# Patent

## Title
Method of programming, erasing and repairing a memory device

## Abstract

A method of programming and erasing a memory device is provided. The memory device includes first and second electrodes and a switching layer therebetween. A first on-state resistance characteristic of the memory device is provided in programming the memory device by application of a first voltage to the gate of a transistor in series with the memory device. Other on-state resistance characteristics of the memory device, different from the first on-state resistance characteristic, may be provided by application of other voltages, different from the first voltage, to the gate of the transistor.

## Background
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to memory devices, and more particularly, to a method of programming, erasing and repairing the state of a metal-insulator-metal (MIM) memory device.
2. Background Art
FIG. 1 illustrates a memory device 30 known as a metal-insulator-metal (MIM) device which includes an electrode 32, an insulating layer 34 (also known as a switching layer) on and in contact with the electrode 32, and an electrode 36 on and in contact with the insulating layer 32, so the insulating layer 34 is between the electrodes 32, 36 (it will be understood that while the electrodes are indicated as metal in the metal-insulator-metal nomenclature, the electrodes can be of any suitable conductive material). The electrode 32 is connected to the drain of an MOS transistor 38, while the source of the transistor 38 is connected to ground, so that the memory device 30 and transistor 38 are in series.
Initially, assuming that the memory device 30 is unprogrammed, in order to program the memory device 30, a programming voltage Vpg is applied to the electrode 36, so that an electrical potential is applied across the memory device 30 from a higher to a lower potential in the direction from electrode 36 to electrode 32, (see FIG. 2, a plot of memory device current vs. voltage applied to the electrode 36 of the memory device 30). This voltage Vpg is sufficient to cause charge carriers to be moved into the insulating layer 34, causing the insulating layer 34 (and the overall memory device 30) to rapidly switch to a low-resistance or conductive state (A). Upon removal of such potential, the charge carriers moved into the insulating layer 34 during the programming step remain therein, so that the insulating layer 34 (and memory device 30) remain in a conductive or low-resistance state, as indicated by the on-state resistance characteristic (B). The voltage Vga applied to the gate of the transistor 38 determines the magnitude of current through the memory device 30 during the programming step.
In order to erase the memory device 30, a positive voltage Ver is applied to the electrode 36, so that an electrical potential is applied across the memory device 30 from a higher to a lower electrical potential in the same direction as in programming the device 30. This potential Ver is sufficient to cause charge carriers to move from the insulating layer 34, in turn causing the insulating layer 34 (and the overall memory device 30) to be in a high-resistance or substantially non-conductive state. This state remains upon removal of such potential from the memory device 30. The gate voltage Vgb again determines the magnitude of current through the memory device 30. As illustrated, the erase voltage Ver is lower than the programming voltage Vpg, and the current provided through the memory device 30 during the erase step (C) is higher than the current through the device 30 during the programming step (based on a higher gate voltage during the erase step than during the programming step).
FIG. 2 also illustrates the read step of the memory device 30 in its programmed (conductive) state and in its erased (nonconductive) state. A voltage Vr is applied to the electrode 36 so that an electrical potential across the memory device 30 from a higher to a lower electrical potential in the same direction as in the programming and erase steps. This voltage Vr is lower than the voltage Vpg applied for programming and is lower than the voltage Ver applied for erasing (see above). In this situation, if the memory device 30 is programmed, the memory device 30 will readily conduct current (level L1), indicating that the memory device 30 is in its programmed state. If the memory device 30 is erased, the memory device 30 will not conduct current (level L2), indicating that the memory device 30 is in its erased state.
As will be seen, the memory device 30 as thus far shown and described is capable of adopting two states, i.e., a first, conductive state, or “on” state, and a second, substantially non-conductive, or “off” state. The memory device 30 thus can include information as to the state of a single bit, i.e., either 0 or 1. However, it would be highly desirable to be able to provide a memory device which is capable of adopting any of a plurality of states, so that, for example, in the case where four different states of the memory device can be adopted, two bits of information can be provided as chosen (for example first state equals 00, second state equals 01, third state equals 10, fourth state equals 11).
Therefore, what is needed is an approach wherein a memory device may adopt each of a plurality of states, each relating to the information held thereby.
DISCLOSURE OF THE INVENTION
Broadly stated, the present method is a method of undertaking a procedure on a memory device, the memory device comprising first and second electrodes and a switching layer between the first and second electrodes, the method comprising providing current through the memory device, the on-state resistance characteristic of the programmed memory device being dependent on the magnitude of current through the memory device.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.



