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Patent Issued for Engineered Magnetic Layer with Improved Perpendicular Anisotropy Using Glassing Agents for Spintronic Applications

May 13, 2014



By a News Reporter-Staff News Editor at Life Science Weekly -- Headway Technologies, Inc. (Milpitas, CA) has been issued patent number 8710603, according to news reporting originating out of Alexandria, Virginia, by NewsRx editors (see also Headway Technologies, Inc.).

The patent's inventors are Jan, Guenole (San Jose, CA); Wang, Yu-Jen (San Jose, CA); Tong, Ru-Ying (Los Gatos, CA).

This patent was filed on February 29, 2012 and was published online on April 29, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with magnetic tunnel junction (MTJ) technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque) magnetization switching described by C. Slonczewski in 'Current driven excitation of magnetic multilayers', J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale.

"Both field-MRAM and STT-MRAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. One of the ferromagnetic layers has a magnetic moment that is pinned in a first direction while the other ferromagnetic layer has a magnetic moment which is free to rotate in a direction that is either parallel or anti-parallel to the first direction. As the size of MRAM cells decreases, the use of external magnetic fields generated by current carrying lines to switch the magnetic moment direction of the free layer becomes problematic. One of the keys to manufacturability of ultra-high density MRAMs is to provide a robust magnetic switching margin by eliminating the half-select disturb issue. For this reason, a new type of device called a spin transfer (spin torque) device was developed. Compared with conventional MRAM, spin-transfer torque or STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a current perpendicular to plane (CPP) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic layer and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic free layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic free layer if the current density is sufficiently high, and if the dimensions of the multilayer are small.

"For STT-MRAM to be viable in the 90 nm technology node and beyond, the ultra-small MTJs (also referred to as nanomagnets) must exhibit a magnetoresistive (MR) ratio that is much higher than in a conventional MRAM-MTJ which uses a NiFe free layer and AlOx as the tunnel barrier layer. Furthermore, the critical current density (Jc) must be lower than about 10.sup.6 A/cm.sup.2 to be driven by a CMOS transistor that can typically deliver 100 .mu.A per 100 nm gate width. A critical current for spin transfer switching (Ic), which is defined as [(Ic.sup.++Ic.sup.-)/2], for the present 180 nm node sub-micron MTJ having a top-down oval shaped area of about 0.2.times.0.4 micron, is generally a few milliamperes. The critical current density (Jc), for example (Ic/A), is on the order of several 10.sup.7 A/cm.sup.2. This high current density, which is required to induce the spin-transfer effect, could destroy a thin tunnel barrier made of AlOx, MgO, or the like. Thus, for high density devices such as STT-MRAM on a gigabit scale, it is desirable to decrease Ic (and its Jc) by approximately an order of magnitude so as to avoid an electrical breakdown of the MTJ device and to be compatible with the underlying CMOS transistor that is used to provide switching current and to select a memory cell.

"Several schemes have been presented to use the spin transfer torque mechanism for magnetic based memory such as STT-MRAM, or current induced domain wall motion based MRAM, logic, and sensor applications. A preferred implementation is to employ a magnetic tunnel junction (MTJ) with a pinned ferromagnetic layer and free ferromagnetic layer separated by a tunneling oxide layer in a TMR configuration. Although this scheme has been widely studied, further improvements in overall performance are needed before a domain wall motion device is used commercially as a memory element in MRAM or as a sensor in hard disk drive (HDD) heads.

"Referring to FIG. 1, the two magnetic layers in a TMR configuration can either have their magnetization pointing in the plane or out of the plane of the film. An example of in-plane magnetization is shown on side (a) of FIG. 1 where a pinned layer 10 has a magnetic moment along an x-axis and the free layer 12 has a magnetic moment free to rotate in a (+) or (-) x-axis direction. An insulating (tunnel barrier) layer 11 separates the aforementioned two ferromagnetic layers. Out of plane magnetization (PMA) is depicted on side (b) of FIG. 1 where pinned layer 20 has a magnetization pointing in a y-axis direction or perpendicular to the film plane and the free layer 21 has a magnetic moment that is free to rotate either in a (+) or (-) y-axis direction. In both examples, the free layer and pinned layer magnetizations are parallel or anti-parallel in a quiescent state. Thus, storage of the digital information as a '1' or '0' magnetic state is provided by the direction of magnetization in the free layer 12 (or 21).

"In the case of a free layer having a magnetization direction perpendicular to the plane of the film, the critical current (i.sub.C) required to switch the magnetic direction in the magnetic element is directly proportional to the perpendicular anisotropy field as shown in equation (1):

".alpha..times..times.e.times..times..perp..times..times. ##EQU00001## where e is the electron charge, .alpha. is a Gilbert damping parameter, Ms is the saturation magnetization of the free layer, h is the reduced Plank's constant, g is the gyromagnetic ratio, H.sub.k.sub.eff.sub.,.perp. is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer. For most applications, the spin polarized current must be as small as possible.

