Abstract:
A method of forming and operating a trench split-gate non-volatile flash memory cell structure. The auxiliary gate of the structure is formed inside a trench on one side of the gate and the source terminal is underneath the auxiliary gate, thereby reducing overall area occupation of the auxiliary gate and the source terminal relative to the cell and increasing packing density. By enclosing the common source terminal inside a deep N-well layer, source resistance for reading data from the cell is reduced and the process of etching out a contact opening is simplified. The structure also ensures the injection of most hot electrons into the floating gate, thereby increasing execution speed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Taiwan application serial No. 90109732, filed Apr. 24, 2001. 
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates to a method of forming and operating a non-volatile flash memory cell structure. More particularly, the present invention relates to a method of forming and operating a trench split gate non-volatile flash memory cell structure. 
     2. Description of Related Art 
     In recent years, the need for non-volatile memory has increased at a rapid rate due to the exponential growth of portable electronic product markets. As technologies for manufacturing flash memories improve, unit cost is greatly reduced stimulating a wider application. Digital cameras, electronic organizers, MP3, electronic answering machines and programmable integrated circuits (IC) often depend on flash memory for data storage. 
     Most flash memory is designed as a type of electrically programmable read-only-memory (EPROM) having an N-channel memory unit. Programming is conducted using channel hot electrons. Among various types of flash memory units, split gate structure has the best hot electron programming efficiency. A conventional stacked flash memory using a channel hot electron injection mechanism or a flash memory using a channel initiated secondary electron injection mechanism through the application of a negative bias voltage on substrate has a hot electron injection efficiency (gate current over drain current) for programming, around 10 −6 ˜10 −8 . However, a split gate flash memory using a source side injection (SSI) hot electron programming mechanism can reach an efficiency of 10 −4 ˜10 −6 . 
     FIG. 1 is a schematic cross-sectional view of a conventional split-gate flash memory unit. This type of split-gate flash memory is also referred to as a sidewall select-gate on the source side (SISOS). To program the split-fate flash memory, a voltage V CG =17V is applied to the control gate  10 , a voltage V D =5V is applied to the drain terminal  12  and a voltage Vs=Vsub=0V is applied to the source terminal  14  and the substrate  16 . In addition, a voltage V SWG =2V is applied to the select gate terminal  18  so that a 5V voltage differential is formed between the source terminal  14  and the neighborhood of node A. Hence, electron injection is boosted and electrons are channeled into interior of the floating gate  20  through the voltage V CG =17V applied to the control gate terminal  10 . The select gate terminal  18  is capable of controlling the electric field created between the source terminal  14  and the node A so that electron injection efficiency is effectively raised. To conduct an erasure, a voltage V D =14V is applied to the drain terminal  12  while other contact points remain at 0V. Therefore, electrons within the floating gate  20  drain away via the drain terminal  12 . 
     Although the aforementioned split-gate flash memory structure has a relatively high efficiency, each memory unit needs to occupy a larger surface area due to the incorporation of a select gate over the source terminal. Thus, each split-gate memory unit occupies more area than a conventional flash memory unit leading to a lowering of packing density and memory capacity. Furthermore, the injection of electrons into the floating gate after crossing over the select gate relies on the hot electrons created by the high electric field at the gap due to a high voltage between the select gate and the floating gate. Rather than focusing at the floating gate, the accelerated electrons focus upon the underlying depletion layer created by the floating gate and the drain voltage. Hence, unless a very high voltage at the gate terminal and a matching drain voltage are applied so that the accelerated electrons converge upon the floating gate region, only a minority of electrons will ultimately end up inside the floating gate. 
     SUMMARY OF INVENTION 
