Ion implantation patterning of high temperature superconducting thin films and multilayers

• Main Author: Wong, Andre Wing Gai
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/9922

This thesis was motivated by the suggestion that selectively implanting YBa₂Cu₃0₇ (YBCO) films with a highly reactive ion, such as Si, could pattern without destroying or removing material. If true, this would greatly simplify conventional methods of patterning multilayer structures. This led to systematic studies on the use of ion implantation to pattern YBCO thin films and multilayers. The samples used were obtained by developing a method referred to as scanning pulsed laser deposition. This technique resulted in the reproducible growth of highly crystalline YBCO thin films with high transition temperatures and critical currents. A study of Si implantation was first done to elucidate the exact mechanism by which it rendered YBCO non-superconducting. Measurements of films implanted at various energies and doses revealed that implantation at required doses severely damaged the films crystalline structure, destroying superconductivity. The growth of high quality multilayers require that the underlying patterned film retain its as-grown crystalline quality. A damaged film can regain that quality (to some extent) with high temperature annealing. Measurements of annealed implanted films revealed that crystalline damage at levels >10 displacements per atom (dpa) could not be completely removed at accessible annealing temperatures. However, when the implantation damage was kept below 1-2 dpa, an annealing temperature of ~ 900°C was successful in recovering most of the original structure. Unfortunately, at these temperatures, the Si-implanted film phase separated, forming islands of a Si-mixed material in a sea of YBCO , and thus regained its original high transition temperature. To address the original claims that Si implantation would not destroy the film, a film was rendered non-superconducting with the appearance that it maintained its crystallinity by implanting only near the films surface. In this case, the film was effectively passivated and a low temperature anneal resulted in oxygen leaving the underlying YBCO structure to the more energetically favored Si0₂ states in the implanted layer. Replenishment of oxygen from the atmosphere was hindered due to the passivating layer capping the film. Patterning films by implanting Si was thus deemed to be unsuitable for multilayer structures. A new technique of substitutional ion implantation patterning was developed using Mg ions. Mg substitutes for the Cu in the Cu-0 planes, drastically reducing transition temperature with very low concentrations. The low required concentrations allowed the use of low implant doses. This, coupled with the relatively low mass of the Mg ion, reduced the implantation damage to levels easily removed with high temperature annealing. As well, the high temperature anneal can incorporate Mg into the YBCO matrix, forming the compound, YBa₂(Cu₁[sub -x]Mg[sub x])₃0₇. The process of implanting Mg into YBCO , followed by a high temperature anneal resulted in the formation of a highly crystalline, non-superconducting material at 77K. Mg implantation was used to successfully pattern films with a resolution of 10 μm. Bilayers with a top YBCO layer and a bottom YBa₂(Cu₁[sub -x]Mg[sub x])₃0₇ were fabricated. Resistivity and x-ray measurements reveal the high quality of both layers.