Fabrication and characterization of Pac-man shaped magnetic tunneling junctions /by Hongmei Han.

As the basic information cell in a magnetic random access memory (MRAM), magnetic tunneling junction (MTJ) and the addressing of its integration issues with the existing silicon technology are critical to improve its performance and competitiveness compared to other emerging RAM technologies. This dissertation uses micromagnetic modeling and microelectronic fabrication method to study the magnetic cell's shape effect on MTJ's magnetic and transport properties. In addition, integration of MTJ on the CMOS line processed metal pads is demonstrated with university facilities.;From micromagnetic simulation, a proposed comma-shaped elongated Pac-man (EPM) shows the best-combined cell selectivity and switching characteristics compared to the existing ellipse and Saturn shapes. Its tilted effective easy axis broke the symmetry of the 'astroid', where enlarged operating window and increased half-selection resistance in the 1st and 3rd quadrants are observed. A back-to-back paired configuration of two 180-EPMs (or half ellipse) and a multi-state MRAM paired cell design are proposed to effectively increase the MRAM density. The results from e-beam patterned magnetic elements match with the micromagnetic simulations.;Various sized MTJs with rectangular, parallelogram, trapezoid and Pac-man shapes are fabricated and studied. Shape mainly affects MTJ's switching characteristics through the variation of its demagnetization field distribution. The integrated rectangular shaped MTJ tends to form vortex which reduces its MR signal up to 60%. MTJ with trapezoid shape shows better properties compared to the parallelogram shaped MTJ due to its confined demagnetic field configuration. Array of 8x8 180-EPM shaped MTJs have narrow switching field distribution. As MTJ's size reduces, it's switching field increases noticeably. The quality of MTJ's barrier layer significantly affects its magnetoresistance (MR). Coupling between the free and fixed layers in a MTJ is detrimental to its MR signal. Through MTJ stack's design with the desired texture and smoothness can effectively reduce this coupling. The MTJs with small barrier height break down at small bias current and are very sensitive to electrostatic charging during the processing.;We concluded that specifically designed shape anisotropy of MTJ cell is an effective way to improve its switching properties and increase the operating window for MRAM devices.Thesis (Ph. D., Materials Science and Engineering)--University of Idaho, December 2008