Shared bit line cross point memory array

[0002] This application is a continuation-in-part of U.S. patent application Ser. No. 09/893,830, filed Jun. 28, 2001, entitled Low Cross-talk Electrically Programmable Resistance Cross Point Memory and of U.S. patent application Ser. No. 09/894,922, also filed Jun. 28, 2001, entitled Electrically Programmable Resistance Cross Point Memory. Both of these applications are incorporated herein by reference.

[0003] This invention relates to nonvolatile memory, and more particularly to a cross point memory structure.

[0004] Materials having a perovskite structure, among them colossal magnetoresistance (CMR) materials and high temperature superconductivity (HTSC) materials are materials that have electrical resistance characteristics that can be changed by external influences.

[0005] For instance, the properties of materials having perovskite structures, especially for CMR and HTSC materials, can be modified by applying one or more short electrical pulses to a thin film or bulk material. The electric field strength or electric current density from the pulse, or pulses, is sufficient to switch the physical state of the materials so as to modify the properties of the material. The pulse is of low enough energy so as not to destroy, or significantly damage, the material. Multiple pulses may be applied to the material to produce incremental changes in properties of the material. One of the properties that can be changed is the resistance of the material. The change may be at least partially reversible using pulses of opposite polarity from those used to induce the initial change.

[0006] Cross point memory arrays along with their accompanying read out circuits can use a significant amount of chip surface area. The reduction in chip size has a corresponding economic benefit.

[0007] Accordingly, a memory structure is provided, which reduces the area needed for a cross point array and accompanying read out circuits. By sharing each bit line with two sets of word lines, the area of each cross point can hold two bits instead of one, and the number of read out circuits is reduced since the number of bit lines per bit is reduced.

[0008] A shared bit line cross point memory array structure is provided, along with methods of manufacture and use. The memory structure comprises a bottom word line with a top word line overlying the bottom word line. A bit line is interposed between the bottom word line and the top word line such that a first cross point is formed between the bottom word line and the bit line and a second cross point is formed between the bit line and the top word line. A material having a property, for example resistance, that can be changed in response to an input voltage is provided at each cross point above and below the bit line.

[0009] Each bit can be programmed by applying a voltage signal between the bit line and the appropriate word line while leaving other word lines floating. Likewise, the bit can be read by applying a read voltage to the desired word line and reading the signal off of the bit line. A block erase can also be achieved by ground all word lines and applying an erase voltage signal to one, or more, bit lines.

[0010] FIG. 1 is an isometric view of a cross point memory array area with a shared bit line.

[0011] FIG. 2 is a cross sectional view of a memory structure during processing.

[0012] FIG. 3 is a cross sectional view of a memory structure during processing.

[0013] FIG. 4 is a cross sectional view of a memory structure during processing.

[0014] FIG. 5 is a cross sectional view of a memory structure during processing.

[0015] FIG. 6 is a cross sectional view of a memory structure during processing.

[0016] FIG. 7 is a cross sectional view of a memory structure during processing.

[0017] FIG. 8 is a cross sectional view of a memory structure during processing.

[0018] FIG. 9 is a cross sectional view of a memory structure during processing.

[0019] FIG. 10 is a cross sectional view of a memory structure during processing.

[0020] FIG. 11 is a cross sectional view of a memory structure during processing.

[0021] FIG. 1 is an isometric drawing illustrating a shared bit line cross point memory array area 10. An embodiment of the memory array area 10 comprises a substrate 12 with a plurality of bottom word lines 14 formed thereon. A layer of oxide 16 overlies the substrate 12 and bottom word lines 14. A plurality of bit lines 18 overly the layer of oxide 16. A second layer of oxide 20 overlies the plurality of bit lines 18. A plurality of top word lines 22 overly the second layer of oxide 20. A passivation layer 24 overlies the plurality of top word lines 22. As is apparent from the isometric view, a cross point configuration is formed between the bottom word lines and the bit lines, as well as, between the top word lines and the bit lines. In this way two sets of word lines share a single set of bit lines. In a low cross talk version, the layer of oxide 16 would be etched so that a perovskite material could be deposited connecting the plurality of bottom word lines 14 to the plurality of bit lines 18 at each cross point. A similar structure could be formed between the bit lines and the plurality of top word lines. Alternatively, continuous active regions could be used in place of the layer of oxide 16 and the second layer of oxide 20.

