Abstract:
A Field Sub-bitline NOR-type (FSNOR) flash array and its operating methods are disclosed. In contrast to the conventional NOR flash array, the FSNOR array is configured in column with multiple 90° rotated NOR pairs linked by field side sub-bitlines to achieve the minimum 4F 2  cell size. The FSNOR flash array is divided into multiple sectors by selection transistors for connecting the even/odd sub-bitlines to the global main first metal bitlines. For each FSNOR sector, the two drain electrodes of column-adjacent NOR pairs form the even/odd sub-bitlines separated by trench field oxides respectively, and the common source electrodes of NOR pairs in a column form the common diffusion source lines tied with metal contacts connected to the first metal common source lines. The FSNOR flash array design has enhanced the electrical isolation of the selected NVM cell devices from the unselected NVM cell devices.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to array architecture of Non-Volatile Memory (NVM) semiconductor cell devices. In particular, the innovative Field Sub-bitline NOR (FSNOR) flash array is configured with multiple NVM semiconductor cell devices, where the drain electrodes of multiple NVM cell pairs in a column are connected together to form two field side sub-bitlines and the common source electrodes of the multiple NVM cell pairs in the column are connected to form a single common source line, and the control gates of multiple NVM cell pairs in rows form the wordlines. 
     Description of the Related Art 
     Non-Volatile Memory (NVM) semiconductor, and particularly Electrically Erasable, Programmable Read-Only Memories (EEPROM), exhibit wide spread applicability in a range of electronic equipment from computers, to telecommunication hardware, to consumer appliances. In general, EEPROM serves a niche in the NVM space as a mechanism for storing firmware and data that can be kept even with power off and can be altered as needed. 
     Non-volatile data represented by the states of threshold voltages (devices&#39; on/off voltages) is stored in EEPROM devices by modulating devices&#39; threshold voltages through the injection of charge carriers into the charge-storage layer of EEPROM devices. For example, with respect to an N-channel EEPROM device, an accumulation of electrons in the floating gate, or in a charge storage dielectric layer, or in a layer of embedded nano-crystals above the channel region, causes the device to exhibit a relatively high threshold voltage. 
     Flash EEPROM may be regarded as specifically configured EEPROM devices into cell array that may be erased only on a global or sector-by-sector basis. Flash EEPROM arrays are also categorized into NOR flash and NAND flash according to the configurations of memory cell connections in the flash arrays. The conventional NOR flash array connects cell devices in parallel-connected pairs  10  in  FIG. 1 , where rows of common source electrodes of the paired cell devices  10  are connected to form multiple horizontal common source lines CS and columns of drain electrodes of the paired cell devices  10  are connected to form multiple vertical bitlines, respectively. As the cell device schematic for an “M×N” NOR flash array shown in  FIG. 1 , each wordline running in x-direction contains “M” NVM cells with the drain electrodes  12  of the NOR cell pairs  10  vertically connected to form bitlines B i  for i=1, . . . M, and each bitline running in y-direction is attached with “N” drain electrodes of the NVM cells. The common source electrodes  11  of rows of NOR cell pairs  10  in the array are horizontally connected to form the common source lines CS. When a wordline is selected, the entire “M” NVM cells of the selected wordline are activated. On the other hand, the NVM cells of the unselected wordlines in the array are electrically detached from the “M” bitlines. The electrical responses at the drain electrodes of the selected “M” NVM cells can be detected through their attached “M” bitlines. Since the applied electrical biases and NVM signals are directly in contact with the drain electrodes of the selected NVM cells in NOR-type flash array without passing any other NMV devices, the read and write access speed are faster and the operation voltages are lower for NOR-type flash array in comparison with NAND-type flash array. 
