Patent application title: Seed-Assisted MOCVD Growth of Threshold Switching and Phase-Change Materials
Smuruthi Kamepalli (Rochester, MI, US)
Tyler Lowrey (Rochester Hills, MI, US)
IPC8 Class: AC23C2800FI
Class name: Integrated circuit, printed circuit, or circuit board multilayer coating hole wall
Publication date: 2008-11-20
Patent application number: 20080286446
A method for forming electrically stimulable materials, including
programmable resistance and electrical switching materials, in high
aspect ratio features. The method includes forming a seed layer in the
recessed portion of a feature and using the seed layer to direct the
vapor phase deposition of an electrically stimulable material. The seed
layer may provide nucleation sites that lead to preferential deposition
of the electrically stimulable material on the seed layer relative to the
sidewalls of the feature. The seed layer may promote the formation of a
finely crystalline morphology of the electrically stimulable material to
facilitate deposition in the recessed portions of a feature and inhibit
blocking of the top of the feature by large crystals.
1. A method for forming an electronic device comprising:providing a
substrate, said substrate supporting a first layer, said first layer
having an opening formed therein, said opening having a sidewall;forming
a seed layer within said opening; andforming an active material within
said opening on said seed layer.
2. The method of claim 1, wherein said first layer comprises a dielectric material.
3. The method of claim 2, wherein said dielectric material is an oxide or nitride.
4. The method of claim 1, wherein said opening is a hole or trench.
5. The method of claim 1, wherein said substrate further supports a second layer, said second layer being interposed between said substrate and said first layer.
6. The method of claim 5, wherein said second layer is a conductive layer.
7. The method of claim 6, wherein said opening exposes the top surface of said second layer.
8. The method of claim 7, wherein said seed layer contacts said second layer.
9. The method of claim 8, wherein said seed layer contiguously covers said second layer.
10. The method of claim 8, wherein said seed layer forms a plurality of spatially-separated discrete regions on said second layer.
11. The method of claim 8, wherein said opening has an aspect ratio of at least 1:1.
12. The method of claim 8, wherein said opening has an aspect ratio of at least 3:1.
13. The method of claim 8, wherein a lateral dimension of said opening is less than 1000 Å.
14. The method of claim 8, wherein a lateral dimension of said opening is less than 500 Å.
15. The method of claim 8, wherein a lateral dimension of said opening is less than 300 Å.
16. The method of claim 1, wherein said seed layer comprises crystalline regions.
17. The method of claim 16, wherein said crystalline regions include nucleation sites, said active material forming on said nucleation sites, said nucleation sites providing a rate of deposition of said active material higher than the rate of deposition of said active material on said sidewall of said opening.
18. The method of claim 1, wherein said seed layer has a first crystallographic structure and said active material has a second crystallographic structure.
19. The method of claim 1, wherein said active material forms as an amorphous material.
20. The method of claim 1, wherein said seed layer has a first chemical composition and said active material has a second chemical composition.
21. The method of claim 20, wherein said seed layer has a first concentration of Ge and said active material has a second concentration of Ge.
22. The method of claim 21, wherein said seed layer further has a first concentration of Sb and said active material further has a second concentration of Sb.
23. The method of claim 1, wherein said active material comprises an electrically stimulable material.
24. The method of claim 23, wherein said active material is a programmable resistance material or an electrical switching material.
25. The method of claim 1, wherein said active material comprises a chalcogen element.
26. The method of claim 25, wherein said active material further comprises Ge, Sb, or In.
27. The method of claim 1, wherein said active material fills the portion of said opening not occupied by said seed layer.
28. The method of claim 1, wherein said active material is formed by a chemical vapor deposition process.
29. The method of claim 28, wherein said seed layer is formed by a physical deposition process.
30. The method of claim 1, wherein the average crystallite size of said active material deposited on said seed layer is smaller than the average crystallite size of said active material when it deposits on said sidewall of said opening.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent application Ser. No. 11/046,114, entitled "Chemical Vapor Deposition of Chalcogenide Materials" and filed Jan. 28, 2005, the disclosure of which is incorporated in its entirety herein.
FIELD OF INVENTION
This invention relates to a process for preparing threshold switching and phase-change materials. More particularly, this invention relates to a chemical vapor deposition process for forming threshold switching and phase-change materials that utilizes a seed material to regulate the as-deposited crystal size. Most particularly, this invention relates to formation of threshold switching and phase-change materials in features with restricted dimensions.
BACKGROUND OF THE INVENTION
Programmable resistance materials and threshold switching materials are promising active materials for next-generation electronic storage, computing and signal transfer devices. A programmable resistance material possesses two or more states that differ in electrical resistance. The material can be programmed back and forth between the states by providing energy to induce an internal chemical, electronic, or physical transformation of the material that manifests itself as a change in resistance of the material. The different resistance states can be used as memory states to store or process data.
Threshold switching materials are capable of being switched between a relatively resistive state (a quiescent low conduction state) and a relatively conductive state. Application of an energy signal, frequently an electrical energy signal, induces the change from the relatively resistive state to the relatively conductive state. The relatively conductive state persists for so long as the energy signal is applied. Once the energy signal is removed, the switching material relaxes back to its quiescent state. Devices that incorporate switching materials are useful as voltage clamping devices, surge suppression devices, signal routing devices, and solid state memory access devices.
Phase change materials are a promising class of programmable resistance materials. A phase change material is a material that is capable of undergoing a transformation, preferably reversible, between two or more distinct structural states. In a common embodiment, a phase change material is reversibly transformable between a crystalline state and an amorphous state. In the crystalline state, the phase change material has lower resistivity; while in the amorphous state, it has higher resistivity. The distinct structural states of a phase change material may be distinguished on the basis of, for example, crystal structure, atomic arrangement, order or disorder, fractional crystallinity, relative proportions of two or more different structural states, a physical (e.g. electrical, optical, magnetic, mechanical) or chemical property etc. Reversibility of the transformations between structural states permits reuse of the material over multiple cycles of operation.
Typically, a programmable resistance material or switching device is formed by placing an active material, such as a phase change material, between two electrodes. Operation of the device is effected by providing an electrical signal between the two electrodes and across the active material. Programmable resistance materials may be used as the active material of a memory device. Write operations in a memory device, which may also be referred to herein as programming operations, apply electric pulses to the memory device. Read operations, which measure the resistance or threshold voltage of the memory device, are performed by providing current or voltage signals across the two electrodes. The transformation between the relatively resistive state and relatively conductive state of a switching material is similarly induced by providing a current or voltage signal between two electrodes in contact with the switching material.
One of the practical challenges that programmable resistance memory devices and switching devices face is a minimization of the energy required to program or switch the active material. Strategies for reducing the energy of operation include modifying the chemical composition of the active material, incorporating an inactive material along with the active material in the region between the electrodes, reducing the volume of active material, and/or reducing the contact area of one or more electrodes contacting the active material.
