Real-Time Studies of Laser Oxidation of Copper: Characteristics of an Optical Heat Source
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REAL-TIME STUDIES OF LASER OXIDATION OF COPPER: CHARACTERISTICS OF AN OPTICAL HEAT SOURCE
L. BAUFAY, F. A. HOULE* and R. J. WILSON IBM Almaden Research Center, 650 Harry Road, San Jose CA 95120 ABSTRACT Oxidation of copper films is carried out using a cw argon ion laser. Time-resolved reflectance measurements during the reaction are performed at probe wavelengths of 632.8 and 514.5 nm. Strong temporal variations in reflectance are observed, and are attributed to interference within the growing oxide film. Measurements carried out as a function of laser power are complemented by Auger electron and scanning electron microscopy. A theoretical model of the system has been developed which treats its optical and thermal properties and the film growth process in an integrated way. The calculations highlight the intimate connection between the time dependence of the optical properties of the metal-metal oxide system and the film growth rate. The highly non-isothermal conditions which prevail account for the rapid rate of oxidation relative to furnace oxidation. Although both laser and furnace oxidation are thermal processes which obey a parabolic rate law, preliminary oxygen pressure dependence data indicate that they may not have the same rate determining step. INTRODUCTION It is a challenge to study the kinetics of very localized reactions such as laser-induced oxidation, doping, etching and deposition under direct writing conditions since the surface areas involved are small and reagent gas pressures are generally elevated. Optical probes are most attractive in this regard since they are sensitive, non-invasive and spatially selective. Moreover, real-time, in situ optical diagnostics can provide direct information on the interaction between the optical energy source and the reacting system. To demonstrate the utility of this approach, we have used reflectance to study cw laser driven oxidation of copper films. The motivation for choosing this system is threefold. First, the optical properties of copper and the copper oxides and the kinetics of furnace oxidation of copper have been extensively studied, providing a basis for interpretation of the optical measurements [1-3]. Second, it is a general model for a larger class of systems in which laser beams induce chemical changes by absorption of light in a solid covered by a planar, transparent overlayer whose thickness changes in time. Finally, the oxidation rate of copper when a laser is used as a heat source has been reported to be higher than that predicted from furnace data using an estimated temperature [4,51. This has been attributed to factors such as enhanced diffusion and time-dependent optical properties of the system, but an experimental investigation of the origin of the phenomenon has not yet been carried out. In this work the kinetics of laser oxidation of copper films are monitored by reflectance measurements at two wavelengths at which significant variations in optical constants exist for Cu, Cu 2 0 and CuO. Surface analyses provide information complementa
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