BRIEF DESCRIPTION OF THE DRAWINGS
The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as said preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
FIG. 1 is a cross-sectional view of the memory device as described above;
FIG. 2 is a plot of current vs. voltage in the programming, reading and erasing of the memory device of FIG. 1;
FIG. 3 is a cross-sectional view of the present memory device, illustrating programming thereof under a first set of programming conditions;
FIG. 4 is a cross-sectional view of the present memory device, illustrating erasing thereof under a first set of erase conditions;
FIG. 5 is a plot of current vs. voltage illustrating the approaches of FIGS. 3 and 4;
FIG. 6 is a cross-sectional view of the present memory device, illustrating programming thereof under a second set of programming conditions;
FIG. 7 is a cross-sectional view of the present memory device, illustrating erasing thereof under a second set of erase conditions;
FIG. 8 is a plot of current vs. voltage illustrating the approaches of FIGS. 6 and 7;
FIG. 9 is a cross-sectional view of the present memory device, illustrating programming thereof under a third set of programming conditions;
FIG. 10 is a cross-sectional view of the present memory device, illustrating erasing thereof under a third set of erase conditions;
FIG. 11 is a plot of current vs. voltage illustrating the approaches of FIGS. 9 and 10;
FIG. 12 is a cross-sectional view of the present memory device, illustrating programming thereof under a fourth set of programming conditions;
FIG. 13 is a cross-sectional view of the present memory device, illustrating erasing thereof under a fourth set of erase conditions;
FIG. 14 is a plot of current vs. voltage illustrating the approaches of FIGS. 12 and 13;
FIG. 15 is a plot of current vs. voltage illustrating the various on-resistance characteristics of the device in its various states, and the reading thereof;
FIGS. 16 and 17 illustrate a repair process for a failed erase of the memory device; and
FIGS. 18-20 are systems incorporating memory devices of the present type.



