US 20060171200 A1
A memory using a mixed valence conductive oxides. The memory includes a mixed valence conductive oxide that is less conductive in its oxygen deficient state and a mixed electronic ionic conductor that is an electrolyte to oxygen and promotes an electric field effective to cause oxygen ionic motion.
1. A memory element comprising:
a mixed valence conductive oxide that is less conductive in its oxygen deficient state; and
an electrolytic tunnel barrier that is an electrolyte to oxygen and promotes an electric field effective to cause oxygen ionic motion.
2. The memory element of
3. The memory element of
4. The memory element of
5. The memory element of
6. The memory element of
7. The memory element of
8. The memory element of
9. The memory element of
10. The memory element of
11. The memory element of
12. The memory element of
13. A memory element, comprising:
a tunneling barrier having a tunnel barrier width; and
a conductive material having a low conductivity region that forms an effective tunnel barrier width greater than the tunnel barrier width, the low conductivity region being responsive to a voltage across the memory element.
14. The memory element of
15. The memory element of
16. The memory element of
17. The memory element of
18. The memory element of
19. The memory element of
20. The memory element of
21. The memory element of
22. The memory element of
23. A two terminal electrical device, comprising:
a tunneling barrier having a tunnel barrier width of less than approximately 50 angstroms; and
a conductive material in series with the tunneling barrier,
wherein the tunneling barrier In series with the conductive material has a first conductivity at a read voltage and a second conductivity at the read voltage after applying a programming voltage.
24. The two terminal electrical device of
25. The two terminal electrical device of
26. The two terminal electrical device of
27. The two terminal electrical device of
28. The two terminal electrical device of
29. The two terminal electrical device of
30. The two terminal electrical device of
31. The two terminal electrical device of
32. The two terminal electrical device of
33. A two terminal electrical device, comprising:
an insulating material of less than approximately 50 angstroms, having a conduction mechanism including tunneling during both reading and writing; and
a conductive material coupled directly or indirectly with the insulating material,
wherein the two terminal electrical device has a first conductivity at a read voltage and a second conductivity at the read voltage after applying a programming voltage.
34. The two terminal electrical device of
35. The two terminal electrical device of
36. The two terminal electrical device of
37. The two terminal electrical device of
38. The two terminal electrical device of
39. The two terminal electrical device of
40. The two terminal electrical device of
41. The two terminal electrical device of
42. The two terminal electrical device of
This application is a continuation of “Memory Using Variable Tunnel Barrier Widths,” U.S. application Ser. No. 10/934,951, filed Sep. 3, 2004, which is incorporated herein by reference in its entirety and for all purposes.
1. Field of the Invention
The present invention relates to computer memory and more specifically to non-volatile memory.
2. Description of the Related Art
Memory can either be classified as volatile or nonvolatile. Volatile memory is memory that loses its contents when the power is turned off. In contrast, non-volatile memory does not require a continuous power supply to retain information. Most non-volatile memories use solid-state memory devices as memory elements.
Since the 1960s, a large body of literature has evolved that describes switching and memory effects in metal-insulator-metal structures with thin insulators. One of the seminal works was “New Conduction and Reversible Memory Phenomena in Thin Insulating Films” by J. G. Simmons and R. R. Verderber in 301 Proc. Roy. Soc. 77-102-(1967), incorporated herein by reference for all purposes. Although the mechanisms described by Simmons and Verderber have since been cast into doubt, their contribution to the field is great.
However, nobody has successfully implemented a metal-insulator-metal structure into a commercial solid-state memory device. In the text “Oxides and Oxide Films,” volume 6, edited by A. K. Vijh (Marcel Drekker 1981) 251-325, incorporated herein by reference for all purposes, chapter 4, written by David P. Oxley, is entirely devoted to “Memory Effects in Oxide Films.” In that text, Oxley says “It is perhaps saddening to have to record that, even after 10 years of effort, the number of applications for these oxide switches is so limited.” He goes on to describe a “need for caution before any application is envisaged. This caution can only be exercised when the physics of the switching action is understood; this, in turn, must await a full knowledge of the transport mechanisms operating in any switch for which a commercial use is envisaged.”
