Intermetallic phase transformations in Au–Al wire bonds

abstract This paper describes the crystallochemical mechanisms that underpin the migration of nano-sizealumina, intermetallic growth and phase transformations in AueAl wire bonds during annealing from175 C to 250 C by utilizing high-resolution transmission electron microscopy (HRTEM). Alumina,encapsulated within the AueAl intermetallic compounds (IMCs), migrates towards the Au ball duringannealing, with Au diffusion into the Al pad. The sequence of AueAl IMC phase development duringannealing was investigated. Initially, Au 8 Al 3 appears as a third phase between the Au 4 Al and AuAl 2 layersthat form during bonding, and gradually becomes the dominant compound. Both AuAl 2 and Au 8 Al 3 aretransformed into the Au-rich alloyAu 4 Al when Al is completely consumed, and this is the terminalproduct. 2011 Elsevier Ltd. All rights reserved. 1. IntroductionThermosonic gold wire bonding is a key technology in electricalinterconnections between integrated circuits and the external cir-cuitryofmicroelectronics,whereby,athingoldwireisbondedtoanaluminum metallization pad using a combination of ultrasonicenergy, pressure (w70 MPa) and heat (150e220 C). It is a complexprocess, for which the physics is not fully understood. The maintheories to describe bonding invoke melting [1], fretting [2]ormicro-slip[3,4],andinvariablystipulatethatremovalofaubiquitousnative alumina over layer, that acts as a barrier to diffusion, isessential for successful bonding. This assumption has been verifiedby high resolution transmission electron microscopy (HRTEM) [5]that showed ultrasonic vibration, under specific pressure andheat, fragments the surficial alumina (w5 nm thick). Pathways forAueAl interdiffusion are created where the oxide detaches andallows the crystallization of Au

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