BEST MODE(S) FOR CARRYING OUT THE INVENTION
Reference is now made in detail to a specific embodiment of the present invention which illustrates the best mode presently contemplated by the inventors for practicing the invention.
FIG. 3 illustrates a memory device of the type generally described above. The particular embodiment of memory device 130, i.e., a metal-insulator-metal (MIM) device 130, includes a copper electrode 132, a CuOx insulating layer 34 (also known as a switching layer) on and in contact with the electrode 132, and a Ni electrode 136 on and in contact with the insulating layer 132, so the insulating layer 134 is between the electrodes 132, 136 (it will be understood that while the electrodes are indicated as metal in the metal-insulator-metal nomenclature, the electrodes can be of any suitable conductive material). The electrode 132 is connected to the drain of an MOS transistor 138, while the source of the transistor 138 is connected to ground, so that the memory device 130 and transistor 138 are in series.
Assuming that the memory device 130 is unprogrammed, in order to program the memory device 130 (see FIGS. 3 and 5, a plot of memory device current vs. voltage applied to the electrode 136 of the memory device 130), a programming voltage Vpg1 (in this example 2.0 volts) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower potential in the direction from electrode 136 to electrode 132. Meanwhile, a selected voltage Vg1, in this example 1.0 volts, is applied to the gate of the transistor 138. This procedure (graphically illustrated in FIG. 5) causes charge carriers in the form of electrons and/or holes to be moved into the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130) to rapidly switch (D) to a low-resistance or conductive state (E). Upon removal of such potential, the charge carriers moved into the insulating layer 134 during the programming step remain therein, so that the insulating layer 134 (and memory device 130) remain in a conductive or low-resistance state, as indicated by the on-state resistance characteristic (E).
In order to erase the memory device 130 (FIGS. 4 and 5), a positive voltage Ver1 (in this example 1.0 volts) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower electrical potential in the same direction as in programming the device 130. Meanwhile, a selected voltage Vg2, in this example 1.8 volts is applied to the gate of the transistor 138. This procedure causes charge carriers to be moved from the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130) to rapidly switch to a high-resistance state. The higher gate voltage Vg2 in the erase step (as compared to gate voltage Vg1 in the programming step) provides substantially higher current (F) through the device 130 during the erase step than during the programming step. This provision of higher current during the erase step as compared to the programming step (based on the higher voltage applied to the gate of the transistor 138 for erase compared to programming, i.e., 1.8 volts versus 1.0 volts) provides an efficient erasing of the memory device 130.
FIGS. 6-8 illustrate a further approach in programming and erasing the memory device 130. Again assuming that the memory device 130 is unprogrammed, in order to program the memory device 130 (FIGS. 6 and 8), a programming voltage Vpg1 (in this example 2.0 volts, the same as in the approach of FIGS. 3-5) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower potential in the direction from electrode 136 to electrode 132. Meanwhile, a selected voltage Vg3, in this example 1.2 volts, is applied to the gate of the transistor 138 (see FIG. 8). This procedure (illustrated in FIG. 8) causes charge carriers in the form of electrons and/or holes to be moved into the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130) to rapidly switch (G) to a low-resistance or conductive state (H). Upon removal of such potential, the charge carriers moved into the insulating layer 134 during the programming step remain therein, so that the insulating layer 134 (and memory device 130) remain in a conductive or low-resistance state, as indicated by the on-state resistance characteristic (H).
In order to erase the memory device 130 (FIGS. 7 and 8), a positive voltage Ver1 (in this example 1.0 volts, the same as in the approach of FIGS. 3-5) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower electrical potential in the same direction as in programming the device 130. Meanwhile, a selected voltage Vg4, in this example 2.0 volts, is applied to the gate of the transistor 138. This procedure causes charge carriers to be moved from the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130 (to rapidly switch to a high-resistance state. The higher gate voltage Vg4 in the erase step (as compared to gate voltage Vg3 in the programming step) provides that substantially higher current (J) during the erase step than during the programming step. This provision of higher current during the erase step as compared to the programming step (based on the higher voltage applied to the gate of the transistor for erase compared to programming, i.e., 2.0 volts versus 1.2 volts) provides an efficient erasing of the memory device 130.
FIGS. 9-11 illustrate yet a further approach in programming and erasing the memory device 130. Again assuming that the memory device 130 is unprogrammed, in order to program the memory device 130 (FIGS. 9 and 11), a programming voltage Vpg1 (in this example 2.0 volts, the same as in the approaches of FIGS. 3-5 and FIGS. 6-8) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower potential in the direction from electrode 136 to electrode 132. Meanwhile, a selected voltage Vg5, in these example 1.5 volts, is applied to the gate of the transistor 138 (see FIG. 11). This procedure (illustrated in FIG. 11) causes charge carriers in the form of electrons and/or holes to be moved into the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130) to rapidly switch (K) to a low-resistance or conductive state (M). Upon removal of such potential, the charge carriers moved into the insulating layer 134 during the programming step remain therein, so that the insulating layer 134 (and memory device 130) remain in a conductive or low-resistance state, as indicated by the on-state resistance characteristic (M).
In order to erase the memory device 130 (FIGS. 10 and 11), a positive voltage Ver1 (in this example 1.0 volts, the same as in the approaches of FIGS. 3-5 and FIGS. 6-9) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower electrical potential in the same direction as in programming the device 130. Meanwhile, a selected voltage Vg6, in this example 2.3 volts, is applied to the gate of the transistor 138. This procedure causes charge carriers to be moved from the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130) to rapidly switch to a high-resistance state. The higher gate voltage Vg6 in the erase step (as compared to gate voltage Vg5 in the programming step) provides that substantially higher current (N) during the erase step than during the programming step. This provision of higher current during the erase step as compared to the programming step (based on the higher voltage applied to the gate of the transistor for erase compared to programming, i.e., 2.3 volts versus 1.5 volts) provides an efficient erasing of the memory device 130.
