# Patent

## Title
HTO offset and BL trench process for memory device to improve device performance

## Abstract

Memory devices having an increased effective channel length and/or improved TPD characteristics, and methods of making the memory devices are provided. The memory devices contain two or more memory cells on a semiconductor substrate and bit line dielectrics between the memory cells. The bit line dielectrics can extend into the semiconductor. The memory cell contains a charge trapping dielectric stack, a poly gate, a pair of pocket implant regions, and a pair of bit lines. The bit line can be formed by an implant process at a higher energy level and/or a higher concentration of dopants without suffering device short channel roll off issues because spacers at bit line sidewalls constrain the implant in narrower implant regions.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Division of application Ser. No. 12/342,008 filed Dec. 22, 2008, the entire contents of which is hereby incorporated by reference.


TECHNICAL FIELD
Described are memory devices having an improved effective channel length (Leffective) and/or improved transport program disturb (TPD) characteristics, methods of forming memory devices, and methods of increasing an effective channel length in memory cells.
BACKGROUND
Modern computing devices utilize a variety of kinds of memory devices to store and access information. Memory devices include general classes of random access memories (RAM) and read only memories (ROM). These classes further contain static RAM (SRAM), dynamic RAM (DRAM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable PROM (EEPROM), as well as FLASH memory, and the like. Most memory devices employ an internal architecture in the form of an array memory of bit cells, containing plural rows and plural intersecting columns.
A memory cell is placed at each intersecting row and column in the array. Typically, a particular memory cell is accessed by activating its row and then reading or writing the state of its column. Memory sizes are defined by the row and column architecture. For example, a 1024 row by 1024 column memory array defines a memory device having one megabit of memory cells. The array rows are referred to as word lines and the array columns are referred to as bit lines.
The trend in semiconductor memory devices has been toward higher circuit density with higher numbers of bit cells per device, lower operating voltages, and higher access speeds. To achieve the high densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels). However, as scaling down device dimensions, hot electrons generated during programming a memory cell may reach adjacent memory cells and disturb programming the adjacent memory cells (e.g., TPD). Thus, the requirement of small features with close spacing between adjacent features requires sophisticated manufacturing techniques.
SUMMARY
The following presents a simplified summary of the innovation disclosed herein in order to provide a basic understanding of some aspects of the innovation. This summary is not an extensive overview of the innovation. It is intended to neither identify key or critical elements of the innovation nor delineate the scope of the innovation. Its sole purpose is to present some concepts of the innovation in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the subject innovation provides methods of making memory devices. Another aspect of the subject innovation provides methods of increasing an effective channel length in memory cells. The methods can involve providing features and first openings therebetween on a semiconductor substrate and forming spacers adjacent side surfaces of the features and in the first openings. The methods can further involve forming bit line trenches between the spacers under the first openings that extend into the semiconductor substrate, thereby forming bit line openings, and forming bit lines in the semiconductor substrate under bit line openings. The bit line can be formed by implantation of dopants using the spacers in the bit line opening as an implant screen. The spacer can be used to offset the implants.
The spacer can constrain the implant in a narrower implant region, thereby increasing an effective channel length of the memory cell. The spacers can also allow the bit line implant with higher energy and/or higher dose without suffering device short channel roll off issues. Higher energy bit line implant can form deeper bit line junction and can effectively block hot electron move to the adjacent memory cell and prevent TPD.
Yet another aspect of the subject innovation provides memory devices having an improved effective channel length and/or improved TPD characteristics. The memory devices contain two or more memory cells on a semiconductor substrate. The memory cell contains a charge trapping dielectric stack and a poly gate on the semiconductor substrate and two pocket implant regions adjacent and under the charge trapping dielectric stack in the semiconductor substrate. The memory devices further contain bit line dielectrics in bit line openings between the memory cells. The bit line dielectrics extend into the semiconductor substrate and bit lines are located under the bit line dielectrics. Extending the bit line dielectric into the semiconductor substrate, the memory device can improve the electrical isolation between memory cells with minimal short channel effects, thereby preventing and/or mitigating TPD.
To the accomplishment of the foregoing and related ends, the innovation, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the innovation. These embodiments are indicative, however, of but a few of the various ways in which the principles of the innovation may be employed. Other objects, advantages and novel features of the innovation will become apparent from the following detailed description of the innovation when considered in conjunction with the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of an exemplary memory device in accordance with an aspect of the subject innovation.
FIG. 2 is a schematic illustration of a portion of a memory core such as may include at least part of one of the cores depicted in FIG. 1 in a virtual ground type configuration in accordance with an aspect of the subject innovation.
FIG. 3 is a top view of at least a portion of a memory core, such as may include at least a part of one of the cores depicted in FIG. 1 in accordance with an aspect of the subject innovation.
FIG. 4 is a cross-sectional isometric illustration of exemplary memory cells, such as that taken along a line A-A of FIG. 3 in accordance with an aspect of the subject innovation.
FIGS. 5-11 illustrate an exemplary method for making a memory device in accordance with an aspect of the subject innovation.
FIG. 12 is a flow diagram of an exemplary methodology of forming a memory device in accordance with an aspect of the subject innovation.
FIG. 13 is a flow diagram of an exemplary methodology of increasing an effective channel length in memory cells in accordance with an aspect of the subject innovation.



