Abstract:
A non-volatile memory device, related memory system, and program method for the non-volatile memory device are disclosed. In the method, memory cells in a memory cell array are accessed through a plurality of word lines by applying a program voltage to a selected word line, wherein the selected word line is not adjacent to an outmost word line, applying a first reduced pass voltage to word lines adjacent to the selected word line, and applying a second reduced pass voltage to the outermost word lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2007-0039873 filed on Apr. 24, 2007, the subject matter of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a semiconductor memory device and more particularly to a flash memory device. 
   Semiconductor memories are vital components in digital logic systems, such as computers and consumer electronics. Therefore, advances in the fabrication of semiconductor memories including process enhancements and technology developments through device scaling to higher densities and faster operating speeds improve the overall performance of digital logic systems. 
   Semiconductor memory devices may be characterized as volatile memory devices, such as Random Access Memory (RAM) devices, or non-volatile memory devices. In RAM devices, digital data is stored by either setting up the logic state of a bi-stable flip-flop in the case of static random access memory (SRAM) devices, or by charging/discharging a capacitor in the case of a dynamic random access memory (DRAM) device. In either case, the data is retained in memory only so long as the power is applied to the device. However, once power is no longer applied, stored data is lost from volatile memories. 
   In contrast, Non-volatile memories, such as Mask Read-Only Memory (MROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM), and Electrically Erasable Programmable Read-Only Memory (EEPROM), are capable of maintaining stored data in the absence of applied power. The non-volatile memory data storage mode may be permanent or reprogrammable depending upon the fabrication technology used to implement the device. 
   A combination of volatile and non-volatile operating modes are available in certain hybrid devices such as the non-volatile SRAM (nvSRAM). These memory devices are particularly well suited for use in systems that require a fast, programmable, non-volatile data storage capability. In addition, dozens of special memory architectures have evolved which contain some additional logic circuitry to optimize their performance of memory devices for application-specific tasks. 
   The incorporation and use of MROM, PROM, and EPROM devices in contemporary applications have proved difficult given the unique conditions necessary to erase and/or data within these devices. On the other hand, the EEPROM is capable of being electrically erased or written to, and as such has been successfully incorporated into many products. Indeed, the application of EEPROMs (e.g., flash memory) has widened recently to include auxiliary memories or system programming memories requiring continuous updates. In particular, so-called flash memory exhibits a higher degree of integration than other types of EEPROM and is thus advantageous in the implementation of large auxiliary memories. 
   SUMMARY OF THE INVENTION 
   Embodiments of the invention provide a flash memory device and operation method capable of improving a pass voltage window. Embodiments of the invention also provide a flash memory device capable of varying a pass voltage based on the relative physical position of a selected word line. 
   In one embodiment, the invention provides a program method for a flash memory device comprising a memory cell array having memory cells accessed through a plurality of word lines and bit lines, comprising; applying a program voltage to a selected word line in the plurality of word lines, wherein the selected word line is not adjacent to an outmost word line in the plurality of word lines, applying a first reduced pass voltage to word lines in the plurality of word lines adjacent to the selected word line, and applying a second reduced pass voltage to the outermost word lines. 
   In another embodiment, the invention provides a program method for a flash memory device comprising a memory cell array having memory cells accessed through a plurality of word lines and bit lines, comprising; applying a program voltage to a selected word line in the plurality of word lines, if the selected word line is not adjacent to first and second outermost word line in the plurality of word lines, applying a first reduced pass voltage to word lines in the plurality of word lines adjacent to the selected word line, and applying a second reduced pass voltage to the outermost word lines, but if the selected word line is adjacent to the first outermost word line, applying a third reduced pass voltage to the first outermost word line, applying the first reduced pass voltage to a word line adjacent to the selected word line and opposite the first outermost word line, and applying the second reduced pass voltage to the second outermost word line, wherein the third reduced pass voltage is less than the second reduced pass voltage which is less than the first reduced pass voltage. 
