# Patent

## Title
Multi-state memory cell with asymmetric charge trapping

## Abstract

A multi-state NAND memory cell includes two drain/source areas in a substrate. An oxide-nitride-oxide structure is formed above the substrate between the drain/source areas. The nitride layer acting as an asymmetric charge trapping layer. A control gate is located above the oxide-nitride-oxide structure. An asymmetrical bias on the drain/source areas causes the drain/source area with the higher voltage to inject an asymmetric distribution hole by gate induced drain leakage injection into the trapping layer substantially adjacent that drain/source area.

## Background
RELATED APPLICATION
This Application is a Divisional of U.S. application Ser. No. 10/785,785, titled “MULTI-STATE MEMORY CELL WITH ASYMMETRIC CHARGE TRAPPING,” filed Feb. 24, 2004, and issued on Jul. 4, 2006 as U.S. Pat. No. 7,072,217 which is commonly assigned and incorporated herein by reference.


TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to memory cells and in particular the present invention relates to multi-state non-volatile memory cells.
BACKGROUND OF THE INVENTION
Memory devices are available in a variety of styles and sizes. Some memory devices are volatile in nature and cannot retain data without an active power supply. A typical volatile memory is a DRAM which includes memory cells formed as capacitors. A charge, or lack of charge, on the capacitors indicate a binary state of data stored in the memory cell. Dynamic memory devices require more effort to retain data than non-volatile memories, but are typically faster to read and write.
Non-volatile memory devices are also available in different configurations. For example, floating gate memory devices are non-volatile memories that use floating gate transistors to store data. The data is written to the memory cells by changing a threshold voltage of the transistor and is retained when the power is removed. The transistors can be erased to restore the threshold voltage of the transistor. The memory may be arranged in erase blocks where all of the memory cells in an erase block are erased at one time. These non-volatile memory devices are commonly referred to as flash memories.
Flash memories may use floating gate technology or trapping technology. Floating gate cells include source and drain regions that are laterally spaced apart to form an intermediate channel region. The source and drain regions are formed in a common horizontal plane of a silicon substrate. The floating gate, typically made of doped polysilicon, is disposed over the channel region and is electrically isolated from the other cell elements by oxide. The non-volatile memory function for the floating gate technology is created by the absence or presence of charge stored on the isolated floating gate. The trapping technology functions as a non-volatile memory by the absence or presence of charge stored in isolated traps that capture and store electrons or holes.
In order for memory manufacturers to remain competitive, memory designers are constantly trying to increase the density of flash memory devices. Increasing the density of a flash memory device generally requires reducing spacing between memory cells and/or making memory cells smaller. Smaller dimensions of many device elements may cause operational problems with the cell. For example, the channel between the source/drain regions becomes shorter possibly causing severe short channel effects. Additionally, possible charge migration from one corner of the cell to the other becomes more of a concern with smaller cell size.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for higher density memory devices.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a cut away view of one embodiment for programming a multi-state NAND memory cell with asymmetric charge trapping of the present invention.
FIG. 2 shows a cut away view of another embodiment for programming a multi-state NAND memory cell with asymmetric charge trapping of the present invention.
FIG. 3 shows a cut-away view of an embodiment for erasing a multi-state NAND memory cell with asymmetric charge trapping of the present invention.
FIG. 4 shows a cut-away view of yet another embodiment of a multi-state NAND memory cell with asymmetric charge trapping of the present invention.
FIG. 5 shows a cut-away view of an embodiment for reading the multi-state NAND memory cell with asymmetric charge trapping of the present invention.
FIG. 6 shows a portion of a multi-state NAND memory cell array of the present invention.
FIG. 7 shows a table of voltages for operation of the embodiment of FIG. 6.
FIG. 8 shows a block diagram of one embodiment of an electronic system of the present invention.



