ICMCTF2000 Session H1: Magnetic Materials

Electrosynthesis of alloy films has been the subject of many meticulous studies. This is due to fact that these materials present in thin film form have several advantages relative to bulk materials such as enhancement of mechanical and magnetic properties. Presently much attention has found on the preparation and characterization of such ultrastructured materials that have giantmagnetotresistance (GMR) properties. The study of electrodeposition of CuFe alloy is particularly interesting because previous studies indicate that this alloy is promising GMR material.

CuFe alloy films were galvanostatically eletrodeposited from aqueous and nonaqueous (etanediol) baths using copper sulfate and ferrous sulfate as initial ingredients. The deposition potentials have determined from polarization curves. All deposition potentials have measured with respect to SCE. Effect of bath composition on composition of Cu and Fe in deposited alloy film for both aqueous and nonaqueous baths has studied using atomic absorption spectroscopy. For 16: 4 (0.1 M FeSO₄ : 0.05 M CuSO₄) the composition of deposit was 2 : 1. Effect of various substrates (stainless steel, brass, copper, titanium), electrolyte temperature, complexing agent (tri-sodium citrate) on deposition potentials and surface morphology has studied. For surface morphology SEM technique has used.

In the present investigation, the CuFe alloy films have electrochemically oxidized at room temperature using simple electrolyte (1 N KOH) cell. The effect of oxidation time and current density on the structural properties of these films has studied using XRD technique. It has found that the oxidized films show CuFe₂O₄ phase with hexagonal crystal structure. Surface morphology of the film has examined using SEM technique. The films are looking smooth and homogeneous.

Magnetoresistive Random Access Memory (MRAM) is based on magnetic memory elements integrated with CMOS. Key attributes of MRAM technology are nonvolatility and unlimited read and write endurance. A leading candidate material for MRAM storage elements is the Magnetic Tunnel Junction (MTJ), a thin-film stack that includes magnetic electrodes separated by an insulating spacer layer. In an MTJ-based element the current flows perpendicular to the layers, tunneling through the thin dielectric spacer layer, typically aluminum oxide. The resistance of the memory bit is either low or high depending on the relative polarization, parallel or anti-parallel, of the magnetic electrodes. A typical MTJ stack also includes a mechanism to pin the polarization of one of the magnetic layers in a fixed direction. The direction of polarization of the unpinned magnetic layer (free layer) switches between parallel and anti-parallel, providing the two resistance states used for information storage. The switching field is produced by current flow through wires patterned below and above the memory element. The memory element itself is patterned to submicron dimensions. Desirable properties of MTJ material, which we have demonstrated, include its high magnetoresistance ratio (MR), in the 30% to 40% range, which results in a large signal, and its relatively high resistance, necessary for high-speed read operations with low operating voltage.

The thickness of the critical layers, including the tunnel barrier, fixed layer and free layer, is typically 15 to 40 Angstrom. Although thickness uniformity and repeatability of all the layers is important, the most sensitive layer in the MTJ stack is the AlOx tunnel barrier. The tunnel barrier is very thin, ? 20 Å, and the resistance of the element is exponentially dependent on the barrier thickness. Uniformity of the MR and the absolute resistance of the cell are critical, since the absolute value of the MTJ resistance is compared with a reference cell during read mode. This sets strict requirements on the AlOx process uniformity since the effect of thickness variation on resistance is amplified by the exponential dependence on thickness. Results are presented for material with AlOx barriers that are pinhole free, very smooth, and extremely uniform over a wafer (1-sigma < 1%). Another important layer to control is the free layer. In patterned bits, the thickness of the free layer is directly related to the field, or current, required for switching the bit. Thin layers are desirable because they result in low switching fields for low power write operations. However, there are fundamental limits to how thin the layer can be made while continuing to function effectively. Ion beam deposition and magnetron sputtering are the preferred methods for depositing the layers. The AlOx tunnel barrier is formed by depositing an Al layer, with thickness < 15 Angstrom, and then oxidizing it with an O2 plasma to form the AlOx.

This presentation will describe the formation and properties of MTJ material appropriate for MRAM, with emphasis on the trends and limits associated with the tunnel barrier and the free layer. Results are presented for MTJ material with very uniform resistance and uniform high MR over 150 mm wafers.

This work was partially supported by DARPA.