The frontiers of magnetic bubble technology

Two areas of major emphasis in the development of magnetic bubble memories are discussed: the effort to increase the bit-packing density and the effort to increase the operating speed of these memories. It is pointed out that increasing the density by scaling down an existing chip design involves consideration of not only lithographic and fabrication problems, but also magnetic materials problems. High density chips may be obtained with at least three different chip configurations: 1) the familiar Permalloy-bar file (PBF) employing T-bar figures, but fabricated with electron-beam lithography and single-level masking; 2) the contiguous-disk file (CDF) which utilizes relatively coarse Permalloy patterns and hence eases lithography requirements; and 3) the bubble-lattice file (BLF) which employs closely packed bubbles with a novel wall-structure data-coding scheme. The relative advantages of these three schemes are compared: the PBF has been developed the farthest and is the only configuration employing single-level masking; the highest density is offered by the CDF if lithography is not the limiting factor, and by the BLF if the achievement of small-diameter bubbles is the limiting factor. Two factors which govern the operating speed of a bubble memory are discussed: the velocity of propagation of a bubble and the organization of the memory so as to achieve the shortest possible access and latency times. Significant progress is being made in the development of bubble materials which support high bubble velocities; this progress is based on understanding of complex changes in wall structure which occur during bubble motion. The advantages of the major/minor loop and coincident-selection organizational schemes in reducing access times and in sharing interconnections and electronics are reviewed. The capabilities of the magnetic bubble memory technology are briefly compared with those of the semiconductor and magnetic disk technologies.