DETAILED DESCRIPTION
In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The terms wafer or substrate, used in the following description, include any base semiconductor structure. Both are to be understood as including silicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI) technology, thin film transistor (TFT) technology, doped and undoped semiconductors, epitaxial layers of a silicon supported by a base semiconductor structure, as well as other semiconductor structures well known to one skilled in the art. Furthermore, when reference is made to a wafer or substrate in the following description, previous process steps may have been utilized to form regions/junctions in the base semiconductor structure, and terms wafer or substrate include the underlying layers containing such regions/junctions. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.
The charge on a floating gate memory forms a Gaussian surface that spreads across the floating gate. The charge in a trapping based memory of the present invention is localized and does not spread. This property permits asymmetric charge and the ability to form multi-state cells.
FIG. 1 illustrates a cut-away view of one embodiment for programming a multi-state NAND memory cell with asymmetric charge trapping. This embodiment is comprised of a substrate 101 with two active areas 105 and 107. Each region 105 and 107 acts alternatively as a drain or source region, depending on the operation performed and voltages that are applied.
In one embodiment, the drain and source regions 105 and 107 are n-type conductive material while the substrate 101 is a p-type conductive material. In an alternate embodiment, these conductive material types are switched.
Above the channel between the drain/source regions 105 and 107 is an oxide-nitride-oxide (ONO) structure 103, 109, and 111. The nitride layer 103 is isolated from the substrate by a first oxide layer 111 and from a control gate 100 by a second oxide layer 109. The nitride layer 103 is the trapping layer that stores the asymmetric charges of the present invention. The present invention is not limited to any certain quantity of dielectric and/or trapping layers.
The present invention is also not limited in the composition of the dielectric/trapping layers. In one embodiment, the oxide material can be aluminum oxide. The trapping layer may be a silicon nanocrystal material. Alternate embodiments use other types of dielectric materials and/or other trapping layer materials.
The embodiment of FIG. 1 illustrates the programming of one data bit in the left side of the trapping layer 103. This is accomplished by applying a relatively high negative voltage to the control gate 100. This voltage turns off the channel in order to prevent leakage from the drain region 105 to the source region 107. In one embodiment, the gate voltage is between −10V and −15V. Alternate embodiments may use other gate voltage ranges.
An asymmetric bias is applied to the drain 105 and source regions 107. In one embodiment, a positive 5V is applied to the drain region 105 and the source region 107 is grounded (i.e., 0V). The large potential on the left side of the junction from both the gate 100 and junction field causes a gate induced drain leakage (GIDL) condition that injects holes into the trapping layer 103 near the left junction. The injected holes neutralize the electrons from a previous erased condition thus resulting in a reduced threshold voltage.
The right junction has a reduced field since the junction bias is zero. This results in a bias condition that does not inject holes. The electrons on the right side of the channel are not compensated by holes thus resulting in the initial programmed or erased condition remaining.
FIG. 2 illustrates a cut-away view of a second embodiment for programming a multi-state NAND memory cell with asymmetric charge trapping. The embodiment of FIG. 2 illustrates the programming of one data bit in the right side of the trapping layer 103. This is accomplished by applying a relatively high negative voltage to the control gate 100. This voltage turns off the channel in order to prevent leakage from the drain region 107 to the source region 105. In one embodiment, the gate voltage is between −10V and −15V. Alternate embodiments may use other gate voltage ranges.
An asymmetric bias is applied to the drain 107 and source regions 105. In one embodiment, a positive 5V is applied to the drain region 107 and the source region 105 is grounded (i.e., 0V). The large potential on the right side of the junction from both the gate 100 and junction field causes a GIDL condition that injects holes into the trapping layer 103 near the right junction. The injected holes neutralize the electrons from a previous erased condition thus resulting in a reduced threshold voltage.
The left junction has a reduced field since the junction bias is zero. This results in a bias condition that does not inject holes. The electrons on the left side of the channel are not compensated by holes thus resulting in the above-described programmed condition remaining.
FIG. 3 illustrates a cut-away view of an embodiment for erasing a multi-state NAND memory cell with asymmetric charge trapping. The erase operation is performed by tunneling electrons into the trapping layer 303 from a uniform sheet of charge in the inversion region 301. This forms a high threshold level by a continuous uniform sheet of trapped charge in the trapping layer 103. The erase operation is accomplished in one embodiment by applying a positive gate voltage in the range of 10-20V. Both the drain and source regions are grounded (i.e., 0V). Alternate embodiments may use other voltages and voltage ranges.
FIG. 4 illustrates a cut-away view of yet another embodiment of a multi-state NAND memory cell with asymmetric charge trapping. This embodiment creates a discontinuous trapping layer 403 by extending the control gate into the trapping layer 403. This results in better sensing, better data retention, and resistance to secondary emissions.
