Abstract:
A flash memory is provided. The flash memory features of having the select gate transistors to include two different channel structures, which are a recessed channel structure and a horizontal channel. Because of the design of the recessed channel structure, the space between the gate conductor lines, which are for interconnecting the select gates of the select gate transistors arranged on the same column, can be shortened. Therefore, the integration of the flash memory can be increased; and the process window of the STI process can be increased as well. In addition, at least one depletion-mode select gate transistor is at one side of the memory cell string. The select gate transistor of the depletion-mode is always turned on.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a memory, and more particularly to the layout and the structure of a NAND flash memory. 
         [0003]    2. Description of the Prior Art 
         [0004]    Recently, as demands for the portable electronic devices are increasing, the market for the flash memory and the electrically erasable programmable read-only memory (EEPROM) is also expanding as well. The aforesaid portable electronic device includes the storage memory for the digital camera, the cell phones, the video game apparatuses, PDAs, telephone answering machines, and the programmable ICs, etc. A flash memory belongs to a non-volatile memory, and has an important characteristic of being able to store data in the memory even though the power is turned off. By changing the threshold voltage of the transistor, the gate can be turned on and off, and the data can be stored in the transistor. Generally speaking, the flash memory can be divided into two types of configurations, namely, a NOR flash memory and a NAND flash memory. The drains of memory cells of a NOR flash memory are connected in parallel for a faster reading speed, which is suitable for code flash memory mainly used for executing program codes. The drains and sources of two neighboring memory cells of a NAND flash memory are serially connected for integrating more memory cells per unit area, which is suitable for a data flash memory mainly used for data storage. Both of the NOR flash memory and the NAND flash memory have a MOS-like memory cell structure, so as to provide advantages of smaller size, higher operation speed, and higher density. 
         [0005]    As the electronic device becomes smaller, integration of the flash memory needs to be increased. Therefore, it is an object of the present invention to provide a new layout and structure for the flash memory to increase the integration of the flash memory. The layout design according to the present invention can make the size of the flash memory smaller. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the flash memory disclosed in the present invention, the flash memory comprises a substrate; a first active area positioned in the substrate, wherein the first active area comprises a first memory cell string comprising a plurality of first storage transistors, a first select gate transistor comprising a first gate length, and a second select gate transistor comprising a second gate length, wherein the first select gate transistor further comprises a first horizontal channel, and the second select gate transistor further comprises a first recessed channel, and wherein each of the first storage transistors comprises a third gate length, respectively; and a second active area positioned in the substrate, wherein the second active area comprises a second memory cell string comprising a plurality of second storage transistors, a third select gate transistor comprising a fourth gate length, and a fourth select gate transistor comprising a fifth gate length, wherein the third select gate transistor further comprises a second recessed channel, and the fourth select gate transistor further comprises a second horizontal channel, and wherein each of the second storage transistors comprises a sixth gate length, respectively, wherein the first select gate transistor and the third select gate transistor are arranged in the same column, and the second select gate transistor and the fourth select gate transistor are arranged in the same column. 
         [0007]    According to a preferred embodiment of the present invention, the length of the first gate length, the length of the second gate length, the length of the third gate length, the length of the fourth gate length, the length of the fifth gate length, and the length of the sixth gate length are substantially equal. 
         [0008]    The select gate transistor of the present invention includes a recessed channel, which can provide a larger process window, and integration of the elements can be increased due to the recessed channel as well. 
         [0009]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a schematic layout of a NAND type flash memory according to the present invention. 
           [0011]      FIG. 2   a  shows a sectional view as viewed along the active area  54  in  FIG. 1  according to the NAND type flash memory of the present invention. 
           [0012]      FIG. 2   b  shows a sectional view as viewed along the active area  68  in  FIG. 1  according to the NAND type flash memory of the present invention. 
           [0013]      FIG. 2   c  shows a sectional view as viewed along the active area  80  in  FIG. 1  according to the NAND type flash memory of the present invention. 
           [0014]      FIG. 2   d  shows a sectional view as viewed along the active area  92  in  FIG. 1  according to the NAND type flash memory of the present invention. 
           [0015]      FIG. 3  to  FIG. 5  show the operating method of the NAND type flash memory according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The structure of the NAND type flash memory disclosed in the present invention features a structure of a plurality of dual gate transistors (dual SG), which is meant to have two ends of the memory cell strings connected to two select gate transistors in series, respectively. In addition, every storage transistor positioned in the memory cell strings is a two-bit storage transistor. 
