Patent Application
20100203709
This invention relates to the deposition of chalcogenide materials. More particularly, this invention relates to a method of depositing chalcogenide materials from a solid source via a thermal process. Most particularly, this invention relates to a method of depositing chalcogenide materials conformally via condensation from a vapor phase ambient formed from a solid source.
Chalcogenide materials are materials that contain a chalcogen element (S, Se, Te) and typically one or more additional elements that serve to modify electronic or structural properties. The II-VI semiconductors (e.g. CdS, ZnTe etc.) are a well-known class of chalcogenide materials. These materials have been widely investigated for their electronic bandgap properties and their potential for providing short wavelength light emission for LED and laser applications.
Another important class of chalcogenide materials includes the chalcogenide compositions that are currently being developed for memory and electrical switching applications. The chalcogenide memory materials are typically chalcogenide phase-change materials and can be used in optical and electrical memory applications. A phase change material is a material that is capable of undergoing a transformation, preferably reversible, between two or more distinct structural states. The distinct structural states 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, or a physical (e.g. electrical, optical, magnetic, mechanical) or chemical property.
The active recording materials in many optical storage technologies are chalcogenide phase-change materials that are reversibly transformable between a crystalline state and an amorphous state through the application of optical energy. These materials can be used to store information by defining a series of two or more distinct structural states, each of which is defined by a characteristic proportion of crystalline and amorphous phase domains within a given volume, and associating a unique information value to each structural state. Storage of data occurs by applying optical energy to the chalcogenide phase-change material in an amount necessary to convert the material to the structural state associated with the input data.
The optical phase change chalcogenide materials are reversibly transformable between different structural states through the controlled application of energy. The proportion of amorphous phase can be increased by applying energy sufficient to create a local temperature in the phase change material that exceeds the melting temperature and removing the energy at a rate sufficient to prevent crystallization upon cooling. The proportion of crystalline phase can be increased by applying energy sufficient to create a local temperature in the phase change material that exceeds the crystallization temperature so that a controlled transformation of amorphous phase material to crystalline phase material is induced. Reading of the information content of the phase change material occurs through the detection of a physical characteristic of the structural state of the material. In optical recording, for example, reflectivity is a widely used as a parameter for detecting the structural state. The reflectivity difference between the crystalline and amorphous states provides sufficient contrast to permit clear resolution of structural states that differ with respect to the relative proportions of crystalline and amorphous phase volume fractions.
Two other important types of chalcogenide materials are the electrical memory and electrical switching materials. In the electrical memory materials, application of electrical energy induces changes in the structural state of a chalcogenide phase-change material. The relative volume fraction of crystalline and amorphous phase domains can be continuously varied by controlling the duration and magnitude of a series of one or more applied electrical current or voltage pulses. As in the optical memory materials, each structural state has a unique resistance and each resistance value can be associated with a unique information value. By applying an appropriate electrical pulse, the electrical chalcogenide memory material can be programmed into the resistance state that corresponds to a particular data value to write data to the material. The chalcogenide electrical memory materials can be reversibly transformed among their different resistance states to provide erasing and rewriting capabilities. Both the electrical and optical chalcogenide memory materials can be incorporated into arrays to provide advanced, high density memory capability.
The chalcogenide electrical switching materials are amorphous phase materials that do not undergo crystalline-amorphous phase transformations. Instead, these materials are switchable between a quiescent resistive state and a dynamic conductive state upon application of a voltage that exceeds a threshold voltage. In the resistive state, the materials inhibit the flow of electrical current and upon application of the thresholding voltage, the material switches nearly instantaneously to a dynamic state that includes a highly conductive filamentary portion that permits the flow of current through high mobility charge carriers believed to include lone pair electrons removed from valence orbitals of chalcogen elements.
As the appreciation of the capabilities of chalcogenide materials grows, greater attention is being placed on further understanding their properties and on developing new chalcogenide materials that exhibit a wider range of properties. The development of new materials requires the production of new compositions or the production of existing compositions in ways that impart unique microstructures. The primary production methods for the chalcogenide memory and switching materials are sputtering and physical vapor deposition. Although these techniques have provided many interesting and useful materials, it is expected that the development of new production methods will expand the range of compositions and properties of chalcogenide materials and will extend the practical application of chalcogenide materials.
In the fabrication of practical chalcogenide devices, it is desirable to reduce the length scale or feature size as much as possible so that a larger number of devices can be formed on a given 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 device fabrication 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 or etching and is subsequently filled with another material. As the dimension or length scale of the opening decreases, it becomes increasingly difficult to satisfactorily fill the opening with another material.
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 lateral dimension of the opening decreases. The deposited layer has a tendency to include gaps, pores, or other unfilled regions. The packing density of material formed in the opening may vary in the depth or lateral dimensions of the opening.
Lack of structural uniformity in the filling of openings compromises device functionality as variations occur from device-to-device across an array of devices on a substrate. In addition, less than optimal performance is achieved for each device due to the defective nature of the deposited material. Imperfections in filling openings become especially pronounced as the aspect ratio (ratio of the dimension normal to the substrate to the dimension parallel to the substrate) of the opening increases. Deep, narrow channels, for example, are more difficult to uniformly fill 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 partially unfilled.
Conformality of deposition is another processing difficulty that becomes exacerbated as feature size decreases. Fabrication of semiconductor 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 a thin film 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. Smooth and uniform coverage is desired.
In addition to difficulties with achieving uniform filling, openings also present complications for achieving conformal deposition that become more pronounced as the lateral dimension 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 thin film 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, steps, sidewalls, and bottom surfaces that form the boundaries of the opening. Achieving conformality over discontinuous features becomes increasingly difficult as the feature size of the opening decreases, or as the aspect ratio of the opening increases.
Fabrication of chalcogenide memory and switching devices often includes a step of forming an opening in a dielectric layer and filling the opening with a chalcogenide material. Miniaturization of chalcogenide memory and switching devices requires methods for conformally depositing chalcogenide materials into openings with small lateral dimensions and/or high aspect ratios. Preferably, the methods would enable the conformal fabrication of active chalcogenide materials in features having dimensions near, at or below the lithographic limit.
Chemical Vapor Deposition (CVD) is one method available in the prior art for filling openings that is expected to remain conformal as the feature size of the opening decreases. In the CVD process, precursors of the constituent elements of a material are reacted to produce a thin film on a substrate. The precursors are introduced into the reactor in gas phase form. Precursors that are in the gas phase at room conditions 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 decomposed (thermally, photochemically, or in a plasma) to provide intermediate species of the constituents that subsequently react to form a thin film of desired composition. 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 precursor; reactor pressure; and the rate of introduction of precursors into the reactor. The molecular dimensions of the gas phase precursors used in CVD allow the precursors to enter small-dimensional openings, where they subsequently react to form a relatively uniform layer.
