Abstract:
A NAND Flash memory device reduces circuitry noise during program operations. The memory includes bit lines that are electrically coupled together to charge share their respective voltage potentials prior to performing a discharge operation on the bit lines. A NAND flash cell is programmed by coupling a first memory array bit line to a program voltage to program the memory cell, biasing a second memory array bit line to a ground potential, wherein the second memory array bit line is located adjacent to the first memory array bit line, activating at least one first transistor to electrically coupling the first and second memory array bit lines together, and activating at least one second transistor to electrically couple the first and second memory array bit lines to a discharge potential.

Description:
RELATED APPLICATION(S) 
     This application is a continuation of U.S. application Ser. No. 12/127,610, filed May 27, 2008, now U.S. Pat. No. 7,995,399, which is a divisional of U.S. application Ser. No. 11/216,739 filed Aug. 31, 2005, now issued as U.S. Pat. No. 7,388,789, both of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to non-volatile memory devices and, more particularly, to NAND flash memory devices. 
     BACKGROUND 
     Flash memory is non-volatile, which means that it stores information on a semiconductor in a way that does not need power to maintain the information in the chip. Flash memory stores information in an array of transistors, called “cells,” each of which traditionally stores one or more bits of information. The memory cells are based on the Floating-Gate Avalanche-Injection Metal Oxide Semiconductor (FAMOS transistor) which is essentially a Complimentary Metal Oxide Semiconductor (CMOS) Field Effect Transistor (FET) with an additional conductor suspended between the gate and source/drain terminals. Current flash memory devices are made in two forms: NOR flash and NAND flash. The names refer to the type of logic used in the storage cell array. 
     A flash cell is similar to a standard MOSFET transistor, except that it has two gates instead of just one. One gate is the control gate (CG) like in other MOS transistors, but the second is a floating gate (FG) that is insulated all around by an oxide layer. The FG is between the CG and the substrate. Because the FG is isolated by its insulating oxide layer, any electrons placed on it get trapped there and thus store the information. 
     When electrons are trapped on the FG, they modify (partially cancel out) an electric field coming from the CG, which modifies the threshold voltage (Vt) of the cell. Thus, when the cell is “read” by placing a specific voltage on the CG, electrical current will either flow or not flow between the cells source and drain connections, depending on the Vt of the cell. This presence or absence of current is sensed and translated into 1&#39;s and 0&#39;s, reproducing the stored data. 
     Memory cells of memory devices are typically arranged in an array with rows and columns. Generally, the rows are coupled via a word line conductor and the columns are coupled together with a bit line conductor. During data read and write functions, voltage coupling between bit lines can influence proper memory operation. 
     For reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need for methods and devices to program non-volatile memory devices. 
     SUMMARY 
     In one embodiment a non-volatile memory device includes an array of non-volatile memory cells having bit lines, and control circuitry to control voltage pre-charging and discharging of the bit lines during a program operation. The control circuitry further controls switching circuitry to charge share the bit lines prior to discharging. 
     In another embodiment, a method comprises biasing a first bit line of a NAND memory array to a first positive voltage during a program operation, and biasing a second bit line, located adjacent to the first bit line, to a second voltage having a potential less than the positive voltage. The first and second bit lines are coupled together to charge share the first positive voltage and second voltage to provide a resultant voltage on the first and second bit lines having a potential between the first positive voltage and the second voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a memory device according to embodiments of the present invention; 
         FIG. 2  illustrates a simplified portion of a prior art NAND flash memory array; 
         FIG. 3  is a block diagram of a prior art NAND flash memory circuitry; 
         FIG. 4  is a simplified prior art multiplex circuit; 
         FIG. 5  is a prior art timing diagram 
         FIG. 6  is a simplified schematic of multiplex and control circuitry according to embodiments of the present invention; and 
         FIG. 7  is a timing diagram of the circuitry of  FIG. 6  according to embodiments of the present invention. 
     
    
    
     DESCRIPTION 
     In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, different embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. 
     As recognized by those skilled in the art, memory devices of the type described herein are generally fabricated as an integrated circuit containing a variety of semiconductor devices. The integrated circuit is supported by a substrate. 
     Integrated circuits are typically repeated multiple times on each substrate. The substrate is further processed to separate the integrated circuits into dice as is well known in the art. The figures are provided to help facilitate an understanding of the detailed description, are not intended to be accurate in scale, and have been simplified. The term line or conductor as used herein is intended to include conductors and semi-conductors, including but not limited to metals, metal alloy, doped silicon and polysilicon. 
     The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
       FIG. 1  is a simplified block diagram of an integrated circuit memory device  100  in accordance with an embodiment of the invention. The memory device  100  includes an array of non-volatile floating gate memory cells  102 , address circuitry  104 , control circuitry  110 , and Input/Output (I/O) circuitry  114 . The memory cells are also referred to as Flash memory cells because blocks of memory cells are typically erased concurrently, in a flash operation. 
     The memory device  100  can be coupled to a processor  120  or other memory controller for accessing the memory array  102 . The memory device  100  coupled to a processor  120  forms part of an electronic system. Some examples of electronic systems include personal computers, peripheral devices, wireless devices, digital cameras, personal digital assistants (PDA&#39;s) and audio recorders. 
