# Patent

## Title
Method of programming memory cell

## Abstract

A method of programming a memory cell is described. First, a first programming operation is performed to inject electrons into a nitride layer adjacent to a side of a drain. The first programming operation includes applying a first gate voltage to a gate, applying a first drain voltage to the drain, applying a first source voltage to a source, and applying a first substrate voltage to a substrate. Then, a second programming operation is performed to inject the electrons into the nitride layer adjacent to a side of the source. The second programming operation includes applying a second gate voltage to the gate, applying a second drain voltage to the drain, applying a second source voltage to the source, and applying a second substrate voltage to the substrate. The second gate voltage is less than the first gate voltage.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of programming a memory cell, and more particularly, to a method of programming a memory cell capable of adjusting voltages automatically.
2. Description of Related Art
Among various types of memory products, the non-volatile memory allows multiple data writing, reading, and erasing operations. One non-volatile memory is a memory that has widely used in personal computers and electronic equipments. In one non-volatile memory, data can be stored, read out or erased numerous times and any stored data can be retained even after power is cut off.
The typical non-volatile memory cell has a floating gate and a control gate made by doped polysilicon. The floating gate disposed between the control gate and the substrate is in a floating state and is not electrically connected to any devices for storing charges. The control gate is used to control the data writing/reading function. Therefore, one non-volatile memory cell can store either “1” or “0” and is a single-bit (1 bit/cell memory cell) memory cell.
With the increase in the integrity of integrated circuit devices, a non-volatile memory cell adopting nitride silicon to fabricate a charge trapping layer as a replacement of a polysilicon floating gate is provided. Please refer to FIGS. 1A and 1B which are schematic views illustrating a programming operation on a conventional 2 bits/cell non-volatile memory cell. First, a memory cell is provided. The memory cell includes a substrate 102, a source 104, a drain 106, an oxide layer 108, a nitride layer 110, another oxide layer 112, and a polysilicon layer 114. The method of a programming operation of the memory cell is that 10 volts of voltage is applied to the polysilicon layer 114, 0 volt is applied to the source 104, 5 ̃7 volts is applied to the drain 106, and 0 volt is applied to the substrate 102, such that hot electrons generated in a channel region are injected into the nitride layer 110 adjacent to a side of the drain 106 so as to store a bit 116. Thereafter, the voltages of the drain 106 and the source 104 are reversely connected, such that the hot electrons generated in the channel region are injected into the nitride layer 110 adjacent to a side of the source 104 for storing a bit 118. The memory cell is a non-volatile memory cell storing 2 bits in one cell (2 bits/cell).
Nevertheless, during the programming operation performed on a conventional 2 bits/cell non-volatile memory cell, if a bit (a first bit) is stored near the drain of the memory cell, the storage of another bit (a second bit) enhances the programming efficiency and influences the performance of devices. Please refer to FIGS. 2A and 2B which are views illustrating a voltage distribution of the conventional 2 bits/cell non-volatile memory cell. The reference number 210 in FIG. 2A refers to a programming threshold voltage (Vt) distribution curve when a storage operation is performed on the bit 116. The reference number 220 in FIG. 2B refers to the programming Vt distribution curve when the storage operation is performed on the bit 118. It can be learned from FIGS. 2A and 2B that during the programming operation performed on the memory cell, the existing bit 116 (the first bit) affects the programming efficiency of another bit 118 (the second bit), leading to an increase in the Vt and a looser Vt distribution curve (as a width labeled as 230 in FIG. 2B). This is the so-called over-programming.
The cross interference of two bits in one memory cell mentioned above may substantially implicate the device operation and even deteriorate the device reliability. Therefore, how to resolve the above issue has become an important topic in the industry.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a method of programming a memory cell. Through said method, a tighter Vt distribution curve is obtained, and the issue of the over-programming is further reduced.
The present invention provides a method of programming a memory cell. The memory cell includes a substrate, a drain and a source disposed in the substrate, and a gate disposed on the substrate between the drain and the source. Here, the gate includes, sequentially from the substrate, a first oxide layer, a nitride layer, a second oxide layer, and a polysilicon layer. The method of programming the memory cell includes performing a first programming operation to inject electrons into a nitride layer adjacent to a side of a drain through a channel hot electron injection (CHEI) effect. The first programming operation includes applying a first gate voltage to the gate, applying a first drain voltage to the drain, applying a first source voltage to the source, and applying a first substrate voltage to the substrate. Then, a second programming operation is performed to inject the electrons into the nitride layer adjacent to a side of the source through a CHEI effect. The second programming operation includes applying a second gate voltage to the gate, applying a second drain voltage to the drain, applying a second source voltage to the source, and applying a second substrate voltage to the substrate. The second gate voltage is less than the first gate voltage.
According to an embodiment of the present invention illustrating the method of programming the memory cell, the first drain voltage is equal to the second source voltage, and the first drain voltage is at a constant value.
According, to an embodiment of the present invention illustrating the method of programming the memory cell, the first drain voltage is equal to the second source voltage, and the first drain voltage is a increased using a step-by-step manner.
According to an embodiment of the present invention illustrating the method of programming the memory cell, the second source voltage is less than the first drain voltage. In one embodiment, the first drain voltage and the second source voltage are at a constant value. In another embodiment, the first drain voltage and the second source voltage are increased using a step-by-step manner.
According to an embodiment of the present invention illustrating the method of programming the memory cell, a pulse width of the second gate voltage is less than that of the first gate voltage. In addition, the pulse width of the second source voltage is less than that of the first drain voltage according to one embodiment.
According to an embodiment of the present invention illustrating the method of programming the memory cell, the difference between the second gate voltage and the first gate voltage is larger than 0, less than or equal to 0.15 times of the first gate voltage.
According to an embodiment of the present invention illustrating the method of programming the memory cell, the first source voltage is 0V.
According to an embodiment of the present invention illustrating the method of programming the memory cell, the second drain voltage is 0V.
According to an embodiment of the present invention illustrating the method of programming the memory cell, the first substrate voltage is 0V.
According to an embodiment of the present invention illustrating the method of programming the memory cell, the second substrate voltage is 0V.
By way of reducing the gate voltage of a second bit during the programming operation, the programming efficiency of the second bit is then reduced, thus leading to a tighter Vt distribution curve and a reduction of the issue of the over-programming.
In order to the make the aforementioned and other objects, features and advantages of the present invention comprehensible, several embodiments accompanied with figures are described in detail below.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are schematic views illustrating a programming operation performed on a conventional 2 bits/cell non-volatile memory cell.
FIGS. 2A and 2B are views illustrating a voltage distribution of the conventional 2 bits/cell non-volatile memory cell.
FIGS. 3A and 3B are schematic views illustrating the programming operation performed on a 2 bits/cell non-volatile memory cell according to a first embodiment of the present invention.
FIGS. 4A and 4B are schematic views illustrating the programming operation performed on the 2 bits/cell non-volatile memory cell according to a second embodiment of the present invention.
FIG. 5 is a view illustrating the voltage distribution of the 2 bits/cell non-volatile memory cell of the present invention.



