forming a first insulating layer over a substrate;

forming a conductive contact in the first insulating layer;

forming a second insulating layer having an opening on the first insulating layer, the opening exposing a top surface of the conductive contact;

forming a conductive line structure in the opening of the second insulating layer, the conductive line structure covering a portion of the exposed top surface of the conductive contact thereby forming a contact void between the second insulating layer and the conductive line structure;

forming a plasma oxide layer to cover exposed surfaces of the conductive line structure, the second insulating layer, and the conductive contact;

performing a wet cleaning process by using an aqueous solution containing negatively charged ions;

forming a capping layer over the plasma oxide layer, the capping layer filling the contact void; and

performing an etching back process to remove the capping layer above the contact void.