A special class of post-development defects, referred as Circular Surface Defects (CSDs), has been reported. Up to now, no resist is immune to CSD printing, including eight commercial KrF resists (from two vendors) and six early samples of ArF resists (from five vendors). An extensive study on the CSDs was conducted on a KrF Resist A, in term of its origin, formation and removal mechanism. Photoacid generators (PAGs) are proved to be a primary contributor to the CSDs. The origin of CSDs is believed to be PAG aggregation along with other hydrophobic components, resulting in formation of microemulsions in the developer. The aggregates have limited solubility in aqueous base developer, and could redeposit on the wafer surface during development. We propose one major defect removal mechanism, or 'Stripping' mechanism. This mechanism is related to resist film thickness loss, which aids in stripping potential defects from the resist surface, or reducing the probability of defect deposition.