Abstract:
A method of modifying the threshold voltages of a plurality of non-volatile memory cells, for example, flash EEPROM memory cells, after an erasure operation, is described. In order to perform the equalization quickly and to optimize the use of the voltage supplies for biasing the columns, the method provides for the following steps: connecting all of the column lines to a voltage supply, monitoring the supply voltage, and applying, to all of the row lines, a voltage variable from a predetermined minimum value to a predetermined maximum value, the rate of change being regulated to maintain the supply voltage of the column lines at a substantially constant, predetermined value. The same method can be used for reliable and quick programming of a memory of the flash EEPROM type, or of another type.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to non-volatile semiconductor memories and, more particularly, to a method and to a circuit structure for modifying the threshold voltages of the cells of a memory.  
         BACKGROUND OF THE INVENTION  
         [0002]    A typical EEPROM memory is formed on a substrate of semiconductor material as a matrix of memory cells, each constituted by a body region with p-type conductivity in which two (source and drain) regions with n-type conductivity are formed, separated by a channel region. A floating-gate electrode is disposed above the channel region and is insulated therefrom by a thin layer of dielectric material. A control gate electrode extends above the floating gate electrode and is insulated therefrom by another layer of dielectric material.  
           [0003]    The cells of the matrix have their source regions connected together to a common terminal which, during programming and reading, is generally connected to the negative (ground) terminal of the supply of the integrated circuit of which the memory forms part. The drain regions of the cells of each column of the matrix are connected to one another by common connection lines, known as bit lines or column lines. The control gate electrodes of the cells of each line are connected to one another by common connection lines known as word lines or row lines. A memory structure of this type is known as a NOR-matrix memory.  
           [0004]    Each cell of the matrix can be selected by a row decoder and a column decoder. Once selected, the cell can be polarized by the application of suitable potentials to its terminals and its state can be investigated by means of a sense amplifier connected in series with the respective bit line.  
           [0005]    In order to write or program a cell of the memory, the bit line and the word line which identify it are brought to predetermined potentials higher than the common source potential, for example 5V and 9V, respectively, such as to bring about a flow of “hot electrons” through the thin dielectric layer from the substrate to the floating-gate electrode. The electrons which accumulate in the floating-gate electrode bring about an increase (of 2-4V) in the threshold voltage of the cell.  
           [0006]    In order to read a cell, the common source terminal is connected to ground, the bit line (drain) is brought to a positive potential, the word line (control gate) is brought to a positive potential higher than that of the bit line, and the drain current is measured by the sense amplifier. A non-programmed cell (logic level “1”) conducts a relatively high current (for example 50 μA) whereas a programmed cell (logic level “0”) conducts a considerably lower current.  
           [0007]    In order to erase a cell, a positive potential (for example, 5V) is applied to the common source terminal, a negative potential (for example, −8V) is applied to the word line (control gate), and the bit line (drain) is left floating. In these conditions, a strong electric field is developed between the floating-gate electrode and the source region so that the negative charge formed by the accumulated electrons is extracted from the floating-gate electrode by the tunnel effect. In a flash EEPROM memory, erasure takes place simultaneously for all of the cells of the matrix or for the cells of a selected section of the matrix.  
           [0008]    More particularly, in order to perform the erasure, a series of voltage pulses is applied to the cells so as to extract electrons from the floating gate by the tunnel effect. An operation is generally performed after each pulse to check the state of each individual cell in order to see whether it has reached the “1” state, that is, the erased cell state. In practice, the cell is biased for reading and its drain current is measured. If the current measured is below a predetermined value, which indicates that the conduction threshold has not been reached, a further erasure pulse is applied. These operations are repeated until the cell with the highest threshold within the matrix is recognized as an erased cell.  
           [0009]    Since the operation to erase a flash EEPROM memory is not selective, the cells which have already been recognized as erased continue to be subjected to erasure pulses. Since each erasure pulse involves the removal of electrons from the floating gate, the thresholds of these cells will continue to be lowered upon each pulse. When all of the cells have been recognized as erased, some of them will therefore have very low or even negative threshold voltages, and will thus behave as depletion transistors. A typical distribution of threshold voltages Vth of the cells after erasure is shown in FIG. 1 in which D.V. and E.V. indicate two predetermined voltage values, as will be explained below, and Log # indicates the logarithm of the number of cells of the memory or of the section in question.  
