Abstract:
The technology relates to a damascene word line for a three dimensional array of nonvolatile memory cells. Partly oxidized lines of material such as silicon are made over a plurality of stacked nonvolatile memory structures. Word line trenches are made in the partly oxidized lines, by removing the unoxidized lines from the intermediate parts of the partly oxidized lines, leaving the plurality of oxidized lines at the outer parts of the plurality of partly oxidized lines. Word lines are made in the word line trenches over the plurality of stacked nonvolatile memory structures.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to high density memory devices, and particularly to memory devices in which multiple planes of memory cells are arranged to provide a three-dimensional memory array. 
         [0003]    2. Description of Related Art 
         [0004]    In an example three-dimensional memory array, multiple ridges each include alternating stacks of semiconductor strips and oxide strips, and the ridges are covered by charge storage layers such as polysilicon or charge trapping dielectric material like ONO. Word lines orthogonal to and conformal to the ridges access the memory cells in the three-dimensional memory array. Insulating lines, such as oxide lines orthogonal to and conformal to the ridges, electrically isolate the neighboring word lines from each other. 
         [0005]    However, it is not trivial to form the oxide lines between the word lines such that neighboring word lines are electrically isolated from each other by the oxide lines.  FIGS. 1-2  show problems in various manufacturing processes for the word lines and the oxide lines of a three-dimensional memory array. 
         [0006]      FIG. 1  is a perspective illustration of a three dimensional memory array device, where the polysilicon word lines are formed prior to the oxide lines separating the word lines, and polysilicon residue forms an undesirable bridge electrically connecting neighboring word lines. 
         [0007]    Semiconductor strips  11 ,  13 , and  15  are separated by oxide strips  10 ,  12 ,  14 , and  16 . The stacks of alternating semiconductor/oxide strips are covered by charge storage layers  26 , such as ONO or ONONO. The polysilicon word line  55  is formed by covering the stacks of alternating semiconductor/oxide strips and charge storage layers with polysilicon, and etching away excess polysilicon between neighboring polysilicon word lines to form trenches between neighboring polysilicon word lines. After etching away the excess polysilicon, oxide lines are formed that isolate neighboring polysilicon word lines. 
         [0008]    A high aspect ratio is presented by the height of the stacks of alternating semiconductor/oxide strips covered by charge storage layers, relative to the desired distance between neighboring word lines. As a result, there is a failure to etch away polysilicon residue  56 . Despite the following oxide fill in the trenches formed by the polysilicon etch, the polysilicon residue  56  electrically connects neighboring word lines (the word line shown in the figure, and a neighboring word line not shown in the figure). 
         [0009]    The charge storage layers fill part of the oxide void in the stack of alternating semiconductor/oxide strips, which is a region  27  of oxide pull-back. Oxide pull-back was caused by cleaning the stacks of alternating semiconductor/oxide strips, in preparation for making the charge storage layers. The region  27  of oxide pull-back in turn results in a void in the charge storage layers. This void is filled by polysilicon residue  57 , which electrically connects neighboring word lines (the word line shown in the figure, and another neighboring word line not shown in the figure). 
         [0010]      FIG. 2  is a perspective illustration of a three dimensional memory array device, where the oxide lines are formed prior to the polysilicon word lines, and an oxide void allows polysilicon residue to form an undesirable bridge electrically connecting neighboring word lines. 
         [0011]    Semiconductor strips  11 ,  13 ,  15  are separated by oxide strips  10 ,  12 ,  14 ,  16 . The stacks of alternating semiconductor/oxide strips are covered by charge storage layers, such as a charge storage structure of an oxide  20 -nitride  21 -oxide  22 . The oxide line  45  is formed by covering the stacks of alternating semiconductor/oxide strips and charge storage layers with oxide, and etching away excess oxide between neighboring oxide lines to form trenches between neighboring oxide lines. After etching away the excess oxide, damascene polysilicon word lines are formed in the trenches between neighboring oxide lines. 
         [0012]    Oxide line  45  has a void  46 . In the step of making polysilicon word lines in the trenches between neighboring oxide lines, the oxide void  46  is filled with polysilicon, electrically connecting the neighboring polysilicon word lines on either side of the oxide line  45 . 
         [0013]    An additional problem is the quality of the charge storage layers covering the alternating stacks of semiconductor strips and oxide strips. When the charge storage layers are formed prior to the word lines and oxide lines, and the oxide lines are formed prior to the word lines, the oxide lines are formed by etching away excess oxide between the oxide lines to form trenches for the word lines. However, after the oxide etch removes excess oxide, the oxide etch damages the charge storage layers under the excess oxide. This damage to the charge storage layers harms memory device performance. For charge storage layers with an outer oxide such as ONO, it is difficult to selectively etch the excess oxide but not the outer oxide of the charge storage layers. 
         [0014]      FIG. 3  is a top view of a three dimensional memory array device, where the ONO storage layers are formed after the oxide lines but prior to the polysilicon word lines, resulting in larger dimensions of the array. The process flow shows: (i) formation of the stacks of alternating oxide/semiconductor strips  18 , (ii) formation of oxide lines  42  orthogonal and conformal to the oxide/semiconductor strips, and (iii) formation of charge storage layers  28  such as ONO or polysilicon. The charge storage layers can cover the stacks of alternating oxide/semiconductor strips  18 . The figure does not show the charge storage layers can cover the stacks of alternating oxide/semiconductor strips  18 , so that the lateral dimensions of the storage layers are visible. This process flow is disadvantageous, because the unit cell size is laterally enlarged by twice the thickness of the storage layers. 
         [0015]    U.S. application Ser. No. 12/347,331 filed 10 Jan. 2012 describes a damascene word line. The present application describes additional technology regarding a damascene word line. Several embodiments in U.S. application Ser. No. 12/347,331 have a relatively shorter process flow. Several embodiments of the present application can be scaled down to a relatively smaller word line pitch, or relatively smaller distance between neighboring word lines. 
         [0016]    It is desirable to provide a structure for three-dimensional integrated circuit memory with a low manufacturing cost, including reliable, very small memory elements and improved process window associated with neighboring stacks of memory cell strings having gate structures. 
       SUMMARY OF THE INVENTION 
       [0017]    Technology is described here that includes a method of making a three dimensional array of nonvolatile memory cells. The method includes the following: 
         [0018]    A first plurality of material lines is made over a plurality of stacked nonvolatile memory structures. The first plurality of material lines are separated by a first plurality of word line trenches that separate neighboring ones of the first plurality of material lines. In some embodiments of the described technology, to form the first plurality of material lines, a layer of material such as silicon is made over the plurality of stacked nonvolatile memory structures, and excess material such as silicon is removed from the layer to leave the first plurality of material lines and form the first plurality of word line trenches between neighboring ones of the first plurality of material lines. 
         [0019]    A plurality of partly oxidized structures is made from the first plurality of material lines by oxidizing both sides of material lines in the first plurality of material lines. The plurality of partly oxidized structures include a plurality of oxide lines bounding a first plurality of narrowed material lines. Narrowed material lines in the first plurality of narrowed material lines have a narrower width than the material lines in the first plurality of material lines. 
         [0020]    The first plurality of narrowed material lines bounded by the plurality of oxide lines are removed, to make a second plurality of word line trenches. 
         [0021]    A plurality of word lines is made over the plurality of stacked nonvolatile memory structures in the first plurality of word line trenches and the second plurality of word line trenches. 
         [0022]    In some embodiments of the described technology, the material lines include silicon lines. In some embodiments of the described technology, the silicon lines include at least one of amorphous silicon, polysilicon, and single crystal silicon. The polysilicon material used in this paper can be replaced by other silicon such as amorphous silicon or single crystal silicon. 
         [0023]    In other embodiments of the described technology, the material lines include metal lines such as W which oxidizes to WOx. Successful oxidation of a metal line to an insulating oxide is process dependent, and varies with the oxygen concentration. 
         [0024]    In yet other embodiments of the described technology, the material lines include semiconductor lines. 