   In another embodiment, the invention provides a flash memory device comprising; a memory cell array comprising a string of non-volatile memory cells series connected between a select transistor connected to a string select line and a ground select transistor connected to a ground select line, wherein each one of the memory cells is respectively connected to a word line arranged in a plurality of word lines, wherein the plurality of word lines comprises a first outermost word line and a second outermost word line, a word line voltage generator configured to generate a program voltage, a pass voltage, a first reduced pass voltage, a second reduced pass voltage, and a third reduced pass voltage, a row decoder circuit receiving the program voltage, pass voltage, first reduced pass voltage, second reduced pass voltage, and third reduced pass voltage from the word line voltage generator, and control logic configured to control the row decoder circuit such that during a program operation the program voltage is applied to a selected word line, and if the selected word line is not adjacent to first and second outermost word lines, a first reduced pass voltage is applied to word lines adjacent to the selected word line, and a second reduced pass voltage is applied to the first and second outermost word lines, but if the selected word line is adjacent to the first outermost word line, a third reduced pass voltage is applied to the first outermost word line, the first reduced pass voltage is applied to a word line adjacent to the selected word line and opposite the first outermost word line, and the second reduced pass voltage is applied to the second outermost word line, wherein the third reduced pass voltage is less than the second reduced pass voltage which is less than the first reduced pass voltage. 
   In another embodiment, the invention provides a memory card comprising a flash memory device such set forth above and a memory controller configured to control the flash memory device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a flash memory device according to an embodiment of the invention. 
       FIG. 2  is a voltage verse bit failure diagram further describing a pass voltage window in the context of the present invention. 
       FIG. 3 , including  FIGS. 3A through 3C , is a collection of voltage waveform diagrams showing a word line bias condition during programming operation for a flash memory device according to an embodiment of the invention. 
       FIG. 4  is a diagram showing pass voltage variation for a programming operation of a flash memory device according to an embodiment of the invention. 
       FIG. 5  is a general block diagram showing a computational system incorporating a flash memory device according to an embodiment of the invention. 
   

   DESCRIPTION OF EMBODIMENTS 
   Embodiments of the invention will now be described with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as being limited to only the illustrated embodiments. Rather, these embodiments are presented as teaching examples. 
   Within contemporary flash memory devices, the distance between a string select line SSL and an adjacent word line (i.e., a first outermost word line in a plurality of word lines within a memory cell array) is generally greater than the distance between adjacent word lines in the plurality of word lines. Likewise, the distance between a ground select line GSL and an adjacent word line (i.e., a second outermost word line in a plurality of word lines opposite the first outermost word line) is also greater than the distance between adjacent word lines in the plurality of word lines. This configuration results in an increased coupling ratio for each one of the first and second outermost word lines. This increase in coupling ratio for the two outermost word lines in the plurality of word lines has material effects related to a pass voltage applied to these word lines. For example, if the pass voltage is relatively high, the channel voltage for a string of memory cells including a program-inhibited memory cell may be boosted high while one or more memory cells in the string suffer from a conventionally understood phenomenon referred to as “soft programming”. Accordingly, the characteristics of a pass voltage must be determined after considering its likely channel boosting effect and related programming characteristics. 
   In accordance with a flash memory device consistent with an embodiment of the invention, a pass voltage applied to an unselected word line in the plurality of word lines may be determined based on its relative physical location or position within the plurality of word lines, and/or the location of a selected word line in the plurality of word lines. In the discussion that follows, the symbol “VPASS” will be used to indicate a pass voltage having a level determined in relation to the position of the word line and the relative position of a selected word line. 
   In one embodiment of the invention, an adjacent word line to a selected word line is supplied with a first reduced pass voltage (hereinafter, VPASS 1 ) reduced by ΔV 1  from a defined pass voltage VPASS. Each of the outermost word lines in the plurality of word lines is supplied with a second reduced pass voltage (hereinafter, VPASS 2 ) reduced by ΔV 2  from the pass voltage VPASS. In a case where a program voltage is supplied to a word line adjacent to an outermost word line (i.e., the word line adjacent to the outermost word line is a selected word line), the outermost word line is supplied with a third reduced pass voltage (hereinafter, VPASS 3 ) reduced by ΔV 3  from the pass voltage VPASS, and a word line adjacent (on the other side from the outermost word line) to the selected word line is supplied with the first reduced pass voltage VPASS 1 . In the foregoing, ΔV 2  is set to be greater than ΔV 1  but less than ΔV 3 , thereby establishing a relative pass voltage reduction relationship of (ΔV 1 &lt;ΔV 2 &lt;ΔV 3 ). 