DETAILED DESCRIPTION
The device and method described herein can improve the scalability of, for example, a single-bit, double-bit, or multi-bit charge trapping dielectric flash memory device, by allowing for a greater effective channel length (Leffctive). A greater effective channel length can provide improved charge separation in memory cells, a reduction in complementary bit disturb effects, as well as improved drain-source break down voltage (BVdss) characteristic. This greater effective channel length can be accomplished due to a decreased lateral dimension for a diffusion region of each bit line and/or due to a bit line dielectric that extends into a semiconductor substrate.
A memory cell, for example, a single bit memory cell or a dual-bit memory cell, is programmed by applying a relatively high voltage to a control gate and connecting a source to ground and a drain to a predetermined potential above the source. During the programming of memory cells, hot electrons in a core cell channel region tunnel through a gate oxide into the floating gate and become trapped in the floating gate since the floating gate is surrounded by an interpoly dielectric and the tunnel oxide. Because of the trapped electrons, the threshold voltage of the cell increases. This change in the threshold voltage (and thereby the channel conductance) of the cell created by the trapped electrons is what causes the cell to be programmed.
The hot electrons generated during programming may be so hot that they go under bit line junction. The hot electrons may reach floating gates or storage elements of adjacent memory cells. Moreover, hot holes generated during programming may have a secondary impact ionization below a bit line region and the secondary electrons diffuse to a gate of adjacent memory cell where they are accelerated by a drain depletion region and injected into a storage element of the adjacent memory cell. As a result, due to the hot electrons and hot holes generated during programming, memory cells may suffer from a TPD phenomenon.
With memory devices shrinking in size, shorter channel lengths of the smaller devices increasingly exhibit short channel behavior that affects program operation. As the memory cells shrink, BVdss generally reduces, resulting in increased column leakage current originated from unselected wordline cells when the same programming voltage is applied at the selected bit line. The increased leakage current may load down the bit line driver and may increase power consumption. In a worst case, the intrinsic programming voltage may be pulled down and the program efficiency may be impaired.
The subject innovation provides memory devices, methods of forming memory devices, and methods of increasing an effective channel length in memory cells. The subject innovation can provide one or more of the following advantages: an increased effective channel length, improved TPD characteristics, and improved drain-source break down voltage (BVdss) characteristics.
An effective channel length of a memory cell is defined between corresponding pairs of buried bit lines. In the subject innovation, the bit lines are formed by implantation of dopants using spacers as an implant screen to offset the implants. The spacers constrain the implant in a narrower implant region, thereby increasing an effective channel length of the memory cell. The spacers can also allow the bit line implant with higher energy and higher dose without suffering device short channel roll off issues. Higher energy bit line implant can form a deeper bit line junction and can effectively block hot electron move to the adjacent memory cell and prevent TPD. The deeper bit line can also prevent and/or mitigate bitline-to-bitline current leakage and increase BVdss.
In the memory device, the memory cells are separated from each other by bit line openings on a semiconductor substrate. The bit line openings extend into the semiconductor substrate and contain a bit line dielectric. Containing the bit line dielectric that extends into the semiconductor substrate, the memory device can improve the electrical isolation between memory cells, thereby preventing and/or mitigating TPD and thereby preventing and/or mitigating bitline-to-bitline current leakage and increasing BVdss.
The subject innovation described herein can be applied to any suitable type of memory cell. For example, the subject innovation can be applied to single-level memory cells, multi-level memory cells, single-bit memory cells, dual-bit memory cells, quad-bit memory cells, multi-bit memory cells, and the like. The dual-bit memory is a relatively modern memory technology and allows multiple bits to be stored in a single memory cell. The dual-bit memory cell is essentially split into two identical (mirrored) parts, each of which is formulated for storing one of two independent bits. Each dual-bit memory cell, like a traditional cell, has a gate with a source and a drain. However, unlike a traditional stacked gate cell in which the source is always connected to an electrical source and the drain is always connected to an electrical drain, respective dual-bit memory cells can have the connections of the source and drain reversed during operation to permit storage of two bits.
A dual-bit memory cell can have a semiconductor substrate with implanted conductive bit lines. The dual-bit memory cell contains a charge trapping dielectric stack that can contain one or more layers and can be formed over the semiconductor substrate. For example, the charge trapping dielectric stack can contain three separate layers: a first insulating layer, a charge trapping dielectric layer, and a second insulating layer. Word lines are formed over the charge trapping dielectric stack substantially perpendicular to the bit lines. Programming circuitry controls two bits per cell by applying a signal to the word line, which acts as a control gate, and changing bit line connections such that one bit is stored by source and drain being connected in one arrangement and a complementary bit is stored by the source and drain being interchanged in another arrangement.
The innovation is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the subject innovation. It may be evident, however, that the innovation can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the innovation.
Although the subject innovation can be applied to any type of memory cell, the innovation is hereinafter illustrated and described in the context of an exemplary semiconductor device having one or more memory arrays containing dual-bit memory cells arranged in a virtual ground type array architecture. FIG. 1 illustrates a top view of an exemplary dual-bit flash memory device 100. The memory device 100 generally includes a semiconductor substrate 102 in which one or more high-density core regions 104 and one or more lower-density peripheral portions are formed. The high-density core regions 104 typically include one or more M by N arrays of individually addressable, substantially identical dual-bit memory cells. The lower-density peripheral portions on the other hand typically include input/output (I/O) circuitry 106 and programming circuitry for selectively addressing the individual memory cells. The programming circuitry is represented in part by and includes one or more x-decoders 108 and one or more y-decoders 110 that cooperate with the I/O circuitry 106 for selectively connecting a source, gate, and/or drain of selected addressed memory cells to predetermined voltages or impedances to effect designated operations on the respective memory cells (e.g., programming, reading, and erasing, and deriving necessary voltages to effect such operations). The memory device 100 contains memory cells that are described in detail below, for example, in FIGS. 4-11.
FIG. 2 is a schematic illustration of a portion 200 of an exemplary memory core such as may include at least a part of one of the M by N array cores 104 depicted in FIG. 1. The circuit schematic shows a line of memory cells, which includes memory cells 201 through 204 in a virtual ground type implementation, for example. The respective memory cells 201 through 204 are connected to a word line 206, which serves as a control gate, and pairs of the memory cells share a common bit line. For instance, in the example shown, the memory cell 201 associates bit lines 208 and 209; the memory cell 202 associates bit lines 209 and 210; the memory cell 203 associates bit lines 210 and 211; and the memory cell 204 associates bit lines 211 and 212. As such, cells 201 and 202 share bit line 209, cells 202 and 203 share bit line 210 and cells 203 and 204 share bit line 211, respectively. The memory cells 201 through 204 are described in detail below, for example, in FIGS. 4-11.
