Selective emitter formation by laser doping of spin-on sources

Abstract

Formation of selective emitter (SE) structures with a controlled dopant profile by laser (KrF Excimer laser at 248nm) annealing of spin-on dopant sources is presented. Different barrier layers (BL) like spin-on glass (SoG), PECVD deposited SiN and SiOx were used as semi-transparent barrier layers for the dopant diffusion. The presence of BL at the interface between silicon substrate and the layer of dopant source controls the dopant profile of shallow emitter (ShE) during thermal diffusion, so that the etch back step could be avoided. This method allows the realization of shallow and selective emitters using a single layer of dopant source. The dopant concentration and depth with respect to the laser parameters and barrier thickness were analyzed using secondary ion mass spectrometry. It was found that the doping profile of phosphorous was precisely controlled in the shallow region upto 200nm with a suitable emitter sheet resistance. Also, the SiN and SoG layers acted as effective phosphorous diffusion barriers for both shallow and selective emitters. On the other hand, the SiOx barrier layers, relatively lower thickness, resulted in the best electrical results at comparably lower laser fluences. In addition, laser induced damage in the silicon crystal at moderate laser fluences is nominal, and is found to be considerable at higher energies due to the enhanced energy absorption of silicon. Periodic structures were observed on the surface of laser treated silicon at the moderate laser fluences. The results were presented in detail in terms of physical behavior of the dopant diffusion with respect to laser fluence in the presence of barrier layer.