   The foregoing application of different pass voltages to various word lines in relation to their absolute position within the plurality of word lines (e.g., an outermost position) and their relative position to a selected word line establishes a useful word line bias condition. In accordance with this bias condition, it is possible to generally and accurately determine a pass voltage window regardless of variations in the coupling ratio between different word lines and associated string select and ground select lines. This enhanced determination ability allows overall channel boosting efficiency and programming characteristics to be improved, as will be described in some additional detail below. 
     FIG. 1  is a block diagram of a flash memory device according to an embodiment of the invention. The embodiment illustrated in  FIG. 1  is a NAND flash memory device, but the dictates and resulting benefits of the present invention may be readily extrapolated by those skilled in the art to other types of memory devices such as MROM, PROM, FRAM, CTF flash memory, NOR flash, and the like. 
   Referring to  FIG. 1 , the NAND flash memory device comprises a memory cell array  100  implemented in conventional fashion with memory cell capable of storing N bits of data per memory cell, where N is a positive integer. As desired for specific implementations, memory cell array  100  may be further organized into a plurality of memory blocks. For convenience of description, only the single memory block illustrated in  FIG. 1  will be discussed. Thus, memory block  100  comprises a plurality of strings  101  each respectively connected to one of a plurality of bit lines BL 0  to BLm−1. 
   Each string  101  comprises (an upper) string select transistor, (a lower) ground select transistor, and a plurality of memory cells serially connected between these select transistors. The string select transistor is controlled by a string select line SSL, and the ground select transistor is controlled by a ground select line GSL. Each one of the plurality of memory cells is respectively controlled by a corresponding word lines WL 0  to WL 31 . The bit lines BL 0  to BLm−1 are connected to a page buffer circuit  110 , which is controlled by control logic  120  and operates as a write driver circuit or a sense amplifier circuit depending on operating mode. For example, page buffer circuit  110  may operate as a write driver circuit during program operations and as a sense amplifier circuit during read operations. Although not shown in figures, page buffer circuit  110  may include individual page buffers connected to respective bit lines BL 0  to BLm−1 or to respective bit line pairs. A column decoder circuit  130  may be controlled by control logic  120  to provide a data transfer path between page buffer circuit  110  and an external data destination (e.g., a memory controller). 
   Continuing with  FIG. 1 , the flash memory device further comprises a word line voltage generator circuit  140  and a row decoder circuit  150 . Word line voltage generator circuit  140  may be controlled by control logic  120  to generate various word line voltages (e.g., a program voltage, a pass voltage, a read voltage, and the like) required to execute program/read operations in the flash memory device. In the illustrated embodiment, word line voltage generator circuit  140  comprises a program voltage generator  141  and a pass voltage generator  142 . Program voltage generator  141  may be used to conventionally generate the program voltage supplied to a selected word line during a program operation. Pass voltage generator  142  may be used to generate the plurality of pass voltages (e.g., VPASS, VPASS 1 , VPASS 2 , and VPASS 3 ) variously supplied to unselected word lines during a program operation. The program and pass voltages VPGM, VPASS, VPASS 1 , VPASS 2  and VPASS 3  may be supplied to word lines traversing memory  100  through row decoder circuit  150 . Consistent with the foregoing discussion, VPASS is greater than VPASS 1  by ΔV 1 , greater than VPASS 2  by ΔV 2 , and greater than VPASS 3  by ΔV 3 . 
   Those skilled in the art will recognize that the functionally conceptually illustrated above in relation to program voltage generator  141  and pass voltage generator  142  may be implemented in many different circuits. For example, it is not necessary for the functionality of program voltage generator  141  and pass voltage generator  142  to be implemented as separate (or independent) circuits. 
