Patent Publication Number: US-8531885-B2

Title: NAND-based 2T2b NOR flash array with a diode connection to cell&#39;s source node for size reduction using the least number of metal layers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application Ser. No. 61/396,553, filed on May 28, 2010, assigned to the same assignee as the present invention, and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the cell array architecture of nonvolatile memory devices, and more particularly to the cell array architecture of nonvolatile NAND-based NOR flash memory devices. 
     2. Description of Related Art 
     Nonvolatile memory is well known in the art. The different types of nonvolatile memory include read-only-memory (ROM), electrically programmable read only Memory (EPROM), electrically erasable programmable read only memory (EEPROM), NOR flash memory, and NAND flash memory. In current applications such as personal digital assistants, cellular telephones, notebook and laptop computers, voice recorders, global positioning systems, etc., the flash memory has become one of the more popular types of nonvolatile memory. Flash memory has the combined advantages of high density, small silicon area, low cost and can be repeatedly programmed and erased with a single low-voltage power supply voltage source. 
     The flash memory structures known in the art employ a charge storage mechanism and a charge trapping mechanism. In the charge storage regime, as with a floating gate nonvolatile memory, the charge representing digital data is stored on a floating gate of the device. The stored charge modifies the threshold voltage of the floating gate memory cell to determine the digital data stored. In a charge trapping regime, as in a silicon-oxide-nitride-oxide-silicon (SONOS) or metal-oxide-nitride-oxide-silicon (MONOS) type cell, the charge is trapped in a charge trapping layer between two insulating layers. The material such as silicon nitride (SiN X ) in the charge trapping layer in the SONOS/MONOS devices has a relatively high dielectric constant (k). 
     Both N-channel and P-channel flash memory cells could be employed to form the memory array. Hereafter, the N-channel flash memory cell is used as an example for description. In general, the unit size of a NAND flash memory cell is relatively smaller than the one of a NOR flash memory cell. In the conventional one transistor (1T) NOR flash memory, it uses channel hot electron (CHE) program scheme and Fowler-Nordheim tunneling erase scheme. Due to the requirement of high enough lateral electric field and electron-hole pairs generated at N+/P+ junction with the biased voltage, the channel length can not be formed by the smallest feature size in the most advanced process because of the punch-through concern. In other words, the scalability becomes worse when the advanced technology has a small geometry such as 130 nm or below. 
     As for the conventional NAND flash memory N-Transistor string, it can take advantage of the smallest feature size in the most advanced process. However, the requirement of two source line (SL) and bit line (BL) select gate transistors along the NAND string to isolate from other NAND string requires certain extra percentage of overhead in the size of the whole memory array. Due to the punch-through concern on those two SL and BL select gate transistors, their channel length can not be made with the smallest feature size in the most advanced process either. In other words, the smallest feature size can only be applied to the memory cell with the cost of more percentage of overhead for the SL and BL select gate transistors. 
     In the NOR flash memory, there is another type called 2T NOR flash memory that is formed by one access transistor and one memory cell. No over-erase issue and less hot carrier injection (HCI) are the advantages of the 2T NOR flash memory because of the access transistor. However, the problem of punch-through effect is just moved from the memory cell to the access transistor. 
     U.S. Pat. No. 6,212,102 discloses a 2T NOR flash cell in which a high voltage is required across the drain and source nodes of the flash cell during FN-edge programming and a longer channel length is also required to prevent the punch-through effect. This causes a physical limitation on how small the cell can be made and in turn limits the use of the cell in ultra high integrated levels of the flash memory below 0.18 μm technology. Furthermore, the negative FN-edge programming causes device oxide degradation because the electron-hole pairs at the biased drain/TPW (triple P-Well) junction are accelerated by the voltage difference between the drain and source nodes. The more holes are trapped in the tunneling oxide, the less P/E endurance cycles will be attained. 
     U.S. Pat. Nos. 6,307,781 and 6,628,544 disclose improvements over U.S. Pat. No. 6,212,102 with uniform channel erase and channel program operations. However, by connecting the common source together in the array, the gate of the access device has to be applied with the most negative voltage, e.g., −3V, to turn off the path to different bit lines through the common source line. Due to this biased condition during the program operation, the program inhibit voltage, i.e., 3V-4V is supposedly isolated by the access device. However, the drain induced leakage current may occur if the channel length is scaled down. Therefore, the flash cell still faces the scaling issue and can only be manufactured with a large memory cell size. 
     U.S. Pat. No. 6,980,472 presents another flash memory in which both source side injection programming and FN channel programming schemes are disclosed. In the memory array, the same scaling issue exists. For the channel programming, it is similar to the one disclosed in U.S. Pat. Nos. 6,307,781 and 6,628,544. The short channel length in the access device cannot stop the drain induced leakage current to the common source line while the program inhibit voltage is applied across the drain region and the source region. In a same manner, for the source side injection programming scheme, the access device needs a longer channel length to prevent the punch-through effect. In addition, compared to the FN channel programming, the flash memory needs more program current because of the hot-electron generation. 
