Picosecond laser pulses weld glass to silicon for MEMS fabrication

March 31, 2015
A picosecond-laser-based microwelding process can join glass to silicon (Si) quickly and with high mechanical strength.

With microelectromechanical systems (MEMS) as well as MOEMS (MEMS with optical function) devices being created for more and more purposes, including biosciences, consumer applications, and micro-optics, fabrication approaches that provide more options in building complex devices are sorely needed. In response, researchers at Osaka University (Japan), Erlangen Graduate School of Advanced Optical Technologies (SAOT; Germany), Okayama University (Japan), and Tampere University of Technology (Finland) have developed a picosecond-laser-based microwelding process that can join glass to silicon (Si) quickly and with high mechanical strength. Most MEMS and MOEMS devices are based on silicon; this new joining process will boost the field of MOEMS in particular.

Light from a fiber laser that emits at 1060 nm, has a beam quality M2 of 1.5, and produces pulses with a 20 ps duration at a rate of 4 MHz is focused through an objective with a numerical aperture of 0.1 to produce a 10 μm spot at the Si/glass interface through the glass. The samples are moved by a 3D stage with a translation speed of up to 2 m/s, as well as a custom-built translation stage with a speed of up to 5 m/s. A frequency-doubled Nd:YAG laser emitting at 532 nm is used to study details of the welding process. Borosilicate glass (Borofloat 33 and D263 by Schott) and aluminosilicate glass (SW-Y by AGC) can be welded. Borofloat 33 is particularly relevant due to the fact that its coefficient of thermal expansion (CTE) is similar to that of Si. The spatial resolution of the process is about 15 to 18 μm with no pre- or post-heating. Maximum strength of the weld joint is 85 MPa for Si/SW-Y and 45 MPa for Si/Borofloat 33. Welded samples can be easily diced, showing the process’ application to wafer-level packaging. Reference: I. Miyamoto et al., Opt. Express 23, 3 (Feb. 2015); doi: 10.1364/OE.23.003427.

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