The present application relates to magnetoresistive random access memory (MRAM). More particularly, the present application relates to a bottom pinned magnetic tunnel junction (MTJ) stack including a top magnetic free layer having a high perpendicular magnetic anisotropy field which can be used in a spin-transfer torque (STT) MRAM device.
STT MRAM devices use a 2-terminal device which includes a MTJ stack that contains a magnetic pinned (reference) layer, a tunnel barrier layer and a magnetic free layer. MTJ stacks can be classified into two types. The first type of MTJ stack is a bottom pinned MTJ stack and the second type of MTJ stack is a top pinned MTJ stack. A typical bottom pinned MTJ stack, which is illustrated in
In the MTJ stacks shown in
Various types of magnetic materials can be used in providing magnetic free layers of a MTJ stack. For example, the magnetic free layer can be composed of at least one magnetic material (such as, for example, a cobalt-iron-boron alloy) with a magnetization that can be changed in orientation relative to the magnetization orientation of the magnetic pinned layer. Such a magnetic material may be used as the magnetic free layer in either a bottom pinned MTJ stack, as shown in
In cases in which a Heusler or half Heusler based alloy is used as the magnetic free layer of a MTJ stack, a specialized metallic seed layer (a so-called chemical templating layer) such as, for example, an aluminum-cobalt (Al—Co) alloy seed layer or an aluminum-iridium (Al—Ir) alloy seed layer with Cs—Cl structure, is needed to promote crystalline ordering of the Heusler or half Heusler based alloy. Due to the strict metallic seed layer requirement, Heusler or half Heusler based alloys have only been traditionally used as a bottom free layer in a top pinned MTJ stack.
Ordered magnetic alloys, such as Heusler or half Heusler based alloys, provide a scalability advantage over traditional cobalt-iron-boron alloys due to their bulk anisotropy properties. This means, that even at a small size, the retention properties of ordered magnetic alloys such as, for example, Heusler or half Heusler based alloys, can be improved by thickening the magnetic free layer; this is not possible for interface anisotropy governed by cobalt-iron-boron alloys.
A bottom pinned MTJ stack containing a top magnetic free layer having a high perpendicular magnetic anisotropy field is provided which can be used as an element/component of a STT MRAM device. The top magnetic free layer is composed of an ordered aluminum-manganese-germanium-containing alloy having a tetragonal crystalline symmetry. The ordered aluminum-manganese-germanium-containing alloy that provides the top magnetic free layer of the present application is formed directly on a tunnel barrier layer of the bottom pinned MTJ stack without the need of a specialized metallic seed layer.
In one aspect of the present application, a bottom pinned MTJ stack is provided. In one embodiment, the bottom pinned MTJ stack includes a tunnel barrier layer located on a magnetic pinned layer, and a magnetic free layer located on the tunnel barrier layer, wherein the magnetic free layer is composed of an ordered aluminum-manganese-germanium-containing alloy having a tetragonal crystalline symmetry.
In another aspect of the present application, a STT MRAM device is provided. In one embodiment, the STT MRAM device includes a bottom pinned MTJ stack located on a surface of a bottom electrode. In one embodiment, the bottom pinned MTJ stack structure includes a tunnel barrier layer located on a magnetic pinned layer, and a magnetic free layer located on the tunnel barrier layer, wherein the magnetic free layer is composed of an ordered aluminum-manganese-germanium-containing alloy having a tetragonal crystalline symmetry.
The present application will now be described in greater detail by referring to the following discussion and drawings that accompany the present application. It is noted that the drawings of the present application are provided for illustrative purposes only and, as such, the drawings are not drawn to scale. It is also noted that like and corresponding elements are referred to by like reference numerals.
In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide an understanding of the various embodiments of the present application. However, it will be appreciated by one of ordinary skill in the art that the various embodiments of the present application may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the present application.
It will be understood that when an element as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “beneath” or “under” another element, it can be directly beneath or under the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly beneath” or “directly under” another element, there are no intervening elements present.
