Thermo-Mechanical Cycling Behavior of Al Thin-Film Metallization

Al-based thin-film metallizations are used as electrical interconnections in integrated circuits and in most of discrete, active components L1]. After deposition at temperatures T between 400 and 450K by evaporat ion or sputtering, different thermal treatments work on the metallizations: The baking of photoresists and the ion etching upto T 320K, the alloying of ohmic contacts with Si upto T-720K, the die bonding and the thermosonic wire bonding at T-500K, the moulding of the encapsulation at T-430K. After the production steps the components are subject to softsoldering on printed-circuit boards with T>520K and thermal cycling during the application between T-220K and T=500K. The time st eps at the named temperatures range between few and 103s, the thermal rates get up to T102Ks-1. Due to the filmn thickness h'1i, the grain sizes d.,5pm and the concentration of lattice defects the thermal treatments cause changes in the film structure and local chemical composition. As the filrns and Si wafers have different coefficients of thermal expansion aF and cr.,.and the substrates are much thicker and mechanically rnore rigid than the films, the films are subjected to thermal compressive or tensile strains. The induced stresses exceed the yield point of the films. The straln relaxation of the metal films during continuous changes in T is explained in terms of diffusional creep [2], of dislocation slip [13] and by comparison of the different driving forces working on mobile grain boundaries and vacancies in the grains [4]. Metals with low stacking fault energy show discontinuous grain growth [5]. Al-based films exhibit the formation of hillocks [6,7] which is not yet explained quantitatively. The purpose of this paper is to learn the kinetics of partially reversible and of irreversible defect reaction mechanisms in Al-based thin-film metallizations on substrates from the measurement of stress relaxation during thermomechanical treatments. It will be shown that the sequence of diffusional creep and of the growth of single grains are sensitively dependent on the thermal and mechanical history. Changes in dc resistance indicate the production of lattice defects under tensile stress and the absorption under compressive stress. initial grain sizes are (100.. . 300)nm of the evaporated and (1...3)pm foir the sputtered films measured by line interception of TEM micrographs. The lateral macrostresses in the films are >1.OMPa at 300K measured by the substrate curvature before and after the removal of the Al films by chemical etching.