Fabrication of electronic devices, including switching, memory, and logic devices, typically includes a number of processes that are used to form various features and layers on a surface of a semiconductor wafer or other appropriate substrate. Physical vapor deposition (PVD), chemical vapor deposition (CVD), and other deposition processes involving the reaction, decomposition or coating of gaseous, liquid, or solid precursors may be used in the formation of semiconductor and other electronic devices. Lithography is a patterning process in the formation of electronic devices that is commonly used to define small-scale features and often sets a limit on the extent of device miniaturization. Additional fabrication processes include chemical-mechanical polishing (CMP), etching, annealing, ion implantation, plating, and cleaning. In normal fabrication, an array containing a large number of electronic devices is formed on a semiconductor wafer.
In electronic device fabrication, it is desirable to reduce the length scale or feature size of devices as much as possible so that a greater number of devices can be formed per unit substrate area. As the feature size of devices is minimized, however, processing of the devices becomes more difficult. Small scale features become more difficult to define as the lithographic limit of resolution is reached and features that are defined become more difficult to process.
A common step in processing involves depositing a layer and forming an opening in it. Openings such as channels, trenches, holes, vias, pores or depressions in layers are commonly employed to permit interconnections between devices or layers of a structure. Typically, the opening is formed by lithography, then etching, and is subsequently filled with another material. As the dimension or length scale of an opening decreases upon miniaturization, it becomes increasingly difficult to fill the opening with another material without compromising performance or durability.
Techniques such as physical vapor deposition (PVD) or sputtering fail to provide dense or complete filling of openings when the dimensions of the opening are reduced below a critical size. Instead of providing a dense, uniform filling, these techniques increasingly incompletely fill openings as the feature size of the opening decreases. As the feature size decreases, there is a tendency for the packing density of the material formed in the opening to vary in the depth or lateral dimensions of the opening and as a result, the layer deposited within the opening may include voids, vacancies, gaps, pores, keyholes, or other regions of non-uniform coverage. Imperfections in the filling of openings become especially pronounced as the aspect ratio (ratio of the depth dimension to the lateral dimension of the feature) of the opening increases. Deep, narrow channels, for example, are more difficult to fill uniformly than channels that are shallow and wide. With deep, narrow features, sputtering and other physical deposition techniques are oftentimes unable to deliver sufficient material to the bottom of the feature. Instead, a layer of material is formed over or only near the top of the feature and the lower part of the feature is blocked and remains largely unfilled. Lack of structural uniformity in the filling of openings compromises performance because: (1) variations in device characteristics occur across an array due to differences in the degree or nature of filling non-uniformities from device-to-device and (2) less than optimal performance is achieved for each device due to the defective nature of the material within the opening.
Conformality of deposition is another processing difficulty that becomes exacerbated as feature size decreases. Fabrication of electronic devices generally involves forming a stack of layers, where the individual layers may differ in dimensions (lateral to or normal to the substrate) and compositions. The process of fabricating an electronic device generally involves sequential deposition of one layer upon a lower (previously formed) layer. Optimal device performance requires conformality of later-formed layers with earlier-formed layers. Each layer in a stack must conform to the shape and contours of the layer in the stack upon which it is formed. Uniform coverage and good adhesion are desired.
In addition to difficulties with achieving uniform filling, openings also present complications for achieving conformal deposition that become more pronounced as the size of the opening decreases. The boundary or perimeter of an opening is frequently defined by an edge, step, or other relatively discontinuous feature. The shape of an opening is generally defined by a sidewall or perimeter boundary and a lower surface or bottom boundary. A trench opening, for example, is defined by generally vertical sidewalls and a bottom surface that is generally parallel to the substrate.
When fabricating electronic devices, it is often necessary to first form a layer with an opening and to subsequently deposit another layer over this layer. Conformality requires that the subsequent layer faithfully conform to the shape and texture of the underlying layer having the opening. The subsequent layer must deposit uniformly over both the portion of the underlying layer in which the opening has not been formed as well as over the opening itself. Conformality over the opening requires uniform coverage of the edges or steps that form the boundary of the opening. Achieving conformality over discontinuous features becomes increasingly difficult as the feature size of the opening decreases or the aspect ratio of the opening increases.
Fabrication of programmable resistance and switching devices often includes a step of forming an opening in a dielectric layer and filling the opening with an active programmable or switching material. Miniaturization of programmable resistance and switching devices requires methods for reducing the dimensions of the active material. A reduction in the volume of active material is beneficial because the energy required for operation of programmable resistance and switching devices decreases with decreasing dimensions of the active material. Accordingly, it is desirable to develop techniques for forming and filling openings with small dimensions without suffering from the imperfections in filling and conformality associated with standard prior art techniques such as sputtering or physical vapor deposition. Ideally, the techniques would enable the fabrication of active materials for programmable resistance and switching devices having dimensions near, at or below the lithographic limit.
Referring to the drawings, FIG. 1 depicts a representative structure of a phase-change material device that illustrates the nature of the type of imperfections that may form in an active programmable resistance or switching material having a sublithographic dimension when the material is deposited via physical vapor deposition. A conductive layer 106 is formed over a substrate 102 and serves as a lower electrical contact. An insulative layer 110 having an opening formed therein is then formed over conductive layer 106. Active material 128 is formed in the opening of insulative layer 110 using a physical vapor deposition process and planarization. A barrier layer 114 is then deposited onto lower electrical contact 128 and a top electrode layer 116 is deposited over barrier layer 114. Active material 128 includes imperfections in the form of internal voids 120 and non-conformal region 112 that detract from device performance and durability.
To improve the quality of active materials in high aspect ratio devices, new methods for forming active materials in openings with reduced dimensionality are needed. The methods must provide uniform filling of openings with active material as well as greater conformality with underlying and surrounding layers than the prevailing methods.
SUMMARY OF THE INVENTION
The instant invention provides electronic devices having logic, memory, switching, or processing functionality based on programmable resistance materials, switching materials, chalcogenide materials, or other active materials and methods of fabricating same.
In accordance with one embodiment of the instant invention, a programmable resistance or switching device includes a substrate with a plurality of stacked layers including a bottom electrode, an insulator layer having an opening formed therein that exposes the bottom electrode, an active material formed in the opening, and a top electrode layer deposited over the active material. The electrode layers may be conductive strips or plugs and may also be in electrical communication with addressing lines or external circuitry.
The active material may be a programmable resistance material, switching material or other electronic material. Representative active materials include chalcogenide materials, pnictide materials, phase-change materials, and threshold switching materials.
The opening may be round, elliptical, bent, rectilinear or other circumferential shape. In one embodiment, the opening is a circular hole that is filled or lined with an active material. In another embodiment, the opening is a trench that is filled or lined with an active material. The opening has an aspect ratio that ranges between 0.25 and 5.
