Fabrication of Stepped and Reflowed 3-D Profiles for Optical Applications by Dose-modulated Electron Beam Lithography and Selective Thermal Reflow

Abstract

Microlens and prism arrays were fabricated using multi-level electron beam patterning combined with thermal reflow. The molecular weight dependent processing allows selective transfer of stepped into sloped resist structures with smooth surfaces.Optical elements with combined diffractive and refractive properties often need surfaces with 3-D profiles, e.g. lenses and prisms, in addition to binary gratings with defined box-type ridges. Normally these elements can only be achieved by the combination of different lithographic steps or with grayscale techniques based on dose-modulated optical or electron beam lithography (DM-EBL) with its limited ability to approximate arbitrary shapes with smooth surfaces. As an alternative, thermal reflow is widely used to generate microlenses with smooth surfaces for optical applications. Spherical surfaces result from the fact that surface tension tends to minimize surfaces and therefore results in convex refractive lenses defined by the outlines and height of the original binary resist structure alone [1,2]. The diversity of 3-D shapes, e.g. prisms with linear slopes and sharp features, lenses with arbitrary convex and concave profiles, or combinations of both, is very restricted. We have now combined DM-EBL with thermal reflow. Starting from 3-D multi-level structures in high molecular weight M w poly(methyl methacrylate) (PMMA) resist, reflow at a moderate temperature does not lead to the formation of the spherical forms defined by surface tension, but profiles matching that of the original stepped structure and with smooth outlines. A multi-level step structure with only a few steps can be easily transferred into a linear slope, simply by smoothening out the surface undulations given by the steps, resulting from lithographically defined local variations of M w [3]. Fig. 1. Process chain or schematic: 3-D resist shapes are achieved by dose-modulated EBL and development. Sharp staircase profiles can be transformed into continuous surface reliefs by thermal annealing at T ~Tg while vertical sidewalls stay unaltered.The main difference to normal reflow is, that local difference in glass transition temperature T g caused by M w reduction in exposed areas can be used to selectively transfer exposed stepped structures into continuous slopes during thermal reflow, while other structures with vertical sidewalls stay unaltered. Prism structures with slopes with 1 µm height and up to 45° inclination were generated using different combinations of doses and reflow temperatures. A variety of slope angles and orientations can be fabricated on the same substrate. Fig. 2. Micrographs of multi-level resist structure (left side, 520 nm high PMMA) with 4-levels of 0.5 µm step size transformed into a structure with sloped sidewalls (right side) with 2 µm slope by thermal post-processing (mid section shows an intermediate reflow state).

Spontaneous Formation of Structures with Micro- and Nano-Scopic Periodic Ripple Patterns

“…Each ripple pattern shown in Figure is created in a single step, with a high degree of repeatability, without requiring any special environment and dedicated instrumentation. On the contrary, creating three-dimensional patterns with sub-micrometer features requires high-end equipment, such as electron-beam lithography (EBL), in conjunction with multiple “masking-etching-writing” steps. , Furthermore, the features sizes, e.g., depth and width of trenches as well as the adjoining hills, can be varied over orders of the length scales (e.g., the trench width of hundreds of nanometers to tens of micrometers), and the patterns can be created over larger distances (e.g., a few micrometers to a few centimeters). Given this versatility and the potential application of these structures in the diverse fields of optics, sensing, microfluidics, die-printing, etc., , we conduct this study to understand the mechanism of the formation of ripples.…”