Abstract:
A method of performing an operation on a flash memory cell device, used when a gate coupling ratio between a floating gate and a control gate of less than 0.4. A potential is required to be applied across the control gate. Electrons are either injected to the floating gate from the control gate or ejected from the floating gate to the control gate. The operation associated with the injection or the ejection is determined by the nature of a silicon channel provided in the device. Devices using a bulk-tied FinFET-like structure are particularly suited to this method. The method is also particularly suited for use on cells in a NAND array.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The use of floating gate technology in flash memory devices is well known. Typically, a Si channel of n or p type semiconductor is provided. The floating gate transistor is surrounded by oxide, enabling a charge stored on the gate to remain there. Program and erase operations take place by a process of channel injection. Electrons undergo Fowler-Nordheim (FN) tunneling and are transferred from the channel to the floating gate, and vice-versa, during operation. 
         [0002]    To provide an efficient channel injection, the possibility of a gate injection (transfer of electrons between the control gate and floating gate via FN tunneling) must be reduced. This is accomplished by maximizing the gate coupling ratio. The gate coupling ratio (GCR) is defined as the ratio of floating gate potential to control gate potential. A GCR of 1 is optimal, but a GCR greater than 0.6 is sufficient for most flash memory devices. 
         [0003]    This result is satisfactory for larger memory devices, but when these devices are scaled down, a high GCR becomes difficult to maintain. For NAND flash in particular, a GCR of less than 0.3 is predicted when the node is below 45 nm. The bottom tunnel oxide will not have a sufficiently large electric field to allow FN tunneling. Moreover, future flash memory devices will require FinFET-like structures to improve device short-channel characteristics. These structures have a naturally large channel to floating gate coupling capacitance, and thus a naturally low GCR. 
         [0004]    Additionally, there is a problem of inter-floating gate coupling in traditional flash memory device arrays. Inter-floating gate coupling capacitance is comparable to the gate coupling capacitance of the channel and floating gate as the density of cells in the array becomes greater. This causes interference among the cells which deteriorates functionality of the flash memory device. Furthermore, electric field stress on the gate oxide affects reliability and endurance of the unit. 
         [0005]    It is therefore desirable to operate the floating gate apparatus in such a way as to enable efficient FN tunneling to the floating gate, particularly when using a FinFET-like structure. It is also desirable to operate the floating gate apparatus in such a way as to increase reliability and endurance, and to reduce inter-cell interference as the devices are scaled down. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    A method of performing an operation on a flash memory cell device is provided when a gate coupling ratio between a floating gate and a control gate is less than 0.4. A potential is required to be applied across the control gate. Electrons are either injected to the floating gate from the control gate or ejected from the floating gate to the control gate. The operation associated with the injection or the ejection is determined by the nature of a silicon channel provided in the device. 
         [0007]    For n-Channel cells, writing is accomplished by ejecting electrons from the floating gate to the control gate. Erasing is accomplished by injecting electrons to the floating gate from the control gate. For p-Channel cells, writing is accomplished by injecting electrons to the floating gate from the control gate. Erasing is accomplished by ejecting electrons from the floating gate to the control gate. 
         [0008]    Devices having a bulk-tied FinFET-like structure are particularly suited to this method, as the structure creates a naturally low gate coupling ratio. This method is also particularly suited for use on cells in a NAND flash memory array since it allows scalability of the cells down to below 20 nm. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0010]      FIG. 1  shows typical cross-sectional views in the channel length direction of flash memory cells for use in one preferred embodiment of the present invention. 
           [0011]      FIGS. 2A ,  2 B,  3 A, and  3 B show electron injection and electron ejection by ±FN tunneling in both n and p Channel cells in accordance with one preferred embodiment of the present invention. 
           [0012]      FIG. 4  shows a cross-sectional view in the channel-width direction of a preferred flash memory cell for use in one preferred embodiment of the present invention. 
