Patent Publication Number: US-9842857-B2

Title: Differential etch of metal oxide blocking dielectric layer for three-dimensional memory devices

Description:
FIELD 
     The present disclosure relates generally to the field of semiconductor devices and specifically to three-dimensional semiconductor devices, such as vertical NAND strings, and methods of making thereof. 
     BACKGROUND 
     Three-dimensional vertical NAND strings having one bit per cell are disclosed in an article by T. Endoh, et. al., titled “Novel Ultra High Density Memory With A Stacked-Surrounding Gate Transistor (S-SGT) Structured Cell”, IEDM Proc. (2001) 33-36. 
     A dielectric metal oxide material can be employed for a blocking dielectric layer that provides electrical isolation between control gate electrodes and memory elements configured to store electrical charges. In case the dielectric metal oxide blocking dielectric layer is formed inside a memory opening, a bottom portion of the dielectric metal oxide blocking dielectric layer needs to be removed by an anisotropic etch to physically expose a semiconductor surface of the substrate prior to forming a vertical semiconductor channel. However, anisotropic etch of the dielectric metal oxide material of the blocking dielectric layer poses a challenge due to low selectivity of the etch process and relative high collateral etch rate of the vertical portions of the blocking dielectric layer. 
     SUMMARY 
     According to an aspect of the present disclosure, a method of manufacturing a semiconductor structure includes forming a stack of alternating layers comprising insulating layers and spacer material layers over a semiconductor substrate, forming a memory opening through the stack, forming an aluminum oxide layer having a horizontal portion at a bottom of the memory opening and a vertical portion at least over a sidewall of the memory opening, where the horizontal portion differs from the vertical portion by at least one of structure or composition, and selectively etching the horizontal portion selective to the vertical portion. 
     According to another aspect of the present disclosure, a monolithic three-dimensional device structure is provided, which comprises a stack of alternating layers comprising insulator layers and electrically conductive layers and located over a semiconductor substrate; a memory stack structure extending through the stack of alternating layers, the memory stack structure comprising a blocking dielectric layer, a memory material layer, and a tunneling dielectric layer; and a semiconductor channel located within the memory stack structure. The blocking dielectric layer comprises at least an aluminum oxide layer laterally surrounding, and contacting, the memory material layer. A bottom surface of the aluminum oxide layer is vertically spaced from a bottom surface of the memory material layer by a horizontal portion of the memory material layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view of an exemplary structure after formation of a stack including an alternating plurality of material layers and memory openings extending through the stack according to an embodiment of the present disclosure. 
         FIGS. 2A-2L  are sequential vertical cross-sectional views of a memory opening within the exemplary structure during various processing steps employed to form a first exemplary memory stack structure according to an embodiment of the present disclosure. 
         FIGS. 2M-2O  are sequential vertical cross-sectional views of a memory opening within the exemplary structure during various processing steps employed to form a variation of the first exemplary memory stack structure according to an embodiment of the present disclosure. 
         FIGS. 3A-3F  are sequential vertical cross-sectional views of a memory opening within the exemplary structure during various processing steps employed to form a second exemplary memory stack structure according to an embodiment of the present disclosure. 
         FIG. 4  is a vertical cross-sectional view of the exemplary structure after formation of memory stack structures according to an embodiment of the present disclosure. 
         FIG. 5  is a vertical cross-sectional view of the exemplary structure after formation of a stepped terrace and a retro-stepped dielectric material portion according to an embodiment of the present disclosure. 
         FIG. 6A  is a vertical cross-sectional view of the exemplary structure after formation of a backside contact trench according to an embodiment of the present disclosure. 
         FIG. 6B  is a partial see-through top-down view of the exemplary structure of  FIG. 6A . The vertical plane A-A′ is the plane of the vertical cross-sectional view of  FIG. 6A . 
         FIG. 7  is a vertical cross-sectional view of the exemplary structure after formation of backside recesses according to an embodiment of the present disclosure. 
         FIG. 8  is a vertical cross-sectional view of the exemplary structure after formation of electrically conductive layers according to an embodiment of the present disclosure. 
         FIG. 9  is a vertical cross-sectional view of the exemplary structure after removing excess conductive material portions according to an embodiment of the present disclosure. 
         FIG. 10  is a vertical cross-sectional view of the exemplary structure after formation of various contact via structures according to an embodiment of the present disclosure. 
         FIGS. 11A and 11B  are transmission electron micrographs (TEMs) of two samples on which a stack of aluminum-carbon-oxygen alloy portion and an aluminum oxide layer was grown on a hydrogen-terminated silicon surface employing an atomic layer deposition process according to an embodiment of the present disclosure. 
         FIG. 11C  is a transmission electron micrograph (TEM) of a sample that was prepared by an atomic layer deposition process to form a stack of aluminum-carbon-oxygen alloy portion and an aluminum oxide layer and was subsequently annealed at 1,000 degrees Celsius to form a carbon-doped aluminum oxygen compound portion according to an embodiment of the present disclosure. 
         FIG. 12  is an electron energy loss spectroscopy (EELS) plot of carbon, nitrogen, oxygen, chromium, aluminum and silicon signal amplitudes (arbitrary units) versus depth in the sample (in nanometers) for the sample of  FIG. 11C . 
     
    
    
     DETAILED DESCRIPTION 
     As discussed above, the present disclosure is directed to three-dimensional memory structures, such as vertical NAND strings and other three-dimensional devices, and methods of making thereof, the various aspects of which are described below. The embodiments of the disclosure can be employed to form various structures including a multilevel memory structure, non-limiting examples of which include semiconductor devices such as three-dimensional monolithic memory array devices comprising a plurality of NAND memory strings. The drawings are not drawn to scale. Multiple instances of an element may be duplicated where a single instance of the element is illustrated, unless absence of duplication of elements is expressly described or clearly indicated otherwise. Ordinals such as “first,” “second,” and “third” are employed merely to identify similar elements, and different ordinals may be employed across the specification and the claims of the instant disclosure. As used herein, a first element located “on” a second element can be located on the exterior side of a surface of the second element or on the interior side of the second element. As used herein, a first element is located “directly on” a second element if there exist a physical contact between a surface of the first element and a surface of the second element. 
     As used herein, a “layer” refers to a material portion including a region having a substantially uniform thickness. A layer may extend over the entirety of an underlying or overlying structure, or may have an extent less than the extent of an underlying or overlying structure. Further, a layer may be a region of a homogeneous or inhomogeneous contiguous structure that has a thickness less than the thickness of the contiguous structure. For example, a layer may be located between any pair of horizontal planes between, or at, a top surface and a bottom surface of the contiguous structure. A layer may extend horizontally, vertically, and/or along a tapered surface. A substrate may be a layer, may include one or more layers therein, and/or may have one or more layer thereupon, thereabove, and/or therebelow. 
     A monolithic three-dimensional memory array is one in which multiple memory levels are formed above a single substrate, such as a semiconductor wafer, with no intervening substrates. The term “monolithic” means that layers of each level of the array are directly deposited on the layers of each underlying level of the array. In contrast, two dimensional arrays may be formed separately and then packaged together to form a non-monolithic memory device. For example, non-monolithic stacked memories have been constructed by forming memory levels on separate substrates and vertically stacking the memory levels, as described in U.S. Pat. No. 5,915,167 titled “Three-dimensional Structure Memory.” The substrates may be thinned or removed from the memory levels before bonding, but as the memory levels are initially formed over separate substrates, such memories are not true monolithic three-dimensional memory arrays. The various three-dimensional memory devices of the present disclosure include a monolithic three-dimensional NAND string memory device, and can be fabricated employing the various embodiments described herein. 
     Referring to  FIG. 1 , an exemplary structure according to an embodiment of the present disclosure is illustrated, which can be employed, for example, to fabricate a device structure containing vertical NAND memory devices. The exemplary structure includes a substrate, which can be a semiconductor substrate. The substrate can include a substrate semiconductor layer  9 . The substrate semiconductor layer  9  is a semiconductor material layer, and can include at least one elemental semiconductor material, at least one III-V compound semiconductor material, at least one II-VI compound semiconductor material, at least one organic semiconductor material, or other semiconductor materials known in the art. The substrate can have a major surface  7 , which can be, for example, a topmost surface of the substrate semiconductor layer  9 . The major surface  7  can be a semiconductor surface. In one embodiment, the major surface  7  can be a single crystalline semiconductor surface. 
     As used herein, a “semiconductor material” refers to a material having electrical conductivity in the range from 1.0×10 −6  S/cm to 1.0×10 5  S/cm, and is capable of producing a doped material having electrical conductivity in a range from 1.0 S/cm to 1.0×10 5  S/cm upon suitable doping with an electrical dopant. As used herein, an “electrical dopant” refers to a p-type dopant that adds a hole to a valence band within a band structure, or an n-type dopant that adds an electron to a conduction band within a band structure. As used herein, a “conductive material” refers to a material having electrical conductivity greater than 1.0×10 5  S/cm. As used herein, an “insulator material” or a “dielectric material” refers to a material having electrical conductivity less than 1.0×10 −6  S/cm. All measurements for electrical conductivities are made at the standard condition. Optionally, at least one doped well (not expressly shown) can be formed within the substrate semiconductor layer  9 . 
     At least one semiconductor device for a peripheral circuitry can be formed on a portion of the substrate semiconductor layer  9 . The at least one semiconductor device can include, for example, field effect transistors. For example, at least one shallow trench isolation structure  120  can be formed by etching portions of the substrate semiconductor layer  9  and depositing a dielectric material therein. A gate dielectric layer, at least one gate conductor layer, and a gate cap dielectric layer can be formed over the substrate semiconductor layer  9 , and can be subsequently patterned to form at least one gate structure ( 150 ,  152 ,  154 ,  158 ), each of which can include a gate dielectric  150 , at least one gate electrode ( 152 ,  154 ), and a gate cap dielectric. A gate electrode ( 152 ,  154 ) may include a stack of a first gate electrode portion  152  and a second gate electrode portion  154 . At least one gate spacer  156  can be formed around the at least one gate structure ( 150 ,  152 ,  154 ,  158 ) by depositing and anisotropically etching a conformal dielectric layer. Active regions  130  can be formed in upper portions of the substrate semiconductor layer  9 , for example, by introducing electrical dopants employing the at least one gate structure ( 150 ,  152 ,  154 ,  158 ) as masking structures. Additional masks may be employed as needed. The active region  130  can include source regions and drain regions of field effect transistors. A first dielectric liner  161  and a second dielectric liner  162  can be optionally formed. Each of the first and second dielectric liners ( 161 ,  162 ) can comprise a silicon oxide layer, a silicon nitride layer, and/or a dielectric metal oxide layer. In an illustrative example, the first dielectric liner  161  can be a silicon oxide layer, and the second dielectric liner  162  can be a silicon nitride layer. The least one semiconductor device for the peripheral circuitry can contain a driver circuit for memory devices to be subsequently formed, which can include at least one NAND device. 