         [0025]    Several embodiments of the described technology can repair the damage to charge storage layers which results from etching. After removing the first plurality of narrowed material lines covering the plurality of stacked nonvolatile memory structures, exposed oxide is etched, which is covering the plurality of stacked nonvolatile memory structures in the second plurality of word line trenches. In some embodiments, the exposed oxide is etched until exposing nitride covering the plurality of stacked nonvolatile memory structures in the second plurality of word line trenches. Such etching of the exposed oxide can remove damaged oxide that would otherwise harm memory device performance. The etched oxide is replaced with new oxide. Oxide is made to cover the plurality of stacked nonvolatile memory structures in the second plurality of word line trenches. This replacement oxide for the etched exposed oxide can result in superior memory device performance. In various embodiments, the oxide can be deposited, and/or oxidized from an exposed nitride layer covering the plurality of stacked nonvolatile memory structures in the second plurality of word line trenches. 
         [0026]    In some embodiments of the described technology, before removing the first plurality of narrowed material lines covering the plurality of stacked nonvolatile memory structures, a second plurality of material lines is made in the first plurality of word line trenches. Prior to making the word lines in the first plurality of word line trenches, the second plurality of material lines are removed from the first plurality of word line trenches, at least partly while removing the first plurality of narrowed material lines. Subsequent to removing the material lines from the word line trenches, the word lines can be made in the word line trenches. 
         [0027]    In some embodiments of the described technology, removing the first plurality of narrowed material lines leaves material such as silicon residue in at least one word line trench of the second plurality of word line trenches. After removing the first plurality of narrowed material lines, material such as silicon residue left in at least one word line trench of the second plurality of word line trenches is oxidized. 
         [0028]    In some embodiments of the described technology, an oxide line of the plurality of oxide lines has a void. After removing the first plurality of narrowed material lines, to at least partly fill the void, material such as silicon residue left in at least one word line trench of the second plurality of word line trenches is oxidized. 
         [0029]    In some embodiments of the described technology, making the word lines is a damascene process. 
         [0030]    In some embodiments of the described technology, prior to oxidizing both opposite sides including first surfaces and second surfaces of the material lines in the first plurality of material lines, oxidation masks are made that cover third surfaces of the material lines in the first plurality of material lines. The third surfaces joining the first surfaces and the second surfaces of the material lines. The oxidation masks can decrease oxidation of the third surfaces while oxidizing the first surfaces and the second surfaces. In some embodiments, material such as silicon is deposited over the oxidation masks at least partly while filling in the first plurality of word line trenches with material such as silicon. After oxidizing the first surfaces and the second surfaces, the oxidation masks can be removed. The third surfaces can be top surfaces with decreased oxidation due to the oxidation masks covering the third surfaces. After removing the oxidation masks, the material such as silicon can be removed from the first plurality of word line trenches at least partly while removing the first plurality of narrowed material lines. 
         [0031]    In some embodiments of the described technology, prior to making the first plurality of word line trenches, the plurality of stacked nonvolatile memory structures are formed, including a plurality of semiconductor strips separated by insulating material, and charge storage structures covering the plurality of semiconductor strips. The charge storage structures can include a silicon nitride layer. Also, the charge storage structures can include an oxide-nitride-oxide-nitride-oxide structure (such as BE-SONOS). In one example, the charge storage structures include a hafnium oxide layer. In yet another example, charge storage structures covering the plurality of semiconductor strips, the charge storage structures include a dielectric layer having a dielectric constant greater than a silicon oxide dielectric constant, also called a high-K layer. 
         [0032]    Further technology described here is an additional method of making a three dimensional array of nonvolatile memory cells. The method includes the following: 
         [0033]    A plurality of partly oxidized silicon lines are formed over a plurality of stacked nonvolatile memory structures. The plurality of partly oxidized silicon lines have a plurality of unoxidized silicon lines at intermediate parts of the plurality of partly oxidized lines and a plurality of oxidized lines at outer parts of the plurality of partly oxidized lines. 
         [0034]    A plurality of word line trenches is made in the plurality of partly oxidized silicon lines, by removing the plurality of unoxidized silicon lines from the intermediate parts of the plurality of partly oxidized lines, leaving the plurality of oxidized lines at the outer parts of the plurality of partly oxidized lines. 
         [0035]    A plurality of word lines is made in the plurality of word line trenches over the plurality of stacked nonvolatile memory structures. 
         [0036]    In some embodiments of the described technology, after removing the plurality of unoxidized silicon lines over the plurality of stacked nonvolatile memory structures, exposed oxide covering the plurality of stacked nonvolatile memory structures in the plurality of word line trenches is etched. Oxide is made to cover the plurality of stacked nonvolatile memory structures in the plurality of word line trenches. 
         [0037]    In some embodiments of the described technology, forming the plurality of partly oxidized silicon lines over the plurality of stacked nonvolatile memory structures includes, the steps of:
       making oxidation masks covering top surfaces of silicon lines in a plurality of silicon lines over the plurality of stacked nonvolatile memory structures; and   oxidizing both opposite exposed surfaces of the silicon lines in the plurality of silicon lines.       
 
         [0040]    Further technology described here is an integrated circuit with a three dimensional array of nonvolatile memory cells. The integrated circuit includes a plurality of double patterned word lines; a plurality of oxide lines separating neighboring ones of the plurality of word lines; and a plurality of stacked nonvolatile memory structures in the three dimensional array. The double patterned word lines follow multiple patterning steps, or multiple exposures and multiple etch steps. 
         [0041]    The plurality of stacked nonvolatile memory structures are covered by the plurality of word lines and the plurality of oxide lines. The plurality of stacked nonvolatile memory structures include a nitride layer. The plurality of stacked nonvolatile memory structures include a first part covered by the plurality of word lines and a second part covered by the plurality of oxide lines. The nitride layer in the first part has a narrower thickness than the nitride layer in the second part. 
         [0042]    Further embodiments of the various technologies are described herein. 
         [0043]    Other aspects and advantages of the present invention can be seen on review of the drawings, the detailed description and the claims, which follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  is a perspective illustration of a three dimensional memory array device, where the polysilicon word lines are formed prior to the oxide lines separating the word lines, and polysilicon residue forms an undesirable bridge electrically connecting neighboring word lines. 
           [0045]      FIG. 2  is a perspective illustration of a three dimensional memory array device, where the oxide lines are formed prior to the polysilicon word lines, and an oxide void allows polysilicon residue to form an undesirable bridge electrically connecting neighboring word lines. 
           [0046]      FIG. 3  is a top view of a three dimensional memory array device, where the ONO storage layers are formed after the oxide lines but prior to the polysilicon word lines, resulting in larger dimensions of the array. 
           [0047]      FIG. 4  illustrates part of a process for manufacturing a memory device, and shows a perspective illustration of forming a plurality of parallel semiconductor strips separated by oxide strips, arranged in a plurality of ridge-shaped stacks. 
           [0048]      FIG. 5  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming oxide-nitride-oxide data storage layers on the plurality of ridge-shaped stacks of  FIG. 4 . 
           [0049]      FIG. 6  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming an oxide and polysilicon, with conformal bottom surfaces arranged over the plurality of ridge-shaped stacks of  FIG. 5 . 
           [0050]      FIG. 7  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming polysilicon lines with oxidation masks from the polysilicon and dielectric layers of  FIG. 6 . 
           [0051]      FIG. 8  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming oxide lines via oxidation, with conformal bottom surfaces arranged over the plurality of ridge-shaped stacks, from the conformal polysilicon lines of  FIG. 7 . 
           [0052]      FIG. 9  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of performing a polysilicon fill over the exposed surfaces, and filling in empty trenches from  FIG. 8 . 
           [0053]      FIG. 10  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing polysilicon to expose the oxidations masks from  FIG. 9 . 
           [0054]      FIG. 11  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing the oxidation masks from  FIG. 10 . 
           [0055]      FIG. 12  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing the polysilicon lines from  FIG. 11 . 
           [0056]      FIG. 13  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of oxidizing polysilicon residue from  FIG. 12 . 