       FIG. 1  is a schematic diagram of a 2T string NAND-based NOR flash memory array  10  with separate bit lines and source lines to overcome the above mentioned drawbacks. In the array, the source lines SL 0 -SL 1  are structured as parallel to the bit lines BL 0 -BL 1  and orthogonal to the word lines WL 0 -WL 3 . With this structure, the program voltage and program inhibit voltage can be applied to each bit line or source line respectively while performing a program operation. Unlike the traditional array with a common source line, there is no voltage difference between the drain node and the source node of a flash cell in the memory array of  FIG. 1  during the program operation. As a result, no punch through problem needs to be taken into consideration and memory size scalability can be attained. In the advanced process such as 90 nm and beyond, the advantage of the small memory cell size in the 2T string NAND-based NOR flash memory is evident. 
       FIG. 2A  is a layout diagram  20  of the 2T string NAND-based NOR flash array shown in  FIG. 1 . In the layout, both the source lines SL 0 -SL 1  and the bit lines BL 0 -BL 1  are formed on the same metal layer. By using M 1  metal layer as the bit lines and source lines, the effective memory cell size is larger because of the M 1  metal layer spacing. The memory cell size can be reduced by using more metal layers to implement the bit lines and source lines. 
       FIG. 2B  is another layout diagram  25  of the 2T string NAND-based NOR flash array shown in  FIG. 1 . In the layout, the source lines SL 0 -SL 1  and the bit lines BL 0 -BL 1  are formed by three different metal layers. By using the connection of M 1  metal layer, M 2  metal layer and M 3  metal layer to place the bit lines and the source lines, the effective memory cell size is reduced because of the utilization of multiple metal layers to save the area. 
     The needs of an improved array architecture for NOR flash memories to be manufactured with the smallest feature size for cell size reduction in the advanced technology without concerns of the drain induced leakage current and punch-through issue are obvious from the above discussion. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the above-mentioned drawbacks of the conventional NOR flash memories. A novel memory array architecture in which a diode connection to the source node of each cell is provided for the nonvolatile two transistor two bit (2T2b) NAND-based fast-random-read NOR flash cell for code storage. Different operations according to the new array architecture are also provided. According to the invention, only one metal layer is required for the preferred 2T2b NAND-based NOR cell array. Furthermore, because the NOR flash cell uses the same technology as the cells of NAND memories, a unified combo NAND and NOR flash design can be integrated into one single chip for both code and data storages in the growing mobile handset applications. 
     Accordingly, an object of the present invention is to provide a novel cell structure with a diode connected between the source line and the source node of the nonvolatile memory cell which can be 1T (one transistor) NOR, 2T string (two transistor string) NAND-based NOR or NT string (N transistor string) NAND flash structure. 
     Another object of the present invention is to provide a floating gate and SONOS type flash memory cell having a smallest feature size in the advanced technology. Device scaling can be achieved without any crucial limitation because of the reversed diode connection along the source line. 
     A further object of the present invention is to use FN channel program and FN channel erase schemes for the floating gate and SONOS type flash memory. A zero voltage difference is provided between the drain node and the source node of the 1T, 2T or NT-string flash cell in erase and program operations. A highly scalable symmetrical memory cell can thus be attained. 
     It is also an object of the present invention to provide preferred voltages for selected word lines, bit lines, source lines and TPW so that the uniform FN channel program and FN channel erase operations can incur less tunneling oxide degradation to achieve high P/E endurance cycles. 
     It is another object of the present invention to provide a preferred voltage for the unselected word lines for a channel program operation so that the disturbance of the threshold voltage (Vt) of the unselected cells can be eliminated or substantially reduced. 
     Yet another object of the present invention is to apply an appropriate voltage to the source line while the flash cell is read. The states of the erase and program operations can be distinguished by the conduction status of the selected memory cell. The forward current of the diode can be detected once the memory cell stays at a conduction mode. 
     It is yet a further object of the invention to provide a Schottky diode between the source line and the source node of the memory cell, wherein the source line is made of a metal layer M 0  and is directly in contact with the source node of the memory cell. The source node of the memory cell is lightly doped with an N− region to form the Schottky barrier diode. 
     Another object of the invention is to provide a Schottky diode between the source line and the source node of the memory cell, wherein the source line is made of a metal layer M 1  and is connected to the source node of the memory cell through a contact. The source node of the memory cell is lightly doped with an N− region to form the Schottky barrier diode. 
     Still an object of the invention is to provide a pillar-structured polysilicon diode between the source line and the source node of the memory cell. The source line is made of a metal layer M 1  or a doped poly layer and is connected to the source node of the memory cell through the polysilicon diode and a contact. The source node of the memory cell is doped with the same concentration as the drain node of the memory cell. 
     A further object of the invention is to provide a P/N+ silicon diode between the source line and the source node of the memory cell. The source line is made of a metal layer M 1  and is in connection with the source node of the memory cell through a contact. The doped P silicon is enclosed by the N+ region at the source node of the memory cell which is doped with the same concentration as the drain node. 
     It is a further object of the invention to provide a P/N+ silicon diode between the source line and source node of the memory cell. The source line is made of a metal layer M 0  and is in contact with the P silicon. The doped P silicon is enclosed by the N+region at the source node of the memory cell which is doped with the same concentration as the drain node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be apparent to those skilled in the art by reading the following description, with reference to the attached drawings brief summarized below. 