     A dielectric material such as silicon oxide can be deposited over the at least one semiconductor device, and can be subsequently planarized to form a planarization dielectric layer  170 . In one embodiment the planarized top surface of the planarization dielectric layer  170  can be coplanar with a top surface of the dielectric liners ( 161 ,  162 ). Subsequently, the planarization dielectric layer  170  and the dielectric liners ( 161 ,  162 ) can be removed from an area to physically expose a top surface of the substrate semiconductor layer  9 . 
     An optional semiconductor material layer  10  can be formed on the top surface of the substrate semiconductor layer  9  by deposition of a single crystalline semiconductor material, for example, by selective epitaxy. The deposited semiconductor material can be the same as, or can be different from, the semiconductor material of the substrate semiconductor layer  9 . The deposited semiconductor material can be any material that can be employed for the semiconductor substrate layer  9  as described above. The single crystalline semiconductor material of the semiconductor material layer  10  can be in epitaxial alignment with the single crystalline structure of the substrate semiconductor layer  9 . Portions of the deposited semiconductor material located above the top surface of the planarization dielectric layer  170  can be removed, for example, by chemical mechanical planarization (CMP). In this case, the semiconductor material layer  10  can have a top surface that is coplanar with the top surface of the planarization dielectric layer  170 . 
     A planar dielectric layer  31  can be formed above the semiconductor material layer  10 . The planar dielectric layer  31  can be a gate dielectric layer that vertically separates horizontal portions of semiconductor channels in an upper portion of the semiconductor material layer  10  and select gate electrodes to be subsequently formed by replacing a bottommost sacrificial material layer  42 . The planar dielectric layer  31  can be, for example, a silicon oxide layer, a dielectric metal oxide layer, or a combination thereof. The thickness of the planar dielectric layer  31  can be in a range from 1 nm to 10 nm, although lesser and greater thicknesses can also be employed. 
     A stack of an alternating plurality of first material layers (which can be insulating layers  32 ) and second material layers (which can be sacrificial material layer  42 ) is formed over the top surface of the substrate, which can be, for example, on the top surface of the planar dielectric layer  31 . As used herein, a “material layer” refers to a layer including a material throughout the entirety thereof. As used herein, an alternating plurality of first elements and second elements refers to a structure in which instances of the first elements and instances of the second elements alternate. Each instance of the first elements that is not an end element of the alternating plurality is adjoined by two instances of the second elements on both sides, and each instance of the second elements that is not an end element of the alternating plurality is adjoined by two instances of the first elements on both ends. The first elements may have the same thickness thereamongst, or may have different thicknesses. The second elements may have the same thickness thereamongst, or may have different thicknesses. The alternating plurality of first material layers and second material layers may begin with an instance of the first material layers or with an instance of the second material layers, and may end with an instance of the first material layers or with an instance of the second material layers. In one embodiment, an instance of the first elements and an instance of the second elements may form a unit that is repeated with periodicity within the alternating plurality. 
     Each first material layer includes a first material, and each second material layer includes a second material that is different from the first material. In one embodiment, each first material layer can be an insulating layer  32 , and each second material layer can be a spacer material layer that provides vertical separation between a vertically neighboring pair of insulator layers  32 . 
     In one embodiment, the spacer material layers can be permanent layers that are not subsequently replaced with a different material. In this case, the spacer material layers can include a conductive material such as doped semiconductor material (such as doped polysilicon or a doped silicon-containing alloy). Alternatively, each first material layer can be an insulating layer  32 , and each second material layer can be a sacrificial material layer. In this case, the stack can include an alternating plurality of insulating layers  32  and sacrificial material layers  42 , and constitutes a prototype stack of alternating layers comprising insulating layers  32  and sacrificial material layers  42 . As used herein, a “prototype” structure or an “in-process” structure refers to a transient structure that is subsequently modified in the shape or composition of at least one component therein. 
     The stack of the alternating plurality is herein referred to as an alternating stack ( 32 ,  42 ). In one embodiment, the alternating stack ( 32 ,  42 ) can include insulating layers  32  composed of the first material, and sacrificial material layers  42  composed of a second material different from that of insulating layers  32 . The first material of the insulating layers  32  can be at least one insulating material. As such, each insulating layer  32  can be an insulating material layer. Insulating materials that can be employed for the insulating layers  32  include, but are not limited to, silicon oxide (including doped or undoped silicate glass), silicon nitride, silicon oxynitride, organosilicate glass (OSG), spin-on dielectric materials, dielectric metal oxides that are commonly known as high dielectric constant (high-k) dielectric oxides (e.g., aluminum oxide, hafnium oxide, etc.) and silicates thereof, dielectric metal oxynitrides and silicates thereof, and organic insulating materials. In one embodiment, the first material of the insulating layers  32  can be silicon oxide. 
     The second material of the sacrificial material layers  42  is a sacrificial material that can be removed selective to the first material of the insulating layers  32 . As used herein, a removal of a first material is “selective to” a second material if the removal process removes the first material at a rate that is at least twice the rate of removal of the second material. The ratio of the rate of removal of the first material to the rate of removal of the second material is herein referred to as a “selectivity” of the removal process for the first material with respect to the second material. 
     The sacrificial material layers  42  may comprise an insulating material, a semiconductor material, or a conductive material. The second material of the sacrificial material layers  42  can be subsequently replaced with electrically conductive electrodes which can function, for example, as control gate electrodes of a vertical NAND device. Non-limiting examples of the second material include silicon nitride, an amorphous semiconductor material (such as amorphous silicon), and a polycrystalline semiconductor material (such as polysilicon). In one embodiment, the sacrificial material layers  42  can be spacer material layers that comprise silicon nitride or a semiconductor material including at least one of silicon and germanium. 
     In one embodiment, the insulating layers  32  can include silicon oxide, and sacrificial material layers can include silicon nitride sacrificial material layers. The first material of the insulating layers  32  can be deposited, for example, by chemical vapor deposition (CVD). For example, if silicon oxide is employed for the insulating layers  32 , tetraethyl orthosilicate (TEOS) can be employed as the precursor material for the CVD process. The second material of the sacrificial material layers  42  can be formed, for example, CVD or atomic layer deposition (ALD). 
     The sacrificial material layers  42  can be suitably patterned so that conductive material portions to be subsequently formed by replacement of the sacrificial material layers  42  can function as electrically conductive electrodes, such as the control gate electrodes of the monolithic three-dimensional NAND string memory devices to be subsequently formed. The sacrificial material layers  42  may comprise a portion having a strip shape extending substantially parallel to the major surface  7  of the substrate. 
     The thicknesses of the insulating layers  32  and the sacrificial material layers  42  can be in a range from 20 nm to 50 nm, although lesser and greater thicknesses can be employed for each insulating layer  32  and for each sacrificial material layer  42 . The number of repetitions of the pairs of an insulating layer  32  and a sacrificial material layer (e.g., a control gate electrode or a sacrificial material layer)  42  can be in a range from 2 to 1,024, and typically from 8 to 256, although a greater number of repetitions can also be employed. The top and bottom gate electrodes in the stack may function as the select gate electrodes. In one embodiment, each sacrificial material layer  42  in the alternating stack ( 32 ,  42 ) can have a uniform thickness that is substantially invariant within each respective sacrificial material layer  42 . 
     Optionally, an insulating cap layer  70  can be formed over the alternating stack ( 32 ,  42 ). The insulating cap layer  70  includes a dielectric material that is different from the material of the sacrificial material layers  42 . In one embodiment, the insulating cap layer  70  can include a dielectric material that can be employed for the insulating layers  32  as described above. The insulating cap layer  70  can have a greater thickness than each of the insulating layers  32 . The insulating cap layer  70  can be deposited, for example, by chemical vapor deposition. In one embodiment, the insulating cap layer  70  can be a silicon oxide layer. 
     Subsequently, a lithographic material stack (not shown) including at least a photoresist layer can be formed over the insulating cap layer  70  and the alternating stack ( 32 ,  42 ), and can be lithographically patterned to form openings therein. The pattern in the lithographic material stack can be transferred through the insulating cap layer  70  and through entirety of the alternating stack ( 32 ,  42 ) by at least one anisotropic etch that employs the patterned lithographic material stack as an etch mask. Portions of the alternating stack ( 32 ,  42 ) underlying the openings in the patterned lithographic material stack are etched to form memory openings  49 . In other words, the transfer of the pattern in the patterned lithographic material stack through the alternating stack ( 32 ,  42 ) forms the memory openings  49  that extend through the alternating stack ( 32 ,  42 ). The chemistry of the anisotropic etch process employed to etch through the materials of the alternating stack ( 32 ,  42 ) can alternate to optimize etching of the first and second materials in the alternating stack ( 32 ,  42 ). The anisotropic etch can be, for example, a series of reactive ion etches. Optionally, the planar dielectric layer  31  may be used as an etch stop layer between the alternating stack ( 32 ,  42 ) and the substrate. The sidewalls of the memory openings  49  can be substantially vertical, or can be tapered. The patterned lithographic material stack can be subsequently removed, for example, by ashing. 
     The memory openings  49  are formed through the planar dielectric layer  31  so that the memory openings  49  extend from the top surface of the alternating stack ( 32 ,  42 ) to the top surface of the semiconductor material layer  10  within the substrate between the lower select gate electrodes. In one embodiment, an overetch into the semiconductor material layer  10  may be optionally performed after the top surface of the semiconductor material layer  10  is physically exposed at a bottom of each memory opening  49 . The overetch may be performed prior to, or after, removal of the lithographic material stack. In other words, the recessed surfaces of the semiconductor material layer  10  may be vertically offset from the undressed top surfaces of the semiconductor material layer  10  by a recess depth. The recess depth can be, for example, in a range from 1 nm to 50 nm, although lesser and greater recess depths can also be employed. The overetch is optional, and may be omitted. If the overetch is not performed, the bottom surface of each memory opening  49  can be coplanar with the topmost surface of the semiconductor material layer  10 . Each of the memory openings  49  can include a sidewall (or a plurality of sidewalls) that extends substantially perpendicular to the topmost surface of the substrate. The region in which the array of memory openings  49  is formed is herein referred to as a device region. The substrate semiconductor layer  9  and the semiconductor material layer  10  collectively constitutes a substrate ( 9 ,  10 ), which can be a semiconductor substrate. Alternatively, the semiconductor material layer  10  may be omitted, and the memory openings  49  can be extend to a top surface of the semiconductor material layer  10 . 
     A memory stack structure can be formed in each of the memory opening employing various embodiments of the present disclosure.  FIGS. 2A-2L  illustrate sequential vertical cross-sectional views of a memory opening within the exemplary structure during formation of a first exemplary memory stack structure according to a embodiment of the present disclosure. Formation of the exemplary memory stack structure can be performed within each of the memory openings  49  in the exemplary structure illustrated in  FIG. 1 . 
