Memory devices, such as random access memory (RAM), read-only memory (ROM), non-volatile memory (NVM) and like, are known in the art. These devices typically provide an indication of the data stored therein by providing a readout electrical signal. A sense amplifier may be used for detecting the signal and determining the logical content of the memory cell being read.
In general, sense amplifiers determine the logical value stored in a cell by comparing the output signal of the cell with a reference level. If the output signal is above the reference level, the cell is determined to be in one state, e.g., erased, and if the output is below the reference level, the cell is determined to be in another state, e.g., programmed. The reference level is typically set as a voltage or current level between the expected erased and programmed output signal voltage or current levels, and is sufficiently far from both output signal levels, so that noise on the output signal of the memory cell being read will not produce false results. As an example, the expected memory cell output signal for its erased and programmed states may be 150 mV and 50 mV, respectively, and the reference level may be 100 mV. In this example, if different sources of noise sources sum up and generate a noise signal of about 50 mV or more then the cell may be erroneously read. To overcome such case, the noise signal generators should be suppressed or the margin between the erased and programmed state signals should be increased.
In a virtual ground (VG) array, the readout of a cell may depend on the state of its neighbor cells. Therefore, a change in the state of the neighbors of a cell may affect the cell readout reliability. This undesired effect is known in the art as the “neighbor effect” (NE). The NE will be better understood by reference to FIG. 1 which is a block diagram of NVM cells in a virtual ground (VG) array. Cell MC3 in FIG. 1 is verified to be in a specific state, e.g., programmed or erased. When MC3 is read, the signal developed at the reading node, for example, either in the drain side or in the source side of the cell, has two components: the current of the cell itself, and the current flowing to or from neighbor cells, depending if cell MC3 is read from the source or from the drain. Neighbor cells may share the same word line of the cell being read and may be connected, either directly or through other cells, to the reading node. For example, in the configuration of FIG. 1, cells MC2 and MC4 are adjacent to cell MC3. When one or more of the neighbor cells changes from an erased state to a programmed state or vice versa, the read-out current from MC3 may exhibit a different signal at the reading node, because the current component of its neighbor cells has changed.
Thus, for example, if cell MC3 is read out from its drain side (shown in FIG. 1) then after the state of MC2 is changed from an erased state to a programmed state, its current “contribution” to the read-out current of MC3 may change, and therefore, the read-out for MC3 may exhibit a different signal at the reading node. A similar effect may happen, for example, after the state of MC4 is likewise changed for the case of source-side read. If farther cells along the same word line change their state (e.g. MC1, MC5, MC6, etc., not shown in FIG. 1) they may also affect the readout of MC3. The influence on MC3 of such changes in the states of cells MC1, MC2, MC4, MC5 and MC6 may not necessarily be of equal magnitude and may depend on the readout scheme, i.e. drain-side read or source-side read.
A memory cell, such as MC3 in FIG. 1, may be read from its drain side or its source side, i.e. the cell current or voltage signal may be sensed or derived from either its drain or source terminals. The NE may reduce the margin of a memory cell causing it to be read incorrectly in either of drain-side or source-side readout schemes.
Several ways to reduce the NE have previously been proposed. One such suggestion to reduce the NE is, for example, to insert a voltage level that nearly equalizes the drain and source voltage of one or more of the neighbor cells. Such method is described in U.S. Pat. Nos. 6,351,415 and 6,510,082. However, since the reading node being sensed is transient, and therefore, its slope and level depend on data, process, and temperature factors, the drain-source voltage of the neighbor cells is typically non-zero, and some neighbor current usually exists, resulting in only partial NE reduction.
Accordingly, there is a need for an efficient and reliable method for reading cells in a memory array.