KR101043384B1 - Magnetoresistive ram using high temperature superconductor - Google Patents

Abstract

The present invention relates to a magnetoresistive ram using a high temperature superconductor, and discloses a technique for increasing the efficiency of switching magnetization and increasing current density by implementing a magnetoresistive ram using a high temperature superconductor. The present invention is an MTJ including a variable ferromagnetic layer, a tunnel junction layer and a fixed ferromagnetic layer, a bit line formed on top of the variable ferromagnetic layer, and one end thereof is connected to the fixed ferromagnetic layer to control the switching operation by a word line. A select transistor, and a source line connected to the other end of the select transistor, wherein the MTJ and the bit line and the source line are made of a high temperature superconductor material.

Description

Magnetoresistive RAM using high-temperature superconductors

1A and 1B are schematic diagrams of a typical MTJ.

2 is a view showing the characteristics of a typical high temperature superconductor.

3A and 3B show an STT-MRAM cell structure using a high temperature superconductor according to the present invention.

Figure 4 is another embodiment of an MRAM using a high temperature superconductor in accordance with the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetoresistive random access memory (MRAM) using a high temperature superconductor. The present invention relates to a magnetoresistive ram using a high temperature superconductor (zero resistance) to increase the efficiency of switching magnetization and current. It is a technology to increase the density.

Most semiconductor memory manufacturers are developing MRAMs using ferromagnetic materials as one of the next generation of integrated information storage devices.

MRAM is a memory device using a quantum mechanical effect called magnetoresistance, and is a memory device that reads and writes data by forming a ferromagnetic thin film in multiple layers to sense a current change according to the magnetization direction of each thin film layer.

In other words, MRAM is a type of memory that stores a magnetic polarization state in a thin film of magnetic material. By using a magnetic field generated by a combination of bit line current and word line current, the MRAM is changed or sensed. A read operation is performed.

The MRAM is a device capable of high speed, low power, and high integration due to the inherent characteristics of the magnetic thin film, and capable of nonvolatile memory operation in which recorded information is not erased even when the power is turned off, such as a flash memory.

MRAM is generally composed of various cell types such as Giant Magneto Resistance (GMR) and Magnetic Tunnel Junction (MTJ).

In other words, MRAM implements a memory device by using a large magnetoresistance (GMR) phenomenon or spin polarization magnetic permeation phenomenon, which occurs because spin has a great influence on electron transfer.

First, an MRAM using a giant magnetoresistance (GMR) phenomenon is implemented using a phenomenon in which the resistance in the case where the spin directions are different in the two magnetic layers having a nonmagnetic layer between them is significantly different.

In addition, the MRAM using the spin polarization magnetic permeation phenomenon is implemented by using the phenomenon that current transmission occurs much better than the case where the spin direction is the same in the two magnetic layers having the insulating layer interposed therebetween.

1A and 1B are schematic diagrams of a general MTJ.

The structure of the conventional MTJ has a characteristic that the resistance varies depending on the alignment of the magnetization directions of the two layers.

That is, the MTJ has a variable free magnetic layer (2), a tunnel junction layer (3), and a magnetization direction fixed in which the magnetization direction is changed by an external magnetic field or the amount and direction of current flowing through the MTJ. Fixed magnetic layer (Fixed magnetic layer) 4 is made of a stack.

Here, the variable ferromagnetic layer 2 and the fixed ferromagnetic layer 4 usually have a material such as NiFeCo / CoFe, and the tunnel junction layer 3 has a material such as Al ₂ O ₃ . The variable ferromagnetic layer 2 and the fixed ferromagnetic layer 4 are separated by a tunnel junction layer 3 which is an insulating layer.

In addition, the variable ferromagnetic layer 2 and the fixed ferromagnetic layer 4 have different thicknesses, and thus the fixed ferromagnetic layer 4 has a changed magnetic polarization state in a strong magnetic field, and the variable ferromagnetic layer 2 has a weak magnetic field. The magnetic polarization state changes at.

When a voltage is applied to the variable ferromagnetic layer 2 and the fixed ferromagnetic layer 4 in the vertical direction, the tunnel junction layer 3 is very thin, so that current flows due to the tunneling phenomenon of electrons.

As shown in FIG. 1A, when the magnetizing directions of the variable ferromagnetic layer 2 and the fixed ferromagnetic layer 4 are the same, the tunneling resistance of the tunnel junction layer 3 is lowered and a large tunneling current flows, thereby increasing the sensing current.