DESCRIPTION OF EMBODIMENTS
Through a method for self-adjusted voltage (SAV), the present invention reduces a gate voltage for programming a second bit. Thereby, a tighter Vt distribution curve can be obtained, and the issue of the over-programming can be further improved. In other words, a gate voltage for programming a second bit is less than a gate-voltage for programming a first bit in the present invention so as to improve the above issues.
Several embodiments are described in detail below to better illustrate the method provided by the present invention.
The First Embodiment
FIGS. 3A and 3B are schematic views illustrating a programming operation performed on a 2 bits/cell non-volatile memory cell according to a first embodiment of the present invention.
Referring to FIGS. 3A and 3B, a memory cell is firstly provided. The memory cell includes a substrate 202, a source 204, a drain 206, and a gate 240. The source 204 and the drain 206 are disposed in the substrate 202. The gate 240 is disposed on the substrate 202 between the source 204 and the drain 206. The gate 240 includes, sequentially from the substrate 202, an oxide layer 208, a nitride layer 211, another oxide layer 212, and a polysilicon layer 214.
As shown in FIG. 3A, a first gate voltage Vg1 is applied to the gate 220 when a first programming operation is performed on the memory cell, such that a channel under the gate 220 is opened. The first gate voltage Vg1 is, for example, 10V more or less. A first drain voltage Vd1 is applied to the drain 206, a first source voltage Vs1 is applied to the source 204, and a first substrate voltage Vsub1 is applied to the substrate 202. The first drain voltage Vd1, the first source voltage Vs1, and the first substrate voltage Vsub1 are, for example, 5V, 0V, and 0V, respectively. Based on the above, electrons in the channel region are moved from the source 204 to the drain 206, and the electrons are accelerated by an electric field in the channel to generate hot electrons. Thereby, the electrons enter the nitride layer 210 adjacent to a side of the drain 206 through a channel hot electron injection (CHEI) effect for storing a bit 216 (a first bit).
In addition, as shown in FIG. 3B, a second gate voltage Vg2 is applied to the gate 220 when a second programming operation is performed on the memory cell, such that the channel under the gate 220 is opened. The second gate voltage Vg2 is, for example, 9V more or less. A second drain voltage Vd2 is applied to the drain 206, a second source voltage Vs2 is applied to the source 204, and a second substrate voltage Vsub2 is applied to the substrate 202. The second drain voltage Vd2 is a constant value and is equal to the first drain voltage Vd1. The second drain voltage Vd2, the second source voltage Vs2, and the second substrate voltage Vsub2 are, for example, 5V, 0V, and 0V, respectively. Based on the above, the electrons in the channel region are moved from the drain 206 to the source 204, and the electrons are accelerated by the electric field in the channel to generate the hot electrons. Thereby, the electrons enter the nitride layer 210 adjacent to a side of the source 204 through a CHEI effect for storing a bit 218 (a second bit).
Particularly, the second gate voltage Vg2 is less than the first gate voltage Vg1. Thereby, under the bit 216 (the first bit) is already stored, the programming efficiency is reduced when the storage operation is performed on another bit 218 (the second bit). Accordingly, the issue of the over-programming is improved, and a tighter Vt distribution curve is obtained.
More specifically, the second gate voltage Vg2 is, for example, 0.1 times of the first gate voltage Vg1 in the above embodiment. In the present invention, the difference between the second gate voltage and the first gate voltage is larger than 0, less than or equal to 0.15 times of the first gate voltage.
The Second Embodiment
Please refer to FIGS. 4A and 4B which are schematic views illustrating a programming operation performed on the 2 bits/cell non-volatile memory cell according to a second embodiment of the present invention. The programming method of the second embodiment is similar to that of the first embodiment. The main difference lies in that the first drain voltage Vd1 is equal to the second source voltage Vs2, and the first drain voltage Vd1 is increased using a step-by-step manner. The first drain voltage Vd1 and the second source voltage Vs2 are predetermined as 5V, for example, and later increased to 7V with time.
