Patent Grant
9997703
Resistive memory elements can be programmed to different resistive states by applying programming energy. After programming, the state of the resistive memory elements can be read and remains stable over a specified time period. Large arrays of resistive memory elements can be used to create a variety of resistive memory devices, including non-volatile solid state memory, programmable logic, signal processing, control systems, pattern recognition devices, and other applications. Examples of resistive memory devices include memristors, phase change memory, and spin-transfer torque.
Memristors are devices that can be programmed to different resistive states by applying a programming energy, for example, a voltage or current pulse. This energy generates a combination of electric field and thermal effects that can modulate the conductivity of both non-volatile switch and non-linear select functions in a memristive element. After programming, the state of the memristor can be read and remains stable over a specified time period. Memristive elements can be used in a variety of applications, including non-volatile solid state memory, programmable logic, signal processing, control systems, pattern recognition, and other applications.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.
In the following description, numerous details are set forth to provide an understanding of the examples disclosed herein. However, it will be understood that the examples may be practiced without these details. While a limited number of examples have been disclosed, it should be understood that there are numerous modifications and variations therefrom. Similar or equal elements in the Figures may be indicated using the same numeral.
As used in the specification and claims herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
As used in this specification and the appended claims, “approximately” and “about” mean a ±10% variance caused by, for example, variations in manufacturing processes.
In the following detailed description, reference is made to the drawings accompanying this disclosure, which illustrate specific examples in which this disclosure may be practiced. The components of the examples can be positioned in a number of different orientations and any directional terminology used in relation to the orientation of the components is used for purposes of illustration and is in no way limiting. Directional terminology includes words such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc.
It is to be understood that other examples in which this disclosure may be practiced exist, and structural or logical changes may be made without departing from the scope of the present disclosure. Therefore, the following detailed description is not to be taken in a limiting sense. Instead, the scope of the present disclosure is defined by the appended claims.
Resistive memory elements can be used in a variety of applications, including non-volatile solid state memory, programmable logic, signal processing, control systems, pattern recognition, and other applications.
As used in the specification and appended claims, the term “resistive memory elements” refers broadly to programmable non-volatile resistors such as resistive random access memory (ReRAM), phase change memory (FORAM), memristor technology based on perovskites (such as Sr(Zr)TiO3), transition metal oxides (such as NiO or TiO2 or TaOx), chalcogenides (such as Ge2Sb2Te5 or AgInSbTe), solid-state electrolytes (such as GeS, GeSe, Cu2S), organic charge transfer complexes (such as CuTCNQ), organic donor-acceptor systems, various molecular systems, or other non-volatile programmable resistive memory elements.
Memristors, or memristive devices, are nano-scale or micro-scale devices that may be used as a component in a wide range of electronic circuits, such as memories, switches, and logic circuits and systems. In a memory structure, a crossbar of memristors may be used. For example, when used as a basis for memories, the memristor may be used to store a bit of information, 1 or 0, corresponding to whether the memristor is in its high or low resistance state (or vice versa). When used as a logic circuit, the memristor may be employed as configuration bits and switches in a logic circuit that resembles a Field Programmable Gate Array, or may be the basis for a wired-logic Programmable Logic Array. It is also possible to use memristors capable of multi-state or analog behavior for these and other applications.
The memristor may either be in a low resistance (“ON”) or high resistance (“OFF”) state. During the last few years, researchers have made great progress in finding ways to make the switching function of these memristors behave efficiently. For example, tantalum oxide (TaOx)-based memristors have been demonstrated to have superior endurance over other nano-scale devices capable of electronic switching. In lab settings, tantalum oxide-based memristors are capable of over 10 billion switching cycles.