The methods for forming the active material include chemical vapor deposition, atomic layer deposition, selective deposition, and solution deposition. The deposition method includes formation of a seed layer in an opening or other region where deposition of the active material is desired. The seed layer may form as a contiguous or continuous layer or as discrete regions in a recessed portion of the opening. The seed layer influences one or more of the growth rate, crystal structure, crystal size, conformality, composition, electrical properties, and adhesion characteristics of the active material. In one embodiment, the seed layer promotes the formation of an active material with finer crystallinity, where the reduced crystal size facilitates the filling of an opening. In another embodiment, the active material forms preferentially on the seed layer relative to the sidewall surface of the opening to permit more unobstructed access of active material deposition precursors to the deeper portions of the opening.
For a better understanding of the instant invention, together with other and further illustrative objects thereof, reference is made to the following description, taken in conjunction with the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic depiction of a conventional two-terminal electronic device having defects in an opening region of the two terminal device.
FIG. 2 depicts a schematic resistance vs. energy/current plot for one type of phase-change material.
FIG. 3 depicts a schematic current-voltage plot of a representative material that exhibits a switching transformation.
FIG. 4 is a schematic depiction of representative CVD precursors for depositing materials containing Ge, Sb, and/or Te.
FIG. 5 is a high magnification (6000×) image of a Ge--Sb--Te ternary chalcogenide deposited by CVD.
FIG. 6A depicts a representative portion of a device structure that includes an opening that exposes the top surface of a supporting layer.
FIG. 6B illustrates obstruction of an opening in a device structure by an active material having a tendency to deposit in the form of large crystallites.
FIG. 6C illustrates obstruction of an opening in a device structure by an active material that preferentially deposits on the sidewall of an opening relative to the exposed surface at the bottom of the opening.
FIG. 6D illustrates obstruction of an opening in a device structure incipient to the formation of an internal void.
FIG. 6E illustrates an internal void or keyhole defect in an active material deposited within an opening.
FIGS. 7A-7C illustrate deposition of an active material on a contiguous seed layer formed at the bottom of an opening of a device structure.
FIGS. 8A-8E illustrate deposition of an active material on discrete regions of seed material formed at the bottom of an opening of a device structure.
FIG. 9 is a cross-sectional FIB-SEM micrograph of a blanket film of a Ge--Sb--Te alloy deposited by CVD on a seed layer.
FIG. 10 is a schematic depiction of a representative device structure including an active material deposited on a seed layer formed in an opening of a dielectric layer.
FIG. 11 is a cross-sectional FIB-SEM micrograph of a Ge--Sb--Te phase-change material deposited by CVD on a Ge2Sb2Te5 seed layer formed by PVD within an opening of a dielectric layer.
FIG. 12 shows I-V characteristics of the device shown in FIG. 11.
FIG. 13 shows the R-I characteristics of the device shown in FIG. 11.
FIG. 14 shows the variation of several electrical parameters of the device shown in FIG. 11 over multiple cycles of operation.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
Although this invention will be described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this invention. Accordingly, the scope of the invention is defined only by reference to the appended claims.
The instant invention relates generally to electronic devices that include two or more electrodes in contact or electrical communication with an active material. As used herein, active material refers generally to an electrically stimulable material such as a programmable resistance material that can, for example, be used for memory, programmable logic, or other applications; other memory material; or electrical switching material. A programmable resistance material is a material having two or more states that are distinguishable on the basis of electrical resistance. The two or more states may be structural states, chemical states, electrical states, optical states, magnetic states, or a combination thereof. A programmable resistance material is transformable ("programmable") between any pair of states by supplying an appropriate amount of energy to the material. The supplied energy may be referred to as a "programming energy". When transformed ("programmed") to a particular state, the programmable resistance material remains in that state until additional energy is supplied to the material. The different states of a programmable resistance material are stable in the absence of external energy and persist for an appreciable amount of time upon removal of the source of programming energy. Programmable resistance materials include phase-change materials, chalcogenide materials, pnictide materials, and other multi-resistance state materials.
Phase change materials include materials that are transformable between two or more crystallographically-distinct structural states. The states may differ in crystal structure, unit cell geometry, unit cell dimensions, degree of disorder, particle size, grain size, or composition. Chalcogenide materials are materials that include an element from Column VI of the periodic table as a significant component along with one or more modifying elements from Columns III (e.g. B, Al, Ga, In), IV (e.g. Si, Ge, Sn), and/or V (e.g. Sb, Bi, P, As) of the periodic table. Transition metals (e.g. Ag, Ni, Cu, Cr, V, Fe) may also be included as modifiers. Representative chalcogenide materials include GaSb, InSb, InSe, Sb2Te3, GeTe, SnSb2Te4, Te81Ge15Sb2S2, Ge--Sb--Te alloys (e.g. Ge2Sb2Te5), Ge--Sb--Te--Se alloys, In--Sb--Te alloys, In--Sb--Ge alloys, Ga--Se--Te alloys, Ag--In--Sb--Te alloys, Ge--Sb--Bi--Te alloys, In--Sb--Bi--Te alloys, Te--Ge--Sb--S alloys, Ge--Sn--Sb--Te alloys.
Pnictide materials are materials that include an element from Column V of the periodic table as a significant component along with one or more modifying elements from Columns III, IV, or VI of the periodic table. Many chalcogenide and pnictide materials are phase-change materials that are transformable between and among a plurality of crystalline, partially crystalline, and amorphous states. Other multi-resistance state materials include metal-insulator-metal structures with thin film insulators, or conductive oxide materials such as a family of CuO materials used in RRAM devices. Programmable resistance materials may serve as the active material in memory devices, including non-volatile memory devices. Representative programmable resistance materials in accordance with the instant invention are described in U.S. Pat. Nos. 5,543,737; 5,694,146; 6,087,674; 6,967,344; 6,969,867; 5,757,446; 5,166,758; 5,296,716; 5,534,711; 5,536,947; 5,596,522; 6,087,674; and 7,020,006 as well as in U.S. patent application Ser. Nos. 11/200,466 and 11/301,211; all of which disclosures are incorporated by reference herein. Co-pending U.S. patent application Ser. No. 11/046,114 incorporated by reference hereinabove also provides relevant information about the basic operational characteristics and compositions of chalcogenide phase-change materials.
For illustrative purposes, a representative depiction of the electrical resistance (R) of a phase-change type of programmable resistance material as a function of electrical energy or current pulse magnitude (Energy/Current) is presented in FIG. 2 herein. FIG. 2 shows the variation of the electrical resistance of a chalcogenide phase-change material resulting from electrical energy or current pulses of various magnitude and may generally be referred to as a resistance plot.
