High T c Superconducting Thin Films
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clude electron-beam and resistanceheated sources, single and multigun sputtering, MBE, laser deposition spin pyrolysis, and plasma spraying. 3 " 15 In this article we will illustrate these developments by discussing electron-beam and sputter deposition techniques, particularly as used at IBM. Early results using these techniques have led to films with reproducible critical temperatures (Tc)15 and high critical current densities (Jc),16 and have been used to fabricate sensitive SQUIDs that operate above 77 K.17 Control of such parameters as choice of substrate, substrate temperature, and annealing temperatures have been demonstrated to be important for fabricating high Tc films. Of course compositional control is vital, especially considering the many phases possible for these materials. These four and fiveelement materials will continue to be the subject of many investigations for years to come. In general, the fabrication of thin films by itself is not sufficient for applications. Of equal importance is the ability to process these thin films into useful forms and patterns. Multilayer capabilities are also important for future practical uses. The fabrication, patterning, a n d m u l t i l a y e r i n g m u s t be accomplished with as little change in the material parameters as possible. Initially, we used ion implantation in our patterning procedure, which generally led to some deterioration of the high Tc material. Patterning with ion implantation also required using lithographic techniques.18 Photo and electron beam lithography techniques have now been applied with m u c h more success to these materials and line widths of a few microns can be generally obtained. Ion milling or wet processing are most commonly used for film patterning although lift-off techniques are being studied. In
fact the application of previously known techniques to the processing of the high Tc materials has been surprisingly successful. This situation will continue to improve as processing t e m p e r a t u r e s decrease. In the next section we will describe the fabrication of high Tc thin films, using YBaCuO as the prime example with reference to similar work in the TIBaCaCuO and BiSrCaCuO compounds. We will also describe some of the measurements being made on such thin films in order to understand their properties. There are far too many property m e a s u r e m e n t s b e i n g m a d e on these films around the world to adequately cover them here and we will only discuss a few which are particularly related to film quality and usefulness. The second main section of this article will deal with applications and processing of high Tc films. Again, not all work can be included. We will mainly discuss some IBM work which is fairly representative of much other work listed in the references. Fabrication and Characterization Although the first films superconducting above 77 K were fabricated in a vapor deposition system containing three separate electron-beam-heated sources, 3 many other techniques have b e e n u s e d to p r o d u c e h i
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