A memristor may comprise a switching material, such as TiOx or TaOx, sandwiched between two electrodes. Memristive behavior is achieved by the movement of ionic species (e.g., oxygen ions or vacancies) within the switching material to create localized changes in conductivity via modulation of a conductive filament and/or tunnel barrier between two electrodes, which results in a low resistance “ON” state, a high resistance “OFF” state, or intermediate states. Initially, when the memristor is first fabricated, the entire switching material may be nonconductive. As such, a forming process may be required to form the conductive channel in the switching material between the two electrodes. A known forming process, often called “electroforming”, includes applying a sufficiently high (threshold) voltage across the electrodes for a sufficient length of time to cause a nucleation and formation of a localized conductive channel (or active region) in the switching material. The threshold voltage and the length of time required for the forming process may depend upon the type of material used for the switching material, the first electrode, and the second electrode, and the device geometry.
Metal or semiconductor oxides may be employed in memristive devices; examples include either transition metal oxides, such as tantalum oxide, titanium oxide, yttrium oxide, hafnium oxide, niobium oxide, zirconium oxide, or other like oxides, or non-transition metal oxides, such as aluminum oxide, calcium oxide, magnesium oxide, dysprosium oxide, lanthanum oxide, silicon dioxide, or other like oxides. Further examples include transition metal nitrides, such as tantalum nitride and titanium nitride.
Prior art memristive devices may include a continuous oxide film between the electrodes. Filaments/ionic diffusion are formed in the oxide film between the electrodes in a random fashion, much like lightning, that may take the path of least resistance. This random path causes variations in the memristor I-V characteristics from switching cycle to cycle and especially from device to device. Prior art memristive or non-volatile resistive memory devices that are either unipolar or bipolar tend to have this random conductive path between the electrodes. This randomness in the conductive channel formation may influence device variability and/or reliability issues.
In a memristor device, the interfaces between the top/bottom electrodes and the switching material may be very sensitive to any contamination because the switching occurs at the interfaces. Presently, in the fabrication of memristors, the interfaces of the electrodes and the active material may suffer contamination from a series of events such as exposing to air, solvent, and deionized water rinsing, plasma treatment, photolithography processes and so on.
In accordance with the teachings herein, a resistive memory device includes a bottom electrode and a top electrode sandwiching a switching layer. The device also includes a field enhancement (FE) feature that extends from the bottom electrode. The FE may extend either into the switching layer or is covered by switching layer, but does not extend all the way to touch top electrode.
The FE feature is to enhance an electric field generated by the two electrodes to confine a switching area of the device at the FE feature. The device further includes a planar interlayer dielectric (ILD) surrounding the device. The ILD electrically isolates the bottom electrode and the top electrode of the device and, being planar, supports the top electrode.
With the FE feature, an electric field is enhanced at the FE feature, thus, a voltage for initial electroforming and/or subsequent driving of the device is reduced. Furthermore, the uniformity and the reliability in operation of the device in an array configuration will be much improved with greater control. The FE features may be formed by a combination of deposition, lithography and etching processes. The term “lithography” may include photolithography, e-beam lithography, ion beam lithography, nanoimprint lithography, nanosphere lithography, and/or block copolymer lithography.
Also in accordance with the teachings herein, a process of fabricating two-dimensional (2D) memristor arrays is provided, using in-situ deposition, e.g., in situ vacuum deposition, of the materials and etching processes to form the bottom electrode and switching element of the resistive memory device. The whole structure is then embedded in a non-conductive material (interlayer dielectric—ILD) which serves multiple purposes of electric isolation, planarization and passivation. A planar surface may be desired for 3D stacking of the memristor arrays for high density devices.
The foregoing process is illustrated in the flow chart depicted in
Other layers may be included as desired, also in the in-situ vacuum deposition. Such other layers may include a diffusion barrier (not shown in
Further in accordance with the teachings herein, solution-based materials such as polymers, monomers, and oligomers may be used as the nonconductive material (ILD) 110 in memristor devices. The materials may also include cross-linkers and photoinitiators, thermal-initiators, or e-beam initiators. SU8, which is a high contrast, epoxy based photoresist designed for micromachining and other microelectronic applications, refers to a series of photoresists available from MicroChem (Newton, Mass.), and may be used in the practice of the teachings herein. These resist materials can be used as a combination insulating layer, planarization layer, and encapsulation layer. Manufacturing methods and processes for using polymeric materials in memristor devices are disclosed.
