Abstract:
A flash memory  100  capable of reducing electric fields applied to the word lines on a memory array and reducing a chip area, includes a memory array  110,  a word line decoder  120  disposed at an end of the memory array on the row direction, selecting a predetermined memory block in the memory array according to an address signal, and outputting a selecting signal to the selected memory block, and a word line drive circuit  130  comprising a switch circuit arranged between the memory arrays  110 A and  110 B and switching the application of the work voltage to a memory cell according to the selecting signal, and a pump circuit raising the voltage level of the selecting signal. The word line decoder  120  has lines WR(i) to transmit the selecting signals. The lines WR(i) are connected to the switch circuit of the word line drive circuit  130.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims priority of Japanese Patent Application No. 2011-213086, filed on Sep. 28 th , 2011, the entirety of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a semiconductor memory device, and in particular relates to a driving method for word lines of an NAND type flash memory. 
         [0004]    2. Description of the Related Art 
         [0005]    As a storage device, flash memories can be widely used in electronic devices, such as digital cameras or smart phones. In satisfaction of the market demand, the flash memory must satisfy needs of: small scale, large capacity, high access speed, and low power consumption, etc. 
         [0006]    A NAND type flash memory comprises a memory block array where a plurality of NAND strings are arranged in the row or column direction. The NAND string comprises a plurality of memory cells connected in series and two selecting transistors respectively connected to the two ends of the series connection of the memory cells. 
         [0007]    Typically, when a memory is written with data, the P-well, drain, and source, of the memory cell substrate, are applied with 0V, and the control gate is applied with a high writing voltage (for example 20V). When a delete operation is performed, the control gate is applied with 0V, and the P-well is applied with a high voltage (for example 20V) to delete the data of at least one memory block. When a read operation is performed, the control gate of the selected memory cell is applied with 0V, and the control gates of the other memory cells are applied with a voltage Vread which is higher than the power voltage Vcc. Therefore, several voltages higher than the power voltage Vcc should be generated in the operation of the flash memory, and then applied to the memory cells via the word lines. 
         [0008]    One of the methods to raise voltage is by using a charge pump. When a word line decoder is provided with the charge pump, the scale of the word line decoder will increase substantially because of capacitors. To solve this problem, Patent document 1 discloses a word line decoder which reduces its layout scale by removing a charge pump. The word line decoder can self boost the voltage level of a word line enable signal to enable a word line, and restrain the voltage level of the word line enable signal from lowering down. 
         [0009]    In addition, when a charge pump is used to raise the voltage level of the program voltage Vpgm or Vread, threshold voltages of NMOS transistors increase because of body effect, and this results in difficulty to sufficiently raise voltages. The word line decoder disclosed in Patent document 2 is used to solve the above problem. The word line decoder disclosed in Patent document 2 applies voltages to the gate and the drain of a pass-transistor connected to the word line at different timings. Thus, self-boost of the pass-transistor is used to avoid the lowering down of operation voltages while reducing circuit scale. 
         [0010]    [Patent document 1] Japan patent application publication 2002-197882 
         [0011]    [Patent document 2] Japan patent application publication 2006-107701 
         [0012]    However, there are still problems for the word line decoder of a conventional flash memory which are as follows.  FIG. 1A  shows a layout of the word line decoder of a conventional flash memory. In the row direction, one end of the memory array is arranged with a word line decoder, a level shifter (both of them are called as a word line decoder  20  thereafter), and a word line drive circuit  22 . In the column direction, one end of the memory array is arranged with a page buffer  30 . In this example, the memory array  10  is divided into  2  memory arrays. The word line decoder  20  provides the required operation voltages to a selected word line and unselected word lines according to an address signal. The operation voltages include a program voltage Vpgm, applied to the selected word line during a writing operation, a pass voltage, applied to the unselected word lines during the writing operation, a ground voltage, applied to the selected word line a during reading operation, and a reading voltage, applied to the unselected word line the during reading operation. 