The resistance plot includes two characteristic response regimes of a chalcogenide phase-change material to electrical energy. The regimes are approximately demarcated with the vertical dashed line 10 shown in FIG. 2. The regime to the left of the line 10 may be referred to as the accumulating regime of the chalcogenide material. The accumulation regime is distinguished by a nearly constant or gradually varying electrical resistance with increasing electrical energy that culminates in an abrupt decrease in resistance at and beyond a critical energy (which may be referred to herein as the set energy). The accumulation regime thus extends, in the direction of increasing energy, from the leftmost point 20 of the resistance plot, through a plateau region (generally depicted by 30) corresponding to the range of points over which the resistance variation is small or gradual to the set point or state 40 that follows an abrupt decrease in electrical resistance. The plateau 30 may be horizontal or sloping. The left side of the resistance plot is referred to as the accumulating regime because the structural state of the chalcogenide material continuously evolves as energy is applied, with the fractional crystallinity of the structural state correlating with the total accumulation of applied energy. The leftmost point 20 corresponds to the structural state in the accumulating regime having the lowest fractional crystallinity and may be referred to as the reset state. This state may be fully amorphous or may be primarily amorphous with some degree of crystalline content.
As energy is added to the reset state, the fractional crystallinity increases, and the chalcogenide material transforms in the direction of increasing applied energy among a plurality of partially-crystalline states along the plateau 30. Selected accumulation states (structural states in the accumulation region) are marked with squares in FIG. 2. Upon accumulation of a sufficient amount of applied energy, the fractional crystallinity of the chalcogenide material increases sufficiently to effect a setting transformation characterized by a dramatic decrease in electrical resistance and stabilization of the set state 40. The structural states in the accumulation regime may be referred to as accumulation states of the chalcogenide material. Structural transformations in the accumulating regime are unidirectional in the sense that they progress in the direction of increasing fractional crystallinity within the plateau region 30 and are reversible only by first resetting the chalcogenide material to reset state 60. The behavior illustrated in FIG. 2 is reproducible over many cycles of setting and resetting a device containing a chalcogenide material by applying the requisite energy or current. Once the reset state is obtained, lower amplitude current pulses can be applied and the accumulation response of the chalcogenide material can be retraced. It is thus possible to cycle between the set and reset states over multiple cycles, a necessary feature for long memory cycle life.
The regime to the right of the line 10 of FIG. 2 may be referred to as the grayscale or direct overwrite regime. The direct overwrite regime extends from the set state 40 through a plurality of intermediate states (generally depicted by 50) to a reset point or state 60. The various points in the direct overwrite regime may be referred to as grayscale or direct overwrite states of the chalcogenide material. Selected direct overwrite states are marked with circles in FIG. 2. Structural transformations in the direct overwrite regime may be induced by applying an electric current or voltage pulse to a chalcogenide material. In FIG. 2, an electric current pulse is indicated. In the direct overwrite regime, the resistance of the chalcogenide material varies with the magnitude of the applied electric pulse. The resistance of a particular direct overwrite state is characteristic of the structural state of the chalcogenide material, and the structural state of a chalcogenide material is dictated by the magnitude of the current pulse applied. In this regime, the fractional crystallinity of the chalcogenide material decreases as the magnitude of the current pulse increases. The fractional crystallinity is highest for direct overwrite states at or near the set point 40 and progressively decreases as the reset state 60 is approached. The chalcogenide material transforms from a structural state possessing a contiguous crystalline network at the set state 40 to a structural state that is amorphous, substantially amorphous or partially-crystalline without a contiguous crystalline network at the reset state 60. The application of current pulses having increasing magnitude has the effect of converting portions of the crystalline network into an amorphous phase and ultimately leads to a disruption or interruption of contiguous high-conductivity crystalline pathways in the chalcogenide material. As a result, the resistance of the chalcogenide material increases as the magnitude of an applied current pulse increases in the grayscale region.
In contrast to the accumulating region, structural transformations that occur in the direct overwrite region are reversible and bi-directional. As indicated hereinabove, each state in the direct overwrite region may be identified by its resistance and a current pulse magnitude, where application of that current pulse magnitude induces changes in fractional crystallinity that produce the particular resistance value of the state. Application of a subsequent current pulse may increase or decrease the fractional crystallinity relative to the fractional crystallinity of the initial state of the chalcogenide material. If the subsequent current pulse has a higher magnitude than the pulse used to establish the initial state, the fractional crystallinity of the chalcogenide material decreases and the structural state is transformed from the initial state in the direction of the reset state along the resistance curve in the direct overwrite regime. Similarly, if the subsequent current pulse has a lower magnitude than the pulse used to establish the initial state, the fractional crystallinity of the chalcogenide material increases and the structural state is transformed from the initial state in the direction of the set state along the resistance curve in the direct overwrite regime.
In OUM (Ovonic Unified (or Universal) Memory) applications, the direct overwrite states of the chalcogenide material are used to define memory states of a memory device. Most commonly, the memory devices are binary memory devices that utilize two of the direct overwrite states as memory states, where a distinct data value (e.g. "0" or "1") is associated with each state. Each memory state thus corresponds to a distinct structural state of the chalcogenide material and readout or identification of the state can be accomplished by measuring the resistance of the material (or device) since each structural state is characterized by a distinct resistance value as exemplified by the range of direct overwrite states 50 in FIG. 2. The operation of transforming a chalcogenide material to the structural state associated with a particular memory state may be referred to herein as "programming" the chalcogenide material or "writing to" the chalcogenide material or storing information in the chalcogenide material.
To facilitate readout and to minimize readout error, it is desirable to select the memory states of a binary memory device so that the contrast in resistance of the two states is large. Typically the set state (or a state near the set state) and the reset state (or a state near the reset state) are selected as memory states in a binary memory application. The resistance contrast depends on details such as the chemical composition of the chalcogenide, the thickness of the chalcogenide material in the device and the geometry of the device. For a layer of phase-change material having the composition Ge22Sb22Te55, a thickness of ˜600 A, and pore diameter of below ˜0.1 μm in a typical two-terminal device structure, for example, the resistance of the reset state is ˜100-1000 kΩ and the resistance of the set state is under ˜10 kΩ. Phase-change materials in general show resistances in the range of ˜50 kΩ to ˜2000 kΩ in the reset state and resistance of ˜0.5 kΩ to ˜50 kΩ in the set state. In the preferred phase-change materials, the resistance of the reset state is at least a factor of two, and more typically an order of magnitude or more, greater than the resistance of the set state. In addition to binary (single bit) memory applications, chalcogenide materials may be utilized as non-binary or multiple bit memory materials by selecting three or more states from among the direct overwrite states and associating a data value with each state, where each memory state corresponds to a distinct structural state of the chalcogenide and is characterized by a distinct resistance value.