Depending upon a signal on the word line and the connection of the bit lines in a memory cell to an electrical source or drain, the memory cells 201 through 204 are capable of writing, reading, and erasing bits at locations 215 through 222. For example, control of the bit at location 215 is achieved through connection of the drain to the bit line 208 and the source to the bit line 209. Similarly, control of the bit at location 216 is achieved through connection of the drain to the bit line 209 and the source to the bit line 208. It will be appreciated that although adjacent memory cells share common bit lines, the adjacent memory cells do not interfere with each other because the memory cells are typically programmed one at a time and in such instances only one memory cell is active at a time while programming.
FIG. 3 illustrates a top view of at least a portion 300 of a memory core, such as may include at least part of one of the M by N array cores 104 depicted in FIG. 1. The memory 300 is formed upon a semiconductor substrate 302 and has a plurality of implanted bit lines 304 extending substantially parallel to one another, and further includes a plurality of formed word lines 306 extending substantially in parallel to one another and at substantially right angles to the plurality of implanted bit lines 304. It will be appreciated that the word lines 306 and bit lines 304 have contacts and interconnections (e.g., bit line junctions, not shown) to programming circuitry such as may be represented, at least in part, by x-decoders and y-decoders. The memory core 300 contains memory cells that are described in detail below, for example, in FIGS. 4-11.
FIG. 4 is a cross-sectional isometric illustration of a portion of a memory device 400 containing a dual-bit memory cell 402 as indicated by a dashed line, such as that taken along a line A-A of FIG. 3. The dual-bit memory cell 402 is formed on a semiconductor substrate 404. The memory cell 402 contains a charge trapping dielectric stack 406; a poly gate 408 on the charge trapping dielectric stack 406; two pocket implant regions 410 adjacent and under the charge trapping dielectric stack 406 in the semiconductor substrate; bit lines 412 in the semiconductor substrate 404 adjacent the charge trapping dielectric stack 406 and under bit line openings 414; and a word line 416. The memory cell 402 is separated from adjacent memory cells by bit line dielectrics 418 in bit line openings 414. The bit line dielectric extends into the semiconductor substrate 404 (e.g., bit line trench 420) and can contain any suitable dielectric material such as oxides (e.g., silicon oxide, high temperature oxide (HTO), high density plasma (HDP) oxide).
The charge trapping dielectric stack 406 can contain two charge storage nodes that can store at least a bit of information, respectively. For example, the memory cell 402 is capable of storing two spatially separated binary data bits, including a left bit 422 represented by the dashed circle A and a right bit 424 represented by the dashed circle B. When the memory cell 402 is such a dual-bit memory cell, the dual-bit memory cell is generally symmetrical, wherein a drain and a source are interchangeable, although asymmetric implementations are possible within the scope of the innovation. In one embodiment, the two charge storage nodes 422, 424 are physically separated by a central dielectric (not shown) in the charge trapping dielectric stack 406. In another embodiment, the two charge storage nodes are not physically separated in a single charge trapping dielectric stack (not shown).
The bit lines 412 typically contain implanted n-type materials, such as arsenic, and may include an oxide portion (not shown) in some examples. The two bit lines 412 are spaced apart and define a channel region 426 therebetween. The channel 426 can have an effective channel length and the bits 422, 424 can be stored apart from each other in the charge trapping dielectric stack 406.
In the exemplary memory cell 402, the left bit line 412 may serve as the source terminal and the right bit line 412 as the drain terminal with respect to the right bit 424. Likewise, the right bit line 412 may serve as the source terminal and the left bit line 412 as the drain terminal for the left bit 422. The subject innovation may be implemented in association with various single or multi-bit memory cell types including a SONOS cell. In addition, the innovation is applicable to such dual-bit memory devices wherein both bits are used for data or information storage, as well as those in which only one bit (e.g., bit A or bit B) of the dual-bit cell is so used.
Since the memory cell 402 has the two charge storage nodes 422, 424, the memory cell 402 can store two physically distinct bits. Each bit within the memory cell 402 serves as a binary unit of data (e.g., either 1 or 0) that can be mapped directly to a memory array. Reading or programming one side of charge storage nodes 422, 424 can occur independently of whatever data is stored on the opposite side of the charge storage nodes 422, 424.
The memory device 400 can contain word lines 416 over the charge trapping dielectric stack 406. Any number of such word lines can be formed over the charge trapping dielectric stack 406 and such word lines may correspond to the word lines 306 depicted in FIG. 3. The word lines can contain a polysilicon material, for example, where the polysilicon material may be deposited over the charge trapping dielectric stack 406 and then patterned and etched.
Referring to FIGS. 5 to 11, one of many possible exemplary embodiments of forming memory devices is specifically illustrated. FIG. 5 illustrates a cross sectional view of an intermediate state of a portion of an exemplary memory device 500. The memory device 500 can contain one or more features 502 on a semiconductor substrate 504. The features 502 can contain a charge trapping dielectric stack 506, a poly layer 508, a first mask layer 510, and two pocket implant regions 512 adjacent and under the charge trapping dielectric stack in the semiconductor substrate 504.
The memory device 500 has first openings 514 between the features 502. A memory cell is formed in the subsequent processes at a position indicated by a dashed line 516. For simplicity of illustration in FIG. 5, two features 502 and three first openings 514 are shown. However, the memory device 500 can have any suitable number of features and first openings. For example, the memory device 500 can have a M×N array of features 502 with M rows and N columns.
The semiconductor substrate 504 may contain any suitable semiconductor material on which electric devices such as memory cell transistors can be formed. Examples of semiconductor materials include silicon, gallium arsenide, indium phosphide, and the like.
The charge trapping dielectric stack 506 is formed over the semiconductor substrate 504. The configuration and/or constituent of the charge trapping dielectric stack 506 may vary and are not critical to the subject innovation. For example, the charge trapping dielectric stack can contain a central dielectric to contain physically separated two charge storage nodes (not shown). In another embodiment, the two charge storage nodes are not physically separated in a charge trapping dielectric stack.
Further, the charge trapping dielectric stack 506 can contain any suitable number of layers. For purposes of this discussion, the charge trapping dielectric stack 506 is described below in terms of three separate layers. However, it is to be appreciated that the charge trapping dielectric stack can include greater than or fewer than three dielectric or non-conductive layers without departing from the scope of the subject innovation.
The exemplary charge trapping dielectric stack 506 contains three separate layers: a first insulating layer 518, a charge trapping dielectric layer 520, and a second insulating layer 522. The charge trapping dielectric layer can contain any suitable dielectric material as long as the material can trap charges. Examples of charge trapping dielectric materials include nitrides (e.g., silicon nitride, silicon oxynitride, and silicon rich nitride), oxides, silicates, a high-k dielectric, for example, having a dielectric constant higher than that of silicon dioxide (SiO2), and the like. In one embodiment, the charge trapping dielectric material contains silicon nitride, silicon oxynitride, and/or silicon rich nitride. In another example, the charge trapping dielectric material contains oxides or silicates containing Al, Ta, Hf, La, Zr, Ti, Nb, Cr, V, Y, Ce, and/or Pr.
The first and second insulating layers 518 and 522 typically contain oxide dielectrics such as silicon dioxide (SiO2) and the charge trapping dielectric layer 520 typically contains nitride dielectrics such as silicon nitride (SixNy). The oxide-nitride-oxide configuration may be referred to as an ONO layer. Especially, when the nitride layer contains silicon rich nitride, the first oxide layer, nitride layer, and second oxide layer may be referred to as an ORO tri-layer.
Specific examples of other charge trapping dielectric stacks include an oxide/nitride bi-layer, a nitride/oxide bi-layer, an oxide/tantalum oxide bi-layer (SiO2/Ta2O5), an oxide/tantalum oxide/oxide tri-layer (SiO2/Ta2O5/SiO2), an oxide/strontium titanate bi-layer (SiO2/SrTiO3), an oxide/barium strontium titanate bi-layer (SiO2/BaSrTiO2), an oxide/strontium titanate/oxide tri-layer (SiO2/SrTiO3/SiO2), an oxide/strontium titanate/barium strontium titanate tri-layer (SiO2/SrTiO3/BaSrTiO2), and the like. In another embodiment, the charge trapping dielectric stack can contain five separate layers, for example, oxide-nitride-polysilicon-nitride-oxide. The oxide-nitride-polysilicon-nitride-oxide configuration may be referred to as an ORPRO layer when the nitride layer contains silicon rich nitride.
The charge trapping dielectric stack 506 can be formed by any suitable technique. For example, the charge trapping dielectric stack 506 can be formed by CVD, lithography, and etching techniques. The oxide-nitride-oxide tri-layer can be fabricated by forming a first silicon oxide layer, forming a silicon nitride layer on the first silicon oxide layer, and forming a second silicon oxide layer on the silicon nitride layer. The first oxide layer can be formed on surfaces of the semiconductor substrate 504 by, for example, oxidation techniques (e.g., thermal oxidation, plasma-assisted oxidation, and the like) and/or deposition techniques (e.g., CVD, spin-on techniques, and the like). A nitride layer is formed on the first oxide layer by, for example, deposition techniques (e.g., CVD, spin-on techniques, and the like). The second oxide layer can be formed in the same manner as the first oxide layer.
The heights of the first oxide layer 518 and second oxide layer 522 may vary and are not critical to the subject innovation. The first oxide layer and second oxide layer have independently any suitable height that depends on the desired implementations of the semiconductor structure 500 being fabricated. In one embodiment, the heights of the first oxide layer and second oxide layer are independently about 1 nm or more and about 10 nm or less. In another embodiment, the heights of the first oxide layer and second oxide layer are independently about 2 nm or more and about 8 nm or less. In yet another embodiment, the heights of the first oxide layer and second oxide layer are independently about 3 nm or more and about 7 nm or less. In still yet another embodiment, the heights of the first oxide layer and second oxide layer are about 5 nm.
The height of the nitride layer 520 may vary and is not critical to the subject innovation. The nitride layer has any suitable height that depends on the desired implementations of the semiconductor structure 500 being fabricated. In one embodiment, the height of the nitride layer is about 1 nm or more and about 20 nm or less. In another embodiment, the height of the nitride layer is about 2 nm or more and about 15 nm or less. In yet another embodiment, the height of the nitride layer is about 3 nm or more and about 10 nm or less. In still yet another embodiment, the height of the nitride layer is about 7 nm.
The poly layer 508 typically contains polysilicon. The height of the poly layer 508 may vary and is not critical to the subject innovation. The poly layer 508 has any suitable height that depends on the desired implementations of the semiconductor structure 500 being fabricated. In one embodiment, the height of the poly layer 508 is about 30 nm or more and about 300 nm or less. In another embodiment, the height of the poly layer 508 is about 50 nm or more and about 200 nm or less. In yet another embodiment, the height of the poly layer 508 is about 70 nm or more and about 150 nm or less. In still yet another embodiment, the height of the poly layer 508 is about 100 nm.
The first mask layer 510 may contain any suitable mask material. Examples of mask materials include oxides such as silicon oxide (e.g., silicon oxide (SixOy) and silicon dioxide (SiO2)); nitrides such as silicon nitride (e.g., silicon nitride (SixNy), silicon rich nitride, and oxygen rich silicon nitride); and the like.
The height of the first mask layer 510 may vary and is not critical to the subject innovation. The first mask layer 510 has any suitable height that depends on the desired implementations of the semiconductor structure 500 being fabricated. In one embodiment, the height of the first mask layer 510 is about 10 nm or more and about 200 nm or less. In another embodiment, the height of the first mask layer 510 is about 20 nm or more and about 150 nm or less. In yet another embodiment, the height of the first mask layer 510 is about 30 nm or more and about 100 nm or less. In still yet another embodiment, the height of the first mask layer 510 is about 60 nm.
Although not shown, an anti-reflective coating (ARC) can be formed on the first mask layer 510 for increasing resolution of a subsequent lithography process. Any suitable ARC material can be employed. For example, ARC contains oxides such as silicon oxide; nitrides such as silicon nitride; carbides such as silicon carbide (SixCy); amorphous carbon; and the like.
The features 502 can be formed on the semiconductor substrate by any suitable technique. For example, a charge trapping dielectric stack is formed on a semiconductor substrate, a poly layer is formed on the charge trapping dielectric stack, and a first mask layer is formed on the poly layer. The first mask layer is patterned by removing portions of the first mask layer via, for example, lithography and etching techniques, thereby forming a patterned first mask layer 510 and exposing portions of the poly layer and the charge trapping dielectric stack. The poly layer and the charge trapping dielectric stack are patterned by removing the exposed portions of the poly layer and the charge trapping dielectric stack that are not covered with the patterned first mask layer, thereby forming the patterned poly layer 508 and the patterned charge trapping dielectric stack 506.
The pocket implant regions 512 typically contain an implanted p-type material, such as boron. The pocket implant regions can facilitate controlling a threshold voltage of the memory cell. The pocket implant regions 512 can be formed in the semiconductor substrate 504 by any suitable technique. For example, the pocket implant region 512 is formed via implantation of one or more dopants at an angle with respect to the surface of the semiconductor substrate 504. The dopants pass through the first opening 514 and are implanted adjacent to and to a certain degree under portions of the charge trapping dielectric stack 508 depending upon the angle of implantation. The dopants can be implanted at an angle of about 5 degrees or more and about 40 degrees or less relative to the surface of the semiconductor substrate 504. Passing the dopants through the first opening between the features with a certain degree, the resultant pocket implant regions are located in the semiconductor substrate under the side surfaces of the features.
Any suitable implant compositions and concentrations for the pocket implants can be employed. For example, the dopants include one or more p-type dopants, such as boron. Such dopants can be implanted at an energy level of about 3 KeV or more and about 30 KeV or less and a dose of about 1E12 atoms/cm2 or more and 5E15 atoms/cm2 or less, for example.
While FIG. 5 illustrate the exemplary method of forming features on a semiconductor substrate, the details of the structure and manufacture of the features are not critical to the subject innovation. The details of the structure and manufacture of the features can be found in, for example, commonly-assigned U.S. Pat. No. 7,157,335, issued Jan. 2, 2007, which is hereby incorporated by reference.