Although CVD in principle is a viable strategy for forming conformal thin film chalcogenide materials at lithographic or sublithographic dimensions, the technique suffers from the limited availability of suitable gas phase precursors for a variety of desired chalcogenide compositions. Many of the most effective chalcogenide compositions are multiple element (ternary and higher) compositions and it becomes difficult to simultaneously control the decomposition or reactivity of multiple precursors to provide uniform films of multi-element materials. Precursor development and qualification is an expensive endeavor. In addition, the purity of material deposited by CVD can be compromised by residual elements released from ligands of the precursors upon reaction or decomposition of the precursors. Also, the reaction conditions (e.g. high temperatures or plasma conditions) needed for reaction of the precursors may damage other layers in the device structure. There is a need, therefore, for alternative methods of forming chalcogenide materials in conformal fashion.
This invention provides a method for forming thin film chalcogenide materials. The method includes condensing a thin film chalcogenide material from the vapor produced by vaporizing a condensed phase chalcogenide source. The condensed phase chalcogenide source may be a solid-phase chalcogenide source, molten-phase chalcogenide source or liquid-phase chalcogenide source. Vaporization may occur through sublimation or evaporation.
In one embodiment, a solid-phase chalcogenide source is heated to a temperature below its melting point and the source material sublimes to form a vapor phase. In another embodiment, a solid-phase chalcogenide source is heated to a temperature at or above its melting point to form a molten or liquid-phase chalcogenide source and the source material evaporates to form a vapor phase. Heating of the chalcogenide source may be achieved with a thermal, electrical, or optical energy source. The heating process may be continuous, pulsed or intermittent in time. In one embodiment, the vapor forms in a non-ionizing environment. In another embodiment, the vapor forms in a plasma-free environment.
The chalcogenide source material is formed on a deposition surface, heated to induce sublimation, and condenses back on the deposition surface to form a condensed chalcogenide product material. To prevent escape of sublimed chalcogenide material, a capping layer is formed over the chalcogenide source material before sublimation. The capping layer seals the deposition surface and contains the chalcogenide-containing vapor phase formed upon sublimation of the chalcogenide source material. The chalcogenide-containing vapor phase is mobile, spreads over the deposition surface, and subsequently condenses to form a product chalcogenide material. The condensed chalcogenide material may form as a solid phase, molten phase or liquid phase on the deposition surface upon condensation. If the condensed chalcogenide material returns to the deposition surface as a molten phase or liquid phase, it is subsequently transformed to a solid phase upon cooling.
The sublimation-condensation process permits a redistribution of the chalcogenide source material across the deposition surface. An initial unfavorable spatial distribution of chalcogenide material on the deposition surface can be transformed to a favorable spatial distribution. Because the sublimed chalcogenide material is in the vapor phase, it is capable of penetrating narrow features on the deposition surface. Subsequent condensation can thus induce formation of a product chalcogenide material at locations not adequately covered by the initial chalcogenide source material.
In one embodiment, the condensation process is controlled to bias the spatial distribution of the product chalcogenide material on the deposition surface. A non-uniform temperature profile can be used to bias condensation toward cooler regions to control the spatial distribution of the product chalcogenide material formed upon condensation. In another embodiment, selective heating of the chalcogenide source material provides control over where sublimation occurs. Selective heating promotes redistribution of the chalcogenide source material to regions of the deposition surface in the vicinity of the localized sublimation process
In one embodiment, the deposition surface is a planar surface. In another embodiment, the deposition surface includes a feature and spatial redistribution of the chalcogenide source material via the instant sublimation-condensation process permits formation of a condensed chalcogenide material within the feature. In another embodiment, the condensed chalcogenide material conformally forms within or fills the opening. The feature may be an opening, hole, via, trench, groove, recess, edge, step, or depression. The cross-sectional shape of the feature may include a portion that is round, curved, elliptical, bent, or rectilinear. The feature may have an aspect ratio between 0.25:1 and 5:1. In one embodiment, the feature has an aspect ratio of at least 1:1. In another embodiment, the feature has an aspect ratio of at least 2:1. In another embodiment, the feature has an aspect ratio of at least 3:1.
In one embodiment, the condensed product chalcogenide thin film material is formed as an amorphous phase material on the deposition surface. In one embodiment, the condensed product forms in a non-ionizing environment. In another embodiment, the condensed product forms in a plasma-free environment.
In one embodiment, the composition of the condensed chalcogenide thin film material is approximately the same as the composition of the chalcogenide source material. In another embodiment, the atomic concentration of the chalcogen element in the condensed chalcogenide material is between 90% and 110% of the atomic concentration of the chalcogen element in the chalcogenide source material. In another embodiment, the atomic concentration of the chalcogen element and at least one other element in the condensed chalcogenide material are both between 90% and 110% of their respective atomic concentrations in the chalcogenide source material. In another embodiment, the atomic concentration of the chalcogen element in the condensed chalcogenide material is between 80% and 120% of the atomic concentration of the chalcogen element in the chalcogenide source material. In another embodiment, the atomic concentration of the chalcogen element and at least one other element in the condensed chalcogenide material are both between 80% and 120% of their respective atomic concentrations in the chalcogenide source material.
In one embodiment, the condensed chalcogenide material is an optical phase change material that is reversibly transformable between a high reflectivity state and a low reflectivity state upon application of optical energy, where the high reflectivity and low reflectivity states differ in fractional crystallinity.
In another embodiment, the condensed chalcogenide material is an electrical switching material that can be switched from a high resistance state to a low resistance state upon application of a threshold voltage, where the low resistance state includes at least a filamentary portion that exhibits high conductivity.
In another embodiment, the condensed chalcogenide material is an electrical memory material in which the relative proportions of crystalline and amorphous phase volumes can be varied through the application of an electrical signal, where the resistivity of the material varies with the relative proportions of crystalline and amorphous phase content.
In one embodiment, the condensed chalcogenide material comprises Te. In another embodiment, the condensed chalcogenide material comprises Te and Ge. In yet another embodiment, the condensed chalcogenide material comprises Te and Sb. In one embodiment, the condensed chalcogenide material comprises Se. In another embodiment, the condensed chalcogenide material comprises Se and Ge. In yet another embodiment, the condensed chalcogenide material comprises Se and Sb.
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.
