Electron beam lithography of microbowtie structures for next-generation optical probe

Abstract

The development of microbowtie structures for a next- generation optical probe called the Wave Interrogated Near-Field Array (WINFA) is presented. The WINFA combines the sensitivity of near-field detection with the speed of optical scanning. The microbowties are de- signed to act as resonant elements to provide spatial resolution well below the diffraction limit with a transmission efficiency approaching unity. Following an introduction of the concept and background informa- tion, the design of the microbowtie is presented. A numerical electromag- netic scattering model is developed and used for better designs of the bowtie structures. The electron-beam lithography process is then used to fabricate the final designed bowties structure. Special fabrication proce- dures have been developed to cope with the charge dissipation problem that arises when lithographing an insulating substrate as is required in the present probe design. Two types of substrates and two types of resists are considered in the present study. The fabricated microstruc- tures have 40 nm bowtie gaps that are more than 200 000 times smaller than the one built previously. All fabricated bowtie microstructures are examined and the results are compared. It has been found that, in addi- tion to the relative ease in fabrication, the bowties on indium-tin-oxide coated glass substrate can not only minimize the charge accumulation in a glass substrate, but also satisfy the functional requirement of optical transparency to the incident wave. Recommendations for making a bowtie structure in the even smaller bowtie array are also included.