FIG. 6 illustrates forming spacers 600 adjacent the side surface of the feature 502 (e.g., the first mask 510, the poly gate 508, and the charge trapping dielectric stack 506) and on the upper surface of the semiconductor substrate 504 in the first opening 514. The spacer 600 can serve as a mask when forming a bit line trench and serve as an implant screen when forming bit lines in subsequent processes.
The spacer 600 can contain any suitable material so that the spacer 600 can serve as an implant screen when forming bit lines in subsequent processes. The spacer 600 can also contain any suitable material so that the spacer 600 can serve as a mask for protecting the covered portion (e.g., an upper portion) of the semiconductor substrate 504 when forming a bit line trench in the semiconductor substrate 504 in subsequent processes. That is, the spacer material can be selected so that there is etch selectivity between the semiconductor substrate 504 and the spacer 600. The spacer material can have a lower etch rate than the semiconductor substrate 504 in a subsequent etching process. For example, an etching rate of an oxide material is markedly slower than an etching rate of a silicon material with respect to a silicon etchant. Accordingly, in one embodiment, the spacer 600 contains an oxide material. Examples of oxides include silicon oxide, tetraethylorthosilicate (TEOS) oxide, high aspect ratio plasma (HARP) oxide, HTO, HDP oxide, oxides (e.g., silicon oxides) formed by an atomic layer deposition (ALD) process, and the like. Other examples of spacer materials include nitrides (e.g., silicon nitride, silicon oxynitride, and silicon rich silicon nitride), silicates, diamond-like carbon, carbide, and the like.
The spacer 600 can be formed by any suitable technique, for example, forming a layer containing the spacer material over the semiconductor substrate 504 and then removing portions of the spacer material layer not near the side surface of the feature 502. The spacer material layer can be formed at least over the side surface of the feature 502.
In one embodiment, the spacer material layer is formed by a HTO process. For example, the HTO is formed by low pressure chemical vapor deposition (LPCVD) using nitrous oxide (N2O) and dichlorosilane (SiH2Cl2) gasses at a temperature of about 600 degrees Celsius or more and about 900 degrees Celsius or less and a pressure of about 100 mTorr or more and about 500 mTorr or less. In another embodiment, the spacer material layer is grown in plasma at low temperatures, e.g., plasma-grown oxide. For example, the spacer material layer contains plasma-grown oxide that is grown at a temperature of about 250 degrees Celsius or more and about 600 degrees Celsius or less. The spacer material layer can be formed by a HDP process. The spacer material layer can be optionally subjected to an anneal, for example, in a nitrogen ambient at about 1000 degrees Celsius for thirty minutes.
After forming the spacer material layer, portions of the spacer material layer can be removed, for example, etching. Any suitable etching can be used as long as the etching can leave a spacer 600 adjacent the side surface of the feature 502 and on the semiconductor substrate 504. Examples of etching include reactive ion etching, chemical plasma etching, or other suitable anisotropic etching utilizing a suitable chemistry.
Choice of a suitable process and reagents of etching depends on, for example, the spacer material, the width and thickness of the spacer material, the desired implementations of the memory device 500 being fabricated, and the like. In one embodiment, reactive ion etching is used with an appropriate etchant for the selected spacer material. For example, when the spacer 600 contains silicon oxides or silicon nitrides, examples of etchants include CF4 or CHF3. In another embodiment, when the spacer 600 contains diamond-like carbon, examples of etchants include oxygen-based plasma. In yet another embodiment, when the spacer 600 contains silicon carbide, examples of etchants include CF4, CH2F2, or C2F6.
The spacer 600 can have any suitable length (SPL) that depends on, for example, the desired width of the subsequently formed trench, the desired implementations of the memory device 500 being fabricated, and the like. Since the spacer 600 can serve as a mask for protecting the covered portion of the semiconductor substrate 504, the length of the spacer 600 can control and/or determine a length of a trench that is formed in the semiconductor substrate 504 in a subsequent process as described below. In one embodiment, the spacer 600 has a length of about 1 nm or more and about 50 nm or less. In another embodiment, the spacer 600 has a length of about 5 nm or more and about 40 nm or less. In yet another embodiment, the spacer 600 has a length of about 10 nm or more and about 30 nm or less.
After forming the two spacers 600 adjacent side surfaces of the features 502, a portion 602 of the upper surface of the semiconductor substrate 504 that is not covered by the spacers 600 is exposed. A trench is formed in the semiconductor substrate 504 between the spacers in a subsequent process.
FIG. 7 illustrates forming bit line openings 700 by removing portions of semiconductor substrate 504 under the first openings 514 between the spacers 600 (e.g., by forming bit line trenches 702 in the semiconductor substrate 504 under the first openings 514). The portion of semiconductor substrate 504 under the first opening 514 can be removed by using the spacer 600 and the first mask layer 510 as a mask.
The portion of the semiconductor substrate 504 can be removed by any suitable technique, for example, etching. A portion of the semiconductor substrate 504 can be removed by contacting the substrate with any suitable etchant that does not substantially damage and/or remove other components of the memory device 500. Wet etching and/or dry etching including isotropic etching and/or anisotropic etching can be employed.
When the semiconductor substrate 504 contains silicon, examples of wet etchants include tetraalkylammonium hydroxides (e.g., tetramethylammonium hydroxide (TMAH)) and alkali metal hydroxides (e.g., a potassium hydroxide (KOH) and cerium hydroxide (CeOH)). Examples of dry etching include reactive ion etching (RIE) using, for example, a mixture gas containing HBr (e.g., HBr and O2 mixture gas, HBr/NF3/He and O2 mixture gas, SF6, HBr and O2 mixture gas). The mixture may further include Cl2.
The bit line opening 700 can have any suitable length (BLOL) that depends on, for example, the desired bit line pitch, the desired implementations of the memory device 500 being fabricated, and the like. In one embodiment, the bit line opening 700 has a length (BLOL) of about 10 nm or more and about 190 nm or less. In another embodiment, the bit line opening 700 has a length (BLOL) of about 20 nm or more and about 100 nm or less. In yet another embodiment, the bit line opening 700 has a length (BLOL) of about 30 nm or more and about 70 nm or less. In still yet another embodiment, the bit line opening 700 has a length (BLOL) of about 50 nm.
The bit line trench 702 in the semiconductor substrate 504 can have any suitable cross-sectional shape. In one embodiment, the bit line trench 702 has relatively sloping side surfaces (e.g., sidewalls) that extend perpendicular to the topological surface of the semiconductor substrate 504. In another embodiment, the bit line trench 702 has relatively vertical side surfaces (not shown).
The bit line trench 702 can have any suitable length and depth. The length (BLTL) and depth (BLTD) of the bit line trench 702 may vary and is not critical to the subject innovation. The length and depth of the bit line trench 702 may depend on, for example, the desired implementations of the memory device 500 being fabricated. In one embodiment, the length of the bit line trench 702 is about 10 nm or more and about 100 nm or less. In another embodiment, the length of the bit line trench 702 is about 20 nm or more and about 90 nm or less. In yet another embodiment, the length of the bit line trench 702 is about 30 nm or more and about 70 nm or less. In one embodiment, the depth of the bit line trench 702 is about 10 nm or more and about 100 nm or less. In another embodiment, the depth of the bit line trench 702 is about 20 nm or more and about 90 nm or less. In yet another embodiment, the depth of the bit line trench 702 is about 30 nm or more and about 70 nm or less.