The details of the fabrication method are now described with reference to
A substrate 112, such as a silicon (Si) wafer, may be provided, on which bottom conductor contact lines and pads 300 may be patterned on an insulating layer 111, such as SiO2, on the substrate. The formation and patterning, such as by photolithography, of the bottom conductor contact lines and pads 300 may be performed by conventional methods. The resulting structure is depicted in
Next, all the materials comprising a memristor bit 304 may be deposited layer by layer in-situ, such as in a sputter vacuum chamber, and the bits may be defined on the bottom conductor 300 such as by photolithography. The bit stack 304 may include the complete set of the materials involved in the memristor device, including, without limitation, a bottom electrode 102, a diffusion barrier 306, the switching material 104, and a non-linear select element 308 (described below). The top electrode 108 on top of the bit stack 304 completes the device. In some examples, one or both of the diffusion barrier 306 and the non-linear select element 308 may be omitted. The patterning of the bits 304 may performed by conventional reactive ion etching, ion beam milling or liftoff process. The resulting structure is depicted in
Thirdly, a planarization and etch back of an insulating layer 110 is carried out as shown in
One example of performing the planarization of the surface of the insulating layer 110 is by conventional chemical mechanical planarization (CMP). Another example of performing the planarization of the surface of the insulating layer 110 is by spin coating or bar coating the insulating material in a liquid form. A solid layer may be formed by evaporating the solvent or by UV/thermal curing. An adhesion promotion treatment may be needed before the coating. The thickness of the coating may be significantly greater to make a planar surface. In some cases, a flat surface (not shown) may be placed on top of the coating with pressure to help to achieve a good planarization. After that, a reactive ion etching may be carried out to etch the insulating material all the way down until the top surface of the memristor stack 304 is exposed, such as at least a portion of the top electrode 108. A slight over etch of a few nanometers may help to ensure that the surface of the top of the memristor stack 304 is clean of any insulating material 110.
Last, the top conductor contact lines and pads 310 may be fabricated in the similar way of the bottom conductor/electrode 300/102, with a diffusion barrier 306′ in some cases. The final structure is depicted in
The bottom conductor and contact pad 300 and bottom electrode 102 may be formed on insulating layer 111 by any of a number of processes, including electroplating, sputtering, evaporation, ALD, co-deposition, chemical vapor deposition, IBD (ion beam deposition), or any other film deposition technology performed in a vacuum. Examples of bottom conductor 300/bottom electrode 102 materials include, but are not limited to, aluminum (Al), copper (Cu), platinum (Pt), tungsten (W), gold (Au), titanium (Ti), silver (Ag), ruthenium dioxide (RuO2), titanium nitride (TiN), tungsten nitride (WN2), tantalum (Ta), tantalum nitride (TaN) or the like, as well as doped silicon. The electrodes 102 may be patterned, if desired. The thickness of the bottom electrode 102 may be in the range of about 10 nm to a few micrometers.
The diffusion barriers 306, 306′ may be any material that prevents metal diffusion, such as tantalum nitride (TaN), titanium nitride (TiN), tungsten silicide (WSi), and tungsten carbide (WC). The diffusion barriers 306, 306′ may be the same or different.
The material of the switching layer 104 may be a metal oxide or semiconductor oxide; examples include either transition metal oxides, such as tantalum oxide, titanium oxide, yttrium oxide, hafnium oxide, niobium oxide, zirconium oxide, or other like oxides, or non-transition metal oxides, such as aluminum oxide, calcium oxide, magnesium oxide, dysprosium oxide, lanthanum oxide, silicon dioxide, or other like oxides. Further examples include transition metal nitrides, such as tantalum nitride and titanium nitride.
The non-linear select element 308 may be a negative differential resistance (NDR) layer, such as NbO2, SnO2, or VO2.
