# Patent

## Title
Nonvolatile memory device using a tunnel oxide layer and oxygen blocking layer as a current limiter element

## Abstract

Embodiments of the invention include a method of forming a nonvolatile memory device that contains a resistive switching memory element with improved device switching performance and lifetime, due to the addition of a current limiting component. In one embodiment, the current limiting component comprises a resistive material configured to improve the switching performance and lifetime of the resistive switching memory element. The electrical properties of the current limiting layer are configured to lower the current flow through the variable resistance layer during the logic state programming steps by adding a fixed series resistance in the resistive switching memory element found in the nonvolatile memory device. In one embodiment, the current limiting component comprises a tunnel oxide layer that is a current limiting material and an oxygen barrier layer that is an oxygen deficient material disposed within a resistive switching memory element in a nonvolatile resistive switching memory device.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to nonvolatile memory elements, and more particularly, to methods for forming resistive switching memory elements used in nonvolatile memory devices.
2. Description of the Related Art
Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments.
Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EEPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has led to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory.
Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
Resistive switching based on transition metal oxide switching elements formed of metal oxide (MO) films has been demonstrated. Although metal oxide (MO) films such as these exhibit bistability, the resistance of these films and/or the ratio of the high-to-low resistance states is (are) often insufficient to be of use within a practical nonvolatile memory device. For instance, the resistance states of the metal oxide film should preferably be significant as compared to that of the system (e.g., the memory device and associated circuitry) so that any change in the resistance state change is perceptible. Since the variation in the difference in the resistive states is related to the resistance of the resistive switching layer, it is often hard to use a low resistance metal oxide film to form a reliable nonvolatile memory device. For example, in a nonvolatile memory that has conductive lines formed of a relatively high resistance metal such as tungsten, the resistance of the conductive lines may overwhelm the resistance of the metal oxide resistive switching element if its resistance was not sufficiently high. This may make it difficult or impossible to sense the state of the bistable metal oxide resistive switching element. Similar issues can arise from integration of the resistive switching memory element with current steering elements, such as diodes and/or resistors. The resistance of the resistive switching memory element (at least in its high resistance state) is preferably significant compared to the resistance of the current steering elements, so that the unvarying resistance of the current steering element does not dominate the resistance of the switching memory element, and thus reduce the measurable difference between the “on” and “off” states of the formed memory device (i.e., logic states of the device). However, since the power that can be delivered to a circuit containing a series of resistive switching memory elements and current steering elements is typically limited in most conventional nonvolatile memory devices (e.g., CMOS driven devices), it is desirable to form each of the resistive switching memory elements and current steering elements in the circuit so that the voltage drop across each of these elements is small, and thus resistance of the series connected elements does not cause the current to decrease to an undesirable level due to the fixed applied voltage (e.g.,  ̃2-5 volts).
As nonvolatile memory device sizes shrink, it is important to reduce the required currents and voltages that are necessary to reliably set, reset and/or determine the desired “on” and “off” states of the device to minimize overall power consumption of the memory chip as well as resistive heating of the device and cross-talk between adjacent devices.
Moreover, as nonvolatile memory device sizes shrink it becomes increasing necessary to assure that the “set” and “reset” currents used to change the state of the memory element are not too large so as to require higher voltage transistors for chip control circuitry, as well as to minimize damage to or alter the electrical or physical properties of the one or more layers found in the formed memory device. A large current flowing through the current carrying lines in a memory array can also undesirably alter or disturb the memory state of other interconnected devices or possibly damage portions of the adjacently connected devices, due to an appreciable amount of “cross-talk” created between them.
There is a need to limit and/or minimize the required current used to sense and program the logic states of each of the interconnected devices in an effort to reduce chip overall power consumption as well as improve device longevity and reduce the possibility of cross-talk between adjacently connected devices, which can alter a nonvolatile memory's device state. It is also desirable to form a nonvolatile memory device that has low programming currents when switching the device between the “on” and “off” states. Certain materials are known and have been used within the device to limit the current across the nonvolatile memory device while trying to resolve the cross-talk issue and lower the programming currents, but these materials through fabrication of the device or through regular device operation can contaminate or alter the properties of the metal oxide switching films and affect the performance of the switching memory element. Therefore, it is desirable to form a nonvolatile memory device that requires low programming currents to change the device between the “on” and “off” states.
SUMMARY OF THE INVENTION
Embodiments of the invention generally relate to a resistive switching nonvolatile memory device having a passive current limiter layer and a barrier layer structure disposed between at least one of the electrodes and a variable resistance layer formed in the nonvolatile memory device. The resistive switching memory elements may be formed as part of a high-capacity nonvolatile memory integrated circuit, which can be used in various electronic devices, such as digital cameras, mobile telephones, handheld computers, and music players. The resistive switching nonvolatile memory device comprises a variable resistance layer and current limiter layer that are configured to adjust the nonvolatile memory device's performance, such as lowering the formed device's switching currents and reducing the device's forming voltage, and reducing the performance variation from one formed device to another. A separation layer is provided between the variable resistance layer and the current limiting layer to prevent the diffusion of mobile ions between the two layers, and thus damaging the switching characteristics of the variable resistance layer. Mobile ions may include one or more elements found in the adjacent layers. In one example, the mobile ions found in the variable resistance layer may be oxygen ions (O2−).
The present invention may provide a nonvolatile memory element, comprising a first electrode layer, a second electrode layer, a variable resistance layer comprising a metal oxide disposed between the first electrode layer and the second electrode layer. A separation layer that inhibits the flow of oxygen ions is disposed between the variable resistance layer and a current limiter layer. The thickness of the separation layer is greater than the average diffusion length of the oxygen ions within the separation layer.
In one embodiment, the separation layer has an electrical resistance that is less than one fifth of the combined electrical resistance of the separation layer, the variable resistance layer, the current limiting layer and the two electrode layers.
Embodiments of the present invention may further provide a method of forming a nonvolatile memory element including depositing a first electrode layer, a second electrode layer, and a variable resistance layer. The method further includes depositing a current limiting layer and a separation layer. The separation layer is disposed between the variable resistance layer and the current limiting layer. The thickness of the separation layer is greater than the average diffusion length of the oxygen ions within the separation layer. The separation layer has an electrical resistance that is less than one fifth of the combined electrical resistance of the separation layer, the variable resistance layer, the current limiting layer and the two electrode layers.



BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG. 1 illustrates an array of resistive switching memory elements in accordance with an embodiment of the invention.
FIG. 2A is schematic representation of a memory device in accordance with an embodiment of the invention.
FIG. 2B is schematic representation of a memory device having a diode type current steering element in accordance with an embodiment of the invention.
FIG. 2C is schematic representation of an array of memory devices in accordance with an embodiment of the invention.
FIG. 2D is schematic representation of an array of memory devices in accordance with an embodiment of the invention.
FIG. 3A is a schematic side cross-sectional view of a standard memory element disposed in a nonvolatile memory device.
FIG. 3B is a schematic representation of an electrical circuit formed in the standard memory element illustrated in FIG. 3A.
FIG. 4A is a graph illustrating the current (I) versus voltage (V) characteristics of the high and low resistance load lines of a variable resistance layer in accordance with an embodiment of the invention.
FIG. 4B is a current versus time plot illustrating the effect of delivering bipolar type switching pulses through a memory element in accordance with an embodiment of the invention.
FIG. 5A is a schematic side cross-sectional view of a memory element disposed in a nonvolatile memory device in accordance with an embodiment of the invention.
FIG. 5B is a schematic side cross-sectional view of a memory element disposed in a nonvolatile memory device in accordance with a further embodiment of the invention.
FIG. 5C is a schematic representation of an electrical circuit formed in the memory element illustrated in FIG. 5A and FIG. 5B in accordance with an embodiment of the invention.
FIG. 6 is a schematic depiction of a process for forming the switching memory device according to one embodiment of the invention.