Likewise, the method provided by the present embodiment is also likely to improve the issue of the over-programming and to obtain a tighter Vt distribution curve.
According to the second embodiment, the method for performing the programming operation on the 2 bits/cell memory cell is by the manner of enabling the second gate voltage Vg2 to be less than the first gate voltage Vg1 and of equalizing the first drain voltage Vd1 and the second source voltage Vs2. Using the way of enabling the second gate voltage Vg2 to be less than the first gate voltage Vg1, the issue of the over-programming resulting from the cross interference of two bits in one memory cell disclosed in the related art is reduced, and a tighter Vt distribution curve is obtained as well.
The Third Embodiment
The programming method of the third embodiment is similar to that of the first embodiment. The main difference lies not only in that the second gate voltage Vg2 is less than the first gate voltage Vg1, but also in that the second source voltage Vs2 is less than the first drain voltage Vd1. And, the first drain Vd1 and the second source voltage Vs2 are at a constant value.
The Fourth Embodiment
The programming method of the fourth embodiment is similar to that of the third embodiment. The main difference lies not only in that the second gate voltage Vg2 is less than the first gate voltage Vg1, but also in that the second source voltage Vs2 is less than the first drain voltage Vd1. And, the first drain Vd1 and the second source voltage Vs2 are increased using a step-by-step manner.
According to the third and the fourth embodiments, the method for performing the programming operation on the 2 bits/cell memory cell is by the manner of enabling the second gate voltage Vg2 to be less than the first gate voltage Vg1 and the second source voltage Vs2 to be less than the first drain voltage Vd1. Said method can also reduce the programming efficiency of the second bit, improve the issue of the over-programming, and obtain a tighter Vt distribution curve.
According to other embodiments, the programming method of the present invention includes performing the storage operation on the 2 bits/cell memory by the manner of enabling a pulse width of the second gate voltage Vg2 to be less than that of the first gate voltage Vg1. In addition, the programming method of the present invention includes performing the storage operation on the 2 bits/cell memory by the same way of enabling the pulse width of the second gate voltage Vg2 to be less than that of the first gate voltage Vg1 and of enabling the pulse width of the second source voltage Vs2 to be less than that of the first drain voltage Vd1. Likewise, said method can reduce the programming efficiency of the second bit, improve the issue of the over-programming, and obtain a tighter Vt distribution curve.
Please refer to FIG. 5 which is a view illustrating a voltage distribution of the 2 bits/cell non-volatile memory cell of the present invention. The voltage distribution shown in FIG. 5 is an experimental result obtained by performing the programming operation according to the first embodiment. A curve 510 is the programming Vt distribution curve of the first bit. A curve 520 is the programming Vt distribution curve of the second bit. It can be learned from FIG. 5 that the method of the present invention can reduce the programming efficiency of the second bit, obtain a tighter Vt distribution curve, and further improve the issue of the over-programming.
Certainly, the method of the present invention can be applied not only to the 2 bits/cell non-volatile memory cell, but also to a 4 bits/cell, 8 bits/cell, or other multi-level non-volatile memory cells.
In summary, through a method for self-adjusted voltage (SAV), in other words, by way of reducing the gate voltage of the second bit during the programming operation, the programming efficiency of the second bit is then reduced, thus leading to a tighter Vt distribution curve and a reduction of the issue of the over-programming.
Although the present invention has been disclosed above by the embodiments, they are not intended to limit the present invention. Anybody skilled in the art can make some modifications and alteration without departing from the spirit and scope of the present invention. Therefore, the protecting range of the present invention falls in the appended claims.