     Referring to  FIG. 2A , a memory opening  49  in the exemplary structure of  FIG. 1  is illustrated in a magnified view. The memory opening  49  extends through the insulating cap layer  70 , the alternating stack ( 32 ,  42 ), the planar dielectric layer  31 , and optionally into an upper portion of the semiconductor material layer  10 . The recess depth of the bottom surface of each memory opening with respect to the top surface of the semiconductor material layer  10  can be in a range from 0 nm to 30 nm, although greater recess depths can also be employed. Optionally, the sacrificial material layers  42  can be laterally recessed partially to form lateral recesses (not shown), for example, by an isotropic etch. 
     Referring to  FIG. 2B , an optional epitaxial channel portion  11  can be formed at the bottom portion of each memory opening  49 , for example, by selective epitaxy. Each epitaxial channel portion  11  comprises a single crystalline semiconductor material in epitaxial alignment with the single crystalline semiconductor material of the semiconductor material layer  10 . In one embodiment, the epitaxial channel portion  11  can be doped with electrical dopants of the same conductivity type as the semiconductor material layer  10 . In one embodiment, the top surface of each epitaxial channel portion  11  can be formed above a horizontal plane including the top surface of a sacrificial material layer  42 . In this case, at least one source select gate electrode can be subsequently formed by replacing each sacrificial material layer  42  located below the horizontal plane including the top surfaces of the epitaxial channel portions  11  with a respective conductive material layer. 
     Referring to  FIG. 2C , an optional outer blocking dielectric layer  501  can be deposited by a conformal deposition method. If employed, the outer blocking dielectric layer  501  is employed as a sub-layer of a blocking dielectric layer. As used herein, a “sub-layer” refers to a layer that is a component layer of another structure that constitutes a layer. In one embodiment, the outer blocking dielectric layer  501  can include silicon oxide, silicon oxynitride, silicon nitride, a dielectric metal oxide other than aluminum oxide (such as hafnium oxide, lanthanum oxide, or zirconium oxide), or a combination thereof. In one embodiment, the outer blocking dielectric layer  501  can include silicon oxide (e.g., SiO 2 ). The outer blocking dielectric layer  501  can be formed by a conformal deposition method such as low pressure chemical vapor deposition, atomic layer deposition, or a combination thereof. The thickness of the outer blocking dielectric layer  501  can be in a range from 1 nm to 20 nm (such as from 2 nm to 4 nm), although lesser and greater thicknesses can also be employed. Alternatively, the outer blocking dielectric layer  501  can be omitted, and a backside blocking dielectric layer can be formed after formation of backside recesses. A cavity  49 ′ is present in the volume of the memory opening that is not filled with the epitaxial channel portion  11  or the outer blocking dielectric layer  501 . 
     Referring to  FIG. 2D , the outer blocking dielectric layer  501  is anisotropically etched, for example, by a reactive ion etch process, to remove horizontal portions. The top surface of the epitaxial channel portion  11  is physically exposed at the bottom of the cavity  49 ′. If the epitaxial channel portion  11  is omitted then the top portion of layer  10  is exposed. The remaining vertical portions of the outer blocking dielectric layer  501  within the memory opening  49  constitute an outer blocking dielectric spacer, which can be homeomorphic to a torus. As used herein, an element is homeomorphic to a torus if the shape of the element can be continuously transformed into a shape of a torus without creating a new hole or without destroying an existing hole. The outer blocking dielectric layer  501  as an outer spacer is formed directly on the sidewall surface of the memory opening  49 . In one embodiment, the outer blocking dielectric layer  501  can comprise a silicon oxide spacer. 
     Referring to  FIG. 2E , an atomic layer deposition process is performed to deposit aluminum oxide or an aluminum-carbon-oxygen alloy depending on surface conditions. The aluminum oxide may be stoichiometric (e.g., Al 2 O 3 ) or non-stoichiometric (e.g., having an Al:O atomic ratio of greater than or less than 2:3, such as 1.5:3 to 2.5:3). The atomic layer deposition process employs an organic (i.e., carbon containing) precursor gas including at least one aluminum atom and an oxidizer gas. The organic precursor gas and the oxidizer gas are alternately flowed into a process chamber including the semiconductor substrate ( 9 ,  10 ) during the atomic layer deposition process. 
     The combination of the organic precursor gas and the oxidizer gas can be selected such that aluminum oxide is deposited on dielectric surfaces such as the inner sidewall of the outer blocking dielectric layer  501  (which is a dielectric spacer), while an aluminum-carbon-oxygen alloy material is deposited on physically exposed semiconductor surfaces such as the top surface of the epitaxial channel portion  11  (or top surface of layer  10  if the epitaxial channel portion  11  is omitted). The aluminum-carbon-oxygen alloy portion  573  can be deposited on the bottom surface of memory opening concurrently with deposition of a vertical portion of the aluminum oxide layer  503  over the sidewalls of the memory opening  49 . 
     The aluminum-carbon-oxygen alloy portion  573  and the aluminum oxide layer  503  are formed simultaneously by atomic layer deposition process employing the same precursor gas and the same oxidizer gas. Incorporation of carbon into the aluminum-carbon-oxygen alloy portion  573  is a phenomenon that is dependent on the surface condition of the top surface of the epitaxial channel portion  11  and on the selection of the precursor gas and the oxidizer gas. In other words, selection of the precursor gas and the oxidizer gas affects the carbon content in the aluminum-carbon-oxygen alloy portion  573 . Thus, the total amount of carbon that can be deposited on the top surface of the epitaxial channel portion can be maximized by selecting an optimal combination of the precursor gas and the oxidizer gas. Generally, a higher carbon concentration and a greater thickness is preferred for aluminum-carbon-oxygen alloy portion  573  so as to maximize the carbon content that is deposited over the top surface of the epitaxial channel portion. 
     Generally, an organic precursor gas including at least one aluminum atom within the molecule can be employed to induce deposition of the aluminum-carbon-oxygen alloy portion  573  with variable carbon content within the aluminum-carbon-oxygen alloy portion  573 . Of aluminum-containing organic precursors, trimethylaluminum (Al 2 (CH 3 ) 6 ) may provide incorporation of a significant amount of carbon into aluminum oxide. In some embodiments, atomic carbon content above 50 atomic % can be achieved. The oxidizer gas can be, for example, oxygen, ozone or water vapor. Water vapor may provide a higher carbon content than oxygen gas as an oxidizing agent. In one embodiment, the organic precursor gas comprises trimethylaluminum, and the oxidizer gas comprises water vapor. 
     Incorporation of carbon can be enhanced by surface treatment of the top surface of the epitaxial channel portion  11 . In one embodiment, the semiconductor substrate ( 9 ,  10 ) includes a silicon-containing semiconductor material (such as silicon or a silicon-germanium alloy), and the semiconductor surface (i.e., the top surface of the epitaxial channel portion  11 ) at the bottom of the memory opening can be a surface of the silicon-containing semiconductor material. In this case, the semiconductor surface at the bottom of the memory opening can be treated to form a hydrogen-terminated surface prior to the atomic layer deposition process. In an illustrative example, a wet etch in a dilute hydrofluoric acid or a HF vapor etch can be employed to remove the native oxide on the top surface of the epitaxial channel portion  11 , and to provide a hydrogen-terminated surface at the top surface of the epitaxial channel portion  11 . 
     The temperature of the atomic layer deposition process can be selected to maximize carbon incorporation in the aluminum-carbon-oxygen alloy portion  573 . For example, if the organic precursor gas comprises trimethylaluminum and the oxidizer gas comprises water vapor, the deposition temperature can be in a range from 100 degrees Celsius to 250 degrees Celsius (for example, between 125 degrees Celsius and 150 degrees Celsius), although lower and higher temperatures can also be employed. The pulse duration for the organic precursor gas flow and the pulse duration for the oxidizer gas flow can also be optimized to increase carbon incorporation. 
     The thickness of the aluminum-carbon-oxygen alloy portion  573  can be self-limiting because the material of the aluminum-carbon-oxygen alloy portion  573  provides a surface that is conductive to deposition of aluminum oxide, i.e., a compound of aluminum and oxygen that is substantially free of carbon. In one embodiment, the thickness of the aluminum-carbon-oxygen alloy portion  573  can be in a range from 1 monolayer to 5 monolayers (e.g., 1-1.5 nm) depending on the selection of the organic precursor gas and the oxidizer gas. The average atomic carbon concentration of the aluminum-carbon-oxygen alloy portion  573  can be in a range from 20 to 70 at. % (such as from 40 to 60 at. %), although lesser and greater atomic concentrations can also be employed. The balance of the atomic composition can be a combination of oxygen atoms and aluminum atoms. In one embodiment, the atomic concentration of aluminum in the aluminum-carbon-oxygen alloy portion  573  can be in a range from 5 to 20 at. %, and the atomic concentration of oxygen in the aluminum-carbon-oxygen alloy portion  573  can be in a range from 10 to 50 at. %, although lesser and greater atomic concentrations can also be employed for each of aluminum and oxygen. The horizontal aluminum-carbon-oxygen alloy portion  573  has more carbon than the remainder (e.g., vertical portion) of the aluminum oxide layer  503 , which may have 0 to 10 at. % carbon. In one embodiment, the horizontal aluminum-carbon-oxygen alloy portion  573  is thinner than the remainder (e.g., vertical portion) of the aluminum oxide layer  503 . For example, the horizontal aluminum-carbon-oxygen alloy portion  573  is 15-30 percent thinner (e.g., 1-1.5 nm thick) than the remainder of the aluminum oxide layer  503  (e.g., 2-2.5 nm thick). 
     Referring to  FIG. 2F , formation of the aluminum-carbon-oxygen alloy portion  573  proceeds directly on, or in the immediate vicinity of, a semiconductor surface. Optional continuation of the atomic layer deposition process forms aluminum oxide composed essentially of aluminum and oxygen only over the aluminum-carbon-oxygen alloy portion  573 . Thus, a horizontal portion of the aluminum oxide layer  503  is deposited over the aluminum-carbon-oxygen alloy portion  573  as the atomic layer deposition process continues. The atomic layer deposition process can be continued until the vertical portion of the aluminum oxide layer  503  directly on the inner sidewall of the outer blocking dielectric spacer  501  reaches a target thickness, which can be in a range from 2 nm to 15 nm, although lesser and greater target thicknesses can also be employed. 
     Referring to  2 G, an anneal process is performed at an elevated temperature to react the aluminum-carbon-oxygen alloy portion  573  and the horizontal portion of the aluminum oxide layer  503  and/or underlying silicon material (e.g., portion  11  or layer  10  if portion  11  is omitted). This anneal process crystallizes the amorphous material of the aluminum oxide layer  503  as deposited into a polycrystalline aluminum oxide material. During the anneal process that converts the vertical portions of the aluminum oxide layer  503  on the inner sidewalls of the outer blocking material layer  501 , the combination of the horizontal portion of the aluminum oxide layer  503  and the aluminum-carbon-oxygen alloy portion  573  is converted into an alloy of C, Al and O. The anneal process is performed at an elevated temperature that is sufficiently high to induce diffusion of the carbon atoms in the aluminum-carbon-oxygen alloy portion  573  employing an anneal process to form the carbon-doped aluminum oxygen compound portion. As used herein, an “aluminum oxygen compound” refers to a compound that includes aluminum and oxygen. As used herein, a “doped aluminum oxygen compound” refers to a compound that includes aluminum, oxygen, and at least another element (e.g., carbon and/or silicon) that is not aluminum or oxygen. The temperature of the anneal process can be, for example, in a range from 700 degrees Celsius to 1,150 degrees Celsius (for example, from 900 degrees Celsius to 1,050 degrees Celsius, including 950 to 1000 degrees Celsius, such as an RTA process conducted for 15-60 seconds, such as 30-45 seconds in an inert ambient, such as nitrogen ambient), although lesser and greater temperatures can also be employed. 
