High rate laser drilling and texturing of silicon

Laser technology offers various opportunities for solar cell manufacturing. The key success factor for many of these is processing velocity. Applications such as surface texturing for light trapping and drilling for wrap through emitters require substantial amounts of features to be processed in the area of a cell with a small spacing. In many cases lasers struggle to deliver productivity required for in-line production and increasing the number of lasers for higher productivity is expensive and requires more space than is available. The more suitable way to extend processing capabilities is improving process efficiency. MOPA fiber lasers allow independent adjustment of pulse parameters, such as pulse energy and pulse width. Experiments showed that the material removal efficiency can be increased significantly by maintaining a proper intensity level by adjusting the pulse width. This resulted in an improvement in the efficiency of drilling and texturing. Through holes i n 200 m silicon wafers were drilled using only 6.5 mJ per hole, resulting in a drilling rate of more than 150 holes per Joule. A FPGA-based pulse synchronization controller was developed for synchronizing the laser pulses with high velocity scanner movement. Using the parallel processing capabilities of the FPGA, the reached pulse positioning accuracy at 5 m/s indicated a temporal resolution of less than 400 nanoseconds. In actuality, the control has a higher resolution of less than 100 ns, as part of the positioning accuracy depends on the scanner. This paper describes the developed MOPA laser system, including the FPGA-based synchronization controller and processing results for high rate drilling and texturing of silicon.