## Claims

1. A method of programming a memory cell including a substrate, a drain and a source disposed in the substrate, and a gate disposed on the substrate between the drain and the source, wherein the gate comprises, sequentially from the substrate, a first oxide layer, a nitride layer, a second oxide layer and a polysilicon layer, the method comprising:
performing a first programming operation, comprising applying a first gate voltage to the gate, applying a first drain voltage to the drain, applying a first source voltage to the source, and applying a first substrate voltage to the substrate, so as to inject electrons into the nitride layer adjacent to a side of the drain through a channel hot electron injection (CHEI) effect; and
performing a second programming operation, comprising applying a second gate voltage to the gate, applying a second drain voltage to the drain, applying a second source voltage to the source, and applying a second substrate voltage to the substrate, so as to inject the electrons into the nitride layer adjacent to a side of the source through a CHEI effect,
wherein the second gate voltage is less than the first gate voltage.

2. The method of claim 1, wherein the first drain voltage is equal to the second source voltage, and the first drain voltage is at a constant value.

3. The method of claim 1, wherein the first drain voltage is equal to the second source voltage, and the first drain voltage is increased using a step-by-step manner.

4. The method of claim 1, wherein the second source voltage is less than the first drain voltage.

5. The method of claim 4, wherein the first drain voltage and the second source voltage are at a constant value.

6. The method of claim 4, wherein the first drain voltage and the second source voltage are increased using a step-by-step manner.

7. The method of claim 1, wherein a pulse width of the second gate voltage is less than that of the first gate voltage.

8. The method of claim 7, wherein a pulse width of the second source voltage is less than that of the first drain voltage.

9. The method of claim 1, wherein the difference between the second gate voltage and the first gate voltage is larger than 0, less than or equal to 0.15 times of the first gate voltage.

10. The method of claim 1, wherein the first source voltage is 0V.

11. The method of claim 1, wherein the second drain voltage is 0V.

12. The method of claim 1, wherein the first substrate voltage is 0V.

13. The method of claim 1, wherein the second substrate voltage is 0V.