While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
DETAILED DESCRIPTION
Embodiments of the invention generally include a method of forming a nonvolatile memory device that contains a resistive switching memory element that has an improved device switching performance and increased usable lifetime, due to the addition of a current limiting component disposed therein. In one embodiment, the current limiting component comprises a tunnel oxide current limiting layer together with a separation layer that acts as an oxygen barrier layer. The electrical properties of the formed current limiting layer are configured to lower the current flow through the variable resistance layer by adding a fixed series resistance in the formed nonvolatile resistive switching memory device. It is generally desirable to form the current limiting layer so that its material and electrical properties will not degrade or breakdown during the often high current “burn-in” type device preparation steps, such as the “electrical forming” process, and also during normal repetitive operation of the nonvolatile resistive switching memory device. The separation layer provides a barrier to the migration of mobile ions between the current limiting layer and the variable resistance layer during both the electrical forming process and during normal operation resulting in improved performance and device longevity.
An illustrative memory array 100 of nonvolatile resistive switching memory devices 200 (hereafter switching memory device 200), which each generally include at least one resistive switching memory element 112, is illustrated in FIG. 1. Memory array 100 may be part of a larger memory device or other integrated circuit structure, such as a system on a chip type device. Read and write circuitry is connected to switching memory devices 200 using word-lines and bit-lines, which are referred to herein generally as electrodes 102 and 118, and are used to read from or write data into the memory element 200. Electrodes 102 and 118, generally include one or more conductive layers that each have a desired function in the array of switching memory devices 200. In some configurations, the electrodes 102 and 118 each comprise two or more conductive layers in which a first conductive layer is used to interconnect the multiple switching memory devices 200 and a second conductive layer is disposed in each switching memory device 200 to provide a desirable electrical interface (e.g., desirable work function) to the adjacent components in the switching memory device 200. Individual switching memory devices 200 or groups of switching memory devices 200 can be accessed using appropriate sets of word-lines and bit-lines, or electrodes 102 and 118. The memory elements 112 in the switching memory devices 200 may be formed from one or more layers 114 of materials, as indicated schematically in FIG. 1. In addition, memory arrays such as memory array 100 can be stacked in a vertical fashion to make multilayer memory array structures. The use of resistive switching memory elements to form memory arrays is merely illustrative, and one skilled in the art will appreciate that the formed devices may be used in other device applications without deviating from the basic scope of the invention described herein.
FIG. 2A schematically illustrates one example of a switching memory device 200 that contains a memory element 112 and an optional current steering device 216, which are both disposed between the electrodes 102 and 118. In one configuration, the current steering device 216 is an intervening electrical component, such as a p-n junction diode, p-i-n diode, transistor, or other similar device that is disposed between electrode 102 and memory element 112, or between the electrode 118 and memory element 112. In one example, the current steering device 216 may include two or more layers of semiconductor material, such as two or more doped silicon layers, that are configured to allow or inhibit the current flow in different directions through the memory element 112 when that memory element is not selected to read.
FIG. 2B schematically illustrates a switching memory device 200 that contains a memory element 112 and a diode type current steering device 216 that preferentially allows current to flow through the memory device 200 in a forward direction (“I+”). However, due to the design of the current steering device 216, a reduced current can also flow in the opposing direction through the device by the application of a reverse bias to the electrodes 102 and 118.
FIG. 2C schematically illustrates an array of switching memory devices 200 that are connected together to form part of a high-capacity nonvolatile memory integrated circuit. Each of the individual switching memory devices 200 can be accessed using appropriate sets of discrete word-lines and bit-lines, which, as noted above, may comprise at least a portion of the electrodes 102 and 118. As illustrated in FIG. 2C, each of the switching memory devices 200 contains a memory element 112 and current steering device 216 (e.g., a diode type) that are connected to at least one of the electrodes 102 and at least one of the electrodes 118. The electrodes 102 and/or 118 are generally biased by circuitry that is connected at the periphery of the memory chip on which the array of memory devices 200 are formed.
FIG. 2D schematically illustrates another embodiment of an array of switching memory devices 200 that are connected together to form part of a high-capacity nonvolatile memory integrated circuit. As shown in FIG. 2D, the current steering device 216, such as a typical MOS type transistor, is used to selectively deliver current through the memory element 112 by use of the appropriate set of word-lines, bit-lines and separate source-lines 119. As illustrated in FIG. 2D, each of the switching memory devices 200 contains a memory element 112 and current steering device 216 (e.g., transistor) that are connected to at least one of the electrodes 102, at least one of the electrodes 118 and at least one of the source lines 119. The source-lines 119 generally comprise one or more patterned conductive layers (e.g., metal) that are adapted to provide a desired amount of current to the memory element 112 when the transistor in the current steering device is turned “on”. The electrodes 102, electrodes 118 and source-lines 119 are typically biased by circuitry that is connected at the periphery of the memory chip on which the array of memory devices 200 are formed.
During operation, such as a read operation, the state of a memory element 112 in the switching memory device 200 can be sensed by applying a sensing voltage (i.e., a “read” voltage VREAD (FIG. 4A)), such as applying about +0.5 volts (V), to an appropriate set of electrodes 102 and 118. Depending on its history, a memory element that is addressed in this way may be in either a high resistance state (HRS) or a low resistance state (LRS). The resistance of the memory element 112 therefore determines what digital data is being stored by the memory element 112. If the memory element 112 is in the high resistance state, for example, the memory element may be said to contain a logic “zero” (i.e., a “0” bit). If, on the other hand, the memory element is in the low resistance state, the memory element may be said to contain a logic “one” (i.e., a “1” bit). During a write operation, the state of a memory element can be changed by application of suitable write signals to an appropriate set of electrodes 102 and 118.
In some embodiments, the memory element 112 uses bipolar switching where opposite polarity set and reset voltages are used to alter the resistance of the memory element between high and low resistance states. FIG. 4A schematically illustrates a log-log plot of current (I) versus voltage (V) of one example of the low-resistance-state (LRS) and high-resistant-state (HRS) curves of a memory element 112 having these two bistable resistive states. The voltage levels VSET and VRESET, shown in FIG. 4A, illustrate typical threshold values used to “set” and “reset” the contents of a memory element 112. In one example, initially, memory element 112 may be in a high resistance state (e.g., storing a logic “zero”). The high resistance state of memory element 112 can be sensed by read and write circuitry 150 (FIG. 2A) using electrodes 102 and 118. For example, read and write circuitry may apply a read voltage VREAD to memory element 112, and can sense the resulting “off” current (IOFF) that flows through memory element 112. When it is desired to store a logic “one” in memory element 112, memory element 112 can be placed into its low-resistance state. This may be accomplished by using read and write circuitry 150 to apply a set voltage VSET (e.g., −1 V to −4 V) across electrodes 102 and 118. In one configuration, applying a negative VSET voltage to memory element 112 causes memory element 112 to switch to its low resistance state. It is believed that the change in the resistive state of memory element 112 may be due to the redistribution or filling of traps (i.e., “trap-mediated”), or defects, in the variable resistance layer 206 (FIG. 3A or 5A), when the device is reverse biased. The defects or traps, which are commonly formed during the deposition or initial burn-in or forming of the variable resistance layer 206, are often created by a non-stoichiometric material composition found in the formed variable resistance layer 206. VSET and VRESET are generally referred to as “switching voltages” herein.
The low resistance state of the memory element 112 can be sensed using the read and write circuitry 150. When a read voltage VREAD is applied to resistive switching memory element 112, the read and write circuitry 150 will sense the relatively high “on” current value (ION), indicating that memory element 112 is in its low resistance state. When it is desired to store a logic “zero” in memory element 112, the memory element can once again be placed in its high resistance state by applying a positive reset voltage VRESET (e.g., +1 V to +5 V) to memory element 112. When read and write circuitry applies VRESET to memory element 112, memory element 112 enters its high resistance state. When the reset voltage VRESET is removed from memory element 112, memory element 112 will once again be characterized by high resistance when the read voltage VREAD is applied. Voltage pulses can be used in the programming of the memory element 112. For example, a 1 microseconds (ms) to 1 nanoseconds (ns) square or trapezoidal shaped pulse can be used to switch the memory element 112. In some embodiments, it may be desirable to adjust the length of the pulse depending on the amount of time needed to switch the memory element 112. In one example, the “set” and “reset” pulses are each about 10 ns in length. While the discussion of the memory element 112 herein primarily provides bipolar switching examples, some embodiments of the memory element 112 may use unipolar switching, where the set and reset voltages have the same polarity, without deviating from the scope of the invention described herein.