         [0013]    The word line drive circuit  22  comprises a pass transistor for transmitting an operation voltage from the word line decoder  20  to the gate of a memory cell. The operation voltage is applied to the memory cell by turning on the pass transistor. The word line drive circuit  22  applies a high voltage to the gate of the pass transistor to restrain the operation voltage from lowering down. 
         [0014]    As shown in  FIG. 1A , the word lines WL connected to the word line drive circuit  22  must be arranged across the memory array  10  in the row direction. The word line WL should be applied with a high program voltage Vpgm (for example, 20V) during a writing operation. If the resistance capacity (RC) of the word line increases, the voltage will take more time to arrive at the end of the word line. In addition, in order to provide the program voltage Vpgm to the memory cell at the end of the word line, a high program voltage Vpgm should be applied to the word line, and this increases power consumption substantially. Moreover, if the width of the word line is wide enough to reduce the wire resistance thereof, it is hard for the memory array to be minimized. 
         [0015]    On the other hand, the pass transistor of the word line drive circuit  22  is constructed by an N-MOS transistor. To restrain from lowering down of the threshold value of the program voltage Vpgm, the gate of the pass transistor must be applied with a voltage higher than the program voltage Vpgm. Therefore, in order to raise the withstand voltage of the gate oxide layer, the thickness of gate oxide layer must be increased (for example, 400 Å), and this increases the size of the transistor and also the circuit area of the word line drive circuit  22 . Furthermore, if the word line drive circuits  22  are arranged with a narrow pitch therebetween, latch-up will easily occur between adjacent pass transistors. In this regard, an appropriate pitch should be kept between the pass transistors, but such arrangement would increase the chip area. 
         [0016]      FIG. 1B  shows another layout of a conventional flash memory. In this example, word line decoders  20 A and  20 B and word line drive circuits  22 A and  22 B are arranged at the left end and the right end of the memory array. The word line decoders  20 A and the word line drive circuit  22 A work for the memory array  10 A, and the word line decoders  20 B and the word line drive circuit  22 B work for the memory array  10 B. The page buffer  30 A below the memory arrays  10 A and  10 B performs data reading or writing for odd bit lines, and the page buffer  30 B above the memory arrays  10 A and  10 B performs data reading or writing for even bit lines 
         [0017]    As shown in  FIG. 1B , though the wire length of the word line WL in the row direction can be reduced to be a half of that shown in  FIG. 1A , a word line decoder and a word line drive circuit should be arranged at both the two ends of the memory array, such that the chip area is increased. 
         [0018]    The purpose of the invention is to solve the problem on the conventional art, and provide a semiconductor memory device capable of reducing the electric fields applied to the word lines on the memory arrays and also reducing the area of the chip including the memory arrays and the peripheral circuits. 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
         [0020]    A semiconductor memory device of the present invention, includes: a memory array constructed by a plurality of cell units, wherein the cell unit is formed by electrically rewritable memory cells connected in series; a word line decoder selecting a specific memory block within the memory array and outputting a selecting signal to the selected memory block according to an address signal; and a word line drive circuit including a switch circuit switching operation voltages to be supplied to the memory cells according to the selecting signal, and a voltage booster circuit boosting the voltage level of the selecting signal, wherein the switch circuit includes at least one transistor self-boosting the voltage level of the selecting signal according to the operation voltage. 
         [0021]    In the semiconductor memory device in accordance with a preferred embodiment, the switch circuit includes a plurality of pass transistors for transmitting the operation voltages to the gates of the memory cells, wherein the gates of the plurality of pass transistors are supplied with the selecting signal and the plurality of pass transistors self-boost the voltage level of the selecting signal according to the operation voltages. The word line drive circuit is arranged between a first memory array and a second memory array extending in the row direction, and the word line drive circuit is common for the first memory array and the second memory array. 