         [0017]      FIG. 1  shows a schematic layout of a NAND type flash memory according to the present invention. As shown in  FIG. 1 , a NAND type flash memory  50  comprises: a substrate  52 , a plurality of active areas  54 ,  66 ,  78 ,  90  positioned in the substrate  52 , wherein the active area  54  comprises a plurality of select gate transistors  58 ,  60 , a memory cell string  56 , and a plurality of select gate transistors  62 ,  64 , and wherein the select gate transistors  58 ,  60 , the memory cell string  56 , and the select gate transistors  62 ,  64  are positioned in a same row. In addition, the select gate transistor  58  is coupled to the select gate transistor  60  in series, and the select gate transistor  62  is coupled to the select gate transistor  64  in series. 
         [0018]    Furthermore, the memory cell string  56  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  114 ,  116 ; and the two-bit storage transistors  114 ,  116  comprise a gate length L 1 . In addition, the select gate transistors  60 ,  62  comprise a recessed channel (not shown), and the select gate transistors  58 ,  64  comprise a horizontal channel (not shown). Each of the select gate transistors  58 ,  60 ,  62 ,  64  comprises a gate length L 2 . 
         [0019]    The active area  66  comprises a plurality of select gate transistors  70 ,  72 , a memory cell string  68 , and a plurality of select gate transistors  74 ,  76 , and wherein the select gate transistors  70 ,  72 , a memory cell string  68 , and the select gate transistors  74 ,  76  are positioned in a same row. In addition, the select gate transistor  70  is coupled to the select gate transistor  72  in series, and the select gate transistor  74  is coupled to the select gate transistor  76  in series. 
         [0020]    Furthermore, the memory cell string  68  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  118 ,  120 ; and the two-bit storage transistors  118 ,  120  comprise a gate length L 1 . In addition, the select gate transistors  70 ,  76  comprise a recessed channel (not shown), and the select gate transistors  72 ,  74  comprise a horizontal channel (not shown). Each of the select gate transistors  70 ,  72 ,  74 ,  76  comprises a gate length L 2 . 
         [0021]    The active area  78  comprises a plurality of select gate transistors  82 ,  84 , a memory cell string  80 , and a plurality of select gate transistors  86 ,  88 , and wherein the select gate transistors  82 ,  84 , a memory cell string  80 , and the select gate transistors  86 ,  88  are arranged in a same row. In addition, the select gate transistor  82  is coupled to the select gate transistor  84  in series, and the select gate transistor  86  is coupled to the select gate transistor  88  in series. Furthermore, the memory cell string  80  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  122 ,  124 ; and the two-bit storage transistors  122 ,  124  comprise a gate length L 1 . In addition, the select gate transistors  84 ,  86  comprise a recessed channel (not shown), and the select gate transistors  82 ,  88  comprise a horizontal channel (not shown). Each of the select gate transistors  82 ,  84 ,  86 ,  88  comprises a gate length L 2 . 
         [0022]    Furthermore, the active area  90  comprises a plurality of select gate transistors  94 ,  96 , a memory cell string  92 , and a plurality of select gate transistors  98 ,  100 , and wherein the select gate transistors  94 ,  96 , the memory cell string  92 , and the select gate transistors  98 ,  100  are arranged in a same row. In addition, the select gate transistor  94  is coupled to the select gate transistor  96  in series, and the select gate transistor  98  is coupled to the select gate transistor  100  in series. Furthermore, the memory cell string  92  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  126 ,  128 ; and the two-bit storage transistors  126 ,  128  comprise a gate length L 1 . In addition, the select gate transistors  94 ,  100  comprise a recessed channel (not shown), and the select gate transistors  96 ,  98  comprise a horizontal channel (not shown). Each of the select gate transistors  94 ,  96 ,  98 ,  100  comprises a gate length L 2 . 
         [0023]    According to the preferred embodiment of the present invention, the length of the gate length L 1  can be equal to the length of the gate length L 2 , but is not limited to this limitation; the length of the gate length L 1  and the length of the gate length L 2  can be different as well. In addition, the select gate transistors  58 ,  64 ,  72 ,  74 ,  82 ,  88 ,  96 ,  98  comprising a horizontal channel is always remained at depletion mode during operation, which means that the select gate transistors  58 ,  64 ,  72 ,  74 ,  82 ,  88 ,  96 ,  98  are always turned on during operation. 
