Abstract:
A magnetic memory cell comprises in-plane anisotropy tunneling magnetic junction (TMJ) and two fixed in-plane storage-stabilized layers, which splits on the both side of the data storage layer of the TMJ. The magnetizations of the said fixed in-plane storage-stabilized layers are all normal to that of the reference layer of TMJ but point to opposite direction. The existing of the storage-stabilized layers not only enhances the stability of the data storage, but also can reduce the critical current needed to flip the data storage layer via some specially added features.

Description:
REFERENCE CITED—U.S. PATENT DOCUMENTS 
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       FIELD OF INVENTION 
       [0018]    The invention is related to memory cell design for magnetoresistive random access memory (MRAM), more specifically a memory cell comprising two in-plane magnetic stabilization enhancement layers locating on opposite side of the data storage layer of an in-plane anisotropy TMR sensing stack structure. The magnetizations of the stability enhancement layers are normal to the magnetic reference layer of MTJ and point to opposite directions. There is also a switching current spin polarization layer built within the stack to reduce the switching current needed to flip the data storage layer. 
       BACKGROUND ART 
       [0019]    Data storage memory is one of the backbones of the modern information technology. Semiconductor memory in the form of DRAM, SRAM and flash memory has dominated the digital world for the last forty years. Comparing to DRAM based on transistor and capacitor above the gate of the transistor, SRAM using the state of a flip-flop with large form factor is more expensive to produce but generally faster and less power consumption. Nevertheless, both DRAM and SRAM are volatile memory, which means they lost the information stored once the power is removed. Flash memory on the other hand is non-volatile memory and cheap to manufacture. However, flash memory has limited endurances of writing cycle and slow write through the read is relatively faster. 
         [0020]    MRAM is a relatively a new type of memory technologies. It has the speed of the SRAM, density of the DRAM and it is non-volatile as well. If it is used to replace the DRAM in computer, it will not only give “instant on” but “always-on” status for operation system and restore the system to the point when the system is power off last time. It could provide a single storage solution to replace separate cache (SRAM), memory (DRAM) and permanent storage (HDD or flash-based SSD) on portable device at least. Considering the growth of “cloud computing”, MRAM has a great potential and can be the key dominated technology in digital world. 
         [0021]    MRAM storage the informative bit “1” or “0” into the two magnetic states in the so-called magnetic storage layer. The different states in the storage layer gives two distinctive voltage outputs from the whole memory cell, normally a patterned TMR or GMR stack structures. The TMR or GMR stack structures provide a read out mechanism sharing the same well-understood physics as current magnetic reader used in conventional hard disk drive. 
         [0022]    There are two kinds of the existing MRAM technologies based on the write process: one kind, which can be labeled as the conventional magnetic field switched (toggle) MRAM, uses the magnetic field induced by the current in the remote write line to change the magnetization orientation in the data stored magnetic layer from one direction (for example “1”) to another direction (for example “0”). This kind of MRAM has more complicated cell structure and needs relative high write current (in the order of mA). It also has poor scalability beyond 65 nm because the write current in the write line needs to continue increase to ensure reliable switching the magnetization of a dimension shrinking magnetic stored layer because of the smaller the physical dimension of the storage layer, the higher the coercivity it normally has for the same materials. Nevertheless, the only commercially available MRAM so far is still based on this conventional writing scheme. The other class of the MRAM is called spin-transfer torque (STT) switching MRAM. It is believed that the STT-RAM has much better scalability due to its simple memory cell structure. While the data read out mechanism is still based on TMR effect, the data write is governed by physics of spin-transfer effect [1, 2]. Despite of intensive efforts and investment, even with the early demonstrated by Sony in late 2005 [3], no commercial products are available on the market so far. One of the biggest challenges of STT-RAM is its reliability, which depends largely on the value and statistical distribution of the critical current density needed to flip the magnetic storage layers within the every patterned TMR stack used in the MRAM memory structures. Currently, the value of the critical current density is still in the range of 10 6  A/cm 2 . To allow such a large current density through the dielectric barrier layer such as AlOx and MgO in the TMR stack, the thickness of the barrier has to be relatively thin, which not only limits the magnetoresist (MR) ratio value but also cause potential risk of the barrier breakdown. As such, a large portion of efforts in the STT-RAM is focused on lower the critical current density while still maintaining the thermal stability of the magnetic data storage layer. Another challenge is related partially to the engineering challenge due to the imperfection of memory cell structure patterning (patterned TMR element) such as edge magnetic moment damage and size variation, as well as uniformity of the barrier thickness during the deposition and magnetic uniformity in the data storage layer and spin polarized magnetic layer (also called reference layer). This non-uniformity leads to variation of the size, edge roughness, magnetic uniformity and barrier thickness for patterned TMR elements, which ultimately cause the statistic variation of critical current density needed for each patterned cell. 