The top electrode 108 may be formed on the non-linear select element 308 (or on the switching layer 104 if the non-linear select element is omitted) by any of the processes listed above for the bottom conductor 300/bottom electrode 102. Examples of top electrode 108 materials include, but are not limited to, the same as those listed above for the bottom conductor 300/bottom electrode 102. The top electrode 108 material may or may not be the same as the bottom conductor 300/bottom electrode, 102 material. The thickness of the top electrode 108 may be in the range of about 10 nm to a few micrometers. The top conductor and contact pads 310 may be supported on the planarized ILD 110, with or without the diffusion barrier 306′ between the top conductor/contact pads and the ILD, to contact the top electrode 108.
A simple photolithographic process may be used to form the interlayer dielectric (ILD) structures 110 by using a photosensitive polymeric material such as SU8 epoxy. The ILD layer 110 can be formed by a simple wet coating technique such as spin casting, gravure coating, spray coating or dip coating. Thickness of the layer can be controlled precisely with a wide range of thicknesses depending on the device requirements. Subsequently, the ILD 110 is exposed to UV light through a photomask and then developed. The ILD structures 110 are formed and cured and become part of the resistive memory device structure.
Alternatively, an imprintable resist may be applied to a wafer, such as by any of the techniques described above. With an imprinted master, the ILD structures 110 can be formed by an imprint process. Once the ILD structures 110 are fully cured, they become part of the resistive memory device structure.
An example process 400 is depicted in
The process 400 continues with depositing 404 all the materials comprising the memristor bit layer 304 layer by layer in-situ on the contact lines 300 and defining the bits 304, including the bottom electrode 102, the diffusion barrier 306, the switching material 104, the non-linear select element 308, the diffusion barrier 306′, and the electrode 108.
The process 400 continues with patterning 406 the bits 304 with a lithography and etching process.
The process 400 then continues with performing 408 a planarization and etch back of the thick insulating layer (ILD) 110, in which the insulating layer is deposited on the wafer that covers all the bit stack 304 materials and then etched back to expose upper portions of the bit stack.
The process 400 then concludes with forming 410 the top contact lines and pads 310 to contact exposed upper portions of the bit stack 304.
Non-uniformity in device forming and operation of a given array of memristors is a well-known issue. This, in turn, leads to concerns about the uniformity and reliability of the array. With field enhanced (FE) features, however, the electric field may be enhanced by as much as three times at the FE features as compared with their surroundings. As a result, the driving voltage to form and activate the switching is reduced accordingly. Furthermore, the switching area of the device is well-defined and confined at the FE feature. This may enhance the uniformity of device performance and reliability.
Manufacturing processes for forming FE features in memristor devices are shown in the attached
A substrate 112 may be provided, with an insulating layer 111, such as SiO2, thereon, as shown in
Bottom electrode 102 may be formed on the insulating layer 111 and then patterned, as shown in
Field enhanced features 106 may be formed on the bottom electrode 102, as shown in
The FE features 106 may have a width (at base) of about 5 to 20 nm and an aspect ratio (height:width) in the range of about 1:1 to 3:1. The composition of the FE features 106 depends on how they are formed. For example, lithography (electron beam—ebeam, ion beam—ibeam—or self-assembly lithography) followed by etching may be employed to form a sharp structure (e.g., needle-like, such as depicted in
The FE features 106 in this example may be more “blocky”, and a few nm in width and height, such as shown in
Switching material 104 may be formed on the patterned bottom electrode 102 with FE feature 106, as shown in
After forming an interlayer dielectric (ILD) 110 on and between memristor bits 100, a continuous top electrode 108 may be formed on the top of the switching material 104 and ILD 110. The ILD 110 may be planarized.
An example process 600 is depicted in
The process 600 continues with forming 606 field enhanced feature 106 on each patterned bottom electrode 102.
The process 600 then continues with forming 608 switching material 104 on the patterned bottom electrode 102 with FE feature 106.