In 2002, over twenty years after writing that chapter, Oxley revisited the subject in “The Electroformed metal-insulator-metal structure: A comprehensive model” by R. E. Thurstans and D. P. Oxley 35 J. Phys. D. Appl. Phys. 802-809, incorporated herein by reference for all purposes. In that article, the authors describe a model that identifies the conduction process as “trap-controlled and thermally activated tunneling between metal islands produced in the forming process.” “Forming” (or “electroforming”) is described as “the localized filamentary movement of metallic anode material through the dielectric, induced by the electric field. Here it is important to note that the evaporated dielectric may contain voids and departures from stoichiometry. When resulting filaments through the dielectric carry sufficient current, they rupture to leave a metal island structure embedded in the dielectric. Electronic conduction is possible through this structure by activating tunneling.”
However, the authors caution, “The forming process is complex and inherently variable. Also tunneling barriers are susceptible to changes in their characteristics when exposed to water vapour, organic species and oxygen . . . . Thus, device characteristics can never be expected to be produced consistently or be stable over long periods without passivation, effective encapsulation and a better understanding of the dynamics of the forming process.”
In seemingly unrelated research, certain conductive metal oxides (CMOs), have been identified as exhibiting a memory effect after being exposed to an electronic pulse. U.S. Pat. No. 6,204,139, issued Mar. 20, 2001 to Liu et al., incorporated herein by reference for all purposes, describes some perovskite materials that exhibit memory characteristics. The perovskite materials are also described by the same researchers in “Electric-pulse-induced reversible resistance change effect in magnetoresistive films,” Applied Physics Letters, Vol. 76, No. 19, 8 May 2000, and “A New Concept for Non-Volatile Memory: The Electric-Pulse Induced Resistive Change Effect in Colossal Magnetoresistive Thin Films,” in materials for the 2001 Non-Volatile Memory Technology Symposium, all of which are hereby incorporated by reference for all purposes.
In U.S. Pat. No. 6,531,371 entitled “Electrically programmable resistance cross point memory” by Hsu et al, incorporated herein by reference for all purposes, resistive cross point memory devices are disclosed along with methods of manufacture and use. The memory device comprises an active layer of perovskite material interposed between upper electrodes and lower electrodes.
Similarly, the IBM Zurich Research Center has also published three technical papers that discuss the use of metal oxide material for memory applications: “Reproducible switching effect in thin oxide films for memory applications,” Applied Physics Letters, Vol. 77, No. 1, 3 Jul. 2000, “Current-driven insulator-conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals,” Applied Physics Letters, Vol. 78, No. 23, 4 Jun. 2001, and “Electric current distribution across a metal-insulator-metal structure during bistable switching,” Journal of Applied Physics, Vol. 90, No. 6, 15 Sep. 2001, all of which are hereby incorporated by reference for all purposes.
There are continuing efforts to incorporate solid state memory devices into a commercial non-volatile RAM.
The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:
It is to be understood that, in the drawings, like reference numerals designate like structural elements. Also, it is understood that the depictions in the FIGs. are not necessarily to scale.
In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the present invention.
The Memory Array
Conventional nonvolatile memory requires three terminal MOSFET-based devices. The layout of such devices is not ideal, usually requiring an area of at least 8 f2 for each memory cell, where f is the minimum feature size. However, not all memory elements require three terminals. If, for example, a memory element is capable of changing its electrical properties (e.g., resistivity) in response to a voltage pulse, only two terminals are required. With only two terminals, a cross point array layout that allows a single cell to be fabricated to a size of 4 f2 can be utilized.
Conductive array line layers 105 and 110 can generally be constructed of any conductive material, such as aluminum, copper, tungsten or certain ceramics. Depending upon the material, a conductive array line would typically cross between 64 and 8192 perpendicular conductive array lines. Fabrication techniques, feature size and resistivity of material may allow for shorter or longer lines. Although the x-direction and y-direction conductive array lines can be of equal lengths (forming a square cross point array) they can also be of unequal lengths (forming a rectangular cross point array), which may be useful if they are made from different materials with different resistivities.
Referring back to
One benefit of the cross point array is that the active circuitry that drives the cross point array 100 or 150 can be placed beneath the cross point array, therefore reducing the footprint required on a semiconductor substrate. However, the cross point array is not the only type of memory array that can be used with a two-terminal memory element. For example, a two-dimensional transistor memory array can incorporate a two-terminal memory element. While the memory element in such an array would be a two-terminal device, the entire memory cell would be a three-terminal device.
Memory Chip Configuration
The reading of data from a memory array 420 is relatively straightforward: an x-line is energized, and current is sensed by the sensing circuits 410 on the energized y-lines and converted to bits of information.
