By I. Miyamoto, Y. Okamoto, A. Hansen, T. Amberla, J. Vihinen, J. Kangastupa

Silicon/glass is one of the most widely used material combinations for sensing and actuating microsystems and micro electronic technologies. Anodic bonding has been most widely used for Si/glass bonding since its invention in 1969, because of its excellent mechanical properties and process throughput. There are, however, some disadvantages in anodic bonding that no space selectivity is available, and high temperature heating is needed for long time with applying high electric field. While several laser-based joining techniques including eutectic bonding, SLB (selective laser bonding) and fusion welding using ns laser pulses have been developed by several groups for utilizing their excellent space selectivity, the joint strength and process throughput are far behind the anodic bonding technique.

The authors have developed a novel fusion welding technique of Si/glass with high spatial resolution down to approximately 10µm at high throughput using ultrashort laser pulses (USLP) of high pulse-repetition rates without pre- and post-heating. Our results indicate that the mechanical strength of the weld joint and process throughput are at least competitive with anodic bonding, showing USLP has brought a new wave not only in welding of glass/glass  but in dissimilar materials like Si/glass.

For welding experiments, USLP (duration=20ps, wavelength=1060nm) focused by a lens of NA0.1 is irradiated to the interface of Si/glass. As the glass samples, Pyrex (Corning) or Borofloat 33 (Schott) with a thickness of 1mm is mainly used. The welding experiment was performed at different welding speeds and pulse repetition rates up to 4MHz, and the mechanical strength of the weld joint is evaluated by a shear test.

The shear strength of the weld joint as high as 70~90MPa has been obtained at a pulse energy of 3µJ, for instance, at repetition rates in a range of 1~2MHz. High mechanical strength of the weld joint is attributed to the anchor effect of intricate Si/glass interface geometry, which is produced by the convection of molten Si and glass at the interface. Welding speed can be increased in proportion to the repetition rate of the laser pulse where no upper speed limit is found in principle.

Applicability to wafer level packaging utilizing excellent space selectivity is demonstrated by drawing a grid-geometry of a dimension of 3mm with a street width of 400μm, and the laser-welded samples are successfully separated along the street using a standard blade dicer without any defects. No damage is also found with keeping hermetic sealing by accelerated life test (500 cycles between -40˚C and 85˚C) in laser-welded silicon-Borofloat 33 sample, showing possibility of versatile applications in different field.