         [0024]    Additionally, the select gate transistors  58 ,  70 ,  82 ,  94  arranged in a same column are coupled to each other in sequence electrically through a gate conductor  102 . The select gate transistors  60 ,  72 ,  84 ,  96  positioned in a same column are coupled to each other in sequence electrically through a gate conductor  104 . The select gate transistors  62 ,  74 ,  88 ,  98  positioned in a same column are coupled to each other in sequence electrically through a gate conductor  106 . The select gate transistors  64 ,  76 ,  88 ,  100  positioned in a same column are coupled to each other in sequence electrically through a gate conductor  108 . In addition, a plurality of bit-line contact pads  110 ,  112  are positioned at a side of the gate conductors  102 ,  108  respectively to send the bit-line signals. 
         [0025]    Because the select gate transistors of the present invention comprise a recessed channel, a larger window process can be obtain during the STI formation, and the integration of the elements can be increased as well. For example, the width of the gate conductor can shrink to 0.09 μm, and the space of the gate conductor can shrink to 0.09 μm. Therefore, the space which the gate conductor has occupied according to the present invention is smaller than the space which the gate conductor occupied according to the conventional process. 
         [0026]      FIG. 2   a  shows a sectional view as viewed along the active area  54  in  FIG. 1  according to the NAND type flash memory of the present invention. 
         [0027]    As shown in  FIG. 2   a , the flash memory  50  comprises a substrate  52 , a memory cell string  56  positioned on the substrate  52 , a select gate transistor  60  comprising a recessed channel and a gate length L 2 , a select gate transistor  58  comprising a horizontal channel and a gate length L 2 , a select gate transistor  62  comprising a recessed channel and a gate length L 2 , and a select gate transistor  64  comprising a horizontal channel and a gate length L 2 . In addition, the memory cell string  56  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  114 ,  116  comprising a gate length L 1 , wherein the number of the two-bit storage transistors comprised in the memory cell string  56  can be 16 or 32, and all of the two-bit storage transistors may be PMOS transistors. 
         [0028]    Furthermore, the select gate transistor  60  is connected directly to one end of the memory cell string  56  in series, and the select gate transistor  58  is connected directly to the select gate transistor  60  in series. The select gate transistor  62  is connected directly to another end of the memory cell string  56  in series, and the select gate transistor  64  is connected directly to the select gate transistor  62  in series. 
         [0029]    According to a preferred embodiment of the present invention, the select gate transistors  58 ,  64  comprising a horizontal channel are always in a depletion mode, which means that the select gate transistors  58 ,  64  are always turned on during operation. 
         [0030]      FIG. 2   b  shows a sectional view as viewed along the active area  66  in  FIG. 1  according to the NAND type flash memory of the present invention. As shown in  FIG. 2   b , the flash memory  50  comprises a substrate  52 , a memory cell string  68  positioned on the substrate  52 , a select gate transistor  72  comprising a horizontal channel and a gate length L 2 , a select gate transistor  70  comprising a recessed channel and a gate length L 2 , a select gate transistor  74  comprising a horizontal channel and a gate length L 2 , and a select gate transistor  76  comprising a recessed channel and a gate length L 2 . In addition, the memory cell string  68  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  118 ,  120 , which comprise a gate length L 1 , wherein the number of the two-bit storage transistors comprised in the memory cell string  68  can be 16 or 32, and all of the two-bit storage transistors may be PMOS transistors. Furthermore, the select gate transistor  72  is connected directly to one end of the memory cell string  68  in series, and the select gate transistor  70  is connected directly to the select gate transistor  72  in series. The select gate transistor  74  is connected directly to another end of the memory cell string  68  in series, and the select gate transistor  76  is connected directly to the select gate transistor  74  in series. 
         [0031]    According to a preferred embodiment of the present invention, the select gate transistors  72 ,  74  comprising a horizontal channel are always in a depletion mode, which means that the select gate transistors  72 ,  74  are always turned on during operation. 
         [0032]      FIG. 2   c  shows a sectional view as viewed along the active area  78  in  FIG. 1  according to the NAND type flash memory of the present invention. As shown in  FIG. 2   c , the flash memory  50  comprises a substrate  52 , a memory cell string  80  positioned on the substrate  52 , a select gate transistor  84  comprising a recessed channel and a gate length L 2 , a select gate transistor  82  comprising a horizontal channel and a gate length L 2 , a select gate transistor  86  comprising a recessed channel and a gate length L 2 , and a select gate transistor  88  comprising a horizontal channel and a gate length L 2 . In addition, the memory cell string  80  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  122 ,  124  comprising a gate length L 1 , wherein the number of the two-bit storage transistors comprised in the memory cell string  80  can be 16 or 32, and all of the two-bit storage transistors may be PMOS transistors. Furthermore, the select gate transistor  84  is connected directly to one end of the memory cell string  80  in series, and the select gate transistor  82  is connected directly to the select gate transistor  84  in series. The select gate transistor  84  is connected directly to another end of the memory cell string  80  in series, and the select gate transistor  88  is connected directly to the select gate transistor  86  in series. 