   Row decoder circuit  150  may be controlled by control logic  120  to drive selected and unselected word lines with corresponding word line voltages (e.g., a program voltage and a selected pass voltage), respectively. In the illustrated embodiment, row decoder circuit  150  comprises a first decoding and driving block  151 , a second decoding and driving block  152 , and a switch block  153 . First decoding and driving block  151  may drive a block word line BLKWL with a high voltage (e.g., a voltage higher than the program voltage) in response to a block address BA. Second decoding and driving block  152  may include a first driver SS or  152   a , a second driver GS or  152   b , and a third driver Si or  152   c . First driver  152   a  may be configured to drive a signal line SS corresponding to a string select line SSL with a power supply voltage or a ground voltage. Second driver  152   b  may drive a signal line GS corresponding to a ground select line GSL with a power supply voltage or a ground voltage. Third driver  152   c  may drive signal lines S 0  to S 31  corresponding to word lines WL 0  to WL 31  with corresponding word line voltages (e.g., a program voltage, a selected pass voltage, a read voltage, etc.) in response to a page address PA. 
   Switch block  153  may be controlled by the block word line BLKWL and electrically connect outputs SS, S 31  to S 0  and GS with corresponding word lines and select lines. The transistors forming switch block  153  may be high-voltage tolerant in a manner well understood in the art. 
   In a flash memory device consistent with an embodiment of the invention, when a program operation is carried out, the word line adjacent to a selected word line may be supplied with the first reduced pass voltage VPASS 1 . In a case where the selected word line is not adjacent to an outermost word line (i.e., WL 0  and WL 31  in the illustrated example), the second reduced pass voltage VPASS 2  is applied to the outermost word lines WL 0  and WL 31 , respectively. On the other hand, in a case where the program voltage Vpgm is applied to a word line (e.g., WL 1  or WL 30 ) adjacent to one of the outermost word lines WL 0  and WL 31  (i.e., when either word line WL 1  or WL 30  is selected), the third reduced pass voltage VPASS 3  is applied to the outermost word line WL 0  or WL 31 , and the first reduced pass voltage VPASS 1  is applied to an opposing word line adjacent to the selected word line. Otherwise, the remaining word lines in the plurality of word lines are driven with the pass voltage VPASS. 
   As is understood by those skilled in the art, one problem may arise when programming selected memory cells connected to a selected word line with unselected memory cells connected to the selected word line which are so-called “program-inhibited.” When the program voltage is applied to the selected word line, it is not only applied to the selected memory cell(s) but also to the unselected memory cells (i.e., program-inhibited memory cells) arranged along the same selected word line. In this case, there may be programmed unselected memory cells connected to the selected word line. Unintended programming of unselected memory cells connected to a selected word line is referred to a program disturb (or disturbance) event. On the other hand, unselected memory cells can be programmed by a pass voltage applied to unselected word lines. Unintended programming of unselected memory cells connected to an unselected word line is referred to as a pass voltage disturb (or disturbance) event. 
   Program disturb and pass voltage disturb events are more fully disclosed, for example, in U.S. Pat. Nos. 5,715,194; 6,061,270; 6,661,707; and 7,031,190, the collective subject matter of which is hereby incorporated by reference. 
   There exists the following relationship between position of a program-inhibited memory cell and an applied pass voltage. As illustrated in  FIG. 2 , if a pass voltage is relatively low, a program preventing effect may be obtained with respect to a program disturb, while program-inhibited memory cells can be unnecessarily soft programmed due to the program disturb. In other words, in a case where a pass voltage is relatively low, since a channel voltage of a program-inhibited memory cell is not boosted to a desired voltage level, the program-inhibited memory cell can be inadvertently programmed. 
   In a case where a pass voltage is relatively high, a program preventing effect may be obtained with respect to program-inhibited memory cells, while program-inhibited memory cells can be unnecessarily programmed due to pass voltage disturb. In other words, if a pass voltage is relatively high, program-inhibited memory cells may be programmed due to the pass voltage. Accordingly, it is important to appropriately determine a pass voltage range considering the above-described relationship. This pass voltage range is referred to as a pass voltage window. 
   A pass voltage may be set to a voltage in the pass voltage window in consideration of possible program disturb and pass voltage disturb events. The following problem may arise due to a pass voltage that is determined in the above-described manner. 