     Accordingly, one object of the present invention is to provide a method of forming and operating a trench split-gate non-volatile flash memory structure. Area occupation of the select gate and the source terminal inside a trench split-gate non-volatile flash memory cell relative to the entire cell is reduced and hence packing density of the memory is increased. Furthermore, the trench split-gate non-volatile flash memory cell structure is also specially fabricated to inject most accelerated electrons into the floating gate and hence operating efficiency of the memory is increased. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a trench split-gate non-volatile flash memory cell structure. The structure comprises a P-type substrate, a deep N-well layer, a shallow P-well layer, a source region, a trench auxiliary gate region, a gate region and a drain region. The P-type substrate, the deep N-well layer and the shallow P-well layer are arranged into a stack with P-type substrate at the bottom, the deep N-well layer in the middle and the shallow P-well layer on top. The source region is embedded inside the deep N-well layer. The trench auxiliary gate region is embedded within the deep N-well layer and the shallow P-well layer above the source region. The gate region is above the shallow P-well layer on one side of the auxiliary gate region. The drain region is embedded within the P-well layer on one side of the gate region. In addition, a metal silicide layer covers the exposed drain region and the trench auxiliary gate region. 
     The trench auxiliary gate includes a polysilicon layer and an oxide layer at the bottom and each side of the polysilicon layer. The gate includes a first polysilicon layer, a second polysilicon layer above the first polysilicon layer and an isolation layer between the first and the second polysilicon layer and on the sidewalls of the first polysilicon layer. The isolation layer can be an oxide-nitride-oxide (ONO) layer, an oxide-nitride (ON) layer or a nitride (N) layer. 
     This invention also provides a method of forming trench split-gate non-volatile flash memory cells that include the following steps. First, a P-type substrate is provided. A deep N-well layer is formed in the P-type substrate and a shallow P-well layer is formed over the deep N-well layer. A gate region is formed over the P-well layer and a drain region and an auxiliary gate region are formed on each side of the gate region. A spacer is formed on each side of the gate region. A trench is formed in the deep N-well layer and the shallow P-well layer within the auxiliary gate region. An oxide layer is formed inside the trench. The deep N-well layer and the drain region of the P-well layer underneath the trench are heavily doped to form a source terminal and a drain terminal. A first polysilicon layer is deposited over the gate region to form a floating gate. An isolation layer is formed over the first polysilicon layer, the drain region and the trench. A second polysilicon layer is formed over the isolation layer to form a control gate. Furthermore, a metal silicide layer may form over the exposed drain terminal and the trench auxiliary gate after the formation of the trench auxiliary gate. Hence, resistance between the auxiliary gate and the drain terminal is lowered, thereby increasing reading current and lowering RC delay. 
     This invention also provides a method of operating a trench split-gate non-volatile flash memory cell. A word line voltage, a source voltage, an auxiliary gate voltage and a bit line voltage are applied to the gate region, the source region, the trench auxiliary gate and the drain region respectively. The bottom section of the flash memory cell structure includes, from top to bottom, a shallow P-well layer, a deep N-well layer and a P-type substrate. The source region is embedded within the deep N-well layer and the trench auxiliary gate region is above the source region embedded within the shallow P-well layer and the deep N-well layer. The operation method includes the following steps. 
     To conduct a programming operation, a high voltage is applied to the word line. A voltage lower than the word line voltage is applied to the bit line. A voltage lower than both the word line voltage as well as the bit line voltage is applied to the source terminal. A voltage lower than both the word line voltage and the bit line voltage is applied to the auxiliary gate terminal. 
     To conduct an erasing operation, a low voltage is applied to the word line. A voltage higher than the word line voltage is applied to the bit line. A voltage higher than the word line voltage but lower than the bit line voltage is applied to the source terminal. A voltage higher than the word line voltage but lower than the bit line voltage is applied to the auxiliary gate terminal. 
     To conduct a reading operation, a higher voltage is applied to the word line and a voltage lower than the word line voltage is applied to the source terminal. A voltage higher than the source terminal voltage is applied to the bit line. Similarly, a voltage higher than the source terminal is also applied to the auxiliary gate terminal. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a schematic cross-sectional view of a conventional split-gate flash memory unit; 
     FIG. 2 is a schematic cross-sectional view of the main body structure of a trench split-gate non-volatile flash memory cell according to one preferred embodiment of this invention; 