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 provides a method for forming chalcogenide memory and electrical switching materials. The method includes vaporizing a condensed phase chalcogenide source material and depositing a chalcogenide product material on a deposition surface spaced apart from the source material. In order to better appreciate the illustrative examples of the instant invention presented hereinbelow, it is helpful to review the basic operational characteristics of phase-change and electrical switching materials.
As indicated above, an important feature of the chalcogenide phase-change family of materials used in optical and electrical memory applications is their ability to undergo a phase transformation between two or more structural states. The chalcogenide phase-change materials have structural states that include a crystalline state, one or more partially-crystalline states and an amorphous state. A partially-crystalline state refers to a structural state in which a volume of chalcogenide or phase-change material includes an amorphous portion and a crystalline portion. Generally, a plurality of partially-crystalline states exists for the chalcogenide or phase-change material that may be distinguished on the basis of the relative proportion of the amorphous and crystalline portions. Fractional crystallinity is one way to characterize the structural states of a chalcogenide phase-change material. The fractional crystallinity of chalcogenide phase-change materials may vary from 0% (the amorphous limit) continuously through a series of partially crystalline states having increased fractional crystallinity up to 100% (the crystalline limit).
Transformations among the structural states are induced by providing energy to the chalcogenide material. Energy in various forms can induce structural transformations of the crystalline and amorphous portions and thus can influence the fractional crystallinity of a chalcogenide material. Suitable forms of energy include one or more of electrical energy, thermal energy, optical energy or other forms of energy (e.g. particle-beam energy) that induce electrical, thermal or optical effects in a chalcogenide material. Continuous and reversible variability of the fractional crystallinity is achievable by controlling the energy environment of a chalcogenide material. A crystalline state can be transformed to a partially-crystalline or an amorphous state, a partially-crystalline state can be transformed to a crystalline, amorphous or different partially-crystalline state, and an amorphous state can be transformed to a partially-crystalline or crystalline state through proper control of the energy environment of a chalcogenide material. Some considerations associated with the use of thermal, electrical and optical energy to induce structural transformations are presented in the following discussion.
The use of thermal energy to induce structural transformations exploits the thermodynamics and kinetics associated with the crystalline to amorphous or amorphous to crystalline phase transitions. An amorphous phase may be formed, for example, from a partially-crystalline or crystalline state by heating a chalcogenide material above its melting temperature and cooling at a rate sufficient to inhibit the formation of crystalline phases. A crystalline phase may be formed from an amorphous or partially-crystalline state, for example, by heating a chalcogenide material above the crystallization temperature for a sufficient period of time to effect nucleation and/or growth of crystalline domains. The crystallization temperature is below the melting temperature and corresponds to the minimum temperature at which crystallization may occur. The driving force for crystallization is typically thermodynamic in that the free energy of a crystalline or partially-crystalline state in many chalcogenide materials is lower than the free energy of an amorphous state so that the overall energy of a chalcogenide material decreases as the fractional crystallinity increases. Formation (nucleation and growth) of a crystalline state or crystalline domains within a partially-crystalline or amorphous state is kinetically enabled up to the melting temperature, so that heating promotes crystallization by providing energy that facilitates the rearrangements of atoms needed to form a crystalline phase or domain. The fractional crystallinity of a partially-crystalline state can be controlled by controlling the temperature or time of heating of the partially-crystalline state or by controlling the temperature or rate of cooling of an amorphous or partially-crystalline state. Through proper control of the peak temperature, time of heating and rate of cooling, structural states over the full range of fractional crystallinity can be achieved for the chalcogenide phase-change materials.
The use of electrical energy to induce structural transformations relies on the application of electrical (current or voltage) pulses to a chalcogenide material. The mechanism of electrically-induced structural transformations includes Joule heating created by resistance to current flow. Joule heating corresponds to a conversion of electrical energy to thermal energy and leads to an increase in the temperature of the chalcogenide material. By controlling the current density, the temperature can be controlled.
The crystalline phase portions of a chalcogenide material are sufficiently conductive to permit current densities that provide efficient Joule heating. The amorphous phase portions, however, are much less conductive and ordinarily would not support current densities sufficient to heat the material to the crystallization temperature. As described more fully hereinbelow, however, the amorphous phase of many chalcogenide materials can be electrically switched to a highly conductive intermediate state upon application of a voltage greater than the threshold voltage. In the intermediate state, the material can support a current density that is high enough to heat the material to the crystallization temperature through Joule heating. By controlling the magnitude and/or duration of electrical pulses applied to a chalcogenide phase-change material, it is possible to vary continuously the fractional crystallinity through controlled interconversion of the crystalline and amorphous phases.
The influence of electrical energy on a chalcogenide material is generally depicted in terms of the I-V (current-voltage) and R-I (resistance-current) relationships of the material. The I-V relationship shows the current response of a chalcogenide material as a function of applied voltage and the R-I relationship shows the variation of the electrical resistance of a chalcogenide material as a function of the amount of electrical energy provided or as a function of the magnitude of the current or voltage pulse applied to a chalcogenide material. A brief discussion of the I-V and R-I characteristics of chalcogenide materials follows.
The I-V response of many chalcogenide materials exhibits an electrical switching event in which the chalcogenide material undergoes a transformation from a more resistive state to a more conductive state. A schematic depiction of the electrical switching event is presented in
The I-V curve includes a resistive branch and a conductive branch. The branches are labeled in
The switching properties of the chalcogenide material are shown in
When the applied voltage equals or exceeds the threshold voltage, the chalcogenide material transforms (switches) from the resistive branch to the conductive branch of the I-V curve. The switching event occurs essentially instantaneously and is depicted by the dashed line in
The switching effect of the instant devices originates from a transformation of the chalcogenide material from a resistive state to a conductive state upon application of a threshold voltage, Vth. According to one model of the switching transformation, application of the threshold voltage causes the formation of a conductive channel or filament within the chalcogenide material. At the threshold voltage, the electric field experienced by the material is sufficiently high to induce a breakdown or avalanche effect whereby electrons are removed from atoms to form a highly conductive, plasma-like filament of charge carriers. Rather than being bound to atoms, some electrons become unbound and highly mobile. As a result, a conductive channel or filament forms. The conductive filament constitutes a conductive volume within the otherwise resistive chalcogenide material. The conductive filament extends through the chalcogenide material between the device terminals and provides a low resistance pathway for electrical current. Portions of the chalcogenide material outside of the filament remain resistive. Since electric current traverses the path of least resistance, the presence of a conductive filament renders the chalcogenide material conductive and establishes a conductive state. The creation of a conductive filament is the event that underlies the switching of the chalcogenide material from a resistive state to a conductive state.