During a write operation, the data is applied from the data bus 460 to the input buffers and data drivers 490 to the selected vertical lines, or bit lines. Specifically, when binary information is sent to the memory chip 400B, it is typically stored in latch circuits within the circuits 495. Within the circuits 495, each y-line can either have an associated driver circuit or a group of y-lines can share a single driver circuit if the non-selected lines in the group do not cause the unselected memory plugs to experience any change in resistance, typically by holding the non-selected lines to a constant voltage. As an example, there may be 1024 y-lines in a cross point array, and the page register may include 8 latches, in which case the y-block would decode 1 out of 128 y-lines and connect the selected lines to block 495. The driver circuit then writes the 1 or 0 to the appropriate memory plug. The writing can be performed in multiple cycles. In a scheme described in PCT Patent Application No. PCT/US04/13836, filed May 3, 2004, incorporated herein by reference, all the 1s can be written during a first cycle and all the 0s can be written during a second cycle. As described below, certain memory plugs can have multiple stable distinct resistive states. With such multi-level resistance memory plugs, driver circuits could program, for example, states of 00, 01, 10 or 11 by varying write voltage magnitude or pulse length.
It is to be noted that such an architecture can be expanded to create a memory where one array handles all the bits of the data bus, as opposed to having multiple arrays, or memory bit blocks as described above. For example, if the data bus, or memory data organization, also called data width, is 16-bit wide, the y-block of one cross point array can be made to decode 16 lines simultaneously. By applying the techniques of simultaneous reads and 2-cycle writes, such a memory chip with only one array can read and program 16-bit words.
Each memory plug contains layers of materials that may be desirable for fabrication or functionality. For example, a non-ohmic characteristic that exhibit a very high resistance regime for a certain range of voltages (VNO− to VNO+) and a very low resistance regime for voltages above and below that range might be desirable. In a cross point array, a non-ohmic characteristic could prevent leakage during reads and writes if half of both voltages were within the range of voltages VNO− to VNO+. If each conductive array line carried ½ VW, the current path would be the memory plug at the intersection of the two conductive array lines that each carried ½ VW. The other memory plugs would exhibit such high resistances from the non-ohmic characteristic that current would not flow through the half-selected plugs.
A non-ohmic device might be used to cause the memory plug to exhibit a non-linear resistive characteristic. Exemplary non-ohmic devices include three-film metal-insulator-metal (MIM) structures and back-to-back diodes in series. Separate non-ohmic devices, however, may not be necessary. Certain fabrications of the memory plug can cause a non-ohmic characteristic to be imparted to the memory cell. While a non-ohmic characteristic might be desirable in certain arrays, it may not be required in other arrays.
Electrodes will typically be desirable components of the memory plugs, a pair of electrodes sandwiching the memory element. If the only purpose of the electrodes is as a barrier to prevent metal inter-diffusion, then a thin layer of non-reactive metal, e.g. TiN, TaN, Pt, Au, and certain metal oxides could be used. However, electrodes may provide advantages beyond simply acting as a metal inter-diffusion barrier. Electrodes (formed either with a single layer or multiple layers) can perform various functions, including to: prevent the diffusion of metals, oxygen, hydrogen and water; act as a seed layer in order to form a good lattice match with other layers; act as adhesion layers; reduce stress caused by uneven coefficients of thermal expansion; and provide other benefits. Additionally, the choice of electrode layers can affect the memory effect properties of the memory plug and become part of the memory element.
The “memory element electrodes” are the electrodes (or, in certain circumstances, the portion of the conductive array lines) that the memory elements are sandwiched in-between. As used herein, memory element electrodes are what allow other components to be electrically connected to the memory element. It should be noted that in both cross point arrays and transistor memory arrays have exactly two memory element electrodes since the memory plug has exactly two terminals, regardless of how many terminals the memory cell has. Those skilled in the art will appreciate that a floating gate transistor, if used as a memory element, would have exactly three memory element electrodes (source, drain and gate).
The memory effect is a hysteresis that exhibits a resistive state change upon application of a voltage while allowing non-destructive reads. A non-destructive read means that the read operation has no effect on the resistive state of the memory element. Measuring the resistance of a memory cell is generally accomplished by detecting either current after the memory cell is held to a known voltage, or voltage after a known current flows through the memory cell. Therefore, a memory cell that is placed in a high resistive state R0 upon application of −VW and a low resistive state R1 upon application of +VW should be unaffected by a read operation performed at −VR or +VR. In such materials a write operation is not necessary after a read operation. It should be appreciated that the magnitude of |−VR| does not necessarily equal the magnitude of |+VR|.