FIG. 5 illustrates a method for reading the left side of the multi-state NAND memory cell of the present invention using asymmetrical biasing of the source/drain regions. The left data bit 500 can be read by applying a relatively high bias to the right source/drain region 501 of the cell. In one embodiment, this drain voltage is in the range of 1-3V. The left drain/source region 503, acting as a source, is grounded and VG is a positive voltage in the range of 3-6V. Alternate embodiments may use other voltages and voltage ranges.
The right data bit 502 is read using an inverse process. In this embodiment, the left drain/source region 503 is grounded while the right source/drain region 501 has a relatively high voltage applied (e.g., 1-3V). VG in this read embodiment is also in the range of 3-6V. Alternate embodiments may use other voltages and voltage ranges.
FIG. 6 illustrates two string arrays of multi-state NAND memory cells of the present invention. A table of voltages for different modes of operation of a selected column of this memory array is illustrated in FIG. 7.
The portion of the NAND memory array of FIG. 6 is comprised of two columns 601 and 602 of multi-state NAND memory cells as described above. One column 601 is selected while the second column 602 is unselected. The selected column 601 is comprised of a select gate 605 for the drain voltage, Vd, and a select gate 606 for the source voltage Vs The selected column 601 is also comprised of three multi-state NAND memory cells 610-612 that are connected to control gate voltages VWL1-VWL3 respectively. The columns of FIG. 6 are for purposes of illustration only since a real memory column is comprised of a substantially larger quantity of cells.
Referring to the voltage table of FIG. 7, two versions of an erase operation are illustrated. In one option, as described above, the drain and source voltages, Vd and Vs, are 0V and the control gate voltage, VH, are in the range of 10-20V. In this embodiment, the control gates of the select gates 605 and 606 are connected to VH/2. Other erase operation embodiments may use GIDL hole injection from both sides of the array simultaneously.
The second option for an erase operation leaves the drain and source connections floating as an open connection (O/C). In this embodiment, the select gates 605 and 606 are also floating.
During a program operation of the left bit in the middle cell 611, VWL2 is −VH (e.g., −10 to −20V), Vd is VDP (e.g., 3 to 6V), and VS is connected to ground. The control gates of the select gates 605 and 606 are connected to VX1 and the control gates of the other cells 610 and 612 in the column 601 are connected to VX2. In one embodiment VX1 is approximately equal to VX2 which is approximately equal to VDP+VT. VT is the threshold voltage of the cell as is well known in the art. The program operation of the right bit in the middle cell 611 uses substantially the same voltages as the left bit but in this case VS is connected to VDP and Vd is connected to ground. Alternate embodiments use other embodiments to achieve substantially similar results.
During a read operation of the left bit in the middle cell 611, VWL2 is VR (e.g., 3-6V), Vd is VDR, and VS is connected to ground. The control gates of the select gates 605 and 606 are connected to VY1 and the control gates of the other cells 610 and 612 in the column 601 are connected to VY2. In one embodiment, VY1 is approximately equal to VY2 which is approximately equal to VDR+VT where VDR in the range of 4-6V. The read operation of the right bit in the middle cell 611 uses substantially the same voltages as the left bit but in this case VS is connected to ground and Vd is connected to VDR. Alternate embodiments use other embodiments to achieve substantially similar results.
FIG. 8 illustrates a functional block diagram of a memory device 800 that can incorporate multi-state NAND memory cells of the present invention. The memory device 800 is coupled to a processor 810. The processor 810 may be a microprocessor or some other type of controlling circuitry. The memory device 800 and the processor 810 form part of an electronic system 820. The memory device 800 has been simplified to focus on features of the memory that are helpful in understanding the present invention.
The memory device includes an array of memory cells 830. In one embodiment, the memory cells are non-volatile floating-gate memory cells and the memory array 830 is arranged in banks of rows and columns.
An address buffer circuit 840 is provided to latch address signals provided on address input connections A0-Ax 842. Address signals are received and decoded by a row decoder 844 and a column decoder 846 to access the memory array 830. It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends on the density and architecture of the memory array 830. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.
The memory device 800 reads data in the memory array 830 by sensing voltage or current changes in the memory array columns using sense/buffer circuitry 850. The sense/buffer circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array 830. Data input and output buffer circuitry 860 is included for bi-directional data communication over a plurality of data connections 862 with the controller 810). Write circuitry 855 is provided to write data to the memory array.
Control circuitry 870 decodes signals provided on control connections 872 from the processor 810. These signals are used to control the operations on the memory array 830, including data read, data write, and erase operations. The control circuitry 870 may be a state machine, a sequencer, or some other type of controller.
The flash memory device illustrated in FIG. 8 has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.
CONCLUSION
In summary, the multi-state NAND cell of the present invention is a trapping based memory that allows asymmetric charges to be stored, thereby providing storage for two bits. The memory cell provides high memory density, low power operation, and improved reliability due to the trapping function.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.