The present application provides a bottom pinned MTJ stack in which a top magnetic free layer having a high perpendicular magnetic anisotropy field is used. Notably, the top magnetic free layer in the bottom pinned MTJ stack of the present application is composed of an ordered aluminum-manganese-germanium-containing alloy which has a tetragonal crystalline symmetry. The top magnetic free layer is formed directly on a tunnel barrier layer of the bottom pinned MTJ stack without the need of a specialized metallic seed layer. Thus, a direct interface between the ordered aluminum-manganese-germanium-containing alloy and the underlying tunnel barrier is obtained in the bottom pinned MTJ stack of the present application.
Referring now to
As is shown in
The bottom electrode 20 may be composed of an electrically conductive material such as, for example, an electrically conductive metal, an electrically conductive metal alloy, or an electrically conductive metal nitride. Examples of electrically conductive metals that can be used to provide the bottom electrode 20 include, but are not limited to, copper (Cu), ruthenium (Ru), cobalt (Co), rhodium (Rh), tungsten (W), aluminum (Al), tantalum (Ta) or titanium (Ti). An example of electrically conductive metal alloy that can be used to provide the bottom electrode 20 includes, but is not limited to, Cu—Al, and an example of electrically conductive metal nitride that can be used to provide the bottom electrode 20 includes, but is not limited to, TaN or TiN. The bottom electrode 20 can be formed utilizing techniques well known to those skilled in the art. The conductive material that provides the bottom electrode 20 can be formed utilizing a deposition process such as, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering or plating. The bottom electrode 20 can have a thickness from 10 nm to 200 nm; although other thicknesses are possible and can be used as the thickness of the bottom electrode 20. The bottom electrode 20 can be formed on a recessed surface or a non-recessed surface of the electrically conductive structure (not shown).
The magnetic pinned layer 22 that is employed in the present application has a fixed magnetization; the magnetic pinned layer 22 can also be referred to as a magnetic reference layer. The magnetic pinned layer 22 can be composed of a metal or metal alloy that includes one or more metals exhibiting high spin polarization. In alternative embodiments, exemplary metals for the formation of the magnetic pinned layer 22 include iron, nickel, cobalt, chromium, boron, and manganese. Exemplary metal alloys may include the metals exemplified above (i.e., iron, nickel, cobalt, chromium, boron, and manganese). In another embodiment, the magnetic pinned layer 22 may be a multilayer arrangement having (1) a high spin polarization region formed from of a metal and/or metal alloy using the metals mentioned above (i.e., iron, nickel, cobalt, chromium, boron, and manganese), and (2) a region constructed of a material or materials that exhibit strong perpendicular magnetic anisotropy (strong PMA). Exemplary materials with strong PMA that may be used include a metal such as cobalt, nickel, platinum, palladium, iridium, or ruthenium, and may be arranged as alternating layers. The strong PMA region may also include alloys that exhibit strong PMA, with exemplary alloys including cobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum, cobalt-platinum, cobalt-palladium, iron-platinum, and/or iron-palladium. The alloys may be arranged as alternating layers. In one embodiment, combinations of these materials and regions may also be employed. The magnetic pinned layer 22 that can be employed in the present application can have a thickness from 3 nm to 20 nm; although other thicknesses for the magnetic pinned layer 22 can be used.
The tunnel barrier layer 24 is composed of an insulator material and is formed at such a thickness as to provide an appropriate tunneling resistance. Exemplary materials for the tunnel barrier layer 24 include magnesium oxide, aluminum oxide, and titanium oxide, or materials of higher electrical tunnel conductance, such as semiconductors or low-bandgap insulators. In one embodiment, magnesium oxide is used as the material that provides the tunnel barrier layer 24. The thickness of tunnel barrier layer 24 can be from 0.5 nm to 1.5 nm; although other thicknesses for the tunnel barrier layer 24 can be used as long as the selected thickness provides a desired tunnel barrier resistance.
The magnetic free layer 26 that is employed in the present application is an ordered aluminum-manganese-germanium-containing alloy (i.e., Al—Mn—Ge) having a tetragonal crystalline symmetry. The term “tetragonal crystalline symmetry” denotes a crystal structure having a unit cell containing three axes, two of which are of the same length and are at right angles to each other, and the third axis is perpendicular to the other two axes. Tetragonal crystalline lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prim with a square base (x by x) and a height (y, which is different from x).
