Photoluminescence Imaging Diagnosis of Particulate Iron Contamination Derived From HF Dip and Thermal Oxidation

Photoluminescence (PL) imaging identifies contamination occurring in thermal oxidation of p-type crystalline silicon. PL images indicate that the contamination decreases the carrier lifetime from ~350 to 50 μs and that it is radially symmetric with a diameter of ~10 mm. By imaging before and after wafers are “light soaked,” the contamination is shown to include Fe, and its net concentration is quantified. By solving the steady-state quasi-neutral semiconductor equations in cylindrical coordinates, the size of the original contaminant Fe particles is determined. This approach accounts for the out-diffusion of Fe during thermal processing and the lateral flow of carriers into the contaminated region; the latter is important due to the large change in lifetime in the vicinity of the defected region. We find that if the particles were pure Fe, their diameter would be just 150-180 nm. Finally, we analyze PL images to identify that the Fe particulates were deposited onto wafers during the post-clean “HF dip” that preceded furnace processing. Thus, we provide a methodology for the analysis of PL images of wafers affected by localized, lifetime-degrading contaminants; a proper treatment of carrier transport in such regions allows us to more accurately determine the size of the contaminating particles.