         FIG. 1  shows a schematic diagram of a 2T string NAND-based NOR flash memory array with separate bit lines and source lines. 
         FIG. 2A  shows a corresponding layout diagram of the 2T string NAND-based NOR flash memory array in  FIG. 1  with separate bit lines and source lines made with M 1  metal layer. 
         FIG. 2B  shows a corresponding layout diagram of the 2T string NAND-based NOR flash array in  FIG. 1  with separate bit lines and source lines made with multiple metal layers. 
         FIG. 3A  shows a schematic diagram of a 1T NOR flash array by using a diode to connect the source of each individual memory cell to one common source line according to the present invention. 
         FIG. 3B  shows a schematic diagram of a modified 2T string NAND-based NOR flash array by using a diode to connect the source of each individual memory cell to one common source line according to the present invention. 
         FIG. 3C  shows a schematic diagram of a modified NT string NAND-based NOR flash array by using a diode to connect the source of each individual memory cell to one common source line according to the present invention. 
         FIG. 4  shows the bias condition applied to the modified 2T string NAND-based NOR flash array for an erase operation according to the present invention. 
         FIG. 5  shows the bias condition applied to the modified 2T string NAND-based NOR flash array for a program operation according to the present invention. 
         FIG. 6  shows the bias condition applied to the modified 2T string NAND-based NOR flash array for a read operation according to the present invention. 
         FIG. 7A  is a cross sectional view of two pairs of the modified 2T string NAND-based NOR flash cells illustrated in  FIG. 3B  according to one embodiment of the present invention. 
         FIG. 7B  is a cross sectional view of the modified 2T string NAND-based NOR flash cells of  FIG. 7A  taken along section A-A′ thereof. 
         FIG. 7C  is a layout diagram of the modified 2T string NAND-based NOR flash cells of  FIG. 7A  with four word lines, four bit lines, one common source line and four individual Schottky diodes. 
         FIG. 8  is an I-V curve of a normal ohmic contact. 
         FIG. 9  is an I-V curve of a Schottky barrier contact. 
         FIG. 10A  is a cross sectional view of two pairs of the modified 2T string NAND-based NOR flash cells illustrated in  FIG. 3B  according to another embodiment of the present invention. 
         FIG. 10B  is a cross sectional view of the modified 2T string NAND-based NOR flash cells of  FIG. 10A  taken along section A-A′ thereof. 
         FIG. 10C  is a layout diagram of the modified 2T string NAND-based NOR flash cells of  FIG. 10A  with four word lines, four bit lines, one common source lines and four individual Schottky diodes. 
         FIG. 11A  is a cross sectional view of two pairs of the modified 2T string NAND-based NOR flash cells illustrated in  FIG. 3B  according to an alternative embodiment of the present invention. 
         FIG. 11B  is a cross sectional view of the modified 2T string NAND-based NOR flash cells of  FIG. 11A  taken along section A-A′ thereof. 
         FIG. 12A  is a cross sectional view of two pairs of the modified 2T string NAND-based NOR flash cells illustrated in  FIG. 3B  according to a further embodiment of the present invention. 
         FIG. 12B  is a cross sectional view of the modified 2T string NAND-based NOR flash cells of  FIG. 12A  taken along section A-A′ thereof. 
         FIG. 13A  is a cross sectional view of two pairs of the modified 2T string NAND-based NOR flash cells illustrated in  FIG. 3B  according to another embodiment of the present invention. 
         FIG. 13B  is a cross sectional view of the modified 2T string NAND-based NOR flash cells of  FIG. 13A  taken along section A-A′ thereof. 
         FIG. 14  is an example of forming Schottky diodes for two pairs of the modified 2T string NAND-based NOR flash cells in the embodiment shown in  FIG. 7A  by forming ohmic contact with plug implant at the drain region and Schottky contact without plug implant at the source region. 
         FIG. 15  is another example of forming Schottky for two pairs of the modified 2T string NAND-based NOR flash cells in the embodiment shown in  FIG. 7A  by using an implant mask to block the source region from the blanket N+ implant. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3A  is a schematic diagram  30  of a 1T NOR flash memory array of the present invention in which a diode is used to connect the source node of each individual memory cell to one common source line. The memory array is composed of bit lines BL 0 -BL 1 , word lines WL 0 -WL 1  and one common source line SL. By connecting the source line at the positive end of the diode, the source node of each memory cell is connected to the negative end of the diode. 
       FIG. 3B  is a schematic diagram  40  of a modified 2T string NAND-based NOR flash memory array of the present invention by using a diode to connect the source node of each individual memory cell to one common source line. This memory array is composed of bit lines BL 0 -BL 1 , word lines WL 0 -WL 3  and one common source line SL. By connecting the source line at the positive end of the diode, the source node of each memory cell is connected to the negative end of the diode. 
       FIG. 3C  is a schematic diagram  50  of a modified NT string NAND flash memory array according to the present invention by using a diode to connect the source node of each individual memory cell to one common source line. This memory array is composed of bit lines BL 0 -BL 1 , word lines WL 0 [0]-WL 0 [N−1], WL 1 [0]-WL 1 [N−1] and one common source line SL. By connecting the source line at the positive end of the diode, the source node of each memory cell is connected to the negative end of the diode. Unlike the conventional NT string NAND flash cell, there is no bit line select gate and source line select gate connected to the drain node and the source node of the flash cell in the present invention. 