On the other hand, as shown in FIG. 1B, when the magnetization directions of the variable ferromagnetic layer 2 and the fixed ferromagnetic layer 4 are different, the tunneling resistance of the tunnel junction layer 3 is increased so that a low tunneling current flows, so the sensing current is small. Lose.

Here, the direction of magnetization polarity of the variable ferromagnetic layer 2 is changed by an external magnetic field, and information of "0" or "1" is stored in accordance with the direction of magnetization polarity of the variable ferromagnetic layer 2. Therefore, the fixed ferromagnetic layer 4 does not change the magnetic polarization state at the time of writing, and only the variable ferromagnetic layer 2 generates a magnetic field in which the magnetic polarization state changes.

However, in the conventional MRAM cell operated as described above, the ferromagnetic layer used for the variable ferromagnetic layer 3 has a feature that the thermal stability decreases rapidly as the volume thereof decreases. If the thermal stability of the MTJ is insufficient, the arranged spin is reversed. That is, the thermal stability of the MTJ decreases in size when the size of the MTJ decreases in proportion to the size of the MTJ.

In addition, there is a feature that the larger the area ratio to the thickness, the more thermally stable. Therefore, in order to apply it to a memory cell of several gigabytes or more, the area of the MTJ element must be made several tens of nanometers or less, and thus, the thickness of the MTJ element must be designed to maintain the volume. However, since the ratio of the area to the thickness decreases rapidly, this cannot be satisfied at the same time.

In addition, since the thickness of the ferromagnetic material has reached several nanometers, it is technically difficult to further reduce the thickness. Therefore, if the area allowed for the MTJ cell is reduced, the stability of the signal is drastically reduced due to the reduction of the area-to-thickness ratio and the double height of volume reduction. As a result, it is very difficult to operate stably in a small memory cell having a few tens of nanometers.

The present invention was created to solve the above problems, and has an object to improve the efficiency of switching magnetization and increase the current density by implementing a magnetoresistive RAM using a high temperature superconductor.

In addition, an object of the present invention is to improve the structure of the MRAM cell to increase the efficiency of writing performance.

Magnetoresistive ram using a high temperature superconductor according to the present invention for achieving the above object, MTJ including a variable ferromagnetic layer, a tunnel junction layer and a fixed ferromagnetic layer; A bit line formed over the variable ferromagnetic layer; A selection transistor whose one end is connected to the fixed ferromagnetic layer and whose switching operation is controlled by a word line; And a source line connected to the other end of the selection transistor, wherein the MTJ and the bit line and the source line are made of a high temperature superconductor material.

And, the present invention is a source region and drain region formed on the silicon substrate; A gate region formed above the channel region of the silicon substrate; A landing pad formed on top of the source and drain regions, respectively; A bypass line connected to the landing pad formed on the drain region; An MTJ formed on the bypass line and including a variable ferromagnetic layer, a tunnel junction layer, and a fixed ferromagnetic layer; And a bit line formed on top of the MTJ, wherein the MTJ and the bit line and the source line are made of a high temperature superconductor material.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 is a view showing the characteristics of a general high temperature superconductor.

Superconductivity is a phenomenon where a material's temperature is very low (typically below -200 ° C), where the electrical resistance becomes '0' and the internal magnetic field is pushed back.

This was discovered in 1911 by Dutch physicist Ones. He found that the mercury's electrical resistance became '0' when the mercury temperature dropped below about 4K (-269 ° C).

The superconductor that exhibits such superconductivity is called a superconductor, and the temperature at which this phenomenon begins to appear is called the critical temperature.

The resistivity of ordinary metal conductors decreases gradually with temperature. However, in the case of a conductor such as copper or silver, there is a limit that the resistance does not decrease by more than a certain value due to impurities or other defects.

Even near absolute temperatures ('0'), the actual copper sample's resistance will be non-zero. On the other hand, the resistance of the superconductor suddenly drops to zero when the temperature drops below the "critical temperature" value. Accordingly, the current flowing through the copper of the superconducting wire can continue to flow even without power supply.

Superconducting phenomena occur in a wide variety of materials, even in one-element materials such as tin and aluminum, and in various metal alloys and doped ceramic materials.

On the other hand, superconductivity does not occur in precious metals such as gold or silver, and does not appear in pure ferromagnetic metals.

