Effect of phosphorous content on phase transformation induced stress in Sn/Ni(P) thin films

Abstract

The film stress evolutions induced by the phase transformation of Sn/Ni(P) films during thermal treatment were investigated using an in situ measurement of wafer curvature by laser scanning. Apparently, tensile stress developed due to the layer-by-layer formation of Ni3Sn4 and Ni3P phases for Sn/Ni(11.7P) films, and a compressive stress evolved for Sn/Ni(3P) films, despite the same phase transformation. The molar volume mismatch and x-ray diffraction analyses before and after the reaction between Sn and Ni(P) films suggested that a compressive stress existed in the Ni3Sn4 layer while the Ni3P layer was under a tensile stress state. The apparent stress states (tensile or compressive) for overall thickness of the films formed by the layer-by-layer transformation in Sn/Ni(P) were determined by the competition between compressive stress related to Ni3Sn4 formation and tensile stress caused by Ni3P formation. The stress states were dependent upon the relative thickness of the product layers with varying P content.