Patent Application
20090134431
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-303663, filed on Nov. 22, 2007, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a nonvolatile semiconductor apparatus using a variable resistive element and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, as nonvolatile memories which enable rewriting electrically, flash memories, in which memory cells having a floating gate structure are NAND-connected or NOR-connected so that a memory cell array is structured, are publicly known. As nonvolatile memories which enable high-speed random access, ferroelectric memories are also known.
On the other hand, as a technique for improving further miniaturization of memory cells, resistance-change type memories in which variable resistive elements are used as memory cells are proposed. As the variable resistive elements, phase-change memory elements in which resistance is changed by a state transition, namely, crystallization/amorphousness of chalcogenide compounds, MRAM elements using resistance change caused by a tunnel magnetoresistance effect, memory elements of polymer ferroelectric RAM (PFRAM) in which resistive elements are formed by conductive polymer, and ReRAM elements in which resistance changes due to application of an electric pulse are known (Patent Document 1: Japanese Patent Application Laid-Open No. 2006-344349, paragraph 0021).
In the resistance-change memory, memory cells can be structured by a series circuit of a schottky diode and a resistance-change element instead of a transistor. For this reason, this memory has advantages in that lamination is easy and higher integration can be realized by a three-dimensional structure (Patent Document 2: Japanese Patent Application Laid-Open No. 2005-522045).
In the conventional resistance-change memory, resistance of a variable resistive element is set to an initial value by an energy given from the outside, but when a sufficient current density is not given, the resetting takes a long time or the resistance is not reset. When heat generation from a non-ohmic element to be connected to the variable resistive element in series increases, a leak current at the time of reverse bias increases, and consumption current in all the memory cells increases.
A nonvolatile semiconductor storage apparatus according to one aspect of the invention includes: a plurality of first wirings; a plurality of second wirings which cross the plurality of first wirings; and a memory cell which is connected between both the wirings at an intersection of the first and second wirings, and includes a variable resistive element operative to store information according to a change in resistance, wherein the memory cell is formed so that a cross section area of the variable resistive element becomes smaller than a cross section area of the other portion.
A nonvolatile semiconductor storage apparatus according to another aspect of the invention comprising: a memory cell array including plural stacked cell array layers, each cell array layer comprising a plurality of first wirings, a plurality of second wirings which cross the plurality of first wirings, and memory cells which are connected at intersections of the first and second wirings, and each memory cell includes a variable resistive element operative to store information according to a change in resistance, wherein the memory cell is formed so that a cross section area of the variable resistive element becomes smaller than a cross section area of the other portion.
A method of manufacturing a nonvolatile semiconductor storage apparatus according to another aspect of the invention includes: forming, on a semiconductor substrate, a laminated body in which at least an interlayer insulating film, a layer for forming a first wiring, a layer for forming a non-ohmic element and a layer for forming a variable resistive element are sequentially laminated; forming a plurality of first grooves on the laminated body, the first grooves extending in a direction where the first wirings are formed, their opening side being wider than their bottom surface side, their depth reaching a lower surface of the layer for forming the first wirings, embedding a first insulating film into the first grooves; forming a plurality of second grooves on the laminated body into which the first insulating film is embedded, the second grooves extending in a direction where the second wirings crossing the first wirings are formed, their opening side being wider than their bottom surface side, their depth reaching an upper surface of the layer for forming the first wirings; embedding a second insulating film into the second grooves; and forming the second wirings on the laminated body into which the second insulating film is embedded.
Embodiments of the present invention will be described below with reference to the drawings.
[Entire Constitution]
The nonvolatile memory includes a memory cell array 1 in which memory cells using ReRAM (variable resistive elements), described later, are arranged into a matrix pattern. A column control circuit 2 is provided on a position adjacent to the memory cell array 1 in a bit line BL direction. The column control circuit 2 controls the bit line BL of the memory cell array 1, erases data in the memory cells, writes data into the memory cells and reads data from the memory cells. A row control circuit 3 is provided on a position adjacent to the memory cell array 1 in a word line WL direction. The row control circuit 3 selects the word line WL of the memory cell array 1, and applies voltages necessary for erasing data in the memory cells, writing data into the memory cells and reading data from the memory cells.
A data input/output buffer 4 is connected to an external host, not shown, via an I/O line, receives writing data and an erase command, outputs reading data, and receives address data and command data. The data input/output buffer 4 transmits the received writing data to the column control circuit 2, and receives the data read from the column control circuit 2 so as to output the read data to the outside. An address supplied from the outside to the data input/output buffer 4 is sent to the column control circuit 2 and the row control circuit 3 via an address register 5. A command supplied from the host to the input/output buffer 4 is sent to a command interface 6. The command interface 6 receives an external control signal from the host, determines whether the data input into the data input/output buffer 4 is writing data, a command or an address. When the input data is the command, the command interface 6 transmits it as a reception command signal to a state machine 7. The state machine 7 manages the entire nonvolatile memory, accepts a command from the host, and manages reading, writing, erasing and input/output of data. The external host receives status information managed by the state machine 7 so as to be capable of determining an operation result. The status information is used for controlling the writing and erasing.
