JP5408983B2 - Multilevel cell data reading method - Google Patents

Description

  The present invention relates to a data reading method for a multi-level cell capable of reading 2-bit data.

  In recent years, research on multi-level cells has been actively conducted in order to further increase the degree of integration of nonvolatile semiconductor memory devices. As disclosed in Patent Document 1, this multi-level cell has one of a plurality of threshold voltages and can read 2-bit data with one memory cell.

  FIG. 3 is a diagram illustrating a relationship between a threshold voltage of a conventional multilevel cell, read data, and a data read voltage applied to the multilevel cell. Referring to this figure, a conventional multilevel cell data read method will be described. As threshold voltages of a multilevel cell, a first threshold voltage Vt1, a second threshold voltage Vt2, a third threshold voltage Vt3, A fourth threshold voltage Vt4 is provided. On the other hand, the data read voltage includes a first level L1 corresponding to the level between the first threshold voltage Vt1 and the second threshold voltage Vt2, and a second level corresponding to the level between the second threshold voltage Vt2 and the third threshold voltage Vt3. It has a level L2, and a third level L3 corresponding to the level between the third threshold voltage Vt3 and the fourth threshold voltage Vt4. The data read voltage is applied to the multi-level cell in a stepped manner in order from the lower first level L1.

Now, if the multi-level cell has the first threshold voltage Vt1, the multi-level cell is supplied with a current only when the data read voltage of the first level L1 is applied, and this state is a state where the data “11” is read. To do. Next, when the multi-level cell has the second threshold voltage Vt2, no current flows in the multi-level cell in a state where the data read voltage of the first level L1 is applied, and the data read voltage of the second level L2 is applied. Then, since a current flows, this state is a state where data “10” is read out. Next, when the multi-level cell has the third threshold voltage Vt3, the current does not flow in the state where the data read voltages of the first and second levels L1 and L2 are applied to the multi-level cell, and the data of the third level L3. Since a current flows when a read voltage is applied, this state is a state where data “00” is read. At this time, if the multi-level cell has the fourth threshold voltage Vt4, no current flows through the multi-level cell even when the data read voltage of the third level L3 is applied, and this state is a state where the data “01” is read. And In this way, the multilevel cell can read 2-bit data.
JP 2005-32431 A

  However, in the conventional data reading method as described above, it is necessary to always apply the data read voltage three times (first level L1, second level L2, and third level L3). There was a problem of long time.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a multilevel cell data reading method capable of shortening the reading time.

  The multilevel cell data read method of the present invention has first to fourth threshold voltages as threshold voltages of the multilevel cell, and when the second threshold voltage or the third threshold voltage is set in the multilevel cell, When the first level information is stored in a part of the address for selecting the multilevel cell and the first threshold voltage or the fourth threshold voltage is set in the multilevel cell, the second level information is set in the part of the address. When the data is read, the level of the information stored in a part of the address is determined. If the level is the first level, reading corresponding to the level between the second threshold voltage and the third threshold voltage is performed. A voltage is applied to determine the threshold voltage of the multi-level cell, read data is determined, and if the level is the second level, the read corresponding to the level between the first threshold voltage and the second threshold voltage Voltage, and applies a read voltage in order to determine the threshold voltage of the multi-level cell that third corresponding to a level between the threshold voltage and a fourth threshold voltage, and wherein the determining the read data.

  According to the present invention as described above, data is read by determining the threshold voltage of a multilevel cell by applying voltage up to two times while referring to information stored in a part of the multilevel cell selection address. Can do. Therefore, the reading time can be shortened.

  Hereinafter, embodiments of a data reading method for a multi-level cell according to the present invention will be described in detail with reference to the drawings. FIGS. 1A and 1B are diagrams for explaining an embodiment of the present invention, and show a relationship between a threshold voltage of a multilevel cell, read data, and a data read voltage applied to the multilevel cell. .