     A doped aluminum oxygen compound portion  583  is formed at a bottom surface of the memory opening by interdiffusion of the materials of the aluminum-carbon-oxygen alloy portion  573  and the horizontal portion of the aluminum oxide layer  503  and/or the silicon substrate material ( 10  or  11 ). In one embodiment, a silicon oxide layer may be formed between the silicon substrate material ( 10  or  11 ) and the portion  573 . Without wishing to be bound by a particular theory, some semiconductor atoms (e.g., silicon) may diffuse into the doped aluminum oxygen compound portion  583  during the anneal process from the silicon portion  11 . The doped aluminum oxygen compound portion  583  can include each element in the aluminum-carbon-oxygen alloy portion  573 , and thus, includes aluminum, carbon, and oxygen and optionally silicon. Due to the presence of the carbon atoms, the doped aluminum oxygen compound portion  583  is carbon doped, and thus, is a carbon-doped aluminum oxide compound portion, such as a silicon and carbon doped aluminum oxide. The thickness of the doped aluminum oxygen compound portion  583  may be substantially the same as the thickness of the stack of the aluminum-carbon-oxygen alloy portion  573  and the horizontal portion of the aluminum oxide layer  503 , or may be greater (e.g., 3-15 nm, such as 4-8 nm) than the thickness of the stack aluminum-carbon-oxygen alloy portion  573  and the horizontal portion of the aluminum oxide layer  503  due to incorporation of additional materials (such as some semiconductor material (e.g., Si) from the epitaxial portion  11  and/or additional gas such as oxygen and/or nitrogen during the anneal process). In one embodiment, the doped aluminum oxygen compound portion  583  can comprise a carbon and silicon doped aluminum oxygen compound portion. The carbon, silicon, oxygen and/or aluminum content in the portion  583  may vary as a function of thickness of the portion  583 . There may be more carbon than aluminum in all or some regions of the portions  583 . 
     The vertical portion of the aluminum oxide layer  503  on the opening sidewall does not significantly change the composition, and may change from an as-deposited amorphous phase to a polycrystalline phase. Alternatively, it may be as-deposited as polycrystalline aluminum oxide. In one embodiment, the aluminum oxide layer  503  on the inner sidewall of the outer blocking dielectric spacer  501  and over the insulating cap layer  70  can consist essentially of aluminum oxide. The horizontal part of layer  503  on portion  573  may be interdiffused with portion  573  during the anneal to form the portion  583 . 
     Referring to  FIG. 2H , a silicon oxide layer (not shown) having a thickness in a range from 0.5 nm to 6 nm can be optionally deposited on the inner sidewalls of the aluminum oxide layer. If a silicon oxide layer is deposited, the silicon oxide layer directly contacts a memory material layer to be subsequently formed. A selective anisotropic etch can be performed to etch a horizontal portion of the optional silicon oxide layer and the doped aluminum oxygen compound portion  583  without significantly etching the vertical portion of the aluminum oxide layer  503  (or the vertical portion of the silicon oxide layer) on the sidewall of the outer blocking dielectric spacer  501 . In one embodiment, the anisotropic etch can employ a plasma of O 2  or a mix of CF 4  and O 2 . Alternative fluorocarbon etchant gas and/or alternative chlorofluorocarbon etchant gas and/or alternative oxygen source may be employed in the anisotropic etch. Dopant atoms, i.e., carbon atoms, in the aluminum oxygen compound portion  583  enhance the etch rate of the aluminum oxygen compound portion  583  relative to aluminum oxide. In other words, the etch rate of the aluminum oxygen compound portion  583  is greater than the etch rate of aluminum oxide in the aluminum oxide layer  503  due to the presence of the dopant atoms in the aluminum oxygen compound portion  583 . 
     A semiconductor surface, i.e., the top surface of the epitaxial portion  11  (or layer  10  if portion  11  is omitted), is physically exposed from underneath the cavity  49 ′ after the doped aluminum oxygen compound portion  583  is anisotropically etched. A suitable cleaning process can be optionally performed to remove any residual material from the top surface of the epitaxial channel portion  11 . The remaining portion of the aluminum oxide layer  503  constitutes an inner blocking dielectric spacer, which can be topologically homeomorphic to a torus. In one embodiment, an undercut region U can be formed underneath the inner blocking dielectric spacer  503 , which can consist essentially of aluminum oxide. 
     Referring to  FIG. 2I , the memory material layer  504  can be formed. In one embodiment, the memory material layer  504  can be a charge trapping material including a dielectric charge trapping material, which can be, for example, silicon nitride. Alternatively, the memory material layer  504  can include a conductive material such as doped polysilicon or a metallic material that is patterned into multiple electrically isolated portions (e.g., floating gates), for example, by being formed within lateral recesses into sacrificial material layers  42 . In one embodiment, the memory material layer  504  includes a silicon nitride layer. 
     The memory material layer  504  can be formed as a single memory material layer of homogeneous composition, or can include a stack of multiple memory material layers. The multiple memory material layers, if employed, can comprise a plurality of spaced-apart floating gate material layers that contain conductive materials (e.g., metal such as tungsten, molybdenum, tantalum, titanium, platinum, ruthenium, and alloys thereof, or a metal silicide such as tungsten silicide, molybdenum silicide, tantalum silicide, titanium silicide, nickel silicide, cobalt silicide, or a combination thereof) and/or semiconductor materials (e.g., polycrystalline or amorphous semiconductor material including at least one elemental semiconductor element or at least one compound semiconductor material). Alternatively or additionally, the memory material layer  504  may comprise an insulating charge trapping material, such as one or more silicon nitride segments. Alternatively, the memory material layer  504  may comprise conductive nanoparticles such as metal nanoparticles, which can be, for example, ruthenium nanoparticles. The memory material layer  504  can be formed, for example, by chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD), or any suitable deposition technique for storing electrical charges therein. The thickness of the memory material layer  504  can be in a range from 2 nm to 20 nm, although lesser and greater thicknesses can also be employed. 
     Subsequently, a tunneling dielectric layer  506  can be formed by a conformal deposition process. The tunneling dielectric layer  506  may include multiple sub-layers, or can include a homogeneous dielectric material layer. The tunneling dielectric layer  506  includes at least one dielectric material through which charge tunneling can be performed under suitable electrical bias conditions. The charge tunneling may be performed through hot-carrier injection or by Fowler-Nordheim tunneling induced charge transfer depending on the mode of operation of the monolithic three-dimensional NAND string memory device to be formed. The tunneling dielectric layer  506  can include silicon oxide, silicon nitride, silicon oxynitride, dielectric metal oxides (such as aluminum oxide and hafnium oxide), dielectric metal oxynitride, dielectric metal silicates, alloys thereof, and/or combinations thereof. In one embodiment, the tunneling dielectric layer  506  can include a stack of a first silicon oxide layer, a silicon oxynitride layer, and a second silicon oxide layer, which is commonly known as an ONO stack. 
     In one embodiment, the ONO stack can be formed by deposition of a silicon oxide layer, nitridation of a surface portion of the silicon oxide layer into a silicon oxynitride layer, and oxidation of a surface portion of the silicon oxynitride layer into the second silicon oxide layer. The portion of the silicon oxide layer that is not converted into the silicon oxynitride layer is the first silicon oxide layer within the ONO stack, and the portion of the silicon oxynitride layer that is not converted into the second silicon oxide layer is the silicon oxynitride layer within the ONO stack. Alternatively, the second silicon oxide layer can be formed by deposition of silicon oxide on the inner sidewalls of the silicon oxynitride layer that is formed by nitridation. In one embodiment, the tunneling dielectric layer  506  can include a silicon oxide layer that is substantially free of carbon and/or a silicon oxynitride layer that is substantially free of carbon. Optionally, at least one additional dielectric material such as a dielectric metal oxide having a dielectric constant greater than 7.9 (i.e., a high-k dielectric) may be deposited as a component of the tunneling dielectric layer  506 . The thickness of the tunneling dielectric layer  506  can be in a range from 2 nm to 20 nm, although lesser and greater thicknesses can also be employed. 
     A first semiconductor channel layer  601  can be optionally deposited on the tunneling dielectric layer  506 . The first semiconductor channel layer  601  includes a semiconductor material such as at least one elemental semiconductor material, at least one III-V compound semiconductor material, at least one II-VI compound semiconductor material, at least one organic semiconductor material, or other semiconductor materials known in the art. In one embodiment, the first semiconductor channel layer  601  includes amorphous silicon or polysilicon. The first semiconductor channel layer  601  can be formed by a conformal deposition method such as low pressure chemical vapor deposition (LPCVD). The thickness of the first semiconductor channel layer  601  can be in a range from 2 nm to 10 nm, although lesser and greater thicknesses can also be employed. A cavity  49 ′ is present in the volume of each memory opening  49  that is not filled with the deposited material layers ( 501 ,  503 ,  504 ,  506 ,  601 ). 
     Referring to  FIG. 2J , the optional first semiconductor channel layer  601 , the tunneling dielectric layer  506 , and the memory material layer  504  are sequentially anisotropically etched employing at least one anisotropic etch process. The portions of the first semiconductor channel layer  601 , the tunneling dielectric layer  506 , the memory material layer  504 , and the blocking dielectric layers ( 501 ,  503 ) located above the top surface of the insulating cap layer  70  can be removed by the at least one anisotropic etch process. Further, the horizontal portions of the first semiconductor channel layer  601 , the tunneling dielectric layer  506 , and the memory material layer  504  at a bottom of each cavity  49 ′ can be removed to form openings in remaining portions thereof. Each of the first semiconductor channel layer  601 , the tunneling dielectric layer  506 , and the memory material layer  504  can be etched by anisotropic etch process. 
     A surface of the epitaxial channel portion  11  can be physically exposed underneath the opening through the first semiconductor channel layer  601 , the tunneling dielectric  506 , and the charge storage element  504 . Optionally, the physically exposed portion of the epitaxial channel portion  11  can be vertically recessed. A tunneling dielectric  506  is surrounded by a charge storage element  504 . The charge storage element  504  can comprise a charge trapping material or a floating gate material. 