The state machine 7 controls a pulse generator 9. This control enables the pulse generator 9 to output a pulse of any voltage at any timing. The generated pulse can be transmitted to any wirings selected by the column control circuit 2 and the row control circuit 3.
A peripheral circuit element other than the memory cell array 1 can be formed on an Si substrate just below the memory cell array 1 formed on a wiring layer. As a result, a chip area of the nonvolatile memory can be made approximately equal to an area of the memory cell array 1.
[Memory Cell Array and its Peripheral Circuit]
Word lines WL0 to WL2 are disposed in parallel as a plurality of first wirings, and bit lines BL0 to BL2 are disposed as a plurality of second wirings so as to cross the word lines WL0 to WL2. A memory cell MC is arranged on their intersection so as to be sandwiched by both the wirings. A material of the first and second wirings is desirably resistant to heat and has low resistance, and for example, W, WSi, NiSi, CoSi or the like can be used.
The memory cell MC is composed of a circuit where a variable resistive element VR and a non-ohmic element NO are connected in series as shown in
The variable resistive element VR can change resistance according to application of a voltage via electric current, heat, or chemical energy. An electrode EL which functions as a barrier metal and an adhesive layer may be arranged on and under the variable resistive element VR. When the electrode is arranged, Pt, Au, Ag, TiAlN, SrRuO, Ru, RuN, Ir, Co, Ti, TiN, TaN, LaNiO, Al, PtIrOx, PtRhOx, Rh/TaAlN or the like is used as an electrode material. A metal film which makes orientation uniform can be inserted. Additionally, a buffer layer, a barrier metal layer, an adhesive layer or the like can be inserted.
In the first embodiment, the non-ohmic element NO, the variable resistive element VR and the electrode EL are arranged in this order from the word line WL side to the bit line BL side, so that a pillar-shaped memory cell MC is formed. The memory cell MC is formed into a tapered shape in which its cross section is gradually reduced from the non-ohmic element NO side to the electrode EL side. That is, when a width of the word line WL arranged on the non-ohmic element NO side is denoted by W1, a width of the bit line BL arranged on the electrode EL side is denoted by W2, a width of the memory cell MC in a bit line BL direction at a connecting terminal on the word line WL side and a width thereof in a word line WL direction are denoted by W1′ and W2′, respectively, and a width of the memory cell MC in the bit line BL direction at the connecting terminal on the bit line BL side and a width thereof in the word line WL direction are denoted by W1″ and W2″, respectively, the following relationship holds:
W1=W1′>W1″
W2,W2′>W2″. [Mathematical formula I]
The variable resistive element VR is composed of a composite compound containing cations to be transition elements, and its resistance changes due to transfer of the cations (ReRAM).
In the example of
In the example of
In the example of
In the variable resistive element VR, an electric potential is applied to the electrode layers 11 and 13 so that the first compound layer 15a becomes an anode side and the second compound layer 15b becomes a cathode side. When potential gradient is generated in the recording layer 15, some of the diffusion ions in the first compound layer 15a transfer in crystal, and enter the second compound layer 15b on the cathode side. Since the gap site which can house the diffusion ions is present in the crystal of the second compound layer 15b, the diffusion ions transferred from the first compound layer 15a side are housed in the gap site. For this reason, the valence of the transition element ions in the first compound layer 15a increases, and the valence of the transition element ions in the second compound layer 15b decreases. In the initial state, when the first and second compound layers 15a and 15b are in the high-resistance state, some of the diffusion ions in the first compound layer 15a transfer into the second compound layer 15b. As a result, conduction carriers are generated in the crystal of the first and second compounds, and both of them have electric conducting property. In order to reset the program state (low-resistance state) into an erase state (high-resistance state), similarly to the former example, a large electric current is applied to the recording layer 15 for sufficient time, and the recording layer 15 is Joule-heated so that the oxidation-reduction reaction of the recording layer 15 may be accelerated. The resetting is enabled also by applying an electric field of the opposite direction to that at the time of the setting.
As shown in
In the first embodiment, the memory cell MC is formed into the tapered shape so that its section area is gradually reduced from the non-ohmic element NO side to the variable resistive element VR side. For this reason, since the cross section area of the variable resistive element VR becomes small, the current density can be improved, and the Joule heat is efficiently generated so that a reset speed can be improved. As a result, the reset operation can be performed by a short pulse. Since the cross section area of the non-ohmic element can be enlarged, a sufficient electric current necessary for the reset can be applied. Overheat of the non-ohmic element is prevented, so that a leak current at the time of reverse bias can be suppressed.
In
The selection circuit 2a is composed of a selection PMOS transistor QP0 and a selection NMOS transistor QN0 which are provided for each bit line BL and in which a gate and a drain are commonly connected. A source of the selection PMOS transistor QP0 is connected to a high-potential power source Vcc. A source of the selection NMOS transistor QN0 is connected to a drive sense line BDS on the bit line side to which a writing pulse and an electric current to be detected at the time of reading data are applied. A common drain of the transistors QP0 and QN0 is connected to the bit line BL, and a bit line selection signal BSi for selecting each bit line BL is supplied to a common gate.