## Claims

1. A method for programming a multi-state NAND memory cell having a control gate, first and second active areas, and a single, continuous nitride trapping layer formed at least a length of the control gate and configured for asymmetrical trapping near each of the active areas, the method comprising:
applying a negative gate voltage, in a range of −10V to −15V, to the control gate; grounding the second active area; and
applying a positive voltage to the first active area to inject an asymmetric distribution hole by gate induced drain leakage injection into the single, continuous nitride trapping layer substantially adjacent the first active area.

2. The method of claim 1 and further including:
grounding the first active area; and
applying a positive voltage to the second active area to inject an asymmetric distribution hole by gate induced drain leakage injection into the trapping layer substantially adjacent the second active area.

3. The method of claim 1 wherein the trapping layer is a discontinuous trapping layer that is divided by an extension of the control gate.

4. A method for programming a multi-state NAND memory cell having a control gate, first and second source/drain regions, and a single, continuous nitride trapping layer formed at least a length of the control gate and configured for asymmetrical trapping of first and second data bits near each of the source/drain regions, the method comprising:
applying a voltage, in a range of −10V to −15V, to the control gate; and
applying asymmetrical voltages to the first and second source/drain regions to program the first and second data bits.

5. The method of claim 4 wherein the first source/drain region is biased at 5V and the second source/drain region is biased at ground potential.

6. The method of claim 4 wherein a voltage on the first source/drain region injects holes into the trapping layer near the first source/drain region.

7. The method of claim 6 wherein injected holes neutralize electrons from a previous erased condition to reduce a threshold voltage of the memory cell.

8. The method of claim 6 wherein a ground potential on the second source/drain region does not inject holes into the trapping layer near the second source/drain region.

9. The method of claim 4 and further including aluminum oxide formed between the trapping layer and the control gate and between the trapping layer and a substrate.

10. The method of claim 4 wherein the memory cell is part of a memory array comprising a plurality of memory cells.

11. The method of claim 10 wherein the memory array is part of a NAND flash memory device.