Electrical switching materials are materials that are switchable between two states that differ in electrical conductivity. The two states range in conductivity from the relatively resistive (e.g. comparable to a dielectric) to the relatively conductive (e.g. comparable to a metal). Electrical switching materials generally have a quiescent or relaxed state, usually a relatively more resistive state, in which they exist in the absence of electrical energy. When electrical energy is applied, the switching material transforms to the more conductive state and persists in that state transitorily for so long as it is subjected to a critical amount of energy from an external source. When the external energy decreases below the critical level, the switching material relaxes back to its quiescent state. Switching materials include OTS (Ovonic Threshold Switch) materials, negative differential resistance materials, and metal-insulator-metal structures. Certain chalcogenide and pnictide compositions exhibit electrical switching, including solely within an amorphous crystallographic phase. Illustrative switching materials include those described in U.S. Pat. Nos. 6,967,344 and 6,969,867, as well as in U.S. patent application Ser. No. 11/046,114, incorporated by reference hereinabove.
The electrical switching properties of one type of electrical switching material in accordance with the instant devices are schematically illustrated in FIG. 3, which shows the I-V (current-voltage) characteristics of a chalcogenide Ovonic threshold switching material. The illustration of FIG. 3 corresponds to a two-terminal device configuration in which two spacedly disposed electrodes are in contact with a chalcogenide switching material and the current I corresponds to the current passing between the two electrodes. The I-V curve of FIG. 3 shows the current passing through the chalcogenide material as a function of the voltage applied across the material by the electrodes. The I-V characteristics of the material are symmetric with respect to the polarity of the applied voltage. For convenience, we consider the first quadrant of the I-V plot of FIG. 3 (the portion in which current and voltage are both positive) in the brief discussion of chalcogenide switching behavior that follows. An analogous description that accounts for polarity applies to the third quadrant of the I-V plot.
The I-V curve includes a resistive branch and a conductive branch. The branches are labeled in FIG. 3. The resistive branch corresponds to the branch in which the current passing through the material increases only slightly as the voltage applied across the material increased. This branch exhibits a small slope in the I-V plot and appears as a nearly horizontal line in the first and third quadrants of FIG. 3. The conductive branch corresponds to the branch in which the current passing through the material increases significantly upon increasing the voltage applied across the material. This branch exhibits a large slope in the I-V plot and appears as a nearly vertical line in the first and third quadrants of FIG. 3. The slopes of the resistive and conductive branches shown in FIG. 3 are illustrative and not intended to be limiting, the actual slopes will depend on the chemical composition of the switching material. Regardless of the actual slopes, the conductive branch necessarily exhibits a larger slope than the resistive branch. When device conditions are such that the switching material is described by a point on the resistive branch of the I-V curve, the switching material or device may be said to be in a resistive state. When device conditions are such that the switching material is described by a point on the conductive branch of the I-V curve, the switching material or device may be said to be in a conductive state.
The switching properties of the material can be described by reference to FIG. 3. We consider a two-terminal device configuration and begin with a device that has no voltage applied across it. When no voltage is applied across the chalcogenide material, the material is in a resistive state and no current flows. This condition corresponds to the origin of the I-V plot shown in FIG. 3. The chalcogenide remains in a resistive state as the applied voltage is increased, up to a threshold voltage (labeled Vt in the first quadrant of FIG. 3). The slope of the I-V curve for applied voltages between 0 and Vt is small in magnitude and indicates that the switching material has a high electrical resistance. The high resistance implies low electrical conductivity and as a result, the current flowing through the material increases only weakly as the applied voltage is increased. Since the current through the material is very small, the resistive state of the material may be referred to as the OFF state of the material.
When the applied voltage equals or exceeds the threshold voltage, the switching material transforms from the resistive branch to the conductive branch of the I-V curve. The switching event occurs nearly instantaneously and is depicted by the dashed line in FIG. 3. Upon switching, the device voltage decreases significantly and the device current becomes much more sensitive to changes in the device voltage. The chalcogenide material remains in the conductive branch as long as a minimum current, labeled Ih in FIG. 3, is maintained.
Chemical vapor deposition, hereinafter referred to as CVD, is a method that offers the prospect of uniform, conformal filling of high aspect ratio openings in electronic device fabrication. As used herein, CVD refers to all forms of chemical vapor deposition, including metalorganic chemical vapor deposition (MOCVD) and plasma enhanced chemical vapor deposition (PECVD). In the CVD process, precursors of the constituent elements of a material are reacted to produce a thin film on a substrate. The reaction of the CVD precursors may occur homogeneously in the gas phase or heterogeneously at the solid-gas interface of the substrate surface. Precursors for many elements are available and a variety of thin film compositions can be synthesized using CVD.
In CVD processing, precursors are introduced into a reactor in gas phase form. Precursors that are in the gas phase at room conditions or upon heating are directly introduced into the reactor, typically in diluted form via a carrier gas. Liquid and solid phase precursors are vaporized or sublimed and then introduced into the reactor, also typically in diluted form in the presence of a carrier gas. Upon introduction into the reactor, precursors containing the chemical constituents of the desired material are reacted or decomposed (thermally, photochemically, or in a plasma) to form a thin film of desired composition on a substrate or underlying structure. The rate of deposition, stoichiometry, composition and morphology of the film can be varied through appropriate control over process parameters such as reaction temperature; substrate; selection of precursors; reactor pressure; and the rate of introduction of precursors into the reactor. CVD offers the advantages of providing high purity thin films at relatively low temperatures. Since CVD precursors or intermediates have molecular dimensions, they can readily access the recessed portions of high aspect ratio features. As a result, CVD processes promote dense, uniform filling of openings. CVD processes are also conformal in nature and provide smooth coverage of underlying, sloped, vertical walled, and adjacent surfaces.
Co-pending U.S. patent application Ser. No. 11/046,114 incorporated by reference hereinabove describes several CVD processes for forming chalcogenide, phase-change, and electrical switching materials. In one example, tris(dimethylamino)antimony (Sb(N(CH3)2)3), diisopropyltellurium (Te(CH(CH3)2)2) and isobutylgermane, (H3Ge(i-C4H9)) were used as precursors for Sb, Te, and Ge, respectively, to form Ge--Sb--Te alloys. The precursors were placed in separate bubblers and delivered in vapor phase form with a carrier gas to a heated substrate in a CVD process. The chemical formulas of the precursors are shown in FIG. 4 and a representative deposition product 150 is shown in FIG. 5. Analysis of the deposition product 150 indicated that its composition was Ge2Sb2Te5.
To further improve the utility of CVD as a deposition method for forming programmable resistance and electrical switching materials, it is desirable to achieve a degree of control over the microstructure of the deposition product. The deposition product 150 shown in FIG. 5 includes crystallites of Ge2Sb2Te5 having a heterogeneous distribution of crystal sizes in the range from below ˜1 μm to ˜10 μm. The presence of micron scale crystallites in the deposition product complicates the objective of achieving a dense, uniform fill of sub-micron features. Formation of a several-micron-sized crystallite over an unfilled feature, for example, blocks access to the feature and prevents deposition within the feature. It is desirable to develop in situ methods for controlling crystallite size through judicious control of processing parameters such as deposition pressure, deposition temperature, flow rates, dilution, precursor selection, time etc.