       FIG. 4  shows the bias condition  60  applied to the modified 2T string NAND-based NOR flash memory array of the present invention for an erase operation. The erase operation is accomplished by using the channel Fowler-Norheim tunneling scheme. During an erase operation, the selected word lines are coupled with Vers=0V in the illustrated diagram. For the P-type well (TPW), it is coupled to a high voltage (HV) level of approximately +20.0V. The common source line and the bit lines are set as floating (FL). After a predetermined time for erasure, the electrons in the memory cell along the selected word lines are expelled from the floating gate to TPW through the insulating gate oxide. 
     For the unselected word lines, they are set to floating. Once TPW is applied to approximately +20.0V, those unselected word lines are also coupled to approximately +20.0V too. No electrons are expelled out of the floating gate of the memory cells. As a result, the threshold voltage of the memory cells will be decreased. According to the present invention, the bias condition for the erase operation can also be accomplished by splitting the voltage applied on TPW and Vers. In other words, TPW can be applied with 10V and the selected word line voltage Vers can be applied with −10V. Similarly, the bias condition can also be achieved by applying the most negative voltage −20V to the selected WL and 0V to TPW. 
     During the erase operation of the present invention, there is no voltage difference between the source node and the drain node of the memory cell. Due to the forward biased condition between TPW and N+ junction of the drain node and source node of the memory cells, no punch-through effect may occur. As for the diode connected to the common source line, it is set with a reversed bias condition. The floating common source line can be coupled with a rational voltage from TPW. Therefore, no break down situation needs to be taken into consideration. 
       FIG. 5  shows the bias condition  70  applied to the modified 2T string NAND-based NOR flash memory array of the present invention for a program operation. The program operation is also accomplished by using the channel Fowler-Norheim tunneling scheme. During a program operation, Vpgm is applied to the selected word line and Vpass is applied to the adjacent word line as illustrated in the diagram. Vpgm is a step-wise pulse voltage from 15V to 20V and Vpass is about 10V. The unselected word lines are applied with Vinh which is approximately one half of the bit line inhibit voltage Vbl_inh. In other words, the bit line inhibit voltage Vbl_inh is set at about 8V-10V, and Vinh is about 4V-5V. In the illustrated diagram, bit line voltage Vbl_pgm is set to 0V for programming the selected memory cell. As for the selected but not to be programmed memory cell, the bit line voltage Vbl_inh is set to 8V-10V. Both the source line SL and the P-type well (TPW) are set to 0V. 
     After a predetermined time for programming, the electrons in the TPW along the selected word lines are injected to the floating gate through the insulating gate oxide because the high electric field has been established for the programmed memory cells. On the contrary, for the selected but non-programmed memory cells, the low electric field can not generate enough electrons to be injected to the floating gate through the insulating gate oxide. In a same manner, for most of the unselected memory cells, the lowest electric field established between the floating gate and TPW can not generate enough electrons to be injected to the floating gate through the insulating gate oxide either. As a result, the threshold voltage of the selected memory cell to be programmed according to the desired program pattern is increased. The threshold voltage of the memory cell selected but to be inhibited is not changed. The threshold voltages of all the unselected memory cells are not changed either. 
     In the program operation, there is no voltage difference between the source nodes and the drain nodes of the memory cells regardless of the program pattern applied to the bit lines. For the selected 2T string NAND-based NOR cell shown in the illustrated diagram, the inhibit voltage Vbl_inh on the bit line can be presented to the source node because of the high voltage Vpgm and Vpass on the word lines. Therefore, no voltage difference exists between the source node and the drain node of the memory cell. As for all the remaining unselected memory cells, the word line voltage Vinh is about 4-5V which is not high enough to turn on the memory cells. Therefore, the punch-through phenomena may occur as long as the channel length is featured as the smallest geometry size in the most advanced technology. The inhibit voltage on the bit lines will be presented to the source nodes of all the remaining unselected memory cells. However, because the common source line is applied with 0V and connected through a diode to the source node of each memory cell, the reversed bias condition always prevents the leakage current to the common source line from occurring and ensures the success of the program operation on the selected memory cells. In the conventional 2T NOR flash cell, a longer channel length has to be chosen to prevent the punch-through problem and the leakage current. In the present invention, the punch-through phenomena can be tolerated without any leakage current during the program operation. 
       FIG. 6  shows the bias condition  80  applied to the modified 2T string NAND-based NOR flash memory array of the present invention for a read operation. In the read operation, the common source line is applied with a certain voltage, e.g., 1.5V. The selected word line of the modified 2T string NAND-based NOR flash memory cell is applied with Vread and the adjacent word line is applied with Vpass. Typically, Vread is a voltage higher than the highest threshold voltage of the erased cell by 1.5V˜2V or VDD as long as it meets the previous criteria. Here, Vpass should be higher than the highest threshold voltage of the programmed cell by 2V. All the other unselected word lines and TPW are applied with 0V. Due to the forward biased diode connection between the common source line and the source node of the memory cell, the read current flows from the source node to the drain node through the memory cell which is in an erased state. On the contrary, there is no or less current flowing from the source node to the drain node through a memory cell in a programmed state. 