   As is well appreciated in the art, the typical distance between a string select line and an adjacent outermost word line is greater than that between adjacent word lines in the plurality of word lines. Thus the coupling ratio between the floating gate of a memory cell corresponding to the outermost word line is relatively increased. Thus, when a pass voltage is applied to the outermost word line, a voltage induced at a floating gate of a corresponding memory cell connected to the outermost word line may be higher than a voltage induced at a floating gate of a corresponding memory cell for each of the remaining word lines. For this reason, an increase in the pass voltage may be limited in relation to the outermost word line. For example, when a pass voltage is defined as 10V, the voltage induced at a floating gate of a memory cell connected to the outermost word line may be higher than the voltage induced at a floating gate of a memory cell connected to the remaining word lines. In this case, a program-inhibited memory cell supplied with the pass voltage may be soft programmed. In response to this potential outcome, the pass voltage may be limited to a relatively lower voltage level. Since the pass voltage is limited to a relatively lower voltage, its corresponding channel boosting efficiency is reduced. This means that a program characteristic, (i.e., the program inhibition characteristic) is reduced. 
   In order to overcome the above-described problem, as illustrated in  FIG. 3A , in a case where a program operation is carried out in an embodiment of the present invention, the first reduced pass voltage VPASS 1  is supplied to word lines WLi−1 and WLi+1 adjacent to a selected word line WLi. At this time, the outermost word lines WL 0  and WL 31  are supplied with the second reduced pass voltage VPASS 2 . Herein, the second reduced pass voltage VPASS 2  may be determined in relation to an understood coupling ratio. Although the second reduced pass voltage VPASS 2  is applied to the outermost word lines WL 0  and WL 31 , the channel voltage of the string supplied with the power supply voltage via a corresponding bit line will be sufficiently boosted by a relatively large coupling ratio. On the other hand, as illustrated in  FIGS. 3B and 3C , in cases where the program voltage Vpgm is applied to either word line WL 1  or WL 30  adjacent to an outermost word line WL 0  or WL 31 , (i.e., when either word line WL 1  or WL 30  is selected), the third reduced pass voltage VPASS 3  is supplied to the adjacent outermost word line WL 0  or WL 31 , and the first reduced pass voltage VPASS 1  is supplied to either word line WL 2  or WL 29  adjacent to the selected word line WL 0  or WL 31 . The remaining word lines may be driven with the pass voltage VPASS. 
   In accordance with these bias conditions, as illustrated in  FIG. 4 , the voltage applied to each word line will not undesirably increase above the pass voltage VPASS. In other words, since the pass voltage is not limited by the effects associated with the position of selected verses unselected word lines, it is possible to increase the maximum level of the pass voltage. That is, it is possible to establish a relatively wide pass voltage window. This is advantageous in the resulting channel boosting efficiency and program characteristic(s) are improved relative to the program disturb and pass voltage disturb events. 
   As noted above, flash memory devices are increasingly used in many host devices, including as selected examples, portable electronics such as cellular phones, personal digital assistants (PDA), digital cameras, portable gaming consoles, MP3 players, etc. Within such devices, flash memory is used to store programming code and payload data (music files, video files, etc.). Additionally, flash memory is also to be used with increasing regularity in home applications such as high-definition TVs, digital versatile disks (DVDs), routers, and global positioning systems (GPSs), etc. 
     FIG. 5  is a block diagram of a general computational system  2000  including one or more flash memory device(s) consistent with an embodiment of the invention. Computational system  2000  comprises a microprocessor  2100 , a user interface  2200 , a memory controller  2400 , a flash memory device  2500 , and a modem  2600  such as a baseband chipset, which are connected via a bus  2001 . Flash memory device  2500  may be configured in a manner consistent with the embodiment shown in  FIG. 1 . Flash memory device  2500  may store N-bit data, where N is a positive integer, to be processed by microprocessor  2100 , as provided by memory controller  2400 . If computational system  2000  shown in  FIG. 5  is a mobile apparatus, a battery  2300  may be further provided to supply an operating voltage. Although not shown in  FIG. 5 , computational system  2000  may further comprise an application chipset, a camera image processor (e.g., CMOS image sensor; CIS), mobile DRAM, etc. In certain computational systems, flash memory device  2500  and memory controller  2400  may be conventionally configured as a memory card. Further, flash memory device  2500  and memory controller  2400  may be configured as a OneNAND™ flash memory device. 
   The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true scope of the invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents.