Oxidation and Nitridation by Pulsed Laser Irradiation of Solids Immersed in Liquids
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OXIDATION AND NITRIDATION BY PULSED LASER IRRADIATION OF SOLIDS IMMERSED IN LIQUIDS S. ROORDA, A. POLMAN, S. B. OGALE* AND F. W. SARIS
FOM-Institutefor Atomic and MolecularPhysics, Kruislaan407, 1098 SJ Amsterdam, the Netherlands
ABSTRACT
Nitridation and oxidation of titanium is achieved by pulsed laser irradiationof Ti immersed in liquid ammonia or water. Rutherford BackscatteringSpectrometry shows that large amountsof nitrogen and oxygen can be incorporatedin the metal surface to a depth of several 1000 A. X-ray diffractionshows evidence of compoundformation.Scanning Electron Microscopy reveals that initialsurface texture is smoothed, and that stress induced cracks and holes may appear.Irradiationof Fe and Si immersed in various liquids shows thatmodification depends on which combinationof solid and liquid is used. Influence of processingparameters such as laser-energy density and number of laser pulses on compoundformation has been investigated.The process is viewed as a reactivesolute incorporationin the lasermelted surface layer,followed by compoundformation. 1.
INTRODUCTION
Thin films of compounds such as nitrides and oxides find applications in diversified fields such as semiconductor technology, mechanical industry, optical component technology and magnetic device industry. Both continuous and pulsed lasers are widely employed for processing of these, often new, materials. The early phase of research in laser processing was primarily focussed on developing an understanding of the physical mechanisms and exploring utilization for annealing and epitaxial regrowth of semiconductors 1 .2 3Recently 4however a considerable emphasis is being laid on the use of lasers for deposition ' , etching , doping5.6
and related applications. In the case of laser induced deposition, the laser is mostly used to products on a induce gas phase chemical reactions and subsequent deposition of reaction closely stationed substrate, by a diffusion limited transport mechanism7-10 . In this paper we explore a new and simple method of compound formation using a pulsed laser to 11 irradiate the surface of a solid which is immersed in a liquid. Two main features set this method apart from more conventional laser treatments 2-7 : Firstly, the liquid above the surface supplies a large amount of reactive species in the immediate vicinity of the metal surface, making this method especially effective for compound formation. Secondly, the treated surface is actually melted by the laser pulses, rather than used for hot-spot generation. In this way, use is made of both enhanced diffusivity and solubility of species in molten metals and of the quenching character typical of pulsed laser melting and solidification. This paper concerns the influence of laser-energy density and the number of laser pulses on compound formation of Ti which is immersed in water or ammonia. Other combinations of solid and liquid will be discussed briefly. 2
EXPERIMENTAL
(X= 308 nm, FWHM = 25 In the experiments reported here a pulsed XeCl excimer laser the beam to
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