In this invention, deposition of active materials, including programmable resistance materials and electrical switching materials, is controlled through the use of a seed layer. The seed layer is formed on a deposition surface or within an opening prior to deposition of the active material. The seed layer functions as a template that directs the growth of the active material. By controlling the characteristics of the seed layer (composition, crystal size, crystal structure, coverage etc.), the instant inventors have discovered that the crystal size and/or crystallographic structure of the active material can be influenced. In particular, use of a seed layer permits a reduction in crystal size of the active material and results in a finer crystallinity that facilitates unobstructed formation of the active material within openings over a wider range of deposition conditions. The seed layer may also direct growth away from the sidewalls of an opening to prevent blockage that impedes formation of the active material in the deeper portions of the opening. Use of a seed layer thus promotes growth of the active material from the bottom or recessed portions of an opening and minimizes the occurrence of voids or other defects within the opening. Greater conformality and uniformity of fill can be achieved as a result.
FIGS. 6A-6E illustrate complications that may arise during active material formation in the absence of a seed layer in a high aspect ratio feature. FIG. 6A depicts a representative electronic device at an intermediate stage of fabrication. The device 150 includes substrate 152, lower conductive layer 155, and dielectric layer 160 with an opening 165 formed therein. Dielectric layer 160 includes a top surface 162 and a sidewall surface 164. FIG. 6B depicts the structure shown in FIG. 6A after deposition of active material 154, where active material 154 forms as a large crystallite. When the crystallite dimensions of active material 154 are comparable to or larger than the lateral dimension of opening 165, active material 154 may occlude opening 165 and prevent formation of active material 154 in the lower portions 166 of opening 165. The net result is a poorly filled opening and a device with inferior properties.
FIG. 6C shows the device of FIG. 6A after deposition of active material 154 having a small crystallite size, but where active material 154 has a tendency to form on sidewalls 164 of opening 165. As the aspect ratio of opening 165 increases, it becomes increasingly difficult to achieve deposition of active material 154 in the lower portions of opening 165. Instead, active material 154 forms along the upper portion of sidewall 164 of opening 165. Once an initial portion of active material 154 forms, subsequent deposition of active material 154 normally occurs preferentially on such initial portion and as a result, opening 165 may fill incompletely as growth of active material 154 in the upper regions of opening 165 occludes the lower regions of opening 165. FIGS. 6D and 6E show the evolution of the structure shown in FIG. 6C upon subsequent growth of active material 154. FIG. 6D illustrates the tendency of the growing active material 154 to pinch access to the lower portions of opening 165 and FIG. 6E illustrates a "keyhole" or internal void 167 that ultimately forms in active material 154 in opening 165. The tendency of active material 154 to form on sidewalls may also lead to device structures having incompletely filled openings such as the structure shown in FIG. 1.
In one embodiment of the instant invention, the seed layer is formed at the bottom of an opening and its presence creates a preferential tendency for subsequent deposition of an active material from a vapor phase growth environment to occur on or near the seed layer toward the bottom of the opening. Preferential formation of the active material in the deeper, recessed portions of an opening promotes complete filling of an opening and diminishes complications that occur when the active material preferential forms on the walls of the opening or at the top of the opening. Preferential formation on the walls or top of an opening impedes access of vapor phase deposition species to the underlying lower portions of the opening and leads to the development of pores, keyholes or gaps within the active material in the opening as described hereinabove.
In another embodiment of the instant invention, the seed layer promotes the formation of the active material with a reduced crystallite size. In this embodiment, the seed layer promotes the formation of active material with a finer grain size and smaller average crystal dimensions than would occur from a given set of growth conditions in the absence of the seed layer. Finer crystallinity facilitates formation of the active material in openings with reduced dimensions and promotes a denser, more continuous filling of openings. The crystallite size of a deposited material depends in part on the relative rates of nucleation and growth. Nucleation refers to the formation of the initial mass of a new phase or material, while growth refers to enlargement of a nucleated phase or material through accumulation of additional mass on the surface of the nucleated phase or material. In the case of CVD deposition of a solid phase material, for example, nucleation corresponds to the initial formation of a solid phase from one or more vapor phase precursors and growth corresponds to the expansion of the initial nucleated solid phase through subsequent deposition of solid phase material on the surface of the initial nucleated solid phase. As a deposition reaction proceeds, the amount of material deposited can increase through addition of material to existing nucleated phases (growth-dominated process) or through the preferential formation of new nucleated phases with slow growth (nucleation-dominated process). If nucleation is energetically disfavored, deposition proceeds primarily through growth and the deposited material tends to have a coarser crystallinity with a larger average crystallite size and a smaller number of crystallites. If nucleation is promoted, in contrast, deposition occurs primarily through the nucleation of a larger number of crystallites, where only limited growth of each crystallite occurs. In one embodiment of the instant invention, finer crystallinity is achieved through enhancement of the rate of nucleation relative to the rate of growth of the active material by the seed layer. Nucleation may be promoted, for example, by the existence of a high concentration of nucleation sites on or within the seed layer. If nucleation sites associated with the seed layer are more numerous and/or more active than nucleation sites associated with the sidewall or bottom surfaces of an opening, the presence of a seed layer can promote smaller grain, more consistent thickness growth on surfaces where the seed layer is located.
In another embodiment of the instant invention, the seed layer regulates the phase or crystallographic structure of the deposited active material. The prevailing use of seed layers or seed crystals in the prior art is to facilitate the growth of a larger quantity of the seed material. In a typical scenario, a particular material may be difficult to grow heterogeneously (e.g. on a foreign material or substrate), but may readily grow homogeneously (e.g. on a seed crystal of the same material). In one embodiment of this application, nucleation of the material is energetically disfavored and growth is favored. As the material grows, it preferentially conforms to the crystal structure and composition of the seed layer. In one embodiment of the instant invention, in contrast, active material deposited on the seed layer has a different composition and/or different crystallographic structure than the seed layer.
The seed layer may be formed by any technique capable of at least partially covering the bottom or lower portion of an opening. The seed layer may be formed, for example, by sputtering, physical vapor deposition (PVD), CVD, electroplating, extrusion, or solution deposition. The active material may be deposited using the same or different technique as the seed layer. In a preferred embodiment, the seed layer is formed of a material or through a process that produces small crystallites of the seed layer to facilitate growth of the seed layer in openings with reduced dimensions.
