Abstract
Hermeticity, reliability and strength of four laminates bonded at different temperatures by Au-Au thermocompression bonding have been investigated. Laminates with a diameter of 150 mm were realized by bonding a wafer containing membrane structures to a Si wafer with patterned bond frames. A bond tool pressure of 2266 mbar was applied for 15 minutes at temperatures ranging from 150–300°C. The hermetic properties were estimated by membrane deflection measurements applying white-light interferometry after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Bond strength was estimated by pull test measurements. A dicing yield above 90% was obtained for all laminates. Laminates bonded at 200°C and above had significantly higher hermetic yield than the laminate bonded at 150°C. No degradation in hermeticity was observed after the reliability tests. The maximum leakage rate (MLR) was estimated from two measurements of membrane deflection executed at two different times and was below 10−11 mbar ⋅ l ⋅ s−1. The average bond strength ranged from 44 to 175 MPa.
Highlights
Micro electro mechanical systems (MEMS) devices, especially inertial sensors, such as mechanical resonators, gyroscopes and accelerometers, can have fragile parts which need to be sealed in a high vacuum environment for high performance and long life time
Inward membrane deflection of the sealed cavities was observed after bonding
After a storage period of 3–5 months, an average change of ∼0.16 μm in membrane deflection of inward deflecting membranes was observed for all laminates with a standard deviation of 0.05 μm
Summary
Micro electro mechanical systems (MEMS) devices, especially inertial sensors, such as mechanical resonators, gyroscopes and accelerometers, can have fragile parts which need to be sealed in a high vacuum environment for high performance and long life time. Hermetic packaging is an essential requirement for such environment sensitive MEMS devices.[1]. Metal thermocompression bonding is a promising technology for hermetic encapsulation of MEMS devices. Successful Au-Au thermocompression bonding has been reported[5,6,7] in the temperature range 260 to 350◦C. Bonding of porous Au has been performed at room temperature.[9] To our knowledge, reports on the reliability and hermeticity of Au-Au thermocompression bonds have not yet been presented. In order to test the reliability of a bonded seal, a sub-set of exposure conditions is chosen. The initial screening of the die seals was done by optically measuring membrane deflection after bonding. To check the reliability of the bonded seals, changes in membrane deflection following steady-state life, thermal shock and moisture resistance tests were measured. The bond strength was investigated by counting the dicing yield of laminates and by pull test measurements on individual dies
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