           [0057]      FIG. 14  is illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing the outer oxide from the oxide-nitride-oxide data storage layers of  FIG. 13 . 
           [0058]      FIG. 15  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming the outer oxide on the nitride-oxide data storage layers of  FIG. 14 . 
           [0059]      FIG. 16  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of performing a polysilicon fill over the exposed surfaces, and filling in word line trenches from  FIG. 15 . 
           [0060]      FIG. 17  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing excess polysilicon from  FIG. 16 . 
           [0061]      FIG. 18  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming silicided polysilicon word lines from the polysilicon word lines of  FIG. 17 . 
           [0062]      FIG. 19  is an alternative of  FIG. 18 , illustrating part of a process for manufacturing a memory device, and is a perspective illustration of removing polysilicon above the oxide-nitride-oxide data storage layers on the plurality of ridge-shaped stacks of  FIG. 17 . 
           [0063]      FIG. 20  is an alternative of  FIG. 18 , illustrating part of a process for manufacturing a memory device, and is a perspective illustration of forming word lines with metal surfaces on the exposed outer oxide of the oxide-nitride-oxide data storage layers and on the exposed polysilicon of  FIG. 19 . 
           [0064]      FIG. 21  is an alternative of  FIG. 18 , illustrating part of a process for manufacturing a memory device, and is a perspective illustration of forming word lines with metal surfaces on exposed polysilicon. 
           [0065]      FIG. 22  is a perspective view comparing the nitride layer of the oxide-nitride-oxide data storage layers, between the areas under the word lines and the areas under the oxide lines. 
           [0066]      FIGS. 23-25  are perspective views of various “M”-shaped gates. 
           [0067]      FIG. 26  is a picture of a cross-section of part of a 3D memory with a stack of alternating channels and oxides, illustrating areas of potential “shadow effect”. 
           [0068]      FIG. 27  is an example 3D memory structure with the damascene gate. 
           [0069]      FIG. 28  is a circuit diagram corresponding to the example 3D memory structure of  FIG. 23  with the damascene gate. 
           [0070]      FIG. 29  is a schematic diagram of an integrated circuit including a 3D NAND-flash memory array with the damascene gate and row, column and plane decoding circuitry. 
           [0071]      FIGS. 30-37  are top views of parts of a process for manufacturing a memory device. 
       
    
    
     DETAILED DESCRIPTION 
       [0072]      FIG. 4  illustrates part of a process for manufacturing a memory device, and shows a perspective illustration of forming a plurality of parallel semiconductor strips separated by oxide strips, arranged in a plurality of ridge-shaped stacks. 
         [0073]    In  FIG. 4 , only 3 layers of semiconductor strips are shown for purposes of illustration. However, the number of layers can be extended to other numbers such as 2 layers, 4 layers, 8 layers, 16 layers, or another number of layers. The memory array is formed on an integrated circuit substrate having an insulating layer over underlying semiconductor or other structures (not shown). The memory array includes a plurality of stacks (2 stacks are shown in the figure) of semiconductor strips  11 ,  13 , and  15  alternating with insulating material strips  10 ,  12 ,  14 , and  16 . The stacks are ridge-shaped extending on the Y-axis as illustrated in the figure, so that the semiconductor strips  11 ,  13 ,  15  can be configured as memory cell strings. Semiconductor strips at the same height can act as memory cell strings in a same memory plane. 
         [0074]    To make the stacks of strips, in one example semiconductor layers alternating with insulating material layers are formed for example in blanket deposition in the array area of the chip. The semiconductor strips are implemented using the semiconductor layers separated by the insulating layers.  FIG. 4  shows the result of a lithographic patterning step used to define a plurality of ridge-shaped stacks of semiconductor strips. Deep, high aspect ratio gaps can be formed between the stacks, supporting many layers, using lithography based processes applying a carbon hard mask and reactive ion etching. 
         [0075]    The semiconductor strips  11 ,  13 ,  15  can be a p-type semiconductor material. For example, the semiconductor strips can be made using p-type polysilicon, or p-type epitaxial single crystal silicon. 
         [0076]    Alternatively, the semiconductor strips  11 ,  13 ,  15  can be n-type semiconductor material. An n-type strip arrangement can result in buried-channel, depletion mode charge trapping dielectric material memory cells. For example, the semiconductor strips  11 ,  13 ,  15  can be made using n-type polysilicon, or n-type epitaxial single crystal silicon. A typical doping concentration for n-type semiconductor strips can be around 10 18 /cm 3 , with usable embodiments likely in the range of 10 17 /cm 3  to 10 19 /cm 3 . The use of n-type semiconductor strips can be particularly beneficial in junction-free embodiments to improve conductivity along the NAND strings and thereby allowing higher read current. 
         [0077]    Inter-level insulating strips  10 ,  12 ,  14 ,  16  can be implemented for example using silicon dioxide, other silicon oxides, or silicon nitride. These layers can be formed in a variety of ways, including low pressure chemical vapor deposition LPCVD processes. 
         [0078]    The insulating material  12  between the semiconductor strips  11  and  13 , and the insulating material  14  between semiconductor strips  13  and  15 , and the insulating material in corresponding insulating material strips in other stacks, has an effective oxide thickness of about 30 nm or greater, where effective oxide thickness EOT is a thickness of the insulating material normalized according to a ratio of the dielectric constant of silicon dioxide and the dielectric constant of the chosen insulation material. The term “about 30 nm” is used here to account for variations on the order of 10% or so, as arise typically in manufacturing structures of this type. The thickness of the insulating material can play a critical role in reducing interference between cells in adjacent layers of the structure. In some embodiments, the EOT of the insulating material can be as small as 20 nm while achieving sufficient isolation between the layers of semiconductor strips. 
         [0079]      FIG. 5  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming oxide-nitride-oxide data storage layers on the plurality of ridge-shaped stacks of  FIG. 4 . 
         [0080]    The data storage layers can include a dielectric charge trapping structure. Shown are the results of blanket deposition of data storage layers oxide  22  (blocking dielectric layer)-nitride  21  (charge storage layer)-oxide  20  (tunneling dielectric layer), which conformally coat the plurality of stacks of semiconductor strips in this example. 
         [0081]    The layer(s) of memory material can comprise other charge storage structures. For example, a bandgap engineered SONOS (BE-SONOS) charge storage structure can be used which includes a dielectric tunneling layer that includes a composite of materials forming an inverted “U” shaped valence band under zero bias. In one embodiment, the composite tunneling dielectric layer includes a first layer referred to as a hole tunneling layer, a second layer referred to as a band offset layer, and a third layer referred to as an isolation layer. The hole tunneling layer in this embodiment comprises silicon dioxide on the side surface of the semiconductor strips formed for example using in-situ steam generation ISSG with optional nitridation by either a post deposition NO anneal or by addition of NO to the ambient during deposition. The thickness of the first layer of silicon dioxide is less than 20 Å, and preferably 15 Å or less. Representative embodiments can be 10 Å or 12 Å thick. 
         [0082]    The band offset layer in this embodiment comprises silicon nitride lying on the hole tunneling layer, formed for example using low-pressure chemical vapor deposition LPCVD, using for example dichlorosilane DCS and NH 3  precursors at 680° C. In alternative processes, the band offset layer comprises silicon oxynitride, made using a similar process with an N 2 O precursor. The band offset layer thickness of silicon nitride is less than 30 Å, and preferably 25 Å or less. 
         [0083]    The isolation layer in this embodiment comprises silicon dioxide, lying on the band offset layer of silicon nitride formed for example using LPCVD high temperature oxide HTO deposition. The thickness of the isolation layer of silicon dioxide is less than 35 Å, and preferably 25 Å or less. This three-layer tunneling layer results in an inverted U-shaped valence band energy level. 
