Untque Features of Laser Ablation
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MITSUGU HANABUSA Toyohashi University of Technology, Department of Electrical and Electronic Engineering, Tenpaku, Toyohashi 441, Japan.
ABSTRACT The advantage and disadvantage of laser ablation as a thin Film deposition technique are discussed on the basis of my own experiences. High instantaneous growth rate is useful to produce amorphous films, and reactive deposition can be achieved by gases introduced in the deposition chamber. Combining these two features, hydrogenated amorphous silicon films were produced. Laser ablation is characterized by the presence of energetic neutrals and ions in beams, which helped to deposit diamond-like carbon films. Also selective-area deposition is made possible by placing a mask between the target and substrate. In pulsed laser ablation the mask must be placed close to the substrate to avoid the spreading of deposits. The thickness profilr can bc controlled by residual gas pressure, and using this technique microlenses were fabricated, in addition to ridge-type optical waveguides. The particle problem which plagues laser ablation is more serious for targets with low fractional ionic character. The usefulness of a molten target as the means to eliminate particles was confirmed for aluminum. INTRODUCTION Recently new thin film deposition techniques based on photo-induced effects have emerged.' They are classified into two different categories. One is the chemical method represented by photochemical vapor deposition (photo-CVD), where photons induce chemical reactions required for deposition in4 place of thermal energy used in traditional chemical vapor deposition.` It is characterized by capabilities of low temperature processing, microprocessing (in case that lasers are used as the light source), and a high degree of control of reactions, which can be utilized to achieve atomic layer epitaxy. On the other hand laser ablation belongs to the second category of a physical method and resembles the sputtering technique used widely in thin film technologies. in this case energetic ions are replaced by photons to eject atoms or molecules out of a solid target placed in front of a substrate. Therefore, this technique is sometimes called laser sputtering. The idea is old, and the work started soon after the laser was invented in 1960. This technique gained real popularity when iL was applied successfully to prepare high-temperature superconducting thin films. I have been interested in photodeposition of various thin films for the last fifteen years. While I placed the emphasis of research on clarification of reaction mechanisms for photo-CVD, I regarded laser ablation as a practical means to produce thin films and even fabricate soie devices, because it was simple enough to be used readily for practical applications. The purpose of this article is to express my view on uniqueness of laser ablation as a thin film deposition technique. It was not intended to review my past experiments and summarize the main results. Rather, I tried to extract the unique feature characteristic of laser abla
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