Abstract:
Nonvolatile memory devices support programming and verify operations that improve threshold-voltage distribution within programmed memory cells. This improvement is achieved by reducing a magnitude of the programming voltage steps and increasing a duration of the verify operations once at least one of the plurality of memory cells undergoing programming has been verified as a “passed” memory cell. The nonvolatile memory device includes an array of nonvolatile memory cells and a control circuit, which is electrically coupled to the array of nonvolatile memory cells. The control circuit is configured to perform a plurality of memory programming operations (P) by driving a selected word line in the array with a first stair step sequence of program voltages having first step height (e.g., ΔV 1 ) and then, in response to verifying that at least one of the memory cells coupled to the selected word line is a passed memory cell, driving the selected word line with a second stair step sequence of program voltages having a second step height (e.g., ΔV 2 ) lower than the first step height.

Description:
REFERENCE TO PRIORITY RELATED APPLICATION  
       [0001]     This patent application claims priority under 35 U.S.C. § 119 to Korean Patent Application 2005-55906 filed on Jun. 27, 2005, the entire contents of which are hereby incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to integrated circuit memory devices and, more particularly, to flash memory devices and methods of operating flash memory devices  
       BACKGROUND OF THE INVENTION  
       [0003]     Flash memory devices are a type of nonvolatile semiconductor memory device. Flash memory devices are regarded as being highly adaptable to large storage capacities and high-frequency operation in applications requiring large-capacity storage devices and coded memories for mobile apparatuses. Flash memory devices are generally classified into NAND and NOR types by the logical pattern of the cell array. The memory cell array of the NOR-type flash memory device is structured such that a plurality of memory cells are arranged as being connected to a single bitline in parallel. In contrast, the NAND-type flash memory device has a memory cell array in which a plurality of memory cells are connected in series from a single bitline. The NOR-type flash memory devices are widely used in applications requiring high-frequency operations because they are operable with high speed in programming and reading operations, relative to the NAND-type flash memory devices. Data values within the flash memory device are defined by the threshold voltages of the memory cells, in which programming operations are carried out by changing the cell threshold voltages. It is now conventional to regulate the cell threshold voltages by the ISPP scheme during programming operations.  
         [0004]      FIG. 1  illustrates the pulses of programming and verifying voltages applied to a wordline of a memory cell during a programming operation with the ISPP scheme in a general flash memory. As shown in  FIG. 1 , in a typical cycle of the ISPP scheme, a unit step of incrementing a program voltage is confined to a ΔV value and a unit verifying time is fixed to Δt. Such fixed ranges for the program voltage step and the unit verifying time are contrary to achieving narrow distribution profiles of cell threshold voltages in multi-level cell (MLC) arrangements. In detail, the fixed range of the incremental program voltage step raises the probability of shifting cell threshold-voltage distribution profiles upward from a verifying voltage Vveri, after the programming operation for those memory cells that have threshold voltages near to the verifying voltage Vveri. This effect arises when the incremental step of the program voltage is excessively high to inject the appropriate amount of hot electrons into a floating gate. The threshold-voltage distribution profiles of memory cells can be improved as the program voltage step becomes lower. However, a too low level of the program voltage step causes the threshold voltages to be reduced in shifting width so much and the number of programming loops to increase thereby, resulting in a longer overall programming time. While a reduction of the program voltage step is effective in improving the threshold-voltage distribution profiles of memory cells, it is inevitable that this method causes a loss in operating speed. To the contrary, an increase in the program voltage step would incur a heavy loss against the threshold-voltage distribution profiles, but it accelerates a programming speed.  
