Theoretical Aspects on Spatial Thickness Variations in Laser-Deposited Thin Films.
- PDF / 341,324 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 27 Downloads / 107 Views
THEORETICAL ASPECTS ON SPATIAL THICKNESS VARIATIONS IN LASER-DEPOSITED THIN FILMS. RAJIV K. SINGH and JAMES CARIGNAN Department of Materials Science and Engineering University of Florida, Gainesville, FL 326112066
ABSTRACT We have studied predicted thickness uniformity of thin films deposited by the pulsed laser ablation technique. Unlike a conventional evaporation process, the spatial thickness characteristics of laser-deposited films is controlled by a number of laser and other deposition variables including laser wavelength, pulse energy density, substrate-target distance, area and shape of the laser irradiated spot, etc. The effect of these parameters on the spatial thickness variations have been analyzed by using a model proposed by Singh et. al. [1]. This model is based on the anisotropic expansion of the laser-generated high-temperature and high-pressure plasma, which is initially confined to small dimensions and then expands anisotropically in vacuum. The results show that spatial film uniformity can be improved by decreasing plasma temperature and specific heat capacity ratio of the plasma gas. For large laser irradiated spot diameters (> 8 mm), the film uniformity was found to decrease with decreasing spot diameters, but this trend reversed at smaller: irradiated spot sizes. Depending on the laser irradiated conditions and substrate-target geometry, the thickness variations have been found to vary from (cos 0)2.5 to (cos 0) 12. Optimization of various parameters to decrease the spatial-thickness inhomogeneities across the film are also discussed. INTRODUCTION The pulsed laser ablation (PLA) technique has gained tremendous interest in the recent years, especially in applications involving use of multicomponent stoichiometric and activated species, and low vapor pressure targets [1-5]. Epitaxial superconducting, semiconducting and ceramic thin films having excellent electrical and microstructural properties have been fabricated by this technique [3]. Despite numerous advantages in growing thin films by PLE, the thickness uniformity of these films is considerably less than that obtained from other physical vapor deposition techniques. Unlike a cosO thickness variation expected from a purely thermal evaporation process, the laser evaporation process is characterized by a sharp variation in the thickness as a function of distance from the center of deposit. Here, 0 refers to the angle subtended by the radial vector and the line perpendicular to the substrate. To overcome large spatial thickness variations in PLE films, new holder target-substrate assemblies have been designed which can lead to uniform film thickness over 4" diameters [6]. There has been some controversy on the thickness variations observed in PLE films. Initial reports by Neifeld et. al [4], Venkatesan et. al [5]., and Singh et. al. [2] etc, found very steep thickness variations in PLE films which were in the range of (cos 0)8-12, whereas Muenchasen et. al [6] observed very diffuse thickness variations in the range between (cos 0) 2 to (cos 0
Data Loading...
