Enabling fine pitch Cu & Ag alloy wire bond assessment for 28nm ultra low-k structure

Abstract

The use of copper wire in IC packaging has been growing steadily driven by cost effectiveness. However, there are concerns and issues that prevent or delay copper wire bonding technology qualification. Copper wire inherent hardness properties induces higher stress on bond pad and underlying pad structure, which results to inevitable pad crack or damage if pad structure is not robustly design or thin aluminum pad thickness of ≤0.8um for copper wire bonding. Silver alloy wire on the other hand showed great potential in IC packaging around ~20% softer compared to harder copper wire. Copper and silver wire are also cost competitive than gold wire. Both wires may potentially take over gold wire bond especially in handheld devices in the coming years. Compared to gold, silver alloy wire is superior in terms of electrical and thermal conductivity and their mechanical properties are quite similar. Silver and copper wire has similar thermal conductivity and both wires still require inert gas environment like pure nitrogen (N 2 ) and forming gas (95% N 2 , 5%H 2 ) during free-air-ball (FAB) formation. An axisymmetric transient nonlinear dynamic model for fine pitch copper wire bonding was developed with FEM simulation software ABAQUS/Explicit in order to assist in understanding the copper and silver wire bond process focusing on stress or strain investigation at aluminum and low k layers. FEM was focused on Cu wire due to its inherent hardness properties compared to softer silver alloy wire. Simulation result showed that majority of ball bond and Al pad deformation is completed in the very beginning when bond force is applied. Similar to the highest stress on low-k layer is also in the very beginning of bond force application and near the bond pad center area. This study aimed to share the learning, challenges and success story for Cu and silver alloy wire bond characterization using different fine wire diameter sizes for 28nm ultra low-k wafer technology on a FBGA 9×9mm package. Fine wire diameter sizes of 18um, 15um and 13um for both palladium coated Cu (PdCu) and silver (Ag) alloy wire types where characterized and compared based on the output responses results gathered during DOE assessment. Over all wire bond DOE results showed good interaction result with no pad crack/damage for optimum wire bond recipe and validated by FIB cut showing no abnormality on low-k layer. Stress test reliability performances for both wire types were assessed and showed no abnormality. Sliver alloy wire sample wet acid decapsulation process has been a concern once failure analysis is required to ascertain bonding wire connection integrity due to wire corrosion, cracking, thinning and wire easily breaks. With the adoption of laser decapsulation process similar to laser type used in laser grooving of wafer and without using any acid have shown a very good result as depicted in figure 12. A more consistent intermetallic assessment for Ag/Al IMC at ball bond side and easier to quantify similar to conventional Au/Al IMC is recommended as compared to bond pad side having lower Al/Ag IMC with wider variance. Finally, with the combined simulation analysis data, experimental DOE's and initial reliability data result, FBGA 9×9mm package was assembled using a more challenging harder wire type of palladium coated Cu with 18um (0.7mil) wire size. Package reliability qualification per industry JEDEC standard for laminate packages passed with no failure as summarized in table 11.