     The set of the tunneling dielectric  506 , the charge storage element  504 , the inner blocking dielectric layer  503 , and the outer blocking dielectric layer  501  within each memory opening  49  collectively constitutes a memory film  50 . In one embodiment, the first semiconductor channel layer  601 , the tunneling dielectric  506 , the charge storage element  504 , the inner blocking dielectric layer  503 , and the outer blocking dielectric layer  501  can have vertically coincident sidewalls. As used herein, a first surface is “vertically coincident” with a second surface if there exists a vertical plane including both the first surface and the second surface. Such a vertical plane may, or may not, have a horizontal curvature, but does not include any curvature along the vertical direction, i.e., extends straight up and down. 
     Referring to  FIG. 2K , a second semiconductor channel layer  602  can be deposited directly on the semiconductor surface of the epitaxial channel portion  11  over the substrate ( 9 ,  10 ), and directly on the first semiconductor channel layer  601 . The second semiconductor channel layer  602  includes a semiconductor material such as at least one elemental semiconductor material, at least one III-V compound semiconductor material, at least one II-VI compound semiconductor material, at least one organic semiconductor material, or other semiconductor materials known in the art. In one embodiment, the second semiconductor channel layer  602  includes amorphous silicon or polysilicon. The second semiconductor channel layer  602  can be formed by a conformal deposition method such as low pressure chemical vapor deposition (LPCVD). The thickness of the second semiconductor channel layer  602  can be in a range from 2 nm to 10 nm, although lesser and greater thicknesses can also be employed. The second semiconductor channel layer  602  may partially fill the cavity  49 ′ in each memory opening, or may fully fill the cavity in each memory opening. 
     The materials of the first semiconductor channel layer  601  and the second semiconductor channel layer  602  are collectively referred to as a semiconductor channel material. In other words, the semiconductor channel material is a set of all semiconductor material in the first semiconductor channel layer  601  and the second semiconductor channel layer  602 . 
     Referring to  FIG. 2L , in case the cavity  49 ′ in each memory opening is not completely filled by the second semiconductor channel layer  602 , a dielectric core layer can be deposited in the cavity  49 ′ to fill any remaining portion of the cavity  49 ′ within each memory opening. The dielectric core layer includes a dielectric material such as silicon oxide or organosilicate glass. The dielectric core layer can be deposited by a conformal deposition method such as low pressure chemical vapor deposition (LPCVD), or by a self-planarizing deposition process such as spin coating. 
     Subsequently, the horizontal portion of the dielectric core layer can be removed, for example, by a recess etch from above the top surface of the insulating cap layer  70 . Further, the horizontal portion of the second semiconductor channel layer  602  located above the top surface of the insulating cap layer  70  can be removed by a planarization process, which can employ a recess etch or chemical mechanical planarization (CMP). A vertical portion of the second semiconductor channel layer  602  remains within each memory opening  49 . 
     Each adjoining pair of a first semiconductor channel layer  601  and a second semiconductor channel layer  602  can collectively form a semiconductor channel  60  through which electrical current can flow when a vertical NAND device including the semiconductor channel  60  is turned on. A tunneling dielectric  506  is embedded within a charge storage element  504 , and laterally surrounds a portion of the semiconductor channel  60 . Each adjoining set of an outer blocking dielectric layer  501 , an inner blocking dielectric layer  503 , a charge storage element  504 , and a tunneling dielectric  506  collectively constitute a memory film  50 , which can store electrical charges with a macroscopic retention time. As used herein, a macroscopic retention time refers to a retention time suitable for operation of a memory device as a permanent memory device such as a retention time in excess of 24 hours. 
     The top surface of the remaining portion of the dielectric core layer can be further recessed within each memory opening, for example, by a recess etch to a depth that is located between the top surface of the insulating cap layer  70  and the bottom surface of the insulating cap layer  70 . Each remaining portion of the dielectric core layer constitutes a dielectric core  62 . 
     Further, drain regions  63  can be formed by depositing a doped semiconductor material within each recessed region above the dielectric cores  62 . The doped semiconductor material can be, for example, doped polysilicon. Excess portions of the deposited semiconductor material can be removed from above the top surface of the insulating cap layer  70 , for example, by chemical mechanical planarization (CMP) or a recess etch to form the drain regions  63 . 
     A memory stack structure  55  extending through the stack of alternating layers is formed. The memory stack structure  55  includes a memory film  50  and a semiconductor channel  60 . The memory film  50  includes a blocking dielectric layer ( 501 ,  503 ), a memory material layer  504 , and a tunneling dielectric layer  506 . 
     Referring to  FIG. 2M , the in-process structure for forming the first exemplary memory stack structure as illustrated in  FIG. 2D  can be modified to convert a surface portion of the semiconductor substrate ( 9 ,  10 ) located directly underneath a cavity  49 ′ into a doped semiconductor portion  117 . The conversion of the surface portion of the semiconductor substrate ( 9 ,  10 ) into the doped semiconductor portion  117  can be performed by implanting dopants into the surface portion of the semiconductor substrate  117 . 
     In one embodiment, the dopants can be selected to facilitate formation of an aluminum-carbon-oxygen alloy portion  573  in a subsequent atomic layer deposition process. In one embodiment, the dopants can include at least one element selected from, or a compound of at least one element selected from, helium, neon, argon, krypton, hydrogen, carbon, nitrogen, oxygen, and fluorine. In one embodiment, the dopants can include carbon. In one embodiment, the dopants can be implanted by a process selected from an ion implantation process that implants the dopants at an implantation angle that is substantially parallel to the sidewall of the memory opening, and a plasma doping process. 
     Referring to  FIG. 2N , the processing steps of  FIGS. 2E and 2F  can be performed. The same process parameters can be employed as described above. The presence of the dopant atoms in the doped semiconductor portion  117  can affect the amount of carbon atoms deposited in the aluminum-carbon-oxygen alloy portion  573  either positively or negatively. The selection of the dopant species and the amount of dopants implanted into the doped semiconductor portion  117  can be employed to optimize the composition and the total amount of carbon in the aluminum-carbon-oxygen alloy portion  573 . In one embodiment, the selection of the dopant species and the amount of dopants implanted into the doped semiconductor portion  117  can be selected to maximize carbon incorporation into the aluminum-carbon-oxygen alloy portion  573 . For example, for carbon implanted into a silicon portion  11  (or silicon layer  10 ), the doped semiconductor portion comprises carbon doped silicon or silicon carbide depending on the amount of carbon doping. For example, a carbon doped silicon or silicon carbide portion  117  may be annealed using rapid thermal annealing (RTA) to diffuse carbon or carbon and silicon from portion  117  into portion  573  to convert portion  573  into aluminum carbonate or silicon aluminum carbonate doped aluminum oxygen compound portion  583 . The RTA may be performed at 900 to 1100° C., such as about 1000 to 1050° C. for 15 to 60 seconds, such as about 30 to 45 seconds. 
     Referring to  FIG. 2O , the processing steps of  FIGS. 2G-2L  can be sequentially performed to provide a variation of the first exemplary memory stack structure. The exemplary structures illustrated in  FIGS. 2L and 2O  includes features that are present in the final device structure to be subsequently formed. For example, a stack of alternating layers comprising insulator layers  32  and spacer layers (as embodied as sacrificial material layers  42  that are subsequently replaced with electrically conductive layers) is located over a semiconductor substrate ( 9 ,  10 ). A memory stack structure  55  extending through the stack of alternating layers is formed. The memory stack structure  55  includes a memory film  50  and a semiconductor channel  60 . The memory film  50  includes a blocking dielectric layer ( 501 ,  503 ), a memory material layer  504 , and a tunneling dielectric layer  506 . The blocking dielectric layer ( 501 ,  503 ) includes at least an aluminum oxide layer  503  laterally surrounding, and contacting, the memory material layer  504 . A bottommost surface of the aluminum oxide layer  503  can is vertically spaced from a semiconductor surface (i.e., the topmost surface) of the epitaxial channel portion  11  by a horizontal portion of the memory material layer  504  that contacts the outer blocking dielectric layer  501  (i.e., the dielectric spacer). The bottommost surface of the memory material layer  504  is in contact with a top surface of an epitaxial channel portion  11  that contacts the semiconductor channel  60 . 
     In one embodiment, the bottommost surface of the memory material layer  504  can be in contact with a doped semiconductor material portion  117  including dopants as illustrated in  FIG. 2O . The dopants can include at least one element selected from helium, neon, argon, krypton, hydrogen, carbon, nitrogen, oxygen, and fluorine. In one embodiment, the doped semiconductor material portion  117  can be a carbon-doped semiconductor material portion (e.g., carbon doped silicon or silicon carbide), and the bottommost surface of the memory material layer  117  can be in contact with a carbon-doped semiconductor material portion. In one embodiment, the blocking dielectric layer ( 501 ,  503 ) can include a silicon oxide layer as an outer blocking dielectric layer  501 , which contacts a sidewall of a memory opening in which the memory stack structure  55  is located. The memory material layer  504  can contact a portion, i.e., a lower portion, of an inner sidewall of the silicon oxide layer. In one embodiment, the aluminum oxide layer  501  can contact another portion, i.e., an upper portion, of the inner sidewall of the silicon oxide layer. In one embodiment, the bottom surface of the silicon oxide layer  501  can be coplanar with (i.e., located within the same plane as) the bottommost surface of the memory material layer  504 . 
     Referring to  FIG. 3A , an in-process structure for forming a second exemplary memory structure is illustrated, which can be derived from the in-process first exemplary memory structure of  FIG. 2D  by depositing an aluminum oxide layer  503  that does not provide a carbon-containing compound material on semiconductor surfaces such as the top surface of the epitaxial channel portion  11 . Thus, an aluminum-carbon-oxygen alloy portion is not formed on the top surface of the epitaxial channel portion  11 . Instead, the aluminum oxide layer  503  is formed directly on the top surface of the epitaxial channel portion  11 . The aluminum oxide layer  503  is formed as a contiguous layer of aluminum oxide directly on the bottom surface of the cavity  49 ′ (i.e., the semiconductor surface of the epitaxial channel portion  11  or layer  10  if portion  11  is omitted) and over the sidewall surface of the memory opening, e.g., directly on an inner sidewall surface of the outer blocking dielectric layer  501 . In this embodiment, layer  503  may be deposited by chemical vapor deposition, sputtering or atomic layer deposition, and in-situ carbon incorporation is not required. The aluminum oxide layer  503  is formed as a single contiguous material layer. In one aspect of this embodiment, layer  503  is deposited with the same composition and crystal structure in its vertical portion  591  and its horizontal portion  592 . 
     In another aspect of this embodiment, layer  503  is deposited with different composition and/or structure in its vertical portion  591  and its horizontal portion  592  due to the difference between aluminum oxide layer deposition on the insulating surface of the stack sidewall in the opening  49  versus deposition on the single crystal silicon ( 11  or  10 ) surface. Specifically, the vertical portion  591  may have a different composition from the horizontal portion  592  by having a different aluminum to oxygen atomic ratio. For example, portion  591  may be stoichiometric while portion  592  may be non-stoichiometric, or both portions may be non-stoichiometric with different aluminum to oxygen ratios. Alternatively or in addition, the vertical portion  591  may have a different structure from the horizontal portion  592  where the vertical portion  591  has an amorphous crystal structure while the horizontal portion  592  has a polycrystalline structure. Alternatively, both portions  591 ,  592  may be polycrystalline, but with portion  591  having a larger average grain size than portion  592 . The composition and/or crystal structure of portion  592  is selected such that it may be selectively etched compared to portion  591 . 
