Pulsed Laser Annealing of Ag-Paste on n-Doped Emitter

Pulsed laser sources are attractive on account of their spatial and temporal controllability at room temperature. Pulsed lasers, in visible (VIS) (300 – 515 nm) and infrared (IR) (900 – 1064 nm) spectral ranges, with pulse widths in micro to femtoseconds range, are used in a wide range of applications including doping, etching, texturing and deposition. In this study, an Nd-YAG dicing laser operating at 1064 nm wavelength with 200 nanosecond pulse duration has been employed to form silver ohmic contacts to an n-type emitter on a p-type silicon substrate. The laser beam was used to anneal screenprinted Ag polymer paste over a broad (~ 7 to 500 mJ/cm^2) range of laser fluences. Computer numerical control software allowed fabrication of geometrical patterns with controllable diameters in 50-150-μm range. Contact resistance measurements were performed using the transmission line method (TLM). Contact resistivity exhibited fast decay from very large values to relatively constant as a function of laser fluence. This variation was attributed to laser energy below the threshold energy which no alloyed Ag/Si contact could be formed. The lowest contact resistivity at 200 mΩ.cm^2 was measured at 35 mJ/cm^2. This value was two orders of magnitude higher than the lowest value for thermally annealed contacts. For the laser parameters investigated here, optimum laser fluences were in 0.2-0.6 J/cm^2 range. It may be possible to attain lower resistivity values trough post-laser annealing.