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

Abstract

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 O 2 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. To the UVML students I worked with over the years. There are actually too many to name going back from when I started at UVa. I just want to say thank you for the collaboration and friendships. List of SymbolsMy predecessor on this project, Bert Herold, thank you for getting me up to speed on it in short order and all the work you put in.To Deborah Kidd, thank you for taking the time to get us started using the e-beam system. Jian Zhang and Jie Wang, your help and advice was not just appreciated but many times needed for processes to work correctly. I am grateful to have worked with you both over the years.To the UVML staff: Joe Beatrice, Alex Lobo, Michael Stogoski, Tim Pernell, and Harry Wade (ret.).Your knowledge, skill, and intuition kept the lab running when it probably shouldn't have. Also, I enjoyed our conversations on a variety of topics while repairing various pieces of equipment.Barbara Graves for help getting my admin items in order especially after I started work.Many thanks to my parents and brothers for their love, support, and encouragement. National Ground Intelligence Center and National Science Foundation for funding the project. . The basic operation of these HEB devices and basic mixer theory, will be described further in chapter 2.In the past decade a novel HEB fabrication method, called the Ti-Line process, was developed at the University of Virginia [7]. This fabrication method was radically different from other...