         [0022]    In the semiconductor memory device in accordance with a preferred embodiment, the voltage booster circuit includes a node where the voltage level is pre-charged to be higher than the power voltage, and a voltage booster transistor of which the gate is connected to the node, wherein the voltage booster transistor boosts the voltage level at the node according to the selecting signal supplied to the drain thereof. The word line decoder includes a voltage booster circuit for supplying the selecting signal, wherein the voltage level is boosted to be higher than the power voltage to the word line drive circuit. The word line decoder includes a drive circuit for driving a bit line selecting transistor and a source selecting transistor of the cell unit. 
         [0023]    In the semiconductor memory device in accordance with a preferred embodiment, the memory array is divided into two divided memory arrays, wherein the word line drive circuit is arranged between the two divided memory arrays, and the word line decoder is arranged at an end of the memory array, and wherein the word line decoder comprises a wire layer for transmitting the selecting signal, wherein the wire layer extends from the word line decoder to the word line drive circuit in the manner of crossing one of the two divided memory arrays in the row direction. The word line extends from the word line drive circuit to respective divided memory array. 
         [0024]    According to the invention, the voltage level of the selecting signal for the word line drive circuit is raised by self-boost of the transistor. In comparison with the prior art, the voltage applied to the transistor can be reduced and the scale of the transistor can also be reduced. Furthermore, in comparison with the prior art, a voltage booster circuit, such as a charge pump is not necessary by utilizing self-boost, and thus the layout scale of the word line drive circuit or the word line decoder can be reduced. Moreover, it is not necessary to boost the selecting signal in consideration of the body effect due to boosting electric charges. In addition, the word line drive circuit can be arranged between the memory arrays in the row direction to reduce the length of the wires extended from the word line drive circuit to reduce load of the wires. The influence of the electric field of the selecting signal on the memory array is also reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0026]      FIG. 1A  shows a layout of a conventional flash memory. 
           [0027]      FIG. 1B  shows another layout of a conventional flash memory. 
           [0028]      FIG. 2  shows a layout of a flash memory in accordance of an embodiment of the invention. 
           [0029]      FIG. 3  shows a block diagram of the word line decoder and the word line drive circuit. 
           [0030]      FIG. 4  shows a circuit diagram of NAND strings formed within the memory block. 
           [0031]      FIG. 5  shows a structure diagram of the switch circuit of the word line driving circuit. 
           [0032]      FIG. 6  shows a circuit diagram of the word line drive circuit. 
           [0033]      FIG. 7  shows a timing chart of the writing operation of the word line drive circuit in accordance with the embodiment. 
           [0034]      FIG. 8A  shows another layout of the word line drive circuit in accordance of an embodiment of the invention. 
           [0035]      FIG. 8B  shows another layout of the word line drive circuit in accordance of an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0037]    Now, embodiments of the invention are described with reference to the drawings. The preferred embodiment of the invention takes a NAND type flash memory as an example. In the drawings, for easy understanding and emphasis, the scale of each part of the memory may be different from that of the practical memory device. 
         [0038]      FIG. 2  shows a layout of a flash memory in accordance of an embodiment of the invention. As shown in  FIG. 2 , the flash memory  100  comprises a memory array  110  divided into at least 2 memory arrays  110 A and  110 B, a word line decoder and a level shifter (both of them are called as a word line decoder  120  thereafter) arranged at an end of the memory array  110  in the row direction, a word line drive circuit  130  arranged between the memory arrays  110 A and  110 B, and page buffers  140  arranged in the column direction of the memory array  110  and having sense amplifiers sensing the data read from bit lines or holding the writing data. Note that although not shown in  FIG. 2 , the flash memory  100  further comprises an input/output buffer transmitting/receiving data to/from external devices, and a controller controlling each part of the flash memory  100  according to the command from the external devices, and etc. 