           [0010]    Cells with thresholds below a certain value are the cause of potential reading errors. In fact, it should be borne in mind that, in order to read a cell, positive voltages are applied both to the column and to the row which contain the cell, and the remaining rows are connected to ground. If the column line selected contains a cell which has been erased excessively until it has a threshold below zero, when the respective row line is grounded, the cell will have its gate electrode at zero and will therefore be conductive, thus potentially causing a false reading.  
           [0011]    The excessively erased cells also cause leakage currents during programming. In fact, for programming, positive voltages are also applied to a selected column and a selected row whilst the non-selected rows are connected to ground. An excessively erased cell in the selected column is also conductive when the respective row is not selected so that, in the selected column, there will therefore be a spurious current which may overload the supply of the column lines.  
           [0012]    To prevent the problems described above, the threshold of each erased cell must be below a maximum value generally known as the erase verify point, indicated E.V. in FIG. 1, but above a minimum value, generally referred to as the depletion verify point, indicated D.V. in FIG. 1. In other words, the condition that the threshold values of the erased cells of the memory should all be within a “range” of values delimited by the two verify points must be satisfied.  
           [0013]    With the tendency to design integrated circuits, and particularly memories, with ever lower supply voltages, the erase verify point falls to very low values. Moreover, since the precise breadth of the distribution of the threshold values of the cells upon completion of the erasure operation is difficult to predict and may in any case undergo variations during the life of the device (because of the variability of the parameters of the manufacturing process, of the erasure method selected, of the ageing state, that is, of the number of programming-erasure cycles to which the memory has been subjected, and of occasional anomalies in the conduction characteristics of the tunnel dielectrics), the condition indicated above is ever more difficult to satisfy.  
           [0014]    In order to bring the thresholds of the erased cells within the “range” of values delimited by the two verify points, a known method provides for “soft” programming operations after the erasure of the cells. In practice, the cells whose thresholds are below the depletion verify point are identified and are subjected to a programming operation, for example, by the application of voltage pulses of increasing amplitude to their gate electrodes, followed by a verification operation each time, until a threshold above the depletion verify point is recognized. Upon completion of the “soft” programming operations, the distribution of threshold voltages should be as shown in FIG. 2.  
           [0015]    The soft programming is performed automatically after an erasure operation by suitable control circuits. To ensure a high level of reliability of the devices, these circuits have to be designed so as to take into account process variability, the conditions of use of the device, and its ageing, so that the initial voltage selected for the corrective programming pulses is very low and the rate of increase of the pulses is prudently low. The time required for the correction operation is therefore quite long. During the design of the device, it is also necessary to take into account the most unfavorable conditions which may arise with regard to the supply, and it is therefore necessary for the voltage supplies, particularly those which supply the column lines, to have dimensions capable of handling the maximum load which can be expected. This generally involves making the voltage supplies, which are normally charge pumps, larger than is necessary for the average supply requirements. Finally, the identification of the cells with thresholds below the depletion verify point may be neither easy nor reliable, particularly in the presence of high leakage currents.  
         SUMMARY OF THE INVENTION  
         [0016]    According to principles of the present invention, a method and a circuit structure are provided for modifying the threshold voltages of the cells of a non-volatile memory quickly. Charge pumps and other related circuits are provided. Increased reliability of the device is achieved using the present invention.  
           [0017]    Aspects of the method and circuit structure of the present invention include a first voltage supply connected to column lines of a plurality of memory cells and a second voltage supply connected to row lines of the plurality of memory cells, the voltage supplies being connected through a feedback path. The output voltage of the first voltage supply is monitored by a sensing circuit, which in turn supplies a signal through the feedback path to an input of the second voltage supply. In one embodiment of the invention, the second voltage supply may be comprised of a variable voltage generator whose output voltage depends on the signal received from the sensing circuit. The method and circuit structure of the present invention provide a means for modifying quickly the threshold voltages of memory cells, such as after an erasure operation or during a programming operation, without excessively loading the first voltage supply or causing the output of the first voltage supply to drop below a predetermined value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention will be understood better from the following detailed description of a non-limiting embodiment thereof, given with reference to the appended drawings, in which:  
         [0019]    [0019]FIGS. 1 and 2 show two distributions of threshold voltages of the cells of a flash EEPROM memory immediately after erasure and after “soft” programming, respectively; and  
         [0020]    [0020]FIG. 3A is a block diagram of a circuit structure according to the invention.  
