Crystallographic Features of Copper Column Growth by Reversible Pulse Current Electrodeposition Jun Liu 1 , Changqing Liu 2 , Paul P Conway 3 Wolfson School of Mechanical and Manufacturing Engineering of Loughborough University Loughborough, Leicestershire, LE11 3TU, UK 1 j.liu3@lboro.ac.uk , +44 (0)1509 227684 2 c.liu@lboro.ac.uk , +44 (0)1509 227681 3 p.p.conway@lboro.ac.uk , +44 (0)1509 227670 Abstract Interconnection with single crystal could be attractive for more reliable, predictable and multifunctional electronics assembly at micro- to nano- scale, if each crystal joint has the same crystal structure and orientation. To achieve single crystal copper column growth, electrodeposition has been carried out with the assistance of reversible pulse plating techniques. Initial experiments to understand how plating process will influence the crystal growth provided a guidance for electrodeposition of single crystal copper columns. It has been found that reversible pulse current in large degree changed the morphology of electrodeposited copper compared to the direct current electrodeposition. Pulse parameter such as peak current density and frequency also affected the crystal growth and morphology of copper column formation. It has appeared that achieving single crystal growth by pulse plating alone is found to be less successful. Therefore, the challenges still exist to achieve single crystal copper column formation by electrodeposition. The possible future approaches may consider the related techniques including substrate treatment prior to deposition and ultrasonic agitation during deposition would be beneficial to eliminate the sites of nucleation. Introduction The interconnects of electronics packaging have been miniaturized to a scale where the size of the interconnect is comparable with the microstructural feature size of the material itself[1]. It has been found that for Sn-3.5Ag solder joints of sub-100 microns in dimension are made up only a few dominant grain boundaries, having a low angle boundary and implying a multicrystalline rather than a polycrystalline mesotexture [2]. Similarly, Henderson et al [3]and LaLonde et al [4]used electron backscatter diffraction (EBSD) and polarized light microscopy (PLM) to image β-Sn dendrite orientation in numerous BGA joints, and they found that an optical or electron micrograph of a BGA solder joint appears to show very few unique crystallographic orientations. They estimated that a typical 900-µm-diameter BGA solder joint contains only eight crystallographically distinct β-Sn dendrites and therefore, the β-Sn microconstituent (and hence the solder joint) is more monocrystalline than poly-crystalline in character. It is believed that the joints for electronics interconnects are more likely to be re-crystallised during processing or life-time service. A joint containing only a small number of indefinite grain orientations will present anisotropic properties. These grain boundaries inside the interconnect materials probably result in a thermodynamically unstable state and unpredictable mechanical properties of the joints. There are generally two approaches to eliminate these effects. One is the nanograin interconnect approach. The other is single crystalline interconnect. Single crystals exhibit numerous attractive characteristics such as high strength, high anisotropic behaviour in terms of mechanical, electrical and thermal performance. If the growth of these crystals can be controlled and manipulated, they could be used to enable reliable interconnections, due to their excellent mechanical strength and electrical and thermal properties. The single crystals may also perform the multiple functions that are required for electronic interconnect: structural integrity, electrical and thermal conductivity, and possibly optical signal transmission. Single crystal nanowires have been fabricated and studied by various methods, such as electrochemical growth, vacuum vapour deposition, hydraulic pressure injection etc[5-8]. Such single crystal nanowires will probably be used as building blocks for nanoelectronic interconnection. At present, however, it is extremely difficult to manipulate these nanowires to make interconnections. Electrochemical deposition has been proved to be an effective method to grown single crystal metallic nanowires[6, 7, 9-12]. And copper wires of up to 5µm or so in diameter has been obtained by electrochemical deposition using reversible electrolysis in an ultrasonic field [13]. Additionally, ultrasonic agitation for electroplating has been found to increase grain size of deposits with increasing ultrasonic intensity[14]. Therefore it is reasonable to expect to obtain single crystal deposits of tens of microns in size, which is more practical for microelectronic interconnection, by controlling the electroplating conditions and optimizing parameters of pulse plating and ultrasonic agitation. Moreover, electrodeposition is compatible with the current electronic fabrication industry. In this paper initial experimental study has been presented on copper column electroplating with reversible pulse current on pre-metallised Si wafer with Au patterned apertures by lithography. Analysis has been carried out to understand how reversible pulse current plating can affect the crystal growth of the copper columns. Experimental details The process flow for fabrication of copper columns is schematically shown in Figure 1. First a test Si wafer is metallised by sputtering Ti of 150nm thick followed by Au of 100nm thick. The titanium layer plays a role of an adhesion layer while the Au one a seed layer for subsequent electroplating. Photolithography technique is applied to 1-4244-0985-3/07/$25.00 ©2007 IEEE 892 2007 Electronic Components and Technology Conference