     FIGS.  3 A˜ 3 F are schematic cross-sectional views showing the progression of steps for producing a trench split-gate non-volatile flash memory cell according to this invention; 
     FIGS.  4 A˜ 4 C are schematic cross-sectional diagrams showing the operations of programming data into, erasing data and reading data from a trench split-gate non-volatile flash memory cell according to this invention; 
     FIG. 5 is a circuit drawing, schematically illustrating an application of the flash memory on the AND-type array, according to this invention; 
     FIG. 6 is a circuit drawing, schematically illustrating an encoding action for multiple bits by once in a BiAND-type array is performed, according to this invention; and 
     FIG. 7 is a circuit drawing, schematically illustrating another encoding action for multiple bits by once in a BiAND-type array is performed, according to this invention. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 2 is a schematic cross-sectional view of the main body structure of a trench split-gate non-volatile flash memory cell according to one preferred embodiment of this invention. As shown in FIG. 2, the flash memory cell includes a P-type substrate  100 , a deep N-well layer  102  and a P-well layer  104  from bottom to top. A heavily doped (with N +  ions) region inside the deep N-well layer forms a source region  106 . 
     A trench auxiliary gate region  108  is above the source region  106  buried inside the N-well layer  102  and the shallow P-well layer  104 . The trench auxiliary gate region  108  includes a polysilicon layer  110  and an oxide layer  112 . The oxide layer  112  is beneath and on each side of the polysilicon layer  110 . A gate  114  is above the P-well layer  104  on one side of the auxiliary gate region  108 . The gate  114  comprises a first polysilicon layer  116  (to serve as a floating gate), a second polysilicon layer  118  and an isolation layer  120 . The second polysilicon layer  118  is above the first polysilicon layer  116  and the isolation layer  120  is between the first and the second polysilicon layer and on the sidewalls of the first polysilicon layer  116 . The isolation layer  120 , for example, can be an oxide-nitride-oxide (ONO) layer, an oxide-nitride (ON) layer or a silicon nitride (N) layer. A heavily doped (with N+ ions) region forms a drain region  107 . The drain region  107  is embedded within the shallow P-well layer  104  on one side of the gate  114 . A metal silicide layer  122  may also be formed on the exposed surface of the drain region  107  and the polysilicon layer  110  to lower resistance between the auxiliary gate and the drain region  107 , thereby increasing reading current and reducing RC delay. 
     In a conventional trench split-gate flash memory cell, the auxiliary gate region (select gate in FIG. 1) and the source region together occupy about 50% of a memory cell. In this invention, however, a self-aligned trench-forming technique is used to bury the auxiliary gate region inside the shallow P-well layer  104  and the deep N-well layer  102  so that the source terminal also uses of the deep N-well layer (the entire component is built on the shallow P-well layer). Consequently, the overall dimension of each memory cell is reduced and packaging density the memory cells is increased. 
     FIGS.  3 A˜ 3 F are schematic cross-sectional views showing the progression of steps for producing a trench split-gate non-volatile flash memory cell of an AND array according to this invention. As shown in FIG. 3A, a P-type substrate  150  is provided. A deep N-well layer  152  and a shallow P-well layer  154  are sequentially formed over the P-type substrate  150 . A channel oxide layer  156  is formed over the shallow P-well layer  154 . A polysilicon layer  158  and a silicon nitride layer  160  are sequentially deposited over the channel oxide layer  156 . 
     As shown in FIG. 3B, a portion of the polysilicon layer  158 , the silicon nitride layer  160  and the channel oxide layer  154  are etched away to form a gate region  162  over the shallow P-well layer  154  and a drain region  164  and an auxiliary gate region  166  on each side of the gate region  162 . As shown in FIG. 3C, a spacer  168  is formed on each side of the gate region  162 . The spacer  168  protects the gate oxide layer during a trench-etching operation so that a higher production yield is obtained. As shown in FIG. 3D, a trench  170  is formed in the deep N-well layer  152  and the shallow P-well layer  154  within the auxiliary gate region  166 . An oxide layer  172  is formed at the bottom and sides of the trench  170 . A heavy ion implantation is carried out implanting dopant ions (N +  ions) into the deep N-well layer  152  and the shallow P-well layer  154  within the drain region  164  underneath the trench  170 , thereby forming a source terminal  174  and a drain terminal  176  respectively. 