         [0084]    The valence band energy level is such that an electric field sufficient to induce hole tunneling through the thin region at the interface with the semiconductor body, is also sufficient to raise the valence band energy level to a level that effectively eliminates the hole tunneling barrier in the composite tunneling dielectric. This structure establishes an inverted U-shaped valence band energy level in the three-layer tunneling dielectric layer, and enables electric field assisted hole tunneling at high speeds while effectively preventing charge leakage through the composite tunneling dielectric in the absence of electric fields or in the presence of smaller electric fields induced for the purpose of other operations, such as reading data from the cell or programming adjacent cells. 
         [0085]    In a representative device, the layer of memory material includes a bandgap engineered composite tunneling dielectric layer comprising a layer of silicon dioxide less than 2 nm thick, a layer of silicon nitride less than 3 nm thick, and a layer of silicon dioxide less that 4 nm thick. In one embodiment, the composite tunneling dielectric layer consists of an ultrathin silicon oxide layer O 1  (e.g. &lt;=15 Å), an ultrathin silicon nitride layer N 1  (e.g. &lt;=30 Å) and an ultrathin silicon oxide layer O 2  (e.g. &lt;=35 Å), which results in an increase in the valence band energy level of about 2.6 eV at an offset 15 Å or less from the interface with the semiconductor body. The O 2  layer separates the N 1  layer from the charge trapping dielectric layer, at a second offset (e.g. about 30 Å to 45 Å from the interface), by a region of lower valence band energy level (higher hole tunneling barrier) and higher conduction band energy level. The electric field sufficient to induce hole tunneling raises the valence band energy level after the second location to a level that effectively eliminates the hole tunneling barrier, because the second location is at a greater distance from the interface. Therefore, the O 2  layer does not significantly interfere with the electric field assisted hole tunneling, while improving the ability of the engineered tunneling dielectric to block leakage during low fields. 
         [0086]    A charge trapping dielectric layer in the layer of memory material in this embodiment comprises silicon nitride having a thickness greater than 50 Å, including for example about 70 Å in this embodiment formed for example using LPCVD. Other charge trapping dielectric materials and structures may be employed, including for example silicon oxynitride (Si x O y N z ), silicon-rich nitride, silicon-rich oxide, trapping layers including embedded nano-particles and so on. 
         [0087]    The blocking dielectric layer in the layer of memory material in this embodiment comprises a layer of silicon dioxide having a thickness greater than 50 Å, including for example about 90 Å in this embodiment, can be formed by wet conversion from the nitride by a wet furnace oxidation process. Other embodiments may be implemented using high temperature oxide (HTO) or LPCVD SiO 2 . Other blocking dielectrics can include high-K materials like aluminum oxide. 
         [0088]    In a representative embodiment, the hole tunneling layer can be 13 Å of silicon dioxide; the band offset layer can be 20 Å of silicon nitride; the isolation layer can be 25 Å of silicon dioxide; the charge trapping dielectric layer can be 70 Å of silicon nitride; and the blocking dielectric layer can be silicon oxide 90 Å thick. The gate material can be p+ polysilicon (work function about 5.1 eV) used in the word line  50  shown in later figures. 
         [0089]    The outer oxide  22  in  FIG. 5  is a sacrifical oxide, to be removed and regrown later. 
         [0090]      FIG. 6  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming an oxide and polysilicon, with conformal bottom surfaces arranged over the plurality of ridge-shaped stacks of  FIG. 5 . 
         [0091]    The polysilicon material  44  is formed conformally over the data storage layers covering the plurality of stacks of semiconductor strips. The polysilicon material  44  fills the gaps between the plurality of stacks of semiconductor strips. 
         [0092]    In a high aspect ratio fill step, conductive material, such as polysilicon having n-type or p-type doping, is deposited to cover the stacks of semiconductor material strips and fill the gaps between the stacks of semiconductor material strips. High aspect ratio deposition technologies such as low-pressure chemical vapor deposition of polysilicon is utilized to completely fill the gaps  220  between the ridge-shaped stacks, even very narrow trenches on the order of 10 nanometers wide with high aspect ratio. 
         [0093]    A dielectric layer  48  can be a silicon nitride film as shown, silicon dioxide, or other silicon oxides. This dielectric layer can be formed in a variety of ways, including a low pressure chemical vapor deposition LPCVD processes. The dielectric layer  48  discourages diffusion of oxidizing species such as oxygen and water from diffusing through the oxidation layer  48 . 
         [0094]      FIG. 7  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming polysilicon lines with oxidation masks from the polysilicon and dielectric layers of  FIG. 6 . 
         [0095]    In a lithographic patterning step, the polysilicon lines  30  and  31  are defined from polysilicon layer  44 . In one representative embodiment, the width of the polysilicon lines is anywhere from 500-1500 angstroms, though this is dependent on specifics of future steps and the desired ultimate width of word lines and spacing between word lines. The lithographic patterning step utilizes a mask for critical dimensions of the array for etching high aspect ratio trenches between the polysilicon lines such as trench  33 , without etching through the ridge-shaped stacks. The high aspect ratio word line trenches separate the polysilicon lines formed in this step, and indicate where the word lines will be formed in a future step, though the word line trenches are narrowed in a future step. Polysilicon can be etched using an etch process that is highly selective for polysilicon over silicon oxides or silicon nitrides. Thus, alternating etch processes are used, relying on the same mask to etch through the conductor and insulating layers, with the process stopping on an underlying insulating layer. Despite the highly selective etch, during the etch there is oxide loss from exposed oxide such as the outer oxide  22 . Such oxide loss varies with position. Oxide  22  is a dummy oxide that later is etched and regrown in subsequent processing discussed below. The oxide  22  protects the SiN memory kernel  21 . 
         [0096]    The polysilicon lines  30  and  31  are formed conformally over the data storage layers covering the plurality of stacks of semiconductor strips. The polysilicon lines  30  and  31  fill the gaps between the plurality of stacks of semiconductor strips. Two polysilicon lines  30  and  31  are shown, but other embodiments have a different number of polysilicon lines. Additional polysilicon lines are formed along the y-axis out of the page and into the page. Neighboring polysilicon lines are separated by a word line trench, such as trench  33 , narrowed in a future step. 
         [0097]    In the same lithographic patterning step that defined polysilicon lines  30  and  31 , the oxidation masks  25  and  29  are defined from dielectric layer  48 . The oxidation masks  25  and  29  discourage diffusion of oxidizing species such as oxygen and water from diffusing through the oxidation masks. Thus, the oxidation masks  25  and  29  discourage oxidation of the top surfaces of the underlying polysilicon lines  30  and  31 . 
         [0098]    The polysilicon residue  32  in  FIG. 7  creates a risk of forming a conductive bridge electrically connecting neighboring word lines. The polysilicon residue  32  is oxidized in  FIG. 8 , removing the risk that the residue  32  shorts together neighboring word lines. The polysilicon line  31  also has a void  47 . 
         [0099]      FIG. 8  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming oxide lines via oxidation, with conformal bottom surfaces arranged over the plurality of ridge-shaped stacks, from the conformal polysilicon lines of  FIG. 6 . During the oxidation process which forms the oxide lines, from the polysilicon surfaces, the polysilicon residue  32  is also oxidized. 
         [0100]    The conformal polysilicon lines  30  and  31  are partly oxidized. Both opposite exposed surfaces of conformal polysilicon line  30  are oxidized, forming oxide lines  34  and  35  on both opposite sides of narrowed polysilicon line  36 . Both opposite exposed surfaces of conformal polysilicon line  31  are oxidized, similarly forming oxide lines  37  and  38  on both opposite sides of a narrowed polysilicon line  39 . However, the oxidation masks  25  and  29  discourage oxidation from the top surfaces of the conformal polysilicon lines  30  and  31  covered by the oxidation masks  25  and  29 . Oxide lines  34 ,  35 ,  37 , and  38  are conformal over the data storage layers covering the plurality of stacks of semiconductor strips, and fill the trenches defined by the plurality of stacks of semiconductor strips. Other embodiments have a number of oxide lines that are sufficient to isolate the number of word lines that access the array. Additional oxide lines are formed along the y-axis out of the page and into the page. This oxidation narrows the trench  33  between oxide lines  35  and  37 , because of the expansion of polysilicon into silicon oxide. An example thickness range of oxide lines  34 ,  35 ,  37 , and  38  is about 400 angstroms down to about 100 angstroms. After the oxidation, the narrowed polysilicon lines have an example thickness of about 350 angstroms down to about 100 angstroms. 