           [0013]      FIGS. 5-6  show a cross-sectional view in the channel-length and width directions of two flash memory cells in NAND arrays for use in accordance with one preferred embodiment of the present invention. 
           [0014]      FIGS. 7A-7C  and  8 A- 8 C show operations on cells in n-Channel and p-Channel NAND arrays for use in accordance with one preferred embodiment of the present invention. 
           [0015]      FIGS. 9A and 9B  show plots of threshold voltage (in volts) vs. time (in seconds, logarithmic scale). 
           [0016]      FIG. 10  shows data for technology nodes of varying sizes indicating scalability down to less than 20 nm. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  shows typical cross-sectional views in the channel length direction of flash memory cells for use in embodiments of the present invention. The left diagram of  FIG. 1  shows an n-Channel device. The structure contains a Si channel  18 , with a p-well  24   a  and n-doped source  20   a  and drain  22   a . In preferred embodiments, a bulk-tied FinFET structure is used. The structure also contains a gate oxide  16 , a charge-storage floating gate  14 , an inter-poly, top-tunneling dielectric  12 , and a control gate  10 . The right side of  FIG. 1  shows a p-Channel device, which is identical to the n-Channel device, except that the Si channel  18  contains an n-well  24   b , and p-doped source  20   b  and drain  22   b.    
         [0018]    The bottom gate oxide  16  is under relatively small electric field stress, typically less than about 7 MV/cm, during program and erase operations. This avoids any FN tunneling, which prevents much of the damage occurring in conventional flash memory devices, and allows a better DC performance. Additionally, the bottom oxide  16  and the tunneling oxide  12  are separate. This enhances reliability and endurance. 
         [0019]    Referring to  FIGS. 2A and 2B , the electron injection from the control gate  10  by −FN tunneling is illustrated. Referring to  FIG. 2A , an erase is affected on the n-Channel device by applying a high threshold voltage to the control gate  10 , for example, −16 V. For a GCR of 0.3, the potential of the floating gate  14  is −4.8 V. The electric field in the bottom oxide  16  is less than 8 MV/cm, but the electric field in the top oxide  12  is greater than 10 MV/cm, and therefore tunneling occurs from control gate  10  to the floating gate  14 .  FIG. 2B , showing a p-Channel device, is similar. However the device is performing a write operation rather than an erase, due to the nature of the Si channel  18 . A GCR of less than about 0.4 is preferred for electron injection in both n-Channel and p-Channel devices. 
         [0020]    Referring to  FIGS. 3A and 3B , the electron ejection from the floating gate  14  by +FN tunneling is illustrated. Referring to  FIG. 3A , a write operation is affected on the n-Channel device by applying a low threshold voltage to the control gate  10 , for example, +16V. For a GCR of 0.3, the potential of the floating gate  14  will be +4.8 V. The electric field in the bottom oxide  16  is less than 8 MV/cm, but the electric field in the top oxide  12  is greater than 10 MV/cm, and therefore tunneling occurs from the floating gate  14  to the control gate  10 .  FIG. 3B , showing a p-Channel device, is similar. However the device is performing an erase operation rather than a write operation, due to the nature of the Si channel  18 . A GCR of less than about 0.4 is preferred for electron ejection in both n-Channel and p-Channel devices. 
         [0021]      FIG. 4  shows a cross-sectional view in the channel-width direction of a flash memory cell for use in one preferred embodiment of the present invention. A high density plasma (HDP) oxide  28  surrounds the channel  18 , gate oxide  16 , and floating gate  14 . This oxide  28  isolates the cell from surrounding cells. The Si channel  18  is a FinFET structure. One method of obtaining a small GCR is to increase the area between the channel  18  and the floating gate  14 , thereby increasing the coupling capacitance between the two. A typical FinFET structure naturally creates a large coupling area between the channel  18  and the floating gate  14 , which makes it particularly suitable for use in accordance with a preferred embodiment of the present invention. 