     The NAND-type flash array connects the NVM cells in series. Unlike the NOR type array with the configuration of source electrode-to-source electrode connection and drain electrode-to-drain electrode connection, NAND-type flash array link the drain electrode of an NVM cell to the source electrode of its next neighboring cell. Usually, the numbers of NVM cells linked in one single NAND string  20  in  FIG. 2  are from 8 cells to 128 cells depending on the generations of the process technology nodes. In  FIG. 2 , the schematic for an “M×N” NAND flash array, the array contains “q*M” NAND cell strings  20  and each NAND cell string  20  contains “p” NVM cells (p=8˜128) and one selection gate to connect the string to the main bitline. Each bitline has “q” NAND cell strings  20  attached. Thus the total NVM cells attached to a single main bitline is p*q=N for an “M×N” NAND array. Since the source electrode and the drain electrode of NVM cells are overlapped each other in the NAND cell string, the NVM cells have no contacts in between the linked NVM cells except one contact  21  placed at the end of the cell string for connecting the NAND string to the main bitline. Usually, a single main bitline connects several NAND strings  20  in y-direction and common source lines CS run in x-direction in the NAND flash array. In contrast, each pair of NVM cells in NOR-type array does have one contact  11  for connecting the cell&#39;s two drain electrodes (one drain electrode equivalently sharing a half contact) to the main bitline. A NOR-type flash array is equivalently to a NAND-type array with p=1. Typically, the NOR-type NVM cell sizes including the area for a single contact  11  in a NOR flash array are 9˜10 F 2  and the NAND-type NVM cell sizes without a contact area in a NAND flash array can achieve the minimum cell area of 4 F 2  respectively, where F is minimum feature size for a process technology node. Therefore, the chip areas of NAND type flash arrays are smaller than those of the NOR type flash arrays (˜40% to ˜50% smaller) for the same memory bits with the same process technology node. In term, the smaller cell array areas for NAND flash would have the advantage of lower manufacturing cost for the same bit storage capacity. 
     Making NOR flash array to be cost competitive as NAND flash array for the same 4F 2  memory cell sizes, we disclosed the NOR flash array using the NVM cell semiconductor devices fabricated with the conventional flash process technology in U.S. Pat. Nos. 8,415,721 B2 and 8,716,138 B2 (the disclosure of which are incorporated herein by reference in their entirety). In the disclosures as shown in  FIG. 3 , the NOR cell pairs  30  of NVM semiconductor devices in  FIG. 3  are arranged by rotating 90° of the conventional NOR cell pairs  10  shown in  FIG. 1 . The drain and source electrodes of the NOR cell pairs  30  form the diffusion sub-bitlines  31  separated by trench field isolation. By twisting the diffusion sub-bitlines along the trench field isolation by a fractional pitch, the diffusion sub-bitlines are able to link their sub-feature diffusion lines (whose features are smaller than the minimum feature F) to the full feature diffusion areas, where full-feature contacts  32  can be landed on. Through the contacts  32  attached to the main bitlines B i  for i=1, . . . , M, in  FIG. 3 , the electrical signals can be picked up from the selected NVM cell devices and the voltage biases can be applied to drain electrodes of the selected NVM cell devices without passing any other NVM cell devices as the NAND flash. For the FSNOR flash array  300  in  FIG. 3 , multiple rows (said 8˜128 rows) of NOR cell pairs  30  are connected with diffusion sub-bitlines to form a NOR flash sector  300   s . The main metal bitlines globally connect multiple sectors through the multiple contacts  32  to form a bank of NOR flash array  300 . Since the extension of multiple sectors in a bank increases the bitline (multiple sub-bitlines+main bitline), capacitance C and resistance R, the electrical signals and voltage biases passing through the bitline to the drain electrodes of the selected NVM devices would be slow and degraded due to the large bitline RC time delay and IR (current-resistance) drops, respectively. Furthermore, the excessive numbers of the unselected NVM devices forming the multiple sector sub-bitlines attached to the single main bitline also increase the bitline leakage currents, i. e., the numbers of unselected cell devices attached to the main bitline multiplied by cell&#39;s junction/channel-diffusion leakage current, resulting in high bitline leakage current noise levels for read operation, and significant applied drain voltage bias drops to the drain electrodes of the selected NVM cell devices in programming operation. For those reasons, the numbers of multiple sectors extended in a bank has to be capped for minimum signal/noise ratio and the applying drain voltage bias integrity. 
