Pulsed Laser Ablation of Boron Nitride
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ABSTRACT The probe beam deflection and optical time-of-flight method have been applied to study the dynamics of Nd:YAG laser (266 nm) ablation of pyrolytic boron nitride. Shock wave generation was observed and its velocity was derived by analysis of the acoustic transit time of the acoustic wave produced by laser ablation. The most probable velocities of B and B+ were obtained by optical time-of flight technique. The deposition of boron nitride on silicon substrate by laser ablation was attempted in nitrogen atmosphere. Cubic phase of boron nitride was observed as well as the hexagonal phase.
INTRODUCTION The laser ablation of solid targets was a research topic of interest since the invention of lasers. With developments of high power lasers in the ultraviolet region, pulsed laser deposition using laser ablation has been intensively studied aiming at the deposition of high quality thin films of wide range including superconductors, semiconductors, ferroelectrics, and dielectrics[ 1I. Laser ablation technique has been known to be especially useful in the stoichiometric deposition of multielement materials like YBCO superconductors. Another useful application of current interest is the deposition of refractory materials such as boron nitridel2l, silicon nitridel3l, carbon nitridel4l, and titanium nitridel5]. In pulsed laser deposition, the quality and properties of films are expected to be highly dependent on the characteristics of the plume generated in the laser ablation process. The information on the production and evolution of laser induced plume from the target and the transport of the ablated species to the substrate will be of critical importance in understanding the dynamics involved in the pulsed laser deposition of materials. Several diagnostic methods including probe beam deflectionl6l, laser induced fluorescencel7l, time-of-flightl8l, optical timeof-flight19l, resonance absorption[ 101, resonance ionizationI 11I,and ultrafast photographyl 121 have been employed to study the plume in the laser ablation. To our knowledge, however, there is still no reliable model to describe the optical and thermal attributes of the laser ablation. In this work, we present experimental results on the optical, photoacoustic, and photothermal studies of laser ablation of pyrolytic boron nitride (pBN). Probe beam deflection[ 131 and optical time-of-flightl 141 technique have been adopted to investigate the dynamics of the formation and expansion of the laser induced plume. Also attempted is the deposition of BN film on the silicon substrate by laser ablation.
EXPERIMENT The overall experimental setup is shown in Fig. 1. The laser ablation of pyrolytic boron nitride (Union Carbide) was done by the fourth harmonic of Nd:YAG laser (266 nm, Quanta-Ray GCR 150) with pulse duration of 5 ns operating at 10 Hz. Probe Beam Deflection Experiment For the probe beam deflection experiment, a He-Ne laser (5 mW) was used as probe laser and a quadrant cell photodiode (UDT SPOT 9D) was employed as a position-sensitive detector. Each two out
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