The conductive filament is maintained between the device terminals as long as the device current remains at or above the holding current. A conductive filament is present for all points along the conductive branch, but the cross sectional area of the filament differs for different points along the conductive branch. Depending on operating conditions within the conductive branch, the filament can be narrow or wide. As the applied voltage is increased along the conductive branch, the cross section of the filament is enlarged as the applied voltage is increased. The enlarged filament indicates a greater volume of the chalcogenide material exhibits high conductivity. During the time the filament enlarges, the voltage across the contacts remains constant. When the filament reaches the dimensions of the pore, it can no longer grow and its resistivity must increase. As a result, the chalcogenide material can support a greater current, as indicated by the conductive branch of the I-V curve, when the applied voltage increases. Variations of the voltage applied to a chalcogenide material operating in the conductive branch modify the width or thickness of the filament in directions lateral to the direction of current flow. The net effect of varying the applied voltage of a chalcogenide material operating in the conductive branch is to modify the volume fractions of the conductive and resistive portions (unless the material is melted and quenched).
Chalcogenide materials of many chemical compositions undergo the foregoing switching effect. Representative chalcogenide materials are those that include one or more elements from column VI of the periodic table (the chalcogen elements) and optionally one or more chemical modifiers from columns III, IV or V. One or more of S, Se, and Te are the most common chalcogen elements included in the active material of the instant devices. The chalcogen elements are characterized by divalent bonding and the presence of lone pair electrons. The divalent bonding leads to the formation of chain and ring structures upon combining chalcogen elements to form chalcogenide materials and the lone pair electrons provide a source of electrons for forming a conducting filament. Trivalent and tetravalent modifiers such as Al, Ga, In, Ge, Sn, Si, P, As and Sb enter the chain and ring structures of chalcogen elements and provide points for branching and crosslinking. The structural rigidity of chalcogenide materials depends on the extent of crosslinking and leads to a broad classification of chalcogenide materials, according to their ability to undergo crystallization or other structural rearrangements, into one of two types: threshold switching materials and phase-change (or memory) materials.
In one embodiment of the instant invention, the switching material includes Se or Te. In another embodiment, the atomic concentration of Se or Te in the switching material is at least 30%. In a further embodiment, the atomic concentration of Se or Te in the switching material is at least 50%. As used herein atomic concentration refers to the percent abundance of an element in molar or atomic terms. In the composition As2Te3Ge, for example, the mole fraction of Te is 0.50 and the atomic concentration of Te is 50%. In another embodiment, the switching material includes Se or Te along with Ge or In. In a further embodiment, the switching material includes Se or Te, Ge or In, and one or more of Si, Sb and As. Representative switching materials include Si14Te39As37Ge9X (X═In or P), As—Te—Ge—Si alloys, As2Te3Ge, As2Se3Ge, As25(Te90Ge10)75, Te40As35Si18Ge6.75In0.25, Te28As34.5Ge15.5S22, Te39As36Si17Ge7P, As10Te21S2Ge15Se50Sb2, and Si5Te34As28Ge11S21Se1. Illustrative compositions for switching materials are discussed in U.S. Pat. Appl. Pub. Nos. 20060118911, 20060171194, 20060291272, 20070096090, 20070105267, and in U.S. Pat. Nos. 3,748,501; 4,845,533; 5,543,737; 5,694,146; 5,757,446; 6,795,338; 6,967,344; and 6,969,867 the disclosures of which are incorporated by reference herein.
The chalcogenide phase-change materials include compositions in the alloy families binary Ge—Te compositions, ternary Ge—Sb—Se compositions, ternary In—Sb—Se compositions, ternary Ge—Sn—Te compositions, binary Ga—Sb compositions, binary In—Sb compositions, binary In—Se compositions, Sb2Te3, Ge2Sb2Te5, ternary Ge—Sb—Te compositions, In2Sb2Te5, ternary In—Sb—Te compositions, ternary GaSeTe compositions, TAG and other ternary Te—As—Ge compositions, GaSeTe, SnSb2Te4, ternary In—Sb—Ge compositions, quaternary Ag—In—Sb—Te compositions, quaternary Ge—Sn—Sb—Te compositions, quaternary Ge—Sb—Se—Te compositions, and quaternary Te—Ge—Sb—S compositions. Chalcogenide phase-change compositions are discussed in U.S. Pat. Nos. 3,748,501 5,335,219 5,406,509 6,967,344; 6,969,867; 7,020,006 5,543,737; 5,694,146; 5,757,446; 5,166,758; 5,296,716; 5,534,711; 5,536,947; 5,596,522; 6,087,674; and 7,186,998; the disclosures of which are incorporated by reference herein.
Both types of chalcogenide materials display the switching behavior shown in
Phase-change chalcogenide materials, on the contrary, are lightly crosslinked and more easily undergo full or partial crystallization. An amorphous phase-change material undergoes filament formation in the presence of a threshold voltage as described in
The R-I response is a meaningful depiction of the characteristics of the chalcogenide phase-change materials and provides a representation of the effect of structural transformations associated with the crystalline-amorphous phase-change process on electrical properties. A representative depiction of the electrical resistance (R) of a chalcogenide material as a function of electrical energy or current pulse magnitude (Energy/Current) is presented in
The resistance plot includes two characteristic response regimes of a chalcogenide material to electrical energy. The regimes are approximately demarcated with the vertical dashed line 10 shown in
The left side of the resistance plot is referred to as the accumulating regime because the structural state of the chalcogenide material cumulatively evolves as energy is applied. The fractional crystallinity of the structural state correlates 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, the chalcogenide material progresses among a plurality of partially-crystalline states with increasing fractional crystallinity along the plateau 30. Selected accumulation states (structural states in the accumulation region) are marked with squares in
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 applied energy within the plateau region 30 and are reversible only by first driving the chalcogenide material through the set point 40 and resetting (melting and quenching the device). 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 possible to reversibly transform between the set and reset states over multiple cycles of operation.
While not wishing to be bound by theory, the instant inventor believes that the addition of energy to a chalcogenide material in the accumulating regime leads to an increase in fractional crystallinity through the nucleation of new crystalline domains or growth of existing crystalline domains or a combination thereof. It is believed that the electrical resistance varies only gradually along the plateau 30 despite the increase in fractional crystallinity because the crystalline domains form or grow in relative isolation of each other so as to prevent the formation of a contiguous crystalline network that spans the chalcogenide material between the two device electrodes. This type of crystallization may be referred to as sub-percolation crystallization. The setting transformation coincides with a percolation threshold in which a contiguous, interconnected crystalline network forms within the chalcogenide material that bridges the space between the two electrodes of the device. Such a network may form, for example, when crystalline domains increase sufficiently in size to impinge upon neighboring domains. Since the crystalline phase of chalcogenide materials is more conductive and less resistive than the amorphous phase, the percolation threshold corresponds to the formation of a contiguous low resistance conductive pathway through the chalcogenide material. As a result, the percolation threshold is marked by a dramatic decrease in the resistance of the chalcogenide material. The leftmost point 20 of the accumulation regime may be an amorphous state or a partially-crystalline state lacking a contiguous crystalline network. Sub-percolation crystallization commences with an initial amorphous or partially-crystalline state and progresses through a plurality of partially-crystalline states having increasingly higher fractional crystallinities until the percolation threshold is reached and the setting transformation occurs.