  As shown in this figure, the threshold voltages of the multi-level cell include a first threshold voltage Vt1, a second threshold voltage Vt2, a third threshold voltage Vt3, and a fourth threshold voltage Vt4 from the lowest. The first threshold voltage Vt1 corresponds to the data “11”, the second threshold voltage Vt2 corresponds to the data “10”, the third threshold voltage Vt3 corresponds to the data “00”, and the fourth threshold voltage Vt4. Corresponds to data “01”.

  The data read voltage has a first level L1, a second level L2, and a third level L3 from the lowest. The first level L1 corresponds to the level between the first threshold voltage Vt1 and the second threshold voltage Vt2, the second level L2 corresponds to the level between the second threshold voltage Vt2 and the third threshold voltage Vt3, and the third level L2 The level L3 corresponds to a level between the third threshold voltage Vt3 and the fourth threshold voltage Vt4.

  In the embodiment of the present invention, when one of the threshold voltages as described above is set in the selected multilevel cell and data is written, it is set in a part of the address for selecting the multilevel cell. Information is stored according to the threshold voltage.

  An address for selecting a multi-level cell has A0 to A7 bits. Since the A0 to A2 bits are assigned to the page address setting, the A3 bit is used for storing the information. If the second threshold voltage Vt2 or the third threshold voltage Vt3 is set as the threshold voltage of the multi-level cell, the “H” level is stored as the information in the A3 bit. When the first threshold voltage Vt1 or the fourth threshold voltage Vt4 is set as the threshold voltage of the multi-level cell, the “L” level is stored as the information in the A3 bit.

  Therefore, when the data read operation is started and a multilevel cell for reading is selected, the content of the A3 bit of the address for selecting the multilevel cell is determined. If the content of the A3 bit is “H” level, the threshold voltage set in the multi-level cell is the second threshold voltage Vt2 or the third threshold voltage Vt3, as shown in FIG. In addition, the data read voltage of the second level L2 corresponding to the level between the second threshold voltage Vt2 and the third threshold voltage Vt3 is applied to the multilevel cell, and the threshold voltage of the multilevel cell is determined. That is, if the threshold voltage of the multilevel cell is set to the second threshold voltage Vt2, and the current flows through the multilevel cell with the data read voltage of the second level L2, the threshold voltage of the multilevel cell is the second threshold voltage. Vt2 is determined, and the read data is determined to be “10”. Further, if the threshold voltage of the multi-level cell is set to the third threshold voltage Vt3 and no current flows through the multi-level cell even when the data read voltage of the second level L2 is applied, the threshold voltage of the multi-level cell is The third threshold voltage Vt3 is determined, and the read data is determined to be “00”.

  If the content of the A3 bit is “L” level in the content determination of the A3 bit, the threshold voltage set in the multi-level cell is the first threshold voltage Vt1 or the fourth threshold voltage Vt4. As shown in (b), the data read voltage at the first level L1 (corresponding to the level between the first threshold voltage Vt1 and the second threshold voltage Vt2) and the data read voltage at the third level L3 (third threshold voltage Vt3). And the fourth threshold voltage Vt4) are sequentially applied to the multilevel cell in a stepped manner, and the threshold voltage of the multilevel cell is determined. That is, if the threshold voltage of the multilevel cell is set to the first threshold voltage Vt1, and a current flows through the multilevel cell when the data read voltage of the first level L1 is applied, the threshold of the multilevel cell The voltage is determined to be the first threshold voltage Vt1, and the read data is determined to be “11”. If the threshold voltage of the multi-level cell is set to the fourth threshold voltage Vt4 and no current flows even when the data read voltage of the third level L3 is applied, the threshold voltage of the multi-level cell is the fourth threshold voltage. The voltage is determined to be Vt4, and the read data is determined to be “01”.