         [0101]    Example oxidation processes are furnace oxidation such as with horizontal furnaces and vertical furnaces, and rapid thermal processors (RTP). 
         [0102]    The oxidation process of  FIG. 8  addresses issues with polysilicon polysilicon residue, as shown in the following figures. The amount of material in the polysilicon line is a limiting source on the oxidation process. In the reaction, Si (from the polysilicon line)+O 2  forms SiO 2 . The volume of the resulting SiO 2  expands by over 120% over the reacted Si source. In terms of volume percentages where the resulting SiO 2  has a volume of 100%, then Si (solid, volume 45%)+O 2  (gas) forms SiO 2  (Solid, volume 100%). So the volume expansion of silicon to silicon oxide is  55 / 45  or about 122% due to oxidation. Small voids in the outer surfaces of polysilicon lines  30  and  31  are filled in the oxidation process. 
         [0103]    The polysilicon residue  32  of  FIG. 7  is oxidized into silicon oxide  42  during the process of oxidizing both opposite sides of polysilicon lines  30  and  31 . Such oxidation reduces the risk of a conductive bridge electrically connecting neighboring word lines. The oxide line  34  also has a void  47 , which was formerly a void in the polysilicon line  30 . 
         [0104]      FIG. 9  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of performing a polysilicon fill over the exposed surfaces, and filling in empty trenches from  FIG. 8 . 
         [0105]    The trench  33  from  FIG. 8  is filled with polysilicon line  41 . The polysilicon fill covers previously exposed oxidations masks. The fill can be formed in a variety of ways, including low pressure chemical vapor deposition LPCVD processes. The polysilicon fill also enters the void  47 . 
         [0106]      FIG. 10  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of performing a polysilicon etch to expose the oxidations masks from  FIG. 9 . 
         [0107]    The removal of excess polysilicon can be performed in a variety of ways, such as dry or wet etching that is selective against etching the underlying oxidation masks, and by chemical mechanical polishing (CMP). Endpoint detection to stop the removal of excess polysilicon can be performed by various processes such as spectrometry laser interferometry, and optical sensing. 
         [0108]      FIG. 11  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing the oxidation masks from  FIG. 10 . 
         [0109]    The oxidation masks  25  and  29  are removed to expose the underlying silicon lines  36  and  39  for subsequent removal. The removal of the oxidation masks can be performed in a variety of ways, such as dry or wet etching such as H 3 PO 4  that is selective against etching the oxide lines  34 ,  35 ,  37 , and  38 , or with various plasmas. 
         [0110]      FIG. 12  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing the polysilicon lines from  FIG. 11 . 
         [0111]    Polysilicon can be etched using an etch process that is highly selective for polysilicon over silicon oxides or silicon nitrides. Despite the highly selective etch, during the etch there is oxide loss from exposed oxide such as the outer oxide  22 . Such loss varies with position. For example, there is a different amount of oxide loss from the top surfaces of oxide lines  34 ,  35 ,  37 , and  38 ; and the side surface of outer oxide  22 . 
         [0112]    The high aspect ratio word line trenches separate the oxide lines  34 ,  35 ,  37 , and  38  from prior steps, and indicate where the word lines will be formed in a future step. The polysilicon residue  43  in void  47  in  FIG. 12  creates a risk of forming a conductive bridge electrically connecting neighboring word lines. The polysilicon residue  43  in void  47  are addressed in connection with  FIG. 13 . 
         [0113]    In addition to filling in voids as discussed in connection with  FIG. 13 , removal of the polysilicon allows removal of the outer oxide  22 . 
         [0114]      FIG. 13  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of oxidizing polysilicon residue from  FIG. 12 . 
         [0115]    Example oxidation processes are furnace oxidation such as with horizontal furnaces and vertical furnaces, and rapid thermal processors (RTP). 
         [0116]    Polysilicon residue  43  is oxidized into oxide  49  During oxidation of the polysilicon which fills in the void, the volume expands. After oxidation, the void  47  will be filled up with oxide, and a portion of the new oxide may overfill the void  47  (not shown). 
         [0117]      FIG. 14  is illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing the outer oxide from the oxide-nitride-oxide data storage layers of  FIG. 13 . 
         [0118]    The sacrificial outer oxide may have been damaged during the lithographic patterning of polysilicon line  30  in connection with  FIG. 6 , as the etching process reached the sacrificial outer oxide. 
         [0119]    The sacrificial outer oxide is removed from the data storage layers covering the plurality of ridge-shaped stacks. For example, HF base solution, is a selective etch with a high SiO 2  etch rate, and very low SiN etch rate. Due to the selective etch, not much SiN will be lost despite over etch of the SiO 2 . Example over etch ranges are 20-50%. For example, if the oxide thickness to be removed is 100 angstroms, and HF etch rate is 100 angstroms/10 min, then dipping in HF for 12 min is a 20% over etch. The extra etch time is 12 minutes−10 minutes=2 minutes, and (2 minutes/10 minutes)=20% over etch. Another selective etch is NH 4 OH. After removing the outer oxide layer  22 , the nitride layer  21  is exposed. 
         [0120]    After the oxide removal, an example thickness range of the oxide lines  34 ,  35 ,  37 , and  38  is 100-350 angstroms, with an example thickness of 300 angstroms. 
         [0121]      FIG. 15  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming the outer oxide on the nitride-oxide data storage layers of  FIG. 14 . 
         [0122]    In the oxidation, SiN+O 2  (high temp) forms SiO 2 (solid)+N 2  (gas). Example high temperature of 1200 deg C. to oxidize SiN is higher than the example high temperature of 900 deg C. to oxidize Si. A strong oxidant such as OH −  and O 2  oxygen atoms help oxidize SiN. 
         [0123]    In this self-aligned process, SiO 2  is formed from the SiN in the word line trenches between neighboring oxide lines  34 ,  35 ,  37 , and  38 . Part of nitride layer  21  is consumed to leave a thinner nitride layer  23 , and the regrown oxide layer  24 . 
         [0124]      FIG. 16  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of performing a polysilicon fill over the exposed surfaces, and filling in word line trenches from  FIG. 15  separating the oxide lines  34 ,  35 ,  37 , and  38 . 
         [0125]    The polysilicon word lines  58 ,  59 ,  61 , and  63  are formed conformally over the data storage layers covering the plurality of stacks of semiconductor strips. The polysilicon word lines  58 ,  59 ,  61 , and  63  fill the trenches defined by the plurality of stacks of semiconductor strips. Additional polysilicon word lines can be formed along the y-axis out of the page and into the page. Neighboring polysilicon word lines are separated by an oxide line. This is a damascene process, as the polysilicon word lines fill previously defined trenches, such as the trenches formed between neighboring oxide lines oxide lines  34 ,  35 ,  37 , and  38 . 
         [0126]      FIG. 17  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of removing excess polysilicon from  FIG. 16 . 
         [0127]    The removal of excess polysilicon can be performed in a variety of ways, such as dry or wet etching that is selective against etching the underlying oxidation masks, and by chemical mechanical polishing (CMP). Endpoint detection to stop the removal of excess polysilicon can be performed by various processes such as spectrometry laser interferometry, and optical sensing. 
         [0128]    The polysilicon word lines  58 ,  59 ,  61 , and  63  define a multi-layer array of interface regions at cross-points between side surfaces of the semiconductor strips  11 ,  13 ,  15  on the stacks and the polysilicon word lines  58 ,  59 ,  61 , and  63 . The damascene process is an alternative to subtractive interconnect structures, such as in  FIG. 1  where the deposited polysilicon word lines are patterned by etching. 
         [0129]    The polysilicon word lines  58 ,  59 ,  61 , and  63  can be a semiconductor material with the same or a different conductivity type as the semiconductor strips  11 ,  13 ,  15  (e.g. heavily doped p+-type polysilicon). 