In one embodiment, the seed layer provides continuous coverage of the underlying surface of a substrate or opening and the active material is deposited with controlled crystallinity directly thereon. FIG. 7A shows electronic device 200 at an intermediate stage of fabrication. Electronic device 200 includes substrate 202, lower conductive layer 205, dielectric layer 210 with opening 215 formed therein and contiguous seed layer 225 formed on the bottom surface 220 of opening 215. Seed layer 225 may be a crystalline, polycrystalline or amorphous material and may have a regular or irregular surface texture. Seed layer 225 permits preferential deposition of an active material thereon relative to sidewall surface 214 of dielectric layer 210 or relative to exposed top surface 216 of lower conductive layer 205 by providing nucleation sites that promote the formation of a solid phase active material from vapor phase deposition precursors. The seed layer thus facilitates initial deposition of the active material at or near the bottom of opening 215. (Active material may also form on top surface 212 of dielectric layer 210 (not shown), but such material is removed in a later processing step and is independent of the deposition that occurs within opening 215.)
Since it is generally expected that subsequent deposition of the active material will preferentially occur on the initially deposited active material relative to the sidewalls of opening 215, growth of the active material occurs in a generally vertical direction from the bottom of the opening 215 to fill the opening 215. In one embodiment, active material 230 forms as a finely crystalline material with a crystal size that is smaller than would form on either top surface 216 of lower conductive layer 205 or surfaces 212 or 214 of dielectric layer 210. FIG. 7B shows the initial formation of active material 230 on seed layer 225 and FIG. 7C shows electronic device 200 after opening 215 has been filled with active material 230. The portion of active material 230 extending above dielectric layer 210 may be removed in a subsequent etching or planarization step and an upper conductive layer may then be formed to provide a second electrical contact to active material 230.
In another embodiment, the seed layer is formed as discrete regions on the substrate or underlying surface of the opening that provide non-contiguous coverage. In this embodiment, the seed layer provides incomplete coverage of its underlying surface and the active material initially forms preferentially along the surface of the seed layer. FIG. 8A, for example, shows seed layer 235 in the form of discrete islands and FIGS. 8B-8E show the accumulation of active material 240 to fill opening 215 formed within dielectric layer 210 of electronic device 200. In FIGS. 8A-8E, seed layer 235 is formed on top surface 216 of lower conductive layer 205. FIG. 8B illustrates the initial nucleation of active material 240 on discrete seed layer 235. Active material 240 forms at nucleation sites at the surface of seed layer 235. FIG. 8C illustrates enlargement of active material 240 as further nucleation and growth occur and regions of active material 240 on adjacent portions of discrete seed layer 235 impinge. In FIG. 8D, active material 240 fills in the spaces between adjacent portions of discrete seed layer 235 and continues growing vertically within opening 215. FIG. 8E depicts electronic device 200 after active material 240 has filled opening 215.
A noteworthy feature of the seed-assisted deposition processes depicted in FIGS. 7A-7C and FIGS. 8A-8E is the preferential nucleation of the active material on the surface of the seed layer. The seed layer directs nucleation of the active material away from the sidewalls of the opening by providing more active or more numerous nucleation sites. After nucleation, continued deposition of the active material occurs preferentially on the initial nucleated region of active material relative to the sidewalls of the opening. The net result is a nucleation and growth process that begins at the seed layer in the recessed portion of the opening and that continues from the recessed portion to the entrance of the opening. The resulting distribution of active material within the opening is more uniform and continuous with fewer voids.
The aspect ratio of opening 215 may be defined as the ratio of the normal dimension of the opening to the lateral dimension of the opening. The normal dimension of opening 215 is generally the physical dimension of the opening perpendicular to substrate 202 (e.g. height dimension) and the lateral dimension of opening 215 is generally a cross-sectional dimension of the opening in a direction parallel to substrate 202 (e.g. width dimension). In FIGS. 7A-7C and 8A-8E, for example, the normal dimension of opening 215 corresponds to the thickness of insulative layer 210 and the lateral dimension of opening 215 is the distance between opposing sidewalls.
In one embodiment of the instant invention, the normal dimension of opening 215 is at least 100 Å. In another embodiment of the instant invention, the normal dimension of opening 215 is at least 500 Å. In yet another embodiment of the instant invention, the normal dimension of opening 215 is at least 1000 Å. In one embodiment of the instant invention, the lateral dimension of opening 215 is less than 1000 Å. In another embodiment of the instant invention, the lateral dimension of opening 215 is less than 500 Å. In yet another embodiment of the instant invention, the lateral dimension of opening 215 is less than 300 Å. In one embodiment of the instant invention, the aspect ratio of opening 215 is at least 0.5:1. In another embodiment of the instant invention, the aspect ratio of opening 215 is at least 1:1. In yet another embodiment of the instant invention, the aspect ratio of opening 215 is at least 2:1. In a further embodiment of the instant invention, the aspect ratio of opening 215 is at least 3:1.
In this example, seed-assisted CVD deposition of a Ge--Sb--Te alloy is demonstrated. The deposition occurred on the SiO2 surface layer of a silicon wafer. The SiO2 surface layer had a thickness of 372.3 nm. A 100 Å thick seed layer of Ge2Sb2Te5 was formed on the SiO2 surface layer using a sputtering process. The wafer was then placed in a CVD deposition chamber for deposition of an active material from vapor phase precursors of Ge, Sb, and Te. Tris(dimethylamino)antimony (Sb(N(CH3)2)3) was used as the antimony (Sb) precursor, diisopropyltellurium (Te(CH(CH3)2)2) was used as the tellurium (Te), and t-butylgermane (H3Ge(C(CH3)3) was used as the germanium (Ge) precursor. The molecular forms of the three precursors are shown in FIG. 4. The three precursors are liquids at ambient conditions and were delivered to the CVD reactor in a vapor phase form via a bubbler. All three precursors were present simultaneously in the CVD reactor and each was introduced into the CVD reactor through a separate feed line. Nitrogen was used as a carrier gas for all three precursors.
The Ge-precursor, Sb-precursor and Te-precursor were placed in separate bubblers heated to 30° C. and delivered to the CVD reactor through separate feed lines, heated to 70° C. Nitrogen was bubbled through the Sb-precursor bubbler at a flow rate of 200 sccm to produce a gas stream containing the Sb-precursor in a vapor phase form diluted in nitrogen. No further dilution was used. Nitrogen was bubbled through the Te-precursor bubbler at a flow rate of 100 sccm to produce a gas stream containing the Te-precursor in a vapor phase form diluted in Nitrogen. No further dilution was used. The Ge-precursor was placed in a separate bubbler. 50 sccm of nitrogen was bubbled through the Ge-precursor bubbler to provide a gas stream containing the Ge-precursor in a vapor phase form diluted in He. No further dilution was used.