The regime to the right of the line 10 of
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 an associated current pulse magnitude, where application of the associated current pulse magnitude induces changes in fractional crystallinity that produce the particular resistance state. Application of a subsequent current pulse may increase or decrease the fractional crystallinity of an existing resistance state of the chalcogenide material. If the subsequent current pulse has a higher magnitude than the pulse used to establish the existing state, the fractional crystallinity of the chalcogenide material decreases and the structural state is transformed from the existing state in the direction of the reset state along the direct overwrite resistance curve. Similarly, if the subsequent current pulse has a lower magnitude than the pulse used to establish the existing state, the fractional crystallinity of the chalcogenide material increases and the structural state is transformed from the existing state in the direction of the set state along the direct overwrite resistance curve.
The direct overwrite states of the chalcogenide material may be 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. 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, 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 Ge22Sb22Te56, a thickness of ˜600 Å, 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 ˜100 kΩ to ˜1000 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.
The instant invention provides a method for forming chalcogenide materials. The method entails vaporizing a condensed phase chalcogenide source material and condensing the resulting vapor on a deposition surface to form a product chalcogenide material. In one embodiment, vaporization is effected by heating the condensed phase chalcogenide source material. The heating may be achieved through a thermal process (e.g. oven heating), an optical process (e.g. laser heating or lamp heating), or electrical process (e.g. resistive heating). In one embodiment, vaporization occurs via a non-ionizing process. In another embodiment, vaporization occurs in a plasma-free environment.
In one embodiment, the heating may occur continuously at a steady temperature. In another embodiment, an interval of continuous heating at a steady temperature may be preceded by a ramp up in temperature and/or followed by a ramp down in temperature. In a further embodiment, the heating may occur intermittently or through cycling, where the heating is variable in time. In one embodiment, variable heating is achieved through pulsing of the source (thermal, optical, electrical) that provides the heat energy. In one embodiment, heating is accomplished through a rapid thermal anneal process. In one embodiment, the source material is heated to a temperature below its melting point. In another embodiment, the source material is heated to a temperature above its melting point. Heating may occur in the presence of a background or inert gas. Background or inert gases include argon or another noble gas, nitrogen, and hydrogen.
The condensed phase chalcogenide source material may be a solid-phase chalcogenide source material or a liquid-phase chalcogenide source material. In one embodiment, the condensed phase chalcogenide source material includes one or more of the chalcogen elements S, Se, or Te. In another embodiment, the condensed phase chalcogenide source material includes one or more of the chalcogen elements S, Se, or Te and one or more elements from column V of the periodic table (e.g. P, Sb, or As). In another embodiment, the condensed phase chalcogenide source material includes one or more of the chalcogen elements S, Se, or Te and one or more elements from column IV of the periodic table (e.g. Si, Ge, or Sn). In another embodiment, the condensed phase chalcogenide source material includes one or more of the chalcogen elements S, Se, or Te and one or more elements from column III of the periodic table (e.g. Ga or In). In another embodiment, the condensed phase chalcogenide source material includes one or more of the chalcogen elements S, Se, or Te along with one or more elements from column V of the periodic table (e.g. P, Sb, or As) and one or more elements from column IV of the periodic table (e.g. Si, Ge, or Sn). In another embodiment, the condensed phase chalcogenide source material includes one or more of the chalcogen elements S, Se, or Te along with one or more elements from column V of the periodic table (e.g. P, Sb, or As) and one or more elements from column III of the periodic table (e.g. Ga or In). In another embodiment, the condensed phase chalcogenide source material includes one or more of the chalcogen elements S, Se, or Te along with one or more elements from column IV of the periodic table (e.g. Si, Ge, or Sn) and one or more elements from column III of the periodic table (e.g. Ga or In). The chalcogenide source material may be a binary, ternary, quaternary, or higher multiplicity composition. In a further embodiment, the chalcogenide source material is substantially free of one or more of the elements carbon, nitrogen, and hydrogen.
In one embodiment, the instant deposition process may be achieved by vaporizing a plurality of condensed phase source materials, each of which contains an element for incorporation into the product. In this embodiment, each condensed phase source material is vaporized to form a vapor containing at least one element that is ultimately incorporated into the product upon condensation. The different condensed phase source materials may be heated to the same or different temperature. Each condensed phase source material may sublime or evaporate to form a vapor and the invention contemplates that some source materials may sublime, while others may evaporate during a particular deposition process. As an example, a Ge—Sb—Te product may be formed by producing vapors from separate Ge, Sb, and Te sources positioned within the deposition apparatus and co-condensing the vapors on a deposition surface. The separate sources may be element sources or multielement sources. A Ge—Sb—Te product may be formed, for example, by vaporizing a Ge—Te source material and a separate Sb source material. Alternatively, a Ge—Sb—Te product may be formed by vaporizing a Ge—Sb source material and a separate Te source material. Similarly, a Ge—Sb—Te product may be formed by vaporizing a Te—Sb source material and a separate Ge source material.
The product chalcogenide material is typically a thin film chalcogenide material. In one embodiment, the product is formed in an amorphous phase. The product includes a chalcogen element that originated from the condensed phase chalcogenide source material. In one embodiment, the atomic concentration of the chalcogen element in the product is between 95% and 105% of the atomic concentration of the chalcogen element in the source material. In another embodiment, the atomic concentration of the chalcogen element in the product is between 90% and 110% of the atomic concentration of the chalcogen element in the source material. In still another embodiment, the atomic concentration of the chalcogen element in the product is between 80% and 120% of the atomic concentration of the chalcogen element in the source material. In a further embodiment, the atomic concentration of the chalcogen element in the product is between 70% and 130% of the atomic concentration of the chalcogen element in the source material.
