A method for cleaning a semiconductor structure includes subjecting a semiconductor structure to an aqueous solution including at least one fluorine compound, and at least one strong acid, the aqueous solution having a pH of less than 1. In one embodiment, the aqueous solution includes water, hydrochloric acid, and hydrofluoric acid at a volumetric ratio of water to hydrochloric acid to hydrofluoric acid of 1000:32.5:1. The aqueous solution may be used to form a contact plug that has better contact resistance and improved critical dimension bias than conventional cleaning solutions.