During the deposition, 1 slm of Hydrogen was delivered into the chamber and 1 slm of nitrogen was delivered from below the substrate through the backfill line. The total pressure in the CVD reactor during deposition was approximately 40 Torr. The substrate was heated to 375° C. during the CVD reaction. The reaction was permitted to run for 1.5 minutes and on conclusion of the reaction, a film having a thickness ranging from about 59 nm to about 67 nm had been prepared on the substrate.
FIG. 9 shows a cross-sectional FIB-SEM micrograph at a magnification of 250,000× of the deposited material and surrounding layers. Deposition structure 300 includes an underlying Si wafer 305 with surface oxide 310. Seed layer 315 is formed on surface oxide 310 and active material 320 is formed on seed layer 315. The micrograph indicates that active material 320 forms as a smooth, continuous layer on seed layer 315. The morphology induced by the seed layer contrasts markedly from the irregular morphology shown in FIG. 5, which is characterized by a wide heterogeneous distribution of crystallite sizes. The presence of seed layer 315 suppresses the tendency of the deposition to form large crystallites and promotes the formation of a conformal, uniform material.
Elemental analysis of active material 320 was completed using energy dispersive spectroscopy (EDS). The EDS results indicated that the ratio of Ge:Sb:Te in active material 320 was 37:30:33, thus indicating a Ge37Sb30Te33 stoichiometric composition. The results indicate that seed layer 315 and active material 320 differ in composition and demonstrate the ability of seed layer 315 to influence the composition of the deposited material. The results also demonstrate the ability to form chalcogenide materials through a seeded deposition process.
In this example, seeded deposition of an active material in an opening of an operable device is demonstrated and performance characteristics of the device are presented. FIG. 10 shows a schematic depiction of the device structure used in this example. Device structure 400 includes a base substrate 405, lower contact 410, dielectric layer 415 having sloped sidewalls 417, seed layer 420, active material 425, upper contact 430, and encapsulating layer 435. In the device used in this example, base substrate 405 is a silicon wafer with surface oxide layer, lower contact 410 is a TiAlN electrode material, dielectric layer 415 is an oxide, seed layer 420 is a sputtered Ge2Sb2Te5 layer having a thickness of ˜100 Å, active material 425 is Ge37Sb30Te33 prepared by CVD with the precursors and conditions described in Example 1 hereinabove to a thickness of ˜750 Å, upper contact 430 is a composite Ti(20 Å)/TiN (600 Å) layer, and encapsulating layer 435 is a Pt layer.
FIG. 11 shows a cross-sectional FIB-SEM micrograph of the device structure. The reference numerals shown in FIG. 11 correspond to those shown in FIG. 10 and the different layers are labeled to the left. FIG. 11 also indicates that the opening in dielectric layer 415 has a width of 133.5 nm at top that tapers to 85.5 nm along sloped sidewalls 417. It is noted that the dimensions of the opening are a design variable that can be selected during fabrication. Routine modifications of processing conditions can create openings having different size (e.g. diameter, lateral dimension, cross-sectional shape or area), depth or taper in the sidewall. The dimensions indicated in FIG. 11 are intended to be illustrative only and do not limit the scope of the instant invention.
The results shown in FIG. 11 indicate that active material 425 formed by CVD fills the opening in dielectric layer 415 in a continuous, uniform fashion without formation of overly large crystallites and without formation of a disadvantageous distribution of crystallite sizes. Instead, active material 425 faithfully conforms to the shape and dimensions of the opening and is densely packed therein. The micrograph indicates an absence of voids or other occupancy defects. The results indicate that inclusion of seed layer 420 facilitates the filling of the opening in dielectric layer 415 with active material 425.
FIGS. 12-14 summarize the electrical performance of electronic device 400 shown in FIG. 11 over multiple cycles of performance. FIG. 12 shows the current-voltage (I-V) relationship of a phase change electronic device 400. I-V response curves are shown for selected cycles of operation. The response curves have the general appearance of the schematic depiction shown in FIG. 3 and include a resistive branch and a conductive branch separated by a switching transformation as described hereinabove. The threshold voltage Vt of the device is ˜1.07V and the holding voltage Vh is ˜0.49V. Although the resistance indicated by the slope of conductive branch is higher than indicated in FIG. 3, this resistance is controlled by resistances in the measurement circuit. The resistance of the active material alone after switching is much lower than indicated by the conductive branch in FIG. 12.
FIG. 13 shows the resistance R of electronic device 400 as a function of the current I passing between lower contact 410 and upper contact 430. R-I curves for selected cycles of operation are shown and generally have the appearance depicted schematically in FIG. 2. The R-I curve for the final cycle of operation (labeled "Cycle 1.6E8") begins at a low initial resistance and does not display the full range of resistances in the direct overwrite region. This result is a consequence of device failure at or shortly before completion of 1.6×108 cycles of operation. (Device failure is also evident from the apparent lack of discontinuity for the Cycle 1.6E9 curve in the I-V plot of FIG. 12). The remaining R-I curves show a plateau region at low current pulse amplitude followed by a set transformation and entry into the direct overwrite region as described hereinabove as the current pulse amplitude increases.
FIG. 14 summarizes electrical performance parameters for phase change electronic device 400 over its operating lifetime. FIG. 14 depicts the reset resistance (uppermost data set plotted on log scale), set resistance (second uppermost data set plotted on log scale), dynamic state (conductive branch) resistance (middle data set), threshold voltage Vt (second lowest data set), and holding voltage Vh (lowest data set). The set and reset pulse durations were 1 μs and 200 ns, respectively. The results indicate that the reset resistance and set resistance remain consistent at about 105.2Ω (˜130 kΩ) and 103.6-104Ω, respectively, until device failure occurred at approximately 8×107 cycles of operation. Over the operation life of the device, the dynamic state resistance (˜1.6V), threshold voltage (˜1.0-1.1 V), and holding voltage (˜0.5V) remained consistent and stable.
This example demonstrates that seeded growth of an active material according to the instant invention provides a fully functional active material that exhibits electrical switching and phase-change operation.
Although the foregoing has described the seeded deposition of active materials in openings, analogous principles apply to the seeded deposition of other types of materials in openings. It is frequently desirable, for example, to deposit electrodes in high aspect ratio openings to achieve reduced contact area. (See U.S. patent application Ser. No. 12/113,566, the disclosure of which is incorporated by reference in its entirety herein.) Small electrode contact area is beneficial because it leads to a reduction in the programming current of phase-change materials. In accordance with the instant invention, formation of a seed layer in the recessed portion of an opening facilitates continuous and uniform deposition of an electrode material within the opening.
Those skilled in the art will appreciate that the methods and designs described above have additional applications and that the relevant applications are not limited to those specifically recited above. Also, the present invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner.
Patent applications by Smuruthi Kamepalli, Rochester, MI US
Patent applications by Tyler Lowrey, Rochester Hills, MI US
Patent applications in class Coating hole wall
Patent applications in all subclasses Coating hole wall