To provide a measurable difference between the logic “zero” and logic “one” states is common to form the variable resistance layer 206 and other memory element 112 components so that the ION and IOFF currents have a difference of at least five times (e.g., current ratio ION/IOFF≧5). In one example, the difference between the logic “zero” and logic “one” states is at least one order of magnitude (e.g., current ratio ION/IOFF≧10). In other words, the ratio of the electrical resistances of the variable resistance layer 206 is decreased by at least 5 to 10 times when switching from the high to the low resistance state. For example, the electrical resistance of the variable resistance layer 206 in the high resistance state is at least 5 to 10 times greater than the electrical resistance of the low resistance state when applying a VREAD read voltage across electrodes 102 and 118 in the device.
In an effort to prepare the memory element 112 for use, it is common to apply a forming voltage (VFORM) at least once across the electrodes 102, 118 to “burn-in” the device. It is believed that the application of a forming voltage, which is typically significantly greater than the VRESET and VSET voltages, causes the defects that are formed within the variable resistance layer 206 during the device fabrication process to move, align and/or collect within various regions of the formed layer, causing the variable resistance layer 206 to consistently and reliably switch between the “on” and “off” resistive states throughout the memory element's life. In one configuration, the forming voltage is between about 1 and about 5 times greater than the VRESET or VSET voltage. In one example, the forming voltage is between about 1.4 and about 2.5 times greater than the VRESET or VSET voltage. In one example, the forming voltage is between about 3 and about 7 volts. However, it is noted that in some cases it is desirable to form the memory element 112 so that the application of a forming voltage is not required at all to assure that the device will perform as desired throughout its life.
FIG. 3A is a schematic side cross-sectional view of a standard un-optimized switching memory device 200A, which contains a memory element 112 and a current steering device 216 (e.g., a diode) that is formed over a substrate 201. In this configuration, the memory element 112 generally contains a top electrode 102, variable resistance layer 206 and intermediate electrode 210. FIG. 3B schematically illustrates an electrical circuit formed in the switching memory device 200A shown in FIG. 3A. As illustrated in FIG. 3B, the electrical circuit within the standard switching memory device 200A includes a top electrode impedance (i.e., resistance RTEL) created by the material layer(s) in the top electrode 102, a variable resistance layer impedance (i.e., resistance RVR) created by the material layer(s) in the variable resistance layer 206, an intermediate electrode impedance (i.e., resistance RIEL) created by the material layer(s) in the intermediate electrode 210, a current steering device impedance (i.e., resistance RCSD) created by the material layer(s) in the current steering device 216 and a bottom electrode impedance (i.e., resistance RBEL) created by the material layer(s) in the bottom electrode 118. The electrodes 102, 210 and 118 are generally formed from a conductive material, such as a highly conductive semiconductor material (e.g., p-type polysilicon, n-type polysilicon) and/or metal (e.g., TiN, Al, W) to minimize the circuit resistance created between interconnected devices in a memory array 100. The variable resistance layer 206 can be a dielectric material, such as a metal oxide material or other similar material that can be switched between at least two or more stable resistive states. One will note that it is assumed that the contact resistances between the various layers in the switching memory device, such as the contact resistance formed between the electrode 102 and the variable resistance layer 206, are negligible to help reduce the complexity of the discussion of the circuit. While the current steering device 216 may include two or more layers of semiconductor material that are adapted to control the flow of current through the formed memory device 200A, the resistance of each of the components in the current steering device 216 are not individually discussed herein to minimize the complexity of the discussion, and thus an overall current steering device resistance RCSD is used to represent the overall impedance of the current steering device 216.
As noted above, FIG. 4A schematically illustrates a log-log plot of current (I) versus voltage (V) of the low-resistance-state (LRS) and high-resistant-state (HRS) curves, or load lines, of a memory element 112 having these two bistable resistive states. As illustrated in FIG. 4A, by sweeping the voltage applied to the electrodes 102 and 118 between the VSET (e.g., −3 volts) and VRESET (e.g., +4 volts), while the device is in the low resistance state, the LRS curve can be created, and by sweeping the voltage applied to the electrodes 102 and 118 between the VSET and VRESET, while the device is in the high resistance state, the HRS curve can be created. As noted above, depending on the physical and electrical characteristics of a formed variable resistance layer 206, it will switch from the HRS to the LRS during a “set” operation when a VSET is applied, and the variable resistance layer 206 will switch from the LRS to the HRS during a “reset” operation when a VRESET is applied.
FIG. 4B is a plot of current versus time for a plurality of bipolar type “set” and “reset” switching pulses, as illustrated by pulses 401-406, that are delivered to a switching memory device in accordance with an embodiment of the invention. In one example, as shown in FIG. 4B, a device programming step may include the delivery of a “set” switching pulse 411, a “reset” switching pulse 413, and two “sensing” pulses 412. To assure that the memory element 112 reliably switches from a high resistance state to a low resistance state and vice versa, one must assure that the “set” switching pulse 411 produces a current that is greater than a minimum “set” currentIMSC, which is defined as the minimum current required to flow through the variable resistance layer 206 to cause it to switch from a high resistance state (e.g., 2.5 MΩ) to a low resistance state (e.g., <<250 kΩ). In one example, the high and low resistance states of the variable resistance layer 206 may be about 2.5 MΩ and about 100 kΩ, respectively.
Similarly, to assure that the memory element 112 reliably switches from a low to a high resistance state the “reset” switching pulse 413 will generally be delivered at a current level that is greater than a minimum “reset” current IMRC, which is defined as the minimum current required to flow through the variable resistance layer 206 to cause it to switch from a low resistance state to a high resistance state. It should be noted that the minimum “set” current IMSC and minimum “reset” current IMRC are related to the physical and/or electrical properties of the material in the variable resistance layer 206, and thus may be adjusted by careful selection of the material(s) and/or physical properties (e.g., thickness) of the variable resistance layer 206 and by performing various post-processing steps on the formed layer. In one example, by controlling the number of defects in the formed variable resistance layer 206, such as by adjusting the stoichiometry of the material(s) found in the variable resistance layer 206 (e.g., HfO1.7 VS. HfO2) during the layer formation process, the minimum switching currents can be adjusted.
Referring to FIG. 4A, in one example, when a “set” switching pulse 411 is delivered through the standard switching memory device 200A the switching memory device will switch from the high-resistance-state (HRS) to the low-resistance-state (LRS), as shown by the arrow 421. One will note that the current flowing through the switching memory device will shift from the initial “set” current IA to the final “set” current IB during the “set” operation, due to the change in resistance (RVR) of the variable resistance layer 206. One will note that the initial “set” current IA will typically equal the minimum “set” current IMSC, which is discussed above.
Alternately, when a “reset” switching pulse 413 is delivered through the standard switching memory device 200A the switching memory device will switch from the low-resistance-state (LRS) to the high-resistance-state (HRS), as shown by the arrow 422. One will note that the current flowing through the switching memory device will shift from the initial “reset” current IC to the final “reset” current ID during the “reset” operation, due to the change in resistance (RVR) of the variable resistance layer 206.
Referring to FIG. 4B, in one example, a “set” switching pulse 411, such as switching pulse 401, is delivered through the standard switching memory device 200A to create a low resistance state or logic “one” state. In this case, a set voltage VSET is applied across electrodes 102 and 118, which creates a first “set” current I1 to flow through the standard switching memory device 200A, due to the impedance of the electrical components found in the memory element 200A. The first “set” current I1 is equal to the applied “set” voltage VSET divided by the sum of the impedances of the standard switching memory device 200A. Therefore, in one example, the first “set” current I1 may equal the following.

