# Patent

## Title
Variable resistive memory punchthrough access method

## Abstract

Variable resistive punchthrough access methods are described. The methods include switching a variable resistive data cell from a high resistance state to a low resistance state by passing a write current through the magnetic tunnel junction data cell in a first direction. The write current is provided by a transistor being electrically coupled to the variable resistive data cell and a source line. The write current passes through the transistor in punchthrough mode.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of U.S. application Ser. No. 12/261,296 filed on Oct. 30, 2008, the contents of which are hereby incorporated by reference in its entirety.


BACKGROUND
Fast growth of the pervasive computing and handheld/communication industry generates exploding demand for high capacity nonvolatile solid-state data storage devices. It is believed that nonvolatile memories, especially flash memory, will replace DRAM to occupy the biggest share of memory market. However, flash memory has several drawbacks such as slow access speed ( ̃microsecond write and  ̃50-100 nanosecond read), limited endurance ( ̃103-105 programming cycles), and the integration difficulty in system-on-chip (SoC). Flash memory (NAND or NOR) also faces significant scaling problems at 32 nm node and beyond.
Magneto-resistive Random Access Memory (MRAM) is another promising candidate for future nonvolatile and universal memory. MRAM features non-volatility, fast writing/reading speed (<10 ns), almost unlimited programming endurance (>1015cycles) and zero standby power. The basic component of MRAM is a magnetic tunneling junction (MTJ). Data storage is realized by switching the resistance of MTJ between a high-resistance state and a low-resistance state. MRAM switches the MTJ resistance by using a current induced magnetic field to switch the magnetization of MTJ.
Recently, a new write mechanism, which is based upon spin polarization current induced magnetization switching, was introduced to the MRAM design. This new MRAM design, called Spin-Transfer Torque RAM (STRAM), uses a (bidirectional) current through the MTJ to realize the resistance switching. Therefore, the switching mechanism of STRAM is constrained locally and STRAM is believed to have a better scaling property than the conventional MRAM.
However, a number of yield-limiting factors must be overcome before STRAM enters the production stage. One challenge is that the transistor utilized to provide the write switching current is sized to accommodate the larger switching current and this increase in size limits the scaling of the memory devices.
BRIEF SUMMARY
The present disclosure relates to variable resistive memory punchthrough access methods. In particular, present disclosure relates to a spin-transfer torque memory that includes a transistor operating in punchthrough mode to assist in resistance state switching and methods of writing and reading the spin-transfer torque memory utilizing the transistor operating in punchthrough mode.
One illustrative method includes switching a variable resistive data cell from a high resistance state to a low resistance state by passing a write current through the variable resistive data cell in a first direction. The write current is provided by a transistor being electrically coupled to the variable resistive data cell and a source line. The write current passes through the transistor in punchthrough mode.



BRIEF DESCRIPTION OF THE DRAWINGS
The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings, in which:
FIG. 1 is a cross-sectional schematic diagram of an illustrative magnetic tunnel junction memory cell in the low resistance state;
FIG. 2 is a cross-sectional schematic diagram of another magnetic tunnel junction memory cell in the high resistance state;
FIG. 3 is a graph of a static R-V (resistance-voltage) curve of a magnetic tunnel junction memory cell;
FIG. 4 is a schematic circuit diagram of an illustrative memory unit;
FIG. 5 is a schematic diagram of a transistor operating in punchthrough mode;
FIG. 6 is a schematic circuit diagram of writing a data state to a selected memory unit of a memory unit array;
FIG. 7 is a flow diagram of an illustrative method of writing to a memory unit array; and
FIG. 8 is a schematic circuit diagram of another memory unit array.



