KR20050002091A - Method for measuring a threshold voltage of flash memory device - Google Patents

Abstract

PURPOSE: A method for measuring threshold voltage of a flash memory device is provided to measure the threshold voltage of NAND flash memory device without using PMOS transistor by sensing the threshold voltage as a relative value. CONSTITUTION: A method for measuring threshold voltage of a NAND flash memory device having plural strings consisting of a drain select transistor(T401), plural flash memory cells(C1-Cn), and a source select transistor(T402) in series, comprises the steps of: increasing threshold voltage equally to higher than 0V in every elements of a memory cell array; increasing read operation voltage as much as the threshold voltage and applying that to the elements; sensing the current in a bit line as elevating word line voltage from 0V; stopping the elevating of word line voltage when there's a current flow in the bit line, measuring the threshold voltage of the NAND flash memory cell with the word line voltage.

Description

Method for measuring a threshold voltage of a flash memory device {Method for measuring a threshold voltage of flash memory device}

The present invention relates to a threshold voltage measuring method of a flash memory device, and more particularly, to a threshold voltage measuring method of a flash memory device capable of measuring a threshold voltage of a flash memory cell in a program state in a NAND type flash memory device.

A NAND flash memory device includes a memory cell array for storing data and a peripheral circuit portion for supplying operating voltages of various potentials to the memory cell array. On the other hand, since no negative voltage is required to drive the NAND flash memory device, a PMOS transistor is not manufactured to simplify the process in the process of manufacturing the NAND flash memory device. For this reason, there is no circuit for applying a negative voltage to the flash memory device.

1 is a characteristic graph illustrating a threshold voltage distribution of a flash memory cell according to a program or erase state.

Referring to FIG. 1, a threshold voltage of a cell programmed in a NAND flash memory device has a positive potential value, and a threshold voltage of an erased cell has a negative potential value. The threshold voltage of the flash memory cell is detected by a read operation to determine data stored in the flash memory cell. A brief description of the read operation is as follows.

2 is a circuit diagram illustrating a read operation of a flash memory device.

Referring to FIG. 2, first, a power supply voltage Vcc is applied to a corresponding drain select line DSL so that a string connected to a flash memory cell for reading stored data is selected during a read operation. The source select transistor T102 is operated by operating T101 and applying a power supply voltage Vcc to the source select line SSL. A predetermined voltage (for example, about 1 V) is applied to the bit line of the selected string. In addition, a voltage of 0 V is applied to the word line of the cell Cs selected for the read operation among all the cells C1 to Cn, and a pass voltage (for example, a current may flow from the drain to the source to the remaining cells not selected). For example, about 4.5) is applied. In this state, the current of the bit line is sensed, and if a current flows in the bit line, the corresponding cell is in an erased state and is determined to be a program state if no current flows.

On the other hand, in the process of detecting the threshold voltage of the cell under the above conditions, since a predetermined pass voltage is applied to the word line of the unselected cells, an electric field is applied to the tunnel oxide film, which results in the floating gate from the channel region of the substrate to the floating gate. Tunneling of the electrons occurs. The amount of electrons tunneled is very small, but as the read operation is repeated, the amount accumulates, and as shown in FIG. 3, the threshold voltage of the erase cell may approach 0V. If the read operation is repeated and the threshold voltage of the erase cell approaches 0V, an error may occur because the cell does not operate in the erase state. Therefore, a case may be required in which the threshold voltage of the erase cell needs to be measured.

In general, after the program operation, the threshold voltage of the programmed cell may be measured using an NMOS transistor. However, in the case of the NAND flash memory device, since the PMOS transistor is not formed, the threshold voltage of the erased cell cannot be measured after the erase operation is performed. Accordingly, in the case of the erase cell, it is difficult to detect a change in the threshold voltage, which may cause an error and deteriorate the reliability of the circuit.

In contrast, in the method of measuring a threshold voltage of a flash memory device according to the present invention, in a read operation for sensing a threshold voltage of an erase cell, the threshold voltage of the erase cell is predetermined in a well in which an array of flash memory cells is formed such that the threshold voltage of the erase cell is higher than at least 0V. With the voltage applied, all operating voltages applied for read operation are increased by the voltage applied to the well, so that the threshold voltage of the erase cell is sensed as a relative value in the voltage range higher than 0V, thereby eliminating the need for NAND without using a PMOS transistor. In the flash memory device, the threshold voltage of the erased cell may be sensed as a relative value, thereby improving circuit reliability.

1 is a characteristic graph illustrating a threshold voltage distribution of a flash memory cell according to a program or erase state.

2 is a circuit diagram illustrating a read operation of a flash memory device.

3 is a characteristic graph illustrating a change in threshold voltage of a flash memory device according to a read operation.

4 is a circuit diagram illustrating a threshold voltage measuring method of a flash memory device according to an exemplary embodiment of the present invention.

In the method of measuring a threshold voltage of a flash memory device according to an embodiment of the present invention, the threshold voltages of the devices are uniformly increased such that the threshold voltages of all devices included in the memory cell array are higher than 0V, and the threshold voltages are increased. Increasing the read operation voltage to the devices; sensing whether a current flows through the bit line while increasing the word line voltage from 0 V to the word line of the flash memory cell to which the threshold voltage is to be sensed; and Stopping the increase of the word line voltage when current flows and measuring the threshold voltage of the flash memory cell with the word line voltage.

In the above, the threshold voltages of the devices may be increased by applying a channel suppression voltage to the well in which the memory cell array is formed. At this time, the channel suppression voltage is applied such that the lowest threshold voltage among the threshold voltages of the devices becomes higher than 0V.