In 1986, high temperature superconductors with a critical temperature above 90K were found in copper perovskite-based ceramic materials, which led to the reactivation of superconductor research.

As a pure research topic, these materials are not explained by existing theories explaining superconductors. In addition, as the superconductivity appears at temperatures above the boiling point (77 K) of liquid nitrogen, an economically important point, more commercial applications open up.

By 1986, physicists believed that superconductivity was not possible at temperatures above 30K, according to Bardeen Cooper Schrieffer (BCS) theory. However, in 1986 Johannes Georg Bednorz and Karl Alexander Muller discovered superconducting materials in lanthanum copper-based perovskite materials, with a critical temperature of 35K.

Later, M.K.Wu et al. Substituted lanthanum with yttrium to produce YBCO (YbBa2Cu3O7), which reached a critical temperature of 92K. This was important in that the temperature was higher than 77 K, the vaporization point of the liquefied nitrogen used as the coolant.

This is commercially important because liquefied nitrogen can be produced anywhere inexpensively without worrying about the raw materials, and free from problems such as solid air plugs, a problem in transporting liquid helium.

Since then, many other copper-based superconductors have been discovered, and the theoretical explanation for the superconductivity of these materials has become one of the most challenging tasks in the field of cohesive materials physics.

Since about 1993, the superconductor with the highest critical temperature is a ceramic consisting of thallium, mercury, copper, barium, calcium and oxygen, with a critical temperature of Tc = 138K.

Superconductivity was found at the critical temperature of 26K in the lanthanum-oxygen-fluorine-iron-arsenic compound (LaO _{1 - x} F _x FeAs), ie oxypnictide. A subsequent group of studies found that the critical temperature increased to 52K when the lanthanum of LaO _{1 - x} F _x FeAs was replaced with other rare earths such as cerium, samarium, neodymium, praseodymium, and the like. In addition, iron-arsenic based superconductors with similar AFe2As2 structures have also been reported.

On the other hand, MRAM using a current induced magnetization switching (CiMS) operation by the spin transfer torque (STT) phenomenon can create a memory cell of 100 nanometers.

Here, the magnetic device technology using the magnetoresistive effect is one of spintronics technologies and has recently implemented a general purpose nonvolatile memory circuit called spin switching torque (STT) random access memory (RAM) by manipulating the spin of electrons.

In the STT RAM, a magnetic material is used to control the spin. When electrons pass through the magnetic material, the spin of the electrons moves in the same magnetization direction as the magnetic material.

Magnetic Tunnel Junction (MTJ) is the most popular spin device because of its potential in applications such as MRAM drive cells or magnetoresistive sensors.

A typical magnetic tunnel junction has a pinned layer and a free layer made of ferromagnetic material, and the two layers are separated by an insulating layer used as a tunnel barrier.

The MRAM by the STT requires a very small operating current compared to a conventional magnetic field, thereby making it possible to create a smaller size memory device.

In addition, the MRAM memory cell by STT has a characteristic of a non-volatile memory that maintains a signal stored for almost 10 years, does not destroy data during a read operation, and can operate at a very high speed (nano second units).

3A and 3B are diagrams illustrating a cell structure of an STT-MRAM using a high temperature superconductor according to the present invention.

The superconducting magnetoresistive RAM according to the present invention includes a bit line 100, an MTJ 105, a contact line 106, a word line 108, a select transistor TR and a source line 110.

The MTJ 105 is formed by stacking a variable free magnetic layer 101, a tunnel junction layer 102, and a fixed ferromagnetic layer 103. The variable free magnetic layer 101 and the fixed magnetic layer 103 are separated from each other by a tunnel junction layer 102 which is a thin insulating layer.

In the present invention, the bit line 100, the variable ferromagnetic layer 101 and the fixed ferromagnetic layer 103 of the MTJ 105, the contact line 106, and the superconducting source line 100 are made of a high temperature superconductor material.

Here, the high temperature superconductor material is lanthanum-based (lanthanum-oxygen-fluorine-iron-arsenic compound (La0 _{1 - x} F _x FeAs)) having a critical temperature of 30K, yttrium-based, bismuth oxide-based, and critical temperature having a critical temperature of 90K. Mercury-based or the like having 134K.