The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.
DETAILED DESCRIPTION
In the following description, reference is made to the accompanying set of drawings that form a part hereof and in which are shown by way of illustration several specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense. The definitions provided herein are to facilitate understanding of certain terms used frequently herein and are not meant to limit the scope of the present disclosure.
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein.
The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” encompass embodiments having plural referents, unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
The present disclosure relates to variable resistive memory punchthrough access methods. In particular, present disclosure relates to a spin-transfer torque memory that includes a transistor operating in punchthrough mode to assist in resistance state switching and methods of writing and reading the spin-transfer torque memory utilizing the transistor operating in punchthrough mode. Thus, the transistor can conduct large write currents as compared to its size. While the present disclosure is not so limited, an appreciation of various aspects of the disclosure will be gained through a discussion of the examples provided below.
Variable resistive memory includes memory cells that switch between at least a low resistance data state and a high resistance data state by passing a write current through the resistive memory cell. In some embodiments the resistive memory cell is a phase change data cell such as, for example, a data cell that includes a chalcogenide material. In some embodiments the resistive memory is a magnetic tunnel junction such as, for example, a spin transfer torque memory cell. These magnetic tunnel junction data cells are further described below.
FIG. 1 is a cross-sectional schematic diagram of an illustrative magnetic tunnel junction data cell 10 in the low resistance state and FIG. 2 is a cross-sectional schematic diagram of another magnetic tunnel junction data cell 10 in the high resistance state. The magnetic tunnel junction data cell 10 includes a ferromagnetic free layer 12 and a ferromagnetic reference (i.e., pinned) layer 14. The ferromagnetic free layer 12 and a ferromagnetic reference layer 14 are separated by an oxide barrier layer 13 or tunnel barrier. A first electrode 15 is in electrical contact with the ferromagnetic free layer 12 and a second electrode 16 is in electrical contact with the ferromagnetic reference layer 14. The ferromagnetic layers 12, 14 may be made of any useful ferromagnetic (FM) alloys such as, for example, Fe, Co, Ni and the insulating barrier layer 13 may be made of an electrically insulating material such as, for example an oxide material (e.g., Al2O3 or MgO). Other suitable materials may also be used.
The electrodes 15, 16 electrically connect the ferromagnetic layers 12, 14 to a control circuit providing read and write currents through the ferromagnetic layers 12, 14. The resistance across the magnetic tunnel junction data cell 10 is determined by the relative orientation of the magnetization vectors or magnetization orientations of the ferromagnetic layers 12, 14. The magnetization direction of the ferromagnetic reference layer 14 is pinned in a predetermined direction while the magnetization direction of the ferromagnetic free layer 12 is free to rotate under the influence of a spin torque. Pinning of the ferromagnetic reference layer 14 may be achieved through, e.g., the use of exchange bias with an antiferromagnetically ordered material such as PtMn, IrMn and others.
FIG. 1 illustrates the magnetic tunnel junction data cell 10 in the low resistance state where the magnetization orientation of the ferromagnetic free layer 12 is parallel and in the same direction of the magnetization orientation of the ferromagnetic reference layer 14. This is termed the low resistance state or “0” data state. FIG. 2 illustrates the magnetic tunnel junction data cell 10 in the high resistance state where the magnetization orientation of the ferromagnetic free layer 12 is anti-parallel and in the opposite direction of the magnetization orientation of the ferromagnetic reference layer 14. This is termed the high resistance state or “1” data state.
Switching the resistance state and hence the data state of the magnetic tunnel junction data cell 10 via spin-transfer occurs when a current, passing through a magnetic layer of the magnetic tunnel junction data cell 10, becomes spin polarized and imparts a spin torque on the free layer 12 of the magnetic tunnel junction data cell 10. When a sufficient spin torque is applied to the free layer 12, the magnetization orientation of the free layer 12 can be switched between two opposite directions and accordingly the magnetic tunnel junction data cell 10 can be switched between the parallel state (i.e., low resistance state or “0” data state) and anti-parallel state (i.e., high resistance state or “1” data state) depending on the direction of the current.
The illustrative spin-transfer torque magnetic tunnel junction data cell 10 may be used to construct a memory device that includes multiple magnetic tunnel junction data cells in an array where a data bit is stored in magnetic tunnel junction data cell by changing the relative magnetization state of the free magnetic layer 12 with respect to the pinned magnetic layer 14. The stored data bit can be read out by measuring the resistance of the cell which changes with the magnetization direction of the free layer relative to the pinned magnetic layer. In order for the spin-transfer torque magnetic tunnel junction data cell 10 to have the characteristics of a non-volatile random access memory, the free layer exhibits thermal stability against random fluctuations so that the orientation of the free layer is changed only when it is controlled to make such a change. This thermal stability can be achieved via the magnetic anisotropy using different methods, e.g., varying the bit size, shape, and crystalline anisotropy. Additional anisotropy can be obtained through magnetic coupling to other magnetic layers either through exchange or magnetic fields. Generally, the anisotropy causes a soft and hard axis to form in thin magnetic layers. The hard and soft axes are defined by the magnitude of the external energy, usually in the form of a magnetic field, needed to fully rotate (saturate) the direction of the magnetization in that direction, with the hard axis requiring a higher saturation magnetic field.