By way of example, if the condensed phase chalcogenide source material is Ge2Sb2Te5, the chalcogen element is Te and its atomic concentration is 55.56%. If this exemplary source material is heated to form a vapor and the vapor is condensed on a deposition surface to form a product in accordance with the instant invention, the product includes Te. In one embodiment, the atomic concentration of Te is between 95% of 55.56% (52.78%) and 105% of 55.56% (58.34%). In another embodiment, the atomic concentration of Te is between 90% of 55.56% (50.00%) and 110% of 55.56% (61.11%). In still another embodiment, the atomic concentration of Te is between 80% of 55.56% (44.45%) and 120% of 55.56% (66.67%). In a further embodiment, the atomic concentration of Te is between 70% of 55.56% (38.89%) and 130% of 55.56% (72.23%).
Independently of or in addition to the chalcogen element, the atomic concentration of one or more other elements (e.g. elements from columns III, IV, or V) in the product condensed from a vapor phase formed from the chalcogenide source material may also be within the ranges indicated above with respect to the atomic concentration of the same element in the chalcogenide source material. To continue with the example of Ge2Sb2Te5 above, Sb is a column V element and Ge is a column IV element. The atomic concentration of each of Sb and Ge is 22.22%. In one embodiment, the atomic concentration of either or both of Sb and Ge is between 95% of 22.22% (21.11%) and 105% of 22.22% (23.33%). In one embodiment, the atomic concentration of either or both of Sb and Ge is between 90% of 22.22% (20.00%) and 110% of 22.22% (24.44%). In one embodiment, the atomic concentration of either or both of Sb and Ge is between 80% of 22.22% (17.78%) and 120% of 22.22% (26.66%). In one embodiment, the atomic concentration of either or both of Sb and Ge is between 70% of 22.22% (15.55%) and 130% of 22.22% (28.89%).
The close fit between the atomic concentration of one or more (up to all) of the elements in the product chalcogenide material and the source chalcogenide material is a benefit of the instant invention. The source material is a chalcogenide material that includes a chalcogen element along with one or more other elements in a particular stoichiometric ratio. Each element is bonded with at least one other element. When elements are present in the form of a compound in the source material, the physical properties (e.g. melting point, vapor pressure, sublimation pressure) are controlled by the compound instead of the elements individually. As a result, when the source material vaporizes, there is a greater tendency for the elements to escape in accordance with the relative proportions defined by the stoichiometric proportions in the compound. As a result, the composition of the condensed product formed from the vapor is more likely to match that of the source material.
If the individual elements are used as source materials in a vaporization process, it may be more difficult to control the relative rates of delivery of the individual elements to the deposition surface to achieve a desired composition in the product phase than if a multielement or compound source material is used. Each element has different vaporization tendencies and to form a product phase having a particular stoichiometry, it is necessary to coordinate delivery of the different elements in space and time to insure that the proportions desired in the product phase are represented at the deposition surface. In the case of Ge2Sb2Te5, for example, use of elemental Ge, Sb, and Te as source materials requires control over the vaporization processes of elements that differ in melting point, vapor pressure, and sublimation pressure. As a result, it may require careful optimization of process conditions to achieve the 2:2:5 stoichiometric ratio present in Ge2Sb2Te5. As demonstrated in the illustrative examples described hereinbelow, when Ge2Sb2Te5 is used as the source material, a product material having a stoichiometric ratio of or close to 2:2:5 is readily achieved. It is similarly possible to achieve product materials having a wide range of stoichiometric proportions that conform closely to the stoichiometric composition of a source material.
A further advantage of the instant invention is the ability to control the phase of the product material. The phase of the chalcogenide source material and the phase of the product material may differ. In one embodiment, the chalcogenide source material is a crystalline or polycrystalline material and the product material is an amorphous phase material having an atomic concentration of one or more (up to all) elements within the ranges indicated hereinabove of their respective atomic concentration in the source material. Formation of an amorphous phase product having a desired stoichiometry is beneficial when it is desired to deposit of a chalcogenide material in or over a feature such as an opening (e.g. hole, trench, via), groove, depression, or step that may be present on the deposition surface. As noted above, conformal deposition in or over features is desirable to avoid gaps or incomplete filling and thus to insure better device performance. Formation of an amorphous phase product on the deposition surface promotes conformality of deposition by avoiding the granularity and non-space-filling tendencies of crystalline phase deposition.
At the same time, the instant invention recognizes that production of commercial quantities of source materials is often most economical for crystalline or polycrystalline forms of the source material. The instant invention therefore provides the benefit of low cost source materials with the advantage of more conformal deposition of a product that is faithful to the composition of the source material. Although a technique such as chemical vapor deposition may provide conformal deposition, it relies on costly precursor sources for the elements.
The deposition surface may be a planar surface or a surface that includes one or more features. Features include openings, holes, vias, trenches, recesses, depressions, grooves, edges and steps. The feature may be round, elliptical, bent, rectilinear or other circumferential shape. In one embodiment, condensation of the vapor phase formed from the source chalcogenide material occurs in a feature of the deposition surface to form a product material therein. In another embodiment, formation of the product material occurs conformally with the feature. In one embodiment, the opening is a circular hole that is filled or lined with a product chalcogenide material. In another embodiment, the opening is a trench that is filled or lined with a product chalcogenide material.
The feature may have an aspect ratio (ratio of the feature dimension normal to the deposition surface to the feature dimension lateral to or parallel to the deposition surface) that ranges between 0.25:1 and 5:1. In one embodiment, the aspect ratio of the feature is at least 0.5:1. In another embodiment of, the aspect ratio of the feature is at least 1:1. In yet another embodiment, the aspect ratio of the feature is at least 2:1. In a further embodiment, the aspect ratio of the feature is at least 3:1.
The deposition surface may be at the same or different temperature than the temperature of the source material. In one embodiment, the temperature of the deposition surface is less than the temperature of the source material. In another embodiment, the temperature of the deposition surface is less than the melting temperature of the source material. In another embodiment, the temperature of the deposition surface is less than the melting temperature of the product material. In another embodiment, the temperature of the deposition surface is less than the melting temperature of the source material. In another embodiment, the temperature of the deposition surface is less than the glass transition temperature of the source material.
Representative performance and benefits of the instant inventions are provided in the following examples. It is noted that the examples are illustrative only and do not serve to limit the scope of the invention. In particular, it is to be appreciated that compositions other than those selected to illustrate the instant invention are operable according to the principles of the instant invention.
In this example, deposition of a thin film chalcogenide material on a planar surface via vaporization of a solid-phase chalcogenide source material is demonstrated. The apparatus used to perform the deposition is shown in
To form product chalcogenide material 55, an inert gas (e.g. N2 or Ar) is introduced to enclosure 15 and the pressure is reduced with a roughing vacuum pump. Deposition was initiated by turning on quartz heaters 25. Quartz heaters 25 were left on for a time window of ˜100 seconds to perform the deposition. During that time window, thermocouple 65 indicated that the temperature of lower support wafer 60 increased to a temperature of about 325° C. (which fluctuated slightly). The temperature increase of lower support wafer 60 lagged the turn on of quartz heaters 25 by about 10 seconds. During the time window of the experiment, solid-phase chalcogenide source material 35 vaporized and product chalcogenide material 55 formed on lower deposition wafer 40.
