Abstract:
A charge trap flash memory device is capable of preventing a data retention fail by ensuring a data retention margin. A method for operating the charge trap flash memory device is provided. A selected memory cell is programmed using a program voltage. The selected memory cell is verified using a first program verify voltage. Date retention states of selected memory cell having passed the program verify step are verified using a retention verify voltage. A read step of determining a program pass or fail by reading data of the selected memory cell having passed the retention verify step is performed using a read voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Priority to Korean application number 10-2009-0000913 filed on Jan. 6, 2009, the entire disclosure of which is incorporated by reference in its entirety is claimed. 
       BACKGROUND 
       [0002]    The disclosure generally relates to a flash memory device, and, more particularly, to a method for operating a charge trap flash memory device. 
         [0003]    In order to prevent mutual interference between adjacent memory cells, a charge trap flash memory having a new cell structure has been developed which performs a program operation and an erase operation by using a charge trapping layer, instead of a floating gate. A charge trap flash memory device having a silicon-oxide-nitride-oxide-silicon (SONOS) or metal-alumina-nitride-oxide-silicon (MANOS) structure, which is widely used as a charge trapping element, includes a tunneling layer, a charge trapping layer, a blocking layer, and a control gate electrode, which are sequentially formed over a substrate with a channel region. 
         [0004]      FIG. 1  illustrates a cross-sectional view of a typical flash memory device having a charge trapping layer. 
         [0005]    Referring to  FIG. 1 , a tunneling layer  110  is formed over a substrate  100 , such as a silicon (Si) substrate. The tunneling layer  110  is formed as an oxide film. Impurity regions  102 , such as source/drain regions, are disposed in the substrate  100  and spaced apart from each other by a predetermined distance. A channel region  104  is disposed between the impurity regions  102 . A silicon nitride film used as a charge trapping layer  120  is formed over the tunneling layer  110 . An insulation film used as a blocking layer  130 , and a control gate electrode  140  are sequentially disposed over the charge trapping layer  120 . 
         [0006]    A program operation is performed by positively charging the control gate electrode  140  and applying an appropriate bias to the impurity regions  102 . Hot electrons from the substrate  100  are trapped in trap sites inside the charge trapping layer  120 . In this way, a memory cell is programmed. On the other hand, an erase operation is performed by negatively charging the control gate electrode  140  and applying an appropriate bias to the impurity regions  102 . Holes from the substrate  100  are trapped in trap sites inside the charge trapping layer  120 . The holes trapped in the charge trapping layer  120  are recombined with extra electrons. In this way, a memory cell is erased. 
         [0007]    In such a flash memory device having the SONOS or MANOS structure, after electric charges are injected into the silicon nitride film used as the charge trapping layer  120  in order to program the memory cell, the injected electric charges may be lost through the tunneling layer  110  or the blocking layer  130 . An electric charge retention characteristic which represents an ability to retain electric charges trapped in the charge trapping layer  120  for a long time largely depends on characteristics of the silicon nitride film used as the charge trapping layer  120 . Therefore, it may be one of the most important factors among data retention characteristics to control the electric charge retention characteristic of the silicon nitride film. 
         [0008]    The improvement in data retention characteristics of the charge trap flash memory device may be achieved by improving the electric charge retention characteristic of an ONO stack structure in a fabrication process, for example, by improving the characteristics of the silicon nitride film or improving the leakage current characteristic of the tunneling layer or the blocking layer. In addition to those methods, the improvement in data retention characteristics of the charge trap flash memory device may be achieved by improving a data retention margin through an electrical operation of memory cells. Generally, in programming flash memory cells, the program operation is followed by a program verify operation at a predetermined threshold voltage (Vt) level. After the program operation, a program pass or fail is determined with reference to a read line, which is relatively lower than a program verify voltage. 
         [0009]      FIG. 2  illustrates an energy distribution of trap sites included in a silicon nitride film used as a charge trapping layer. 