FIG. 8 illustrates forming bit lines 800 in the semiconductor substrate 504 under the bit line openings 700 adjacent the charge trapping dielectric stack 506. The buried bit lines can act as respective sources and drains for corresponding memory cells. As such, respective effective channel lengths are defined between corresponding pairs of buried bit lines 800.
Any suitable implant compositions and concentrations can be employed for the bit lines 800. For example, the bit lines 800 include one or more n-type dopants (e.g., arsenic, phosphorous, antimony). The bit lines 800 can be formed by any suitable technique. The bit line 800 can be formed by implantation of one or more dopants. The dopants pass through the bit line opening 700 between the spacers 600 and are implanted into the semiconductor substrate 504 under the bit line opening 700, leaving a channel region 802 of the semiconductor substrate 504 between the bit lines 800. The first mask layer 510 and the spacer 600 can serve as an implant screen. That is, the spacer is used to offset the implants and the spacer can constrain the implant in a narrower implant region. As a result, the memory cell can advantageously have a decreased bit line length and an increased effective channel length.
In one embodiment, the bit line length (BLL) is about 70% or more and about 140% or less of the length of the bit line opening (BLOL). In another embodiment, the bit line length (BLL) is about 70% or more and about 120% or less of the length of the bit line opening (BLOL). In yet another embodiment, the bit line length (BLL) is about 70% or more and about 110% or less of the length of the bit line opening (BLOL). In still yet another embodiment, the bit line length (BLL) is about 70% or more and about 100% or less of the length of the bit line opening (BLOL).
In one embodiment, the effective channel length is about 70% or more and about 100% or less of the length of the poly gate (PGL). In another embodiment, the effective channel length is about 80% or more and about 100% or less of the length of the poly gate (PGL). In yet another embodiment, the effective channel length is about 90% or more and about 100% or less of the length of the poly gate (PGL). In still yet another embodiment, the effective channel length is about 95% or more and about 100% or less of the length of the poly gate (PGL).
The spacers 600 can allow the bit line implant with higher energy and higher dose without suffering device short channel roll off issues. This is because the spacer at the bit line sidewall can constrain the implant in a narrower implant region. The spacer can tolerate higher energy bit line implant (which usually have high implant strangle) without leading to too wide bit line and device short channel effect. Higher energy bit line implant can form deeper bit line junction and can effectively block the hot electron move to the adjacent cell and prevent TPD.
The bit line 800 can be formed at a relatively higher energy level and/or a relatively higher dose of dopants. In one embodiment, the bit line 800 is formed at an energy level of about 5 KeV or more and about 100 KeV or less and a dose of about 2E13 atoms/cm2 or more and about 4E15 atoms/cm2 or less. In another embodiment, the bit line 800 is formed at an energy level of about 10 KeV or more and about 100 KeV or less and a dose of about 5E13 atoms/cm2 or more and about 4E15 atoms/cm2 or less. In yet another embodiment, the bit line 800 is formed at an energy level of about 20 KeV or more and about 100 KeV or less and a dose of about 1E14 atoms/cm2 or more and about 4E15 atoms/cm2 or less.
The resultant bit line 800 can have a higher concentration of dopants because of the implant with the higher energy and/or higher dose. In one embodiment, the bit line 800 has a concentration of dopants of about 5E17 atoms/cm3 or more and about 1E21 atoms/cm3 or less. In another embodiment, the bit line 800 has a concentration of dopants of about 1E18 atoms/cm3 or more and about 1E21 atoms/cm3 or less. In yet another embodiment, the bit line 800 has a concentration of dopants of about 5E18 atoms/cm3 or more and about 1E21 atoms/cm3 or less.
The bit line 800 can have an increased depth because of the implant with the higher energy and/or higher dose. In one embodiment, the bit line 800 has a depth (BLD) of about 20 nm or more and about 300 nm or less. In another embodiment, the bit line 800 has a depth of about 30 nm or more and about 250 nm or less. In yet another embodiment, the bit line 800 has a depth of about 40 nm or more and about 200 nm or less.
A portion of the resultant bit line 800 can overlap with a portion of the pocket implant region 512. For example, an upper portion of the bit line adjacent to the charge trapping dielectric stack 506 overlaps with a portion of the pocket implant region. In one embodiment, at least about 20% of the area of the pocket implant region overlap with the bit line. In another embodiment, at least about 30% of the area of the pocket implant region overlap with the bit line. In yet another embodiment, at least about 40% of the area of the pocket implant region overlap with the bit line.
FIG. 9 illustrates forming a bit line dielectric layer 900 over the semiconductor substrate 504. The bit line dielectric layer 900 can contain any suitable dielectric material such as oxides. Examples of oxides include silicon oxide, TEOS oxide, HARP oxide, HTO, HDP oxide, ALD oxide, and the like. In one embodiment, the bit line dielectric layer contains the same material as the spacer 600.
The spacer 600 may or may not be removed before forming a bit line dielectric layer 900. In one embodiment, the spacer is not removed before forming the bit line dielectric layer. When the spacer is not removed, the bit line opening 700 contains at least a portion of the spacer 600. In another embodiment, the spacer 600 is removed before forming a bit line dielectric layer (not shown). When the spacer is removed, the bit line opening does not contain spacers. The spacer 600 can be removed by any suitable technique. For example, the spacer 600 is removed by etching.
FIG. 10 illustrates removing the first mask layer 510 and removing an upper portion of the bit line dielectric layer 900, thereby forming a bit line dielectric 1000 in the bit line opening 700. When the spacer 600 exists in the bit line opening, an upper portion of the spacer may or may not be removed. When the bit line dielectric layer 900 contains the same material as the spacer 600, the upper potions of the bit line dielectric layer 900 and spacer 600 can be removed at the same time.
The first mask layer 510 can be removed by any suitable technique. For example, when the first mask layer 510 contains nitride materials, the first mask layer 510 can be removed by contacting the first mask layer 510 with any suitable nitride etchant that does not substantially affect or damage the integrity of other layers in the memory device 500 such as the poly layer 508. Examples of nitride etchants include phosphoric acid. Other nitride etchants can also be used as long as they are capable of removing the first mask layer 510 selective to other layers.
The upper portion of the bit line dielectric layer 900 can be removed by any suitable technique. For example, the upper portion of the bit line dielectric layer is removed by chemical-mechanical polishing (CMP) and/or etching. For example, the upper portion of the bit line dielectric layer is polished by CMP under any suitable condition to facilitate removing/polishing the upper portion. The conditions generally depend upon, for example, the thickness of the bit line dielectric layer, the composition of the bit line dielectric layer, the desired implementations of the memory device 500 being fabricated, and the like.
The upper portion of the bit line dielectric layer 900 can be further removed by etching. For example, when the bit line dielectric layer 900 contains oxide materials, the upper portion of the bit line dielectric layer 900 can be removed by contacting the bit line dielectric layer 900 with any suitable oxide etchant that does not substantially affect or damage the integrity of other layers in the memory device 500 such as the poly layer 508. Examples of oxide etchants include halogen acids such as hydrofluoric acid. In one embodiment, the oxide etchant is a hydrofluoric acid solution such as a buffered hydrofluoric acid (BHF: e.g., hydrofluoric acid-ammonium fluoride buffered solution). In another embodiment, the oxide etchant is a vapor of hydrofluoric acid. Other oxide etchants can also be used as long as they are capable of removing portions of the bit line dielectric layer 900 selective to other layers.