FIG. 3 is a graph of a static R-V sweep curve of a magnetic tunnel junction data cell. When applying a positive voltage on the second electrode 16 in FIG. 1 or 2, the MTJ 10 enters the positive applied voltage region in FIG. 3 and switches from the high resistance state (FIG. 2) to the low resistance state (FIG. 1). When applying a positive voltage on the first electrode 15 in FIG. 1 or 2, the magnetic tunnel junction data cell 10 enters the negative applied voltage region in FIG. 3. The resistance of the magnetic tunnel junction data cell switches from the low resistance state (FIG. 1) to the high resistance state (FIG. 2).
Let RH and RL denote the high and low magnet resistance, respectively. We define the Tunneling Magneto Resistance Ratio (TMR) as TMR=(RH−RL)/RL. Here RH, RL and TMR are determined by also the sensing current or voltage, as shown in FIG. 3. Generally, a large TMR makes it easier to distinguish the two resistance states of the magnetic tunnel junction data cell.
FIG. 4 is a schematic circuit diagram of a memory unit 20 and FIG. 5 is a schematic diagram of a transistor 30 operating in punchthrough mode. The memory unit 20 includes a variable resistive data cell (e.g., magnetic tunnel junction data cell MTJ) electrically coupled to a bit line BL and a source line SL. The variable resistive data cell or magnetic tunnel junction data cell MTJ is configured to switch between a high resistance state and a low resistance state by passing a write current through the data cell. A transistor 30 is electrically connected between the variable resistive data cell or MTJ and the source line SL. The transistor can be a metal-oxide-semiconductor field effect transistor (MOSFET). In many embodiments, the MOSFET is a NMOS or a PMOS transistor. In many embodiments, the transistor includes a gate contact that is in electrical connection to a word line WL.
In FIG. 5 the transistor 30 includes a gate 31, and in many embodiments the gate 31 is in electrical connection with the WL, although in some embodiments the gate 31 is not in electrical connection with the WL. The transistor 30 includes a semiconductor substrate 32 having a p or n doped source region 33 and drain region 35. The source region 33 is in electrical connection to a source electrode 34 and the drain region 35 is in electrical connection to a drain electrode 36. A channel region 37 separates the source region 33 and drain region 35. The transistor 30 is illustrated as being between the source line SL and the variable resistive data cell or magnetic tunnel junction data cell MTJ. However, the transistor 30 can be between the bit line BL and the variable resistive data cell or magnetic tunnel junction data cell MTJ. The transistor 30 operates as a bi-directional switch to allow read and write currents to flow through the variable resistive data cell or magnetic tunnel junction data cell MTJ.
The transistor 30 is configured to operate in punchthrough mode. Punchthrough mode occurs when a sufficient voltage is applied across the source region 33 and drain region 35 to merge a source depletion region 33D and a drain depletion region 34D. The merged area 38 allows the transistor 30 to conduct a large current from the source electrode 34 to the drain electrode 36. The transistor 30 allows punchthrough at relatively low voltages (known as the punchthrough voltage Vp). Current can flow in either direction through the transistor 30 in punchthrough mode. In punchthougth mode, the gate of the transistor 30 is not activated by the word line WL. Thus, in some embodiments, the word line WL can be omitted from the memory unit 20.
Thus for example, the transistor 30 can provide a first data state write current I1 to the variable resistive data cell or magnetic tunnel junction data cell MTJ to switch the variable resistive data cell or magnetic tunnel junction data cell MTJ from a high resistance state to a low resistance state and the transistor 30 can provide a second data state current I2 to the variable resistive data cell or magnetic tunnel junction data cell MTJ to switch the variable resistive data cell or magnetic tunnel junction data cell MTJ from a low resistance state to a high resistance state or vice versa. The first data state write current I1 and the second data state current I2 flow through the transistor 30 in punchthrough mode. In many embodiments, first data state write current I1 has a similar or the same magnitude as the second data state current I2.
In some embodiments, the transistor (described throughout) is replaced with a bidirectional switch, where the material of the gate is polysilicon, metal or a dielectric such as, for example, silicon nitride or silicon oxide. In these embodiments, the read operation is accomplished via a short read cycle, as described below.
FIG. 6 is a schematic circuit diagram of writing a data state to a selected memory unit of a memory unit array 40. The memory array 40 includes a plurality of bit lines BL1, BL2, BL3 and a plurality of source lines SL1, SL2, SL3 intersecting with the plurality of bit lines BL1, BL2, BL3 and forming a cross-point array. The memory array 40 includes a plurality of word lines WL1, WL2, WL3. While only three bit lines, source lines, and word lines are illustrated, it is understood that the memory unit array 40 can have any useful number of bit lines, source lines, and word lines.
