Gamma rays challenge e-beams for writing waveguides

Researchers at National Taiwan University (Taipei, Taiwan) and National Tsing-Hua University (Hsinchu, Taiwan) are showing that gamma-ray irradiation may be superior to electron-beam (e-beam) direct writing for the fabrication of polymer optical waveguides.

Although e-beam irradiation can be used to fabricate waveguides on polymers, in particular, benzocyclobutene (BCB), a popular polymer used for flat-panel displays and microelectronics packaging, the process is complicated and time-consuming for large-area applications. A simple one-shot irradiation dose of gamma rays from a Cobalt 60 source on a gold/chrome-masked BCB substrate, however, can easily create 1.2-µm-thick and 2-µm-wide waveguides with TE- and TM-mode losses of 0.82 and 1.08 dB/cm, respectively, after annealing the waveguides. These loss values are smaller than those of waveguides fabricated using e-beam technology. Furthermore, gamma rays are capable of inducing refractive-index changes as high as 0.16 in BCB materials, and 0.06 in. polymethylmethacrylate materials. The use of tungsten as a mask material (more resistant to gamma rays than gold) should further improve waveguide performance. Contact Way-Seen Wang at [email protected].