Revolution in Large-Area Curved Surface Lithography: Nanofilm Sculpting by Thermocapillary Modulation

Citation

Lim, Soon Wei Daniel (2017) Revolution in Large-Area Curved Surface Lithography: Nanofilm Sculpting by Thermocapillary Modulation. Senior thesis (Major), California Institute of Technology. doi:10.7907/Z9BP00TZ. https://resolver.caltech.edu/CaltechTHESIS:05162017-143757221

Abstract

Conventional lithography excels in producing blocky structures but has difficulty producing out-of-plane curvature. Such curvature is necessary for optical elements such as microlens arrays. Spatiotemporal control of the surface tension of liquid films offers a powerful method for sculpting myriad 3D shapes, thereby meeting this deficiency. In the Thermocapillary Lithography (TCL) project, we modulate thermocapillary forces by local control of surface temperature to deform a flat nanofilm into a variety of structures, which are then solidified in situ. In this thesis, we present two facile means of projecting the required temperature field, which we call Conduction TCL and Laser-induced TCL. In the former, which is a detailed expansion of the work performed in this group, the Laboratory of Interfacial and Small Scale Transport {LIS2T}, we place an array of chilled, prefabricated pins in close proximity to the film to provide precise thermal control via conduction. In the latter, which is new and has not been realized in literature yet, we project a spatially-modulated laser light field onto a horizontal heated fluid to achieve the same film deformation. Laser-induced TCL is shown to be a fully non-contact means of fabrication that admits real-time monitoring of the film profile. We demonstrate that the resultant temperature gradient field is capable of sculpting complex structures such as refractive optical elements, multiscale protrusions and depressions, arbitrary 2D images, as well as waveguides. By varying the pattern width, pitch and evolution time, we have also fabricated plano-convex, plano-concave, caldera-like, and hierarchical Microlens Arrays (MLAs) with ultrasmooth surfaces. As a proof of concept, the diverging arrays were incorporated in an adaptive optics component for wavefront sensing. This is the first functional optical device fabricated by modulation of the thermocapillary instability. Furthermore, the ultrasmooth out-of-plane curvature accessible through TCL is ideal for fabricating curved mirrors at the microscale. We exploit this property to fabricate the first large-scale optical microcavity array with curved mirrors for optical filtration. In the process, we developed a conformal, room-temperature metallization protocol for thermosensitive surfaces. In all, TCL is shown to be a facile, single-step means of fabricating complex ultrasmooth topologies, and opens up the possibility of printing planar optical circuit elements and beam shaping topologies on demand.

Thesis Files