A memory unit 20 (as described above) is adjacent to at least selected cross-points of the cross-point array. The memory unit 20 includes a magnetic tunnel junction data cell MTJ electrically coupled to a bit line BL1, BL2, or BL3 and a source line SL1, SL2, or SL3. The variable resistive data cell or magnetic tunnel junction data cell MTJ is configured to switch between a high resistance state and a low resistance state by passing a write current through the magnetic tunnel junction data cell, as described above.
A transistor 30 is electrically connected between the variable resistive data cell or magnetic tunnel junction data cell MTJ and the source line (SL3 for the illustrated cross-point memory unit 20). The transistor 30 provides current to the variable resistive data cell or magnetic tunnel junction data cell MTJ in punchthrough mode, as described above.
FIG. 6 also illustrates a writing access method. The bit lines BL1, BL2, BL3 and the source lines SL1, SL2, SL3 are precharged to a specified precharge voltage level (Vp/2). This specified precharge voltage level can be any useful level. In many embodiments, the specified precharge voltage level is in a range 40 to 60% of the punchthrough voltage Vp, or about 50% of the punchthrough voltage Vp.
The selected memory unit 20 to write to is located at the intersection of BL2 and SL2. Thus, a punchthrough voltage Vp is applied across the selected memory unit 20 in a first direction (to write a first resistance state). In this example, the punchthrough voltage Vp is applied to the bit line BL2 and the source line SL2 is grounded. The precharge voltage is selected so that the voltage difference between any other (non-selected) two source lines or bit lines is always less than the punchthrough voltage Vp of the transistor 30. Therefore, current does not flow through any other transistor in the array 40. Switching the ground and punchthrough voltage Vp provides current in a second direction (to write a second resistance state) opposing the first direction.
Reading the memory unit 20 can be accomplished by activating the gate of the transistor 30 via the word line WL and allowing a read current to pass though the transistor 30 via an inversion layer between the source region and drain region of the transistor 30. Alternatively, the memory unit 20 can be read by conducting a current through the transistor 30 in punchthrough mode. In punchthrough reading mode, a punchthrough voltage Vp is applied across the transistor 30 and variable resistive data cell or magnetic tunnel junction data cell MTJ for a period of time that is not long enough to switch the data state of the variable resistive data cell or magnetic tunnel junction data cell MTJ.
FIG. 7 is a flow diagram of an illustrative method of writing to a memory unit array 100, described above. The method includes precharging the memory array to a specified precharge voltage level (less than the punch through voltage level) at block 101, as described above. If writing a low resistance state, then the punchthrough writing voltage Vp is applied to a selected source line and a selected bit line is grounded at block 102 to place the selected variable resistive data cell or MTJ in a low resistance state at data state 104. If writing a high resistance state, then the punchthrough writing voltage Vp is applied to a selected bit line and a selected source line is grounded at block 103 to place the selected variable resistive data cell or MTJ in a high resistance state at data state 105. In other embodiments, the data resistance states can be reversed based on the configuration of the variable resistive data cell or MTJ.
FIG. 8 is a schematic circuit diagram of another memory unit array 200. The exemplary memory array 200 includes, at least, a first transistor T1 electrically connected between a first variable resistive data cell or first magnetic tunnel junction data cell MTJ1 and a first bit line BL1, and a second transistor T2 electrically connected between a second variable resistive data cell or second magnetic tunnel junction data cell MTJ2 and a second bit line BL2. The illustrated memory unit array 200 further includes a third transistor T3 electrically connected between a third variable resistive data cell or third magnetic tunnel junction data cell MTJ3 and a third bit line BL3, and a fourth transistor T4 electrically connected between a fourth variable resistive data cell or fourth magnetic tunnel junction data cell MTJ4 and a fourth bit line BL4. The memory array 200 can have any useful number of variable resistive data cells or magnetic tunnel junction data cells, as desired. For example, two bit lines, two transistors, and two variable resistive data cells could be used.
The variable resistive data cell or magnetic tunnel junction data cells are electrically connected to a common conductive line 202. A common transistor 204 electrically couples the common conductive line 202 to a source line SL. A word line WL activates a gate of the common transistor 204 to allow current to flow from the source line SL the selected bit line BL1, BL2, BL3, or BL4. A punchthrough voltage Vp (at least) is applied across, for example, a first bit line BL1 and the source line SL. Current does not flow into any of the other variable resistive data cell or magnetic tunnel junction data cells (i.e., MTJ2, MTJ3, MTJ4) since it takes at least two times the punchthrough voltage to punchthrough any of the other transistors (i.e., T2, T3, T4).
Thus, embodiments of the VARIABLE RESISTIVE MEMORY PUNCHTHROUGH ACCESS METHOD are disclosed. The implementations described above and other implementations are within the scope of the following claims. One skilled in the art will appreciate that the present disclosure can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.

