Preparation and sealing of polymer microchannels using electron beam lithography to pattern absorber for laser welding

Abstract

Laser welding can make very precise joints in plastics products, both in terms of joint location and amount of heating applied. These welding methods allow complex products such as microfluidic devices to be made, where channels and structure resolution below 100μm is regularly used. However, to date, the dimension of welds made using lasers has been limited by the focus spot size that is achievable from the laser source. Theoretically, the minimum spot size possible from a good quality laser beam is comparable to the wavelength of the radiation emitted. Practically, with reasonable focal length optics, the spot size achievable is a few factors larger than this. The resulting weld even larger than this. The narrowest welds feasible to date have therefore been 10 to 20μm wide using a near-infrared laser source. The aim of this work was make welds less than 10μm wide in PMMA thermoplastic, using EB lithography to prepare laser absorber tracks and channels, followed by laser welding to carry out welds of the order of 1 μm wide. This technique should allow welds to be made below the resolution limit of the near-infrared laser. Welded joints with a width of 1 μm have been achieved and channels with a width of 5 μm. The procedure was based on the principle of transmission laser welding using a thin coating of infrared absorbent material at the joint interface. The coating was patterned using electronbeam lithography to obtain the required resolution in a reproducible manner, and that resolution was retained after the transmission laser welding process. The joint strength was ratified using larger scale samples. The results demonstrate that plastics products could be made with a high density of structure with resolution below 1 μm, and that welding can be applied without excessively heating regions outside the weld lines. This may be applied to smaller scale sensor and analysis chips, micro-bio and chemical reactors using either liquids or gases, and to microelectronic packaging.