The variation of stress and resistivity with time, the phenomenon of self-annealing, is studied in a copper interconnection layer deposited by electroplating. In a conventional copper sulfate layer deposited on a TaN barrier layer, a high stress copper layer, the resistivity and stress are high and decrease rapidly with self-annealing. For example, the resistivity decreased from 2.9 to 2.4 muOmega cm as the stress fell from 33 to 28 MPa over 1 month. However, a self-annealing effect such as this cannot be observed clearly when low stress and low resistivity copper layers are deposited. Lower stress and lower resistivity copper layers may be deposited by electroplating in a copper hexafluorosilicate electrolytic solution. A self-annealing phenomenon cannot be observed in this layer. (C) 2003 The Electrochemical Society.