         [0130]    Memory cells comprising field effect transistors having charge storage structures are formed in the 3D array of cross-points. Using dimensions for the widths of the semiconductor strips and word lines on the order of 25 nanometers, with gaps between the ridge-shaped stacks on the order of 25 nanometers, a device having a few tens of layers (e.g. 32 layers) can approach terabit capacity (10 12 ) in a single chip. 
         [0131]    In one embodiment charge trapping dielectric memory cells are formed at the intersection of polysilicon word lines  58 ,  59 ,  61 , and  63  and semiconductor strips  11 ,  13 ,  15 . Active charge trapping dielectric regions are formed on the both sides of the strip semiconductor strips  11 ,  13 ,  15 , between the polysilicon word lines  58 ,  59 ,  61 , and  63  and the semiconductor strips  11 ,  13 ,  15 . In the embodiment described here, each memory cell is a double gate field effect transistor having active charge storage regions, one on each side of the semiconductor strip. Electron current flows along the semiconductor strips, to sense amplifiers where it can be measured to indicate the state of a selected memory cell. 
         [0132]    The source/drain regions on either side of the polysilicon word lines  58 ,  59 ,  61 , and  63  along the semiconductor strips can be “junction-free”, without source and drain doping having a conductivity type opposite that of the channel regions along the semiconductor strips beneath the word lines. In the junction free embodiment, the charge trapping dielectric field effect transistors can have a p-type channel structure. Also, source and drain doping could be implemented in some embodiments, in a self-aligned implant after word line definition. 
         [0133]    In alternative embodiments, the semiconductor strips  11 ,  13 ,  15  can be implemented using a lightly doped n-type semiconductor body in junction free arrangements, resulting in a buried-channel field effect transistor which can operate in depletion mode, with naturally shifted lower threshold distributions for the charge trapping dielectric cells. 
         [0134]      FIG. 18  illustrates part of a process for manufacturing a memory device, and is a perspective illustration of forming silicided polysilicon word lines from the polysilicon word lines of  FIG. 17 . 
         [0135]    A layer of silicide (e.g. tungsten silicide, cobalt silicide, titanium silicide)  51  can be formed over the top surface of the word line  58 . Deposition is followed by RTP (Rapid Thermal Processing). 
         [0136]      FIG. 19  is an alternative of  FIG. 18 , illustrating part of a process for manufacturing a memory device, and is a perspective illustration of removing polysilicon above the oxide-nitride-oxide data storage layers on the plurality of ridge-shaped stacks of  FIG. 17 . After removal, vertical polysilicon members  52  remain on the sides of the plurality of ridges. 
         [0137]      FIG. 20  is an alternative of  FIG. 18 , illustrating part of a process for manufacturing a memory device, and is a perspective illustration of forming word lines with metal surfaces on the exposed outer oxide of the oxide-nitride-oxide data storage layers and on the exposed polysilicon of  FIG. 19 . 
         [0138]    TiN is deposited conformally, followed by W deposition and removal of excess material, such as by etching or CMP. 
         [0139]      FIG. 21  is an alternative of  FIG. 18 , illustrating part of a process for manufacturing a memory device, and is a perspective illustration of forming word lines with metal surfaces on exposed polysilicon. 
         [0140]    Unlike  FIG. 18 , following the polysilicon etch, a horizontal polysilicon member is left that connects the vertical polysilicon members on the sides of the plurality of ridges. 
         [0141]    The metal is deposited on the exposed horizontal polysilicon member. More particularly, TiN is deposited conformally, followed by W deposition and removal of excess material, such as by etching or CMP. 
         [0142]      FIG. 22  is a perspective view comparing the nitride layer of the oxide-nitride-oxide data storage layers, between the areas under the word lines and the areas under the oxide lines. 
         [0143]    The nitride layer  23  for the kernel area, or the region under a word line  58 , is thinner than the nitride layer  21  for the region under an oxide line  34 . The thinner nitride layer  23  results from oxidation of part of the nitride layer  21  as discussed in connection with  FIG. 12 . The thicker nitride layer  21  under the oxide line  34  helps reduce unwanted operations due to fringing electric fields. 
         [0144]      FIGS. 23-25  are perspective views of various “M”-shaped gates.  FIG. 23  shows the “M”-shaped gate from  FIG. 18 .  FIG. 24  shows the “M”-shaped gate from  FIG. 20 .  FIG. 25  shows the “M”-shaped gate from  FIG. 21 . In  FIGS. 24 and 25 , the proximal surfaces of the respective metal lines  64  and  62  are covered with the conformal liner such as TiN. In contrast, in  FIGS. 20 and 21  the proximal surfaces of the respective metal lines  64  and  62  were shown uncovered by the conformal liner such as TiN for illustration purposes. 
         [0145]      FIG. 26  is a picture of a cross-section of part of a 3D memory with a stack of alternating channels and oxides, illustrating areas of potential “shadow effect”. 
         [0146]    The TEM cross-section shows a portion of an 8-layer vertical gate, thin-film-transistor, BE-SONOS charge trapping dielectric NAND device which has been fabricated and tested. The device was made with a 75 nm half pitch. The channels were n-type polysilicon about 18 nm thick. No additional junction implant was used, resulting in a junction free structure. The insulating material between the strips to isolate the channels in the Z-direction was silicon dioxide was about 40 nm thick. The gates were provided by a p+-polysilicon line. The test device implemented  32  word line, junction-free NAND strings. The width of the lower strip is greater than the width of the upper strip because the trench etch used to form the structure resulted in a tapered side wall with progressively wider strips as the trench becomes deeper, and with the insulating material between the strips being etched more than the polysilicon. The narrower width of the insulating material, and the broader width of the adjacent channels, results in a surrounding gate effect that benefits device control and performance. However, another result is the potential shadow effect areas which induce unwanted polysilicon residues in the shadow effect areas. Fortunately, in various disclosed embodiments, such polysilicon residues are oxidized so that the polysilicon residues do not remain and create undesirable electrical connection between neighboring word lines. 
         [0147]    After an interlayer dielectric is formed over the top of the array, vias are opened in which contact plugs, using tungsten fill for example, are formed reaching to the top surfaces of the gate structures. Overlying metal lines are patterned to connect as SSL lines, to column decoder circuits. A three-plane decoding network is established, accessing a selected cell using one word line, one bit line and one SSL line. See, U.S. Pat. No. 6,906,940, entitled Plane Decoding Method and Device for Three Dimensional Memories. 
         [0148]    As a result, a 3D array of SONOS-type memory cells configured in a NAND flash array can be formed. The source, drain and channel are formed in the silicon (S) semiconductor strips, the layer of the memory material includes a tunneling dielectric layer which can be formed of silicon oxide (O), a charge storage layer which can be formed of silicon nitride (N), a blocking dielectric layer which can be formed of silicon oxide (O), and the gate comprises polysilicon (S) of the word lines. 
         [0149]    In other embodiments, the NAND memory cells are replaced by other types of memory cells. For example, manowire MOSFET type cells can also be configured in this manner, by providing nanowire or nanotube structures in channel regions on word lines  111 - 114 , like those described in Paul, et al., “Impact of a Process Variation on Nanowire and Nanotube Device Performance”, IEEE Transactions on Electron Devices, Vol. 54, No. 9, September 2007, which article is incorporated by reference as if fully set forth herein. 
         [0150]      FIG. 27  is an example 3D memory structure with the damascene gate. A 3D NAND-flash memory array structure has metal layers of string select lines (a lengthwise orientation layer parallel to the semiconductor material strips, a widthwise orientation layer parallel to the word lines) and bit lines with a lengthwise orientation parallel to the semiconductor material strips. Insulating material is removed from the drawing to expose additional structure. For example, insulating layers are removed between the semiconductor strips, in the ridge-shaped stacks, and are removed between the ridge-shaped stacks of semiconductor strips. 