         [0023]    The success of the STT-RAM largely depends on the breakthrough on the material used in STT-RAM, which give a fair balance between the barrier thickness (related to broken down voltage and TMR ratio), critical current density and thermal stability of the magnetic storage layer. Currently, Based on the anisotropy of the data storage layer, the STT-RAM can be classified into in-plane anisotropy cell and perpendicular cell. The in-plane anisotropy cell has much high magnetoresistance value (MR value) than that of the perpendicular cell but suffers from the thermal stability issue when the size of the cell is reduced, particularly when the magnetization of the storage layer (SL) is parallel to the fixed reference magnetic layer (RL), the magnetostatic coupling between the SL and RL will low the energy barrier and cause large noise or even SL flips. 
         [0024]    In this invention, we propose a stabilization scheme to enhance the thermal stability of in-plane MRAM cell with spin-polarization layer, which could also low the critical current needed to flip the data storage layer. 
       SUMMARY OF THE INVENTION 
       [0025]    The present invention of the proposed memory cells for MRAM to enhance the thermal stability while maintaining low switching current, which comprises an in-plane anisotropy magnetic tunneling junction (MTJ), two within stack magnetic stabilization layers whose magnetization point to opposite direction and all normal to the that of the reference layer of the MTJ as well as spin polarization layer for switching current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  illustrates one of the embodiments of proposed magnetic memory cell. 
           [0027]      FIG. 2  illustrates one of the embodiments of proposed magnetic memory cell with spin polarization layer. 
           [0028]      FIG. 3  Illustrates one of the embodiments of proposed magnetic memory cell with synthetic antiferromagnetic spin polarization layer. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The following description is provided in the context of particular designs, applications and the details, to enable any person skilled in the art to make and use the invention. However, for those skilled in the art, it is apparent that various modifications to the embodiments shown can be practiced with the generic principles defined here, and without departing the spirit and scope of this invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles, features and teachings disclosed here. 