     In order to be extendable for the numbers of sectors attached to the single main metal bitlines in a bank, not limited by the above reasons, and reduce the line resistance from the larger resistance of sub-bitlines to the smaller resistance of common source lines, we has disclosed a new type of 4F 2  FSNOR flash array separated by sectors with the even/odd sub-bitline selections to the global main bitlines for the even/odd number NVM cell devices of the NVM cell pairs and the low resistance global common source lines. In the new FSNOR array architecture of the invention, one and only one selected NVM device is electrically connected to the single global main metal bitline for the accessing operations of read and programming such that the selected NVM cell devices are fully immune from the interferences of other unselected NVM cell devices in the array. This interference immunity for the new FSNOR flash array of the invention is proven to be much better performance on NVM signal/noise ratio, applying drain voltage bias integrity, and programming disturbance to the neighboring cells than any other existing flash arrays. 
     SUMMARY OF THE INVENTION 
       FIG. 4  shows the schematic of the “i” sector  400   i  in new 4F 2  FSNOR flash array  400  according to an embodiment of the invention. For the sector “i”  400   i  in the flash array  400 , the control gates of NVM cells form wordlines W j , for j=1, . . . , I, in the x-direction with the minimum control gate pitch of a specific process technology and the first metal bitlines in the y-direction with the minimum first metal line pitch of a specific process technology form the global main first metal bitlines and the global first metal common source lines in the alternating common-source-line/bitline sequence of ---, CS, B n−2 , CS, B n−1 , CS, B n , CS, B n+1 , CS, B n+2 , CS, ---. The odd field side sub-bitlines  41  formed by the drain electrodes of the odd number NVM devices of the columned NVM cell pairs  40  are connected to the source electrodes of the selection MOSFET devices  46  controlled by the odd selection line S io  (top) and the even field side sub-bitlines  42  formed by the drain electrodes of the even number NVM devices of the columned NVM cell pairs  40  are connected to the source electrodes of the selection MOSFET devices  48  controlled by the even selection line S ie  (bottom). The drain electrodes of the selection MOSFET devices  46  and  48  landed with the contacts  45  are connected to the global main first metal bitlines B k , for k=1, . . . , n, . . . , M, where M is the number of NVM cell pairs  40  in a row. The common source electrodes of the columned NVM cell pairs  40  forms the vertical diffusion common source lines  43 , which are tied with the contacts  47  connected to the global first metal common source lines CS. Please note that the NVM cells in each sector comprise no electrical contact inside the sector array area. For example, the NVM cells in sector “i”  400   i  comprise no electrical contact inside the sector array area (represented by dashed rectangle  400   i ). 
     To access the odd number NVM cell devices of a selected wordline (the control gates of a row of NVM devices) in the selected sector “i” for reading and programming, the odd selection line S io  is applied with the “on” voltage bias V s  to electrically connect the odd field side sub-bitlines  41  with the global main metal bitlines, while the control gate voltages V CG  is applied to activate the selected wordline. For accessing the even number NVM cell devices of a selected wordline in the selected sector “i”  400   i , the even selection line S ie  is applied with the “on” voltage bias V s  to electrically connect the even field side sub-bitlines  42  with the global main metal bitlines, while the control gate voltage V CG  is applied to activate the selected wordline. The charge storing material of NVM devices of the invention is made of conducting floating gate, charge storage dielectric film or a layer of embedded nano-crystal grains. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made to the following drawings, which show the preferred embodiments of the present invention, in which: 
         FIG. 1  shows a typical schematic for a conventional NOR-type flash array. 
         FIG. 2  shows a typical schematic for a conventional NAND type flash array. 
         FIG. 3  shows the schematic of FSNOR flash array according to the prior art. 
         FIG. 4  shows the schematic of FSNOR flash array according to an embodiment of the present invention. 
         FIG. 5  shows a top view of a portion of the FSNOR flash array of  FIG. 4 . 
         FIG. 6  shows the top view of the silicon surfaces containing P-type silicon active areas, N+sub-bitline ( 41  and  42 ) and CS line ( 43 ) diffusion areas, and the field oxide areas for the FSNOR flash array according to the invention. 
         FIG. 7  shows the cross section view of the cut line “A” in  FIG. 6  and assuming the charge storing material is made of conducting floating gate. 