I1=VSET/(RTEL+RVR+RIEL+RCSD+RBEL).

Since the most damage to the elements contained in the standard switching memory device 200A will generally occur when the largest current is delivered through the switching memory device, the pulse 411 schematically illustrated in FIG. 4B focuses on the later stages of the “set” operation, and thus the first “set” current I1 achieved during the later stages of the “set” switching pulse 411 will generally equal the final “set” current IB. In general, the first “set” current I1 will vary during the time that the “set” pulse 411 is applied across the electrodes 102 and 118. The first “set” current I1 may have low current region 409 that is created due to the electrical properties of the material as it switches from the HRS to the LRS, and also have the final “set” current region as discussed above. Therefore, since the actual impedance of the electrodes is generally small, due to the need to reduce the power loss in the device, and the impedance of the variable resistance layer 206 is desirably low at the end of the “set” operation (e.g., I1=IB) to achieve a logic “one” state the impedance of the current steering device will dominate the circuit (e.g., RCSD<<RTEL+RIEL+RBEL+RVR) and the impedance of the circuit in this state is approximately equal to the impedance of the current steering device (i.e., RCSD). Therefore, the magnitude of the set current I1 created by the switching pulse 401 will equal the maximum current, or load current IL (FIG. 4B), which is approximately equal to the set voltage divided by the impedance of the current steering device (i.e., I1=IL= ̃VSET/RCSD). One will note that the difference between the “set” current I1 and the minimum IMSC current is much larger than necessary to cause the device to reliably switch to the logic “one” state. However, in practice it has been found that the high currents delivered through a standard type switching memory device 200A can permanently damage the memory element components and cause cross-talk to occur between adjacently connected memory devices. One will note that the magnitude of the “set” current is particularly important for bipolar switching applications that require the current steering element 216 to be reverse biased to “set” the resistance of the memory element into a low resistance state. In this case, the act of driving a high current through the current steering device 216, in a non-forward direction, can breakdown, generate heat within and ultimately damage the material layers used to form the current steering element 216 and memory element 112, which will reduce the current steering element's and/or memory element's effective lifetime. It has been found that, since the current steering device 216 provides the primary voltage drop in the standard switching memory device 200A during the “set” operation (e.g., switch to “on” state), the current steering device 216 often is required to operate near its breakdown voltage to reliably cause the variable resistance layer 206 to switch. The application of the current steering device 216 in this regime will cause its impedance to drop over time due to damage to the materials in the formed layer. Typically the resistance (RCSD) of an undamaged reverse biased diode type current steering device, for example, may be in a range between about 1 and about 100 mega-ohms (MΩ), while the resistance of a forward biased diode type current steering device may be between about 1 and about 20 kilo-ohms (kΩ).

Therefore, after performing the “set” operation by applying the “set” switching pulse 411, it is common to apply a “sensing” pulse 412 to assure that the logic “one” state has been achieved. The application of a sensing pulse 412, such as sensing pulse 404 in FIG. 4B, is generally performed by applying a VREAD voltage (e.g., +0.5V) across the electrodes 102, 118. If the “set” operation was performed correctly, the current through a standard switching memory device 200A during this sensing step will equal the ION current, which equals the VREAD voltage divided by the impedance of the circuit. For a standard switching memory device 200A that has a variable resistance layer 206 that is in a low resistance state, the ION current will approximately equal to the VREAD voltage divided by the impedance of the current steering device (e.g., ION= ̃VREAD/RCSD).
Next, in cases where it desirable to change the memory element 112 from a low resistance state (i.e., logic “one” state) to a high resistance state (i.e., logic “zero” state) a “reset” switching pulse 413, such as reset switching pulse 405, is delivered through the standard switching memory device 200A. One will note that the largest current that is delivered through the switching memory device during the “reset” operation will be achieved when the initial “reset” current IC flows through the device. The current flowing through the device during the “reset” operation will then tend to drop as the variable resistance layer 206 switches from a LRS to a HRS. Therefore, the pulse 413, which is schematically illustrated in FIG. 4B, may have a high current portion 419 at the start of the delivered pulse 413 and a stable region that equals the “reset” current I4 during the later stages of the “reset” operation. Therefore, the “reset” current I4 achieved during the “reset” switching pulse 413 will generally equal the final “reset” current ID and the maximum current achieved during the pulse 413 will equal the initial “reset” current Ic. It has been found that the magnitude of the current required to switch the memory element 112 to a high resistance state from a low resistance state is dependent on the magnitude of the current used to “set” the device in the low resistance state. If a high “set” current, such as current I1, is delivered to the memory element 112, then a higher “reset” current is required to achieve a desirable high resistance state. Stated another way, the difference between the initial “reset” current IC, and/or the final “reset” current ID, and the minimum “reset” current IMRC current needs to be larger than necessary to cause the device to switch from the “on” to the “off” state if the magnitude of the prior applied “set” current is too far from the minimum “set” current IMSC. The larger than necessary swings in the current used to switch between the “on” and “off” states can damage the materials and components in the switching memory device, thus affecting the memory element's lifetime and reliability.
Next, after delivering the “reset” switching pulse 413 it is common to apply a “sensing” pulse 412, such as sensing pulse 406 in FIG. 4B, to assure that the logic “zero” state has been achieved. The sensing pulse 412 is generally performed by applying a VREAD voltage (e.g., +0.5V) across the electrodes 102, 118. If a “reset” operation was performed correctly, the current through a standard switching memory device 200A during this sensing step will equal the IOFF current, which for the standard switching memory device 200A will equal to the VREAD voltage divided by the sum of the current steering device impedance resistance (RCSD) and the resistance of the variable layer (RVR). Therefore, in one example, the IOFF current for the standard memory device 200A will be as follows.

IOFF= ̃VREAD/(RCSD+RVR)

FIG. 5A is a schematic side cross-sectional view of one embodiment of an improved switching memory device 200B that contains a memory element 112 and a current steering device 216 that are disposed between electrodes 102 and 118 and are formed over a portion of a substrate 201. The switching memory device 200B contains a current limiting component, such as current limiting layer 204 and separation layer 205 that is configured to improve the device's switching performance and lifetime. In this configuration, the memory element 112 will generally contain a top electrode 102, a current limiting layer 204, a separation layer 205, a variable resistance layer 206 and an optional intermediate electrode 210, or additional conductive layer. In one embodiment, the current limiting layer 204 is disposed within the improved switching memory device 200B close to the variable resistance layer 206 and/or current steering device 216 to effectively limit or prevent the propagating programming current pulses (e.g., “set” or “reset” pulses) delivered through the switching memory device 200B from damaging the layers formed therein during normal device operation. At the same time, the separation layer 205 disposed between the current limiting layer 204 and the variable resistance layer 206 provides an effective barrier preventing the migration of mobile ions from migrating from the current liming layer 204 into the variable resistance layer 206 damaging the integrity of the vacancies and thus the switching properties of the variable resistance layer 206. Positioning the current limiting layer 204 near the variable resistance layer 206 and/or current steering device 216 can be important in switching memory devices 200B that utilize high speed transient programming pulses, such as square or trapezoidal shaped pulses that are less than about 1 ms in length. It is believed that the use of an externally positioned resistive element in a circuit in which the switching memory device 200B is formed, such as resistive layers or structures formed on other parts of the chip in which the switching memory device 200B is formed, will not effectively prevent the delivered high speed programming pulse energy from causing the materials in the variable resistance layer 206 and/or current steering device 216 from breaking down when the high speed transient programming pulses are delivered through the switching memory device 200B. It is believed that the propagation delay created by the transmission of the high speed programming pulse through the length of the electrical circuit formed between the external resistive element and the switching memory device 200B components (e.g., variable resistance layer 206 and current steering device 216) will generally prevent the externally positioned resistive element from effectively reducing or dropping the instantaneous amount of energy passing through the variable resistance layer 206 and current steering device 216 as the high speed programming pulse passes through the switching memory device 200B in the forward and/or reverse bias directions. In one embodiment, the current limiting layer 204 is disposed in close proximity to the variable resistance layer 206, such as substantially adjacent to the variable resistance layer 206 with a separation layer 205 between the current limiting layer 204 and the variable resistance layer 206. One will note that the position of the current limiting layer 204 in the switching memory devices 200B need not be limited to the position shown in FIG. 5A, and thus the configuration as shown is not intended to be limiting as to the scope of the invention described herein. In one embodiment, the current limiting layer 204 is disposed between the variable resistance layer 206 and the current steering device 216. In one embodiment, the current limiting layer 204 can be placed between any adjacently positioned layers in the formed switching memory device 200B, such between the intermediate electrode 210 and the variable resistance layer 206 or between the intermediate electrode 210 and the current steering layer 216. It should be understood that in each of these embodiments mentioned above, the separation layer 205 is disposed between the current limiting layer 204 and the variable resistance layer 206 to prevent cross-contamination between these respective layers
In one embodiment, as illustrated in FIG. 5A, the electrodes 102 and 118 may each comprise more than one layer of conductive material. In one configuration, the top electrode 102 may comprise a first conductive layer 102A and a second conductive layer 1028, and the bottom electrode 118 may comprise a first conductive layer 118A and a second conductive layer 1188. In this case, the first conductive layer 102A in the top electrode 102 and the first conductive layer 118A in the bottom electrode 118 can be used to interconnect multiple switching memory devices 200B in an array of formed devices, and thus may act as word-lines or bit-lines. The second conductive layer 1028 and the second conductive layer 1188 may each comprise a material that has desirable electrical properties (e.g., work function) so that these layers can help improve the electrical characteristics of the memory element 200B. The first conductive layer 102A and/or first conductive layer 118A may comprise, for example, tungsten (W), aluminum (Al) or copper (Cu), while the second conductive layer 102B and/or the second conductive layer 118B may comprise, for example, titanium (Ti), titanium nitride (TiN), or doped poly-silicon. One will note that the configuration shown in FIG. 5A and discussed herein is not intended to limiting as to the scope of the invention described herein, since, for example, the electrodes 102 and 118 may comprise a single conductive layer, and the position of the various layers, or number of layers, in the stack of layers used to form switching memory device may be altered without deviating from the basic scope of the invention described herein.
FIG. 5B is a schematic side cross-sectional view of another embodiment of an improved switching memory device 200B as previously shown in FIG. 5A. The switching memory device 200B includes optional variable resistance layer top electrode 203 that serves as a conductive layer between the variable resistance layer 206 and the separation layer 205. Variable resistance layer top electrode 203 may comprise any one of the materials as noted above with regards to the discussion of electrodes 102 and 118
FIG. 5C schematically illustrates an electrical circuit formed by the switching memory device 200B shown in FIG. 5A. As illustrated in FIG. 5C, the electrical circuit within the switching memory device 200B includes a top electrode impedance (i.e., resistance RTEL) created by the top electrode 102 layer(s), a current limiting layer impedance (i.e., resistance RCLL) created by the current limiting layer 204, a variable resistance layer impedance (i.e., resistance RVR) created by the variable resistance layer 206, an intermediate electrode impedance (i.e., resistance RIEL) created by the intermediate electrode 210 layer(s), a current steering device impedance (i.e., resistance RCSD) created by the current steering device 206 and a bottom electrode impedance (i.e., resistance RBEL) created by the bottom electrode 118 layer(s).
Referring back to FIG. 4B, in one example, a “set” switching pulse 411, or set pulse 403, is delivered through the switching memory device 200B to create a low resistance state, or logic “one” state. In this configuration, a “set” voltage VSET is applied across electrodes 102 and 118, which creates a set current I3 to flow through the switching memory device 200B, due to the impedance of the components in the switching memory device 200B. The set current I3 will equal the VSET voltage divided by the sum of the impedances in the switching memory device 200B. Therefore, in one example, the set current I3 will equal the following.

