Fabrication method of micro RF coaxial transmitter on metal substrate combining positive and negative photoresist processes

Abstract

Based on lithography and electrochemical deposition technology, an integrative fabrication method of micro RF coaxial transmitter on copper substrates combining positive and negative photoresist processes is proposed in this paper. In this method, positive photoresist (AZ50XT) was used for fabricating sacrificial layer which worked as master mold for electroforming copper; negative photoresist (SU-8) was used for fabricating insulating layer. However, due to the distribution of light intensity inside the AZ50XT photoresist is uneven during the lithography process, which will cause the sidewall of the film tilted. Thus, a method of multiple exposures and developments was proposed to improve the sidewall inclination of the AZ50XT photoresist and provide an accurate master mold for electrochemical deposition of the structure. Meanwhile, contrast experiments were conducted to verify the practical effect. The experiments results show that when the thickness of AZ50XT film is 65 μm, the sidewall inclination can be increased from 67.4° to 84.5°. In addition, the soft-bake parameter was adjusted to further improve the sidewall inclination of AZ50XT film. Ultimately, based on the experimental results, a micro RF coaxial transmitter was fabricated. The outline size of the coaxial transmitter is 3000 μm × 400 μm × 200 μm.