Interconnect Bonding


coining at room temperature with 40kgf force, bonding at 210°C for 180s at 5x106 kg/m2 pressure (32.2kgf) over a 512 x 640 array on 10µm pitch. Post bond annealing was performed at 200°C for 20 minutes.


Fig. 6. Optical micrographs of a 10µm pitch die after shear test. Left: Top die. Right: Bottom die


After annealing, electrical testing on the seven bonded devices was done and the median channel yield (1272 daisy chained interconnects/channel) was 92.2% and the median channel resistance was 86Ω. The average resistance of each interconnect including the wiring and pad structures was 68mΩ. Fig. 10 shows a cross-sectional SEM of a 10µm pitch device bonded at 210°C showing similar structure to devices bonded at 300°C. Quantitative energy dispersive x-ray spectroscopy (EDS) was performed on the cross- section to identify the Cu/Sn intermetallic compound present in the bond. EDS spectra and quantitative results for 10µm pitch bonded sample, at the bondline, showed 61% wt. Cu and 39% wt. Sn, indicating


the Cu3Sn intermetallic phase. There is no Cu6Sn5 intermetallic phase or unreacted Sn present.


18


Fig. 7. Optical micrographs of a typical plated Cu (bottom die, left) to CMP Cu (top die, right, same scale) after shear testing


then oven annealed at 200°C for 20 minutes in N2 atmosphere without pressure. Quantitative energy dispersive x-ray


spectroscopy (EDS) was performed on cross- sections to identify the different Cu/Sn intermetallic compounds that are present in the bonds. The four different material regions, labeled A, B, C, and D in Fig. 9, were analyzed by EDS.


The sample bonded at 300°C did not


exhibit unreacted Sn, whereas the sample bonded at 210°C exhibited a region of


unreacted Sn (region D in Fig. 18). The Cu3Sn phase (B) and Cu6Sn5 phase (C) are present in both. In the 210°C sample, the Cu3Sn phase on top and bottom Cu pillars is separated by a thin


(0.2µm) Cu6Sn5 intermetallic.


Shear testing of the 300°C and 210°C bonded devices was performed. The shear test result was ≥10 kg for both types.


Low Temperature Bonding at 10µm To demonstrate the repeatability and test the robustness of the low temperature Cu/Sn-Cu bonding process at 10µm pitch, a qualification of seven bonding runs was done. This run used the low temperature bonding conditions: in situ


Shear Testing of Cu/Sn-Cu Bonded at 210°C Fig. 11 shows representative top and bottom die surfaces from a Cu/Sn-Cu part at 10µm pitch bonded at 210°C after shear testing. The bottom die, right, shows the mechanical BCB key (dark circles around copper landing pads). Cu/Sn intermetallic is visible on the copper pads. Shear failure occurred in the Cu/Sn IMC. The shear strength was 6.0 kg, exceeding the 2.5kg specification without underfill.


Fig. 8. CMP Cu - CMP Cu device after shear testing, showing oxidized bond pads along the edges and corner of the Plated Cu bottom die after shear testing


www.euroasiasemiconductor.com  Issue III 2011


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