I3=VSET/(RTEL+RCLL+RSL+RVR+RIEL+RCSD+RBEL).

Therefore, since the impedance of the electrodes and the separation layer are generally small, due to the need to reduce the power loss in the device, and the impedance of the variable resistance layer is desirably low to achieve a logic “one” state, the impedance of the current steering device and the current limiting layer will dominate the circuit (e.g., (RCLL+RCSD*)<<RTEL+RIEL+RBEL+RSL+RVR) and the impedance of the circuit in this state is effectively equal to the sum of the impedances of the current steering device and the current limiting layer (i.e., RCLL+RCSD). Therefore, referring to FIG. 4B, the magnitude of the set current I3 created by a “set” pulse 403 will equal a current (I3), which can be adjusted by the selection of a desired fixed impedance value of the current limiting layer 204. One will note that due to the presence of the added impedance (RCLL) of the current limiting layer 204 in the switching memory device 200B, versus the standard switching memory device 200A (FIG. 3A), the actual impedance (RCSD*) of the current steering device 216 will generally be greater than the impedance of a current steering device 216 disposed in the standard current steering device 200A, since the added voltage drop of the current limiting layer 204 in the device circuit will prevent the current steering device 216 from being damaged by the application of the programming currents during normal operation. As noted above, since the current steering device 216 in a standard switching memory device 200A (FIG. 3A) is the primary voltage drop during the “set” operation (e.g., switch to “on” state), the current steering device 216 often is required to operate near its breakdown voltage to reliably cause the variable resistance layer 206 to switch, which will generally not be the case in the switching memory device 200B due to the added voltage drop provided by the current limiting layer 204. The addition of the current limiting layer 204 in the switching memory device 200B reduces the voltage applied across the current steering device 216, and thus prevents the impedance of the current steering device 216 from dropping due to the application of a voltage near the breakdown state of the material and/or degrading over time due to damage created by the repetitive application of the programming voltages.

