Abstract:
Methods to implant ions into the sidewall of a three dimensional high aspect ratio feature, such as a trench or via, are disclosed. The methods utilize a phenomenon known as knock-in, which causes a first species of ions, already disposed in the fill material, to become implanted in the sidewall when these ions are struck by ions of a second species being implanted into the fill material. In some embodiments, these first species and second species have similar masses to facilitate knock-in. In some embodiments, the entire hole is not completely filled with fill material. Rather, some fill material is deposited, an ion implant is performed to cause knock-in to the sidewall adjacent to the deposited fill material, and the process is repeated until the hole is filled.

Description:
Embodiments of the present disclosure relate to methods of implanting the sidewalls of high aspect ratio features, specifically to doping a sidewall of a trench or via in a non volatile memory or other device. 
     BACKGROUND 
     As the desire to integrate more and more transistors onto a single substrate continues to grow, new technologies are developed. Previously, increases in transistor density were largely achieved by the miniaturization of the transistor itself. However, as geometries have continued to shrink, the widths of certain features, such as transistor gates may be less than ten atomic layers. Thus, there is a physical limit to the degree of miniaturization that is possible. 
     In an attempt to continuing integrating more transistors on a single device, the concept of vertical devices, also known as 3D devices, has gained momentum. Briefly, traditional transistors are made with the source, drain and gate region horizontally oriented. Vertical gates stack these features in the vertical direction, thereby reducing the horizontal footprint of each device. Various techniques are being proposed by various semiconductor manufacturers. 
     However, there are challenges associated with vertical devices. Specifically, with respect to certain vertical non-volatile memory (NVM) devices, such as NAND FLASH devices, the concept of string current has been discussed as a potential issue. The string current, or current in the vertical direction in a NVM memory device, is a function of the doping concentration of a polycrystalline channel. Inadequate or non-uniform doping of this channel may degrade the device operating parameters and performance. In addition, non-uniform doping of this channel may affect the threshold voltage of the different cells along that channel. These issues may impact certain types of NVM memory devices, including NAND FLASH devices. Similar challenges requiring controlled doping of vertical layers also occur in other types of NVM memory devices, such as resistive memory cells, which include but are not limited to ReRAM, PCRAM, and CBRAM devices. Additionally, other type of devices may have similar challenges. 
     Therefore, it would be beneficial if there were a method of doping a sidewall in a high aspect ratio feature, such as the trenches and vias in a vertical NVM device, such that performance parameters were optimized. 
     SUMMARY 
     Methods to implant ions into the sidewall of a three dimensional high aspect ratio feature, such as a trench or via, are disclosed. The methods utilize a phenomenon known as knock-in, which causes a first species of ions, already disposed in the fill material, to become implanted in the sidewall when these ions are struck by ions of a second species being implanted into the fill material. In some embodiments, these first species and second species have similar masses to facilitate knock-in. In some embodiments, the entire feature is not completely filled with fill material. Rather, some fill material is deposited, an ion implant is performed to cause knock-in to the sidewall adjacent to the deposited fill material, and the process is repeated until the feature is filled. 
     According to one embodiment, a process of doping a sidewall in a three-dimensional structure is disclosed, where the three-dimensional structure comprises a stack of alternating layers of different compositions. This method comprises etching a hole through the stack, the hole having a depth and a width; depositing a wall material along the sides of the hole; depositing a thickness of fill material in the hole after depositing of the wall material, where the fill material comprises a first species to be implanted in the wall material via knock-in; and implanting a second species into the deposited fill material, the second species having a mass and energy so as to cause the first species to be laterally dispersed into the wall material adjacent the deposited fill material. 
     According to a second embodiment, a process of doping a sidewall in a three-dimensional structure is disclosed, where this process comprises depositing an alternating pattern of first and second layers on a substrate, to form a stack; etching a hole through the stack; depositing a wall material on walls of the hole; depositing a fill material into the hole, where the fill material comprises a first species to be implanted in the wall material via knock-in; implanting a second species into the deposited fill material, wherein the ions are implanted into the fill material and on top of the stack; and removing at least one layer from the top of the stack after the implanting. 