         [0039]    The memory arrays  110 A and  110 B are divided into a plurality of memory blocks BLK( 0 ), BLK( 1 ), . . . , and BLK(m) in the column direction, wherein each memory block is constructed by several pages.  FIG. 4  shows a circuit diagram of NAND strings formed within the memory block. At the first page, a plurality of memory cells are connected in series in the column direction to form an NAND string (a cell unit NU is called hereafter). In the example shown in  FIG. 4 , a cell unit NU comprises  32  memory cells MCi (i=0, 1, . . . , 31), a bit line selecting transistor BST, and a source selecting transistor SST. The bit line selecting transistor BST and the source selecting transistor SST are respectively connected to the two ends. The drain of the bit line selecting transistor BST is connected to a corresponding bit line GBL, and the source of the source selecting transistor SST is connected to a common source line SL. The control gate of the memory cell MCi is connected to a corresponding word line WLi. The gates of the bit line selecting transistor BST and the source selecting transistor SST are respectively connected to gate lines SGD and SGS, which are parallel with the word lines WLi. 
         [0040]    Typically, the memory cell has a MOS structure comprising a source/drain of N type diffusion area, a tunnel-oxide layer formed on the tunnel between the source and the source, a floating gate formed on the tunnel-oxide layer, a dielectric layer formed on the floating gate, and a control gate formed on the dielectric layer. Commonly speaking, when the floating gate is accumulated with no electric charges, namely, data “1” is written, the threshold voltage is negative, and the memory cell is normally on. When the floating gate is accumulated with electric charges, namely, data “0” is written, the threshold voltage is shifted to be positive, and the memory cell is normally off. 
         [0041]      FIG. 3  shows a block diagram of the word line decoder  120  and the word line drive circuit  130 . For convenience for description, assume that a memory block is constructed by a left page and a right page (2 pages in total) of the memory arrays  110 A and  110 B.  FIG. 3  shows two adjacent memory blocks BLK( 0 ) and BLK( 1 ). 
         [0042]    The word line decoder  120  comprises a block selecting circuits  122  selecting a memory block according to an address signal Ax, a level shifter  124  generating necessary operation voltages according to a control signal from a controller (not shown in  FIG. 3 ), and SGS/SGD drive circuits  126  connected to the gates of the bit line selecting transistor BST and the source selecting transistor SST and supplying gate selecting signals SGS/SGD. 
         [0043]    According to the address signal Ax and the control signal C, the word line decoder  120  uses the level shifter  124  to generate an operation voltage GWL( 0 : 31 ) to a corresponding word line WL ( 0 : 31 ). Namely, when data is written, the program voltage Vpgm (for example, 20V) is applied to a selected word line and a pass voltage (for example, 10V) is applied to unselected word lines. When a read operation is performed, a ground level voltage is applied to the selected word line and a read voltage Vread (for example, 4.5V) is applied to the unselected word lines 
         [0044]    The block selecting circuits  122 - 0  transmits a selecting signal PASSV( 0 ) to the switch circuit  132 - 0  of the word line drive circuit  130  when the memory block BLK( 0 ) is selected. The selecting signal PASSV( 0 ) has a voltage level (for example, 10V) which is raised by the level shifter  24  to be higher than the power voltage Vcc. The SGS/SGD drive circuit  126 - 0  supplies gate selecting signals SGS/SGD which is raised to about 5˜6V to the bit line selecting transistor BST and the source selecting transistor SST of the memory block BLK( 0 ). Similarly, when the memory block BLK( 1 ) is selected, the block selecting circuits  122 - 1  transmits a selecting signal PASSV( 1 ) to the switch circuit  132 - 1  of the word line drive circuit  130 . The SGS/SGD drive circuit  126 - 1  supplies gate selecting signals SGS/SGD, which are raised to about 5˜6V, to the bit line selecting transistor BST and the source selecting transistor SST of the memory block BLK( 1 ). As shown in  FIG. 3 , the gate selecting signal SGD — 01 transmitted by the SGS/SGD drive circuits  126 - 0  and  126 - 1  to the bit line selecting transistor BST is common. 