"Thermal stability is a function of the perpendicular anisotropy field as shown in equation (2):

".DELTA..times..perp..times..times..times. ##EQU00002## where k.sub.B is the Boltzmann constant and T is the temperature. In both of the in-plane and out-of-plane configurations represented in FIG. 1, the perpendicular anisotropy field of the magnetic element is expressed in equation (3) as:

".perp..times..times..perp..times..chi. ##EQU00003## where D (approximately 4.pi.) is the demagnetizing factor of the structure, M.sub.s is the saturation magnetization, d is the thickness of the magnetic element, H.sub.k,.chi. is the crystalline anisotropy field, and K.sub.U.sup..perp.,s is the surface perpendicular anisotropy of the top and bottom surfaces of the magnetic element. In polycrystalline materials where grains are randomly oriented, H.sub.k,.chi. is the sum of the crystalline anisotropy of all the grains defining the region of interest. When the grains are large, H.sub.k,.chi. can be significant whereas when the grains are small or the material is amorphous, this crystalline contribution to the total anisotropy field is small. From equation (3), one can see that the crystalline anisotropy of the material plays a detrimental role when the origin of the perpendicular anisotropy is mostly interfacial.

"In the absence of strong crystalline anisotropy, the perpendicular anisotropy field of a magnetic layer is dominated by the shape anisotropy field on which little control is available. At a given thickness, lower magnetization saturation decreases shape anisotropy and the spin-polarized switching current but also decreases thermal stability which is not desirable. Therefore, an improved configuration for a magnetic element is needed that provides improved thermal stability for a free layer with perpendicular magnetic anisotropy. In other words, it is desirable to increase the perpendicular anisotropy field in a perpendicular-to-plane structure if one wants to increase thermal stability independently of moment or volume of the magnetic layer, and without affecting the critical current. There is presently no teaching as to how perpendicular magnetic anisotropy can be enhanced at first and second free layer interfaces with adjoining layers in a MTJ stack while selectively crystallizing portions of the free layer adjacent to these interfaces, and maintaining amorphous character (lower magnetic moment) in a middle portion of the free layer."

Supplementing the background information on this patent, NewsRx reporters also obtained the inventors' summary information for this patent: "One objective of the present disclosure is to provide a MTJ with one or both of a free layer and reference layer having perpendicular magnetic anisotropy (PMA) wherein a middle region of the magnetic layer remains amorphous while regions along interfaces with adjoining MTJ layers are selectively crystallized during annealing which results in higher thermal stability and tolerance to higher annealing temperatures.

"A second objective of the present disclosure is to provide a MTJ according to the first objective wherein the materials used for the PMA layers are compatible with a variety of magnetic device applications including STT-MRAM, current induced domain wall motion based MRAM, logic, and read head sensors.

"According to one embodiment, these objectives are achieved with a MTJ structure comprised of a pinned magnetic layer, a magnetic free layer (hereafter referred to as free layer), and a tunnel barrier layer formed between the pinned layer and free layer. The free layer has a first surface that forms a first interface with the tunnel barrier layer thereby inducing PMA in a first region of the free layer adjacent to the first interface. Moreover, there is preferably a perpendicular Hk enhancing layer that forms a second interface with a second surface of the free layer which faces away from the tunnel barrier and thereby induces PMA in a second region of the free layer that is adjacent to the second interface. Between the first and second regions is a middle or third region of the free layer which has an amorphous character that is maintained even at elevated annealing temperatures by the presence of one or more glassing agents such as Si, Ta, P, Nb, Hf, Ti, Pd, Be, Cr, Zr, Cu, Os, V, or Mg in a concentration of 1% to 30% of the ferromagnetic free layer. In one aspect, one or more glassing agents are codeposited or 'doped' into the free layer which is comprised of a plurality of layers. In one embodiment that involves a bottom spin valve structure, the first through third regions may comprise three distinct layers in a composite free layer configuration. For example, the first region may be a CoFeB layer formed on the tunnel barrier, the second region may be a CoFeBG (or CoFeB:G) layer where G is a first glassing agent, and the third region may be a second CoFeB layer. B is considered to be a second glassing agent. Thus, the glassing agents are distributed such that a first or higher concentration (B+G) is formed in the middle region of the free layer and a second or lower concentration (B+G) is formed in the free layer proximate to the first and second interfaces. In the second concentration, G may be zero.

"Another important factor is the diffusion coefficient of the one or more glassing agents. Preferably, Ta which has a low diffusion coefficient is not deposited proximate to the first or second interfaces. On the other hand, B has a relatively high diffusion coefficient and tends to migrate out of the first and third regions near the first and second interfaces thereby enabling a lower crystallization temperature in those regions.

"In an alternative embodiment, there may be a plurality of 'n' layers in the composite free layer wherein the bottom and top layers in the free layer stack have a lower concentration of glassing agents than the other 'n-2' layers. Furthermore, the concentration of the glassing agents becomes greater with increasing distance from the first and second interface until the highest concentration is found in the middle one or more layers of the stack of 'n' layers.