The manganese-germanium-containing (i.e., Al—Mn—Ge) alloy that can be used as the magnetic free layer 26 typically has an atomic ratio of 1:1:1 between the Al, Mn and Ge. In some embodiments, the manganese-germanium-containing (i.e., Al—Mn—Ge) alloy that can be used as the magnetic free layer 26 can have an atomic ratio that deviates 10% or less from the 1:1:1 ratio mentioned above. In some embodiments, up to 20 atomic percent of the total manganese content of the ordered aluminum-manganese-germanium-containing alloy is replaced with chromium (Cr); an ordered Cr—Mn—Al—Ge alloy is provided which also has the tetragonal crystalline symmetry. The ordered Cr—Mn—Al—Ge alloy can also be used as the magnetic free layer 26. In one embodiment, from 1 to 20 atomic percent of the total manganese content of the ordered aluminum-manganese-germanium-containing alloy is replaced with Cr.
The magnetic free layer 26 of the present application can have a thickness from 3 nm to 10 nm. The magnetic free layer 26 of the present application which includes the ordered manganese-germanium-containing (i.e., Al—Mn—Ge) alloy, with or without Cr replacement, has a magnetic moment area from 0.035 milli-emu/cm2 to 0.15 milli-emu/cm2. Magnetic moment area was determined by magnetometry using a vibrating sample magnetometer (VSM).
The magnetic free layer 26 of the present application which includes the ordered manganese-germanium-containing (i.e., Al—Mn—Ge) alloy, with or without Cr replacement, typically has a perpendicular magnetic anisotropy field that is greater than 2 Tesla. In some embodiments, the perpendicular magnetic anisotropy field of the magnetic free layer 26 of the present application can be from 0.5 Tesla to 5 Tesla. Magnetic anisotropy field was determined by magnetometry using a vibrating sample magnetometer (VSM).
The MTJ capping layer 28 is present on the magnetic free layer 26. The MTJ capping layer 28 is preferentially composed of magnesium oxide (MgO). Other materials for the MTJ capping layer 28 include aluminum oxide (Al2O3), calcium oxide (CaO), tantalum oxide (Ta2O5), niobium oxide (Nb2O5) or ternary oxides such as, for example, MgyTi1-ypx. The MTJ capping layer 28 can have a thickness from 0.3 nm to 2 nm; other thicknesses are possible and can be used in the present application as the thickness of the MTJ capping layer 28.
The etch stop layer 30 is composed of a metal such as, for example, ruthenium (Ru) or iridium (Ir) that has a higher etch rate compared to the hard mask 32 which prevents the magnetic pinned layer 22, the tunnel barrier layer 24 and the magnetic free layer 26 from being exposed to the etchant materials used to pattern the hard mask 32. The etch stop layer 30 can have a thickness from 5.0 nm to 30 nm; although other thicknesses for the etch stop layer 30 can be used in the present application.
The hard mask 32 can be composed of a metal nitride such as, for example, tantalum nitride (TaN) or titanium nitride (TiN) or a metal such as, for example, titanium (Ti) or tantalum (Ta), which is compositionally different from the material used to provide the etch stop layer 30. In some embodiments, the hard mask 32 can be employed as a top electrode of the STT MRAM device. In other embodiment, a separate top electrode (composed of one of the electrically conductive materials mentioned above for the bottom electrode 20) can be formed on the hard mask 32. The hard mask 32 can have a thickness from 50 nm to 1500 nm; although other thicknesses for the hard mask 32 can be used in the present application.
The MTJ stack shown in
The various materials that provide the MTJ stack of the present application can be deposited by chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD) or sputtering. The various materials that provide the MTJ stack of the present application can be deposited utilizing the same or different deposition process. In one embodiment of the present application, the magnetic free layer 26 described above can be formed using one or more stoichiometrically adjusted targets by sputtering or co-sputtering. In some embodiments of the present application, the tunnel barrier layer 24 and/or the MTJ capping layer can be formed by sputtering MgO from a stoichiometeric MgO target, or by oxidation of a grown Mg layer, or by utilizing a combination of both.
Referring now to
While the present application has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present application. It is therefore intended that the present application not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.