Laser drilling up to15,000 holes/sec in silicon wafer for PV solar cells

One approach to realize a back contact solar cell design is to 'wrap' the front contacts to the backside of the cell [1]. This results in significantly reduced shadowing losses, possibility of simplified module assembly process and reduced resistance losses in the module; a combination of measures, which are ultimately expected to lower the cost per watt of PV modules. A large number of micro-vias must be drilled in a silicon wafer to connect the front and rear contacts. Laser drilling was investigated using a pulsed disk laser which provided independent adjustment of pulse width, repetition rate and laser power. To achieve very high drilling rates, synchronization of the laser pulses with the two-axis galvanometer scanner was established using a FPGA controller. A design of experiments (DOE) was developed and executed to understand the key process drivers that impact the average hole size, hole taper angle, drilling rate and hole quality. Laser drilling tests were performed on wafers with different thicknesses between 120 mu m and 190 mu m. The primary process parameters included the average laser power, pulse length and pulse repetition rate. The impact of different laser spot sizes (34 mu m and 80 mu m) on the drilling results was compared. The results show that average hole sizes between 30 - 100 mu m can be varied by changing processing parameters such as laser power, pulse length, repetition rate and spot size. In addition, this study shows the effect of such parameters on the hole taper angle, hole quality and drilling rate. Using optimized settings, 15,000 holes per second are achieved for a 120 mu m thick wafer with an average hole diameter of 40 mu m.