Observation of Extremely Low Defect Densities in Silicon Wafers

Defects in p- polished silicon wafers originating from Czochralski (Cz)-pulled crystals are commonly delineated by hot SC1 treatment with immersion times in the range from about 20 min to 4 h corresponding to silicon removals of 20 nm to 200 nm. This procedure is no longer applicable for wafers with defect densities that are orders of magnitude lower than in Cz p- wafers due to their very low count rates. However, a low defect density, such as in homoepitaxial silicon layers, can be investigated by sequentially treating wafers in hot SC1 solution several times so that the overall duration of the preparation is on the order of twenty hours resulting in a silicon removal of about 1 µm. Defects delineated by hot SC1 etching can, in addition, be investigated with an atomic force microscope. In the case of homoepitaxial silicon layers, these defects are identified as single as well as dual pits with a morphology characteristic of so-called crystal originated particles (COPs) as observed on p- Cz wafers. These COPs are related to voids generated by vacancy agglomeration in the growing ingot. The morphology of the delineated defects indicates that such voids exist in homoepitaxial layers as well, which is supported by the fact that the temperature range relevant for void formation in p- Cz silicon crystals is also used in silicon homoepitaxial growth. The oxygen backpressure during homoepitaxial silicon growth is negligible. Thus, formation of voids in the epitaxial layer is not influenced by oxygen, specifically when an oxygen-free silicon substrate, such as a wafer cut from a floating-zone (FZ) crystal, is used. However, not only single but also dual-pit COPs are observed on such epitaxial layers after delineation, which questions assumptions and models in the literature attributing the formation of multiple voids in p- Cz silicon crystals to the role of oxygen.