The selection circuit 3a is composed of a selection PMOS transistor QP1 and a selection NMOS transistor QN1 which are provided for each word line WL and in which a gate and a drain are commonly connected. A source of the selection PMOS transistor QP1 is connected to a drive sense line WDS on the word line side to which a writing pulse and an electric current to be detected at the time of reading data are applied. A source of the selection NMOS transistor QN1 is connected to a low-potential power source Vss. The common drain of the transistors QP1 and QN1 is connected to the word line WL, and a word line selection signal /WSi for selecting each word line WL is supplied to the common gate.
The above-described example is suitable for selecting the memory cells individually. When data in the plurality of memory cells MC connected to the selected word line WL is collectively read, a sense amplifier is arranged for each of the bit lines BL0 to BL2. The bit lines BL0 to BL2 are connected to the sense amplifiers, respectively, via the selection circuit 2a by the bit line selection signal BS.
In the memory cell array 1, polarity of the diode SD is made to be opposite to that of the circuit shown in
[Manufacturing Method According to the First Embodiment]
A method for manufacturing the nonvolatile memory according to the embodiment shown in
An FEOL (Front End Of Line) process for forming a transistor or the like composing the necessary peripheral circuit on the silicon substrate 21 is executed, and the first interlayer insulating film 25 is deposited on the silicon substrate 21. The via 26 is also fabricated at this time.
Thereafter, the upper layer portion after the first metal 27 is formed.
The process for forming the upper layer portion will be described with reference to
After the first interlayer insulating film 25 and the via 26 are formed, deposition of a layer 27a to be the first metal 27 in the memory cell array, formation of a layer 28a to be the barrier metal 28, deposition of a layer 29a to be the non-ohmic element 29, deposition of a layer 30a to be the first electrode 30, deposition of a layer 31a to be the variable resistive element 31, and deposition of a layer 32a to be the second electrode 32 are executed thereon in this order. A laminate body 40 of the upper layer portion shown in
Thereafter, a nanoimprint technique is used for forming tapered grooves in this embodiment. Liquid resist 41 with low viscosity is dropped onto an upper surface of the laminated body 40, and a template 42 made of quartz is pushed against the upper surface by very weak strength. A plurality of parallel grooves 42a are formed on a lower surface of the template 42. The grooves 42a have a trapezoidal cross section in which an opening side has a wider width. The template 40 is processed by a normal method such as photolithography, but since microfabrication in L/S up to 10 nm order is enabled, a minute cross point structure can be created by using the template 40. The template 42 is pushed against the laminated body 40 so that a direction in which the grooves 42 extend becomes parallel with the word line WL. The inside of the grooves 42a is filled with the resist 41 without a gap.
As shown in
Thereafter, the formed resist pattern 43 is used as a mask to carry out first anisotropic etching, and grooves 44 are formed along the word lines WL as shown in
A second interlayer insulating film 34 is embedded into the grooves 44. A material of the second interlayer insulating film 34 may have insulating property, and suitably has low capacity and satisfactory embedding property. A flattening process is executed by CMP or the like, so that an excessive portion of the second interlayer insulating film 34 is removed and the upper electrode 32 is exposed. The cross-sectional view after the flattening process is shown in
Second etching is carried out in L/S in a direction crossing the first etching. In this case, as shown in
The third interlayer insulating film 35 is then embedded into the grooves 54. A material of the third interlayer insulating film 35 suitably has satisfactory insulating property, low capacity and satisfactory embedding property. Then, the flattening process is executed by CMP or the like, so that an excessive portion of the third interlayer insulating film 35 is removed and the upper electrode 32 is exposed. A cross-sectional view after the flattening process is shown in
As shown in
A multi-layer cross point type memory cell array can be formed by repeating the formation of the multi-layered structure. At this time, when the step of the deposition of the barrier metal layer 28 and subsequent steps are repeated, the memory cell array where wiring is shared by the adjacent memory cell arrays on upper and lower layers can be realized. When the step of the formation of the first interlayer insulating film 25 and subsequent steps are repeated, the memory cell array, where the wiring is not shared by the memory cell arrays adjacent on the upper and lower layers, can be realized.
Thereafter, the nonvolatile semiconductor storage apparatus according to the embodiment is formed by forming the metal wiring layer 38.
As a result, the memory cells MC can be formed into the tapered shape in which their cross section area on the variable resistive element 31 side becomes smaller than that on the non-ohmic element 29 side. For this reason, the current density of the variable resistive element 31 and the current value of the non-ohmic element 29 can be heightened.
In order to form such a tapered shape, besides the above manufacturing method, etching by means of normal resist film formation, etching using a hard mask such as TEOS, SiO2, SiN or amorphous Si may be used. In these etching methods, etching conditions are variously changed so that the memory cells MC can be formed into the tapered shape.
As shown in
The memory cell array 1 can be divided into MATs in some memory cell groups. The column control circuit 2 and the row control circuit 3 may be provided for each MAT, each sector or each cell array layer MA, or may be shared by them. Further, the circuits may be shared by a plurality of bit lines BL in order to reduce the area.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-303663 | Nov 2007 | JP | national |
