Laser Melting and Recrystallization of Bulk Si by Nanosecond UV Laser Pulses
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ABSTRACT Laser ablation of semiconductors presents an increasing interest for both thin film growth and surface modification. We present herein a study of the damage produced in bulk silicon by nanoseconds UV laser pulses with energy above the melting threshold. This study is carried out with a Raman microprobe. Polarized microRaman was used to reveal the main changes in the melted and recrystallized volume. These changes were observed in the liquid/solid boundaries, where tensile stress due to the induced thermal wave is more important. The morphology of the melted region evidences matter accumulation at such a boundary. INTRODUCTION Laser processing of semiconductors is becoming very attractive since it does not require lithographic preparation of the surtace. The feasibility of different laser procedures has been demonstrated; i.e. laser annealing [1], doping [2], etching [3] and contact formation [4]. Generally laser matter interaction results in changes, both morphologic and structural, which the amplitude depends on the specific conditions of the laser process and the characteristics of the material, such as optical absorption, reflectivity, melting temperature and thermal conductivity. Both the control of the ideal conditions for each application, as well as the changes induced in the material by the laser beam, need to be studied. During this process, the laser fluence can be below or above the melting threshold, which is critical to the damage induced by the laser processing. We focused this work on the damage produced by short UV laser pulses on bulk Si with fluences above the melting threshold. This study is based upon the study of the morphologic and structural changes induced by the laser pulses. The study of the morphology is performed by means of different microscopies (Nomarski, Scanning Electron Microscopy and Phase Stepping Microscopy). The structural features accompanying the morphologic changes are studied by Raman microspectroscopy, allowing a spatially resolved assessment of the subtle structural transformations induced by the laser beam pulse. EXPERIMENT Sample preparation Laser ablation was performed with a pulsed YAG laser (frequency quadrupled, X = 266 nm). The typical duration of the pulse was 10 ns. The laser beam was focused with a quartz lens in order to get a spot diameter at the sample surface ranging from 5 to 15 Vtm by adjusting the focus. The ablations were carried out with several incident energies (8.6 ýtJ/10- 8 s < E < 112 4tJ/10- 8 s) and a variable number of shots. This experiment was made at LSLA (Laboratoire de 407 Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
Spectroscopie Laser Analytique) in CEA (Centre d'Energie Atomique) (Saclay, France). All the ablations were performed in air. Morphology Morphological aspects were studied by Nomarski Microscopy, SEM (Scanning Electron Microscopy, JEOL JSM-820) and PSM (Phase Stepping Microscopy) for the topographic assessment. This technique consists of optical interferometry and image processing for fringe c
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