Photoluminescence imaging diagnosis of particulate iron contamination derived from HF dip and thermal oxidation

Photoluminescence (PL) imaging is used to identify a contamination source that is introduced during a thermal oxidation of crystalline p-type silicon. The PL images indicate that the contamination severely decreases the carrier lifetime from ~350 ms to 50 ms, and that it is localised and radially symmetric with a diameter of ~10 mm. By taking PL images before and after the wafers are `light-soaked´, the contamination source is shown to be 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 takes into account the out-diffusion of the Fe during thermal processing, as well as the lateral flow of carriers into the contaminated region; the latter is important due to the rapid and large change in lifetime in the vicinity of the defected region. We find that if the original particles were pure Fe, their diameter would be just 150-180 nm. Finally, we analyse the PL images of a comprehensive experiment to identify that the Fe particulates were deposited onto the wafer during the `HF dip´, which followed the RCA clean, and preceded the loading of the wafers into the furnace. Thus, we provide a methodology for the analysis of PL images of wafers affected by localised, lifetime-degrading contaminants; a proper treatment of carrier transport in such regions allows us to more accurately determine the size of the contaminating particles.