The time window of the experiment was kept short to prevent excessive heating of lower deposition wafer 40. To promote condensation of product chalcogenide material 55 on lower deposition wafer 40, it is desirable to insure that lower deposition wafer 40 remains cool relative to the upper wafer 30. When quartz heaters 25 are turned on, upper wafer 30 and solid-phase chalcogenide source material 35 heat up first. The heating of lower support wafer 60 is delayed and since lower deposition wafer 40 is shielded, its heating is delayed even more. If the time window of the experiment is kept short relative to the time needed to achieve full temperature equilibration, lower deposition wafer 40 (and product chalcogenide material 55) remained cooler than upper wafer 30 and solid-phase chalcogenide source material 35 during the deposition.
To conclude the deposition, quartz heaters 25 were turned off and lower deposition wafer 40 and product chalcogenide material 55 were allowed to cool to room temperature. A carbon layer was sputtered over product chalcogenide material 55 for subsequent SEM analysis. Inspection of upper wafer 30 revealed depletion of chalcogenide source material 35. The central portion of chalcogenide source material 35 was essentially fully depleted. A ring of chalcogenide material having a diameter of several inches was observed to surround the depleted source material. The surrounding ring may be the result of redeposition of chalcogenide material on upper wafer 30 during the deposition process.
Product chalcogenide material 55 was characterized with EDX (energy dispersive x-ray) analysis and an SEM (scanning electron microscope) image on a cleaved layer. The EDX spectrum is shown in
The SEM image is shown in
The experiment was repeated for a second trial on a planar lower deposition wafer and the resulting SEM image is shown in
This example demonstrates that the instant invention permits deposition of a thin film product chalcogenide material through a process of vaporization of a solid-phase chalcogenide source material and condensation of the vapor on a remote deposition surface. Material from the source material is successfully transferred to a deposition surface to form a film that faithfully adheres to the composition of the source material.
In this example, deposition of a thin film chalcogenide material within a patterned opening on a deposition surface via vaporization of a solid-phase chalcogenide source material is demonstrated. Except for lower deposition wafer 40, the apparatus, chalcogenide source material, and procedure of deposition are as described in Example 1 hereinabove. In this experiment, lower deposition wafer 40 was replaced with a deposition wafer that included a patterned feature. Specifically, the lower deposition wafer used in this experiment included a plurality of pore feature and deposition of a product chalcogenide material in these pores was achieved.
The pore feature of lower deposition wafer 140 shown schematically in
The chalcogenide product material of a representative structure was analyzed by SEM imaging and EDX compositional analysis. To complete the analysis, the structure was cleaved. The atomic concentrations of Ge, Sb, and Te in the chalcogenide product material at a representative location were determined to be 22.4%, 19.0%, and 58.6%; respectively. These concentrations are very close to the composition of chalcogenide source material 35.
In this example, the electrical characteristics of a thin film chalcogenide material formed within a patterned opening of a deposition surface via vaporization of a solid-phase chalcogenide source material are determined.
The lower deposition wafer used for the experiments of this example was similar to lower deposition wafer 140 shown schematically in
Selected devices from an uncleaved region of the lower deposition wafer were chosen to perform electrical testing to characterize performance attributes. The electrical tests were performed by attaching external leads to lower electrode layer 255 and upper electrode layer 285. The leads were connected to an electrical testing apparatus capable of delivering pulsed or constant electrical signals over a wide range of voltages, currents and waveforms. The electrical tests included measurements of the I (current)-V (voltage) and R (resistance)-I (current) characteristics of the tested device as described hereinabove in connection with
Beginning with the third cycle of operation, the switchback response characteristic of the electrical switching event described hereinabove was observed. The switchback response was also observed for the fourth and fifth cycles of operation. A device conditioning period of a few cycles of operation following fabrication is commonly observed for phase-change devices. The data indicate that after conditioning, the device has a threshold voltage (Vt) of about 1.18 V, a holding voltage (Vh) of about 0.74 V and a dynamic resistance (derived from the slope of the conductive branch of the I-V response curve) of about 1.57 kΩ.
In the third cycle of operation, the R-I response of the device exhibited a high resistance (˜1 MΩ) at low currents followed by a pronounced decrease in resistance at ˜0.15 mA. The decrease in resistance corresponds to the set transformation of the device as described hereinabove. At still higher currents, the resistance of the device increases markedly and levels off at about 1 MΩ for currents above ˜0.95 mA. The leveling of the device resistance at the high end of the current range corresponds to the reset transition described in greater detail hereinabove. The R-I data indicate that the device has a set resistance of ˜10 kΩ and a reset resistance of ˜1 MΩ.
Testing of the device was continued beyond five operating cycles to test the endurance of the device. For practical devices, it is desirable for the device characteristics to remain stable over a large number of operating cycles.
The results of this example show that a chalcogenide material formed in accordance with the instant vaporization process displays the operational characteristics expected of phase-change memory materials. The conditioning response, set current, set resistance, reset current, reset resistance, cycling, and endurance characteristics of devices prepared in accordance with the instant invention are comparable to those observed for devices having a similar structure in which other methods were used to deposit the active chalcogenide material.
The foregoing examples demonstrate the ability of the instant sublimation process to redistribute a chalcogenide source material to fill a feature on a deposition surface. In the embodiments described hereinabove, the chalcogenide source material was spaced apart from the deposition surface. In further embodiments, the chalcogenide source material may be initially formed on the deposition surface itself and the instant sublimation process may be used to redistribute the chalcogenide source material to produce a different spatial distribution of chalcogenide material on the deposition surface. These embodiments provide a further technique for filling features (including high aspect ratio features), forming conformal layers within or over features, and/or maintaining similarity in the composition of a source and a condensed product material, as described hereinabove.