         [0005]     The problems due to the fixed verifying time are generated when the threshold voltages of memory cells are close in value to the verifying voltage Vveri. In general, sense amplifiers employed in the flash memory devices detect and amplify drain voltages discharged while the verifying voltage is being applied to wordline. If a threshold voltage of memory cell is higher than the verifying voltage Vveri, the memory cell is detected as an off-cell. However, if a threshold voltage of a memory cell is lower than the verifying voltage Vveri, the memory cell is detected as an on-cell. However, when there is only a small difference between a cell threshold voltage and the verifying voltage, (i.e., when the threshold voltage is positioned around an intermediate level between the on-cell and off-cell) a sensing time interval may need to become substantial to obtain accurate verification of the program state of a cell. If a memory cell undergoes an insufficiently long verification operation, then it may be judged as a passed cell even though its threshold voltage is at a relatively low “fail” level. As will be understood by those skilled in the art, the use of a verification operation having an insufficiently long verification time interval may cause “failed” cells to appear as “passed” cells and thereby increase a width of a threshold-voltage distribution profile.  
       SUMMARY OF THE INVENTION  
       [0006]     Nonvolatile memory devices according to embodiments of the present invention support programming and verify operations that improve threshold-voltage distribution within programmed memory cells. This improvement is achieved by reducing a magnitude of the programming voltage steps and increasing a duration of the verify operations once at least one of the plurality of memory cells undergoing programming has been verified as a “passed” memory cell. According to some of these embodiments, the nonvolatile memory device includes an array of nonvolatile memory cells and a control circuit, which is electrically coupled to the array of nonvolatile memory cells. The control circuit, which may include a plurality of functional circuit elements, is configured to perform a plurality of memory programming operations (P) by driving a selected word line in the array with a first stair step sequence of program voltages having first step height (e.g., ΔV 1 ) and then, in response to verifying that at least one of the memory cells coupled to the selected word line is a passed memory cell, driving the selected word line with a second stair step sequence of program voltages having a second step height (e.g., ΔV 2 ) lower than the first step height. The control circuit may also be configured to perform a plurality of memory verify operations (V), which are interleaved in time with the plurality of memory programming operations.  
         [0007]     According to aspects of these embodiments, a first plurality of memory verify operations are associated with the first stair step sequence of program voltages and a second plurality of memory verify operations are associated with the second stair step sequence of program voltages. In order to support a narrowing of the threshold-voltage distribution within the programmed memory cells, a first duration (t 1 ) of the first plurality of memory verify operations is less than a second duration (t 2 ) of the second plurality of memory verify operations. Moreover, a first interleaved sequence of program and verify operations associated with the first stair step sequence of program voltages commences with a program operation and terminates with a verify operation having the first duration (t 1 ) and a second interleaved sequence of program and verify operations associated with the second stair step sequence of program voltages commences with a verify operation having the second duration (t 2 ). Thus, two immediately consecutive verify operations of different duration are performed during an interval when at least one memory cell is verified as a passed memory cell.  
         [0008]     The control circuit according to embodiments of the invention may include a row decoder electrically coupled to a plurality of word lines in the array and a voltage generator electrically coupled to the row decoder. The voltage generator is responsive to a voltage control signal (V_CONT). A program controller is also provided within the control circuit. The program controller is configured to switch the voltage control signal (V_CONT) from a first logic state to a second logic state in response to a first pass signal (e.g., SPF) and is further configured to switch the voltage control signal from the second logic state to the first logic state in response to a last pass signal (e.g., MPF). This control circuit also includes a sense amplifier and a pass/fail detector electrically coupled to an output of the sense amplifier. The pass/fail detector is configured to generate the first and last pass signals and the sense amplifier is responsive to a sense amplifier enable signal generated by the program controller. In particular, the pass/fail detector may include a first pass detector having OR logic therein that generates the first pass signal SPF and a last pass detector having AND logic therein that generates the last pass signal MPF. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0009]     Non-limiting and non-exhaustive embodiments of the present invention will be described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified. In the figures.  