         [0021]    [0021]FIG. 3B is a more detailed block diagram of a circuit structure according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    [0022]FIG. 3A illustrates a general block diagram of a memory circuit  8  according to principles of the present invention. The memory circuit  8  includes a first voltage supply circuit  10 , and a second voltage supply circuit  14  for providing power to the memory array  20  on the same integrated circuit chip.  
         [0023]    The memory array  20  has a plurality of memory cells  26  accessed by column lines  22  and row lines  24 . The output of the first voltage supply circuit  10  is provided to a column decoder circuit  12 . The column decoder circuit  12  contains the appropriate circuitry and input terminals for receiving the address and decoding the address for a particular column to be accessed within a memory array. Included within the column decoder circuit  12  is also the appropriate circuitry and connection for providing the correct voltage at the appropriate timing sequence on each of the respective column lines  22 , the details of which are not shown because these are well known in the art. The voltage output of the first voltage supply circuit  10  is therefore provided, through the column decoder circuitry  12 , to the column lines  22 .  
         [0024]    The row lines  24  are driven by the row decoder circuit  16 . The row decoder circuit  16  contains the appropriate address input terminals and circuitry for properly addressing and putting the correct voltage supply on the proper row line  24 . The second voltage supply circuit  14  is coupled to the row decoder  16  for providing the appropriate voltage level to be placed on the row lines  24  (the details of which are known in the art and therefore not shown).  
         [0025]    A sensing circuit  18  has its input line  19  coupled to the output of the first voltage supply circuit  10 . The sensing circuit  18  receives the output of the first voltage supply circuit at its input  19 . The sensing circuit  18  provides at an output line  21  a signal which corresponds to the value of the input signal on input line  19 . The signal on the output line  21  is received by the second voltage supply circuit  14  and is used to modify its output voltage level to the row decoder circuit  16 . The sensing circuit  18  can be any acceptable sensing circuit for recognizing the output of the first voltage supply circuit  10 . In one embodiment, it is a voltage sensing circuit which senses the value of the voltage and provides, on its output, a signal proportional to the value of the voltage which has been sensed. Alternatively, it may sense the current output of the first voltage supply circuit, or some other parameter which is indicative of the power being provided to the column lines  22 . For those memories whose column lines are current driven rather than voltage driven, a current sensing circuit for circuit  18  would be more applicable while in other memories a voltage sensing circuit is preferred.  
         [0026]    The output signal on line  21  may be of any acceptable type, for example, it may be a digital signal having the appropriate number of bits for providing the input to the second supply circuit  14 . Alternatively, it may be an analogue voltage signal, current signal, or any other acceptable signal which provides an indication of the power being output by the first voltage supply circuit  10 . One particular embodiment of the inventive circuit will be described in more detail with respect to FIG. 3B.  
         [0027]    In FIG. 3B, the memory circuit  8  includes a block NOR-MX  20  that represents a matrix of flash EEPROM memory cells  26  having their respective gate electrodes connected to one another by row lines, and their respective drain electrodes connected to one another by column lines. The source electrodes of the cells are connected together to a common terminal, not shown. A column decoder C-DEC  12  contains controllable connection means, not shown, for example, transistors having the function of electronic switches, for connecting the column lines to a column polarizing voltage supply. All or some of the column lines or individual column lines can be selected for connection. The column supply is represented in the drawing by a charge pump CH-P  30  connected to a constant-voltage supply Vdd, and by a voltage regulator V-REG  32 . A line decoder R-DEC contains controllable connection means, not shown, for connecting the row lines to a row polarizing voltage supply. It is also possible to select all or some of the row lines or individual row lines for connection. The row supply is represented by a ramp voltage generator R-GEN  36  with a regulator R-REG  34  for regulating the rate of voltage increase. A voltage sensor V-SEN  18  is connected to the voltage regulator V-REG  32  in order to monitor its output voltage and to supply a corresponding signal to the rate of increase regulator R-REG  34 . A stepped voltage generator may be used instead of the ramp voltage generator.  
