Development of an industrial tool to make passivation layers for UV sensors improvement

摘要

Today, the collection of generated charges is a limiting problem for the realization of UV sensors. Indeed, the native silicon oxide of the surface acts as a region of recombination centers [1]. Then, the sensors exhibit a low sensitivity in the UV wavelengths. An approach to overcome this drawback is the realization of a few nanometers thick passivation layer at the surface by creating an ultra-shallow junction (USJ) with a high activation level. The realization of such junctions requires two steps: first, the implantation of dopants which consists in introducing impurities at the surface of the substrate, then the thermal activation of these dopants to obtain the electrical characteristics of the junction. The Plasma Immersion Ion Implantation (PHI) process allows us to implant dopants in a very thin layer (10-20 nm) into the silicon substrate [2-4]. These impurities are located in interstitial sites in the silicon, and need an activation process to modify the electrical properties of the layer. The step is performed by means of an excimer laser annealing process (ELA) [5-8] to melt a very thin layer of the silicon substrate and then activate the dopants without diffusion. In the framework of the ALDIP project (Laser Activation of Dopants implanted by Plasma Immersion), IBS Company has developed with its partners a cluster to realize these two steps with industrial production rates and cleanliness. Four-point probe measurements and SIMS analyzes have been used to characterize the junctions realized with this process. We have reached a sheet resistance lower than 500 Ω/sq for a junction depth of 29 nm and an abruptness of 3 nm/dec. Nevertheless, electrical measurements on diodes have revealed a significant leakage current of around 10~(-5) A/cm~2, revealing the presence of defects inside the junction. Light Beam Induced Current (LBIC) [9-12] characterization has shown that these defects are localized at the edge of the laser beam.