     Referring to  FIG. 3B , a horizontal portion  592  of the contiguous aluminum oxide layer  503  on the bottom surface of the cavity  49 ′ can optionally be converted into a doped aluminum oxygen compound portion  593  by implanting dopants into the horizontal portion  591  of the contiguous aluminum oxide layer  503 . If portion  592  has a different composition or structure than portion  591  as described above, then the doping of portion  592  may be omitted. The dopants can be selected from elements that can increase the etch rate of the doped aluminum oxygen compound portion  593  relative to aluminum oxide by changing the composition of aluminum oxide and/or by introducing structural defects into the doped aluminum oxygen compound portion  593 . Thus, the doped aluminum oxygen compound portion  593  can be formed at a bottom surface of the cavity  49 ′ and a vertical portion  591  of aluminum oxide layer  503  consisting essentially of aluminum oxide can remain least over a sidewall of the memory opening. 
     In one embodiment, the dopants can be implanted by an ion implantation process at an implantation angle that is substantially parallel to the sidewall of the memory opening. In one embodiment, the dopants can be implanted along a direction that is substantially perpendicular to the top surface  7  of the substrate (i.e., substantially perpendicular to a horizontal interface between the aluminum oxide layer  503  and the epitaxial channel portion  11 ). The vertical portion  591  of the contiguous layer of aluminum oxide that is not implanted with the dopants constitutes a remaining portion of the aluminum oxide layer  503 . In one embodiment, the dopants can include at least one element selected from, or a compound of at least one element selected from, helium, neon, argon, krypton, hydrogen, carbon, nitrogen, and fluorine having a dose of at least 10 16  cm −2 . The energy of the ion implantation process can be selected such that a predominant portion of the implanted dopants remain in the doped aluminum oxygen compound portion  593 . The dose of the ion implantation process can be selected such that the implanted dopants have an atomic concentration in a range from 0.1% to 30% of portion  593 , although lesser and greater percentages can also be employed. 
     Referring to  FIG. 3C , an anisotropic etch can be performed to etch the doped aluminum oxygen compound portion  593  without significantly etching the vertical portion of the aluminum oxide layer  503  on the sidewall of the outer blocking dielectric spacer  501 . In one embodiment, the anisotropic etch can employ a plasma of O 2  and optionally a mix of O 2  and CF 4 . Generally, plasma of O 2  is effective in removing carbon based materials. Alternative fluorocarbon etchant gas and/or alternative chlorofluorocarbon etchant gas and/or alternative oxygen source may be employed in the anisotropic etch which selectively etch aluminum carbonate compared to aluminum oxide. Dopant atoms in the aluminum oxygen compound portion  593  enhance the etch rate of the aluminum oxygen compound portion  593  relative to aluminum oxide portion  591 . In other words, the etch rate of the aluminum oxygen compound portion  593  is greater than the etch rate of aluminum oxide portion  591  of the aluminum oxide layer due to the presence of the dopant atoms in the aluminum oxygen compound portion  593  and/or due to the implanted dopant atoms causing disorder of the crystal structure of portion  593 . For example, implanted ions such as hydrogen or noble gas ions cause portion  593  to become amorphous and thus allow portion  593  to be selectively etched compared to polycrystalline portion  591 . Alternatively, implanted ions such as fluorine and carbon cause a change in composition of portion  593  which allows portion  593  to be selectively etched compared to the aluminum oxide portion  591 . For example, as discussed above, carbon implantation may cause the formation of aluminum carbonate portion  593  which may be selectively etched with respect to the aluminum oxide portion. Fluorine implantation may generate a volatile aluminum trifluoride compound which sublimates during a subsequent crystallization anneal, such as an anneal at a temperature above 1238° C. The sublimation of aluminum trifluoride from portion  593  causes portion  593  to become aluminum poor compared to portion  591 , and permits selective etching of portion  593  compared to portion  591  due to the difference in composition (e.g., aluminum content) between the portions. Implantation of nitrogen into portion  593  may retard the crystallization of portion  593  during the subsequent crystallization anneal. In this case, the nitrogen doped portion  593  may remain amorphous after the crystallization anneal, while portion  591  is converted from amorphous to polycrystalline aluminum oxide. The amorphous portion  593  may be selectively etched compared to the polycrystalline portion  591 . 
     A semiconductor surface, i.e., the top surface of the epitaxial portion  11 , is physically exposed from underneath the cavity  49 ′ after the doped aluminum oxygen compound portion  593  is anisotropically etched. A suitable cleaning process can be optionally performed to remove any residual material from the top surface of the epitaxial channel portion  11 . The remaining portion  591  of the aluminum oxide layer  503  constitutes an inner blocking dielectric spacer, which can be topologically homeomorphic to a torus. In one embodiment, an undercut region U can be formed underneath the inner blocking dielectric spacer  503 , which can consist essentially of aluminum oxide. 
     Referring to  FIG. 3D , the processing steps of  FIG. 2I  can be performed to optionally deposit the first semiconductor channel layer  601 . 
     Referring to  FIG. 3E , the processing steps of  FIGS. 2J and 2K  can be performed to etch horizontal portions of the first semiconductor channel layer  601 , the tunneling dielectric layer  506 , and the memory material layer  504 , and to recess a physically exposed horizontal surface of the epitaxial channel portion  11 . Subsequently, the second semiconductor channel layer  602  can be deposited by a conformal deposition method. 
     Referring to  FIG. 3F , the processing steps of  FIG. 2L  can be performed to form a dielectric core  62  and the drain region  63  within each memory opening. 
     Multiple instances of the first or second exemplary memory stack structure (or any variation thereof) can be embedded into the memory openings  49  in the exemplary structure illustrated in  FIG. 1 .  FIG. 4  illustrates the exemplary structure that incorporates multiple instances of the exemplary memory stack structure of  FIG. 2L ,  FIG. 2O , or  FIG. 3F . The exemplary structure includes a semiconductor device, which comprises a stack ( 32 ,  42 ) including an alternating plurality of material layers (e.g., the sacrificial material layers  42 ) and insulating layers  32  located over a semiconductor substrate ( 9 ,  10 ), and a memory opening extending through the stack ( 32 ,  42 ). The semiconductor device further comprises blocking dielectric layers ( 501 ,  503 ) vertically extending from a bottommost layer (e.g., the bottommost sacrificial material layer  42 ) of the stack to a topmost layer (e.g., the topmost sacrificial material layer  42 ) of the stack, and contacting a sidewall of the memory opening and a horizontal surface of the semiconductor substrate. While the present disclosure is described employing the illustrated configuration for the memory stack structure, the methods of the present disclosure can be applied to alternative memory stack structures including a polycrystalline semiconductor channel. 
     Referring to  FIG. 5 , an optional first contact level dielectric layer  71  can be formed over the substrate ( 9 ,  10 ). As an optional structure, the first contact level dielectric layer  71  may, or may not, be formed. In case the first contact level dielectric layer  71  is formed, the first contact level dielectric layer  71  includes a dielectric material such as silicon oxide, silicon nitride, silicon oxynitride, porous or non-porous organosilicate glass (OSG), or a combination thereof. If an organosilicate glass is employed, the organosilicate glass may, or may not, be doped with nitrogen. The first contact level dielectric layer  71  can be formed over a horizontal plane including the top surface of the insulating cap layer  70  and the top surfaces of the drain regions  63 . The first contact level dielectric layer  71  can be deposited by chemical vapor deposition, atomic layer deposition (ALD), spin-coating, or a combination thereof. The thickness of the first contact level dielectric layer  71  can be in a range from 10 nm to 300 nm, although lesser and greater thicknesses can also be employed. 
     In one embodiment, the first contact level dielectric layer  71  can be formed as a dielectric material layer having a uniform thickness throughout. The first contact level dielectric layer  71  may be formed as a single dielectric material layer, or can be formed as a stack of a plurality of dielectric material layers. Alternatively, formation of the first contact level dielectric layer  71  may be merged with formation of at least one line level dielectric layer (not shown). While the present disclosure is described employing an embodiment in which the first contact level dielectric layer  71  is a structure separate from an optional second contact level dielectric layer or at least one line level dielectric layer to be subsequently deposited, embodiments in which the first contact level dielectric layer  71  and at least one line level dielectric layer are formed at a same processing step, and/or as a same material layer, are expressly contemplated herein. 
     Optionally, a portion of the alternating stack ( 32 ,  42 ) can be removed, for example, by applying and patterning a photoresist layer with an opening and by transferring the pattern of the opening through the alternating stack ( 32 ,  42 ) employing an etch such as an anisotropic etch. An optional trench extending through the entire thickness of the alternating stack ( 32 ,  42 ) can be formed within an area that includes a peripheral device region  200  and a portion of a contact region  300 , which is adjacent to a device region  100  that includes an array of memory stack structures  55 . Subsequently, the trench can be filled with an optional dielectric material such as silicon oxide. Excess portions of the dielectric material can be removed from above the top surface of the first contact level dielectric layer  71  by a planarization process such as chemical mechanical planarization and/or a recess etch. The top surfaces of the first contact level dielectric layer  71  can be employed as a stopping surface during the planarization. The remaining dielectric material in the trench constitutes a dielectric material portion  64 . 
     A stepped cavity can be formed within the contact region  300 , which can straddle the dielectric material portion  64  and a portion of the alternating stack ( 32 ,  42 ). Alternatively, the dielectric material portion  64  may be omitted and the stepped cavity  69  may be formed directly in the stack ( 32 ,  42 ). The stepped cavity can have various stepped surfaces such that the horizontal cross-sectional shape of the stepped cavity changes in steps as a function of the vertical distance from the top surface of the substrate ( 9 ,  10 ). In one embodiment, the stepped cavity can be formed by repetitively performing a set of processing steps. The set of processing steps can include, for example, an etch process of a first type that vertically increases the depth of a cavity by one or more levels, and an etch process of a second type that laterally expands the area to be vertically etched in a subsequent etch process of the first type. As used herein, a “level” of a structure including alternating plurality is defined as the relative position of a pair of a first material layer and a second material layer within the structure. 
     The dielectric material portion  64  can have stepped surfaces after formation of the stepped cavity, and a peripheral portion of the alternating stack ( 32 ,  42 ) can have stepped surfaces after formation of the stepped cavity. As used herein, “stepped surfaces” refer to a set of surfaces that include at least two horizontal surfaces and at least two vertical surfaces such that each horizontal surface is adjoined to a first vertical surface that extends upward from a first edge of the horizontal surface, and is adjoined to a second vertical surface that extends downward from a second edge of the horizontal surface. A “stepped cavity” refers to a cavity having stepped surfaces. 