  As described above, according to the above method, the threshold voltage of the multilevel cell is determined by applying the voltage twice at the maximum in FIG. 1B while referring to the information stored in a part of the cell selection address. Data can be read. Therefore, the reading time can be shortened. Further, according to the above method, when the first threshold voltage Vt1 and the fourth threshold voltage Vt4 are determined when the content of the A3 bit is “L” level, the determination of the second threshold voltage Vt2 and the third threshold voltage Vt3 is performed. Therefore, even when the distribution of the second threshold voltage Vt2 and the third threshold voltage Vt3 is widened and the interval between the second threshold voltage Vt2 and the third threshold voltage Vt3 is narrowed, the threshold voltage is accurately determined. be able to. Furthermore, when the content of the A3 bit is “H” level and the determination of the second threshold voltage Vt2 and the third threshold voltage Vt3 is performed, since the data read voltage is applied only once at the second level L2, time Since the data read voltage can be set to an accurate level over time, the distribution of the second threshold voltage Vt2 and the third threshold voltage Vt3 is widened, and the interval between the second threshold voltage Vt2 and the third threshold voltage Vt3 is narrowed. Even so, it is possible to accurately determine the threshold voltage.

  FIG. 2 is a block diagram showing a circuit for realizing the data reading method as described above. In this figure, reference numeral 11 denotes a core portion, which comprises a memory cell array 12 having multi-level cells, a row decoder 13, a column decoder 14, and a sense amplifier 15. Reference numeral 16 denotes an address decoder, and 17 denotes a read voltage generation circuit. Addresses A0 to A7 are input to the address decoder 16. The address decoder 16 and the row decoder 13 and column decoder 14 select a multilevel cell for reading from the memory cell array 12. The address decoder 16 decodes the content of the A3 bit among the addresses A0 to A7 and outputs the decoded content to the read voltage generation circuit 17. Therefore, the read voltage generation circuit 17 generates only the data read voltage of the second level L2 or the stepwise data read voltage of the first level L1 and the third level L3 according to the contents of the input A3 bit. The data read voltage is supplied to the selected multi-level cell of the memory cell array 12. Therefore, the data read operation as in the above embodiment is executed.

The figure for demonstrating embodiment of the data read method of the multilevel cell of this invention, and is a figure which shows the relationship between the threshold voltage of a multilevel cell, read data, and the data read voltage applied to a multilevel cell. The block diagram which shows the circuit which implement | achieves the data reading method of embodiment of this invention. The figure which shows the threshold voltage of the conventional multilevel cell, read data, and the relationship of the data read voltage applied to a multilevel cell.

Explanation of symbols

Vt1 first threshold voltage Vt2 second threshold voltage Vt3 third threshold voltage Vt4 fourth threshold voltage L1 first level of data read voltage L2 second level of data read voltage L3 third level of data read voltage

Claims (1)

Having first to fourth threshold voltages as threshold voltages of the multi-level cell;
The first to fourth threshold voltages have a first threshold voltage, a second threshold voltage, a third threshold voltage, and a fourth threshold voltage from the lowest, and the first threshold voltage corresponds to data “11”. The second threshold voltage corresponds to data “10”, the third threshold voltage corresponds to data “00”, and the fourth threshold voltage corresponds to data “01”.
Wherein when setting the multi-level cell to the second threshold voltage or said third threshold voltage, stores the first level of information on a part of the address that selects the multi-level cell,
Wherein when setting the multi the level cell first threshold voltage or said fourth threshold voltage, stores the second level of information on a part of the address,
When reading data, the level of information stored in a part of the address is determined, and if the level is the first level, the read voltage corresponding to the level between the second threshold voltage and the third threshold voltage To determine the threshold voltage of the multi-level cell, determine the read data,
If the level is the second level, the first threshold voltage and a read voltage corresponding to a level between the second threshold voltage, and a read voltage corresponding third threshold voltage level between said fourth threshold voltage It is applied in order to determine the threshold voltage of the multi-level cell, the data reading method of a multi-level cell, which comprises determining the read data.

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