## Claims

1. A method comprising:
precharging a plurality of bit lines and a plurality of source lines to a precharge voltage being less than a punchthrough voltage of a transistor, the plurality of source lines intersecting with the plurality of bit lines and forming a cross-point array, and a memory unit adjacent to at least selected cross-points of the cross-point array, the memory unit comprising a variable resistive data cell and the transistor, the transistor is electrically connected between the variable resistive data cell and one of the plurality of source lines; and
writing a first data state to one or more variable resistive data cells along a selected bit line by applying the punchthrough voltage across the selected bit line and one or more selected source lines to pass a first data state write current through the transistor by merging a source depletion region and a drain depletion region in the transistor substrate in punchthrough mode.

2. A method according to claim 1, further comprising writing a second data state to one or more variable resistive data cells along a selected bit line by applying the punchthrough voltage across the selected bit line and one or more selected source lines to pass a second data state write current through the transistor in punchthrough mode, the second data state write current opposing the first data state write current.

3. A method according to claim 2, wherein the first data state write current has about the same magnitude as the second data state write current.

4. A method according to claim 1, wherein the transistor is a NMOS transistor or a PMOS transistor.

5. A method according to claim 1, wherein the variable resistive data cell is a magnetic tunnel junction data cell.

6. A method according to claim 5, wherein the magnetic tunnel junction data cell is a spin-transfer torque data cell.

7. A method according to claim 1, further comprising passing a read current through the variable resistive data cell by activating a gate of the transistor.

8. A method according to claim 1, further comprising passing a read current through the variable resistive data cell, the read current passing through the transistor in punchthrough mode.

9. A method according to claim 1, wherein the first data state write current and the second data state write current is formed by a voltage difference across the source depletion region and the drain depletion region and having a magnitude equal to or greater than the punchthrough voltage for the transistor.

10. A method according to claim 1, wherein the precharge voltage is in a range from 40 to 60% of the punchthrough voltage.

11. A method according to claim 1, wherein the precharge voltage is about 50% of the punchthrough voltage.

12. A method comprising:
precharging a plurality of bit lines and a plurality of source lines to a precharge voltage being less than a punchthrough voltage of a transistor, the plurality of source lines intersecting with the plurality of bit lines and forming a cross-point array, and a memory unit adjacent to at least selected cross-points of the cross-point array, the memory unit comprising a variable resistive data cell and the transistor, the transistor is electrically connected between the variable resistive data cell and one of the plurality of source lines; and
writing a first data state to one or more variable resistive data cells along a selected bit line by applying the punchthrough voltage across the selected bit line and one or more selected source lines.

13. A method according to claim 12, wherein the precharge voltage is in a range from 40 to 60% of the punchthrough voltage.

14. A method according to claim 12, wherein the transistor is a NMOS transistor or a PMOS transistor.

15. A method according to claim 12, wherein the variable resistive data cell is a magnetic tunnel junction data cell.

16. A method according to claim 15, wherein the magnetic tunnel junction data cell is a spin-transfer torque data cell.

17. A method according to claim 12, further comprising passing a read current through the variable resistive data cell, the read current passing through the transistor in punchthrough mode.