Low-Temperature Solid-State Bonding Using Hydrogen Radical Treated Solder for Optoelectronic and MEMS Packaging

Many optoelectronic and MEMS devices require hermetic sealing for protection of the optoelectronic elements and movable fragile parts to ensure stable and reliable performance characteristics. Most commonly used bonding techniques for hermetic sealing are glass-frit bonding, anodic bonding, solder bonding, and eutectic bonding. For many applications, low temperature bonding is desirable because the temperature sensitive materials or components degrade and thermally induced stresses can be created during bonding. Furthermore, since the hermetic sealing is typically the final processing step in device fabrication, bonding method that is tolerant to surface roughness is desirable.

In this study, a low-temperature solid-state bonding process at bonding temperature lower than 200 °C using hydrogen radical treated Sn-3.0Ag-0.5Cu (wt%) solder without flux is investigated for potential application as hermetic packaging. The interfacial reaction between solder and Au is studied by means of scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). The bonding quality is evaluated by means of die-shear test and leak rate test.

The re-oxidation of hydrogen radical treated Sn was studied by X-ray photoelectron spectroscopy (XPS). Fig. 1 shows the typical XPS spectra of Sn plate (purity: 5N) treated with hydrogen radicals (a) and argon fast atom beam (FAB) (b). The XPS Sn (3d_2/5) spectra have been used to follow the surface oxide growth on Sn surfaces as a function of exposure time to air after removal of surface oxide. Two peaks are due to the presence of Sn metal and SnO_x. The surface of Sn plate treated with Ar FAB contained a large amount of SnO_xthan that with hydrogen radicals after exposure time to air of over 20 h. These results indicate differences between the removal of surface oxide by chemical reaction of hydrogen radicals and that by physical bombardment using fast atom beams. The inhibitory effect of re-oxidation of hydrogen radical treated solder [1] was confirmed by XPS measurement.

Fig. 2 shows the process flow of the low-temperature solid-state bonding. Sn-Ag-Cu solder paste (melting point: 217 °C) is deposited by screen printing technique. Then, the surface oxides of Sn-Ag-Cu particles in the paste are reduced by hydrogen radical treatment at 180 ºC. After that, the solder is reflowed at 230 °C for 90 s with additional hydrogen radical treatment for 3 sec. During the reflow process, the Sn-Ag-Cu particles completely coalesce and solder pattern is formed on the wettable metal parts of the substrate [2]. Then, the Sn-Ag-Cu patterns are exposed to air and bonded with Au films without flux at temperatures below the melting point.

Successful bonding was obtained at a bonding temperature above 150 °C for 10 min. Fig. 3 shows the SEM image and EDX map of the cross section of the bonded interface under the application of a bonding pressure of 150 MPa and held at 170 °C for 30 min. The Sn-Ag-Cu solder pattern was deformed to achieve enough contact with Au thin film (thickness: 500 nm) and Au-Sn intermetallic compounds were formed during bonding process as shown in Fig. 3(b). No clear interface corresponding to the original Au surfaces is visible. To evaluate the bonding strength, die-shear test was performed and the die shear strength was over 30 MPa. Fracture mainly occurred in the inside of solder layer, which indicates that solder layer was well bonded with Au.

The Sn-Ag-Cu sealing ring pattern (width: 100 μm, size: 2.4 mm × 2. 4 mm, thickness: 5-10 μm) was formed on a glass substrate (2.8 × 2.8 mm²) by hydrogen radical reflow process for hermetic leak rate testing. The cavity structure (cavity volume: 1.9 × 10^-3 cm³) with Ti (thickness: 50 nm)/Au (thickness: 500 nm) sealing ring was also fabricated on a Si substrate. Hermeticity was evaluated by measuring the resonance characteristics of photothermally excited microcantilevers inside the cavity [3]. The samples bonded at the low temperature of 170 °C under the application of a bonding pressure of 150 MPa showed leakage rates of less than 1.0 × 10⁻⁹ Pa·m³ s⁻¹, which is the rejection limit defined by the MIL-STD-883G specification.

In conclusion, hydrogen radical treated Sn-Ag-Cu solder patterns were bonded with Au thin films at a lower temperature than the melting point of Sn-Ag-Cu without flux. This process is expected to be applicable for chip-size packaging of optoelectronic and MEMS devices.

References

[1] T. Hagihara et al., Electronics Packaging Technology Conference 2008, pp. 595-600.

[2] S. Nishi et al., International Conference on Solid State Devices and Materials 2006, pp. 320-321.

[3] S. Yamamoto et al., J. Micromech. Microeng., vol. 22, 055026 (2012).