     A retro-stepped dielectric material portion  65  (i.e., an insulating fill material portion) can be formed in the stepped cavity by deposition of a dielectric material therein. A dielectric material such as silicon oxide can be deposited in the stepped cavity. Excess portions of the deposited dielectric material can be removed from above the top surface of the first contact level dielectric layer  71 , for example, by chemical mechanical planarization (CMP). The remaining portion of the deposited dielectric material filling the stepped cavity constitutes the retro-stepped dielectric material portion  65 . As used herein, a “retro-stepped” element refers to an element that has stepped surfaces and a horizontal cross-sectional area that increases monotonically as a function of a vertical distance from a top surface of a substrate on which the element is present. If silicon oxide is employed for the retro-stepped dielectric material portion  65 , the silicon oxide of the retro-stepped dielectric material portion  65  may, or may not, be doped with dopants such as B, P, and/or F. 
     Referring to  FIGS. 6A and 6B , at least one dielectric support pillar  7 P may be optionally formed through the retro-stepped dielectric material portion  65  and/or through the first contact level dielectric layer  71  and/or through the alternating stack ( 32 ,  42 ). The plane A-A′ in  FIG. 6B  corresponds to the plane of the vertical cross-sectional view of  FIG. 6A . In one embodiment, the at least one dielectric support pillar  7 P can be formed in the contact region  300 , which is located adjacent to the device region  100 . The at least one dielectric support pillar  7 P can be formed, for example, by forming an opening extending through the retro-stepped dielectric material portion  65  and/or through the alternating stack ( 32 ,  42 ) and at least to the top surface of the substrate ( 9 ,  10 ), and by filling the opening with a dielectric material that is resistant to the etch chemistry to be employed to remove the sacrificial material layers  42 . 
     In one embodiment, the at least one dielectric support pillar can include silicon oxide and/or a dielectric metal oxide such as aluminum oxide. In one embodiment, the portion of the dielectric material that is deposited over the first contact level dielectric layer  71  concurrently with deposition of the at least one dielectric support pillar  7 P can be present over the first contact level dielectric layer  71  as a second contact level dielectric layer  73 . Each of the at least one dielectric support pillar  7 P and the second contact level dielectric layer  73  is an optional structure. As such, the second contact level dielectric layer  73  may, or may not, be present over the insulating cap layer  70  and the retro-stepped dielectric material portion  65 . The first contact level dielectric layer  71  and the second contact level dielectric layer  73  are herein collectively referred to as at least one contact level dielectric layer ( 71 ,  73 ). In one embodiment, the at least one contact level dielectric layer ( 71 ,  73 ) can include both the first and second contact level dielectric layers ( 71 ,  73 ), and optionally include any additional via level dielectric layer that can be subsequently formed. In another embodiment, the at least one contact level dielectric layer ( 71 ,  73 ) can include only the first contact level dielectric layer  71  or the second contact level dielectric layer  73 , and optionally include any additional via level dielectric layer that can be subsequently formed. Alternatively, formation of the first and second contact level dielectric layers ( 71 ,  73 ) may be omitted, and at least one via level dielectric layer may be subsequently formed, i.e., after formation of a backside contact via structure. 
     The second contact level dielectric layer  73  and the at least one dielectric support pillar  7 P can be formed as a single contiguous structure of integral construction, i.e., without any material interface therebetween. In another embodiment, the portion of the dielectric material that is deposited over the first contact level dielectric layer  71  concurrently with deposition of the at least one dielectric support pillar  7 P can be removed, for example, by chemical mechanical planarization or a recess etch. In this case, the second contact level dielectric layer  73  is not present, and the top surface of the first contact level dielectric layer  71  can be physically exposed. Memory contact via structures  88  can be formed through the first and second contact level dielectric layers ( 73 ,  71 ). Specifically, a photoresist layer can be applied over the second contact level dielectric layer  73 , and can be lithographically patterned to form openings overlying the drain structures  63 . An anisotropic etch can be performed to transfer the pattern in the photoresist layer through the first and second contact level dielectric layers ( 73 ,  71 ) to form memory contact via cavities that extend through the first and second contact level dielectric layers ( 73 ,  71 ). The memory contact via cavities can be filled with at least one conductive material. Excess portions of the at least one conductive material can be removed from above a horizontal plane including a top surface of the second contact level dielectric layer  73 . Each remaining contiguous portion of the at least one conductive material constitutes a memory contact via structure  88 , which contacts a top surface of an underlying drain region  63 . The photoresist layer can be subsequently removed, for example, by ashing. Alternatively, the memory contact via structure  88  may be formed at a later step in the process shown in  FIG. 10 . 
     Another photoresist layer (not shown) can be applied over the alternating stack ( 32 ,  42 ) and/or the retro-stepped dielectric material portion  65 , and optionally over the and lithographically patterned to form at least one backside contact trench  79  in an area in which formation of a backside contact via structure is desired. The pattern in the photoresist layer can be transferred through the alternating stack ( 32 ,  42 ) and/or the retro-stepped dielectric material portion  65  employing an anisotropic etch to form the at least one backside contact trench  79 , which extends at least to the top surface of the substrate ( 9 ,  10 ). In one embodiment, the at least one backside contact trench  79  can include a source contact opening in which a source contact via structure can be subsequently formed. A source region  61  can be formed by implanting electrical dopants through each backside contact trench into a semiconductor portion located on, or within, the substrate ( 9 ,  10 ). For example, a source region  61  may be formed by implantation of dopant atoms into a portion of the semiconductor material layer  10  through each backside contact trench  79 . Alternatively, a semiconductor portion can be formed on the substrate ( 9 ,  10 ) by deposition of a semiconductor material, for example, by selective epitaxy, and by implantation of electrical dopants into the deposited semiconductor portion. 
     Referring to  FIG. 7 , an etchant that selectively etches the second material of the sacrificial material layers  42  with respect to the first material of the insulating layers  32  can be introduced into the at least one backside contact trench  79 , for example, employing an etch process. Backside recesses  43  are formed in volumes from which the sacrificial material layers  42  are removed. The removal of the second material of the sacrificial material layers  42  can be selective to the first material of the insulating layers  32 , the material of the at least one dielectric support pillar  7 P, the material of the retro-stepped dielectric material portion  65 , the semiconductor material of the semiconductor material layer  10 , and the material of the outermost layer of the memory films  50 . In one embodiment, the sacrificial material layers  42  can include silicon nitride, and the materials of the insulating layers  32 , the at least one dielectric support pillar  7 P, and the retro-stepped dielectric material portion  65  can be selected from silicon oxide and dielectric metal oxides. In another embodiment, the sacrificial material layers  42  can include a semiconductor material such as polysilicon, and the materials of the insulating layers  32 , the at least one dielectric support pillar  7 P, and the retro-stepped dielectric material portion  65  can be selected from silicon oxide, silicon nitride, and dielectric metal oxides. In this case, the depth of the at least one backside contact trench  79  can be modified so that the bottommost surface of the at least one backside contact trench  79  is located within the planar dielectric layer  31 , i.e., to avoid physical exposure of the top surface of the semiconductor substrate layer  10 . 
     The etch process that removes the second material selective to the first material and the outermost layer of the memory films  50  can be a wet etch process employing a wet etch solution, or can be a gas phase (dry) etch process in which the etchant is introduced in a vapor phase into the at least one backside contact trench  79 . For example, if the sacrificial material layers  42  include silicon nitride, the etch process can be a wet etch process in which the first exemplary structure is immersed within a wet etch tank including phosphoric acid, which etches silicon nitride selective to silicon oxide, silicon, and various other materials employed in the art. The at least one dielectric support pillar  7 P, the retro-stepped dielectric material portion  65 , and the memory stack structures  55  provide structural support while the backside recesses  43  are present within volumes previously occupied by the sacrificial material layers  42 . 
     Each backside recess  43  can be a laterally extending cavity having a lateral dimension that is greater than the vertical extent of the cavity. In other words, the lateral dimension of each backside recess  43  can be greater than the height of the backside recess  43 . A plurality of backside recesses  43  can be formed in the volumes from which the second material of the sacrificial material layers  42  is removed. The memory openings in which the memory stack structures  55  are formed are herein referred to as front side recesses or front side cavities in contrast with the backside recesses  43 . In one embodiment, the device region  100  comprises an array of monolithic three-dimensional NAND strings having a plurality of device levels disposed above the substrate ( 9 ,  10 ). In this case, each backside recess  43  can define a space for receiving a respective word line of the array of monolithic three-dimensional NAND strings. 
     Each of the plurality of backside recesses  43  can extend substantially parallel to the top surface of the substrate ( 9 ,  10 ). A backside recess  43  can be vertically bounded by a top surface of an underlying insulating layer  32  and a bottom surface of an overlying insulating layer  32 . In one embodiment, each backside recess  43  can have a uniform height throughout. Optionally, a backside blocking dielectric layer can be formed in the backside recesses. 
     Physically exposed surface portions of epitaxial channel portions  11  and the source regions  61  can be converted into dielectric material portions by thermal conversion and/or plasma conversion of the semiconductor materials into dielectric materials. For example, thermal conversion and/or plasma conversion can be employed to convert a surface portion of each epitaxial channel portion  11  into a dielectric spacer  116 , and to convert a surface portion of each source region  61  into a sacrificial dielectric portion  616 . In one embodiment, each dielectric spacer  116  can be topologically homeomorphic to a torus, i.e., generally ring-shaped. As used herein, an element is topologically homeomorphic to a torus if the shape of the element can be continuously stretched without destroying a hole or forming a new hole into the shape of a torus. The dielectric spacers  116  include a dielectric material that includes the same semiconductor element as the epitaxial channel portions  11  and additionally includes at least one non-metallic element such as oxygen and/or nitrogen such that the material of the dielectric spacers  116  is a dielectric material. In one embodiment, the dielectric spacers  116  can include a dielectric oxide, a dielectric nitride, or a dielectric oxynitride of the semiconductor material of the epitaxial channel portions  11 . Likewise, each sacrificial dielectric portion  616  includes a dielectric material that includes the same semiconductor element as the source regions  61  and additionally includes at least one non-metallic element such as oxygen and/or nitrogen such that the material of the sacrificial dielectric portions  616  is a dielectric material. In one embodiment, the sacrificial dielectric portions  616  can include a dielectric oxide, a dielectric nitride, or a dielectric oxynitride of the semiconductor material of the source region  61 . 
     Referring to  FIG. 8 , a backside blocking dielectric layer (not shown) can be optionally formed. The backside blocking dielectric layer, if present, comprises a dielectric material that functions as a control gate dielectric for the control gates to be subsequently formed in the backside recesses  43 . In case a blocking dielectric  502  is present within each memory opening, the backside blocking dielectric layer is optional. In case a blocking dielectric layer  502  is omitted, the backside blocking dielectric layer is present. 