         [0045]    Here, referring to  FIG. 2 , a wire layout of the bit line decoder  120  and the bit line drive circuit  130 (i) of the i-th memory block is taken as an example. The i-th block selecting circuit  120 -i of the word line decoder  120  is connected to the switch circuit  132 -i of the word line drive circuit  130  via a metal wire WR(i) which extends across the memory array  110 B. The metal wire WR(i) is used to transmit the selecting signal PASSV(i). The metal wire WD/WS which extends across the memory arrays  110 A and  110 B in the row direction is used to transmit the gate selecting signal SGD/SGS from the SGS/SGD drive circuit  126 -i. Note that the metal wire WD/WS extends across the entire memory array  110  in the row direction and does not contact the word line drive circuit  130 . 
         [0046]      FIG. 5  shows a structure diagram of the switch circuit of the word line driving circuit. As shown in  FIG. 5 , a switch circuit  132 A- 0  connected to the memory array  110 A is formed at the left side of the word line drive circuit  130 - 0 , and a switch circuit  132 B- 0  connected to the memory array  110 B is formed at the right side of the word line drive circuit  130 - 0 . Similarly, a switch circuit  132 A- 1  is formed at the left side of the word line drive circuit  130 - 1 , and a switch circuit  132 B- 1  is formed at the right side of the word line drive circuit  130 - 1 . The switch circuits  132 A- 0 ,  132 B- 0 ,  132 A- 1 , and  132 B- 1  have the same structure, so only the switch circuit  132 A- 0  is described. 
         [0047]    The switch circuit  132 A- 0  comprises a plurality of N-channel pass transistors connected to the word lines WL( 0 )˜WL( 31 ) of the cell unit NU. The gates of these pass transistors are commonly supplied with a selecting signal PASSV_INT from the word line drive circuit  130 - 0 . The selecting signal PASSV_INT is generated according to the selecting signal PASSV of the word line decoder  120 . Therefore, when the memory block is selected, the selecting signal PASSV_INT has a voltage level capable of fully conducting the pass transistors such that the operation voltages GWL( 0 : 31 ) from the word line decoder  120  can be transmitted to the corresponding word lines WL( 0 : 31 ). On the other hand, when the memory block is not selected, the selecting signal PASSV has a non-activating voltage level (L voltage level) such that the selecting signal PASSV_INT also has a non-activating voltage level and the pass transistors are not conducted. 
         [0048]      FIG. 6  shows a circuit diagram of the word line drive circuit  130 . The word line drive circuit  130  comprises a switch circuit  132  switched by the signal PASSV_INT and a voltage booster circuit  134  raising the voltage at a node according to the selecting signal PASSV. The voltage booster circuit  134  comprises an N-channel first transistor TR 1  with a high withstand voltage, and an N-channel second transistor TR 2  with a high withstand voltage, of which the gate is connected to the N-channel first transistor TR 1 . In operation, the gate of the first transistor TR 1  receives a signal VXD having a boosted voltage level Vp which is higher than the power voltage Vcc (for example, 3V). When the drain is applied with a signal LPVBST having the same voltage level Vp, a voltage level Vp−Vt (Vt is the threshold voltage of the first transistor TR 1 ) will be generated at the node LPVBST — 1 connected to the source. 
         [0049]    Regarding the second transistor TR 2 , the gate is connected to the node LPVBST — 1, the drain is supplied with a selecting signal PASSV from the word line decoder  120 , and the source is connected to the gates of the transistors PTR of the switch circuit  132 . When the voltage level Vp−Vt is generated at the node LPVBST — 1 and the drain of the second transistor TR 2  is applied with the selecting signal having voltage level high than Vp−Vt, self-boost will happens at the node LPVBST — 1 by the capacitive coupling between the gate and the drain of the second transistor TR 2 . The second transistor is then conducted because of the self-boosted gate voltage. Accordingly, the selecting signal PASSV_INT can be generated without lowering down the voltage level of the selecting signal PASSV. 
