Fabrication of Terahertz Coupling Structures by Electron Beam Lithography

Abstract

In order to design electromagnetic radiation coupling structures such as antennas and photonic crystals the critical dimensions of the structures must be commensurate with the wavelengths of the radiation of interest. In the case of Terahertz radiation the characteristic wave- length is in the range 1 mm to 30 µm. To achieve high quality structures the fabrication technique must have a resolution capability considerably finer than the wavelength. For Terahertz coupling structures this constraint is at the limit of many conventional machining based fabrication capa- bilities. One technique to overcome this resolution issue is to pattern structures using electron beam lithography (EBL) and then use semiconductor fabrication techniques to produce the final structures. This technique provides a flexible patterning capability for fabricating devices with a wide range of topologies and sizes. The principle advantages of EBL are the ability to pattern features with dimensions in the micron and sub-micron range as well as the ability to directly pattern structures on a substrate without the need for a prefabricated mask. The technique also affords the flexibility to change the pattern topology by altering the lithography data file and it provides the ability to fabricate structures containing elements ranging in size from microns up to millimetres on the same substrate. This enables structures which operate over a wide band of Terahertz frequencies to be patterned. The structures are defined using conventional CAD drafting packages and then converted into lithography data files for exposure. The structures are patterned using a scanning electron microscope which has been modified for direct write EBL. The planar structures are exposed in a layer of electron sensitive resist, which after developing produces an opening in the resist in the shape of the desired structure. This opening is used for the deposition of metal. The resist is then removed leaving the structure on the substrate. In situations where electrical contact is required part of the structure can be patterned to form the contacts for bonding to other components in a system.