         [0033]    According to a preferred embodiment of the present invention, the select gate transistors  82 ,  88  comprising a horizontal channel are always in a depletion mode, which means that the select gate transistors  82 ,  88  are always turned on during operation. 
         [0034]      FIG. 2   d  shows a sectional view as viewed along the active area  92  in  FIG. 1  according to the NAND type flash memory of the present invention. As shown in  FIG. 2   d , the flash memory  50  comprises a substrate  52 , a memory cell string  92  positioned on the substrate  52 , a select gate transistor  96  comprising a horizontal channel and a gate length L 2 , a select gate transistor  94  comprising a recessed channel and a gate length L 2 , a select gate transistor  98  comprising a horizontal channel and a gate length L 2 , and a select gate transistor  100  comprising a recessed channel and a gate length L 2 . In addition, the memory cell string  92  comprises a plurality of two-bit storage transistors, such as the two-bit storage transistors  126 ,  128 , which comprise a gate length L 1 , wherein the number of the two-bit storage transistors comprised in the memory cell string  92  can be 16 or 32, and all of the two-bit storage transistors may be PMOS transistors. Furthermore, the select gate transistor  96  is connected directly to one end of the memory cell string  92  in series, and the select gate transistor  94  is connected directly to the select gate transistor  96  in series. The select gate transistor  98  is connected directly to another end of the memory cell string  92  in series, and the select gate transistor  100  is connected directly to the select gate transistor  98  in series. According to a preferred embodiment of the present invention, the select gate transistors  96 ,  98  comprising a horizontal channel are always in a depletion mode, which means that the select gate transistors  96 ,  98  are always turned on during operation. 
         [0035]      FIG. 3  to  FIG. 5  show the operating method of the NAND type flash memory according to the present invention. 
         [0036]      FIG. 3  shows the operating method in which the memory cell strings  68 ,  92  are read simultaneously. 
         [0037]    First, the select gate transistor having the symbol “◯” depicted on it means that the select gate transistor is turned on, and the select gate transistor having the symbol “X” depicted on it means that the select gate transistor is turned off. 
         [0038]    As shown in  FIG. 3 , the gate conductors  102 ,  108  are turned off, the gate conductors  104 ,  106  are turned on, 1 volt is applied to the memory cell strings  56 ,  68 ,  80 ,  92 , 0 volt is applied to the bit-line contact pad  110 , 2.5 volts is applied to the bit-line contact pad  112 , and 0 volt is applied to the substrate  52  (not shown). 
         [0039]    Notably, the select gate transistors  58 ,  64 ,  72 ,  74 ,  82 ,  88 ,  96 ,  98  are always turned on, because they are in the depletion mode. Therefore, the turning on and off of the gate conductors  102 ,  104 ,  106 ,  108  are to only control the on and off of the select gate transistors  76 ,  84 ,  86 ,  94 ,  100 . In this way, the data stored in the memory cell strings  68 ,  92  can be read. 
         [0040]    In  FIG. 4  and  FIG. 5 , the select gate transistor having the symbol “◯” depicted on it means that the select gate transistor is turned on, and the select gate transistor having the symbol “X” depicted on it means that the select gate transistor is turned off. 
         [0041]      FIG. 4  shows the operating method in which memory cell strings  68 ,  92  are programmed. As shown in  FIG. 4 , the gate conductors  102 ,  108  are turned off, the gate conductors  104 ,  106  are turned on, 6 volts is applied to memory cell strings  56 ,  68 ,  80 ,  92 , −3 volts is applied to the bit-line contact pad  112 , 0 volt (not shown) is applied to the substrate  52 , and the bit-line contact pad  110  is floating. In this way, data can be programmed into the two-bit storage transistors  114 ,  116 ,  126 ,  128  which are comprised in the memory cell strings  68 ,  92 , respectively. 
         [0042]      FIG. 5  shows the operating method in which the memory cell strings  56 ,  68 ,  80 ,  92  are block erased. As shown in  FIG. 5 , all of the gate conductors  102 ,  104 ,  106 ,  108  are turned on, −7 volts is applied to the memory cell strings  56 ,  68 ,  80 ,  92 , 8 volts is applied to the bit-line contact pad  112 , 8 volts is applied to the bit-line contact pad  110 , and 8 volts (not shown) is applied to the substrate  52 . In this way, data stored in the memory cell strings  56 ,  68 ,  80 ,  92  can be block erased. 
         [0043]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.