Hermeticity and Reliability of Al-Al Thermocompression Wafer Bonding
Authors: Malik Nisant , Poppe Erik , Schjolberg-Henriksen Kari , Taklo Maaike Margrete Visser , Finstad Terje G 
Published: February 2020

Thermocompression bonding using deposited metal films as bonding material is a promising method for realizing a conductive and hermetic seal for device encapsulation [1]. Al is of high interest as bonding material due to its CMOS process compatibility [2]. This paper reports on the hermeticity and aspects of reliability of Al-Al thermocompression bonds.
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. The bottom wafer had 3 µm high protruding frame structures of width 20, 40, 80 and 200 µm, which defined the bonding area. The total bonding area on a 150mm wafer was 590.17 mm2. Both wafers had a SiO2 layer of 7500 Å. A 1 µm thick layer of Al was sputter deposited and patterned on all wafers. Top and bottom wafers were aligned and bonded in an EVG®510 bonder. The ambient pressure was maintained below 1e-3 mbar during bonding. The selected range of bonding parameters is given in Table 1.

After bonding, the inward deflection of the membranes was measured by white light interferometry. The wafers were stored for 3 months after bonding and then 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 checking the membrane deflection after each test.
The inward deflection of the membranes was measured after bonding and after 3 months of storage. 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 6.6 µm. Laminate H-4 had the highest hermeticity yield of 91.61 %, indicating that good bonding can be achieved by bonding wafers for only 15 minutes with a bond force of 60 kN at a bonding temperature of 350 °C. Laminates bonded at a temperature ≥350 °C with a bond force of 60 kN for at least 15 min (i.e. H-4 to H-8) also had high hermetic yield. The low hermeticity yield of laminates H-6 and H-7 is probably due to misalignment (for the widest frames the yield was almost 100 % whereas it was low for the narrower frames). The low hermeticity yield of laminates H-1 to H-3 shows that a bonding temperature of 300 °C or a bond force of 36 kN is not sufficient to produce hermetic bonds.
No changes in membrane deflection were observed in any of the bonded laminates after exposure to the steady-state life test or thermal shock test. Flat membranes were observed in a few dies after exposure to moisture resistance test (MRT) as shown in Table 1. The reason of the leak is unknown. Laminates H-1 to H-3 and the misaligned laminates H-6 and H-7 had a higher percentage of leaky dies supporting the low hermeticity yield results. Laminates H-4, H-5 and H-8 had a lower percentage of leaky dies supporting the high hermeticity yield results.
The above presented results indicate that for our laminates, Al-Al thermocompression bonding at 350 °C, applying 60 kN for 15 min can produce hermetic bonds that can withstand some key environmental stress conditions.

References

[1] S.Farrens,“Metal based wafer level packaging”,Suss white paper,2005. http://www.suss.com/fileadmin/user_upload/technical_publications/WP_WLP_MetalBasedWaferLevelPackaging_0810.pdf
[2] N. Malik,K. Schjølberg-Henriksen,E. Poppe,M.M.Visser Taklo,T.G. Finstad, “Al Al thermocompression bonding for wafer-level MEMS sealing”, Sens. Actuators A: Phys.(2014), http://dx.doi.org/10.1016/j.sna.2014.02.030.