FIGS. 12-14 illustrate yet a further approach in programming and erasing the memory device 130. Again assuming that the memory device 130 is unprogrammed, in order to program the memory device 130 (FIGS. 12 and 14), a programming voltage Vpg1 (in this example 2.0 volts, the same as in the previous embodiments) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower potential in the direction from electrode 136 to electrode 132. Meanwhile, a selected voltage Vg7, in this example 1.8 volts, is applied to the gate of the transistor 138 (see FIG. 14). This procedure (illustrated in FIG. 14) causes charge carriers in the form of electrons and/or holes to be moved into the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130) to rapidly switch (P) to a low-resistance or conductive state (Q). Upon removal of such potential, the charge carriers moved into the insulating layer 134 during the programming step remain therein, so that the insulating layer 134 (and memory device 130) remain in a conductive or low-resistance state, as indicated by the on-state resistance characteristic (Q).
In order to erase the memory device 130 (FIGS. 13 and 14), a positive voltage Ver1 (in this example 1.0 volts, the same as in the previous approaches) is applied to the electrode 136, so that an electrical potential is applied across the memory device 130 from a higher to a lower electrical potential in the same direction as in programming the device 130. Meanwhile, a selected voltage Vg8, in this example 2.7 volts, is applied to the gate of the transistor 138. This procedure causes charge carriers to be moved from the insulating layer 134, causing the insulating layer 134 (and the overall memory device 130) to rapidly switch to a high-resistance state. The higher gate voltage Vg8 in the erase step (as compared to gate voltage Vg7 in the programming step) provides that substantially higher current (R) during the erase step than during the programming step. This provision of higher current during the erase step as compared to the programming step (based on the higher voltage applied to the gate of the transistor for erase compared to programming, i.e., 2.7 volts versus 1.8 volts) provides an efficient erasing of the memory device 130.
It will be seen that the on-resistance characteristics (E, H, M, Q) in the various approaches described above are different from each other. The particular determined on-resistance characteristic is dependent on the applied gate voltage (Vg1, Vg3, Vg5, Vg7) for the approaches of FIGS. 3-5, 6-8, 9-12, and 13-15 respectively (it is noted that the programming voltage Vpg1 remains constant at 2.0 volts in each of the approaches). In fact, it will be seen that providing a higher gate voltage determines a lower-resistance on-resistance characteristic. That is, in the specific examples set forth above, providing successively higher gate voltages Vg1, Vg3, Vg5, Vg7 in programming results in successively lower resistance on-state resistance characteristics (E, H, M, Q)
FIG. 15 is a composite of FIGS. 5, 8, 11 and 14 illustrating the various on-resistance states (E, H, M, Q) of the memory device 130 in a single Figure. As noted above, each different above-described approach results in and determines a corresponding, different on-resistance characteristic of the memory device 130. With a read voltage Vr (lower in magnitude than Vpg1 and Ver1) applied to the electrode 136, the level of current through the memory device 130 will be determined by the particular on-resistance characteristic. As illustrated, the on-resistance characteristic (E) provides a current level La, the on-resistance characteristic (H) provides a current level Lb, the on-resistance characteristic (M) provides a current level Lc, and the on-resistance characteristic (Q) provides a current level Ld, with Ld>Lc>Lb>La.
In the present example, the memory device 130 is shown and described as adopting any one of four distinct, individual states. As described above, these four different states of the memory device 130 can provide two bits of information.
It will be understood that the difference in gate voltage level when programming the memory device 130 can be set more closely than in the present example, so that the memory device 130 would be capable of adopting more than four individual states.
FIGS. 16 and 17 Illustrate a repair process in the event that a memory device 130 is improperly erased. Assuming that programming has been undertaken in a manner similar as described above with relation to FIGS. 3 and 5, i.e., Vpg1=2.0 volts, Vg1=1.0 volts, the on-resistance characteristics (E) is provided. Then, as described above with regard to FIGS. 4 and 5, in the erasing step, i.e., Ver1=1.0 volt is applied to electrode 136, and Vg1=1.8 volts is applied to the gate of the transistor 138. However, it may happen that for a variety of reasons, an erase malfunction occurs and instead of being erased the memory device 130 is reprogrammed based on the voltage applied to the gate of the transistor 138 during this attempted erase step. The resulting on-resistance characteristic would be as that shown in FIG. 14 (Q). When the improper erasing is realized upon a subsequent read step (indicating a current level that would be expected when taking the programming approach of FIGS. 12 and 14), the erase step next undertaken is as that described above in relation to FIGS. 13 and 14, i.e., the erase approach is based on having programmed the memory device 130 with the on-resistance characteristics (Q), which in fact undesirably occurred. Using this approach, an improperly erased memory device can be properly erased in a later step.
FIG. 18 illustrates a system 200 utilizing memory devices as described above. As shown therein, the system 200 includes hand-held devices in the form of cell phones 202, which communicate through an intermediate apparatus such as a tower 204 (shown) and/or a satellite. Signals are provided from one cell phone to the other through the tower 204. Such a cell phone 202 with advantage uses memory devices of the type described above for data storage, for example names, telephone number and other data. One skilled in the art will readily understand the advantage of using such memory devices in other hand-held devices which utilize data storage, such as portable media players, personal digital assistants, digital cameras and the like.
FIG. 19 illustrates another system 300 utilizing memory devices as described above. The system 300 includes a vehicle 302 having an engine 304 controlled by an electronic control unit 305. The electronic control unit 305 with advantage uses memory devices of the type described above for data storage, for example data relating to engine and vehicle operating conditions.
FIG. 20 illustrates yet another system 400 utilizing memory devices as described above. This system 400 is a computer 402 which includes an input in the form of a keyboard, and a microprocessor for receiving signals from the keyboard through an interface. The microprocessor also communicates with a CDROM drive, a hard drive, and a floppy drive through interfaces. Output from the microprocessor is provided to a monitor through an interface. Also connected to and communicating with the microprocessor is memory which may take the form of ROM, RAM, flash and/or other forms of memory. The memory with advantage uses memory devices of the type described above for storage of any data which is of use.
The foregoing description of the embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications or variations are possible in light of the above teachings.
The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill of the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.

