Dislocation accumulation and strengthening in Cu thin films

An analysis that addresses the strain-hardening behavior of thin metallic f ilms on substrates is presented. Stress measurements were made on 0.5 mum t hick Cu films on Si substrates during thermal cycling, during stress relaxa tion at room temperature (RT), and after quenching in liquid nitrogen. Sign ificant strengthening was observed in the thermal cycle during cooling. The stress relaxation at RT shows a decrease of the stress from 360 MPa to 290 MPa within 15 months. A theoretical approach to the strengthening phenomen on is made on the basis of the Peach-Koehler dislocation-interaction forces . It shows that adding threading dislocations into a parallel array of disl ocations at the film-substrate interface can contribute significantly to th e strain hardening of thin films. The calculated strain hardening accounts for a large portion of the observed strengthening. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.