"In an alternative embodiment, a glassing agent with a relatively low diffusion coefficient such as Ta is deposited in the form of one or more layers. For example, the free layer may be formed by sequentially depositing a first CoFeB layer, a Ta glassing agent layer with about a nanolayer thickness or less, and then a second CoFeB layer. Moreover, the glassing layer may be comprised of a plurality of layers that form a laminated structure with one or more glassing agents such as Ta/Mg/Ta, or Ta/Ta/Ta. It should be understood that during a subsequent annealing step, the glassing agent layer with low diffusion coefficient may form nano-crystals along the CoFeB grain boundaries. Since the nano-crystals typically have a size of less than 10 Angstrom diameter with randomly oriented magnetic moments, there is a net moment contribution=0 to the free layer magnetization. In this case, there is a continuous concentration gradient of glassing agent with a first or higher concentration at the middle of the free layer where both B and the low diffusion coefficient glassing agent are present to a second or lower concentration proximate to the first and second interfaces where there is only a small concentration, if any, of B present and none of the low diffusion coefficient glassing agent. Thus, the glassing agent concentration gradually increases as one approaches the middle of the free layer from either the first or second interfaces.

"The free layer may be comprised of an alloy of Co, Fe, Ni, and B with a thickness between 5 and 25 Angstroms. Preferably, the free layer is thin enough so that the perpendicular surface anisotropy field is significant compared with the shape anisotropy field. Furthermore, the free layer may be engineered such that the interfacial perpendicular anisotropy dominates the anisotropy field in an out-of-plane magnetization configuration also known as a perpendicular magnetic anisotropy (PMA) structure. In one aspect, the MTJ may have a bottom spin valve structure represented by seed layer/AFM layer/pinned layer/tunnel barrier/free layer/Hk enhancing layer/capping layer.

"The perpendicular Hk enhancing layer is made of any material that provides additional interfacial perpendicular anisotropy when contacting a surface of the free magnetic layer, and is formed on an opposite side of the free layer with respect to the tunnel barrier layer interface with the free layer. In a preferred embodiment, both of the tunnel barrier layer and perpendicular Hk enhancing layer are made of MgO. In this case, the thickness and oxidation state of the MgO perpendicular Hk enhancing layer are controlled to give a resistance.times.area (RA) product smaller than that of the tunnel barrier layer to minimize the reduction in the magnetoresistive (MR) ratio. In an alternative embodiment, the perpendicular Hk enhancing layer may be comprised of other oxides including SiOx, SrTiOx, BaTiOx, CaTiOx, LaAlOx, MnOx, VOx, AlOx, TiOx, and HfOx. In yet another embodiment, the perpendicular Hk enhancing layer may be one of Ru, Ta, Ti, B, V, Mg, Ag, Au, Cu or Cr. Moreover, an oxide perpendicular Hk enhancing layer may be embedded with conductive particles made of one or more of Fe, Co, Ni, Ru, Cr, Au, Ag, and Cu to lower the resistivity therein.

"Preferably, the capping layer which is also referred to as the buffer layer is made of Ru. If the perpendicular Hk enhancing layer is an oxide, it is important to select a buffer layer to have a free energy of formation substantially higher than that of the Hk enhancing layer so that the buffer layer does not change the oxidation state of the Hk enhancing layer during an anneal step. In other words, the metal selected for the buffer layer should not attract oxygen from an oxide Hk enhancing layer during an anneal step which would undesirably decrease the induced perpendicular anisotropy at the interface of the free layer and perpendicular Hk enhancing layer.

"Alternatively, the free layer may be formed below the tunnel barrier in a top spin valve structure represented by seed layer/Hk enhancing layer/free layer/tunnel barrier/pinned layer/AFM layer/capping layer. In this case, the seed layer is preferably Ru so that the seed layer has a free energy of oxide formation substantially less than that of a perpendicular Hk enhancing layer comprised of an oxide.

"The present disclosure also anticipates that both of the free layer and pinned layer may be comprised of one or more glassing agents having a first or higher concentration in a middle region of the free layer and pinned layer, and a second or lower concentration in a region adjacent to interfaces formed with adjoining layers in the MTJ stack."

For the URL and additional information on this patent, see: Jan, Guenole; Wang, Yu-Jen; Tong, Ru-Ying. Engineered Magnetic Layer with Improved Perpendicular Anisotropy Using Glassing Agents for Spintronic Applications. U.S. Patent Number 8710603, filed February 29, 2012, and published online on April 29, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=73&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=3616&f=G&l=50&co1=AND&d=PTXT&s1=20140429.PD.&OS=ISD/20140429&RS=ISD/20140429

Keywords for this news article include: Crystallins, Electronics, Engineering, Eye Proteins, Nano Crystal, Nanotechnology, Magnetic Moment, Emerging Technologies, Headway Technologies Inc..

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Source: Life Science Weekly


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