       FIG. 7A  is a cross sectional view  100  of two pairs of modified 2T string NAND-based NOR flash cells  41  as shown in  FIG. 3B  according to the present invention. The gates of transistors C 00 , C 10 , C 20  and C 30  are made of poly2 conduction layer. G 0   104   a , G 1   104   b , G 2   104   c , and G 3   104   d  are the poly2-gates of the 2-poly storage transistors C 00 , C 10 , C 20  and C 30 . As described above, all seven nodes BL 0   122 ,  102   a ,  102   b ,  102   c ,  102   d , SL  126  and TPW  124  of the two pairs of modified 2T string NAND-based NOR flash cells have to be coupled with appropriate bias conditions in the circuit for respective operations. 
     Underneath the poly2-gates  104   a ,  104   b ,  104   c  and  104   d , those NMOS device transistors have ONO as the gate dielectric shown as  106   a ,  106   b ,  106   c  and  106   d , poly1 floating gates  108   a ,  108   b ,  108   c  and  108   d  and tunneling oxides  110   a ,  110   b ,  110   c  and  110   d . The electrons injected from TPW  124  can be retained in the floating gates  108   a ,  108   b ,  108   c  and  108   d  while the flash cell is programmed. On the contrary, the stored electrons in the floating gates  108   a ,  108   b ,  108   c  and  108   d  are pulled out toward TPW  124  while an erase operation is performed. SL  126  is made of M 0  metal layer and is directly connected to lightly doped N− region  116  to form the Schottky barrier diode. The heavily doped N+ drain regions  114 ,  120  and drain/source regions  112 ,  118  of the two pairs of modified 2T string NAND-based NOR flash cells are implanted with phosphorus on TPW  124 . BL 0   122  is made of M 1  metal layer and connected to the drain regions  214  and  220  through the ohmic contacts  121   a  and  121   b.    
       FIG. 7B  is a cross sectional view of the modified 2T string NAND-based NOR flash cell in  FIG. 7A  taken at section A-A′ thereof. There are three Schottky diodes  128   a ,  128   b  and  128   c  formed between M 0  metal layer  126  and N− source regions  116   a ,  116   b  and  116   c  on TPW  124 . These diodes are isolated by the insulators  130   a ,  130   b ,  130   c  and  130   d  respectively. 
       FIG. 7C  is a layout diagram of the modified 2T string NAND-based NOR flash cell array corresponding to  FIG. 7B  with four word lines, four bit lines, one common source line and four individual Schottky diodes according to the present invention. The four word lines WL 0 , WL 1 , WL 2  and WL 3  are made of poly2. The four bit lines BL 0 , BL 1 , BL 2  and BL 3  are formed in M 1  metal layer. The overlapped area between word lines and bit lines is the flash memory cell, where the floating gate (FG) is used to store the charge. One common SL is made of M 0  metal layer and is in parallel with the word lines. Four individual Schottky diodes are formed at the overlapped area between the four bit lines and one common source line SL. 
       FIG. 8  is an I-V curve of a normal ohmic contact. According to the characteristics of the semiconductor, the current-voltage (I-V) curve is linear and symmetric on the region of a semiconductor device. As shown  FIG. 7C  and  FIG. 10C , there are eight ohmic contacts located at overlapped area between the four bit lines and the drain regions of the memory cells. 
       FIG. 9  is an I-V curve of a Schottky barrier contact. As can be seen, the current-voltage (I-V) curve is nonlinear and asymmetric on the region of a semiconductor device. A Schottky barrier is a potential barrier at the junction between metal and semiconductor. As shown in the I-V curve, it provides the capability to rectify the current and is suitable as a diode. Compared to a P-N junction diode, it has a lower junction voltage and a decreased depletion width in the metal. Proper control for the concentration of the dopants in the semiconductor, the metal&#39;s work function, the gap of the intrinsic semiconductor and other manufacturing factors can achieve the rectifying I-V curve as shown in  FIG. 9 . 
       FIG. 10A  is a cross sectional view  200  of two pairs of modified 2T string NAND-based NOR flash cells  41  as shown in  FIG. 3B  according to another embodiment of the present invention. The gates of transistors C 00 , C 10 , C 20  and C 30  are made of poly2 conduction layer. G 0   204   a , G 1   204   b , G 2   204   c , and G 3   204   d  are the poly2-gates of the 2-poly storage transistors C 00 , C 10 , C 20  and C 30 . As described above, all seven nodes BL 0   222 ,  202   a ,  202   b ,  202   c ,  202   d , SL  226   c  and TPW  224  of the two pairs of modified 2T string NAND-based NOR flash cells have to be coupled with appropriate bias conditions in the circuit for respective operations. 
