Laser micro-welding of silicon and borosilicate glass using nonlinear absorption effect induced by 1558-nm femtosecond fiber laser pulses

The micro-welding technique based on the nonlinear absorption via focused femtosecond laser pulses is useful for welding transparent materials without introducing a light-absorbing intermediate layer. In fact, it has been successful to weld a wide variety of glass materials using 800-nm or 1045-nm pulses. In this paper, we show that this technique can be extended to semiconductor materials, which are opaque in the above wavelength regions, by demonstrating the welding of silicon and borosilicate glass. The key is the use of long-wavelength pulses. We used 1558-nm, 947-fs, 500-kHz pulses from an amplified femtosecond Er-fiber laser. We used a 20× objective lens with a numerical aperture of 0.40 to focus the pulses at the interface of silicon and borosilicate glass, which were mounted on a two-dimensional translation stage. By translating the stage perpendicular to the optical axis in the two-dimensional plane, we produced a 3 × 3 array that consists of welding areas of 100 &mgr;m × 100 &mgr;m. After welding, we performed a simple tensile test. The joint strength was found to be 3.74 MPa, which was on the same order as that between borosilicate glasses (9.87 MPa). Although the welding between silicon substrates is currently hindered by the difficulty of observing focal point with visible light, our result is an important step toward the welding of semiconductor materials, which may have various applications such as three-dimensional stack of electronic devices and the fabrication of micro-electro-mechanical systems.