Tuesday, June 29, 2010

UV laser scattering on Laser Spike Annealed wafers: an introduction

As devices continue to shrink towards a true nm region, ultra-shallow and low-restivity junctions become vital to suppress short-channel effects and improve device performance. These junctions are created in part from annealation across the wafer; temperature uniformity and the minimization of pattern density effects are crucial within the 45nm node and beyond. A recently introduced method that does this is the Laser Spike Anneal (LSA). This process enables highely localized elevated temperatures for rapid annealing of implant layers without impacting the process thermal budget. The catch: the rapid heating (inherent within the LSA process) induces slip line defects and other surface damage. This is not conducive toward understanding and characterizing the surface morphology of post-anneal wafers. This problem doesn't arise in more traditional methods such as Atomic Force Microscopy (AFM), which does provide accurate and quantitative surface information, but it's too slow for impatient corporate clients.

Scattering of lasers has long been used for monitoring slip lines and other defects on amorphous substrates. Scattering is highly sensitive to changes in substrate morphology and surface roughness. Researchers commonly call the surface roughness "haze". Full-wafer haze information allows characterization and monitoring of surface quality at production-worthy throughput.

In a paper called "A Novel Method of Characterizing Post-laser Anneal Surface Conditions for the 45nm Process Technology Node" by W-Y Teng, J-H Yeh(United Microelectronics Corporation) and P. Chen, S. Radovanovic, D.K. Chen, H. Cheng, and U. Mahajan (KLA-Tencor Corporation), advanced UV laser scattering is applied to characterize the surface. The authors used high-resolution haze to capture whole-wafer surface data at sub-nm resolution. This surface condition presented good correlation with the LSA processing conditions. The results were further confirmed by scanning electron microscope, illustrating the potential of using haze for process development, characterization, and monitoring. The next entry will detail the experimental details.