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|Large Cu Wire Wedge Bonding|
|Keywords: Cu wire bonding , Large Cu wire , wedge bonding|
|As the demand for longer lifetime and/or higher power density for power devices and especially power modules is increasing, a high current carrying, highly conductive, efficient heat transfer interconnect is a must to enable the device functionality and reliability. Following the significant success of the improvements of die attach heat conductivity  and the revolution of Copper (Cu) die metallization , a critical link now becomes the die-substrate interconnect, namely, wire bonding. Cu wire bonding is increasingly used in high pin count IC devices with fine pitch due to the soaring gold price. Medium size Cu wire (> 1mil) bonding on power devices has been used for years, but was never adopted as a mainstream interconnect. Recently, successful bonding of large 16mil Cu wire on specifically metallized IGBTs was reported [3, 4]. Such devices showed up to ten times higher lifetime in power cycling when compared to a standard Al wire interconnect on Al die metallization. K&S’ Wedge Bonder Business Unit has been aggressively supporting this industry migration through active development programs. In a first phase of an extended feasibility program, 12mil and 20mil 4N Cu wire without coating was bonded on Direct Bonded Copper substrates (DBC), bare Cu plates and Silicon wafers with various in-house developed metallization structures. A comprehensive set of bond quality criteria was developed based on the experience with Al wire bonding. Bond process windows were then derived to determine the necessary equipment specifications for large Cu wire bonding. The K&S Wedge Bonder Business Unit is striving to develop a robust production method involving equipment, process, and materials that will make the Cu-Cu interconnect a viable bonding alternative. In the first phase of an extensive feasibility study, 8-, 12- and 20-mil 4N uncoated Cu wire was bonded to Direct Bonded Copper (DBC) substrates, bare Cu plates, and silicon wafers with various in-house-developed metallization structures. During the study, a comprehensive set of bond-quality criteria for Cu-Cu bonding was developed from which bond-process windows were generated to determine the process-parameter ranges needed to create equipment specifications for large-Cu-wire bonding. K&S has developed and thoroughly tested a Cu-only bond head that provides significantly higher bond force, wire-clamp force, and ultrasonic power than is typical of Al-wire bonding. This rear-cut head is compatible with the preferred die-bond configuration and most current power-module designs. V-groove bond tools enable bond shear strengths and pull strengths of 1.0 and 1.5 times the wire’s tensile strength respectively. Tests also revealed that bond-force compression caused substrate protrusion, the severity of which depends on the hardness and roughness of the material and the level of applied force. Large-Cu-wire bonding on Cu die pads is no less essential to higher product reliability than Cu-Cu bonding; however, adhering stiff, large-diameter Cu wire to a fragile, metallized wafer without cratering is difficult. The type of die metallization and the stack structure are critical. In theory, a pad material as hard as or harder than the Cu wire would be needed to withstand the high stress and friction of bonding and to protect the active device structure beneath the bond pad. Nickel (Ni) in a stack with a noble-metal (palladium and/or gold) top layer has been widely used in assembling electronic packaging for wire-bonding applications . Ni has a higher modulus and is harder than Cu, which makes it ideal for withstanding the compression and abrasion that occur during Cu-wire bonding to protect the structures underneath the bond pad. In this study, various nickel-based metallization structures on Si wafers placed on thin Al or Cu films, with a Cu top layer, were analyzed and compared to assess their suitability for large-Cu-wire bonding. It was found that, even under bond forces >8,000gf, a hard, thin film of metal beneath a top Cu layer enables crater-free bonding in certain applications if an optimized bond tool is used. (Figure 1) Fig. 1: 12-mil Cu-wire bonding on Si wafers with Cu top metal  G.Lu, J.N.Calata, Z.Zhang, J.G.Bai, A’ lead-free, low-temperature sintering die-attach technique for high-performance and high-temperature packaging’, Proc. IEEE HDP 2004, pp.42-46;  C.Chen, S.Zhang, S.Lee, L.Mohamed, ‘Investigation on copper diffusion depth in copper wire bonding’, Microelectronics Reliability, 51 (2010), p.166-170;  P.Luniewski, K.Guth, D.Siepe, ‘Cu Bonds and Chip-to Substrate Joints Beyond Silver Sintering’, Bodo’s Power Systems, (www.bodospower.com), August 2010, pp.32-33;  D.Siepe, R.Bayerer, R.Roth, ‘The Future of Wire Bonding Is? Wire Bonding!’, Proceeding of CIPS 2010, Mar 16-18 2010, Nuremberg-, Germany, Paper3.7;|
|Jamin Ling, Director
Irvine , CA