     Underneath the poly2-gates  204   a ,  204   b ,  204   c  and  204   d , those NMOS device transistors have ONO as the gate dielectric shown as  206   a ,  206   b ,  206   c  and  206   d , poly1 floating gates  208   a ,  208   b ,  208   c  and  208   d  and tunneling oxides  210   a ,  210   b ,  210   c  and  210   d . The electrons injected from TPW  224  can be retained in the floating gates  208   a ,  208   b ,  208   c  and  208   d  while the flash cell is programmed. On the contrary, the stored electrons in the floating gates  208   a ,  208   b ,  208   c  and  208   d  are pulled out toward TPW  224  while an erase operation is performed. SL  226   c  is made of M 1  metal layer and is connected to lightly doped N− region  216  through contact CT to form the Schottky diode. The heavily doped N+ drain regions  214 ,  220  and drain/source regions  212 ,  218  of the two pairs of modified 2T string NAND-based NOR flash cells are implanted with phosphorus on TPW  224 . BL 0   222  is made of M 2  metal layer and connected to the drain regions  214  and  220  through the ohmic contacts  221   a  and  221   b , M 1  metal  226   a ,  226   b , and vias  223   a  and  223   b.    
       FIG. 10B  is a cross sectional view of the modified 2T string NAND-based NOR flash cell in  FIG. 10A  taken at section A-A′ thereof. There are three Schottky diodes  228   a ,  228   b  and  228   c  formed between M 1  metal layer  226   c  and N− source regions  216   a ,  216   b  and  216   c  through contacts  221   c - 1 ,  221   c - 2 ,  221   c - 3  on TPW  224 . These diodes are isolated by the insulators  230   a ,  230   b ,  230   c  and  230   d  respectively. 
       FIG. 10C  is a layout diagram of the modified 2T string NAND-based NOR flash cell array corresponding to  FIG. 10B  with four word lines, four bit lines, one common source lines and four individual Schottky diodes according to the present invention. The four word lines WL 0 , WL 1 , WL 2  and WL 3  and made of poly2. The four bit lines BL 0 , BL 1 , BL 2  and BL 3  are formed in M 2  metal layer. The overlapped area between word lines and bit lines is the flash memory cell, where the floating gate (FG) is used to store the charge. One common SL is made of M 1  metal layer and is in parallel with the word lines. Four individual Schottky diodes are formed at the overlapped area between four bit lines and one common source line SL. 
       FIG. 11A  is a cross sectional view  300  of two pairs of modified 2T string NAND-based NOR flash cells  41  as shown in  FIG. 3B  according to an alternative embodiment of the present invention. The gates of transistors C 00 , C 10 , C 20  and C 30  are made of poly2 conduction layer. G 0   304   a , G 1   304   b , G 2   304   c , and G 3   304   d  are the poly2-gates of the 2-poly storage transistors C 00 , C 10 , C 20  and C 30 . As described above, all seven nodes BL 0   322 ,  302   a ,  302   b ,  302   c ,  302   d , SL  326  and TPW  324  of the two pairs of modified 2T string NAND-based NOR flash cells have to be coupled with appropriate bias conditions in the circuit for respective operations. 
     Underneath the poly2-gates  304   a ,  304   b ,  304   c  and  304   d , those NMOS device transistors have ONO as the gate dielectric shown as  306   a ,  306   b ,  306   c  and  306   d , poly1 floating gates  308   a ,  308   b ,  308   c  and  308   d  and tunneling oxides  310   a ,  310   b ,  310   c  and  310   d . The electrons injected from TPW  324  can be retained in the floating gates  308   a ,  308   b ,  308   c  and  308   d  while the flash cell is programmed. On the contrary, the stored electrons in the floating gates  308   a ,  308   b ,  308   c  and  308   d  are pulled out toward TPW  324  while an erase operation is performed. SL  326  is made of polysilicon or a metal layer and is connected to N+ junction  316  through the pillar-structured polysilicon diode  328  and one conductive layer  317 . The heavily doped N+ drain regions  314 ,  320  and drain/source regions  312 ,  318  of the two pairs of modified 2T string NAND-based NOR flash cells are implanted with phosphorus on TPW  324 . BL 0   322  is made of M 1  metal layer and connected to the drain regions  314  and  320  through the ohmic contacts  321   a  and  321   b.    
       FIG. 11B  is a cross sectional view of the modified 2T string NAND-based NOR flash cell in  FIG. 11A  taken at section A-A′ thereof. There are three pillar-structured polysilicon diodes  328   a ,  328   b  and  328   c  formed and connected between polysilicon  326  and N+ source regions  316   a ,  316   b  and  316   c  through conductive layers  317   a ,  317   b , and  317   c  on TPW  324 . These diodes are isolated by the insulators  330   a ,  330   b ,  330   c  and  330   d  respectively. 
       FIG. 12A  is a cross sectional view  400  of two pairs of modified 2T string NAND-based NOR flash cells  41  as shown in  FIG. 3B  according to another embodiment of the present invention. The gates of transistors C 00 , C 10 , C 20  and C 30  are made of poly2 conduction layer. G 0   404   a , G 1   404   b , G 2   404   c , and G 3   404   d  are the poly2-gates of the 2-poly storage transistors C 00 , C 10 , C 20  and C 30 . As described above, all seven nodes BL 0   422 ,  402   a ,  402   b ,  402   c ,  402   d , SL  426  and TPW  424  of the two pairs of modified 2T string NAND-based NOR flash cells have to be coupled with appropriate bias conditions in the circuit for respective operations. 
