Modeling of Metal Contact Formation on Silicon with a Scanning Rectangular Laser Beam

Laser processing of semiconductor materials to form metal contacts on the wafer surface has been studied intensively due to the potential applications in the photovoltaics industry.  The characteristic traits of laser beams, high power intensities and small beam diameters (< 100 µm), provide local heating, melting, and alloying of the material without subjecting the rest of the wafer to extreme thermal cycles or temperatures.  In addition, laser processes can be automated with robotic arms or stages, which allows for faster throughput in the competitive photovoltaic industry.  Recently, the use of elongated rectangular or line beams have been used for metal alloying of silicon.  Compared to typical circular beams with Gaussian intensity distributions, these beams may offer some unique benefits including less ‘laser-induced defects’ and shorter melt durations.  Modeling of heat transfer and fluid flow during the laser beam interaction with silicon has been limited.  This work will carry out an analysis of moving pulsed rectangular beams with different length to width ratios, intensity distributions, powers, and pulse repetition rates in order to determine the thermal cycles, processing throughput, and alloy concentration profiles experienced with a rectangular beam.  A moving circular beam case will also be given for comparison.  Modifying the shape of the beam produces considerable differences in peak temperatures and amount of melted material when compared to a circular beam.  The peak temperature varies inversely with the circumference of the beam, so if two beams have the same intensity, the circular beam will produce a higher peak temperature compared to the rectangular beam.  The differences are much more pronounced as the length to width ratio increases towards 40:1.