the dielectric and interfacial oxygen oxidizes Cu. The copper oxide acts as a source for Cu ions available for diffusion. A quantitative analysis of Cu drift in nanoporous dielectrics that shows the importance of surface chem...

Polymer Penetration and Pore Sealing in Nanoporous Silica by CHF3 Plasma Exposure

The polymerization and pore sealing that occurs during fluorocarbon plasma treatment of nanoporous silica xerogel was investigated experimentally by Rutherford backscattering spectroscopy and successfully modeled using a diffusion-reaction analysis. CHF 3 was used as a reactant gas to expedite the rate of polymerization due to the presence of hydrogen in its structure and its low C/F ratio. Knudsen diffusion was assumed to be the dominant mechanism for the motion of polymer precursor species through the nanoporous material over the range of pressures used in the plasma experiments. The amount of fluorine atoms deposited on the sidewalls of the pores was measured as a function of depth in the dielectric film and that amount was assumed to correspond with the mass of the polymer layer formed inside the pores. By applying a Thiele-type analysis to this system, we successfully matched model calculations with measured fluorine amounts, predicted the time required to reach a steady-state concentration of the polymer precursor in a pore (∼10 - 7 s) and predicted the time required to seal off pore necks at the surface of the dielectric (∼70 s). Both the model and experimental results show a greater depth of penetration and an enhanced deposition of polymer at higher porosities, confirming the need for pore sealing during back-end-of-the-line processing of nanoporous materials.

A Model for the Etching of Nanoporous Silica in C 4 F 8 Plasmas Based on Pore Geometry and Porosity Effects

Plasma etching of nanoporous materials (NPMs) is a complicated phenomenon and depends upon the NPMs parameters, such as the overall porosity, the pore size and structure, and the concentration of organic groups on the surface of the film and inside its pores. Polymerization during fluorocarbon plasma exposure is ubiquitous and suppresses the net etching rate. The model developed here accounts for the polymerization that occurs. In this study, a new plasma etching model is developed that applies in the high-polymerization-rate regime. This new model includes pore structure factors (pore shape and size) as well as mass and volume effects in the form of the films overall porosity. According to the model, at low porosities the etching rate varies directly with the total porosity. However, as the porosity of the film increases, surface effects become important and the etching rate is affected by both total porosity and pore geometry. Finally, we correlate the corrected etching rate, including the porosity and the average pore size effect, with the etching of solid SiO 2 over a wide range of bias voltage. In the fluorocarbon suppression regime, the corrected etching rate expression agrees with the experimental results and collapses all etch rate data onto a single curve.

Barrier Layer Morphological Stability and Adhesion to Porous Low-κ Dielectrics

Two particularly important reliability issues facing the integration of low- κ dielectric films are the fracture energy of the barrier-dielectric interface and the barrier layer integrity during processing. We have noticed that the compressive stresses in the barrier layers on low- κ dielectrics lead to spontaneous delamination and formation of telephone-cord like morphologies. These morphologies allow the measurement of fracture energy and are advantageous over artificially contrived features to yield realistic debonding parameters. The fracture energy of common barrier films, TaN and Ta, was determined using this method for varying porosity nanoporous silica and MSQ. Detailed characterization of the telephone cord morphology using a combination of Optical Microscopy, SEM and Profilometry was done. The fracture energy for Ta on different low-κ dielectrics was evaluated using a 1-D model for straight buckles. The kinetic coefficient of buckling was also evaluated.