     At least one metallic material can be deposited in the plurality of backside recesses  43 , on the sidewalls of the at least one the backside contact trench  79 , and over the top surface of the second contact level dielectric layer  73 . As used herein, a metallic material refers to an electrically conductive material that includes at least one metallic element. 
     The metallic material can be deposited by a conformal deposition method, which can be, for example, chemical vapor deposition (CVD), atomic layer deposition (ALD), electroless plating, electroplating, or a combination thereof. The metallic material can be an elemental metal, an intermetallic alloy of at least two elemental metals, a conductive nitride of at least one elemental metal, a conductive metal oxide, a conductive doped semiconductor material, a conductive metal-semiconductor alloy such as a metal silicide, alloys thereof, and combinations or stacks thereof. Non-limiting exemplary metallic materials that can be deposited in the plurality of backside recesses  43  include tungsten, tungsten nitride, titanium, titanium nitride, tantalum, tantalum nitride, cobalt, and ruthenium. In one embodiment, the metallic material can comprise a metal such as tungsten and/or metal nitride. In one embodiment, the metallic material for filling the plurality of backside recesses  43  can be a combination of titanium nitride layer and a tungsten fill material. In one embodiment, the metallic material can be deposited by chemical vapor deposition or atomic layer deposition. 
     A plurality of electrically conductive layers  46  can be formed in the plurality of backside recesses  43 , and a contiguous metallic material layer  46 L can be formed on the sidewalls of each backside contact trench  79  and over the at least one contact level dielectric layer ( 71 , 73 ). Thus, each sacrificial material layer  42  can be replaced with an electrically conductive layer  46 . A backside cavity  79 ′ is present in the portion of each backside contact trench  79  that is not filled with the contiguous metallic material layer  46 L. 
     Referring to  FIG. 9 , the deposited metallic material of the contiguous metallic material layer  46 L is etched back from the sidewalls of each backside contact trench  79  and from above the second contact level dielectric layer  73 , for example, by an isotropic etch. Each remaining portion of the deposited metallic material in the backside recesses  43  constitutes an electrically conductive layer  46 . Each electrically conductive layer  46  can be a conductive line structure. Thus, the sacrificial material layers  42  are replaced with the electrically conductive layers  46 . 
     Each electrically conductive layer  46  can function as a combination of a plurality of control gate electrodes located at a same level and a word line electrically interconnecting, i.e., electrically shorting, the plurality of control gate electrodes located at the same level. The plurality of control gate electrodes within each electrically conductive layer  46  are the control gate electrodes for the vertical memory devices including the memory stack structures  55 . In other words, each electrically conductive layer  46  can be a word line that functions as a common control gate electrode for the plurality of vertical memory devices. Optionally, the sacrificial dielectric portions  616  can be removed from above the source regions  61  during the last processing step of the anisotropic etch. 
     In case the spacer material layers (i.e., the second material layers) are conductive material layer at the time of formation (i.e., as formed at the processing steps of  FIG. 1 ), replacement of the spacer material layers with electrically conductive layers not necessary. In this case, the processing steps of  FIGS. 7-9  can be omitted. 
     Referring to  FIG. 10 , an insulating material layer  74  can be formed in the at least one backside contact trench  79  and over the second contact level dielectric layer  73  by a conformal deposition process. Exemplary conformal deposition processes include, but are not limited to, chemical vapor deposition and atomic layer deposition. The insulating material layer  74  includes an insulating material such as silicon oxide, silicon nitride, a dielectric metal oxide, an organosilicate glass, or a combination thereof. The thickness of the insulating material layer  74  can be in a range from 1.5 nm to 60 nm, although lesser and greater thicknesses can also be employed. 
     An anisotropic etch is performed to remove horizontal portions of the insulating material layer  74  from above the second contact level dielectric layer  73 . Each remaining portion of the insulating material layer  74 L inside a backside contact trench  79  constitutes a vertically elongated annular structure with a vertical cavity therethrough, which is herein referred to as an insulating spacer  74 . In one embodiment, an annular bottom surface of the insulating spacer  74  contacts a top surface of the source region  61 . 
     Each insulating spacer  74  can be formed over the sidewalls of the backside contact trench  79 , and directly on the sidewalls of the electrically conductive layers  46 , i.e., directly on the sidewalls of the metallic material portions  46 . The thickness of each insulating spacer  74 , as measured at a bottom portion thereof, can be in a range from 1.5 nm to 60 nm, although lesser and greater thicknesses can also be employed. In one embodiment, the thickness of the insulating spacer  74  can be in a range from 3 nm to 10 nm. 
     Various contact via structures can be formed through dielectric material layers/portions of the exemplary structure. For example, peripheral device contact via structures ( 8 G,  8 A) can be formed in the peripheral device region to provide electrical contact to various nodes of the peripheral devices. The peripheral device contact via structures ( 8 G,  8 A) can include, for example, at least one gate contact via structure  8 G and at least one active region contact via structure  8 A. The memory contact via structures  88  may also be formed during the same step as via structures ( 8 G,  8 A). Likewise, word line contact via structures (not shown for clarity) to word lines may also be formed during the same step. 
     The exemplary structure includes a monolithic three-dimensional memory device. The monolithic three-dimensional memory device comprises a stack of alternating layers comprising insulating layers  32  and electrically conductive layers  46  and located over a substrate ( 9 ,  10 ), an array of memory openings extending through the stack, and a plurality of memory stack structures  55  located within a respective memory opening. Each electrically conductive layer  46  can laterally surround the plurality of memory stack structures  55 . Each memory stack structure  55  extends through the stack of alternating layers. Each memory film  50  comprising a memory material layer  504 , and a tunneling dielectric layer  506 . A semiconductor channel  60  extends through the memory film  50 . The tunneling dielectric layer  506  can comprise an ONO stack (as embodied as a tunneling dielectric layer  506 ). 
     In one embodiment, the memory device of the present disclosure can be a monolithic three-dimensional memory device comprising a vertical NAND device located over the substrate ( 9 ,  10 ), and the electrically conductive layers  46  can comprise, or are electrically connected to, a respective word line of the vertical NAND device. The substrate ( 9 ,  10 ) can comprise a silicon substrate. The vertical NAND device can comprise an array of monolithic three-dimensional NAND strings located over the silicon substrate. At least one memory cell in a first device level of the three-dimensional array of NAND strings is located over another memory cell in a second device level of the three-dimensional array of NAND string. The silicon substrate can contain an integrated circuit comprising a driver circuit for the memory device located thereon. 
     The array of monolithic three-dimensional NAND strings can comprise a plurality of semiconductor channels. At least one end portion of each of the plurality of semiconductor channels extends substantially perpendicular to a top surface of the substrate ( 9 ,  10 ). In one embodiment, the plurality of semiconductor channels can comprise a common horizontal semiconductor channel layer that is a portion of the semiconductor material layer  10  between a source region  61  and the epitaxial channel portions  11 , the epitaxial channel portions  11 , and the vertical semiconductor channels  60  that are portions of the memory stack structures  55 . The array of monolithic three-dimensional NAND strings can comprise a plurality of charge storage elements (which can be embodied as sections of a memory material layer  504  that is present within each memory stack structure  55 ). Each charge storage element can be located adjacent to a respective one of the plurality of semiconductor channels, i.e., adjacent to a respective vertical semiconductor channel  60 . The array of monolithic three-dimensional NAND strings can comprise a plurality of control gate electrodes having a strip shape extending substantially parallel to the top surface of the substrate ( 9 ,  10 ). The plurality of control gate electrodes comprise at least a first control gate electrode located in the first device level and a second control gate electrode located in the second device level. 
     The present disclosure provides various methods for eliminating, or reducing, charge scattering trap density within a semiconductor channel inside a memory stack structure. Thus, the methods of the present disclosure can be employed to enhance the on-current of three-dimensional memory devices. 
     Referring to  FIGS. 11A and 11B , transmission electron micrographs (TEM&#39;s) of two samples are shown. Each of the two samples includes a stack of aluminum-carbon-oxygen alloy portion  573  and an aluminum oxide layer  503  that was grown on a hydrogen-terminated silicon surface of a semiconductor material layer  10  (which is a single crystalline silicon layer) employing the atomic layer deposition process according to an embodiment of the present disclosure. The atomic layer deposition process employed trimethylaluminum and water vapor as an organic precursor and an oxidizer, respectively. The deposition temperature was about 125 degrees Celsius. The first thickness t 1  of the aluminum-carbon-oxygen alloy portion  573  is about 1.5 nm, and the second thickness t 2  of the aluminum oxide layer  503  is about 1.5 nm. A first cap layer  1110  (including sputter chromium) and a second cap layer  1120  (including silicon oxide) was deposited for the purpose of sample preparation. 
       FIG. 11C  shows a transmission electron micrograph (TEM) of a sample that was prepared by the atomic layer deposition process employed to form a stack of aluminum-carbon-oxygen alloy portion and an aluminum oxide layer (in the same manner as the samples illustrated in  FIGS. 11A and 11B ), and was subsequently annealed at 1,000 degrees Celsius for 3 seconds by a rapid thermal anneal (RTA) process to form a carbon-doped aluminum oxygen compound portion  583  according to an embodiment of the present disclosure. The carbon-doped aluminum oxygen compound portion  583  formed by the RTA process includes aluminum, silicon, carbon, and oxygen, and has a thickness of about 5 nm, which is greater than the thickness of the stack of aluminum-carbon-oxygen alloy portion  573  and an aluminum oxide layer  503  as grown on a hydrogen-terminated silicon surface of a semiconductor material layer  10 . Without wishing to be bound by a particular theory, it is believed that incorporation of silicon atoms and possibly additional oxygen atoms during the RTA process may have induced the increase of the thickness of the carbon-doped aluminum oxygen compound portion  583  over the thickness of the stack of aluminum-carbon-oxygen alloy portion  573  and an aluminum oxide layer  503  as grown on a hydrogen-terminated silicon surface of a semiconductor material layer  10 . 
       FIG. 12  illustrates an electron energy loss spectroscopy (EELS) plot of carbon, nitrogen, oxygen, chromium, aluminum and silicon signal amplitudes (arbitrary units) versus depth in the sample (in nanometers) for the sample of  FIG. 11C .  FIG. 12  shows that carbon atoms are present within the aluminum-carbon-oxygen alloy portion  583  formed by the atomic layer deposition process employing the organic aluminum precursor and water vapor. In  FIG. 12  the carbon content (area under curve) is greater than aluminum content suggesting that the portion  583  comprises predominantly of carbon. The carbon content is highest at center of portion  583  but is not confined to a small 1 nm layer as shown in  FIGS. 11A and 11B  prior to the RTA crystallization anneal which is believed to be due to intermixing. 
     Although the foregoing refers to particular preferred embodiments, it will be understood that the disclosure is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the disclosure. Where an embodiment employing a particular structure and/or configuration is illustrated in the present disclosure, it is understood that the present disclosure may be practiced with any other compatible structures and/or configurations that are functionally equivalent provided that such substitutions are not explicitly forbidden or otherwise known to be impossible to one of ordinary skill in the art. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.