Referring to FIG. 4B, in general, it is desirable to form the current limiting layer 204 so that its impedance (RCLL) limits the current through the memory element 112 to a value (e.g., current I2) that is just greater than the minimum “set” current IMSC, as illustrated by pulse 402, and still allow the “on” logic state to be reliably “set” by the applied VSET voltage. It is believed that by adding the current limiting layer 204 to a memory element 112 can also help reduce the apparent minimum IMSC current required to cause the variable resistance layer 206 to change to a low resistive state, since the addition of the current limiting layer impedance (RCLL) in the circuit will reduce the swing in current between the “set” and “reset” switching currents at the same fixed applied voltage, thus affecting the density and movement of the traps in the variable resistance layer 206. Not intending to be bound by theory, but it is believed that when a smaller “on” state switching current is applied to a device, that the formed filament(s), or aligned traps, in the variable resistance layer will be smaller in size than if a higher “on” current is applied, thus making the filament(s) easier to alter during the “reset” phase of the resistive switching process.
In some embodiments, it is desirable to form the current limiting layer 204 from a material that will not significantly vary in resistance when the “set” and “reset” switching currents are applied to the switching memory device 200B. Forming the current limiting layer 204 from a material that has a generally constant resistance will assure that the switching characteristics of the device will not change over the life of the switching memory device 200B, due to changes in the material in the formed layer. Also, forming the current limiting layer 204 from a material that does not vary in resistance during the programming steps, due to the use of a non-resistive switching material, has many advantages that include: 1.) less variability in the electrical properties of the formed layer due to variations in the deposition process (e.g., defect density variations) 2.) less variability in the electrical properties of the formed layer over the lifetime of the formed memory device due to any physical or chemical change in the current limiting layer material and 3.) a lower device performance variability that is created by differences in the “forming” process (i.e., application of the forming voltage (VFORM)). It is desirable to form the current limiting layer 204 so that its material and electrical properties will not degrade or breakdown during the “forming” process, and also during normal repetitive operation of the switching memory device 200B.
To maintain the integrity of the current limiting layer 204 and switching memory device 200B through the “electrical forming” process and over the expected life of the device, a separation layer 205 may be disposed between an oxygen containing current limiting layer 204 and the variable resistance layer 206 as shown in FIG. 5. The separation layer 205 acts as an oxygen barrier layer so the oxygen ions from the current limiting layer 204 do not contaminate the variable resistance layer 206 and degrade its performance. The protection of the variable resistance layer 206 is crucial as the resistive switching capabilities of the variable resistance layer 206 are enabled by only a small number of oxygen vacancies within the variable resistance layer 206. The migration of even a small amount of oxygen ions has a negative impact on the switching characteristics of the variable resistance layer 206, and thus will limit the lifetime of the device. Because the addition of another layer will add additional resistance to the switching memory device 200B, it is desirable to form the separation layer 205 from materials that are resistant to oxygen diffusion, but that will not add a significant burden of resistance to the switching memory device 200B, and that are also compatible with the switching memory device manufacturing process.
Device Structure and Formation Processes
In one embodiment, as discussed above, a memory array 100 (FIG. 1) comprises a plurality of switching memory devices 200B that are each interconnected by the electrodes 102 and 108. As illustrated in FIG. 5A, a switching memory device 200B may comprise a top electrode 102, a current limiting layer 204, a separation layer 205, a variable resistance layer 206, an intermediate electrode 210, a current steering device 216 and an electrode 118. In one configuration, as noted above, the current steering device 216 comprises a p-n junction diode, p-i-n diode, transistor, or other similar device that is disposed between electrode 102 and memory element 112, or between the electrode 118 and memory element 112. In one example, the current steering device 216 may include two or more layers of a semiconductor material, such as two or more doped silicon layers, that are configured to direct the flow of current through the device. In one example, the current steering device is a diode that comprises a p-doped silicon layer (not shown), an un-doped intrinsic layer (not shown), and an n-doped silicon layer (not shown) that has an overall resistance between about 1 kΩ and about 100 MΩ. The overall resistance will generally depend on the type of current steering device that is formed and in what direction current is flowing through the device (e.g., forward or reversed biased).
The electrodes 102, 210 and 118 disposed in the switching memory device 200B are generally formed from a conductive material that has a desirable conductivity and work function. In some configurations, the electrode 102, 210 and/or 118 disposed in the switching memory device 200B are each formed from different materials, which may include, but are not limited to p-type polysilicon, n-type polysilicon, transition metals, transition metal alloys, transition metal nitrides, and transition metal carbides. In one example, the electrode 102 and the electrode 118 comprise a metal, metal alloy, metal nitride or metal carbide formed from an element selected from a group consisting of titanium (Ti), tungsten (W), tantalum (Ta), cobalt (Co), molybdenum (Mo), nickel (Ni), vanadium (V), hafnium (Hf) aluminum (Al), copper (Cu), platinum (Pt), palladium (Pd), iridium (Ir), ruthenium (Ru), and combination thereof. In one example, the electrodes 102 and 118 comprise a metal alloy selected from the group of a titanium/aluminum alloy, or a silicon-doped aluminum (AlSi). In one embodiment of the switching memory devices 200B, the electrodes 102 and 118 comprise a metal, such as a transition metal, transition metal alloy, transition metal carbide, transition metal nitride (e.g., TiN), non-mobile metal such as gold (Au) or platinum (Pt), and the intermediate electrode 210 comprises a heavily doped semiconductor material, such as a heavily doped silicon material (e.g., n-type polysilicon material) that interfaces well with the current steering device 216. In one example, the intermediate electrode 210 comprises polysilicon and is between about 50 Å and about 500 Å thick, and the electrodes 102 and 118 are between about 50 Å and 5000 Å thick and comprise a metal, such as titanium nitride (TiN).
The variable resistance layer 206 disposed in a switching memory device 200B can be a dielectric material, such as a metal oxide material or other similar material that can be switched between at least two or more stable resistive states. In some embodiments, the variable resistance layer 206 is a high bandgap material (e.g., bandgap>4 electron volts (eVs)), such as hafnium oxide (HfxOy), tantalum oxide
(TaxOy), aluminum oxide (AlOy), lanthanum oxide (LaxOy), yttrium oxide (YxOy), dysprosium oxide (DyxOy), ytterbium oxide (YbxOy) and zirconium oxide (ZrxOy). It has been found that using high band gap variable resistance layer materials will improve data retention in the memory element 112, and reduce the leakage current in the formed memory element device, since the amount of trapped charge in the variable resistance layer material will be less than a lower band gap material, and the high band gap materials create a large barrier height that the carriers have to cross during the read, set and reset operations. In other embodiments, lower bandgap metal oxide materials can be used, such as titanium oxide (TiOx), nickel oxide (NiO)x or cerium oxide (CeOx) may be advantageous for some embodiments. In some cases, a semiconductive metal oxide (p-type or n-type) such as zinc oxides (ZnxOy), copper oxides (CuxOy), and their nonstoichiometric and doped variants can be used. The variable resistance layer 206 may comprise a metal oxide (e.g., HfO2) layer formed to a thickness of between about 10 Å and about 100 Å. In one configuration, the variable resistance layer 206 is doped with a material that has an affinity for oxygen (e.g., transition metals (Al, Ti, Zr)) to form a metal-rich variable resistance layer (e.g., HfO1.7 vs. HfO2), which is deficient in oxygen, and thus has a larger number of oxygen vacancy type defects. The additional vacancy defects can reduce the required switching and forming voltages, reduce the device operating current(s), and reduce the device to device variation in a formed memory element. In one example, the variable resistance layer 206 may comprise a metal oxide layer, such as HfxOy, TaxOy, AlxOy, LaxOy, YxOy, DyxOy, YbxOy and/or ZrxOy, formed to a thickness of between about 20 Å and about 100 Å, such as between about 30 Å and about 50 Å. The variable resistance layer 206 can be deposited using any desired technique, but in some embodiments described herein is deposited using an ALD process. In other embodiments, the variable resistance layer 206 can be deposited using a CVD (e.g., LPCVD, PECVD) or ALD (e.g., PEALD), physical vapor deposition (PVD), liquid deposition processes, and epitaxy processes. It is believed that PEALD processes can be used to control defects and improve switching and forming voltages in some embodiments. In one example, an ALD process using tetrakis(dimethylamino)hafnium (TDMAH) and an oxygen containing precursor at a temperature of about 250° C. is used to form an 50 Å thick hafnium oxide (HfxOy) containing variable resistance layer 206.
In one embodiment, the current limiting layer 204 comprises a tunnel oxide that can be reliably and consistently formed within the switching memory devices 200B. In one configuration of the memory element 112, the formed tunnel oxide type current limiting layer 204 creates a barrier that is used to adjust the ease with which current will flow through the formed device when a voltage is applied across the electrodes. The added barrier to current flow will tend to reduce the magnitude of the IoN and IOFF current that flow through the device during operation, due to the increased energy required to move the electrons over and/or tunnel through the tunnel oxide layer. A tunnel oxide layer comprises a dielectric layer that allows minimal current to flow through the layer at voltages below the tunneling voltage threshold. The tunneling phenomenon generally occurs when a high enough electric field is created across the thickness of the tunnel oxide layer, due to the application of a biasing voltage. The electron tunneling through the formed tunnel oxide layer may occur by hot electron (HE) injection type tunneling, Fowler-Nordheim tunneling or other similar tunneling phenomena. One will note that the increase in barrier height will generally not affect the current ratio (ION/IOFF), and thus not affect one's ability to detect the different logic states in the switching memory device.
To achieve a current limiting layer that has desirable electrical and/or physical properties, one or more steps in a current limiting layer 204 deposition process can be adjusted to form a tunnel oxide layer that has desirable properties. As discussed above, in some cases it is desirable to adjust the resistance of the current limiting layer 204, so that it matches the resistance of the formed current steering device 216 in the formed switching memory device 200B. One skilled in the art will appreciate that the resistance (R) to current flow through the tunnel oxide layer can be adjusted by varying the thickness, material composition, or crystalline structure. Since the cross-sectional area (A) of the device is generally fixed by the size and lateral spacing of the switching memory devices 200B, and thus is generally not easily varied from one switching memory device to the next, the resistance R of the current limiting layer 204 can be controlled by the adjustment of the thickness “t” (FIGS. 5A, 5B). Typical deposition processes may include ALD, PVD and CVD processes that can be tailored to adjust the electrical properties and thickness of the deposited current limiting layer 204. In one example, for a memory element 112 that is about 150 nm×150 nm is size, a current limiting layer 204 that is about 23 Å thick, and has an equivalent resistivity of 100 Ω-cm will achieve a resistance of about 1 MΩ, so as to match the resistance found in a current steering device that has a resistance of about 1 MΩ. It should be noted that tunnel oxides are generally not resistive in a traditional sense, since there is generally no electron transfer through the material in the tunnel oxide layer when current is flowing through the tunnel oxide. In another example, the current limiting layer 204 is formed so that its impedance (RCLL) is between about 10 kΩ and about 10 MΩ, such as between about 100 kΩ and about 1 MΩ.
Materials used to form the tunnel oxide type current limiting layer 204 include various dielectric materials that generally include various metal oxides. In one configuration, the current limiting layer 204 includes a tunnel oxide that has a breakdown voltage that exceeds the breakdown voltage of the variable resistance layer 206. Current limiting layer 204 materials that have a breakdown voltage that is less than the variable resistance layer 206 material's breakdown voltage will become damaged during the application of the forming voltage (VFORM), which is discussed above. Therefore, in one embodiment of the invention, the material in a formed current limiting layer 204 disposed in a formed memory element 112 has a breakdown voltage that is greater than the breakdown voltage of the material found in the variable resistance layer 206. In one example, the current limiting layer 204 comprises a aluminum oxide (Al2O3) layer that is formed using a PVD, CVD or ALD type process. Current limiting layers comprising aluminum oxide can be advantageous due to their high breakdown voltage. In yet another example, the current limiting layer 204 comprises a tunnel oxide, that comprises a silicon oxide (SiO2), tantalum oxide (Ta2O5), magnesium oxide (MgO) or zirconium oxide (ZrO2) layer that is formed using an ALD, CVD or PVD process. In some configurations, the composition of the metal containing layer can be adjusted to change the resistivity of the formed layer. In one configuration, the oxygen level in a deposited layer, such as a silicon oxide, a tantalum oxide or a zirconium oxide containing layer, is adjusted to control the current flow through the formed layer. In some deposition processes, the resistivity of the formed oxygen containing layer is controlled by adjusting the partial pressure of oxygen in the processing region of the deposition chamber during a CVD, PVD, or ALD deposition process.
It is generally desirable to form the current limiting layer 204 so that its material and electrical properties will not degrade or breakdown during the often high current “electrical forming”, “burn-in” type device preparation steps, and also during normal repetitive operation of the nonvolatile resistive switching memory device.
The use of tunnel oxides as described above, when used in series with the variable resistance layer 206 acts as a current limiting element that will reduce leakage. However, when the tunnel oxide is formed so that it is in direct contact with the variable resistance layer 206, the process of forming the tunnel oxide can contaminate the variable resistance layer 206, and thus degrade the switching performance and longevity of the device. To prevent the formed tunnel oxide layer or other subsequent processing and operating currents from damaging the variable resistance layer 206's interface or properties, a separation layer 205 can be used as a contamination barrier layer to prevent the diffusion of the components used to form the tunnel oxide layer from migrating into the variable resistance layer 206.
An effective separation layer 205 with oxygen blocking characteristics must include conductive materials that enable the current flow through the switching memory device 200B yet prevent the flow of oxygen ions from the current limiter layer 204 from migrating to and damaging the switching characteristics of the variable resistance layer 206. The benefit of the addition of the separation layer 205 to the switching memory device 200B includes the increased longevity of the device by inhibiting the movement of the oxygen ions between the two oxygen containing layers, such as the current limiter layer 204 and the variable resistance layer 205. However, consideration must also be given to the resistance of separation layer 205 and the impact of the additional resistance to the performance of the switching memory device 200B. Additional layers within the switching memory device 200B will increase the resistance of the series connected elements and cause the current to decrease to undesirable levels. Although the separation layer 205 provides the benefit of being a barrier to oxygen diffusion, it must not be so thick as to cause an undue burden to the resistance of the device. Therefore, the materials selected for the separation layer must be considered for both their mobile ions blocking properties as well as their contribution to the resistance of the switching memory device. Materials that act as an effective oxygen contamination barrier comprise at least one layer of a conductor material resistant to oxygen diffusion such as noble metals or nitride rich materials including titanium nitride (TiN), tungsten nitride (WN) and tantalum nitride (TaN), aluminum nitride (AlN), hafnium nitride (HfN), zirconium nitride (ZrN), boron nitride (BN), calcium nitride (CaN), ruthenium titanium nitride (RuTiN), silicon oxy nitride (SiON), aluminum oxy nitride (AlON), titanium aluminum oxy nitride (TiAlON) and other metals or metal alloys such as iridium (Ir), Iridium oxide (IrO2), gold (Au), platinum (Pt), platinum beryllium (PtBe) and zirconium (Zr). The resistance of the separation layer is dependent on both the resistivity of the selected material and its thickness. The optimal thickness of the separation layer 205 can be determined by both the material's oxygen diffusivity as a lower limit, and its electrical conductivity as an upper limit for each particular selected material.
One method for determining the thickness of separation layer 205 is based on the mobile ion diffusivity in the separation layer material. In one example, the lower limit of the film thickness must be greater than the mean free path of oxygen in the separation layer 205 over the life of the device (e.g., ten years). This can be determined by calculating the diffusion length, or mean free, path of the oxygen ions using the following equation:

I(mean free path)=(D*t)1/2 

Here, D is the diffusivity of oxygen in the material of interest, and t is the time scale of operation (10 years or 3×108 s). A separation layer with a thickness greater than the diffusion length of oxygen in the material over the expected life of the switching memory device 200B will insure that the oxygen ions in the current limiting layer 204 do not migrate through the separation layer 205 into the variable resistance layer 206 and degrade or inhibit the performance of switching characteristics of the variable resistance layer within the lifetime or useable life of the device. In another embodiment, the minimum thickness of the separation layer 205 is selected to be equal to or greater than the thickness of the resistive switching layer (e.g., 100 Å). The device specification will generally dictate the thickness of the resistive switching layer 206 that is sufficient to prevent the diffusion of oxygen to maintain the requisite number of oxygen vacancies in the variable resistance layer over the useable life of the memory element. In one example, separation layer 205 is sized so that the oxygen concentration change in the variable resistance layer 206 is less than approximately 0.1% over the expected useable life (10 years) of the device that is maintained at a temperature of 27° C. In another example, separation layer 205 is sized so that the oxygen concentration change in the variable resistance layer 206 is less than approximately 0.1% over the expected useable life of one million cycles of the device that is maintained at a temperature of 27° C.
The maximum thickness of the separation layer 205 must also be considered so as to prevent the separation layer from introducing significant resistance into the memory device, and thus inhibiting the performance of the switching memory device 200B. It has been found that the separation layer 205 should contribute no more than 20% of the total resistance of the switching memory device 200B using the following equation:

RSL≧(RTEL+RCLL+RSL+RVR+RIEL+RCSD+RBE)/20

One will note that the maximum thickness of the separation layer will be determined by the resistivity of the material used to form the separation layer 205. in some embodiments the separation layer should contribute no more than 10% of the total resistance of the top electrode layer, the current limiter layer, the separation layer, the variable resistance layer, the intermediate electrode layer, the current steering device and the bottom electrode. Other considerations for the material selection include the ease of manufacturing and the integration into the device structure. Generally, a thickness of between 10 Å and 100 Å of a nitride rich material is appropriate to obtain oxygen-blocking benefits while retaining compatibility to manufacturing processes and techniques. In one example, oxygen deficient materials suitable as an oxygen blocker for separation layer 205 are amorphous or polycrystalline and with a grain size of 30 nm or smaller and include tungsten nitride (WN), titanium nitride (TiN), ruthenium titanium nitride (Rux(TiN)y) (x=0.67, y=0.33), tantalum nitride (TaN), Iridium (Ir), tungsten nitride, (TN), Iridium oxide (IrOx) (0≦x≦2), titanium aluminum oxy nitride (TiWAlXOYNZ) (0.4≦w≦0.6) (0.3≦x≦0.5) (0<y≦0.2) (0.8≦z≦1).