     As shown in FIG. 3E, a polysilicon layer  178  is formed inside the trench  170  to form a trench auxiliary gate electrode. A metal silicide layer  179  is formed over the exposed surface of the drain region  176  and the polysilicon layer  178  to lower resistivity between the auxiliary gate  178  and the drain terminal  176 , thereby increasing reading current and reducing RC delay. An isolation layer  180  is formed over the auxiliary gate  178  and the drain terminal  176 . The silicon nitride layer  160  as shown in FIG. 3D is removed. A first polysilicon layer  181  is formed over the polysilicon layer  158  within the gate region  162 , thereby forming a floating gate terminal. Another isolation layer  182  is formed over the first polysilicon layer  181 , the drain terminal  176  and the trench  170  region. As shown in FIG. 3F, a second polysilicon layer  184  serving as a control gate is formed over the isolation layer  182 . The aforementioned spacers  168  and isolation layer  182  can be ON layers or ONO layers, for example. 
     FIGS.  4 A˜ 4 C are schematic cross-sectional diagrams showing the operations of programming data into, erasing data and reading data from a trench split-gate non-volatile flash memory cell according to this invention. 
     The configuration for programming data into a flash memory cell is shown in FIG. 4A. A high voltage between 8V˜16V (16V in FIG. 4A) is applied to a word line that leads to the gate terminal. A voltage lower than the word line voltage, for example, between 4V˜10V (6V in FIG.  4 A), is applied to a bit line that leads to the drain terminal. A voltage lower than the word line voltage, for example, between 1V˜5V (2.5V in FIG. 4A) is applied to the auxiliary gate terminal. A voltage lower than both the word line voltage and the bit line voltage such as 0V is applied to the source terminal. A voltage such as 0V is applied to the substrate. Notice that the direction of electron injection from the source terminal into the floating gate is identical to the path for electron acceleration. Hence, operating efficiency of the memory cell may increase up to 10 −3  compared with 10 −4 ˜10 −6  of a conventional split-gate flash memory cell. 
     The configuration for erasing data from the flash memory cell is shown in FIG. 4B. A low voltage, for example, between 8V˜−16V (−8V in FIG. 4B) is applied to the word line. A voltage higher than the word line voltage, for example, between 4V˜10V (9V in FIG. 4B) is applied to the drain terminal. A voltage higher than the word line voltage, for example, around 0V (or a floating state voltage), is applied to the auxiliary gate terminal. A voltage higher than the word line voltage but lower than the bit line voltage, for example, around 0V (or a floating state voltage), is applied to the source terminal. A voltage of around 0V is also applied to the substrate. 
     The configuration for reading data from a flash memory cell is shown in FIG. 4C. A high voltage, for example, between 0V˜6V (2V in FIG.  4 C), is applied to the word line. A voltage lower than the word line voltage, for example, between 0V˜5V (1V in FIG.  4 C), is applied to the drain terminal. A voltage higher than the word line voltage, for example, between 0V˜5V (4V in FIG.  4 C), is applied to the auxiliary gate terminal. A voltage higher than the word line voltage but lower than the bit line voltage such as 0V is applied to the source terminal. Similarly, 0V is applied to the substrate. 
     A few of actual applications of the invention are described as follows. FIG. 5 is a circuit drawing, schematically illustrating an application of the flash memory on the AND-type array, according to this invention. In FIG. 5, a ballistic injection mechanism is employed for encoding action, where it means that hot electrons are injected into the source terminal by a ballistic manner. This mechanism is therefore called as Ballistic injection AND-type flash EEPROM (BiAND). The BiAND array and the typical AND-type array have the same arrangement about the fixing terminal of source terminal and gate terminal, but the insulating oxide layer in the BiAND array is similar to manner of the virtually grounded AND-type array, in which the insulating oxide layer along the channel is removed. Therefore, the structure of the BiAND array is between the typical AND-type array and virtually grounded AND-type array. In this manner, the area of the insulating oxide layer can be saved so as to greatly reduce the occupation area of the flash memory cells. However, it then cannot increase the wing of the control gate and the floating gate due to the insulating oxide layer being saved. This means that the coupling coefficient between the floating gate and the control gate could be reduced. In order to increase the coupling coefficient, a special design on the floating gate is necessary for the whole BiAND device. That is why the floating gate is designed in 3D structure, so as to increase the coupling coefficient. This principle of the invention can also be applied to the designs of NOR, NAND, AND array. 
     The encoding action for the BiAND-type is shown in FIG.  6 . The corresponding working voltages for encoding are also listed in Table 1: 
     [t1] 
     