     Underneath the poly2-gates  404   a ,  404   b ,  404   c  and  404   d , those NMOS device transistors have ONO as the gate dielectric shown as  406   a ,  406   b ,  406   c  and  406   d , poly1 floating gates  408   a ,  408   b ,  408   c  and  408   d  and tunneling oxides  410   a ,  410   b ,  410   c  and  410   d . The electrons injected from TPW  424  can be retained in the floating gates  408   a ,  408   b ,  408   c  and  408   d  while the flash cell is programmed. On the contrary, the stored electrons in the floating gates  408   a ,  408   b ,  408   c  and  408   d  are pulled out toward TPW  424  while an erase operation is performed. SL  426   c  is made of M 1  metal layer and is connected to P/N+ junction diode  428  through the contact CT  421   c . The heavily doped N+ drain regions  414 ,  420  and drain/source regions  412 ,  418  of the two pairs of modified 2T string NAND-based NOR flash cells are implanted with phosphorus on TPW  424 . BL 0   422  is made of M 2  metal layer and connected to the drain regions  414  and  420  through the ohmic contacts  421   a  and  421   b , M 1  metal  426   a ,  426   b , and vias  423   a  and  423   b.    
       FIG. 12B  is a cross sectional view of the modified 2T string NAND-based NOR flash cell in  FIG. 12A  taken at section A-A′ thereof. There are three P/N+ junction diodes  428   a ,  428   b  and  428   c . The negative ends of those three P/N+ junction diodes  428   a ,  428   b  and  428   c  are the source regions of the memory cells on TPW  424 . The positive ends of those three P/N+ junction diodes  428   a ,  428   b  and  428   c  are connected to the common source line  426   c  by contacts  421   c - 1 ,  421   c - 2 , and  421   c - 3 . These diodes are isolated by the insulators  430   a ,  430   b ,  430   c  and  430   d  respectively. 
       FIG. 13A  is a cross sectional view  500  of two pairs of modified 2T string NAND-based NOR flash cells  41  as shown in  FIG. 3B  according to a further embodiment of the present invention. The gate of transistors C 00 , C 10 , C 20  and C 30  are made of poly2 conduction layer. G 0   504   a , G 1   504   b , G 2   504   c , and G 3   504   d  are the poly2-gates of the 2-poly storage transistors C 00 , C 10 , C 20  and C 30 . As described above, all seven nodes BL 0   522 ,  502   a ,  502   b ,  502   c ,  502   d , SL  526  and TPW  524  of the two pairs of modified 2T string NAND-based NOR flash have to be coupled with appropriate bias conditions in the circuit for respective operations. 
     Underneath the poly2-gates  504   a ,  504   b ,  504   c  and  504   d , those NMOS device transistors have ONO as the gate dielectric shown as  506   a ,  506   b ,  506   c  and  506   d , poly1 floating gates  508   a ,  508   b ,  508   c  and  508   d  and tunneling oxides  510   a ,  510   b ,  510   c  and  510   d . The electrons injected from TPW  524  can be retained in the floating gates  508   a ,  508   b ,  508   c  and  508   d  while the flash cell is programmed. On the contrary, the stored electrons in the floating gates  508   a ,  508   b ,  508   c  and  508   d  are pulled out toward TPW  524  while an erase operation is performed. SL  526  is made of M 0  metal layer and is directly connected to P/N+ junction diode  528 . The heavily doped N+ drain regions  514 ,  520  and drain/source regions  512 ,  518  of the two pairs of modified 2T string NAND-based NOR flash cells are implanted with phosphorus on TPW  524 . BL 0   522  is made of M 1  metal layer and connected to the drain regions  514  and  520  through the ohmic contacts  521   a  and  521   b.    
       FIG. 13B  is a cross sectional view of the modified 2T string NAND-based NOR flash cell in  FIG. 13A  taken at section A-A′ thereof. There are three P/N+ junction diodes  528   a ,  528   b  and  528   c . The negative ends of those three P/N+ junction diodes  528   a ,  528   b  and  528   c  are the source regions of the memory cells on TPW  524 . The positive ends of those three P/N+ junction diodes  528   a ,  528   b  and  528   c  are connected directly to the common source line  526 . These diodes are isolated by the insulators  530   a ,  530   b ,  530   c  and  530   d  respectively. 
       FIG. 14  is a cross sectional view  600  of two pairs of modified 2T string NAND-based NOR flash cells  41  as shown in  FIG. 3B  according to an example of the embodiment shown in  FIG. 7A  of the present invention. In this example, the Schottky contact of the Schottky barrier diode is formed without plug implant at the source region and the ohmic contact is formed with plug implant at the drain region. In the non-salicided contact process, the plug implant is used to reduce the contact resistance. Furthermore, the plug implant can prevent the contact etch from punching through the junction and causing a short to TPW. 
