Development of high-power VCSEL emitter of size 30mm x 30mm, scalable in two dimensions and applicable to Si wafer annealing

Abstract

Two-dimensionally scalable vertical cavity surface emitting laser (VCSEL) emitter of size 30mm×30mm was designed and fabricated for practical applications in the rapid heating processes of large-area targets. Nine emitters were arranged in a 3×3 array for an experimental VCSEL module, which was used for annealing of a 4” Si wafer. Each emitter was composed of 120 chips of size 1.26mm×1.26mm, and each chip was composed of 770 VCSEL diodes of 30μm in diameter, emitting a continuous beam with wavelength of 940nm and maximum optical power density of 47.6W/cm2. Since VCSELs generated a significant amount of Joule heat during operation, an effective cooling system was required for reliable thermal management of the emitter. A water cooling block of 126 microchannel flow passages with the heat transfer area-to-volume ratio approximately 1270m2/m3 was designed and fabricated for each emitter, resulting in the thermal resistance of 0.012K/W, equivalent to the overall heat transfer coefficient of 92,000W/m2K at water flow rate of 1L/min, which was the minimum value for the normal operation of the emitter. By comparing the experimental images on the 4” Si wafer irradiated by the single emitter and by the module with the numerical predictions based on the ray-tracing method, the laser beam emitted from a single VCSEL diode of 30μm in diameter was estimated to have the mode of TEM01* (doughnut profile, multimode) and a divergence angle of 20∘. These were two factors that affected the irradiation uniformity on the target. In addition, to validate the applicability of the high-power VCSEL system to anneal large-area targets, the heating process of 4” Si wafers using the module of nine emitters was experimentally carried out at atmospheric pressure, and was compared with numerical predictions.