This paper presents the results of a systematic TEM investigation to understand the chemical, morphological and microstructural makeup of photoresist residues following oxygen plasma ashing. The investigation was carried out on generic non-product control wafers and product wafers with a view to help aid in the identification of a suitable post-ashing cleaning process. Specimens in planview were prepared by mechanical grinding and Argon ion milling from the substrate end of the sample. Energy Dispersive X-ray Spectrometry (EDS) was used to obtain the composition and selected area electron diffraction (SAD) was used to obtain information on crystallinity. The residues were essentially amorphous and were found to be arsenic-rich in composition and exhibited a dropletlike morphology decorating areas where the photoresist was used as an implantation mask. In addition, hexagonally shaped crystals and spherical particulates of varying dimensions were seen in several regions and were found to be carbon rich.