In these embodiments, the redistribution of chalcogenide source material afforded by the instant sublimation process permits a transformation of chalcogenide material from an initial, unfavorable spatial distribution to an optimal or more favorable spatial distribution. As noted hereinabove, physical vapor deposition is a non-conformal deposition technique that provides poor coverage of steps and edges and poor fills of features. Physical vapor deposition is advantageous, however, from the standpoint that it is a simple and versatile technique capable of providing a wide range of compositions. In accordance with one aspect of the instant invention, a chalcogenide source material may be deposited on a deposition surface with physical vapor deposition and subsequently spatially redistributed via the instant sublimation process. Although a chalcogenide source material deposited by physical vapor deposition is inadequate at filling or conformal covering of a feature, subsequent sublimation and condensation induces a spatial redistribution of chalcogenide material that enables conformal coverage and more complete filling of features. Accordingly, the instant invention provides a way to achieve conformal or nearly conformal deposition and/or dense or nearly dense filling of features from physical vapor deposition or other inherently non-conformal or poorly conformal deposition techniques. The method provides the benefit of quick and convenient deposition of a chalcogenide source material via non-conformal or poorly conformal technique and corrects for deficiencies in conformality, coverage, or filling density via a spatial redistribution of the chalcogenide source material via a sublimation-condensation process. The product chalcogenide material formed after spatial redistribution in accordance with this aspect of the invention may be referred to herein as a redistributed chalcogenide material.
In order to prevent significant losses of the chalcogenide source material during sublimation, a capping layer is formed over the chalcogenide source material before sublimation. The capping layer acts to seal the deposition surface after deposition of the chalcogenide source material to inhibit or prevent escape of the vaporized chalcogenide material that forms upon sublimation. The capping layer may include internal pores to facilitate the distribution of the vaporized chalcogenide material. Contrasting thermal expansion coefficients of the source chalcogenide material and the capping layer may also create voids or gaps at the interface to permit internal redistribution of the vaporized chalcogenide source material. The capping layer can be formed from any material that provides a good seal with the source chalcogenide material and/or deposition surface. Representative capping materials include polymers, photoresist materials, oxides, metals, and sol-gel materials.
Once the capping layer is formed, the source chalcogenide material is heated to induce sublimation and formation of a vapor phase. The vapor phase chalcogenide material is retained internally and prevented from escaping by the capping layer. The vapor phase chalcogenide material is then condensed back onto the deposition surface by cooling to form a redistributed chalcogenide material. Cooling can be achieved by uniformly cooling the deposition surface or selectively cooling certain portions of it. In one embodiment, selective cooling is performed in the vicinity of a feature on the deposition surface to bias the condensation process to fill or cover the feature.
Due to the lack of conformality, feature 312 is incompletely filled and the surface of initial source material 315 is textured or otherwise irregular. The extent to which feature 312 is filled depends on the circumstances of deposition, composition of the material deposited, and aspect ratio. Panel (b) shows an embodiment in which a relatively deep penetration of initial source material 315 into feature 312 occurs. In other embodiments, the depth of penetration of initial source material 315 into feature 312 upon non-conformal deposition is less than indicated in panel (b).
Following deposition of initial source material 315, capping layer 320 is next deposited to provide the structure shown in panel (c) of
Filling of feature 312 is facilitated by controlling the cooling of the sublimed source material. Since condensation occurs preferentially on cooler surfaces, filling of feature 312 can be promoted by maintaining cooler temperatures in the vicinity of feature 312. To enable redistribution of sublimed source material in the lower (deeper) portions of feature 312, it is necessary to insure that access to the bottom surface of 312 is not blocked during the condensation process. If, for example, condensation occurs more quickly along the upper sidewall surfaces of feature 312, occlusion of the bottom surface of feature 312 by the condensed material may occur and prevent sublimed material from accessing the lower portion of feature 312. Accordingly, in one embodiment, condensation of sublimed source material at the bottom surface and lower regions of feature 312 is promoted by establishing a temperature gradient in the direction normal to substrate 305. By maintaining a higher temperature at the entrance of feature 312 and a cooler temperature at the bottom of feature 312, condensation can be preferentially induced in the lower portions of feature 312 to promote more complete filling.
One approach for establishing a favorable temperature gradient is to effect sublimation by using a heat source placed above the device.
The temperatures established in the different parts of the structure will depend on the thermal conductivity of the intervening layers of the device. Dielectric material 310, for example, has a low thermal conductivity and as a result, a large temperature drop per unit thickness across dielectric material 310 is expected. Conductive layer 307, in contrast, is typically metallic and exhibits a high thermal conductivity. The temperature gradient across conductive layer 307 is accordingly reduced and may be negligible. The right side of
In one embodiment, the heat source is an electromagnetic heat source, such as an infrared lamp, that provides thermal electromagnetic radiation. In this embodiment, antireflective layer 340 transmits infrared or thermal radiation from the heat source and mask layer 345 reflects infrared or thermal radiation from the heat source. Antireflective material 340 may, for example, be a material having a bandgap energy that exceeds the energy of the radiation provided by the electromagnetic heat source. Many metal oxides and metal nitrides, for example, have bandgaps higher in energy than the energy of infrared electromagnetic radiation. Representative antireflective materials include transition metal oxides (e.g. TiO2, ZnO), transition metal nitrides (e.g. TiN), nitrides or oxides of aluminum (e.g. AlN, Al2O3), and nitrides or oxides of silicon (e.g. Si3N4, SiO2). Mask layer 345, in contrast, is generally a metal or low energy bandgap material that efficiently reflects or absorbs radiation produced by the electromagnetic heat source. Metals such as Al, Ti, or Cu, for example, efficiently reflect infrared or thermal radiation. In one embodiment, mask layer 345 reflects or absorbs at least 80% of the electromagnetic radiation provided by the electromagnetic heat source. In another embodiment, mask layer 345 reflects or absorbs at least 90% of the electromagnetic radiation provided by the electromagnetic heat source. In a further embodiment, mask layer 345 reflects or absorbs at least 95% of the electromagnetic radiation provided by the electromagnetic heat source.
Upon conclusion of the sublimation-condensation process, antireflective layer 340 and mask layer 345 may be removed and device processing may continue with formation of conductive layer over the redistributed source material to provide a second electrical contact.
While not wishing to be bound by theory, the instant inventor surmises that the instant sublimation methods are more effective for source materials that sublime congruently or that sublime in molecular form. If the source material sublimes non-congruently or by element, and the rate of sublimation differs appreciably for different elements, the sublimed vapor phase will differ in composition from the original source material and subsequent condensation may produce a product material that differs in composition from the original source material. Chalcogenide materials are believed to be especially amendable to the instant sublimation process because of an expected similarity of the compositions of the sublimed vapor phase and the original source material.
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.
This application is a continuation in part of U.S. patent application Ser. No. 12/369,807, entitled “Deposition of Chalcogenide Material via Vaporization Process”, and filed on Feb. 12, 2009, the disclosure of which is hereby incorporated by reference in its entirety herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12369807 | Feb 2009 | US |
| Child | 12494933 | US |