     According to a third embodiment, a process of doping a sidewall in a non-volatile memory device, is disclosed. This process comprises depositing an alternating pattern of first and second layers on a substrate, to form a stack; etching a hole through the stack, the hole have a depth and a width; depositing a wall material on walls of the hole; depositing a thickness of fill material in the hole after depositing of the wall material, the thickness being less than the depth of the hole, where the fill material comprises a first species to be implanted in the wall material via knock-in; implanting a second species into the deposited fill material, the second species being of a mass and energy so as to cause the first species to be laterally dispersed into the wall material adjacent the deposited fill material; performing a subsequent deposition of a thickness of fill material into the hole after the implanting; performing a subsequent implanting of the second species into the fill material, whereby a different portion of the wall material is implanted by knock-in; and repeating the deposition and implanting steps until the hole is filled with fill material. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  shows an intermediate process step in the creation of a NVM memory device; 
         FIG. 2  shows an intermediate process step in accordance with a first embodiment of the creation of a doped sidewall in the device of  FIG. 1 ; 
         FIG. 3  shows a second intermediate process step in accordance with the first embodiment of the creation of a doped sidewall in the device of  FIG. 1 ; 
         FIG. 4  shows a third intermediate process step in accordance with a first embodiment of the creation of a doped sidewall in the device of  FIG. 1 ; 
         FIG. 5  shows an intermediate process step in accordance with a second embodiment of the creation of a doped sidewall in the device of  FIG. 1 ; 
         FIG. 6  shows a second intermediate process step in accordance with the second embodiment of the creation of a doped sidewall in the device of  FIG. 1 ; 
         FIG. 7  shows a third intermediate process step in accordance with the second embodiment of the creation of a doped sidewall in the device of  FIG. 1 ; 
         FIG. 8  is a representative flowchart of the first embodiment of the fabrication process shown in  FIGS. 2-4 ; and 
         FIG. 9  is a representative flowchart of the fabrication process shown in  FIGS. 5-7 . 
     
    
    
     DETAILED DESCRIPTION 
     The creation of a vertical NVM device requires a plurality of process steps to build the three-dimensional structure. During an intermediate process step, shown in  FIG. 1 , the vertical NVM device  100  has a plurality of alternating first layers  110  and second layers  120 , deposited on top of the substrate  105 . These alternating layers may be used to form an ONO (oxide-nitride-oxide) stack. This stack may comprise alternating layers of silicon nitride and silicon oxide. In other embodiments, this stack may comprise alternating layers of different compositions, such as an oxide or other dielectric and a metal. In some embodiments, the top layer or layers of the stack may be made thicker than the rest of the layers so that they can block ions that are implanted during a subsequent step from penetrating the stack. This top layer or layers may be used as a sacrificial layer, in that it will later be removed. The thickness of this sacrificial layer may be dictated by the depth to which ions penetrate the stack during subsequent process steps, as described in more detail below. In other embodiments, after the stack has been created, a deposition of another material, different from that used in the first layer  110  and the second layer  120 , is applied to act as a sacrificial layer. This sacrificial layer may be made using a heavier material, due to its ability to inhibit the passage of implanted ions. 
     Vertical holes  130  are then created in this stack, such as by using high resolution lithography and etching techniques. These vertical holes  130  may extend through the stack and to the substrate  105 . These vertical holes  130  may be circular, rectangular or have any other desired shape and may have a depth of 2 μm and a width of only 20 nm, thereby having an aspect ratio, defined as the depth of the feature divided by its width, of 100. In other embodiments, these vertical holes may have an aspect ratio greater than 50. High aspect ratio (HAR) features may be any feature having an aspect ratio of 10 or more. 
     After the vertical holes  130  have been created, a material which will ultimately become doped, is deposited on the sidewalls of the vertical holes  130 . This material, referred to as the wall material  140 , may be deposited using traditional deposition processes, or may be deposited using atomic layer deposition (ALD). In some embodiments, this wall material  140  may be polycrystalline silicon, amorphous silicon, crystalline silicon, tantalum pentoxide (Ta 2 O 5 ), tantalum, a tantalum oxide (TaO x ), tungsten, a tungsten oxide (WO x ), hafnium or a hafnium oxide (HfO x ), although the composition of the wall material  140  is not limited by this disclosure. In some embodiment, such as that shown in  FIG. 1 , the wall material  140  may also be deposited on the upper surface of the stack. In some embodiments, the wall material  140  may be a plurality of vertical layers. For example, the wall material  140  may include a first material deposited against the side walls of the hole  130 , and a second material deposited on that first material. 