     The memory device  100  receives control signals across control lines  122  from the processor  120  to control access to the memory array  102  via control circuitry  110 . Access to the memory array  102  is directed to one or more target memory cells in response to address signals received across address lines  124 . Once the array is accessed in response to the control signals and the address signals, data can be written to or read from the memory cells across data, DQ, lines  126 . 
     In addition to general memory functions, control circuit  110  performs a write operation on the memory cells. As explained below, the write operation includes controlling a write multiplex circuit. 
     It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device of  FIG. 1  has been simplified to help focus on the invention. It will be understood that the above description of a memory device is intended to provide a general understanding of the memory and is not a complete description of all the elements and features of a typical memory device. 
       FIG. 2  illustrates a simplified portion of a prior art NAND flash memory array. NAND Flash uses tunnel injection for writing and tunnel release for erasing. The NAND memory includes floating gate memory cells  220  coupled to source line  224 , word lines  226  and a bit line  230 . The cells are coupled in a string, or series of cells between the bit line and source line. One or more bit line select transistors  240  are used to selectively isolate the cells from the bit and source lines. 
     In a read operation, a word line of a target (selected) memory cell can be maintained at a low voltage level. All unselected cell word lines are coupled to a voltage sufficiently high to activate the unselected cells regardless of their floating gate charge. If the selected cell has an uncharged floating gate, it is activated. The bit line and source line are then coupled through the series of memory cells. If the selected cell has a charged floating gate, it will not activate. The bit line and source lines, therefore, are not coupled through the series of memory cells. 
     Because of the close proximity of the memory cells, bit line coupling can be a problem during reading/sensing operations. That is, the length and close spacing of adjacent bit lines results in voltage noise on bit lines. Of particular concern is bit line coupling during write verify operations. As known to those skilled in the art, a write operation typically includes one or more program steps and one or more read/verify steps. 
     During read/verify operations, the prior art couples the bit lines of the inactive page to a low potential, such as ground, to provide shielding against bit line coupling. As illustrated in  FIG. 3 , the prior art NAND memory word lines are divided into even and odd ‘pages.’ A latch and multiplex circuit  310  are used to couple data to an active one of the pages and bias bit lines of the non-active page. Optionally, a second latch  320  can be provided in the prior art devices to cache data for the second page, while writing to the first page. 
     The multiplex circuit  310  is a bi-directional switching circuit to couple bit lines to I/O circuit through a sense amplifier/latch circuit. Alternatively, the multiplex circuit couples bit lines to bias voltages during program and verify operations. Unselected bit lines are coupled to either a high bias potential, such as Vcc, during program operations of adjacent bit lines, or coupled to a low bias potential, such as ground, during verify operations of adjacent bit lines. 
     The pages are interwoven such that alternating bit lines  330  and  332  of array  300  belong to different pages. During operation, one page can be active and the other page can be inactive. The bit lines of the inactive page are coupled to a high potential, such as Vcc, during a program operation. The Vcc biased bit lines, therefore, prevent memory cells coupled to a common word line from being programmed. 
       FIG. 4  is a simplified schematic diagram of a prior art write multiplex circuit  400 . The circuit couples the bit lines  402 ,  404 ,  406  and  408  to write lines  490  and  492 . The circuit is also used to couple bias voltages to bit lines. For example, odd bit lines  402  and  406  (BLOn and BLOn+1) form a logical odd page and even bit lines  404  and  408  (BLEn and BLEn+1) form a logical even page. When programming the odd bit lines, the even bit lines are coupled to Vcc. When programming the even bit lines, the odd bit lines are coupled to Vcc. Likewise, during verify (read) operations the non-active bit lines are coupled to a ground bias. 
     The input signals and voltages of the multiplex circuit include write signals (W-even and W-odd) on nodes  430  and  440  to selectively activate transistors  410 ,  412 ,  414  and  416  coupled between the bit lines and the write lines. Pre-charge signals (Precharge-even and Precharge-odd) on nodes  450  and  460  control transistors  420 ,  422 ,  424  and  426  coupled between the bit lines and bias signals (Bias-even and Bias-odd) on nodes  470  and  480 . 
     Referring to  FIG. 5 , a prior art sample timing diagram of an example operation of the circuit of  FIG. 4  is described. In the illustrated example, data is written to the odd bit lines  404  and  408 . The program operation can be divided into three basic phases, Pre-charge  500 , Program  510  and Discharge  520 . 
     In the Pre-charge phase  500  the even bit lines BLEn  404  and BLEn+1  408  are coupled to the Bias-even voltage of Vcc through transistors  422  and  426  by precharge-even (node  460 ) being at a high state (Vcc+Vth). This pre-charges BLEn  404  and BLEn+1  408  to Vcc. 
     In the Program phase  510  the data on Wn  490  and Wn+1  492  are coupled to the odd bit lines, BLOn  402  and BLOn+1  406 , when transistors  410  and  414  are activated by W-odd (node  440 ) transitioning to a high state such as Vcc+Vth. In this example, the write data on Wn  490  is low and the write data on Wn+1  492  is high. As such, BLOn  402  remains low and BLOn+1  406  is coupled high. 