## Claims

1. A method of programming a memory device, the memory device comprising first and second electrodes and a switching layer between the first and second electrodes, the method comprising:
providing the memory device having a plurality of selectable conductive on-resistance states;
programming the memory device, wherein the programming includes
applying a constant voltage to the first electrode of the memory device, wherein the second electrode of the memory device is coupled to a transistor;
applying a selected voltage to a gate of the transistor in series with the memory device, wherein the selected voltage is one of a plurality of predetermined voltages; and

reading a programmed state of the memory device, wherein both erasing of the memory device and the programming of the memory device include applying across the memory device a higher to a lower electrical potential in the direction from the first electrode to the second electrode.

2. The method of claim 1 and further comprising erasing the programmed state of the memory device to provide an erase state of the memory device.

3. The method of claim 1, wherein a selected one of the plurality of selectable conductive on-resistance states of the memory device is determined by the selected voltage applied to the transistor gate, wherein the selected one of the plurality of selectable conductive on-resistance states provides one of a plurality of programmed states of the memory device, wherein each programmed state of the memory device corresponds to one of the selectable conductive on-resistance states.

4. The method of claim 3 wherein the switching layer is in contact with the first and second electrodes.

5. The method of claim 1 and further comprising said memory device incorporated in a system.

6. The method of claim 5 wherein the system is selected from the group consisting of a hand-held device, a vehicle, and a computer.

7. The method of claim 1 wherein a relatively higher gate voltage provides a relatively lower-resistance conductive state, as indicated by the selectable conductive on-resistance state of the memory device.