Any suitable amount of the upper portion of the bit line dielectric layer 900 can be removed. In one embodiment, the upper surface of the poly layer 508 is higher than the resultant upper surface of the bit line dielectric layer 900. In another embodiment, the upper surface of the poly layer 508 is substantially coplanar with the resultant upper surface of the bit line dielectric layer 900 (not shown). In yet another embodiment, the upper surface of the poly layer 508 is lower than the resultant upper surface of the bit line dielectric layer 900 (not shown). Due to the removal of the upper portion of the bit line dielectric layer, the upper surface of the poly layer 508 can be exposed.
FIG. 11 illustrates forming a word line 1100 over the semiconductor substrate 504, thereby forming memory cells 1102 on the semiconductor substrate 504. The word line 1100 can contain any suitable conductive material such as polysilicon. The word line 1100 can be formed by, for example, forming a layer of word line material over the semiconductor substrate 504 and patterning (e.g., etching) the layer to establish the word line 1100 over the bit lines 800. The bit lines 800 and the word line 1100 can be oriented at substantially right angles relative to one another. When the spacers 600 are not removed and present in the bit line opening 700, the memory cells are offset by the spacers 600 and the bit line dielectrics 1000.
The memory cell 1102 contains the charge trapping dielectric stack 506 on the semiconductor substrate 504 and the poly gate 508 on the charge trapping dielectric stack 506. The memory cell 1102 contains a pair of the bit lines 800 in the semiconductor substrate 504 adjacent the charge trapping dielectric stack 506 and under the bit line openings 700. The memory cells 1102 are separated by the bit line dielectrics 1300 in the bit line openings 700 that extend into the semiconductor substrate 504.
The memory cell 1102 contains one or more charge storage nodes. The charge storage nodes can store at least a bit of information, respectively. For example, the memory cell 1102 is capable of storing two spatially separated binary data bits, including a left bit represented by the dashed circle A and a right bit represented by the dashed circle B. When the memory cell 1102 is such a dual-bit memory cell, the dual-bit memory cell is generally symmetrical, wherein a drain and a source are interchangeable, although asymmetric implementations are possible within the scope of the innovation. In the exemplary memory cell 1102, the left bit line 800 may serve as the source terminal and the right bit line 800 as the drain terminal with respect to the right bit. Likewise, the right bit line 800 may serve as the source terminal and the left bit line 800 as the drain terminal for the left bit.
The subject innovation may be implemented in association with various single or multi-bit memory cell types including a SONOS cell. In addition, the innovation is applicable to such dual-bit memory devices wherein both bits are used for data or information storage, as well as those in which only one bit (e.g., bit A) of the dual-bit cell is so used. Although not shown in Figures, the charge trapping dielectric stack contains a dielectric portion between the charge storage nodes, and the charge storage nodes are physically separated from each other.
FIG. 12 illustrates an exemplary methodology 1200 of forming memory cells. At 1202, features are provided on a semiconductor substrate and first openings are provided between the features on the semiconductor substrate. The feature contains a charge trapping dielectric stack and a poly gate. At 1204, pocket implant regions are formed adjacent and under the charge trapping dielectric stack in the semiconductor substrate. At 1206, spacers are formed adjacent side surfaces of the features and in the first openings. The spacers contain oxides or nitrides. At 1208, bit line trenches are formed between the spacers under the first openings that extend into the semiconductor substrate, thereby forming bit line openings. At 1210, bit lines are formed under the bit line openings in the semiconductor substrate.
Although not shown in FIG. 12, in one embodiment, the bit line is formed by implantation of one or more dopants using the spacers as an implant screen. The bit line can be formed so that a portion of the bit line overlaps with a portion of the pocket implant region. The spacers can be formed by a high temperature oxide formation process. The method can further involve forming bit line dielectrics in the bit line openings. In another embodiment, the method can further involve forming bit line dielectrics in the bit line openings with the proviso that the spacers are not removed from the first opening. The bit line dielectrics and the spacers can contain the same material (e.g., oxides or nitrides).
FIG. 13 illustrates an exemplary methodology 1300 of increasing an effective channel length in memory cells. At 1302, features are provided on a semiconductor substrate and first openings are provided between the features on the semiconductor substrate. The feature contains a charge trapping dielectric stack and a poly gate. At 1304, pocket implant regions are formed adjacent and under the charge trapping dielectric stack in the semiconductor substrate. At 1306, spacers are formed adjacent side surfaces of the features and in the first openings. The spacers contain oxides or nitrides. At 1308, bit line trenches are formed between the spacers under the first openings that extend into the semiconductor substrate, thereby forming bit line openings. At 1310, bit lines are formed under the bit line openings in the semiconductor substrate by using the spacers as an implant screen to constrain a length of the bit lines.
Although not shown in FIG. 13, in one embodiment, the spacer is formed by a high temperature oxide formation process. In another embodiment, the length of the bit line is constrained so that an effective channel length is about 90% or more and about 100% or less of the length of the poly gate. In yet another embodiment, the bit line is formed so that a portion of the bit line overlaps with a portion of the pocket implant region. In still yet another embodiment, the method further involves forming bit line dielectrics in the bit line openings with the proviso that the spacers are not removed from the bit line openings.
Further, although not shown, the methodologies of FIGS. 12 and 13 may include any suitable memory device fabrication processes. General examples of memory device fabrication processes include masking, patterning, etching, planarization, cleaning, thermal oxidation, implant, annealing, thermal treatment, and deposition techniques normally used for making memory devices.
The resultant memory cell formed herein can be employed for central processing units (CPUs); volatile memory devices such as DRAM devices, SRAM devices, and the like; input/output devices (I/O chips); and non-volatile memory devices such as EEPROMs, EPROMs, PROMs, and the like.
The resultant memory cell formed herein is useful in any electronic device. For example, the resultant memory cell is useful in computers, appliances, industrial equipment, hand-held devices, telecommunications equipment, medical equipment, research and development equipment, transportation vehicles, radar/satellite devices, and the like. Hand-held devices, and particularly hand-held electronic devices, achieve improvements in portability due to the small size and lightweight of the memory devices. Examples of hand-held devices include cell phones and other two way communication devices, personal data assistants, Palm Pilots, pagers, notebook computers, remote controls, recorders (video and audio), radios, small televisions and web viewers, cameras, and the like.
What has been described above includes examples of the subject innovation. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the subject innovation, but one of ordinary skill in the art may recognize that many further combinations and permutations of the subject innovation are possible. Accordingly, the subject innovation is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” and “involves” are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim.