         [0010]    The silicon nitride film used as the charge trapping layer has characteristics of an insulation film, and includes a large number of traps capable of trapping electric charges therein. The traps inside the silicon nitride film are distributed randomly and exhibit a certain range of energy level distribution as illustrated in  FIG. 2 . Although the silicon nitride film includes a plurality of traps  210  with a relatively high energy level, it also includes traps  220  with a relatively low energy level. The traps  220  with a relatively low energy level trap electric charges through the program and erase operations of the flash memory device, and then the trapped electric charges are easily detrapped or released as a certain time elapses. Consequently, as illustrated in  FIG. 3 , a threshold voltage of a memory cell is greatly lowered. 
         [0011]      FIG. 3  illustrates a variation in a threshold voltage of a memory cell due to loss of electric charges in a charge trap flash memory device. 
         [0012]    Since the electric charges trapped in the charge trapping layer by the program operation are detrapped with the elapse of time, the threshold voltage of the memory cell is gradually lowered from a threshold voltage  310  which is given when the memory cell is programmed. Typically, the initial reduction width of the threshold voltage is greatest, and the threshold voltage is gradually lowered with the elapse of time. When the threshold voltage becomes lower than a voltage of a read line, the corresponding memory cell is determined as an unprogrammed memory cell in the read operation. Consequently, a data retention fail may increase. 
       SUMMARY 
       [0013]    Examples of a method for operating a charge trap flash memory device, which is capable of preventing a data retention fail, are disclosed herein. 
         [0014]    In one aspect, a method for operating a charge trap flash memory device includes programming a selected memory cell using a program voltage, verifying the selected memory cell using a first program verify voltage, verifying date retention states of the selected memory cell using a retention verify voltage when the verifying is passed, reading a cell data of the selected memory cell when the verifying data retention states is passed and determining a program pass or fail of the selected memory cell. 
         [0015]    In another aspect, a method for operating a charge trap flash memory device includes programming a selected memory cell using a program voltage, verifying the selected memory cell using a first program verify voltage, verifying date retention states of the selected memory cell using a retention verify voltage when the verifying is passed, reprogramming the selected memory cell having failed the retention verify step, verifying the reprogrammed memory cell using a second program verify voltage, reading a cell data of the reprogrammed memory cell when the verifying data retention states is passed, reading a cell data of the selected memory cell when the second verifying the reprogrammed selected memory cells is passed and determining a program pass or fail of the selected memory cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  illustrates a cross-sectional view of a typical prior art flash memory device having a charge trapping layer; 
           [0017]      FIG. 2  illustrates an energy distribution of trap sites included in a silicon nitride used as a charge trapping layer of a typical prior art flash memory device; 
           [0018]      FIG. 3  illustrates a variation in a threshold voltage of a memory cell due to loss of electric charges in a typical prior art charge trap flash memory device; 
           [0019]      FIG. 4  illustrates a flowchart of a method for operating a charge trap flash memory device; 
           [0020]      FIG. 5  illustrates a threshold voltage distribution of memory cells in the method for operating the charge trap flash memory device; 
           [0021]      FIG. 6  illustrates a retention read level in the method for operating the charge trap flash memory device; and 
           [0022]      FIG. 7  illustrates the individual operations of a page and a block in the method for operating the charge trap flash memory device. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Hereinafter, examples of a method for operating a charge trap flash memory device, which is capable of preventing a data retention fail, will be described in detail with reference to the accompanying drawings. 
         [0024]    In one example, an appropriate threshold voltage level between a cell program verification line and a read line is set, and an appropriate program bias is applied to a memory cell whose threshold voltage is lowered below the set threshold voltage level. In this way, the memory cell is reprogrammed to have a threshold voltage higher than the cell program verification line. Consequently, a data retention margin may be ensured, and a data retention fail may be prevented. 
         [0025]      FIG. 4  illustrates a flowchart of a method for operating a charge trap flash memory device according to one example.  FIG. 5  illustrates a threshold voltage distribution of memory cells in the method for operating the charge trap flash memory device.  FIG. 6  illustrates a concept of a retention read level in the method for operating the charge trap flash memory device.  FIG. 7  illustrates the individual operations of a page and a block in the method for operating the charge trap flash memory device. 
