Electrical, Mechanical, and Hermeticity Properties of Low-Temperature, Plasma Activated Direct Silicon Bonded Joints

Low-temperature silicon direct bonding enabled by plasma activation (plasma bonding) is widely used in the fabrication of silicon-on-insulator (SOI) material. We expand the current knowledge by electrical and mechanical characterization of plasma bonded interfaces. Further, we report the first assessments on hermeticity properties of plasma bonds subjected to environmental testing. C

Two types of test laminates were prepared: hermeticity laminates (Herm) and laminates for electrical and mechanical measurements (ElMech). Cross-sectional sketches of dies of both types are shown in Figure 1. Herm wafers had 481 membranes of dimension 2500 × 2500 × 36 µm³. ElMech wafers had 169 frame structures of widths 100, 200, or 400 µm protruding 6 µm above the silicon surface. Wafer ElMech1 was boron implanted on both sides for electrical contact. Laminates were made by silicon direct bonding between Herm or ElMech wafers and plane silicon wafers, which was boron implanted for laminate ElMech1. All wafers were activated for 1 min in an O₂ plasma in an AMS-200 (Alcatel) with a source power of 2500W and a bias power of 180W. After activation, the ElMech and Herm wafers were dipped in DI-water for 1 minute and dried. Laminates were bonded applying a bonding pressure of 350 mbar for 2 minutes at 50^oC in an ambient pressure below 5×10^-3mbar. Aluminium contacts for electrical measurements were made on both sides of laminate ElMech1.

ElMech laminates were diced into individual dies. The dicing yield was recorded for each frame design. Dies from laminate ElMech1 were characterized by capacitance and I-V measurements. The deflection of individual membranes was measured by white light interferometry three times over a period of four months. Pieces containing 25 membranes were subjected to environmental testing consisting of 1000 hours of storage at 150° C, 50 cycles of thermal shock from -65° C to +200° C, and 10 cycles in 90-100% humidity from 25° C to 65°C.

The dicing yield of laminates ElMech1 and ElMech2 is shown in Figure 2. Frames of width 100 µm had low yield of 17 – 30%, while frames of widths 200 and 400 µm had yield of 39 – 58%. Considering that the applied frame design is very poorly suited for the continuous propagation of a bonding wavefront [1], the dicing yield indicates that satisfactory bond strength was obtained.

Capacitance values were obtained for all measured samples, corresponding to an SiO₂ layer of thickness 11 – 14 nm at the bonding interface. However, the I-V measurements showed an ohmic behavior without hysteresis, showing a resistance of 2.2 W which was independent of frame width, i.e. bonding area. This result contradicts earlier results that the current is restricted at the bonded interface [2]. The calculated oxide thickness of 11 – 14 nm is in agreement with the 8 – 10 nm oxide thickness found by capacitance-voltage measurements in a previous study [2]. The same study reports a discrepancy between ellipsometry and capacitance measurements [2]. Further investigation is needed to verify the physical thickness of any SiO₂at the bonding interface of our samples. In addition, the simultaneous capacitive and resistive behavior of the bonded dies must be explained.

No significant change in the membrane deflection was observed over the course of 4 months. This means that the leak rate was at least below 1.54×10^-11 mbar l s^-1, which is agreement with earlier results [3]. Longer storage time can reveal that the actual leak rate is even lower. No visible leaks were found in any of the 2 × 25 membranes that had undergone environmental testing. The presented results indicate that the bonds are conductive, hermetic, and that they can sustain environmental stresses.

[1] Q-Y.Tong, U. Gösele, Semiconductor Wafer Bonding, Wiley, 1998

[2] P. Amirfeiz et al., J. Electrochem Soc 147(7) 2693, 2000

[3] M.M.V. Taklo et al., Sens. Act. A 97-98, 434, 2002