Capillary-Driven Reflow of Thin Cu Films with Submicron, High Aspect Ratio Features

Citation

Brain, Ruth Amy (1996) Capillary-Driven Reflow of Thin Cu Films with Submicron, High Aspect Ratio Features. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/nt1r-kp46. https://resolver.caltech.edu/CaltechTHESIS:11052019-121746850

Abstract

Conventional sputtering techniques are no longer sufficient for the fabrication of interconnects as trench widths enter the submicron regime and aspect ratios become greater them 1:1. The goal of this thesis is to investigate Cu as a potential interconnect metal for use in integrated circuit technology. Since sputtering is well established and widely used in the integrated circuit industry, we have used current sputtering technology as our deposition technique of choice. An alternative approach to modify the nonconformal deposition profiles obtained by sputtering is to reflow (planarize by capillary-driven surface diffusion) the metal film during a post-deposition anneal. In particular, reflow is performed for thin Cu films deposited on refractory metal barrier layers (Mo, Ta, and W) at temperatures ≤ 500°C.

With a goal of developing and understanding a post-deposition reflow process for Cu, we have studied the following topics. Chapter 1 introduces relevant current concepts in ultra-large scale integration (ULSI) for interconnect technology to motivate the approaches described in this thesis. Chapter 2 investigates several techniques to improve the initial Cu coverage obtained from a magnetron sputtering source. This was found to be necessary since thin Cu films agglomerate on many underlayers, and this constrains the initial Cu thickness requirements for successful reflow. Chapter 3 describes an investigation of the atomic transport mechanism during reflow of Cu films, to understand the kinetics of reflow and measure the appropriate kinetic constants. Extensive transmission electron microscope (TEM) work was done to examine reflowed profiles and to relate the extent of reflow to the morphology, texture, grain size, and orientation of the Cu films. Hot-stage TEM experiments were performed to observe dynamically the reflow of a very low aspect ratio film. Chapter 4 develops a finite-element model to study surface diffusion mediated reflow in high aspect ratio trenches. We have considered (i) reflow of typical continuum, as-deposited profiles from a magnetron sputtering source, (ii) reflow of continuum profiles including an anisotropic surface energy, and (iii) reflow with the inclusion of grain boundaries. We also discuss some limitations of a post-deposition reflow process, and we make recommendations to facilitate the ability to reflow Cu in high aspect ratio trenches. Chapter 5 examines a non-infrared annealing technique to reflow Cu films. In particular, we have examined the possibility of annealing Cu films in a single-mode microwave cavity, which can be advantageous because it is possible to thermally isolate the substrate. Chapter 6 provides a summary of the work in this thesis and suggests some possibilities for future work.

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