## Claims

1. A memory device comprising:
two or more memory cells on a semiconductor substrate separated by an opening, each memory cell comprising a charge trapping dielectric stack and a poly gate on the semiconductor substrate and two pocket implant regions adjacent and under the charge trapping dielectric stack in the semiconductor substrate;
spacers, comprising oxides or nitrides, adjacent to side surfaces of the charge trapping dielectric stack and the poly gate and in the opening;
bit line trenches between the spacers under the opening that extend into the semiconductor substrate and form bit line openings;
bit line dielectrics in the bit line openings between the memory cells, the bit line dielectrics extending into the semiconductor substrate; and
bit lines in the semiconductor substrate adjacent to the charge trapping dielectric stack and under the bit line openings.

2. The memory device of claim 1, wherein the bit line has a concentration of dopants of about 5E17 atoms/cm3 or more and about 1E21 atoms/cm3 or less.

3. The memory device of claim 1, wherein the bit line has a depth of about 20 nm or more and about 300 nm or less.

4. The memory device of claim 1, wherein the pocket implant region comprises boron and the bit line comprises arsenic.

5. The memory device of claim 1, wherein an effective channel length of the memory cell is about 70% or more and about 100% or less of a length of the poly gate.

6. The memory device of claim 1, wherein the length of the bit line is constrained so that an effective channel length of the memory cell is about 70% or more and about 100% or less of the length of the poly gate.

7. The memory device of claim 1, wherein the bit line is formed so that a portion of the bit line overlaps with a portion of the pocket implant region.

8. The memory device of claim 1, wherein the bit line dielectrics and the spacer comprise the same material.

9. A memory device comprising:
two or more memory cells on a semiconductor substrate separated by an opening, each memory cell comprising a charge trapping dielectric stack and a poly gate on the semiconductor substrate and two pocket implant regions adjacent and under the charge trapping dielectric stack in the semiconductor substrate, the charge trapping dielectric stack comprising a silicon nitride charge trapping dielectric;
spacers, comprising oxides or nitrides, adjacent to side surfaces of the charge trapping dielectric stack and the poly gate and in the opening;
bit line trenches between the spacers under the opening that extend into the semiconductor substrate and form bit line openings;
bit line dielectrics in the bit line openings between the memory cells, the bit line dielectrics extending into the semiconductor substrate; and
bit lines in the semiconductor substrate adjacent to the charge trapping dielectric stack and under the bit line openings.

10. The memory device of claim 1, wherein the silicon nitride charge trapping dielectric comprises silicon rich silicon nitride.

11. The memory device of claim 1, wherein the bit line length is about 70% or more and about 140% or less of the length of the bit line opening.

12. The memory device of claim 1, wherein the bit line has a concentration of dopants of about 1E18 atoms/cm3 or more and about 1E21 atoms/cm3 or less.

13. The memory device of claim 1, wherein the length of the bit line is constrained so that an effective channel length of the memory cell is about 80% or more and about 100% or less of the length of the poly gate.

14. A dual-bit memory device comprising:
two or more dual-bit memory cells on a semiconductor substrate separated by an opening, each dual-bit memory cell comprising a charge trapping dielectric stack and a poly gate on the semiconductor substrate and two pocket implant regions adjacent and under the charge trapping dielectric stack in the semiconductor substrate;
spacers, comprising oxides or nitrides, adjacent to side surfaces of the charge trapping dielectric stack and the poly gate and in the opening;
bit line trenches between the spacers under the opening that extend into the semiconductor substrate and form bit line openings;
bit line dielectrics in the bit line openings between the dual-bit memory cells, the bit line dielectrics extending into the semiconductor substrate; and
bit lines in the semiconductor substrate adjacent to the charge trapping dielectric stack and under the bit line openings.

15. The dual-bit memory device of claim 14, wherein the charge trapping dielectric comprises silicon nitride.

16. The dual-bit memory device of claim 14, wherein the bit line length is about 70% or more and about 140% or less of the length of the bit line opening.

17. The dual-bit memory device of claim 14, wherein the bit line has a concentration of dopants of about 1E18 atoms/cm3 or more and about 1E21 atoms/cm3 or less.

18. The dual-bit memory device of claim 14, wherein the length of the bit line is constrained so that an effective channel length of the memory cell is about 80% or more and about 100% or less of the length of the poly gate.

19. The dual-bit memory device of claim 14, wherein the bit line is formed so that a portion of the bit line overlaps with a portion of the pocket implant region.

20. The dual-bit memory device of claim 14, wherein the bit line dielectric and the spacer comprise the same material.