     According to one embodiment, shown in  FIG. 2 , a fill material  150  is deposited using a deposition technique, such as by using high density plasma chemical vapor deposition (HDPCVD), atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), sputtering or other techniques. The fill material  150  fills a portion of the vertical hole  130  and also accumulates on the top surface of the stack. In some embodiments, an etch process is performed at the top of the vertical hole  130  so that the deposited fill material  150  does not constrict the width of the vertical hole  130 ; a condition known as necking. 
     In some embodiments, the fill material  150  that is deposited on the top surface of the stack is removed, such as by a preferential etch or a planarization step, such as CMP. In other embodiments, the fill material  150  may remain on the top surface of the stack to protect the stack from the subsequent ion implantation. 
     The choice of fill material  150  may be determined based on the dopant ions which are to be implanted into the wall material  140 . For example, the fill material  150  may be comprised of a compound which has the desired dopant as one of its elements. For example, if the desired dopant to be used in the wall material  140  is aluminum, the fill material may be Al 2 O 3 . In another example, if the desired dopant is oxygen, the fill material  140  may be SiO 2 . In another embodiment, the desired dopant may be co-deposited with another species to form the fill material  150 . For example, boron or phosphorus may be co-deposited with a silicon-based material, such as silicon or silicon dioxide, as the fill material  150 . In this example, the boron or phosphorus is the desired dopant. In another embodiment, the fill material  150  is deposited and the dopant species is implanted thereafter to allow the introduction of the desired species into the fill material  150 . Thus, the fill material  150  contains the desired dopant ions, either as a part of the molecular structure, as a co-deposited species, or as a subsequently implanted species. These desired dopant ions will be implanted into the wall material  140  in a subsequent step. It is noted that while the term dopant is used to describe ions  151  that are implanted in the wall material  140 , it is understood that this term also includes any species that affects the properties of the wall material  140 , such as conductivity, stoichiometry, damage profile, diffusion characteristics or other properties. Thus, in addition to traditional p-type and n-type silicon dopants, such as boron, aluminum and phosphorus, dopant ions  151  (see  FIG. 3 ) may also comprise other species, such as oxygen. 
     After the fill material  150  has been deposited into a portion of the vertical hole  130 , an implantation step using ions  160  of a first species is performed, as shown in  FIG. 3 . In this embodiment, the fill material  150  has been removed from the top surface of the stack, but this is not required in all embodiments. The purpose of the ion implant is to cause ions  151  in the fill material  150  to disperse laterally into the adjacent wall material  140 . This concept is known as knock-in, where a vertical ion implant also causes movement of ions  151  in the implanted fill material  150 . 
     The ion implant may be performed using the same species of ions  160  as is to be implanted in the wall material  140 . For example, in some embodiments, knock-in may be maximized when the implanted ions  160  and the ions  151  intended for lateral movement are the same mass and/or the same species. Thus, if it is desirous to implant aluminum into the wall material  140 , the fill material  150  that is deposited may be Al 2 O 3 , and the ion implantation may be performed using Al+ ions. Alternatively, if it is desirous to implant boron or phosphorus, the fill material  150  may be p- or n-doped silicon dioxide or silicon, and the ion implantation may be performed using B or P ions, respectively. In another embodiment, the ion implantation may be performed with silicon. Other ions  151  may be implanted into the wall material  140 , using a variety of fill materials  150  and ion species  160 . This ion implant is preferably performed at an angle normal to the top surface of the stack. 
     In addition, the use of the same species for ions  160  and ions  151  may have other benefits. For example, as ions  160  are implanted in the fill material  150 , they cause ions  151  to knock-in to the adjacent wall material  140 . This necessarily decreases the number of ions  151  that are available for knock-in during subsequent implants. However, if ions  160  are the same species as ions  151 , the previously implanted ions  160 , which now are disposed in the fill material  150 , effectively become ions  151 , available for knock-in during subsequent implants. Furthermore, ions  160  may be implanted in the wall material  140  via lateral straggle as well. These ions  160  may also contribute to the total dose required in the wall material  140 . 
     In another embodiment, the species  160  may be selected such that they do not change the wall material  140  in an appreciable way. For example, the wall material  140  may comprise a particular atom, such as silicon, and the ion species  160  may also include that ionized atom. In this way, lateral straggle of ions  160  may not significantly impact the properties of wall material  140 . In one further embodiment, the ions  151  may be boron or phosphorus. In this embodiment, the ions  160 , which may be silicon, may cause the boron ions  151  to penetrate more deeply into the wall material  140  due to their higher mass. In addition, any silicon ions  160  that experience lateral straggle do not affect the wall material  140 . 
