Cu bonding to co lowK wafers: a systematic study of the mechanical bonding process

Wire bonding using copper wire

“…There is also a need to investigate the effects of the process parameters on the hardness of Cu FABs (Zhong et al, 2007b), because Cu exhibits a larger strain-hardening effect at a higher strain rate (Bhattacharyya et al, 2005). Degryse et al (2005), Hong et al (2005), Tian et al (2005) Chen et al (2006a), Hung et al (2006), Kaimori et al (2006b), Ratchev et al (2006), England and Jiang (2007), Goh and Zhong (2007a) and Zhong et al (2007b) Since copper wire is harder than gold wire, to improve stitch bondability, higher parameter settings have to be used, causing heavy cap marks and potential short tails or wire open. Cu/Au stitch bonds are weak, and thus copper wire bonding has wire open and short tail defects, poor process control, and low stitch pull readings (Goh and Zhong, 2007a).…”

“…The bonding position significantly affects the local stress near the bond, and the wire should be bonded at the pad center Chen et al (2004) The stress is large if the pad size is close to the wire ball size Cu and Au bond wires have a work-hardening effect and higher forces are needed to form a Cu bond, leading to higher stresses in the pad structure Degryse et al (2004Degryse et al ( , 2005 Slip was the major mechanism involved in the overall deformation of polycrystalline copper Hong et al (2005) Higher bonding force and power are needed for the second bond than the first bond to have strong pull force Tian et al (2005Tian et al ( , 2008) Decreased hardness and strength of the HAZ led to breakage sites of the wires to be in the HAZ near Cu balls Hung et al (2006) Annealed Cu wires exhibited tensile strength and elongation characteristics comparable to those of Au wires Chen et al (2006a) With a TiW barrier layer adopted, Cu or Au diffusion to Si is decreased Zhang et al (2006Zhang et al ( , 2007 Cu-Al IMC is thinner than Au-Al IMC at the bonding interface, resulting better bond strength and smaller electrical resistance Pd-plated Cu wire demonstrated excellent reliability and bondability Kaimori et al (2006a, b) A new capillary with a new surface morphology leads to satisfactory results in ball shear and stitch pull tests Goh and Zhong (2007a) Increasing the temperature can enlarge the bondability window and less bonding force can be used England and Jiang (2007) Lower ultrasonic power and bonding force can help minimise pad cratering Plasma cleaning of lead-frames before bonding increases the tail breaking stability significantly Lee et al (2007) Asperity plastic deformation is the most significant factor for good bonding Murali et al (2007) Ultrasonic energy breaks the oxide film and deforms asperities, while the bonding force increases the asperity proximity To have shorter firing time during FAB formation, use a lower contact velocity and provide sufficient inert-gas coverage is recommended for a softer FAB and minimised stress induced during ball bond impact Zhong et al (2007b) Cu/Al IMCs are mainly Cu 9 Al 4 and CuAl 2 , with CuAl present in smaller amounts Hang et al (2008) increasing research on applying FEA to wire-bonded packages (Chen et al, 2006b;Ishiko et al, 2006;Saiki et al, 2006;Fiori et al, 2007a, b;van Driel, 2007;Viswanath et al, 2007;…”

“…The traditional configuration with SiO 2 dielectric and Al interconnection layers is being replaced with low-k dielectrics and Cu interconnection layers (Degryse et al, 2004). Numerical investigations reveal that the yield stress of the wire bond determines the pressure on the pad structure (Degryse et al, 2005). A stiffer and thicker capping lowers local stresses under the bond edge.…”

Wire bonding of low‐k devices

“…A stiffer capping redistributes the deformation over a larger area, leading to a smaller local deformation in the metal layer at the bond edge, and the stress peak decreases Degryse et al (2005) Low-k materials need longer bond time to overcome the energy loss Tan et al (2005) Stiffer wires require higher USG power than a softer wire to achieve equivalent ball sizes and ball shear Soft 4 N wire needs lower USG power and is suitable for bonding of low-k devices The low-k material type is the most effective determinant for the pad deformation Viswanath et al (2005) Smaller copper pad thickness and wire diameter lead to a higher pad deformation Viswanath et al (2007) Thicker copper deposition reduces the pad cupping and leads to better bonding and reduction of cratering problems Decreasing low-k modulus leads to stress increases in the Cu/low-k structure and pad sinking Increasing the cap metallization thickness results in a better Cu/low-k structural behavior and minimises cratering problems The ultrasonic energy transfer of a capillary with a small radius transition is higher than that of a capillary with a sharp transition, resulting in satisfactory ball shear, size and height, and stitch pull Goh and Zhong (2006) A capillary with a smaller chamfer angle, a larger inner chamfer and a larger chamfer diameter can increase the percentage of the IMC in the bond interface, eliminating metal pad peeling and ball lift failures Peeling defects can be reduced by squashing FABs using initial force and bonding force just before bonding starts Kim et al (2006) A larger squashed area can distribute and reduce the stress originated by power onto the bond pads Initial force larger than bonding force is effective to reduce defects The pull force increases with increased bond force Yeh et al (2006) Peeling failure rates significantly depend on wire types used and their respective bonding parameters Fiori et al (2007) Increasing the contact area of the wire to the narrow-pitched pad is effective Inoue et al (2007) 630 mm 2 or the bonding force was above 8 gf. No failure was detected during the reliability tests.…”

“…Higher forces are required to form a Cu bond, resulting in higher stresses in the pad structure. A stiffer capping redistributes the deformation over a larger area, leading to a smaller local deformation in the metal layer at the bond edge, and the stress peak decreases (Degryse et al, 2005). Transient non-linear FEAs of thermosonic wire bonding on Al-capped Cu/low-k structures reveal that the type of low-k materials is the most effective determinant for the pad deformation (Viswanath et al, 2005).…”