         [0151]    The multilayer array is formed on an insulating layer, and includes a plurality of word lines  425 - 1 , . . . ,  425 - n - 1 ,  425 - n  conformal with the plurality of ridge-shaped stacks, and which act as word lines WLn, WLn- 1 , . . . WL 1 . The damascene word lines are discussed herein. An example word line pitch is 75 nm, divided into a 40 nm word line thickness and a 35 nm space between adjacent word lines, which is a ratio of the word line thickness to the space between adjacent word lines of about 1.14. An example word line pitch range extends from 75 nm down to about 30 nm. The ratio of the word line thickness to the space between adjacent word lines has a range of about 0.8-1.2. The plurality of ridge-shaped stacks includes semiconductor strips  412 ,  413 ,  414 ,  415 . Semiconductor strips in the same plane are electrically coupled together by stairstep structures. 
         [0152]    The shown word line numbering, ascending from 1 to N going from the back to the front of the overall structure, applies to even memory pages. For odd memory pages, the word line numbering descends from N to 1 going from the back to the front of the overall structure. 
         [0153]    Stairstep structures  412 A,  413 A,  414 A,  415 A terminate semiconductor strips, such as semiconductor strips  412 ,  413 ,  414 ,  415 . As illustrated, these stairstep structures  412 A,  413 A,  414 A,  415 A are electrically connected to different bit lines for connection to decoding circuitry to select planes within the array. These stairstep structures  412 A,  413 A,  414 A,  415 A can be patterned at the same time that the plurality of ridge-shaped stacks are defined. 
         [0154]    Stairstep structures  402 B,  403 B,  404 B,  405 B terminate semiconductor strips, such as semiconductor strips  402 ,  403 ,  404 ,  405 . As illustrated, these stairstep structures  402 B,  403 B,  404 B,  405 B are electrically connected to different bit lines for connection to decoding circuitry to select planes within the array. These stairstep structures  402 B,  403 B,  404 B,  405 B can be patterned at the same time that the plurality of ridge-shaped stacks are defined. 
         [0155]    Any given stack of semiconductor strips is coupled to either the stairstep structures  412 A,  413 A,  414 A,  415 A, or the stairstep structures  402 B,  403 B,  404 B,  405 B, but not both. A stack of semiconductor strips has one of the two opposite orientations of bit line end-to-source line end orientation, or source line end-to-bit line end orientation. For example, the stack of semiconductor strips  412 ,  413 ,  414 ,  415  has bit line end-to-source line end orientation; and the stack of semiconductor strips  402 ,  403 ,  404 ,  405  has source line end-to-bit line end orientation. 
         [0156]    The stack of semiconductor strips  412 ,  413 ,  414 ,  415  is terminated at one end by the stairstep structures  412 A,  413 A,  414 A,  415 A, passes through SSL gate structure  419 , gate select line GSL  426 , word lines  425 - 1  WL through  425 -N WL, gate select line GSL  427 , and terminated at the other end by source line  428 . The stack of semiconductor strips  412 ,  413 ,  414 ,  415  does not reach the stairstep structures  402 B,  403 B,  404 B,  405 B. 
         [0157]    The stack of semiconductor strips  402 ,  403 ,  404 ,  405  is terminated at one end by the stairstep structures  402 B,  403 B,  404 B,  405 B, passes through SSL gate structure  409 , gate select line GSL  427 , word lines  425 -N WL through  425 - 1  WL, gate select line GSL  426 , and terminated at the other end by a source line (obscured by other parts of figure). The stack of semiconductor strips  402 ,  403 ,  404 ,  405  does not reach the stairstep structures  412 A,  413 A,  414 A,  415 A. 
         [0158]    A layer of memory material separates the word lines  425 - 1  through  425 - n , from the semiconductor strips  412 - 415  and  402 - 405  as described in detail in prior figures. Ground select lines GSL  426  and GSL  427  are conformal with the plurality of ridge-shaped stacks, similar to the word lines. 
         [0159]    Every stack of semiconductor strips is terminated at one end by stairstep structures, and at the other end by a source line. For example, the stack of semiconductor strips  412 ,  413 ,  414 ,  415  is terminated at one end by stairstep structures  412 A,  413 A,  414 A,  415 A, and terminated on the other end by source line  428 . At the near end of the figure, every other stack of semiconductor strips is terminated by the stairstep structures  402 B,  403 B,  404 B,  405 B; and every other stack of semiconductor strips is terminated by a separate source line. At the far end of the figure, every other stack of semiconductor strips is terminated by the stairstep structures  412 A,  413 A,  414 A,  415 A, and every other stack of semiconductor strips is terminated by a separate source line. 
         [0160]    Bit lines and string select lines are formed at the metals layers ML 1 , ML 2 , and ML 3 . 
         [0161]    Transistors are formed between the stairstep structures  412 A,  413 A,  414 A and the word line  425 - 1 . In the transistors, the semiconductor strip (e.g.  413 ) acts as the channel region of the device. SSL gate structures (e.g.  419 ,  409 ) are patterned during the same step that the word lines  425 - 1  through  425 - n  are defined. A layer of silicide can be formed along the top surface of the word lines  425 - 1  through  425 - n , the ground select lines  426  and  427 , and over the gate structures  409  and  419 . The layer of memory material  415  can act as the gate dielectric for the transistors. These transistors act as string select gates coupled to decoding circuitry for selecting particular ridge-shaped stacks in the array. 
         [0162]    The first metal layer ML 1  includes string select lines with a lengthwise orientation parallel to the semiconductor material strips. These ML 1  string select lines are connected by short vias to different SSL gate structures. 
         [0163]    The second metal layer ML 2  includes string select lines with a widthwise orientation parallel to the word lines. These ML 2  string select lines are connected by short vias to different ML 1  string select lines. 
         [0164]    In combination, these ML 1  string select lines and ML 2  string select lines allow a string select line signal to select a particular stack of semiconductor strips. 
         [0165]    The first metal layer ML 1  also includes two source lines with a widthwise orientation parallel to the word lines. 
         [0166]    Finally, the third metal layer ML 3  includes bit lines with a lengthwise orientation parallel to the semiconductor material strips. Different bit lines are electrically connected to different steps of the stairstep structures  412 A,  413 A,  414 A,  415 A and  402 B,  403 B,  404 B,  405 B. These ML 3  bit lines allow a bit line signal to select a particular horizontal plane of semiconductor strips. 
         [0167]    Because a particular word line allows a word line to select a particular row plane of memory cells, the threefold combination of word line signals, bit line signals, and string select line signals is sufficient to select a particular memory cell from the 3D array of memory cells. 
         [0168]      FIG. 28  is a circuit diagram corresponding to the example 3D memory structure of  FIG. 27  with the damascene gate. 
         [0169]    The schematic diagram shows 2 planes of memory cells having 9 charge trapping dielectric cells arranged in a NAND configuration, which is representative of a cube which can include many planes and many word lines. The 2 planes of memory cells are defined at the cross-points of word lines  160 ,  161  acting as word lines WLn- 1 , WLn, with a first stack of semiconductor strips, a second stack of semiconductor strips and a third stack of semiconductor strips. 
         [0170]    The first plane of memory cells includes memory cells  70 ,  71  in a NAND string on a semiconductor strip, memory cells  73 ,  74  in a NAND string on a semiconductor strip, and memory cells  76 ,  77  in a NAND string on a semiconductor strip. Each NAND string is connected to a ground select transistor on either side (e.g., ground select devices  90 ,  72  on either side of NAND string  70 ,  71 ). 
         [0171]    The second plane of memory cells corresponds with a bottom plane in the cube in this example, and includes memory cells (e.g.  80 ,  82 ,  84 ) arranged in NAND strings in a similar manner those in the first plane. 
         [0172]    As shown in the figure, the word line  161  acting as word line WLn includes vertical extensions which correspond with the material in the trench  120  shown in  FIG. 5  between the stacks, in order to couple the word line  161  to the memory cells (cells  71 ,  74 ,  77  in the first plane) in the interface regions in the trenches between the semiconductor strips in all of the planes. 
