Hole formation in thin aluminium films under controlled variation of strain and temperature

Abstract

This paper reports a systematic study of local stress relaxation phenomena in aluminium films deposited by evaporation and sputtering in the thickness range 0.25–2.2 μm, with special emphasis on relaxation by hole formation. Tensile film stresses were generated during the cooling phase of an annealing cycle as well as by micromechanical bending of the substrate in a heating stage mounted in situ in a scanning electron microscope. Hence the relaxation phenomena could be observed as they happened, and characterized with respect to number, size, shape and critical formation temperature. Critical formation stresses were estimated. Relaxation by hole formation was observed only in evaporated and sputtered films of submicron thickness; thicker films relaxed by grain collapse. Hole formation occurred only at elevated temperature, in evaporated films during the first heating cycle, in sputtered films not until the second cycle. The holes were rather extended and richly branched out in the thinnest films, but displayed more rounded shapes in the thickest of the submicron films. A mechanism for the hole formation phenomenon, based on creep cavitation and interfacial slip, is suggested. The results show for the first time that hole formation does not require that the aluminium film be coated with a passivation layer, nor does it require the aluminium film to be shaped into narrow stripes. Tensile stress alone gives rise to holes. In a special study it was shown that the original, thin aluminium surface oxide is extraordinarily pliable. During film stress relaxation by hole formation the oxide film remains intact and is in fact suspended like a drumhead over the holes, some of which are several micrometres in size. The oxide film does not break until it is punctured by violent external action.