         [0030]    With reference of the  FIG. 1  shows an embodiment of proposed magnetic memory cell  100 . The proposed MRAM memory cell  100 , counted from the material growth plane from the bottom, comprises a bottom electrode  101 ; in-plane-anisotropy magnetic stabilization layer  102  with fixed magnetization orientation; an non-magnetic metallic layer  103 ; antiferromagnetic layer  104  such as IrMn; synthetic antiferromagnetic layer (SAF)  105  with balanced or closely balanced moment for magnetic layers (for example CoFeB20/CoFe10/Ru/CoFe10/CoFeB20); tunneling barrier  106  such as MgO, TiOx, AlOx; in-plane anisotropic data storage layer  107  such as CoFeB; non-magnetic metallic layer  108  with long spin diffusion length such as Cu, Al; in-plane-anisotropy stabilization layer  109  with fixed magnetization orientation and top electrode  110 . The magnetic stabilization layer  102  and  109  have their magnetizations pointing at opposite direction and being normal to the magnetization of magnetic layers in SAF layer  105 . The net magnetic moment of the layer  102  and  109  prefers to be very close or the same amount so that they can form a flux close loop with edge magnetic charge canceling each other. If the magnetic moment of layer  102  and  109  is not the same, the individual distance from the layer  102  or layer  109  to the data storage layer  107  need to be adjusted accordingly to ensure the force acts on the data storage layer from the layer  102  and layer  109  is close to balance. By doing so, an energy barrier is established along the direction normal to the magnetization of magnetic layers in SAF layer  105 , which prohibits the magnetization of data storage layer  107  to align into this direction at static state because this breaks the magnetic balance and established close flux loop between the layer  102  and  109 . As such, we use the in-stack layer  102  and  109  to establish a magnetic anisotropy in the memory cell structure, which can enhance the magnetic stability against thermal agitation. Since the magnetizations of layer  102  and  109  need point to opposite direction, the coercivity of layer  102  and  109  should be significantly different so that the magnetizations of layer  102  and  109  can be set independently by external field with little interference. The layer  102  can be made of the hard magnetic materials such as CoCr, CoPt, CoCrPt or bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFe, CoCrPt/NiFe etc. For layer  109 , it is preferable to be made of bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFe, CoCrPt/NiFe etc because the layer  109  can also act as a spin polarization layer for write current  111 . As said previously, a non-magnetic metallic layer  108  is made of long spin diffusion length such as Cu, Al separating the layer  109  from the data storage layer  107 . When the write current  111  through layer  109  get polarized, the polarized write current  111  will preserve this polarized state when move into the data storage layer  107 . Based on theory [1,2,8], the magnetization of data storage layer  107  will be switched direction. This reduces the critical current needed to flip the data storage layer  107  comparing to a based MTJ at the same conditions. 
         [0031]    With reference of the  FIG. 2  shows an embodiment of proposed magnetic memory cell  200 , the proposed MRAM memory cell  200 , counted from the bottom, comprises a bottom electrode  201 ; in-plane-anisotropy magnetic stabilization layer  202  with fixed magnetization orientation; an non-magnetic metallic layer  203 ; antiferromagnetic layer  204  such as IrMn; synthetic antiferromagnetic layer (SAF)  205  with balanced or closely balanced moment for magnetic layers; tunneling barrier  206  such as MgO, TiOx, AlOx ; in-plane anisotropic data storage layer  207  such as CoFeB; non-magnetic dielectric layer  208  such as MgO, TiOx, AlOx or the combination of dielectric with metal such as Cu, Al, Ag such as MgO/Cu with significant low value of resistance-area product RA compared to the barrier  206 ; fixed in-plane-anisotropy spin polarization layer  209 ; metallic spacer layer  210 ; fixed in-plane-anisotropy stabilization layer  211  and top electrode  212 . 
         [0032]    The magnetic stabilization layer  202  and  211  has their magnetizations pointing at opposite direction and being normal to the magnetization of magnetic layers in SAF layer  205 . The magnetization of the spin polarization  209  also points to opposite to that of the stabilization layer  211  with the moment of the layer  211  is noticeably larger than that of layer  209 . Overall, the design of the materials of layers  202 ,  209  and  211  follows the rule that the data storage data  207  sees balanced magnetic torque from layer  202 ,  209  and  211  when it slightly rotates from its stable positions. One of the way to achieve the design rule is to balance the overall distance between the data storage layer  207  to layer  209 ,  211  and  202  and keep overall the net moment of these three layers, considering the orientation of the magnetization of each layer, is zero or very close to zero so that they can form a flux close loop with edge magnetic charge canceling each other. The layer  210  separates the layer  211  from the spin polarization layer  209  and can be made of metallic layer with short pin diffusion length. The thickness of layer  210  need to large enough to destroy the spin memory of the electrons obtained from the magnetic layer  211 . 
         [0033]    The layer  202  and  211  can be made of the hard magnetic materials such as CoCr, CoPt, CoCrPt or bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFe, CoCrPt/NiFe etc. For layer  209 , it is preferable to be made of bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFeB, CoCrPt/CoFeB etc because the layer  209  is a fixed spin polarization layer for write current  213 . 