       
         
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Byte 1 
                 Byte 2 
                 Byte 3 
                 Byte 4 
               
               
                   
                 Program 
                 Program 
                 Program 
                 Program 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Word-Line 
                 12 V 
               
               
                 0 
               
               
                 Word-Line 
                  0 V 
               
               
                 Others 
               
             
          
           
               
                 Bit-Line 
                 5 V/0 V 
                   0 V 
                   0 V 
                 5 V/0 V 
               
               
                 Odd 
               
               
                 Bit-Line 
                   0 V 
                 5 V/0 V 
                 5 V/0 V 
                   0 V 
               
               
                 Even 
               
               
                 AG-Line 
                 2.5 V 
                 2.5 V 
                   0 V 
                   0 V 
               
               
                 Odd 
               
               
                 AG-Line 
                   0 V 
                   0 V 
                 2.5 V 
                 2.5 V 
               
               
                 Even 
               
             
          
           
               
                 Source-Line 
                  0 V 
               
               
                 P-Sub 
                  0 V 
               
             
          
           
               
                 Cell Location 
                 4(n − 1) + 1 
                 4(n − 1) + 2 
                 4(n − 1) + 3 
                 4(n − 1) + 4 
               
               
                 in 1 st  WL 
               
               
                   
               
               
                 Note: n = 1˜8  
               
             
          
         
       
     
     If one wants to encode a word for the BiAND-type array, that is, an 8-bit encoding action is intended. In this case, the word line is applied with a voltage of 12V. Then, the devices at the 1 st , 5 th , 9 th , 13 th , . . . 4(n−1)+1, where n is 8 in the case, are selected, and all of the drains are connected and applied by a voltage of 5V. The auxiliary word line is applied by 2.5V. The other devices without connection remain 0V. In this manner, the 8-bits can be encoded. Likewise, for the second word is stored on the devices at 2, 6, 10, 14, . . . 4(n−1)+2. The third word is stored at 3, 7, 11, 15, . . . 4(n−1)+3. Multiple bits can be simultaneously operated in the BiAND arrary. 
     FIG. 7 describes how to read multiple bits by once action. The corresponding working voltages for encoding are also listed in Table 2: 
     [t2] 
     
       
         
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Byte 1 
                 Byte 2 
                 Byte 3 
                 Byte 4 
               
               
                   
                 Read 
                 Read 
                 Read 
                 Read 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Word-Line 
                 3.3 V 
               
               
                 0 
               
               
                 Word-Line 
                   0 V 
               
               
                 Others 
               
             
          
           
               
                 Bite-Line 
                 2 V 
                 0 V 
                 0 V 
                 2 V 
               
               
                 Odd 
               
               
                 Bit-Line 
                 0 V 
                 2 V 
                 2 V 
                 0 V 
               
               
                 Even 
               
               
                 AG-Line 
                 5 V 
                 5 V 
                 0 V 
                 0 V 
               
               
                 Odd 
               
               
                 AG-Line 
                 0 V 
                 0 V 
                 5 V 
                 5 V 
               
               
                 Even 
               
             
          
           
               
                 Source-Line 
                   0 V 
               
               
                 P-Sub 
                   0 V 
               
             
          
           
               
                 Cell Location 
                 4(n − 1) + 1 
                 4(n − 1) + 2 
                 4(n − 1) + 3 
                 4(n − 1) + 4 
               
               
                 in 1 st  WL 
               
               
                   
               
               
                 Note: n = 1˜8  
               
             
          
         
       
     
     When the channel hot electrons are used to encode the device in the flash memory device, the voltages applied for reading action is just reducing the voltage level from that of encoding action. It is sufficient for the voltage to read the current and without injecting the hot electrons into the floating gate, i.e., the device is not set to work at the saturation condition. If a reading action for a word on the BIAND array is to intended, that is, 8 bits are to be read simultaneously. The selected word lines of 1, 5, 9, 13, . . . 4(n−1)+1 are applied by 4V, where n is equal to 8 in the case. The connected bit line is applied by 2V. The auxiliary bit line is set to 5V. The other devices without connection are set to 0V. Likewise, the other words can be Read by the same manner. 
     If the BiAND array is to be erased, all of the word lines are applied by −10V. the drains terminals are applied by 4V, and the auxiliary gate, source terminal, and substrate are applied by 0V, so as to have the tunneling effect between the drain terminal and the floating gate. The electrons are drawn out from the floating gate, so as to reduce the threshold voltage and the erasing action is accomplished. 
     In summary, one major aspect of this invention is the construction of a source terminal and an auxiliary gate inside a shallow P-well layer and a deep N-well layer so that area occupation of these regions is greatly reduced. Moreover, the hot electrons that inject into the floating gate during programming follow a path identical to the path of acceleration for the electrons. Therefore, the operating efficiency of the memory cell structure is increased and resistivity at the source terminal is reduced when data is read from the cell. In addition, a metal silicide layer may form over the drain terminal and the auxiliary gate to reduce the resistance between the auxiliary gate and the drain terminal. Hence, reading current is increased and RC delay is reduced. Ultimately, average operating speed of the flash memory structure increases. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.