The bit line 100 is connected to the variable ferromagnetic layer 101 of the MTJ 105, and the contact line 106 is connected to the fixed ferromagnetic layer 103 of the MTJ 105.

In addition, one end of the selection transistor TR is connected to the fixed ferromagnetic layer 103 of the MTJ 105 through the contact line 106, and the other end is connected to the source line 110. In addition, the switching operation of the selection transistor TR is controlled by the word line 108.

In the STT-MRAM using the high temperature superconductor of the present invention having such a configuration, the spin transfer switching operation switches the state of the superconducting MTJ 105 from parallel (0) to unfair (0) or vice versa. This is done by the current flowing from the top to the bottom of the superconducting MTJ 105 or vice versa.

That is, when a low voltage is applied to the source line 110 and a high voltage is applied to the bit line 100, current flows from the bit line 100 toward the source line 110. Accordingly, as in FIG. 3A, the data "0" of the low resistance state is written to the MTJ 105.

On the other hand, when a high voltage is applied to the source line 110 and a low voltage is applied to the bit line 100, current flows from the source line 110 toward the bit line 100. Accordingly, as in FIG. 3B, the data "1" of the high resistance state is written to the MTJ 105.

The present invention allows each cell to be individually addressed by allowing the cell current to flow directly through the superconducting bit line 100 and the superconducting source line 110.

When the current is polarized through the superconducting STT method, the current flowing through the fixed ferromagnetic layer 103 polarizes the electrons. The polarized electrons affect the switching operation of the variable ferromagnetic layer 101 so that the MTJ 105 performs parallel and uneven switching. Here, the switching operation means magnetization inversion of the variable ferromagnetic layer 101. Accordingly, data is transferred from the fixed ferromagnetic layer 103 (polarizer) to the variable ferromagnetic layer 101.

The difference in conductivity depends on the spin polarization of the ferromagnetic layer used as the variable ferromagnetic layer 101 and the fixed ferromagnetic layer 103 and the tunneling effect of electrons through the barrier material.

The change in resistance according to parallel and antiparallel states is called Tunneling Magnetoresistance (TMR).

Therefore, the development of ferromagnetic materials and tunnel barriers with high spin polarization is the most important key technology for increasing the reading margin of MRAM.

In addition, an important technical challenge of STT-RAM is the switching problem of magnetic tunnel junctions. As the degree of integration increases to Gb, switching of specific cells becomes more difficult or interference problems occur due to adjacent cells depending on the driving current limit.

The present invention implements a cell structure using superconductivity for a cell structure that increases the efficiency of switching magnetization inversion and increases the current density. In addition, an MRAM cell structure using a superconducting material layer having low saturation magnetization can be implemented to increase the efficiency of writing margin.

4 is another embodiment of an MRAM using a high temperature superconductor according to the present invention.

The present invention forms a source region 202a and a drain region 202b on the silicon substrate 200. The barrier film 204 is formed on the channel region of the silicon substrate 200. The gate region 206 is formed on the barrier film 204.

Contact layers 208a and 208b are formed on the source and drain regions 202a and 202b, respectively, and landing pads 210a and 210b are formed on the contact layers 208a and 208b.

In addition, a cladding region 212 is formed above the gate region 206, and a word line 214 formed to be surrounded by the cladding region 212 is formed on the cladding region 212.

The contact layer 216 is formed on the landing pad 210b formed above the drain region 202b. The bypass line 218 is formed on the contact layer 216 extending to the word line 214 region.

The MTJ 220 is formed in an upper region of the word line 214 on the bypass line 218. The bit line 225 is formed on the MTJ 220.

Here, the MTJ 220 is formed by stacking the fixed ferromagnetic layer 223, the tunnel junction layer 222, and the variable ferromagnetic layer 221.

In the present invention, the bit line 225, the variable ferromagnetic layer 221, the fixed ferromagnetic layer 223, and the word line 214 of the MTJ 220 are made of a superconducting material.

Here, the high temperature superconductor material is lanthanum-based (lanthanum-oxygen-fluorine-iron-arsenic compound (La0 _{1 - x} F _x FeAs)) having a critical temperature of 30K, yttrium-based, bismuth oxide-based, and critical temperature having a critical temperature of 90K. Mercury-based or the like having 134K.

According to the present invention, magnetization reversal is performed by using a magnetic field generated from a superconducting external conductor during a recording operation for recording data in the MTJ 220.