Switching Memory Device Fabrication Processes
FIG. 6 illustrates a process sequence 700 that can be used to form a memory element 112 components in a switching memory device 200B illustrated in FIGS. 5A & 5B, according to embodiments of the invention. While omitted from the discussion below, the electrode 118 and current steering device 216 elements can be formed over a portion of a substrate 201, such as a silicon substrate, by use of a physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), or other similar process that is well known in the art. In some configurations, it is desirable to determine the empirical or theoretical resistance of the current steering device 216 structure in the memory element 112, so that the resistance of the current limiting layer 204 can be adjusted relative to the expected resistance of the formed current steering device 216. In one example, the current steering device 216 is a diode that comprises a p-doped silicon layer (not shown) that is formed by a CVD process, an un-doped intrinsic layer (not shown) that is formed by an CVD process, and an n-doped silicon layer (not shown) that is formed by a CVD process. In one example, the electrode 118 comprises a layer of titanium nitride (TiN) that is between about 500 Å and 1 μm thick and is formed by use of a PVD process.
Referring to FIGS. 5A and 6, at step 701 an intermediate electrode 210 is formed over a substrate 201. In one embodiment, the intermediate electrode 210 is a highly doped polysilicon layer that is formed using a conventional CVD or ALD type polysilicon deposition technique. In some cases, an optional native oxide layer removal step may be performed after forming the intermediate electrode layer 210 by use of a wet chemical processing technique, or conventional dry clean process that is performed in a plasma processing chamber. In one example, the intermediate electrode 210 comprises polysilicon that is between about 50 Å and about 5000 Å thick, which is formed by use of a CVD or ALD polysilicon deposition process.
Referring to FIGS. 5A and 6, at step 702, the variable resistance layer 206 is deposited over the intermediate electrode 210 using a PVD, CVD or ALD deposition process. The variable resistance layer 206 may comprise a metal oxide layer, such as HfxOy, TaxOy, AlxOy, LaxOy, YxOy, DyxOy, YbxOy and/or ZrxOy, formed to a thickness of between about 20 Å and about 100 Å, such as between about 30 Å and about 50 Å. The variable resistance layer 206 can be deposited using any desired technique, but in some embodiments described herein is deposited using an ALD process. In one example, an ALD process using tetrakis(dimethylamino)hafnium (TDMAH) and an oxygen containing precursor (e.g., water vapor) at a temperature of about 250° C. is used to form a 30 Å thick hafnium oxide (HfxOy) which acts as the variable resistance layer 206.
At step 703, as depicted in FIGS. 5A and 6, a separation layer 205 is formed over the variable resistance layer 206 as shown in FIG. 5A. In one example, the separation layer 205 includes a nitride rich material such as titanium nitride (TiN) or tantalum nitride (TaN). The separation layer 205 may be deposited using a deposition process, such as PVD, CVD, ALD or other similar process. In one example, the separation layer 205 is between about 10 Å and 100 Å thick. In one example, a PVD process is used to form a separation layer 205 that comprises titanium nitride (TiN) that is between about 10 Å and 100 Å thick.
At step 704, as depicted in FIGS. 5A and 6, a current limiting layer 204 is formed over the separation layer 208 using a deposition process, such as a PVD, CVD, ALD or other similar process. In one embodiment, the current limiting layer 204 is a tunnel oxide layer (e.g., Al2O3, ZrO2, SiO2,), or semiconductor layer (e.g., doped Si, SixNy) that is formed by use of a PVD, CVD or ALD process. In one example, the current limiting layer 204 may be formed to a thickness between about 20 Å and about 30 Å, and comprise a material such as Al, Ta, Mg, Si, AlXOY, TaXOY, MgXOY and SiXOY. In one example, the current limiting layer 204 layer is formed using a PVD process that deposits an AlxOy layer at a deposition rate of between about 0.1 to 1 Å/minute using a pure aluminum target and maintaining the processing environment during the PVD deposition process to between about 10 and about 50% oxygen (O2) and the balance being argon (Ar) gas. It has been found that maintaining the oxygen concentration in a PVD processing environment to a concentration of greater than about 10% will form a dielectric layer. Therefore, one can adjust the layer thickness and resistivity to form an aluminum oxide layer containing current limiting layer 204 that has a desirable resistance. In one process example, the oxygen concentration in the processing environment during deposition is controlled to form an aluminum oxide (Al2O3) layer that has a desirable thin-film resistivity within a range of 100-10,000 Ohm-cm.
At step 705, the electrode 102 is formed over the current limiting layer 204 as shown in FIG. 5A using one or more of the materials that are discussed above. The electrode 102 layer may be deposited using a deposition process, such as a PVD, CVD, ALD or other similar process. In one example, the electrode layer 102 is between about 100 Å and 1000 Å thick. In one example, a PVD process is used to form an electrode 102 layer that comprises titanium nitride (TiN) and is between about 100 Å and 1000 Å thick.
At step 706, the formed switching memory device 200B is optionally annealed at a temperature of greater than about 450° C. In one example, the formed switching memory device 200B is annealed at a temperature of greater than about 700° C. In another example, the formed switching memory device 200B is annealed at a temperature of between about 450° C. and about 1000° C. for a period of time between about 30 seconds and about 20 minutes. The process(es) performed at step 708, are generally configured to cause the layers disposed in the switching memory device 200B to form a desirable interface between adjacent layers as well as activate and/or desirably process the other layers formed in the switching memory device.
Process and Device Examples
In one example of a process of forming a switching memory device, after performing the steps 701-706 in the processing sequence 700, a memory element 112 is formed that comprises: an intermediate electrode 210 comprising an n-doped polysilicon layer, a variable resistance layer 206 that is about 50 Å thick and comprises hafnium oxide (HfOx), a separation layer 205 that is between 10 Å and 100 Å thick and comprise tantalum nitride (TaN), a current limiting layer 204 that is between about 10 Å and 50 Å thick and comprises aluminum oxide (Al2O3), and an electrode 102 that comprises a layer of titanium nitride (TiN). After forming the switching memory device 200B (FIG. 5A), then at least one thermal processing step is performed, such as step 708, to form switching memory device 200B.
In another example of a process of forming a switching memory device, after performing the steps 701-708, a memory element 112 is formed that comprises: an intermediate electrode 210 comprising an n-doped polysilicon layer, a variable resistance layer 206 that is about 50 Å thick and comprises hafnium oxide (HfOx), a separation layer 205 that is between 10 Å and 100 Å thick and comprise tantalum nitride (TaN), a current limiting layer 204 that is between about 10 Å and 50 Å thick and comprises aluminum oxide (Al2O3), and an electrode 102 that comprises a layer of titanium nitride (TiN). After forming the switching memory device 200B, then at least one thermal processing step is performed, such as step 710, to form switching memory device 200B.
The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention as defined by the claims that follow.

## Claims

1. A nonvolatile memory element comprising:
a first layer operable as a first electrode;
a second layer operable as a second electrode;
a third layer disposed between the first layer and the second layer,
wherein the third layer is operable as a variable resistance layer and comprises a non-stoichiometric metal oxide;

a fourth layer operable as a current limiting layer,
wherein the fourth layer comprises second metal oxide,
wherein the fourth layer is disposed between the first layer and the third layer; and

a fifth layer disposed between the third layer the fourth layer,
wherein the fifth layer comprises a metal oxy-nitride and blocks oxygen migration between the third layer and the fourth layer,
wherein the fifth layer is amorphous or polycrystalline and has a grain size of 30 nanometers of less.

2. The nonvolatile memory element of claim 1, wherein the thickness of the fifth layer is greater than or equal to a thickness of the third layer.

3. The nonvolatile memory element of claim 1, wherein the first metal oxide of the third layer is selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide, and aluminum oxide.

4. The nonvolatile memory element of claim 1, wherein the third layer has a thickness of between 20 angstroms and 100 angstroms.

5. The nonvolatile memory element of claim 1, wherein the second metal oxide of the fourth layer is selected from the group consisting of aluminum oxide, zirconium oxide, silicon oxide, and tantalum oxide, and magnesium oxide.

6. The nonvolatile memory element of claim 1, wherein the metal oxy-nitride of the fifth layer comprises a material selected from the group consisting of aluminum oxy-nitride, or titanium aluminum oxy-nitride.

7. The nonvolatile memory element of claim 1, wherein the third layer further comprises a dopant.

8. The nonvolatile memory element of claim 1, wherein the first metal oxide of the third layer comprises oxygen vacancies.

9. The nonvolatile memory element of claim 1, wherein a breakdown voltage of the fourth layer is higher than a breakdown voltage of the third layer.

10. The nonvolatile memory element of claim 1, wherein the first metal oxide of the third layer is hafnium oxide, wherein the second metal oxide of the fourth layer is aluminum oxide, and wherein the fifth layer comprises titanium aluminum oxy-nitride.

11. The nonvolatile memory element of claim 1, wherein the fifth layer is amorphous.

12. The nonvolatile memory element of claim 1, wherein the fifth layer is polycrystalline and has a grain size of 30 nanometers or less.

13. A nonvolatile memory element, comprising:
a first layer operable as a first electrode;
a second layer operable as a second electrode;
a third layer disposed between the first layer and the second layer,
wherein the third layer is operable as a variable resistance layer and comprises a non-stoichiometric metal oxide;

a fourth layer disposed between the first layer and the third layer,
wherein the fourth layer has a single resistive state,

wherein the fourth layer comprises a second metal oxide; and
a fifth layer disposed between the third layer the fourth layer,
wherein the fifth layer comprises a metal oxy-nitride and blocks oxygen migration, and
wherein the fifth layer has an electrical resistance that is less than one fifth of a combined electrical resistance of the first layer, the second layer, the third layer, the fourth layer and the fifth layer.

14. The nonvolatile memory element of claim 13, wherein the electrical resistance of the fifth layer is no greater than one tenth of the combined electrical resistance of the first layer, the second layer, the third layer, the fourth layer and the fifth layer.

15. The nonvolatile memory element of claim 13, wherein the metal oxide of the third layer is selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide and aluminum oxide.

16. The nonvolatile memory element of claim 13, wherein the third layer has a thickness of between 20 angstroms and 100 angstroms.

17. The nonvolatile memory element of claim 13, wherein the second oxide comprises a material selected from the group consisting of group consisting of aluminum oxide, zirconium oxide, silicon oxide, and tantalum oxide, and magnesium oxide.

18. The nonvolatile memory element of claim 13, wherein the fourth layer has a thickness of between 20 angstroms and 500 angstroms.

19. The nonvolatile memory element of claim 13, wherein the metal oxy-nitride of the fifth layer comprises a material selected from the group consisting of aluminum oxy-nitride, and titanium aluminum oxy-nitride.

20. The nonvolatile memory element of claim 9, wherein the fifth layer has a thickness of between 10 angstroms and 100 angstroms.