         [0173]    Memory cell strings in adjacent stacks alternate between a bit line end-to-source line end orientation and a source line end-to-bit line end orientation. 
         [0174]    Bit lines BL N  and BL N-1    96  terminate the memory cell strings, adjacent to the string select devices. For example, in the top memory plane, bit line BL N  terminates the memory cell strings which have string select transistors  85  and  89 . By contrast, the bit line is not connected to trace  88 , because the strings of adjacent stacks alternate between a bit line end-to-source line end orientation and a source line end-to-bit line end orientation. So instead for this string, the corresponding bit line is connected to the other end of the string. In the bottom memory plane, bit line BL N-1  terminates the memory cell strings which have corresponding string select transistors. 
         [0175]    String select transistors  85 ,  89  are connected between respective NAND strings and string select lines SSL n−1  and SSL n  in this arrangement. Likewise, similar string select transistors on a bottom plane in the cube are connected between respective NAND strings and string select lines SSL n−1  and SSL n  in this arrangement. String select lines  106 ,  108 , are connected to different ridges, to the gates of string select transistors in each memory cell string, and provide in this example string select signal SSL n−1 , SSL n  and SSL n+1 . 
         [0176]    By contrast, a string select transistor is not connected to trace  88 , because the strings of adjacent stacks alternate between a bit line end-to-source line end orientation and a source line end-to-bit line end orientation. So instead for this string, the corresponding string select transistor is connected to the other end of the string. The NAND string with memory cells  73 ,  74  also has a string select device, not shown, on the other end of the string. The trace  88  is terminated by a source line  107 . 
         [0177]    Ground select transistors  90 - 95  are arranged at the first ends of the NAND strings. Ground select transistors  72 ,  75 ,  78  and corresponding second plane ground select transistors are arranged at the second ends of the NAND strings. Accordingly, ground select transistors are on both ends of the memory strings. Depending on the particular end of the memory string, the ground select transistor couples the memory string to a source line, or to a string select device and bit line. 
         [0178]    The ground select signal GSL (odd)  159  and ground select signal GSL (even)  162  are on opposite sides of the word lines  160 ,  161  acting as word lines WLn- 1 , WLn. The ground select signal GSL (odd)  159  in this example is coupled to the gates of the ground select transistors  90 - 95 , and can be implemented in the same manner as the word lines  160 ,  161 . Similarly, the ground select signal GSL (even)  162  in this example is coupled to the gates of ground select transistors  72 ,  75 ,  78  and corresponding second plane ground select transistors, and can be implemented in the same manner as the word lines  160 ,  161 . The string select transistors and ground select transistors can use the same dielectric stack as a gate oxide as the memory cells in some embodiments. In other embodiments, a typical gate oxide is used instead. Also, the channel lengths and widths can be adjusted as suits the designer to provide the switching function for the transistors. 
         [0179]      FIG. 29  is a schematic diagram of an integrated circuit including a 3D NAND-flash memory array with the damascene gate and row, column and plane decoding circuitry. 
         [0180]    The integrated circuit line  975  includes a 3D NAND flash memory array  960 , implemented as described herein, with a damascene gate or word line. A row decoder  961  is coupled to a plurality of word lines  962 , and arranged along rows in the memory array  960 . A column decoder  963  is coupled to a plurality of SSL lines  964  arranged along columns corresponding to stacks in the memory array  960  for reading and programming data from the memory cells in the array  960 . A plane decoder  958  is coupled to a plurality of planes in the memory array  960  via bit lines  959 . Addresses are supplied on bus  965  to column decoder  963 , row decoder  961  and plane decoder  958 . Sense amplifiers and data-in structures in block  966  are coupled to the column decoder  963  in this example via data bus  967 . Data is supplied via the data-in line  971  from input/output ports on the integrated circuit  975  or from other data sources internal or external to the integrated circuit  975 , to the data-in structures in block  966 . In the illustrated embodiment, other circuitry  974  is included on the integrated circuit, such as a general purpose processor or special purpose application circuitry, or a combination of modules providing system-on-a-chip functionality supported by the NAND flash memory cell array. Data is supplied via the data-out line  972  from the sense amplifiers in block  966  to input/output ports on the integrated circuit  975 , or to other data destinations internal or external to the integrated circuit  975 . 
         [0181]    A controller implemented in this example using bias arrangement state machine  969  controls the application of bias arrangement supply voltage generated or provided through the voltage supply or supplies in block  968 , such as read, erase, program, erase verify and program verify voltages. The controller can be implemented using special-purpose logic circuitry as known in the art. In alternative embodiments, the controller comprises a general-purpose processor, which may be implemented on the same integrated circuit, which executes a computer program to control the operations of the device. In yet other embodiments, a combination of special-purpose logic circuitry and a general-purpose processor may be utilized for implementation of the controller. 
         [0182]      FIGS. 30-37  are top views of parts of a process for manufacturing a memory device. 
         [0183]      FIG. 30  shows a polysilicon layer, indicated by the “plus” pattern, covering semiconductor strips such as semiconductor strip  112  connected to BL PAD structure  109 , which corresponds to stairstep structures  402 B,  403 B,  404 B,  405 B in  FIG. 27 .  FIG. 30  shows a top view which corresponds to the perspective view of  FIG. 6 , prior to etching the polysilicon. Throughout  FIGS. 30-37 , to acts as a visual aid, the SSL gate structure and the semiconductor strips are shown by the alternating “dot dash” line, regardless of whether there is a covering layer. In  FIG. 31 , the polysilicon is etched.  FIG. 31  shows a top view which corresponds to the perspective view of  FIG. 6 . The blank areas in  FIG. 31  which are areas from which polysilicon has been removed, correspond to the trenches  33  in  FIG. 6 . 
         [0184]      FIG. 32  shows a top view which corresponds to the perspective view of  FIG. 8 , after oxidation of exposed polysilicon surfaces. The oxide lines such as oxide lines  151  in  FIG. 8  correspond to the oxide lines  34 ,  35 ,  37 , and  38  in  FIG. 8 . Oxidation masks are not shown in  FIG. 32 . Multiple polysilicon lines are shorted together at this time; connecting polysilicon between the multiple polysilicon lines is eventually removed as shown in  FIG. 36 . 
         [0185]      FIG. 33  shows a top view which corresponds to the perspective view of  FIGS. 9-11 , after performing polysilicon fill-in of the empty trenches between oxide lines following  FIG. 32 . Oxidation masks and overlying polysilicon which cover the oxide lines are not shown in  FIG. 33 . 
         [0186]      FIG. 34  shows a top view which corresponds to the perspective view of  FIGS. 12-13 , after removing the polysilicon fill following  FIG. 33 . Oxide voids are filled in by oxidation of polysilicon which had filled or partly filled such oxide voids. 
         [0187]      FIG. 35  shows a top view which corresponds to the perspective view of  FIGS. 16-17 , after moving the outer oxide layer from the oxide-nitride-oxide data storage layers covering the semiconductor strips, and converting exposed nitride to an outer oxide layer of the data storage layers covering the semiconductor strips following  FIG. 34 . Polysilicon fill is performed on the empty trenches between oxide lines. Multiple polysilicon lines are shorted together at this time; connecting polysilicon is eventually removed as shown in  FIG. 36 . 
         [0188]      FIG. 36  shows a top view which corresponds to the perspective view of  FIG. 18  prior to the silicide, after removing excess polysilicon which shorts together multiple polysilicon lines. Also shown are varying widths of polysilicon structures. The word lines such as word lines WL  125 - 1 , WL  125 - 2  WL  125 - 3 , and WL  125 - 4  are roughly L-shaped. One segment of the L-shape has a narrower width covering the semiconductor strips, and another segment of the L-shape has a larger width to connect to interlevel contacts. This larger width is supported by an additional mask in a self aligned double isolation process. Also, certain structures such as the SSL  127  and GSL  128  have a larger width than the narrow segment of the word lines. 
         [0189]      FIG. 37  shows a top view which corresponds to the perspective view of  FIG. 18  after the silicide, shown as a diagonal line pattern. 
         [0190]    While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.