         [0050]    In the switch circuit, the gate of each pass transistor PTR is applied with the selecting signal PASSV_INT. When the drain of the pass transistor PTR is applied with an operation voltage GWL (for example, a program voltage Vpgm), the selecting signal PASSV_INT applied to the gate of the pass transistor PTR will be self-boosted. Therefore, lowering down of voltage due to the pass transistor PTR can be avoided and the operation voltage can be transmitted to the corresponding word line. 
         [0051]      FIG. 7  shows a timing chart of the writing operation of the word line drive circuit in accordance with the embodiment. First, at timing t 1 , the gate of the first transistor TR 1  is applied with a signal VXD which has been boosted to, for example, 6V. Following, at timing t 2 , the drain of the first transistor TR 1  is applied with a signal LPVBST which has been boosted to, for example, 6V, such that the node LPVBST — 1 is pre-charged to 6V−Vt. Next, at timing t 3 , the drain of the second transistor TR 2  is applied with a program voltage Vpgm (for example, 16V) as a selecting signal PASSV, and the node LPVBST — 1 is self-boosted (6V−Vt+Boost). Therefore, without lowering down the program voltage Vpgm, the voltage booster circuit  134  can supply a selecting signal PASSV_INT which has voltage level equal to the program voltage Vpgm to the switch circuit  132 . 
         [0052]    Next, at timing t 4 , by lowering down the signal LPVBST to the voltage level Vcc, the voltage level at the node LPVBST — 1 is discharged via the first transistor TR 1  to the voltage level Vcc. Following, at timing t 5 , the operation voltage GWL is applied to the drain of the pass transistor PTR. Namely, a selected word line WL_SEL is applied with a pass voltage Vpass and the program voltage Vpgm in sequence, and unselected word line WL_USEL is applied with the pass voltage Vpass. The pass voltage is, for example, 10V. According to the operation voltage GWL applied to the pass transistors PTR, the selecting signal PASSV_INT is self-boosted to Vpgm+Boost. Therefore, the pass transistors PTR are conducted strongly and the operation voltage GWL is transmitted to the corresponding word line. After this operation, the next operation is performed in the same way. Note that in the reading operation the unselected word lines are supplied with a reading voltage (for example, 4.5V) higher than the power voltage Vcc, but the operation is still performed in the same way. 
         [0053]    According to the embodiment, the selecting signal PASSV_INT from the voltage booster circuit  134  is applied to the gate of the pass transistor PTR and the operation voltage GWL is applied to the drain of the pass transistor. In this way, capacity coupling between the gate and the drain/source is utilized to self-boost the selecting signal PASSV_INT. Therefore, the voltage applied to the pass transistors PTR in the embodiment can be reduced to be lower than the high voltages applied to the gate and the source of selecting transistor without utilizing self-boost in the prior art. The pass transistors PTR can also be reduced such that the circuit area of the switch circuit  132  is reduced. In addition, the boost voltage applied to the word line can be lowed down more than the prior art does. 
         [0054]    In the above embodiment, one word line decoder  120  is arranged for all memory blocks BLK( 0 ) . . . BLK(m) of the memory arrays  110 , but it is also possible to arrange a plurality of word line decoders for memory blocks BLK( 0 ) . . . BLK(m). In this situation, a specified word line decoder can be selected out from all word line decoders according to the address signal. 
         [0055]    In the above embodiment, the word line decoder  120  is arranged between two memory arrays  110 A and  110 B in the row direction, but the arrangement is not limited thereto. As shown in  FIG. 8A , the word line decoder  130  is arranged to an end of the memory array  110 . As shown in  FIG. 8B , a memory array  110  is divided to 4 memory arrays  110 A,  110 B,  110 C, and  110 D, and two word line drive circuits  130 A and  130 B are arranged between adjacent memory arrays in the row direction. 
         [0056]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.