         [0010]      FIG. 1  is a diagram showing variations of voltage levels versus time during programming and verifying operations by an ISPP scheme in a conventional flash memory device;  
         [0011]      FIG. 2  is a block diagram illustrating a memory device that performs programming and verifies operations according to embodiments of the present invention;  
         [0012]      FIG. 3  is a block diagram illustrating a functional structure of a pass/fail detector shown in  FIG. 2 ;  
         [0013]      FIG. 4  is a timing diagram illustrating sensing operations performed by the memory device of  FIG. 2 ;  
         [0014]      FIG. 5A  is a circuit diagram illustrating a bias condition in a verifying operation for a NOR-type memory cell;  
         [0015]      FIG. 5B  is a diagram illustrating relative positions of threshold and verifying voltages for a unit memory cell;  
         [0016]      FIG. 5C  is a graphic diagram illustrating features of verifying processes in accordance with variations of threshold voltages along the drain voltage and verifying time in memory cells;  
         [0017]      FIG. 6  is a flow chart illustrating programming operations performed by embodiments of the present invention;  
         [0018]      FIG. 7  is a diagram illustrating variations of wordline voltages versus time by an adaptive ISPP scheme in accordance with embodiments of the invention; and  
         [0019]      FIG. 8  is a diagram showing improved distribution profiles of threshold voltages by the programming scheme according to embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]     The present invention now will be described more fully herein with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout and signal lines and signals thereon may be referred to by the same reference characters.  
         [0021]      FIG. 2  is a block diagram illustrating a functional structure for a programming operation in a flash memory according to an embodiment of the present invention. The flash memory device includes a memory cell array  20  coupled to a row decoder and a column selector, a voltage generator  10  supplying a program voltage Vpgm and a verifying voltage Vveri to a wordline of memory cells, a writing driver  30  setting bitline voltages of memory cell to be programmed in a programming operation, a sense amplifier  40  detecting channel currents of memory cells in verifying steps, a pass/fail detector  50  checking up pass states with at least one memory cell and with all the memory cells, and a program controller  60  regulating the program voltage and sensing time therein. With such an organization, the programming operation is carried out using a first mode that repeats a cycle with a larger incremental step range ΔV 1  and a shorter verifying time t 1 , and a second mode that repeats a cycle with a smaller incremental step range ΔV 2  and a longer verifying time t 2 . These functional elements that are shown in  FIG. 2  as being coupled (directly or indirectly) to the cell array  20  collectively define a memory content circuit.  
         [0022]     The voltage generator  10  generates voltages for the programming and verifying operations (e.g., V WL , V BL , and so forth), which are supplied to the wordline and the writing driver  30 . The voltage V WL  applied to a wordline includes the program voltage Vpgm, which is applied to a wordline while programming memory cells, and the verifying voltage Vveri that is applied to a wordline while verifying memory cells. The voltages used for programming the flash memory device are applied to wordlines in the ISPP scheme so as to shift up threshold voltages of memory cells over the verfying voltage Vveri. The ISPP scheme is conducted by repeating a loop in which stepping-up program-voltage pulses of Vpgm are applied to the wordline and then the verifying voltage Vveri is applied next thereto. The voltage generator  10  is designed to supply program-voltage pulses to wordlines in variable stepping-up ranges, which differs from the uniform ranges shown in  FIG. 1 . Further, the bitline voltage V BL  for activating a selected bitline during the programming operation is also generated in sync with the program voltage Vpgm.  
         [0023]     The memory cell array  20  includes a plurality of memory cells arranged in a NOR logical pattern, being connected with the row and column devices. The memory cells described herein are referred as being operable with the characteristics of NOR-type flash memory cells.  
         [0024]     The writing driver block  30  activates bitlines of selected memory cells at a point of applying the program voltage Vpgm to a wordline of the selected memory cell. During the programming operation, the writing driver block  30  transfers the bitline voltage V BL  to the bitlines from the voltage generator  10  in response to a bitline-enabling signal BLEN provided from the program controller  60 . While applying the program voltage Vpgm to the wordline, the writing driver block  30  controls a drain side of the selected memory cell on a level of the bitline voltage V BL  (e.g., 5V) so as to induce the injection of hot electrons into a floating gate electrode of the memory cell.  