On the other hand, the step of raising and applying the operating voltage may include selecting a string by applying a voltage that is increased as the threshold voltage is increased to the drain select line and the source select line of the string to which the flash memory cell is connected, and the bit of the string. Applying the increased bit line voltage to the line as the threshold voltage is increased, and increasing the threshold voltage so that current flows from the drain to the source in the word line of the flash memory cell, which is not the threshold voltage measurement target among the devices. Applying an elevated pass voltage.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various forms, and the scope of the present invention is not limited to the embodiments described below. Only this embodiment is provided to complete the disclosure of the present invention and to fully inform those skilled in the art, the scope of the present invention should be understood by the claims of the present application.

On the other hand, when a film is described as being "on" another film or semiconductor substrate, the film may exist in direct contact with the other film or semiconductor substrate, or a third film may be interposed therebetween. In the drawings, the thickness or size of each layer is exaggerated for clarity and convenience of explanation. Like numbers refer to like elements on the drawings.

4 is a circuit diagram illustrating a threshold voltage measuring method of a flash memory device according to an exemplary embodiment of the present invention.

Referring to FIG. 4, in a NAND flash memory device, a flash memory array is composed of a plurality of strings (only one is shown in the figure). Here, each string includes a drain select transistor T401 connected in series, a plurality of flash memory cells C1 to Cn, and a source select transistor T402. Here, the drain of the drain select transistor T401 is connected to the bit line, and the source of the source select transistor T402 is connected to the common source line.

In the above configuration, while a voltage of 0 V is conventionally applied to a well in which a flash memory array is formed, the present invention suppresses a predetermined channel in a well so that a threshold voltage of an erased flash memory cell (for example, Cs) is higher than 0 V. Apply voltage.

More specifically, the channel suppression voltage is applied to the well so that the channel is suppressed from being formed in the channel region of the flash memory cell and a higher voltage is applied to the word line so that the channel is formed. As a result, the threshold voltages of all the flash memory cells C1 to Cn and the turn-on voltages of the select transistors T401 and T402 are increased by the channel suppression voltage applied to the well. Therefore, when the well applies the channel suppression voltage, the operating voltages applied to the flash memory cells C1 to Cn and the select transistors T401 and T402 must be increased by the channel suppression voltage.

A method of measuring the threshold voltage of an erase cell under the above conditions will be described below.

First, when a threshold voltage of an erase cell having a negative voltage threshold is measured, a string connected to a flash memory cell for reading stored data is selected. The string is selected when a voltage is applied to the corresponding drain select line DSL and the source select line SSL so that the drain select transistor T401 is turned on. In this case, since the channel suppression voltage is applied to the well, the voltage of the drain select line DSL and the source select line SSL is generally increased by the channel suppression voltage applied to the well (for example, the voltage applied during the read operation). , About 7V) to turn on. In addition, a voltage (for example, about 5 V) that is increased by a channel suppression voltage is applied to the bit line of the selected string. In addition, a pass voltage (for example, about 10 V) that is increased by a channel suppression voltage is applied to a word line of a flash memory cell that is not a threshold voltage measurement target among all the cells C1 to Cn so that current flows from the drain to the source. do.

In this state of increasing the threshold voltage of the cell above 0V, it is detected whether a current flows through the bit line while increasing the word line voltage of the cell Cs to sense the threshold voltage from 0V. Continue to increase the word line voltage of the cell and stop increasing the word line voltage when current is detected on the bit line. In this case, the word line voltage is the threshold voltage of the cell in which the channel suppression voltage is applied, and the voltage obtained by subtracting the channel suppression voltage from the word line voltage is the threshold voltage of the actual cell in which the channel suppression voltage is not applied. As a result, the threshold voltage of the erase cell may be measured in a voltage range higher than 0V.

As described above, the present invention is a state in which a predetermined voltage is applied to a well in which an array of flash memory cells is formed such that the threshold voltage of the erase cell is higher than at least 0V during a read operation for sensing the threshold voltage of the erase cell. By increasing all the operating voltages applied for the read operation by the voltages applied to the wells, the threshold voltages of the erase cells are sensed relative to each other in the voltage range higher than 0V, thereby eliminating cells even in a NAND flash memory device without using a PMOS transistor. The threshold voltage of can be sensed as a relative value to improve the reliability of the circuit.

Claims (5)

Uniformly raising the threshold voltages of the devices so that the threshold voltages of all devices included in the memory cell array are higher than 0V; Increasing a read operation voltage by increasing the threshold voltage and applying the read voltage to the devices; Sensing whether a current flows in the bit line while increasing the word line voltage from 0 V to the word line of the flash memory cell to which the threshold voltage is to be sensed; Stopping the increase of the word line voltage when a current flows in the bit line, and measuring the threshold voltage of the flash memory cell using the word line voltage. The method of claim 1, A threshold voltage measuring method of a flash memory device, wherein the memory cell array is a NAND type flash memory cell array. The method of claim 1, And applying a channel suppression voltage to a well in which the memory cell array is formed to increase the threshold voltages of the devices. The method of claim 3, wherein The threshold voltage measuring method of the flash memory device to which the channel suppression voltage is applied such that the lowest threshold voltage of the threshold voltages of the devices is higher than 0V. The method of claim 1, wherein the increasing of the operating voltage is performed by: Selecting the string by applying a voltage increased by the threshold voltage to a drain select line and a source select line of a string to which the flash memory cell is connected; Applying an increased bit line voltage to the bit line of the string by increasing the threshold voltage; And A threshold voltage of a flash memory device includes applying a pass voltage that is increased by increasing the threshold voltage to a word line of a flash memory cell that is not a threshold voltage measurement target among the devices so that current flows from a drain to a source. How to measure.