     In the Discharge phase  520 , all of the bit lines are discharged through transistors  420 ,  422 ,  424  and  426 . That is, the Bias-even (node  480 ) and Bias-odd (node  470 ) signals are grounded and transistors  420 ,  422 ,  424  and  426  are activated by high signals on precharge-even (node  460 ) and precharge-odd (node  450 ). 
     The coupling capacitance between bit lines has an adverse influence during the discharge phase. For example, a bit line at zero volts can be coupled negative by an adjacent bit line. Specifically, a bit line voltage of 0−Vcc*(2Cc/(Ci+2Cc)) can be realized where Cc is a bit line coupling capacitance, and Ci is the bit line intrinsic capacitance. In a NAND memory, the actual bit line voltage may be clamped at or near −0.5 volts (diode junction drop) depending upon fabrication techniques. This negative bit line voltage can create noise and possible malfunction of circuitry located near the memory array. 
     Referring to  FIGS. 6 and 7  a NAND memory and example timing diagram of embodiments of the present invention are described. A multiplex circuit  600  of one embodiment can include switching circuitry to selectively couple bit lines  612 ,  614 ,  616  and  618  to either write data lines  620  and  630  or bias voltages on nodes  680  and  682 . Each bit line is coupled to two transistors  640 / 650 ,  642 / 652 ,  644 / 654  and  646 / 656 . The first transistor  640 ,  642 ,  644  and  646  has a gate coupled to a gate signal, W-odd (node  662 ) or W-even (node  660 ), and selectively couples the bit line to the write line, Wn  620  or Wn+1  630 . The second transistor  650 ,  652 ,  654  and  656  has a gate coupled to a gate signal, precharge-odd (node  670 ) or precharge-even (node  672 ), and selectively couples the bit line to a bias voltage on connection bias-odd (node  680 ) or bias-even (node  682 ). The illustrated multiplex circuit  600  has been simplified to illustrate four bit lines. It will be appreciated by those skilled in the art that each multiplex circuit can be coupled to thousands of bit lines. 
     Control circuitry  610  is provided to control the gate signals and bias voltages  660 ,  662 ,  670 ,  672 ,  680  and  682 . Control circuitry  610  can be generally incorporated into the memory control circuitry  110  but is separately illustrated in  FIG. 6  for purposes of explanation. The control circuitry activates the multiplex circuit  600  to charge share adjacent bit lines  612 / 614  and  616 / 618  prior to discharging the bit lines. In the illustrated embodiment of  FIG. 6  the control circuitry activates transistors  640 ,  642 ,  644  and  646  to electrically couple the even and odd bit lines. As such, if a pre-charged bit line at Vcc potential is coupled to a bit line at ground potential, the bit lines move to ½ Vcc prior to discharge. 
     Referring to  FIG. 7  a sample timing diagram of an example operation of the circuit of  FIG. 6  is described. In the illustrated example, data is written to the odd bit lines. The program operation can be divided into four basic phases, Pre-charge  700 , Program  710 , Charge Share  720  and Discharge  730 . 
     In the Pre-charge phase  700  the even bit lines  614  and  618  are coupled to the Bias-even voltage of Vcc, on node  682 , through transistors  652  and  656  by precharge-even, node  672 , being at a high state (Vcc+Vth). This pre-charges BLEn  614  and BLEn+1  618  to Vcc. 
     In the Program phase  710  the data on Wn  620  and Wn+1  630  is coupled to the odd bit lines, BLOn  612  and BLOn+1  616 , when transistors  640  and  644  are activated by W-odd, node  662 , transitioning to a high state such as Vcc+Vth. In this example, the write data on Wn is low and the write data on Wn+1 is high. As such, BLOn  612  remains low and BLOn+1  616  is coupled high. 
     In the Charge Share phase  720  W-even on node  660  and W-odd on node  662  both transition to Vcc to activate transistors  640 ,  642 ,  644  and  646 . Because the transistors are couple to an even and an odd bit line, bit lines  612  and  614  share their charges and bit lines  616  and  618  share their charges. The resultant charge may be less than Vcc. As illustrated, the Wn  620  write voltage is zero (ground) while the Wn+1  630  write voltage is Vcc. Bit lines BLOn  612  and BLEn  614 , therefore, equilibrate during charge sharing to about ½Vcc. In contrast, bit lines BLOn+1  616  and BLEn+1  618  are both at Vcc and remain at Vcc during charge sharing. 
     In the Discharge phase  730 , all of the bit lines  612 ,  614 ,  616  and  618  are discharged through transistors  650 ,  652 ,  654  and  656 . That is, the Bias-even (node  682 ) and Bias-odd (node  680 ) signals are grounded and transistors  650 ,  652 ,  654  and  656  are activated by high signals on precharge-even (node  672 ) and precharge-odd (node  670 ). By adding the Charge Share phase to the NAND memory program operations the noise experienced by circuitry near the memory array during the Discharge phase can be reduced.