         [0026]    At the step  410 , memory cells are programmed by applying a predetermined program voltage (Vpgm). At the step  420 , it is verified whether or not the programmed memory cells are programmed to an appropriate level. At this time, a verify voltage set to a predetermined level is used. At the step  430 , when the memory cells are not programmed to the appropriate level (that is, the memory cells do not pass the verify step  420 ), the memory cells are programmed using an incremental step pulse programming (ISPP) scheme until they pass the verify step  420 . That is, the program voltage is incremented by a step voltage (Vstep) with a predetermined level, and the memory cells are programmed until they pass the verify step  420 . When the memory cells are programmed to the appropriate level, they have a threshold voltage distribution  510  as shown in  FIG. 5 . 
         [0027]    At the step  440 , when the memory cells are programmed to the appropriate level and thus pass the verify step  420 , retention states of data programmed in the memory cells are verified after a certain time elapses. As described above, since electric charges trapped in the silicon nitride charge trapping layer are detrapped with the elapse of time, the threshold voltages of the memory cells are gradually lowered and have a threshold voltage distribution  520 / 540  as shown in  FIG. 5 . Therefore, when a read operation is performed after the completion of the program operation on the entire memory cells, the programmed memory cells are determined as being unprogrammed, thus causing a program fail. Therefore, it is necessary to perform an operation of verifying data retention states of the programmed memory cells after a certain time elapses and then recovering the threshold voltages of the memory cells. 
         [0028]    To verify the data retention states of the memory cells, a retention read level is set to determine the upper limit of the threshold voltage, which is used as a reference to determine whether the memory cells pass the data retention verify step  440 . As illustrated in  FIGS. 5 and 6 , the retention read level may be set to a level between the program verify level and the read level. The retention read level may be set to an appropriate level according to characteristics of the memory cells. Specifically, the retention read level may be set to a level ranging from the read voltage +10% of the read voltage to the program verify voltage −10% of the program verify voltage. 
         [0029]    After the retention read level is set, the threshold voltage of the memory cell is compared with the retention read level. When the threshold voltage of the memory cell is lower than the retention read level, it is determined that the memory cell does not pass the verify step. On the other hand, when the threshold voltage of the memory cell is higher than the retention read level, it is determined that the memory cell passes the verify step. 
         [0030]    At the steps  460  and  470 , when the memory cell does not pass the verify step because its threshold voltage of the memory cell is lower than the retention read level (that is, the threshold voltage of the memory cell is lowered because electric charges are lost due to detrapping or the like), the corresponding memory cell is reprogrammed to have a threshold voltage higher than the set program verify level, as indicated by a reference numeral “ 530 ” as shown in  FIG. 5 . At this time, the operation of reprogramming the memory cell may be performed using an ISPP scheme. That is, the memory cell is programmed while incrementing the program voltage by a step voltage until it passes the program verify step  470 . The program verify level may be set within the range of the verify level of the step  420  ±1.0 V. 
         [0031]    At the step  450 , when the threshold voltage of the memory cell is higher than the retention read level, data of the memory cell is read. At the step  480 , it is determined whether the memory cell passes the program step, or fails the program step because it is not programmed to a desired level. Then, the program operation is terminated. 
         [0032]    When a certain time elapses after the completion of the program operation, the threshold voltage of the programmed memory cell is confirmed. When the threshold voltage of the programmed memory cell is lower than a predetermined level, the corresponding memory cell is reprogrammed. In this way, it is possible to prevent a retention fail which is caused when the threshold voltage of the programmed memory cell is lowered by loss of electric charges. 
         [0033]    Meanwhile, the operation for ensuring the retention margin may be individually performed on pages or blocks sharing the same page buffer. That is, while the typical program operation and program verify operation are performed on memory cells included in a page or block of a designated plane, the retention read operation and the reprogram operation may be performed on memory cells included in a page or block of another plane. For example, as illustrated in  FIG. 7 , while the program operation and program verify operation are performed on memory cells included in a page or block  710  of a plane 0, the operation of determining a program pass or fail is performed on a page or block  720  of another plane, e.g., a plane 1. The operation of determining whether to reprogram a memory cell is performed by performing a retention read operation on memory cells included in a page or block  730  of a plane N−1. The reprogram operation is performed on memory cells included in a page or block  740  of a plane N. Hence, it is possible to prevent the increase of an operating time of the device due to the operation of ensuring the retention margin. 
         [0034]    The examples of the claimed invention have been disclosed above for illustrative purposes. Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.