     The implant energy of the ion implant affects the degree of knock-in. For example, a higher implant energy may cause ions  151  to move a greater distance laterally. In some embodiments, it may be desirous that the ions  151  are implanted in the wall material  140 , and do not pass beyond the wall material  140  and into the stack. In other embodiments, if the wall material  140  comprises a plurality of vertical layers, it may be desirous for the ions  151  to penetrate some of these layers, but not other layers. Therefore, the implant energy may be determined based on producing a knock-in distance such that ions  151  penetrate the wall material  140  but do not pass through that material. Thus, in one embodiment, the implant energy may be determined based on the desired knock-in. Once the implant energy has been determined, the thickness of the deposition of fill material  150  can be determined. For a given implant energy, the implanted ions  160  will penetrate into the fill material  150  to a determined depth. This relationship also depends on the selection of ion species  160  and the composition of the fill material  150 . For example, a lighter mass fill material  150 , such as carbon, may allow ions  160  to penetrate to a greater depth than a heavily material, such as silicon dioxide. 
     In some embodiments, the ion implant is performed as a chained implant, where the implant energy is varied to allow knock-in to occur at a plurality of depths. Thus, the term “implant” as used herein, is used to describe the implantation of ions  160  at one or more implant energies when a specific amount of fill material  150  is deposited. In other words, all implants that are performed while the fill material  150  is at a given depth, regardless of implant tool, ion species or implant energy, is described herein as an implant. 
     Chained implants may allow the creation of controlled dopant concentration profiles in the vertical direction along wall material  140 . For example, high implant energies may allow the ions  160  to penetrate the fill material  150  more deeply, and cause knock-in in this region. Lower implant energies do not penetrate the fill material  150  as deeply and may cause knock-in at a region closer to the top surface of the fill material  150 . The dose at each implant energy may be chosen to result in the required dopant profile. For example, if the deposition process results in a concentration profile of atoms  151  in fill material  150  that varies with depth, the dose of ions  160  at different energies can be adjusted to account for this. 
     The depth to which the ions  160  penetrate may define the thickness of the fill material  150  deposition, as knock-in cannot occur below this depth. After the ion implantation has been performed, the wall material  140  that is disposed adjacent to the fill material  150  will have been doped with the desired dopant. 
     In other embodiments, the penetration of ions  151  past the wall material  140  may not be detrimental. In these embodiments, the implant energy used for the ion implant may be determined based on the capabilities of the ion implanter or other factors. For example, the implant energy may be limited to minimize damage to the surrounding stack. As described above, a chained implant, using multiple implant energies and/or ion species, may be performed at each ion implantation step. As described above, the thickness of the fill material  150  deposition is determined by the depth to which the ions  160  penetrate the fill material  150  at the specified implant energy. 
     This sequence is then repeated, by depositing another layer of fill material  150  in the vertical hole  130  and the top surface of the stack, as shown in  FIG. 4 . The thickness of this layer of fill material  150  may be the same as the previous layer and may be chosen based on the ability for ions to penetrate the entire thickness of the newly deposited layer. However, in other embodiments, the thickness of the newly deposited layer may be greater than or less than the previously deposited layer. 
     The fill material  150  deposited on the top surface of the stack may be removed as described previously. The wall material  140  that was previously implanted via knock-in is now shown in crosshatch in  FIG. 4 . A subsequent ion implant may be performed using the same implant energies and ion species  160  as was used during the first ion implant. In other embodiments, different implant energies or ion species may be used. As a result of this second implant, another portion of the wall material  140  becomes implanted by ions  151  via knock-in within the fill material  150 . This process of depositing a thickness of fill material  150  into the hole  130 , and implanting ions  160  into the newly deposited fill material  150  to cause knock-in in the adjacent wall material  140  is repeated until the entire hole  130  has been filled and the entire height of the wall material  140  is doped. In some embodiments, this sequence of deposition and implantation may be repeated ten times to achieve the desired pattern. In other embodiments, this sequence may be repeated in a greater or fewer number of times. In one embodiment, the entire hole  130  may be filled and implanted in a single sequence, such as by using a chained implant utilizing multiple implant energies. 
