The effect of Cu on morphological instabilities in thin Al/Pt films
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B. Blanpain Department of Metallurgy and Materials Engineering, KULeuven, de Croylaan 2, B-3001 Heverlee, Belgium (Received 22 July 1991; accepted 17 January 1992)
Al films deposited on Pt layers developed voids after annealing at 250 °C. The amount of Al in the area surrounding the voids increased relative to the as-deposited film. The addition of 1 or 4% Cu to the Al suppressed the void formation and lateral Al migration. The void formation is related to compound formation with the Pt. The addition of Cu to the Al did not modify the Pt 2 Al 3 formation. I. INTRODUCTION
II. EXPERIMENTAL
A variety of morphological instabilities have been studied in Al films. Included among these are the well known "annealing hillocks"1 which can form as a result of the thermal expansion mismatch between the film and the substrate and the voids formed in stressed lines.2 In both these instances, the morphological change is driven by mechanical, not chemical, processes. Another morphological instability that has been observed in Al films is the formation of voids in Al/Pt or Pt/Al structures.3"8 This phenomenon was first observed by Murarka et al.3 With Pt (200 nm) films on preannealed Al (1500 nm) an uneven surface with voids in the Al underlaying the nonuniformities was observed after annealing. They found the size of the voids increased with annealing time and attributed the void formation to a surface migration controlled interaction. The void formation kinetics and morphology have been studied in detail.4'5 At low annealing temperatures (150 °C to 200 °C) a solid state amorphization (a-AlPt) reaction occurs in the Al-Pt system,7'8 during which voiding is also evident. The amorphous Al-Pt phase has a composition range between that of the equilibrium Pt8Al2i and Pt2Al3 phases and has been observed to form during evaporation (%70 °C) at the Al/Pt interface. The reaction phase at temperatures higher than 225 °C is the crystalline Pt2Al3 phase.9 The mechanism of void formation seems to be the Al dissolution into the reaction phase (a-AlPt or Pt 2 Al 3 ) at the Al/Pt interface. With Al/Ti or Hf bilayers, the reaction (which forms TiAl3 or HfAl3) is nonuniform and proceeds along the grain boundaries of the Al. It has been found by Wittmer et a/.10'11 that the addition of Cu to the Al results in a more uniform layer-by-layer reaction. This was attributed to Cu segregation at the Al grain boundaries (as CuAl2) which retarded Al grain growth and reduced discontinuous precipitation of the aluminide along the Al grain boundaries. The purpose of this work is to examine the effect of Cu additions to Al on the void formation in Al/Pt bilayers.
Thin-film samples of Al, Al(l% Cu), and Al(4% Cu)/Pt were evaporated at room temperature in an oilfree vacuum on oxidized (100) Si. To ensure adhesion of the Pt, a thin layer (
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