Front side metallization of crystalline silicon solar cells using selectively laser drilled vias

In this paper we will clearly demonstrate the selective removal of the silicon nitride dielectric layer even using nanosecond range laser pulses, through carefully controlling the energy density of the laser pulses. For this work we use quadrupled Nd:YAG Coherent AVIA laser with 266nm wavelength and 10ns pulse length, and planar silicon wafers with 80nm PECVD deposited nitride layer. It has been found that the laser pulse with a peak energy density of 4.3 J/cm2 or lower can remove the nitride without damaging the underlying silicon while the pulse with a peak energy density of 4.8 J/cm2 or higher will damage the silicon substantially. Moreover, when the laser energy density is maintained below the threshold for silicon ablation, multiple laser pulses can be used to more completely remove the nitride layer. On the other hand, several methods are being studied to form the metal contact with the n+ emitter layer which is exposed after selectively opening the laser vias through the nitride layer. One of them is to use blanket sputtered nickel layer and screen printed silver paste as etching mask. We have found that the sputtered nickel layer can be patterned using the screen printed silver gridlines as a protection mask and ferric chloride as the etching solution. This avoids using standard photolithography method to pattern the nickel contact layer and thus reduces the number of process steps and cost. With high quality sputtered nickel as the contact metal, the contact resistivity can be reduced by about two orders of magnitude compared to standard screen printed and fired silver contacts, and the firing temperature can be lowered to about 500°C. We have applied these technologies to fabricate silicon solar cells and the details of electrical characterization will be presented at the Conference.