         [0022]      FIG. 5  shows a cross-sectional view in the channel-length direction of two flash memory cells in a NAND array for use in accordance with a preferred embodiment of the present invention. A well  24  runs along a bit line (not shown in  FIG. 5 ) in the array. The cells share a junction  26 . The distance between the first comer of the control gate  10  and the corresponding comer of control gate  10 ′ is  2 F, where F is the width of the cell, also called a technology node. 
         [0023]      FIG. 6  shows a cross-sectional view in the channel-width direction of two flash memory cells in a NAND array for use in accordance with a preferred embodiment of the present invention. The HDP oxide  28  isolates the channel fins  18  and  18 ′ from each other. The control gate  10  runs along a word line (not shown in  FIG. 6 ) in the array. The cells share this gate  10 . The distance between the first edge of the fin  18  and the corresponding edge of fin  18 ′ is  2 F, where F is as defined for  FIG. 5 . 
         [0024]      FIGS. 7A ,  7 B, and  7 C show operations in an n-Channel NAND array in accordance with a preferred embodiment of the present invention. In  FIG. 7A , the threshold voltage for cell A is lowered by applying, for example,  15  V along the adjoining word line and grounding the corresponding bit line. +FN tunneling occurs and cell A is programmed. Neighboring cells B, C, and D have program disturbances at acceptable levels. In  FIG. 7B , the threshold voltage is increased to, for example, −18 V on all word lines. −FN tunneling occurs and programmed cells are erased. In  FIG. 7C , cell A is read out by applying an appropriate potential to the corresponding word line, and applying a pass voltage to other word lines to allow a read through current. 
         [0025]      FIGS. 8A ,  8 B, and  8 C show operations in a p-Channel NAND array in accordance with a preferred embodiment of the present invention. In  FIG. 8A , the threshold voltage for cell A is increased by applying, for example, −18 V along the adjoining word line and grounding the corresponding bit line. −FN tunneling occurs and cell A is programmed. Neighboring cells B, C, and D have program disturbances at acceptable levels. In  FIG. 8B , the threshold voltage is lowered to, for example, +15 V on all word lines. +FN tunneling occurs and programmed cells are erased. In  FIG. 8C , cell A is read out by applying an appropriate potential to the corresponding word line, and applying a pass voltage to other word lines to allow a read through current. 
         [0026]      FIGS. 9A and 9B  are plots of threshold voltage (in volts) vs. time (in seconds, logarithmic scale). The plot in  FIG. 9A  shows the drop in threshold voltage associated with +FN tunneling from the floating gate. The GCR is fixed at 0.3, the bottom oxide thickness (O 1 ) is set to 7 nm, and the tunneling oxide thickness (O 2 ) is set to 10 nm. Three different potentials were applied to the control gate. The results demonstrate that a large memory window can be obtained using a medium range voltage. The plot in  FIG. 9B  shows the increase in threshold voltage associated with −FN tunneling from the control gate. The simulation sets the same GCR, O 1 , and O 2  parameters as in  FIG. 9A , and applies the same potentials across the control gate. 
         [0027]      FIG. 10  shows a table and plot of coupling ratio vs. technology node size (in nm). The simulation was conducted with decreasing node size down to about 20 nm. α G  is the GCR. α B  is the coupling ratio between the floating gate and the channel. α WL-WL  is the interference coupling ratio between cells on the same word line. α BL-BL  is the interference coupling ratio between cells on the same bit line. The remaining process parameters in the left-hand column of the table are defined and labeled in  FIGS. 5 and 6 . 
         [0028]    The data from  FIG. 10  shows that a floating gate device utilizing gate-injection methods as described above has scalability down to a technology node size of less than about 20 nm. The gate coupling ratio can be maintained at about 0.3. Additionally, the interference coupling ratios from adjacent cells can be limited to below 0.1, eliminating much of the deterioration on cell function. 
         [0029]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention.