# Patent

## Title
Non-volatile memory device including nitrogen pocket implants and methods for making the same

## Abstract

In a non-volatile memory structure, the source/drain regions are surrounded by a nitrogen-doped region. As a result, an interface between the substrate and the charge trapping layer above the nitrogen-doped region is passivated by a plurality of nitrogen atoms. The nitrogen atoms can improve data retention, and performance of cycled non-volatile memory devices.

## Background
BACKGROUND
1. Field of the Invention
The embodiments described below are related to non-volatile memory devices, and more particularly to systems and methods for improving the reliability of non-volatile memory devices.
2. Background of the Invention
Conventional non-volatile memory devices rely on floating gate technology, or a trapping layer construction to store charge that can then be read out of the device. Such non-volatile memory devices can be viewed as enhancements to the standard metal oxide semiconductor (MOS) transistor. As is well understood, a MOS transistor comprises three main terminals: a source, a drain, and a gate. A potential applied to the gate directly controls the formation and conductivity of a channel formed below the gate between the source and drain. The conductivity of the channel affects the flow of current between the source and drain terminals. The channel becomes significantly conductive as the gate potential exceeds a certain threshold, referred to as the transistor's threshold voltage.
A floating gate memory cell differs from a standard MOS transistor in that it has an additional electrically isolated gate, a floating gate, below the standard control gate and above the transistor channel. Such a floating gate is typically composed of a conducting material, such as a polysilicon layer. A floating gate memory device stores information by holding electric charge within a floating gate. Adding or removing charge from the floating gate changes the threshold voltage of the cell, thereby defining whether the memory cell is in a programmed or erase state. Charge flowing in the channel can then be attracted to, or injected into the floating gate or trapping layer.
Charge trapping structures, such as nitride read-only memory structures, on the other hand, include a trapping nitride layer configure to store the charge instead of a floating gate suspended above the cell. FIG. 1 is a diagram illustrating a conventional nitride read-only memory structure 100. As can be seen, nitride read-only memory 100 is constructed on a silicon substrate 102. The silicon substrate can be a P-type silicon substrate or an N-type silicon substrate; however, for various design reasons P-type silicon substrates are often preferred. Drain/source regions 104 and 106 can then be implanted in substrate 102. A trapping structure 108 is then configured on substrate 102 between source/drain regions 104 and 106. Control gate 110 is then formed on top of trapping layer 108.
Drain/source regions 104 and 106 are silicon regions that are doped to be the opposite type as that as substrate 102. For example, where a P-type silicon substrate 102 is used, N-type drain/source regions 104 and 106 can be implanted therein.
Charge trapping structure 108 comprises a nitride trapping layer as well as an isolating oxide layer between the trapping layer and the channel in substrate 102. In other embodiments, trapping structure 108 can comprise a nitride trapping layer sandwiched between two isolating, or dielectric layers, such as oxide layers. Such a configuration is often referred to as an Oxide-Nitride-Oxide (ONO) trapping layer.
Charge can be accumulated and confined within trapping structure 108 next to drain/source regions 104 and 106, effectively storing two separate and independent charges. Each charge can be maintained in one of two states, either programmed or erased, represented by the presence or absence of a pocket of trapped electrons. This enables the storage of two bits of information without the complexities associated with multilevel cell technology.
Each storage area in nitride read-only memory cell 100 can be programmed independently of the other storage area. A nitride read-only memory cell is programmed by applying a voltage that causes negatively charged electrons to be injected into the nitride layer of trapping structure 108 near one end of the cell. Erasing is accomplished by applying voltages that cause holes to be injected into the nitride layer where they can compensate for electrons previously stored in the nitride layer during programming.
In general, because the stored charge is confined next to the drain/source regions within the trapping layer, numerous program —recycles can be performed without significantly degrading the cell's performance. Unfortunately, the interface trap density (Dit) increases as nitride read-only memory device 100 is cycled. The cycles induce interface traps, i.e., traps at the interface between charge trapping structure 108 and substrate 102. These interface traps will then trap charge at the interface between trapping structure 108 and substrate 102. The interface trap density (Dit) is a measure of the amount of trapped charge per cm2.
The cycling-induced interface traps will cause swings in the I-V curve for nitride read-only memory device 100 as well as threshold voltage shifts. The I-V swings and threshold voltage shifts can degrade the performance of nitride read-only memory device 100.
For example, the interface traps can cause nitride read-only memory device 100 to pass a program verify operation with much fewer electrons trapped in charge trapping structure 108 than it would typically take for a newer device to pass a program verify operation. Further, high temperature baking anneals the traps, leading to further threshold voltage loss. Thus, cycled nitride read-only memory devices cannot retain their data well in a high temperature environment.
Similar cycle induced interface traps can occur in other non-volatile memory devices, such as the floating gate devices described above. As a result, the performance of non-volatile memory devices can be improved by reducing the interface trap density.
SUMMARY
In a non-volatile memory structure, the source/drain regions are surrounded by a nitrogen-doped region. As a result, an interface between the substrate and the charge trapping structure above the nitrogen-doped region is passivated by a plurality of nitrogen atoms. The nitrogen atoms can improve data retention, and performance of cycled non-volatile memory devices.
In one aspect, the nitrogen-doped regions are implanted around, or adjacent to the source/drain regions and in the path of carrier movement during program and erase cycles.
These and other features, aspects, and embodiments of the invention are described below in the section entitled “Detailed Description.”



