Activation of silicon wafer by excimer laser

The application of lasers for annealing wafer-based and thin-film microelectronic devices is steadily increasing. Excellent control of material characteristics such as the dopant activation profile are achieved through proper selection of the laser parameters which directly influence the laser material interaction; these include wavelength, pulse duration and fluency. Nanosecond pulses at short UV wavelengths, as emitted by excimer lasers, are particularly beneficial for shallow activation and the increasing demand to keep the overall temperature budget low during the anneal. The short wavelength of e.g. 308nm or 248nm leads to an absorption depth of less than 10 nm in crystalline silicon that enables the deposition of the laser energy in a small confined volume. In this paper, we look at lasers, optics and annealing systems that have proved themselves in the annealing of semiconductor wafers. The application of a system for the uniform shallow activation of Back-Side-Illuminated (BSI) CMOS image sensors will be discussed in detail. This annealing system is based on a proven industrial excimer laser and yields a high uniformity of sheet resistance that is better than 0.5% (sigma). A unique optics system has been developed to provide a highly uniform thin line beam of 300 mm length. The wafer is covered in a single scan so that the process time for a 12" wafer is reduced to only 20 seconds. Results from recent application work involving dopant activation for silicon wafers will also be presented.