     After the process is completed, the wall material  140  may have a uniform dopant concentration throughout the entire depth of the vertical hole  130 . However, in other embodiments, it may be desirable to tune the dopant concentration of the wall material  140  as a function of depth in the vertical hole  130 . For example, more (or less) dopant may preferably be implanted in the wall material  140  near the bottom of the deep hole  130 . In this case, the implant energies and dose of the various implants may differ. A greater implant energy and dose may cause a greater concentration of dopant in the wall material  140 . 
     Once the entire height of the wall material  140  has been implanted, the fill material  150  may optionally be removed from the hole  130 , such as by using an etching process. In other embodiments, it may be acceptable or preferable to allow the fill material  150  to remain in the hole  130 . Once these steps are completed, fabrication of the NVM device may continue in accordance with known techniques. 
     This first embodiment discloses a process where a thickness of fill material  150  is added, an ion implant is performed to cause knock-in of ions  151  in the fill material  150 , and these steps are repeated until the hole  130  has been filled with fill material  150 . A summary of this process is shown in  FIG. 8 . First, a stack of alternating layers, such as oxide and nitride, or dielectric and metal, are created, as shown in step  210 . Then, a vertical hole  130  is etched in the stack, as shown in step  210 . The sidewalls of the hole  130  are deposited with a wall material  140 , as shown in step  220 . A thickness of fill material  150  is then deposited on the structure, thereby filling a portion of the hole  130 , as shown in step  230 . Optionally, the fill material  150  that was deposited on top of the stack is removed, as shown in step  240 . An ion implantation is then performed using ions  160  to cause knock-in of ions  151  within the fill material  150 , as shown in step  250 . This serves to create a dopant concentration in the wall material  140  disposed adjacent to the implanted fill material  150 . If the hole is not yet filled, process steps  230 ,  240  and  250  are repeated. Once the hole is filled, the process is complete, as shown in step  260 . 
     In a second embodiment, this process is performed in a different order. In this embodiment, the fill material  150  is deposited so as to completely fill the vertical hole  130 , as shown in  FIG. 5 . The composition of the fill material  150  may be the same as that described earlier with respect to the first embodiment. 
     As described above, the fill material  150  that is deposited on the top surface of the stack may be removed such as by using an etching process or CMP. In other embodiments, the fill material  150 , or a portion of the fill material  150  may be left on the top surface of the stack to protect the underlying stack from the subsequent ion implants. 
     After the deposition of fill material  150 , an ion implantation using ion species  160  is performed as shown in  FIG. 6 . As described above, the ion implant may be a single implant or a set of chained implants at different implant energies. This first ion implant causes knock-in in the region near the top surface of the fill material  150 , such that ions  151  are deposited laterally into the wall material  140 . This ion implant is preferably performed at an angle normal to the top surface of the stack. As described above, the ions  160  used for this implant may be the same mass and/or species as those ions  151  that are introduced via knock-in. In one embodiment, the implant energies used for ion implant may be determined based on the desired knock-in of ions  151 . As stated earlier, the amount and distance of knock-in may be related to the implant energies of the ion implant. 
     After the ion implant is completed, as determined by the doping concentration of the wall material  140 , some of the fill material  150  is removed, as shown in  FIG. 7 . This thickness of fill material  150  may be removed using an etching process. The exposed wall material  140 , shown now in crosshatch in  FIG. 7  has been implanted to the desired dopant concentration via knock-in during the previous ion implant. The thickness of fill material  150  removed may be related to the depth to which knock-in occurred during the previous ion implantation. After this thickness of fill material  150  has been removed, a second ion implant is performed so as to dope the wall material  140  adjacent to the fill material  150  remaining in the hole  130 . This sequence of removing fill material  150  and performing an ion implantation process is repeated until the entirety of the wall material  140  has been implanted. The number of these sequences may vary. For example, ten sequences may be required to dope the entirety of the wall material  140 . In other embodiments, fewer sequences are needed. In one embodiment, the entirety of the wall material  140  may be implanted via knock-in through the use of chained implants performed while the fill material  150  fills the entire hole  130 . 
