Laser process optimization for improving Emitter Wrap Through drilling rates

Manufacturing of Emitter Wrap Through (EWT) solar cells relies on fast laser drilling of silicon wafers. A single cell may comprise up to tens of thousands of holes, which need to be drilled at cycle times of a few seconds per wafer. Typically these holes are drilled using pulsed infrared lasers. Combined laser power for requested production rate is a few hundred watts. Master Oscillator Power Amplifier fiber laser concept allows for independent adjustment of pulse width, energy and temporal shape. This capability can be exploited for improving the drilling process by pulse parameter optimization. Combined with a FPGA controller, one can use a high beam scanning velocity with precise pulse synchronization to drill holes at unprecedented speeds. The laser beam is scanned along a grid path repeatedly. As each repetition increases the depth of the forming hole, complete holes are pierced through the wafer after a sufficient number of scans. Presented results establish optimal pulse parameters and temporal shapes for EWT drilling. It was shown that a single hole can be drilled through a 210 μm silicon wafer using less than 3 mJ of energy; consequently 6,250 holes per second could be drilled using 18.2 W laser power. The entrance diameter of the hole was 30 μm and the exit 15 μm. Investigated parameters included pulse width, pulse energy and pulse shaping for ideal material removal and maximized energy efficiency. Using the capabilities of the MOPA laser, the results were compared to q-switched lasers by mimicking the shape of a high peak power q-switched pulse of a similar width.