That is, when a predetermined amount or more of current flows in the MTJ 220 to cross the critical magnetic field, the magnetization direction of the superconducting variable ferromagnetic layer 221 is reversed. In particular, as the size of the magnetic tunnel junction cell becomes smaller, the coercivity (Coercivity, Hc), a force that resists switching, becomes larger. Accordingly, the superconducting MTJ 220 and the superconducting external conductor are used because a large current must flow through the conductor.

The present invention uses the word line 214 and the bit line 225 formed at positions perpendicular to each other as superconducting external conductors. That is, as much current as necessary for the superconducting word line 214 and the superconducting bit line 225 is applied. Then, desired information is recorded by selectively inverting the magnetization direction of the superconducting variable ferromagnetic layer 221 in a specific cell at the intersection of the two conductive lines.

As described above, when a current flows through the superconducting wire, a circular magnetic field is induced around the superconducting wire. Then, a current above the threshold flows through the superconducting wire to form a magnetic field larger than the coercive force of the superconducting magnetic thin film near the cell. Then, the magnetization direction of the superconducting variable ferromagnetic layer 221 made of the superconducting magnetic thin film can be determined according to the direction of the current flowing through the conductive wire.

In this case, the reason why the current is applied to the two vertical superconducting wires is that the threshold of the current required to switch the magnetization direction of the superconducting magnetic layer at the intersection point is about half of the threshold required to switch the current through one superconducting wire. Because it is small. Accordingly, current is applied to the two superconducting wires to selectively switch only the desired cells.

As described above, according to the present invention, data is written according to the change of the magnetization state of the MTJ 220 according to the polarity of the current supplied to the word line 214 and the bit line 225. The amount of current flowing from the drain to the source is determined by the tunneling current generated differently according to the magnetization state of the MTJ 220 to read data.

As described above, the present invention implements a magnetoresistive RAM using a high temperature superconductor to increase the efficiency of switching magnetization inversion and increase the current density.

It also provides the effect of improving the structure of the MRAM cell to increase the efficiency of writing performance.

Claims (9)

MTJ including a variable ferromagnetic layer, a tunnel junction layer, and a fixed ferromagnetic layer; A bit line formed on the variable ferromagnetic layer; A select transistor whose one end is connected to the fixed ferromagnetic layer and whose switching operation is controlled by a word line; And A source line connected to the other end of the selection transistor, And the MTJ, the bit line and the source line are made of a high temperature superconductor material. Claim 2 has been abandoned due to the setting registration fee. The magnetoresistive RAM of claim 1, wherein the MTJ reads and writes data using a spin change torque (STT) phenomenon. Claim 3 was abandoned when the setup registration fee was paid. The method of claim 1, wherein the MTJ writes data '0' according to the current flowing from the bit line to the source line, and writes data '1' according to the current flowing from the source line to the bit line. Magnetoresistive RAM using high temperature superconductor. Claim 4 was abandoned when the registration fee was paid. The method of claim 1, wherein the high temperature superconductor material Claim 5 was abandoned upon payment of a set-up fee. 2. The magnetoresistive ram of claim 1, wherein the MTJ comprises the variable ferromagnetic layer and the fixed ferromagnetic layer made of the high temperature superconductor material. A source region and a drain region formed on the silicon substrate; A gate region formed above the channel region of the silicon substrate; Landing pads formed on the source and drain regions, respectively; A bypass line connected to the landing pad formed on the drain region; An MTJ formed on the bypass line, the MTJ including a variable ferromagnetic layer, a tunnel junction layer, and a fixed ferromagnetic layer; A bit line formed on top of the MTJ; And the MTJ, the bit line and the source line are made of a high temperature superconductor material. Claim 7 was abandoned upon payment of a set-up fee. 7. The magnetoresistive RAM of claim 6, wherein the MTJ selectively inverts the magnetization direction according to currents applied to the bit line and the word line to read and write data. Claim 8 was abandoned when the registration fee was paid. The method of claim 6, wherein the high temperature superconductor material Magnetoresistive RAM using a high-temperature superconductor, characterized in that it comprises any one of lanthanum-based, yttrium-based, bismuth oxide-based, mercury-based. Claim 9 was abandoned upon payment of a set-up fee. 7. The magnetoresistive ram of claim 6, wherein the MTJ comprises the variable ferromagnetic layer and the fixed ferromagnetic layer made of the high temperature superconductor material.

Images