         [0028]    When the memory is in operation, first of all, the actual erasure of the matrix NOR-MX or of a section thereof defined by a predetermined number of rows and by a predetermined number of columns is performed. For example, all of the cells are erased, that is, are brought to the “1” state as described above. In order to equalize the threshold voltages of the erased cells, according to the invention, all of the rows or a predetermined number of rows and all of the columns or a predetermined number of columns are selected and simultaneously connected to the respective supplies. For simplicity of description, it is assumed that all of the rows and all of the columns are selected and biased.  
         [0029]    The output voltage of the ramp generator R-GEN  36 , applied to the row lines at the beginning of the equalization treatment, is set at a predetermined minimum value at the design stage. This minimum voltage is selected so as to be below the minimum threshold voltage which can be expected for the erased cells and, in any case, low enough to ensure that the total current absorbed by the cells in these initial polarization conditions, that is, the sum of the leakage currents of the cells, is not such as to load the column supply beyond its capacity. In other words, it is ensured that the total current absorbed does not cause a decrease in the column biasing voltage below a predetermined value and, more precisely, that value required for correct biasing at the programming stage.  
         [0030]    The maximum rate of variation of the row voltage, that is, the maximum slope of the ramp of the generator R-GEN  36 , is also set at the design stage. In particular, it may be selected so as to be very high, for example, 1 V/μs, in view of the fact that the true rate of change is regulated continuously in order to be adapted to contingent requirements, as described below.  
         [0031]    As the ramp voltage, and with it the gate voltage of the cells, gradually increases, the cells which have lower threshold voltages are subjected to the same process as is used in the programming stage, that is, their floating-gate electrodes are enriched with electrons so that their threshold voltages are increased. The number of cells with thresholds which are too low is thus gradually reduced, and the overall leakage current is also reduced therewith.  
         [0032]    The sensing circuit V-SEN  18  detects the output voltage of the voltage regulator V-REG  32  and supplies a corresponding signal to the rate of increase regulator RREG  34  which in turn determines the effective slope of the ramp voltage of the ramp generator R-GEN  36 . The effect of the feedback loop is that the output voltage of the column supply is kept constant by varying the rate of increase of the row voltage based on the overall current-absorption of the cells.  
         [0033]    Thus, the rate of increase of the row voltage may reach a maximum possible rate which is compatible with the current-output capacity of the charge pump.  
         [0034]    The equalization process terminates when the ramp voltage reaches a maximum value which is set at the design stage. At this point, the threshold voltages of the cells will be distributed within a very narrow band of values disposed below the erase verify level.  
         [0035]    The method according to the invention thus enables the row voltage to be adapted automatically to the capacity of the charge pump used for biasing the columns and, indirectly, to the threshold distribution of the cells after erasure. This enables devices to be designed with smaller charge pump dimensions and, thus, with considerable savings in area in comparison with the prior art. The method also at the same time ensures an increased speed of equalization of the threshold voltages of the cells. In this connection, it should also be noted that the minimum voltage of the ramp generated can be selected so as to be very low, in order to bring all of the cells within the predetermined range, without this appreciably affecting the equalization time. This contrasts with conventional methods in which the minimum starting voltage is one of the main parameters which affects the time taken to perform the operation.  
         [0036]    If the maximum ramp voltage is selected so as to be fairly low to avoid the risk of any cell being programmed, that is, brought to zero logic value, the equalization process may require an additional, selective treatment. For example, conventional threshold regulation operations may be used, that is, the cells may be selected individually and suitable corrections may be applied to each of them, as explained at the beginning. In this case, however, there will be neither the problems of identifying the cells or of parasitic absorption in the column lines, nor the other problems described above with reference to the prior art, since the distribution of the threshold voltages is already sufficiently narrow.  
         [0037]    Embodiments of the invention has been described and illustrated, clearly many modifications are possible within the scope of the same inventive concept. For example, instead of applying the method described to all of the cells of a matrix together, it could be applied to subsets of cells or to sections of the matrix in succession; in this case, the dimensions of the subsets could be selected automatically on the basis of the effective capacity of the column-polarization voltage supply.  
         [0038]    Moreover, the invention may be used not only with a two-level flash EEPROM memory such as that described, but with any other memory that can be used in those types in which erasure takes place simultaneously for all the cells or for blocks of cells or other types as appropriate. It is also applicable, both for threshold equalization after erasure and for actual programming of the memory cells, to all memory in which the current required for the programming is controllable by means of the gate voltages of the cells.  
         [0039]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.