Investigation of Ti-line and E-beam Fabrication Processes for Superconducting, Phonon Cooled Hot Electron Bolometers

Two processes were devised utilizing electron beam as well as optical lithography for fabricating niobium nitride (NbN) hot electron bolometers (HEB). This process was developed due to issues with an HEB fabrication method known as the “Ti-line” method. The Ti-line process utilized only optical lithographic means for making HEB devices. This was revolutionary in that HEB’s need nano-scale features which usually necessitate the use of electron beam lithography. By being able to use optical lithographic mean to obtain nano-scale dimensions allows for faster throughput among other things. In all, the Ti-line method was an excellent means for fabricating Nb HEB devices. Unfortunately, when trying to make NbN HEBs the NbN material would have higher than normal resistances as well as resistances that would rise over time. After much work investigating the reason for the heightened resistance, it was determined that an oxygen plasma step was a major factor. Unfortunately there did not appear to be a way to process the HEBs using the Ti-line method without using the O2 plasma.
Electron beam lithography (ebl) is the current state of the art for fabricating HEB devices. In this work two mask designs were devised with the same basic end design as the Ti-line device design except that the new mask sets incorporate an ebl step. Along with this, the two processes have different ebl techniques for obtaining nano-scale dimensions. The reason for having two methods was the ebl system at the University of Virginia is an older system and was not always able to resolve small enough features. Thus two variations were devised in the event the e-beam resolution was not optimal.