The process 600 concludes with forming 610 interlayer dielectric (ILD) 110 between resistive memristor devices 100 and then forming continuous top electrode 108 contacting the memristor devices, the FE features, and the ILD.
In the first example process, a substrate 112 supporting an insulating layer 111 in turn supporting a bottom electrode 102 of the resistive memory device may be provided, as shown in
In the second example process, in place of the use of a photolithography mask, as described above with reference to
Yet another example process is depicted in
“Block copolymer” refers to a polymer having two or more chemically differentiated polymer blocks. In some examples the blocks have different physical properties such as different degrees of hydrophilicity or hydrophobicity, and are non-miscible over a range of temperatures, which means that the blocks form separate domains in the melt phase. This type of block copolymer can be synthesized by living polymerization methods such as atom transfer radical polymerization (ATRP), fragmentation chain transfer polymerization (RAFT), nitroxide-mediated polymerization (NMP), etc. The molecular weight of the copolymer may be selected so that the end-to-end distance of the block is commensurate with smallest feature of the pattern. In some examples, molecular weights for each block of a block copolymer range from about 200 to 1,000,000 (weight average). The feature sizes may be as small as a few nm. Examples of such block co-polymers include poly(styrene-b-dimethylsiloxane) and poly(methyl methacrylate-b-styrene).
After spin-coating, a baking is carried out, and the block copolymer is phase-segregated. A UV exposure is then followed to break the bonds between blocks. The layer is then developed in a solvent that removes only one material and leave another material (with nano holes or pores) to serve as a mask. See, for example, Craig J. Hawker and Thomas P. Russell, “Block Copolymer Lithography: Merging “Bottom-Up” with “Top-Down” Processes”, MRS BULLETIN⋅VOLUME 30⋅December 2005, p. 952.
The porous polymer layer 800 is used as deposition mask, An FE layer 106′ is then deposited on the surface of the polymer layer 800 and into the pores 802. Once the deposition process is done, the polymer layer 800 is removed, such as by dissolving in solvent, leaving FE features 106 behind. With the FE features 106 on the bottom electrode 102, the switching layer 104, ILD 110, and top electrode 108 are then formed to complete the resistive memory device process as shown above for
The resistive memory device 100 may be used in a memory array.
According to one illustrative example, the intermediate layer 910 may be a dielectric layer, such as ILD 110. A number of the resistive memory devices 912-918 may be formed at the intersections, or junctions, between nanowires 902 in the top layer 906 and nanowires 904 in the bottom layer 908. The nanowires 902, 904 may serve as the top and bottom electrodes 108, 102, respectively, in the resistive memory device 100. For example, when forming a resistive memory device similar to the example shown in
To avoid complicating
For purposes of illustration, only a few of the resistive memory devices 912-918 are shown in
The example above is only one illustrative example of the nanowire memory array 900. A variety of other configurations may be used. For example, the memory array 900 may incorporate nonlinear elements that have different structures. The different structures could include more or less layers, layers that have different compositions than described above, and layers that are ordered in different ways than shown in the example given above. For example, the memory array may include memristors or other memory elements. Further, the memory array may use a wide range of conductors to form the crossbars.
It should be understood that the resistive memory devices, and memristors, described herein, such as the example memristors depicted in the Figures, may include additional components and that some of the components described herein may be removed and/or modified without departing from the scope of the resistive memory device disclosed herein. It should also be understood that the components depicted in the Figures are not drawn to scale and thus, the components may have different relative sizes with respect to each other than as shown therein. For example, the upper, or second, electrode 114 may be arranged substantially perpendicularly to the lower, or first, electrode 106 or may be arranged at some other non-zero angle with respect to each other. Further, deposited layers may or may not be conformal with respect to underlying features.
| Filing Document | Filing Date | Country | Kind |
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| PCT/US2013/052050 | 7/25/2013 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
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| WO2015/012839 | 1/29/2015 | WO | A |
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