BRIEF DESCRIPTION OF THE DRAWINGS
Features, aspects, and embodiments of the inventions are described in conjunction with the attached drawings, in which:
FIG. 1 is a diagram illustrating an exemplary nitride read-only memory device;
FIG. 2 is a diagram illustrating an exemplary process for implanting source/drain regions within the nitride read-only memory device of FIG. 1;
FIG. 3 is a diagram illustrating an example method for implanting source/drain regions in nitride read-only memory device of FIG. 1 in accordance with one embodiment;
FIG. 4 is a diagram illustrating an example of non-volatile memory structure formed using the process of FIG. 3;
FIG. 5 is a diagram illustrating the concentration of atoms within the device of FIG. 4; and
FIG. 6 is a diagram illustrating charge pump current (ICP) curves for non-volatile memory devices formed using the method of FIG. 2 versus the method of FIG. 3.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments described below are directed to nitride read-only memory devices; however, it will be apparent that the methods described herein are not limited to nitride read-only memory devices and can apply generally to non-volatile memory devices, e.g., including floating gate flash memory devices, Nbit devices, BE-SONOS devices, PHINES devices, BP-PHINES devices, as well as logic transistors.
In the embodiments described below the source/drain regions are surrounded by a nitrogen-doped region. The interface between the silicon substrate and the trapping structure is then passivated by a plurality of nitrogen atoms. For example, the trapping structure can actually comprise a dielectric layer and a nitrogen-trapping layer. The dielectric layer, i.e., a tunnel oxide, can reside between the trapping layer and the silicon substrate. Thus, the interface between the silicon substrate and the tunnel oxide layer can be passivated by a plurality of nitrogen atoms.
Nitride read-only memory devices store charge in a trapping layer near the source/drain junctions. Accordingly, enhancement of the interface between the silicon substrate and the insulating layer separating the silicon substrate from the charge trapping layer near the source/drain junctions can improve nitride read-only memory reliability. Nitrogen implantation, i.e., implantation of N2/N, followed by annealing can cause interface traps to be passivated by nitrogen atoms. This passivation can strengthen the oxide-substrate interface and reduce damage resulting from hot carrier stress.
In the embodiments described below, a nitrogen passivation is achieved by forming a nitrogen pocket implant before forming the buried diffusion lines. FIG. 2 is flowchart illustrating an exemplary process for forming the buried diffusion lines in a nitride read-only memory device. First, in step 202, a photoresist will be formed on the substrate for forming the buried diffusion lines. Next, in step 204, a P-pocket will be implanted in the areas surrounding the buried diffusion lines, e.g., using a dopent such as boron. In step 206, the buried diffusion lines will be implanted e.g., using a dopent such as As or P. In step 208, the photoresist is removed.
In the process according to the embodiments described herein, a nitrogen pocket is implanted within the P-pocket and around the area comprising the buried diffusion lines. FIG. 3 is a diagram illustrating an example method for forming an nitride read-only memory device in accordance with one embodiment. The method of FIG. 3 can be used, for example, to form nitride read-only memory device 400 in FIG. 4. Thus, in step 302, a photoresist can be applied for forming buried diffusion lines 406 and 412. Impurities, in this case P-type impurities, can then be implanted to form pockets 402 and 408, in step 304, using a dopent such as boron.
In step 306, nitrogen pocket implants 404 and 410 can then be implanted around the area where buried diffusion lines 406 and 412 will be formed. For example, nitrogen pockets 404 and 410 can be implanted using a dopant such as N2(28) or N(14), with an energy in the range of about 1 KeV to about 100 KeV, a dose in a range of approximately 1E12 to 1E15 cm−2, and a tilt angle of approximately 0° to 60°. In other embodiments, nitrogen implants can be formed near buried diffusion lines 406 and 412, but not necessarily around them as illustrated in FIG. 4.
In step 308, buried diffusion lines 406 and 412 can be implanted, e.g., using a dopent such as As or P. In step 310 the photoresist applied in step 302 can be removed.
After formation of the buried diffusion regions 406 and 412, trapping structure 414 can be formed over substrate 401. In the embodiment of FIG. 4, trapping structure 414 comprises an ONO structure. The ONO structure comprises a dielectric layer, or barrier oxide 420, a nitrogen-trapping layer 418, and another dielectric, or oxide layer 416.
As can be seen, nitrogen implants 404 and 410 contact oxide layer 420. As a result, nitrogen implants 404 and 410 can be used to passivate the interface between silicon substrate 401 and oxide layer 420 with nitrogen atoms. For example, after implantation of nitrogen implants 404 and 410, in step 306, the thermal budget in the sequential fabrication process can be configured so as to allow nitrogen atoms to pile up at the interface between oxide layer 420 and silicon substrate 401. This leads to the nitrogen atom passivation.
FIG. 5 is a diagram illustrating the concentration of various atoms within silicon substrate 401. Curve 506 illustrates the concentration of nitrogen atoms after implantation of nitrogen implants 404 and 410. Curve 504 illustrates that the amount of nitrogen atoms near the interface of the oxide layer 420 and silicon substrate 401 increases after receiving thermal treatment in the sequential process.
With the nitrogen passivation, nitride read-only memory device 400 will illustrate a smaller ICP, which is representative of a stronger interface between oxide layer 420 and silicon substrate 401, e.g., and interface with fewer dangling silicon bonds at the interface between oxide layer 420 and silicon substrate 401.
FIG. 6 is a graph illustrating charge pump current (ICP) curves 602 and 604 for an nitride read-only memory device without nitrogen implants 404 and 410, and with nitrogen implants for 404 and 410 respectively. The graph of FIG. 6 illustrates the ICP for the devices with initial interface traps. As can be seen, the nitride read-only memory device with nitrogen implants 404 and 410 has a lower ICP, representing a better interface.
Accordingly, by including nitrogen implants, e.g., nitrogen implants 404 and 410, near the buried diffusion lines, or source/drain regions, the reliability of an nitride read-only memory device, such as nitride read-only memory device 400, can be improved by the elimination of charged traps at the interface between oxide layer 420 and silicon substrate 401.
While certain embodiments of the inventions have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the inventions should not be limited based on the described embodiments. Rather, the scope of the inventions described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.

## Claims

1. A method for fabricating a non-volatile memory device, comprising:
providing a patterned photoresist layer on a silicon substrate defining a buried diffusion region;
implanting conductive impurities in the silicon substrate to form an impurity pocket;
implanting nitrogen in the silicon substrate so as to form a nitrogen implant that surrounds the buried diffusion region; and
forming a charge trapping structure on the silicon substrate adjacent to the nitrogen implant,
wherein the buried diffusion region is completely contained within the nitrogen implant and the nitrogen implant is completely contained within the impurity pocket.

2. The method of claim 1, further comprising causing nitrogen atoms to pile up at the interface between the charge trapping structure and the silicon substrate.

3. The method of claim 1, wherein implanting conductive impurities comprises implanting P-type impurities in the silicon substrate.

4. The method of claim 1, wherein the nitrogen implant is implanted within the impurity pocket.

5. The method of claim 1, wherein a N2(28) dopant is used to form the nitrogen implant.

6. The method of claim 1, wherein a N(14) dopant is used to form the nitrogen implant.

7. The method of claim 1, wherein a dopant with an energy in the range of about 1 KeV-about 100 KeV is used to form the nitrogen implant.

8. The method of claim 1, wherein a dopant dose in the range of about 1E12 cm−2-about 1E15 cm−2 cm is used to form the nitrogen implant.

9. The method of claim 1, wherein the trapping structure comprises a dielectric layer formed on the silicon substrate between first and second ones of the buried diffusion region and a nitrogen charge trapping layer formed on the dielectric layer.

10. The method of claim 9, wherein the dielectric layer is in contact with the nitrogen implant.

11. The method of claim 1, wherein the trapping structure comprises an ONO structure.

12. A method for fabricating a non-volatile memory device, comprising:
providing a patterned photoresist layer on a silicon substrate to define a diffusion region;
implanting conductive impurities in the silicon substrate to form an impurity pocket;
implanting nitrogen to form a nitrogen pocket within the impurity pocket, the nitrogen pocket surrounding the diffusion region;
forming a diffusion line in the diffusion region after the implanting nitrogen;
forming a charge trapping structure on the silicon substrate, the charge trapping structure contacting the nitrogen implant; and
causing nitrogen atoms to pile up at the interface between the charge trapping structure and the silicon substrates,
wherein the buried diffusion region is completely contained within the nitrogen implant and the nitrogen implant is completely contained within the impurity pocket.

13. The method of claim 12, wherein the conductive impurities are P-type impurities.

14. The method of claim 12, wherein a N2(28) dopant is used to implant the nitrogen implant.

15. The method of claim 12, wherein a N(14) dopant is used to implant the nitrogen implant.

16. The method of claim 12, wherein a dopant with an energy in the range of about 1 KeV-about 100 KeV is used to implant the nitrogen implant.

17. The method of claim 12, wherein a dopant dose in the range of about 1E12 cm−2-about 1E15 cm−2 is used to implant the nitrogen implant.

18. The method of claim 12, wherein the trapping structure comprises a dielectric layer formed on the silicon substrate between first and second ones of the buried diffusion region and a nitrogen charge trapping layer formed on the dielectric layer.

19. The method of claim 18, wherein the dielectric layer is in contact with the nitrogen implant.

20. The method of claim 12, wherein the trapping structure comprises an ONO structure.