         [0025]     The sense amplifier block  40  is connected to both the writing driver block  30  and the bitlines of memory cells, detecting cell states during reading and verifying operations. The sense amplifier block  40  includes a plurality of sense amplifiers, which are coupled to corresponding bitlines in the cell array  20 . During the reading operation, a read voltage Vread is applied to a selected wordline of memory cells and the sense amplifiers of the block  40  detect data in accordance with pass or fail states of the memory cells. In the verifying operation, the verifying voltage is applied to a selected wordline and the sense amplifiers of the block  40  detect pass or fail states of the memory cells in response to a sense-enabling signal SAEN. The verifying time is controlled in response to the sense-enabling signal SAEN that is applied to the sense amplifier block  40  from the program controller  60 .  
         [0026]     The pass/fail detector  50  outputs first and last pass signals, SPF and MPF, in response to monitoring detection results of the sense amplifier block  40 . In a general case, a pass signal is generated to the program controller  60  only when all memory cells are detected as being in pass states even if there are inputs of pass-informing signals or data from sense amplifiers. In contrast, the flash memory device of the present invention includes circuitry for detecting a point at which at least one memory cell has been verified as being passed. When this occurs, the first pass signal SPF goes to a high level and the program controller  60  identifies that there is one or more memory cell passed in the ISPP loop. The last pass signal MPF goes to high level only when all the memory cells, which have been programmed, are detected as being passed by the sense amplifiers. The structures and operations of the pass/fail detector  50  will be further described in detail with reference to  FIG. 3 .  
         [0027]     The program controller  60  monitors and regulates conditions of the programming voltages and detecting operations with the aforementioned components. The program controller  60  regulates the voltage generator  10  using a voltage control signal V_CONT. This regulation causes a transfer of the program voltage by the ISPP scheme with the larger incremental step range ΔV 1  to a selected wordline in the first programming mode. In addition, the program controller  60 , while the program voltage Vpgm of pulse is being applied to the selected wordline, controls the writing driver block  30  to activate the bitline-enabling signal BLEN in sync with the pulses of the program voltage Vpgm. In a step of verifying a programmed result after applying the program voltage pulses Vpgm, the program controller  60  outputs the sense-enabling signal SAEN for the time t 1 , regulating a verifying time by the sense amplifier block  40 . If there is a receipt of the first pass signal SPF, which goes to a high level when at least a memory cell is detected as being passed during the period of the first programming mode, the program controller  60  changes operation to the second programming mode that is characterized with the smaller incremental step range ΔV 2  and the longer verifying time t 2 .  
         [0028]     Here, in the second programming mode, it is required that a verifying operation occur over a longer verifying time t 2  that supports accurate detection of whether the corresponding memory cells are being in the pass states, for memory cells detected as being passed during the first programming mode. Thus, when the first pass signal SPF goes to a high level, the program controller  60  activates the sense-enabling signal SAEN during the time t 2  to conduct a re-verifying operation for the first passed cells, at the beginning of the second programming mode. After this re-verify operation, in a step of applying the program voltage pulse Vpgm, the voltage generator  10  is controlled to supply the program voltage Vpgm with the smaller incremental step range ΔV 2  to the selected wordline. The second programming mode is terminated when all of the memory cells are detected as being completely programmed after the generation of the first passed cells. After completing the programming operation for all of the memory cells to be programmed, the pass/fail detector  50  detects the completion of the programming operation and sends the last pass signal MPF to the program controller  60 . The program controller  60  terminates the program operation cycle in response to the last pass signal MPF. The program controller may be comprised of a timer to output an alternative one of the first and second verifying times.  
         [0029]     As such, the programming operation is carried out by way of the first programming mode with repeated high-frequency programming/verifying loops and the second programming mode with repeated high-resolution programming/verifying loops, using the functional components shown in  FIG. 2 . The second programming mode begins in response to detecting at least one passed memory cell by the first programming mode. While the first programming mode is terminated with the verifying operation, the second programming mode begins with resuming the verifying operation that is carried out in the larger verifying time so as to correctly check out whether the memory cells are conditioned in the pass states. The reason for conducting the verifying operation of the second programming mode subsequent to the verifying operation at the end of the first programming mode will be described in detail with reference to  FIG. 5 .  
         [0030]      FIG. 3  is a block diagram illustrating a functional structure of the pass/fail detector  50  shown in  FIG. 2 . Referring to  FIG. 3 , the pass/fail detector  50  is comprised of first and last pass detectors,  51  and  52 , inputting cell detection results from the sense amplifiers of the block  40 . The first pass detector  51  generates and supplies the first pass signal SPF to the program controller  60  when there is at least one memory cell detected as being passed. For instance, if the pass state corresponds to when an output of the sense amplifier block  40  is logically ‘1’, the first pass detector  51  has the same input/output characteristics as an OR logic gate. If there is detected at least a single passed memory cell (i.e., at least one of the lines SA 1 , SA 2 , . . . SAn is a logical “1”), the first pass signal SPF goes to high level and is transferred to the program controller  60 .  
         [0031]     The last pass detector  52  generates and applies the last pass signal MPF to the program controller  60  when all of the memory cells are detected as being passed. The last pass detector  52  may be formed of an AND logic gate, outputting the last pass signal MPF at a high level only when all the memory cells are detected as being passed and outputs of the sense amplifiers are all logically “1”. If all the memory cells are completely programmed and all the sense amplifier output signals indicate the pass states, the last pass signal MPF will go to a high level and be transferred to the program controller  60 .  
         [0032]     Thus, the pass/fail detector  50  provides circuitry for switching the operation to the second programming mode in response to generating the first pass signal SPF and terminating the whole programming operation in response to generating the last pass signal MPF.  
         [0033]      FIG. 4  is a timing diagram illustrating the sensing operation by means of the components shown in  FIG. 2 , which are regulated by the program controller  60 . Referring to  FIG. 4 , a turning point between the first and second programming modes is a point when the first pass signal SPF rises up to high level. In the first programming mode, it can be seen that the turning point corresponds to the time period t 1  for which the sense-enabling signal SAEN is set to a high level to activate the sense amplifier block  40 . In this description, the high-level period of the sense-enabling signal SAEN is also referred to as a verifying time for detecting a pass state of a memory cell by the sense amplifier. The bitline-enabling signal BLEN is logically the reverse to the sense-enabling signal SAEN. When the bitline-enabling signal BLEN is set to a high level, it is the programming period for which the program voltage pulse Vpgm is being applied to a selected wordline. The incremental range of the program voltage in the first programming mode is the larger incremental step range ΔV 1 . When it detects a first passed cell while the first programming mode is being conducted with programming and verifying cycles by the ISPP scheme, the first pass signal SPF goes to a high level to turn the operation into the second programming mode. As aforementioned with reference to  FIG. 2 , the second programming mode begins with the operation for correctly detecting the first passed cells. In other words, the second programming mode begins with the verifying operation for the longer verifying time t 2  that permits precise verification for the already passed memory cells, and does not begin immediately with a programming operation subsequent to the last verifying time t 1  of the first programming mode. Such subsequent verifying operations over the first and second programming modes are provided to prevent miss-detection of a programmed memory cell due to an insufficient verifying time although the programmed memory cell has a threshold voltage lower than the verifying voltage Vveri. Namely, subsequent to the verifying operation of the first programming mode, the second programming mode verifies the first passed cells in the longer verifying time t 2  that is sufficiently extended more than the time t 1 . If the first passed cell is detected as a failed one through the verifying operation at the beginning of the second programming mode, the memory cell will be excluded from the next programming operation. For this, the second programming mode begins with a verifying operation. And, the second programming mode is terminated in response to the last pass signal MPF of high level by normal completion of program for all the memory cells.  
         [0034]     As such, the program controller  60  enables the high-frequency (high-speed) programming operation in the first programming mode, and the second programming mode, after the generation of at least one first passed cells, with the smaller incremental step range ΔV 2  and the longer verifying time t 2 , in sequence. Therefore, embodiment of the present invention are able to effectively restrain the upper and lower distribution profiles of threshold voltages after completing the whole programming operation by means of the organization for adaptively changing the incremental range of the program voltage step and the verifying time in accordance with the generation of the first passed cells.  
         [0035]      FIGS. 5A, 5B , and  5 C explain reasons for carrying out the verifying operation with the longer verifying time t 2  at the beginning of the second programming mode after the detection (or generation) of the first passed cell (or cells) by the first programming mode as shown in  FIG. 4 .  FIG. 5A  is a circuit diagram illustrating a bias condition in the verifying operation for a NOR-type memory cell. Referring to  FIG. 5A , in the NOR-type memory cell programmed in a predetermined threshold voltage Vth, the source electrode S is grounded to a reference voltage while the drain electrode D is precharged in a voltage Vdrain offered by the sense amplifier. The control gate G is supplied with the verifying voltage Vveri through the wordline thereof so as to find the pass state. The sense amplifier compares the drain voltage Vdrain with the reference voltage to detect whether the memory cell is being held in an “on” state or an “off” state in response to application of the verifying voltage Vveri.  
         [0036]     Now, it will be described about the benefit of conducting the sequential verifying operations while switching from the first programming mode to the second programming mode.  FIG. 5B  is a diagram illustrating positions of threshold and verifying voltages for a unit memory cell. From  FIG. 5B , it is possible to find the positions of a first threshold voltage Vth 1  sufficiently lower than the verifying voltage Vveri, a second threshold voltage Vth 2  lower than the verifying voltage Vveri but near thereto, and a third threshold voltage Vth 3  sufficiently higher than the verifying voltage Vveri. The first and second threshold voltages, Vth 1  and Vth 2 , which are lower than the verifying voltage Vveri, correspond to an on-state cell, while the third threshold voltage Vth 3 , which is higher than the verifying voltage Vveri, corresponds to an off-state cell.  
         [0037]     FIG. SC is a graphic diagram illustrating features of verifying processes in accordance with variations of threshold voltages (along the drain voltage) and verifying time in memory cells. Referring to  FIG. 5C , in the case of the first and third threshold voltages Vth 1  and Vth 3  with sufficient intervals from the verifying voltage Vveri, it is possible to correctly detect a cell state regardless of the verifying time t 1  or t 2 . But, in the case of the second threshold voltage Vth 2 , which is approximately equal to the verifying voltage Vveri, the cell state would be detected as passed if the short verifying time t 1  was used or failed if the longer verifying time was used. This anomaly may result in miss-verification due to the insufficient verifying time although a cell threshold voltage is still lower than the verifying voltage Vveri. Since a memory cell once detected as being passed is excluded from the next programming cycle, it results in extension of the lower threshold-voltage distribution profile to a level below the verifying voltage Vveri.  
         [0038]     Thus, embodiments of the invention include a first programming mode where the verifying operation is conducted in a shorter verifying time t 1 , and the second programming mode resumes the verifying operation for the passed cells in the longer verifying time t 2 .  
         [0039]      FIG. 6  is a flow chart illustrating a programming procedure by an adaptive ISPP scheme in accordance with embodiments of the invention. During the beginning of the adaptive ISPP programming operation, the wordlines of the memory cells are supplied with the program voltage pulse with the larger incremental step range ΔV 1  (i.e., a first program voltage), in order to shift the threshold voltages up near to the verifying voltage (step S 10 ), which is the first programming mode. Next, a first verifying step S 20  is carried out for the shorter verifying time t 1  (i.e., a first verifying time), after once applying the program voltage pulse. If there is no passed cell after the first verifying step, the procedure returns to the step S 10  of programming with a voltage pulse in the larger incremental step range ΔV 1 . However, if there is at least one passed cell after the first verifying step, then operations pass from step S 30  to step S 40  where the second programming mode begins.  
         [0040]     The second programming mode initially verifies the first passed cells in the longer verifying time t 2  that is sufficiently extended more than the time t 1 , S 40 . If the first passed cell is detected as a failed one through the verifying operation at the beginning of the second programming mode, the memory cell should not be excluded in the next programming operation.  
         [0041]     The second programming mode begins with re-verifying the first passed cell with the second verifying time t 2  (step S 40 ). And, it continues to detect whether all the memory cells are being passed until the last pass signal MPF is applied to the program controller  60  shown  FIG. 2  (step S 50 ). If all the memory cells are detected as not being passed entirely, it repeats the steps of programming the memory cells in the program voltage pulse with the incremental step range ΔV 2  smaller than ΔV 1  used in the first programming mode (S 60 ) and verifying the programmed memory cells in the second verifying time t 2  (S 40 ). The steps S 40  through S 60  are a programming and verifying loop that corresponds to the second programming mode. If there is an input of the last pass signal MPF at the program controller  60 , indicating that all the memory cells are detected as being passed during the second programming mode, the whole programming procedure according to the adaptive ISPP scheme is terminated.  
         [0042]     Accordingly, the adaptive ISPP scheme including the aforementioned steps is able to quickly shift threshold voltages of the memory cells close to the verifying voltage Vveni by means of the program voltage with the larger incremental step range ΔV 1  and the shorter verifying time t 1 . Further, after the generation of the first passed cell, the second programming mode is carried out to accomplish the high-resolution programming result, responding to the first pass signal SPF, by means of the program voltage with the smaller incremental step range ΔV 2  and the longer verifying time t 2 . Especially, the second programming mode is controlled to start, after detecting at least a passed cell through the first programming mode, with the verifying operation in the time t 2  that extends longer than the former verifying time t 1 .  
         [0043]      FIG. 7  is a diagram illustrating variations of wordline voltages versus time by the adaptive ISPP scheme in accordance with embodiments of the invention, showing stepping-up pulses of the program voltage and variable verifying times. In  FIG. 7 , the hatched portions denote the program voltage pulses and the others denote pulses of the verifying voltage. The first programming mode is the period where the program voltage pulse with the larger incremental step range ΔV 1  and the verifying voltage pulse for the shorter verifying time t 1  are alternately applied to the wordlines of the memory cells prior to the generation (or detection) of the first passed cell. After detecting at least the first passed cell, operations switch to the second programming mode in which the program voltage pulse with the smaller incremental step range ΔV 2  and the verifying voltage pulse for the longer verifying time t 2  are alternately applied to the wordlines of the memory cells. The turning point changing from the first programming mode to the second programming mode corresponds to the last verifying operation of the first programming mode, at which the first passed cell is detected at least, as noticed by the broken line shown in  FIG. 7 . Then, after the generation of the first passed cell, the second programming mode performs a first verifying operation with the verifying time t 2  that is sufficient 1 y longer than the verifying time t 1  of the first programming mode. Thereafter, in the second programming inode, the programming and verifying loop is repeated using the program voltage pulses with the smaller incremental step range ΔV 2  and the longer verifying time t 2 , until all of the memory cells are completely programmed.  
         [0044]      FIG. 8  graphically shows an improved distribution profile of threshold voltages by the programming scheme according to the invention, compared to the conventional one  200  with the present profile  210  by the adaptive ISPP scheme of the invention. From  FIG. 8 , it can be seen that the cell threshold-voltage distribution profile  210  by the invention is configured to be more tightly constrained in width compared with the conventional distribution profile  200 .  
         [0045]     The improvement on the lower side of the present distribution profile, ΔV low , results from the process of accurately detecting the cell states by re-verifying the passed cells for the extended verifying time (i.e., the longer verifying time t 2 ), after the generation of the first passed cell. On the other hand, The improvement on the upper side of the present distribution profile, ΔV up , is obtained because the memory cells are restricted in the higher range of threshold voltage by conducting the high-resolution programming operation with the smaller incremental step range ΔV 2  after the generation of the first passed cell.  
         [0046]     With the aforementioned organizations and operating steps for programming and verifying, the cell threshold-voltage distribution profile after programming is remarkably improved relative to the conventional distribution profile. The invention may be applicable to other types of flash memory devices, besides the NOR-type flash memory device.  
         [0047]     The programming operations performed by embodiments of the invention area able to restrain the upper and lower threshold-voltage distribution profiles that result from programming operations. Thus, embodiments of the invention may be used in memory devices that require narrow threshold-voltage characteristics, such as multi-level cells.  
         [0048]     In the drawings and specification there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.