Laser-Enhanced sputter or vapor deposition of thin metallic films on ceramic substrates
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INTRODUCTION
T H E energy of a nearly coherent laser beam incident on a material is partly absorbed by the electrons as a result of electron-photon interactions, m In particular, in metallic materials, which have a high density of conduction electrons, laser light is absorbed within the first few tens of nanometers. ~21 For semiconductors, the absorption is also within a similar distance when the wavelength of the laser light is in the ultraviolet (UV) range. [3] This energy is transferred as heat to the lattice in a very short time (10 -11 t o 1 0 - 1 2 s ) . [4[ The laser can thus act as a surface heating source. Metallic films reflect a large portion of the light in the visible range. However, in the UV range, the reflectivity of metals decreases strongly. Only 10 to 30 pct of the incident UV light is reflected by a polished copper surface, while the remainder is absorbed within the first 20 nanometers (nm). [5,6[ Pulsed excimer lasers operate in the UV range of the spectrum.t7] Due to recent advances in laser technology, which include increases in laser power and repetition rate, these lasers are today widely used as industrial processing tools. Applications in the semiconductor industry range from the fabrication of premium solar cells tS] to doping, t9j etching, t~~ photolithography Im and laser-assisted chemical vapor deposition.t121 Excimer laser ablation of polymers is used for fine drilling of circuit boards. I131 These lasers are also being used as an important tool for deposition of high T, superconductor f i l m s . Hal
In this article, we describe a controlled process that combines an excimer laser and a sputtering module in A.J. P E D R A Z A , Professor, and M.J. G O D B O L E , Postdoctoral Research Associate, are with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200. This invited paper is based on a presentation made in the symposium "Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces" presented as part of the 1989 Fall Meeting of TMS, October 1-5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of A S M / M S D . METALLURGICAL TRANSACTIONS A
order to deposit highly adherent metallic films on ceramic substrates. One of the main characteristics of this class of lasers is that the pulse duration is of the order of tens of nanoseconds. A xenon chloride (308-nm wavelength) excimer laser with a pulse duration (full width at half maximum) of 41 nanoseconds (ns) was used in this research. The energy deposited per unit area of the specimen (energy density) was in the range of 0.1 to 3.0 J / c m 2. This implies that 2.5 to 75 M W were deposited in a volume of 2 . 1 0 - 6 c m 3. This extremely large power, that has to be dissipated by heat conduction, limits the conditions for nondestructive pulsed-laser processing of materials. Two important features of the excimer laser are relevant for the present process. First, it can melt thin layers in a reproducible and controllable way. Second, beca
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