Surface tension assisted lithography (STAL): a novel microfabrication techniques for microfluidics

摘要

Many fundamental fields of research have highly advanced in the last three decades due to the unprecedented precision and complexity enabled by the microfabrication technology. Fabrication of 3D microstructures such as simple spherical and cylindrical shapes is highly desired to accurately mimic the natural phenomena in a research environment. Surprisingly, 3D microstructures are commonly avoided if devices are to be realised using typical, planar microfabrication methods due to the limited capabilities of producing 3D structures. In fact, photolithography, the traditional and the most common method for mass-production of microfabricated systems, allows definition of almost arbitrarily complex shapes on planar surfaces, but has limited capability of producing 3D structures. Several other non-planar microfabrication techniques have been reported such as direct laser writing, inclined UV lithography, and the surface wrinkling. However, none can be considered as a true contender to the photolithography, due to the fact that each of them is subject to some combination of the following problems: costly infrastructure, long write times, poor feature addressing, poor resolution, and lack of control. This thesis explores the potential of utilising surface tension driven techniques for 3D microfabrications. The surface tension driven techniques appear promising, due to the key advantages namely, exceptionally smooth surface, cost effectiveness, and self alignment properties. Until now, little attention has been given to the surface tension driven techniques. The major contributions of this thesis include introducing, characterising, and implementing of the novel Surface Tension Assisted Lithography (STAL) technique for 3D microfabrication technique. STAL consists of a sequence of the following steps: soft lithography physically patterns the polymer, then UV exposure defines the reflow container, then a thermal treatment solidifies the container and reflows the unexposed region of the polymer, and finally an exposure ensures that the reflowed structures retain their shape. It is shown that STAL provides independent control over the height and diameter of the semi-spherical structures. There are many possible applications for 3D structures, even in the form of simple spherical caps. One of the applications of semi-spherical structures is demonstrated by fabricating novel semi-spherical microelectrodes for dielectrophoretic manipulation cells. Advantages of semi-spherical microelectrodes over 2D configurations are demonstrated through a series of experiments and numerical simulations. The potential of STAL to produce more complicated systems such as hybrid structures with planar posts integrated into STAL structures is also explored. A simplified model has been developed to predict the defection of the posts under surface tension. In the closing chapter of this thesis, the opportunities to extend STAL for producing more complex 3D structures are identified. Fabrication of convex 3D features with complex containers in the scale of conventional microfluidic structures is investigated. And also, the concept of patterning STAL structures photolithographically is explored. This combination can offer an opportunity to produce structures such as suction cups for hydrodynamic cell trapping.