Hermeticity and Reliability of Au-Au Thermocompression Bonds, Realized at Low Temperature

Au-Au thermocompression bonding is a reliable and space-efficient method for wafer-level sealing [1]. It is commonly performed at bond temperatures of 300 – 400 ^oC [1-3] and bond pressures of 0.5 – 15 MPa [1, 3]. He leak rates of 2.7 ×10^-10 atm cm³ s^-1 have been presented [2]. We have explored the lower temperature limits for realizing Au-Au thermocompression bonds with high yield. Further, we have investigated the reliability of the bonds when subjected to environmental stress test conditions. 

Test laminates were prepared by bonding bottom Si wafers to top Si wafers with 481 membranes with a nominal thickness of 36 µm and nominal dimension 2.5 × 2.5 mm². A cross-sectional view of a single die is shown in Figure 1. Both wafers had a SiO₂ layer of 7500 Å. Metal layers of 400 nm TiW and 1.2 µm Au were sputter deposited and patterned to form frames on all wafers. The nominal width of the bonding frames was 20, 40, 80, and 200 µm.  The total bonding area on a 150mm wafer was 504 mm². Top and bottom wafers were aligned and bonded in vacuum ambient in a SB6e (Suss MicroTec.) bonder applying a tool pressure of 2266 mbar for 15 minutes at temperatures ranging from 150 ^oC to 300 ^oC. The targeted bond pressure was 8.5 MPa. The bonded laminates and parameters are given in Table 1.

After bonding, the inward deflection of the membranes was measured by white light interferometry. The wafers were subjected to environmental stress conditions consisting of steady-state life test (exposure to 150 °C for 1000 h), 50 cycles of thermal shock from -65 °C to +200 °C, and 10 cycles in 90 – 100 % humidity from 25 °C to 65 °C. Characterization was done by investigating the membrane deflection after each test.

The hermeticity yield was defined as the percentage of membranes deflecting inwards by more than 2 µm after bonding, and is shown in Figure 1 and Table 1. An average membrane deflection was 5.5 µm. Laminates Au200 and Au250 had hermeticity yields of 65 – 75 % for all frame widths, except 200 µm wide frames on Au200. Laminate Au150 had yields of 3 – 11 % for all frame widths. Laminate Au300 appeared to be misaligned since the yield was below 2 % for frames of widths 20 – 80 µm and 54 % for 200 µm wide frames. The results indicate that at 8.5 MPa bond pressure, there seems to exist a threshold temperature for bonding between 150 ^oC and 200 ^oC. Bonding at 200 ^oC and 250 ^oC seem to be of equal quality.

The membrane deflection measurements provided an estimate of the maximum air leak rate of 7.4×10^-12 atm cm³ s^-1. Longer storage times can reveal more accurate leak rates. No changes in membrane deflection were observed in any of the laminates after exposure to the steady-state life test, thermal shock test or moisture resistance test. The results indicate that Au-Au thermocompression bonds realized at 200 ^oC and 250 ^oC are hermetic and can withstand environmental stress test conditions.

References

[1] R. Fraux, J. Baron, 3D packaging, 21, 24-27, 2011

[2] G.-S. Park et al., Electrochemical and Solid-State Letters, 8 (120), G330-G332, 2005

[3] Xu et al.,   IEEE Transactions on Advanced Packaging, 33( 4), 904-911, 2010