[0022] Note that the terms “top” and “bottom” are for ease of explanation with respect to the drawings and should not be construed as requiring a specific orientation. The device can assume any spatial orientation during manufacture and operation.

[0023] FIG. 1 shows just the memory array area. It should be clear that in an actual device, the substrate 12, the bottom word lines 14, the bit lines 18, and the top word lines 22 may extend well beyond the memory array area to other areas containing other device structures.

[0024] A method for forming a low cross talk resistive memory array is provided. FIG. 2 shows a cross-sectional view of a cross point memory array area 10 following some initial processing. The memory array area 10 comprises a substrate 12 with a bottom word line 14 formed thereon. A layer of oxide 16, which is deposited over the substrate to a thickness of between approximately 500 nm and 1000 nm above the bottom word line 14, is planarized to a thickness of between approximately 50 nm and 500 nm over the bottom word line. The layer of oxide 16 is etched to form openings 15 to allow access to the bottom word line 14.

[0025] The substrate 12 is any suitable substrate material, whether amorphous, polycrystalline or crystalline, such as LaAlO₃, Si, SiO₂, TiN or other material.

[0026] The bottom word lines 14 are made of conductive oxide or other conductive material. In a preferred embodiment, the conductive material is a material, such as YBa₂Cu₃O₇(YBCO), that allows the epitaxial growth of an overlying perovskite material. In another preferred embodiment, the conductive material is platinum or iridium. The bottom word lines are a thickness in the range of between about 5 nm and about 500 nm.

[0027] Referring now to FIG. 3, a layer of perovskite material 17 is deposited over the oxide 16 to fill the openings 15. The perovskite material 17 is a material capable of having its resistivity changed in response to an electrical signal. The perovskite material is preferably a colossal magnetoresistive (CMR) material or a high temperature superconducting (HTSC) material, for example Pr_0.7Ca_0.3MnO₃(PCMO). Another example of a suitable material is Gd_0.7Ca_0.3BaCo₂O₅₊₅. The perovskite material is preferably between about 50 nm and 500 nm thick following polishing. The perovskite material 17 can be deposited using any suitable deposition technique including pulsed laser deposition, rf-sputtering, e-beam evaporation, thermal evaporation, metal organic deposition, sol gel deposition, and metal organic chemical vapor deposition. The perovskite material is preferably polished using CMP.

[0028] FIG. 4 shows the memory array area 10 following deposition and patterning of bit lines 18. The bit lines 18 comprise a conductive material, preferably YBCO, platinum, iridium, copper, silver, or gold. The perovskite material that is now interposed between the bottom word line 14 and the bit lines 18 is now a set of resistive memory bits 25.

[0029] FIG. 5 the memory area 10 following deposition of a second oxide layer 20, patterning, deposition and polishing of perovskite material 27. The process is similar to that described above in connection with FIG. 3.

[0030] FIG. 6 shows a memory array area 10 following formation of a top word line 22, and passivation of the memory array area. The perovskite material that is now interposed between the top word line 22 and the bit lines 18 is now a second set of resistive memory bits 29. The top word lines 22 and the bottom word lines 14 are each preferably substantially parallel rows. The top word lines 22 and the bottom word lines 14 are arranged in a cross point arrangement with respect to the bit lines 18, such that they each cross the bit lines in a regular pattern. A cross point refers to each position where a word line, either a top word line or a bottom word line, crosses a bit line. As shown, the word lines and the bit lines are arranged at substantially 90 degrees with respect to each other. Although as shown and described the top word lines and the bottom word lines are directly aligned with respect to each other, it is also possible to have them offset with respect to each other. In which case, any bit formed between the bottom word line and the bit line would not be aligned with a corresponding bit formed between the bit line and the top word line.

