Deposition and Planarization of Epitaxial Oxide Thin Films for High Temperature Superconducting Device Applications.

Planarization processing of interlayer films is required in the fabrication of any multilayer device in order to achieve complete step coverage over patterned surfaces. This issue becomes more complex for multilayer epitaxial structures, however, since epitaxial growth and planarization need to take place simultaneously. A generally applicable epitaxial planarization process is important to fabrication of epitaxial device structures. Realization of such a process will permit complete step coverage as well as prevent growth of misoriented grains over underlying sharp steps. The effects of both macroscopic and microscopic surface relief on epitaxial dielectric film growth and planarization processes were investigated in this thesis. It was found in the course of studying epitaxial planarization of {~}1500 A high steps that the presence of microscopic steps one to two unit cells high also affect epitaxial development of the dielectric film. Both the individual and combined effects of these two types of surface relief were examined specifically in the context of multilayer high temperature superconducting Ba_2YCu_3O _{rm 7-x} (BYC) device processing. The three dielectric materials used in this thesis were yttria-stabilized ZrO_2, CeO _2, and MgO. All three are lattice -matched to BYC and possess materials properties appropriate to device fabrication and performance. Ion beam assisted deposition (IBAD) was used to investigate planarization processing of YSZ and CeO_2 while off-axis rf magnetron sputtering was used to study MgO film growth. Homoepitaxial planarization of IBAD YSZ on patterned YSZ single crystal substrates was demonstrated. BYC films deposited subsequently on planarized YSZ exhibited high J_{rm c} values. A similar trend towards planarization was observed for heteroepitaxial planarization of LBAD CeO_2 films on patterned LaAlO_3. The presence of substrate microsteps in the substrate, however, disrupted epitaxial film growth due to the large lattice mismatch that existed between CeO_2 and LaAlO _3 in the (001) direction. Oriented MgO films were found to grow differently on SrTiO_3 and LaAlO_3. MgO grew epitaxially on SrTiO_3 while film growth of MgO on LaAlO_3 was found to be affected by the presence of native substrate steps. These steps appeared to act as preferential nucleation sites and if properly oriented, helped to reinforce oriented film growth. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.).