         FIG. 8  shows the read operation for the odd number cell devices of a selected row in the selected sector according to the invention. 
         FIG. 9  shows the read operation for the even number cell devices of a selected row in the selected sector according to the invention. 
         FIG. 10  shows the programming operations for the odd number cell devices of a selected row in the selected sector according to the invention. 
         FIG. 11  shows the programming operations for the even number cell devices of a selected row in the selected sector according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Field Sub-bitline NOR (FSNOR) flash arrays of the invention have the same cell array area as those in the NAND-type flash for a specific technology nodes, while preserving the advantages of read/write accessing speed and low operation voltages. Those of ordinary skill in the art will immediately realize that the embodiments of the present invention described herein in the context of schematics and fabrication methods are illustrative only and are not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefits of this disclosure. 
     For illustration purpose, we shall apply N-type NVM cells to demonstrate the new FSNOR flash arrays of the invention. However, the FSNOR flash array architectures of the invention are not limited to N-type NVM cells, but are applicable to P-type NVM cells. The NVM cell size in the array demonstrated is determined by Control-Gate pitch and First-Metal line pitch according to a process technology node. The minimum feature size of a process technology node is defined by F. The Control-Gate pitch and First-Metal line pitch can be the minimum 2F for a specific technology node. Thus, the cell feature size for a FSNOR flash array can be the minimum 4F 2  in contrast to conventional NOR cell feature sizes between 9˜10F 2 . 
     The array top view for the sector schematic in  FIG. 4  on silicon is shown in  FIG. 5  for the N-type sub-bitline NVM devices. The FSNOR flash arrays  400  of the present invention are fabricated with the conventional CMOS process technology. Examples of the process module for forming the field side sub-bitlines and integrated process fabrication are described in U.S. Pat. Nos. 8,415,721 B2 and 8,716,138 B2. We shall not repeat the fabrication process here. The only differences between the FSNOR flash array  400  of the invention and the prior FSNOR flash array (described in U.S. Pat. Nos. 8,415,721 B2 and 8,716,138 B2) are the different mask drawings for forming the selection gates and the diffusion CS lines.  FIG. 6  shows the top view of the silicon surfaces containing P-type silicon active areas, N+ sub-bitline ( 41  and  42 ) and CS line ( 43 ) diffusion areas, and the first and second field oxide areas ( 61 ,  62 ) for the FSNOR array. Each of the first field oxide areas  61  has straight portions  61 A and bending portions  61 B. The second field oxide areas  62  and the bending portions  61 B are arranged in a pattern that corresponds to locations of the selection transistors  46 ,  48 . The second field oxide areas  62  and the bending portions  61 B are used to define the sectors and separate the pairs of selection transistors  46 ,  48  from their adjacent diffusion common source lines  43 . The straight portions  61 A are used to isolate adjacent sub-bit lines  41 ,  42  of column-adjacent NVM cell pairs. 
       FIG. 7  shows the cross section view of the cut line “A” in  FIG. 6 . In the embodiment of  FIG. 7 , each N-type NVM cell comprises a control gate  705 , a coupling dielectric  704 , a floating gate  703 , a tunneling dielectric  702 , a source electrode (i.e., part of the CS line  43 ) and a drain electrode (i.e., part of N+ sub-bitline  41  or  42 ). The junction depth of diffusion sub-bitlines  41  and  42  are required to be above the bottom of trench isolation  701  (or field oxide areas  61 ,  62 ) such that the two sub-bitlines  41  and  42  along the two sides of trench walls are electrically isolated one from the other. 