     According to the two pairs of modified 2T string NAND-based NOR flash cells  41  shown in  FIG. 3B , the gates of transistors C 00 , C 10 , C 20  and C 30  are made of poly2 conduction layer in  FIG. 14 . G 0   604   a , G 1   604   b , G 2   604   c , and G 3   604   d  are the poly2-gates of the 2-poly storage transistors C 00 , C 10 , C 20  and C 30 . As described above, all seven nodes BL 0   622 ,  602   a ,  602   b ,  602   c ,  602   d , SL  626  and TPW  624  of the two pairs of modified 2T string NAND-based NOR flash cells have to be coupled with appropriate bias conditions in the circuit for respective operations. 
     Underneath the poly2-gates  604   a ,  604   b ,  604   c  and  604   d , those NMOS device transistors have ONO as the gate dielectric shown as  606   a ,  606   b ,  606   c  and  606   d , poly1 floating gates  608   a ,  608   b ,  608   c  and  608   d  and tunneling oxides  610   a ,  610   b ,  610   c  and  610   d . The electrons injected from TPW  624  can be retained in the floating gates  608   a ,  608   b ,  608   c  and  608   d  while the flash cell is programmed. On the contrary, the stored electrons in the floating gates  608   a ,  608   b ,  608   c  and  608   d  are pulled out toward TPW  624  while an erase operation is performed. SL  626  is made of M 0  metal layer and is directly connected to lightly doped N− region  616  to form the Schottky diode. 
     In order to form the Schottky diode at the source line, the lightly doped N− blanket implant is performed to all the active regions in the memory array first. Therefore, the N− drain regions  614 ,  620  and drain/source regions  612 ,  618  of the two pairs of modified 2T string NAND-based NOR flash cells are also lightly doped with phosphorus on TPW  624 . Because a Schottky diode is not to be formed on the drain regions  614  and  620 , the plug implant is highly doped to regions  613  and  619  as N+ junction regions enclosed by N− junction regions  614  and  620 . BL 0   622  is made of M 1  metal layer and connected to the N+ drain regions  613  and  619  through the ohmic contacts  621   a  and  621   b.    
       FIG. 15  is another example of forming Schottky diode in the two pairs of modified 2T string NAND-based NOR flash cells  41  of  FIG. 3B  according to the embodiment shown in  FIG. 7A  of the present invention. With reference to  FIG. 15 , the cross sectional view  700  shows one implant mask is used to block N+ implant at the source region. In the salicided contact process, plug implant is not required. 
     According to two pairs of modified 2T string NAND-based NOR flash cells  41  shown in  FIG. 3B , the gates of transistors C 00 , C 10 , C 20  and C 30  are made of poly2 conduction layer in  FIG. 15 . G 0   704   a , G 1   704   b , G 2   704   c , and G 3   704   d  are the poly2-gates of the 2-poly storage transistors C 00 , C 10 , C 20  and C 30 . As described above, all seven nodes BL 0   722 ,  702   a ,  702   b ,  702   c ,  702   d , SL  726  and TPW  724  of the two pairs of modified 2T string NAND-based NOR flash cells have to be coupled with appropriate bias conditions in the circuit for respective operations. 
     Underneath the poly2-gates  704   a ,  704   b ,  704   c  and  704   d , those NMOS device transistors have ONO as the gate dielectric shown as  706   a ,  706   b ,  706   c  and  706   d , poly1 floating gates  708   a ,  708   b ,  708   c  and  708   d  and tunneling oxides  710   a ,  710   b ,  710   c  and  710   d . The electrons injected from TPW  724  can be retained in the floating gates  708   a ,  708   b ,  708   c  and  708   d  while the flash cell is programmed. On the contrary, the stored electrons in the floating gates  708   a ,  708   b ,  708   c  and  708   d  are pulled out toward TPW  724  while an erase operation is performed. SL  726  is made of M 0  metal layer and is directly connected to lightly doped N− region  716  to form the Schottky diode. 
     In order to form the Schottky diode, the heavily doped N+ blanket implant is performed to all the active regions in the array first except the active region along the source region  716  which is masked by the implant mask. Therefore, the N+ drain regions  714 ,  720  and drain/source regions  712 ,  718  of the two pairs of modified 2T string NAND-based NOR flash cells are also heavily doped with phosphorus on TPW  724 . BL 0   722  is made of M 1  metal layer and connected to the N+ drain regions  714  and  720  through the ohmic contacts  721   a  and  721   b . SL  726  is made of M 0  metal layer and is directly connected to lightly doped N− region  716  to form the Schottky diode. 
     Although the above description has been using N-channel flash memory cells as examples, it is obvious that the principle of the invention is also applicable to P-channel flash memory cells. Similarly, a person of ordinary skill in the art can extend the erase, program and read operations, the diode connection from each source line to the source region, and the construction of the Schottky diode described above for the 2T string NAND-based NOR flash cells to a flash memory array which comprises a 1T NOR flash cell or an NT string NAND-based NOR flash cells. For simplicity, they will not be repeated in this description. Furthermore, the flash cell structure can also be extended to a traditional 2-dimensional flash cell structure or the 3-dimensional stacked flash cell structure. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the flash memory array by using a diode to connect the source node of each individual memory cell to one common source line according to the present invention. It is intended that the embodiments described above be considered as exemplary only, with a true scope of the disclosed embodiments being indicated by the following claims and their equivalents.