Furthermore, it is possible to have a memory cell that can be switched between resistive states with voltages of the same polarity. For example, in the paper “The Electroformed metal-insulator-metal structure: a comprehensive model,” already incorporated by reference, Thurstans and Oxley describe a memory that maintains a low resistive state until a certain VP is reached. After VP is reached the resistive state can be increased with voltages. After programming, the high resistive state is then maintained until a VT is reached. The VT is sensitive to speed at which the program voltage is removed from the memory cell. In such a system, programming R1 would be accomplished with a voltage pulse of VP, programming R0 would be accomplished with a voltage pulse greater than VP, and reads would occur with a voltages below VT. Intermediate resistive states (for multi-level memory cells) are also possible.
The R1 state of the memory plug may have a best value of 10 kΩ to 100 kΩ. If the R1 state resistance is much less than 10 kΩ, the current consumption will be increased because the cell current is high, and the parasitic resistances will have a larger effect. If the R1 state value is much above 100 kΩ, the RC delays will increase access time. However, workable single state resistive values may also be achieved with resistances from 5 kΩ to 1 MΩ and beyond with appropriate architectural improvements. Typically, a single state memory would have the operational resistances of R0 and R1 separated by a factor of 10.
Since memory plugs can be placed into several different resistive states, multi-bit resistive memory cells are possible. Changes in the resistive property of the memory plugs that are greater than a factor of 10 might be desirable in multi-bit resistive memory cells. For example, the memory plug might have a high resistive state of R00, a medium-high resistive state of R01, a medium-low resistive state of R10 and a low resistive state of R11. Since multi-bit memories typically have access times longer than single-bit memories, using a factor greater than a 10 times change in resistance from R11 to R00 is one way to make a multi-bit memory as fast as a single-bit memory. For example, a memory cell that is capable of storing two bits might have the low resistive state be separated from the high resistive state by a factor of 100. A memory cell that is capable of storing three or four bits of information might require the low resistive state be separated from the high resistive state by a factor of 1000.
Creating the Memory Effect with Tunneling
Tunneling is a process whereby electrons pass through a barrier in the presence of a high electric field. Tunneling is exponentially dependent on both a barrier's height and its width. Barrier height is typically defined as the potential difference between the Fermi energy of a first conducting material and the band edge of a second insulating material. The Fermi energy is that energy at which the probability of occupation of an electron state is 50%. Barrier width is the physical thickness of the insulating material.
The barrier height might be modified if carriers or ions are introduced into the second material, creating an additional electric field. A barrier's width can be changed if the barrier physically changes shape, either growing or shrinking. In the presence of a high electric field, both mechanisms could result in a change in conductivity.
Although the following discussion focuses mainly on purposefully modifying the barrier width, those skilled in the art will appreciate that other mechanisms can be present, including but not limited to: barrier height modification, carrier charge trapping space-charge limited currents, thermionic emission limited conduction, and/or electrothermal Poole-Frenkel emission.
Referring back to
Fundamentally, the electrolytic tunnel barrier 505 is an electronic insulator and an ionic electrolyte. As used herein, an electrolyte is any medium that provides an ion transport mechanism between positive and negative electrodes. Materials suitable for some embodiments include various metal oxides such as Al2O3, Ta2O5, HfO2 and ZrO2. Some oxides, such as zirconia might be partially or fully stabilized with other oxides, such as CaO, MgO, or Y2O3, or doped with materials such as scandium.
The electrolytic tunnel barrier 505 will typically be of very high quality, being as uniform as possible to allow for predictability in the voltage required to obtain a current through the memory element 500. Although atomic layer deposition and plasma oxidation are examples of methods that can be used to create very high quality tunnel barriers, the parameters of a particular system will dictate its fabrication options. Although tunnel barriers can be obtained by allowing a reactive metal to simply come in contact with an ion reservoir 510, as described in PCT Patent Application No. PCT/US04/13836, filed May 3, 2004, already incorporated herein by reference, such barriers may be lacking in uniformity, which may be important in some embodiments. Accordingly, in a preferred embodiment of the invention the tunnel barrier does not significantly react with the ion reservoir 510 during fabrication.
With standard designs, the electric field at the tunnel barrier 505 is typically high enough to promote tunneling at thicknesses between 10 and 50 angstroms. The electric field is typically higher than at other points in the memory element 500 because of the relatively high serial electronic resistance of the electrolytic tunnel barrier 505. The high electric field of the electrolytic tunnel barrier 505 also penetrates into the ion reservoir 510 at least one Debye length. The Debye length an be defined as the distance which a local electric field affects distribution of free charge carriers. At an appropriate polarity, the electric field within the ion reservoir 510 causes ions (which can be positively or negatively charged) to move from the ion reservoir 510 through the electrolytic tunnel barrier 505, which is an ionic electrolyte.