     In the reading mode, all the global first metal common source lines CS are electrically connected to the common ground voltage. As illustrated in  FIG. 8 , the odd selection line S io  in the selected sector “i” is applied with the “on” voltage bias V s  to electrically connect the odd field side sub-bitlines  41  with the global main first metal bitlines. When the control gates of the selected wordline are applied with a read voltage V CGR , the “on/off” signals between the drain electrodes and the source electrodes of the odd number NVM cell devices passing through the “on” odd selection MOSFET devices  46  to electrically connect the odd field side sub-bitlines  41  to the main first metal bitlines B k  are detected by the sensing amplifiers (not shown). The sensing amplifiers sense the voltage signals at the global main first metal bitlines. As illustrated in  FIG. 9 , the even selection line S ie  in the selected sector “i” is applied with the “on” voltage bias V s  to electrically connect the even field side sub-bitlines  42  with the global main first metal bitlines. When the control gates of the selected wordline are applied with a read voltage V CGR , the “on/off” signals between the drain electrodes and the source electrodes of the even number NVM cell devices of the selected wordline passing through the “on” even selection MOSFET devices  47  to electrically connect the even field side sub-bitlines  42  to the global main first metal bitlines B k , are detected by the sensing amplifiers (not shown). The sense amplifiers sense the voltage signals at the global main first metal bitlines. 
     As illustrated in  FIG. 10 , for programming operations, the entire global first metal common source lines CS are initially biased with the non-programming voltage V NP  or floating. For programming the odd number NVM cell devices of the selected wordline in the sector “i”, the odd selection MOSFET devices  46  are turned on by applying the “on” voltage bias V s  to the odd selection line S io  to electrically connect the odd field side sub-bitlines  41  with the main first metal lines. The programming drain voltage bias V P  for the NVM cell devices to be programmed and the non-programming drain voltage V NP  or floating for the NVM cell devices not to be programmed are applied to their corresponding global main first metal bitlines. When a control gate voltage pulse with the amplitude of V CGP  (&gt;V p ) is applied to the selected wordline, the odd number NVM cells of the selected wordline with drain voltage bias V p  are programmed to the high threshold voltage state and meanwhile the odd number NVM cells of the selected wordline with the drain voltage bias V NP  or floating remain at the low threshold voltage state accordingly. Note that the conventional Hot Carrier Injection (HCl) method for V NP =0 V, Channel Induced Secondary Electron (CHISEL) method for V NP &lt;V P  as disclosed in U.S. Pat. No. 7,733,700 B2 (the disclosure of which is incorporated herein by reference in its entirety), and Band to Band Hole Induced Secondary Electron (B2BHISEL) method for floating source node as disclosed in U.S. Pat. No. 9,082,490 B2 (the disclosure of which is incorporated herein by reference in its entirety). 
     As illustrated in  FIG. 11  for programming operations, the entire global first metal common source lines CS are initially biased with the non-programming voltage V NP  or floating. For programming the even number NVM cell devices of the selected wordline in the sector “i”  400   i , the even selection MOSFET devices  48  are turned on by applying the “on” voltage bias V s  to the even selection line S ie  to electrically connect the even field side sub-bitlines  42  with the global main first metal lines. The programming drain voltage bias V P  for the NVM cell devices to be programmed and the non-programming drain voltage V NP  or floating for the NVM cell devices not to be programmed are applied to their corresponding global main first metal bitlines. When a control gate voltage pulse with the amplitude of V CGP  (&gt;V p ) is applied to the selected wordline, the even number NVM cells of the selected wordline with drain voltage bias V p  are programed to the high threshold voltage state and meanwhile the even number NVM cells of the selected wordline with the drain voltage bias V NP  or floating remain at the low threshold voltage state accordingly. 
     To conclude the three basic flash operations, i. e., read, programming, and erase, we apply the conventional Fowler-Nordheim tunneling method for erasing a page or sector(s) as for the conventional flash erase operation. During the erase operation, the global first metal common source lines CS and the substrate are both biased with ground voltage bias or negative voltage bias, and a high control gate voltage pulse with voltage amplitude high enough to generate tunneling electrical fields between the charge storing layer and the silicon substrate (usually E˜0.1 volt per angstrom oxide thickness) for the stored charges to be tunneled out is applied to the selected wordline (page erase) or wordlines (sector erase and block erase). In summary, we have disclosed the new Field Sub-bitline NOR-type (FSNOR) Non-Volatile Memory (NVM) flash array and the methods of operations for the FSNOR flash array. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention should not be limited to the specific construction and arrangement shown and described, since various other modifications may occur to those ordinarily skilled in the art.