         [0034]    As said previously, non-magnetic layer  208  is made of MgO, TiOx, AlOx or the combination of dielectric with metal such as Cu, Al, Ag such as MgO/Cu with significant low value of resistance-area product RA compared to the barrier  206 . When the write current  213  through layer  209  get polarized, the polarized write current  213  will preserve this polarized state when move into the data storage layer  207 . Based on theory [1,2,8], the magnetization of data storage layer  207  will be switched direction. This reduces the critical current needed to flip the data storage layer  207  comparing to a based MTJ at the same conditions. 
         [0035]    Layers  208 ,  209  and  210  build up the separating layer between the layer  211  and data storage layer  207 . 
         [0036]      FIG.3  shows an embodiment of proposed a magnetic memory cell  300 . the proposed MRAM memory cell  300 , counted from the bottom, comprises a bottom electrode  301 ; in-plane-anisotropy magnetic stabilization layer  302  with fixed magnetization orientation; an non-magnetic metallic layer  303 ; antiferromagnetic layer  304  such as IrMn; synthetic antiferromagnetic layer (SAF)  305  with balanced or closely balanced moment for magnetic layers; tunneling barrier  306  such as MgO, TiOx, AlOx ; in-plane anisotropic data storage layer  307  such as CoFeB; non-magnetic layer  308  such as MgO, TiOx, AlOx or the combination of dielectric with metal such as Cu, Al, Ag such as MgO/Cu with significant low value of resistance-area product RA compared to the barrier  306 ; a SAF spin polarization layer  309  with structure such as CoFe/Ru/CoFe; a SAF polarizer stabilizing layer  310 ; an metallic spacer layer  311 ; a fixed in-plane-anisotropy stabilization layer  312  and top electrode  213 . 
         [0037]    The magnetic stabilization layer  302  and  312  has their magnetizations pointing at opposite direction and being normal to the magnetization of magnetic layers in SAF layer  305 . 
         [0038]    The magnetization directions of the magnetic layers for the SAF spin polarization layer  309  points also normally to the magnetization of magnetic layers in SAF layer  305 . SAF polarizer stabilizing layer  310  is above the SAF spin polarization layer and it can be made of either permanent magnetic layer such as CoPt or CoCr-based hard magnetic layer or antiferromagnetic layer such as IrMn or PtMn, whose Neel temperature is significantly different from the one of the layer  304 . Regardless of the materials used for layer  310 , the design rule is that the magnetic moment from layer  309  and layer  310  on both sides of the Ru layer in SAF layer  309  should be equal or very closely to be equal. As such, the magnetic layers, including layer  310 , on both sides of the Ru layer of SAF layer  309  will form a close flux loop and give zero combined edge magnetic charges. 
         [0039]    The layer  311  separates the layer  312  from the layer  310  and can be made of metallic layer with short pin diffusion length. The thickness of layer  311  need to large enough to destroy the spin memory of the electrons obtained from the magnetic layer  312 . 
         [0040]    The layer  302  and  312  can be made of the hard magnetic materials such as CoCr, CoPt, CoCrPt or bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFe, CoCrPt/NiFe etc. The coercivity of layer  302  and layer  312  need widely different so that they can be set by external magnetic field independently. 
         [0041]    As said previously, non-magnetic layer  308  is made of MgO, TiOx, AlOx or the combination of dielectric with metal such as Cu, Al, Ag such as MgO/Cu with significant low value of resistance-area product RA compared to the barrier  306 . When the write current  314  through layer  309  get polarized, the polarized write current  314  will preserve this polarized state when move into the data storage layer  307 . Based on theory [1,2,8], the magnetization of data storage layer  307  will be switched direction. This reduces the critical current needed to flip the data storage layer  307  comparing to a based MTJ at the same conditions. 
         [0042]    Layers  308 ,  309 ,  310  and  311  build up the separating layer between the layer  312  and data storage layer  307 .