The ion reservoir 510 is a material that is conductive enough to allow current to flow and has mobile ions. The ion reservoir 510 can be, for example, an oxygen reservoir with mobile oxygen ions. Oxygen ions are negative in charge, and will flow in the direction opposite of current.
When an electric field is applied across the electrolytic tunnel barrier 505, the electric field would penetrate at least one Debye length into the oxygen reservoir 635. The negatively charged oxygen ions migrate through the electrolytic tunnel barrier 505 to combine with positively charged metal ions in the complementary reservoir 615, creating a low conductivity oxide 640. This low conductivity oxide 640 is cumulative with the electrolytic tunnel barrier 505, forcing electrons to tunnel a greater distance to reach the conductive complimentary reservoir 615. Because of the exponential effect of barrier width on tunneling, the low conductivity oxide 640 can be just a few angstroms wide and still have a very noticeable effect on the memory element's effective resistance.
Those skilled in the art will appreciate that redox reaction can occur at either the top or bottom surface of the electrolytic tunnel barrier 505. The low conductivity oxide 640 will form at the top of the electrolytic tunnel barrier 505 if the mobility of the complementary ions is greater than the mobility of the oxygen ions through the electrolytic tunnel barrier 505. Conversely, if the mobility of oxygen ions is greater than the mobility of the complementary ions through the electrolytic tunnel barrier 505, then the low conductivity oxide 640 will form at the bottom of the electrolytic tunnel barrier 505.
The stability of metal oxides will depend on its activation energy. Reversing the redox reaction for many metal oxides, such as Hf and Al, requires a great amount of energy, making such high activation energy cells convenient for use as one-time programmable memories. Oxides with low activation energy, such as RuOx and CuOx, are usually desirable for reprogrammable memories.
One optimization would be to use the polarity that is less sensitive to read disturbs during reads. For write once memory this may be complementary to the write polarity. Alternatively, alternating read polarities can be used. Another optimization for certain embodiments ould be to limit the size of the complementary reservoir 615.
In most cases the effective width of the tunneling barrier is limited only by the availability of ions in the reservoirs 615 and 635. Since many different barrier widths can be formed multiple bits per cell can be easily implemented with different resistive states.
Referring back to
Accordingly, as shown in
Creating the Memory Effect with Oxygen Depletion
In these embodiments, ion deficiency (which, in the embodiment of
The mixed electronic ionic conductor 705 is similar, and in some cases identical, to the electrolytic tunnel barrier 505 of
In one specific embodiment that is similar to an inverted embodiment of what is shown in
The mixed valence oxide 710 might be a 500 Angstrom layer of a PCMO perovskite, rf magnetron sputtered in 10 mTorr of argon at 550° C. by applying 120 watts to a Pr0.7Ca0.3MnO3 target (made with hot isostatic pressing or HIP), afterwards cooled in-situ for 10 minutes in the sputter ambient gas environment of 10 mTorr of argon, then cooled for another 10 minutes in a load lock chamber at 600 Torr of oxygen.
The mixed electronic ionic conductor 705 might be 20 or 30 Angstroms of some type of AlOx, rf magnetron sputtered in 4 mTorr of argon with 1% oxygen at 300° C. by applying 150 watts to an Al2O3 target (also made with HIP), and then annealed for 30 minutes at 250° C. in the sputter ambient gas environment of 4 mTorr of argon with 1% O2.
If an embodiment similar to
The top electrode 515 might be 500 Angstroms of platinum, DC magnetron sputtered with 180 watts applied to a platinum target in 4 mTorr of argon at 25° C.
Although the invention has been described in its presently contemplated best mode, it is clear that it is susceptible to numerous modifications, modes of operation and embodiments, all within the ability and skill of those familiar with the art and without exercise of further inventive activity. For example, although the ion reservoir was described as being negative in connection with the oxygen reservoir, a positively charged ion reservoir may have the same functionality, as long as the other physical requirements of the specific embodiments are met. Furthermore, while the theories provided above are one possible explanation of how the various materials interact, the inventors do not wish to be bound by any theoretical explanation. Accordingly, that which is intended to be protected by Letters Patent is set forth in the claims and includes all variations and modifications that fall within the spirit and scope of the claims.