Patent Application
20120241878
This disclosure relates generally to the field of magnetoresistive random access memory (MRAM), and more specifically to materials for use in fabrication of magnetic tunnel junctions for spin torque transfer (STT) MRAM.
MRAM is a type of solid state, non-volatile memory that uses tunneling magnetoresistance (TMR) to store information. MRAM is made up of an electrically connected array of magnetoresistive memory elements, referred to as magnetic tunnel junctions (MTJs). Each MTJ includes a free layer and fixed layer that each include a layer of a magnetic material, and that are separated by a non-magnetic insulating tunnel barrier. The free layer has a variable magnetization direction, and the fixed layer has an invariable magnetization direction. An MTJ stores information by switching the magnetization state of the free layer. When the magnetization direction of the free layer is parallel to the magnetization direction of the fixed layer, the MTJ is in a low resistance state. Conversely, when the magnetization direction of the free layer is antiparallel to the magnetization direction of the fixed layer, the MTJ is in a high resistance state. The difference in resistance of the MTJ may be used to indicate a logical ‘1’ or ‘0’, thereby storing a bit of information. The TMR of an MTJ determines the difference in resistance between the high and low resistance states. A relatively high difference between the high and low resistance states facilitates read operations in the MRAM.
The magnetization direction of the free layer may be changed by a spin torque switched (STT) write method, in which a write current is applied in a direction perpendicular to the film plane of the magnetic films forming the MTJ. The write current has a tunneling magnetoresistive effect, so as to change (or reverse) the magnetization direction, or state, of the free layer of the MTJ. In STT magnetization reversal, the write current required for the magnetization reversal is determined by the current density. As the area of the surface in an MTJ on which the write current flows becomes smaller, the write current required for reversing the magnetization of the free layer of the MTJ becomes smaller. Therefore, if writing is performed with fixed current density, the necessary write current becomes smaller as the MTJ size becomes smaller. Inclusion of material layers that exhibit perpendicular anisotropy (PMA) in a MTJ also lowers the necessary write current density relative to MTJs having in-plane magnetic anisotropy, lowering the total necessary write current. However, MTJs that include PMA materials may not exhibit sufficient coercivity (Hc) to meet reliability and retention requirements for an MRAM made up of the PMA MTJs.
In one aspect, a magnetic tunnel junction (MTJ) for a magnetic random access memory (MRAM) includes a magnetic free layer having a variable magnetization direction; an iron (Fe) dusting layer formed on the free layer; an insulating tunnel barrier formed on the dusting layer; and a magnetic fixed layer having an invariable magnetization direction, disposed adjacent the tunnel barrier such that the tunnel barrier is located between the free layer and the fixed layer; wherein the free layer and the fixed layer have perpendicular magnetic anisotropy.
In another aspect, a method of forming a magnetic tunnel junction (MTJ) for a magnetic random access memory (MRAM) includes forming a magnetic free layer having a variable magnetization direction; forming an iron (Fe) dusting layer over the free layer; forming a tunnel barrier comprising an insulating material over the Fe dusting layer; and forming a magnetic fixed layer having an invariable magnetization direction over the tunnel barrier, wherein the free layer and the fixed layer have perpendicular magnetic anisotropy.
Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings.
Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:
Embodiments of an MTJ with an iron (Fe) dusting layer located between the free layer and the tunnel barrier are provided, with exemplary embodiments being discussed below in detail. The addition of the Fe dusting layer increases the Hc in MTJs that include PMA materials. The Fe dusting layer may be relatively thin, for example, from about 0.2 angstroms ({acute over (Å)}) to about 2 {acute over (Å)} thick in some embodiments. A PMA MJT stack that includes an Fe dusting layer may be grown at room temperature, reducing manufacturing complexity for an MRAM comprising the PMA MTJs.
Referring initially to
The presence of the Fe dusting layer 103 on top of a CoFeB free layer 102 significantly increases the Hc of the MTJ devices. For example, in an MTJ 100 with a free layer 102 made of 7CoFe20B20 and a dusting layer 103 that is about 0.4{acute over (Å)} thick, the Hc of a MTJ having a diameter of about 120 nanometer (nm) is about 600 to 700 Oersteds (Oe), compared to about 200 Oe for an MTJ with a 7CoFe20B20 free layer and no dusting layer, as illustrated by graphs 200a and 200b of
For a given thickness of CoFeB in the free layer 102, as the Fe dusting layer is made thicker (e.g., greater than about 2 {acute over (Å)}), the Hc of the MTJ eventually decreases because of the increase of total moment and weaker PMA. A relatively thick Fe dusting layer 103 may also increase the switching voltage (i.e., the voltage required to change the magnetization direction of the free layer, Vc) of the MTJ. Depending on the specific requirements for Hc (for retention) and Vc (for switching) for the MRAM comprising the MTJs, optimal CoFeB and Fe relative thicknesses may be selected. The thickness of the Fe dusting layer 103 may be from about 0.2 {acute over (Å)} to about 2 {acute over (Å)} thick in some embodiments.
The MgO tunnel barrier 104 may be formed by radiofrequency (RF) sputtering in some embodiments. Alternatively, the MgO tunnel barrier 104 may be formed by oxidation (either natural or radical) of a layer of Mg in other embodiments. After oxidation, the MgO layer may then be capped with a second layer of Mg. The second layer of Mg may have a thickness of about 5 {acute over (Å)} or less in some embodiments. The Hc of the free layer 102 may vary based on the method chosen to form the MgO tunnel barrier 104. For example, in the case of an MgO tunnel barrier 104 made by radical oxidation and capped with a second layer of Mg, the thickness of the second Mg layer may significantly impact the Hc of the free layer. For a first exemplary MTJ, when the barrier is made of 9 {acute over (Å)} Mg|Radical Oxidation|3 {acute over (Å)} Mg, an Hc of about 120 Oe is observed. For a second exemplary MTJ having the same free layer and fixed layer materials as the first exemplary MTJ, when the barrier is made of 9 {acute over (Å)} Mg|Radical Oxidation|2 {acute over (Å)} Mg, a Hc of about 270 Oe is observed. This is illustrated in graphs 300a and 300b of
Referring now to
As further depicted in
It should be appreciated that the exemplary MTJ embodiments 100, 400, 500, and 600 discussed above with respect to FIGS. 1 and 4-6 are shown for illustrative purposes only, and it is contemplated that other suitable MTJ structures may be formed in which an Fe dusting layer is disposed between a free layer and a tunnel barrier so as to provide sufficient coercivity (Hc) to meet reliability and retention requirements for an MRAM made up of the PMA MTJs.
The technical effects and benefits of exemplary embodiments include increased coercivity and magnetoresistance in a MTJ through addition of the Fe dusting layer.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