       FIG. 9  shows a representative process flow for this embodiment. The stack is created, a vertical hole  130  is etched through this stack and a wall material  140  is deposited on the sidewalls of the hole  130 , as described above and shown in steps  300 - 320 . The hole  130  is then completely filled with fill material  150 , as shown in step  330 . Optionally, the fill material  150  that was deposited on the top of the stack may be removed, as shown in step  340 . After this, an implant using ions  160  is performed to cause knock-in, as shown in step  350 . A thickness of fill material  150  is then removed from the hole  130 , as shown in step  360 . This thickness may be related to the depth to which knock-in occurred during the previous implant. If all of the fill material  150  has not yet been removed from the hole  130 , the process steps  350 ,  360  are repeated. Once all of the wall material  140  has been doped, the process ends, as shown in step  370 . 
     In each of the embodiments described above, the ion implants are performed using appropriate implant energies and doses so as to create a uniform dopant concentration profile in the wall material  140 . In other embodiments, the implants energies and doses of the ion implants may be varied to create a variable dopant concentration profile. For example, the dopant concentration may be a gradient where the wall nearest the bottom of the hole  130  is most heavily (or lightly) doped. 
     In other embodiments, a device parameter of the NVM device may be optimized by varying the various ion implants. For example, the threshold voltage of each cell in a FLASH device may be a critical device parameter. For example, a higher threshold voltage may be used to compensate for a thin charge trap stack or a shorter device channel length. In other embodiments, higher threshold voltages may be required near the top (or bottom) of the hole  130 . In other embodiments, NVM cell device parameters such as high and low resistance state, set voltage, reset voltage and forming voltage may be critical parameters that may be optimized. This can be readily accomplished by varying the implant energies and durations of the various ion implants. 
     For example, in some embodiments, the important device parameters of the NVM device may be measured after the device has been completely processed. Variations in these measured device parameters can be used to feedback changes to the implant energy and dose applied to the fill material  150  to alter the threshold voltages for subsequently processed NVM devices. In other words, the process also includes measuring the device parameters of a previously fabricated structure, and optimizing the doping concentration profile of a subsequent device based on these measured device parameters. This feedback may be used to affect an operating parameter of the implant, such as the implant dose, implant energy or another operating parameter. 
     After the completion of this process, the wall material  140  will have the desired doping concentration, which may be about 1E17 atoms/cm 3  or more for a NAND FLASH channel. In addition, the doping concentration may be uniform throughout the height of the vertical channel. In other embodiments, the doping concentration of the wall material  140  may vary as a function of the height to optimize device parameters, such as the threshold voltage of the charge traps, high or low resistance states, set or reset voltages or forming voltages in a resistive memory cell. 
     In another embodiment, metrology information from a partially completed device may be used to influence implant parameters. For example, the doping concentration of the deposited material  140  may be measured during the fabrication process. The implant parameters may be modified based on this measured concentration. In another example, the thickness of the wall material  140  may be measured during the fabrication process, and the implant parameters may be modified based on this measured thickness. 
     Note that, in the sequences shown in  FIGS. 8-9 , one or more ion implants may be performed. These ion implants may have a deleterious effect on the uppermost layers of the stack. For example, the implanting of ions  160  into the stack may impact the performance of the NVM device. As described earlier, sacrificial layers may be included in the stack. These sacrificial layers may be the same material as used for the first and second layers, or may be a different material. In one embodiment, these sacrificial layers may be removed after the ion implantations steps are completed, such as by MCP or etching. The removal of these sacrificial layers results in an stack that has not been contaminated by the ion implants. In other words, if the desired stack is to have 8 sets of alternating oxide and nitride layers or 8 sets of alternating dielectric and metal layers, it may be advantageous, in some embodiments, to apply at least one extra layer or set of alternating layers. These extra layers are added with the expectation that they will be contaminated by the ion implants and will thereafter be removed. In other words, rather than introducing another process step to add a mask on top of the stack, the stack is simply extended such that the top layer or layers serve as a mask (albeit a more porous mask). As described above, the sacrificial layer or layers may be thicker than the other layers in the stack. 
     While the disclosure describes the use of ion implants to cause knock-in at a particular step in the fabrication sequence, the disclosure is not limited to this embodiment. 
     The present disclosure shows the process steps associated with the formation of a NVM device in accordance with a particular process. It is understood that this includes NAND FLASH devices, and resistive memory devices, such as ReRAM, CBRAM and PCRAM devices. In addition, this technique can be used in conjunction with other three-dimensional semiconductor structures which have deep sidewalls or other high aspect ratio features that need to be doped. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.