[0031] In a preferred embodiment of the present method, one, or more, of transistor structures, interconnects or other components of a memory circuit may be formed prior to the formation of the memory array area 10. By forming components of the memory circuit prior to the memory array area 10, possible degradation of the perovskite material due to subsequent processing is reduced, or eliminated.

[0032] FIGS. 7 through 11 illustrate another embodiment of a shared bit line memory structure and method of processing. FIG. 7 shows a cross-sectional view of a cross point memory array area 10 following some initial processing. The memory array area 10 comprises a substrate 12 with a bottom word line 14 formed thereon. A first layer of active material 76 is deposited over the bottom word line. The active material is preferably a perovskite material, such as a colossal magnetoresistive (CMR) material or a high temperature superconducting (HTSC) material, for example Pr_0.7Ca_0.3MnO₃(PCMO). Another example of a suitable material is Gd_0.7Ca_0.3BaCo₂O₅₊₅. The first layer of active material 76 is preferably between about 5 nm and 500 nm thick. The active material can be deposited using any suitable deposition technique including pulsed laser deposition, rf-sputtering, e-beam evaporation, thermal evaporation, metal organic deposition, sol gel deposition, and metal organic chemical vapor deposition. The active material is removed from outside the memory array area by ion milling or other suitable process as shown in FIG. 8. It is also possible to form a large recessed area to deposit perovskite material over and then use chemical mechanical polishing (CMP) to form the first layer of active material 76.

[0033] FIG. 9 shows the memory array area 10 following the formation of bit lines 18. The bit lines 18 are formed by depositing a layer of oxide and patterning it to form openings to the first layer of active material 76. A suitable conductive material is then deposited and polished forming the bit lines 18.

[0034] FIG. 10 shows the memory array area 10 following the deposition and patterning of a second layer of active material 80. The second layer of active material 80 is formed using a method described above in connection with the formation of the first layer of active material 76.

[0035] FIG. 11 shows the memory array area 10 following formation of the top word line 22 and deposition of a passivation oxide 90. Each top word line 22 forms a cross point with each bit line 18. The second layer of active material is a high resistivity material as formed. An upper resistive bit can be formed at each cross point by applying a voltage signal between each word line and each bit line to convert a region of active material to a low resistance state. Likewise a lower resistive bit can be formed between a bottom word line and a bit line. Each region at a cross point generally corresponds to a bit.

[0036] The bit lines also form a cross point array with the bottom word lines. By applying voltage signals a lower set of resistive bits can be formed. Each bit line will then have an upper set of resistive bits connected to one of the top word lines, and a lower set of resistive bits connected to one of the bottom word lines. This allows two sets of word lines to share a single set of bit lines. This also allows fewer bit line read circuits to be used for a given number of desired bits.

[0037] Once a device is completed and in operation, it can be programmed and read. The resistivity of each bit may be changed to program, or erase, a single bit. The resistivity is changed by applying a programming voltage between one word line and one bit line, while allowing the remaining word lines to float so that no signal flows between other word lines and the bit line. This includes setting the bottom word line to float when the programming voltage is on a top word line, or vice versa. This allows an upper bit for example to be programmed without affecting a lower bit.

[0038] The programming voltage is a voltage capable of changing the resistivity of a bit without damaging the bit. In some cases, it may not be able to apply fixed voltage capable of changing the resistivity of the bit without damaging the bit. The programming voltage may need to be a series of voltage pulses which can change the resistivity without damaging the bit.

[0039] Once a bit is programmed, it would be useful to be able to read the bit. The bit can be read by applying a voltage across a word line and a bit line while allowing the remaining word lines to float so that not current flows between the bit line and the remaining word lines. The output of the bit is then read at the bit line using a read out circuit.

[0040] It would also be possible to set all bits along a single bit line to the same resitance state, high or low, by grounding all the word lines, top and bottom, and applying a programming voltage to at least one bit line. If a programming voltage is applied to all bit lines a blanket programming of all the bits would be achieved effectively simultaneously. This may be useful for achieving a block erase.

[0041] Although a preferred embodiment, and other embodiments have been discussed above, the coverage is not limited to these specific embodiments. Rather, the claims shall determine the scope of the invention.