Abstract:
A non-volatile semiconductor memory cell structure and method of manufacture. The method includes the steps of forming a shallow first-type well layer, a second-type well layer and a deep first-type well layer over a substrate, forming stack gates over the shallow first-type well layer and finally forming source terminals and drain terminals. The source terminals penetrate through the shallow first-type well layer and connect with the second-type well layer. The drain terminals are close to the surface of the shallow first-type well layer. Both the source terminals and the drain terminals contain second type dopants.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The present invention relates to a non-volatile semiconductor memory cell structure and method of manufacture. More particularly, the present invention relates to a flash memory cell structure and method of manufacture.  
           [0003]    2. Description of Related Art  
           [0004]    Non-volatile memory is used inside various electronic components for holding structural data, programming data or repeatedly accessed data. Non-volatile programmable memory such as electrically erasable programmable read-only-memory (EEPROM) is now routinely used inside personal computers and electronic equipment. A conventional EEPROM employs a floating gate transistor structure to write data into or erase data from a memory cell. However, erasure speed of this type of memory cell is relatively slow. Memory having a fast erasure speed, commonly referred to as flash memory, has now become a mainstream product in the market. In general, flash memory can be roughly divided into two major types, namely, NAND type and NOR type. In the NAND type flash memory, memory cells are connected in series by connecting a drain terminal of a previous memory cell with a source terminal of a following serial-connected memory cell. That is, the drain terminal of each of the NAND type memory cells is commonly using a same region of the source terminal of the following serial-connected memory cell. In the NOR type flash memory, a source region is commonly used by NOR type memory cells, for example 6. That is, the source terminals of the NOR type memory cells are connected to each other by the commonly used source region.  
           [0005]    FIGS.  1  to  3  are cross-sectional views showing the steps for producing a conventional non-volatile semiconductor memory cell. First, as shown in FIG. 1, a substrate  100  is provided. A deep P-well layer  102 , an N-well layer  104  and a channel-doped region  106  are formed over the substrate  100 . The channel-doped region  106  is a p-doped region near the surface of the substrate  100 . The N-well layer  104  is beneath the channel-doped region  106  and the deep P-well layer  102  is beneath the N-well layer  104 .  
           [0006]    As shown in FIG. 2, a stack gate  108  is formed over the channel-doped region  106 . The stack gate  108  comprises of a first dielectric layer  108   a , a floating gate  108   b , a second dielectric layer  108   c  and a control gate  108   d.    
           [0007]    After the fabrication of the stack gate  108 , ion implantation and heat drive-in processes are carried out to form a P-well  114 , a source terminal  112  and a drain terminal  110  as shown in FIG. 3. The source terminal  112  and the drain terminal  110  are heavily N-doped (n + ) regions. The distribution of the source terminals localizes the channel-doped regions  106  into separate blocks such that each pair of stack gates  108  in a block uses a common drain terminal  110 . The P-well  114  is under the drain terminal  110 . Due to the heat drive-in process, the P-well  114  expands into regions under the stack gate  108  and overlaps with a portion of the channel-doped region  106 .  
           [0008]    In a conventional method of manufacturing non-volatile semiconductor memory cell, the distribution of dopants inside the channel-doped/P-well overlapping region underneath the stack gate is often laterally non-uniform. The variation in dopant concentration inside the channel-doped/P-well overlapping region will lead to threshold voltage deviation of memory cells.  
           [0009]    In addition, the spread of the P-well region is often subjected to the effect caused by the thermal budget. If the P-well regions expand further towards the source terminals due to drive-in, the blocks normally localized by the source terminals may be too close or conduct leading to reliability problem for the memory cells.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, one object of the present invention is to provide a nonvolatile semiconductor memory cell structure and method of manufacture capable of improving the uniformity of dopant distribution inside a region underneath a stack gate structure of the memory cell.  
           [0011]    A second object of this invention is to provide a non-volatile semiconductor memory cell structure and method of manufacture capable of improving the reliability of the memory cell.  
           [0012]    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 non-volatile semiconductor memory cell structure. The memory cell structure is built over a substrate. The substrate comprises of, from top to bottom, a shallow first-type well layer, a second-type well layer and a deep first-type well layer. A multiple-layered stack gate is over the substrate. The substrate further contains a multiple of source terminals and drain terminals. The source terminal and the drain terminal together form an adjacent pair. Each of the source terminal and the drain terminal is formed between a pair of stack gates. The source terminal has a depth great enough to pass through the shallow first-type well layer and connect with the second-type well layer. The drain terminal is close to the surface of the shallow first-type well layer. Both the source terminal and the drain terminal are second-type doped.  
           [0013]    This invention also provides a method of fabricating a non-volatile semiconductor memory cell. The method includes the following steps. A deep first-type well layer, a second-type well layer and a shallow first-type well layer are sequentially formed over a substrate. A stack gate is formed over the shallow first-type doped well layer. A source terminal and a drain terminal is formed such that the source terminal passes through the shallow first-type well layer and connects with the second-type well layer and the drain terminal is formed close to the surface of the first-type well layer. Both the source terminal and the drain terminal are second-type doped.  
           [0014]    In this invention, if the shallow first-type well layer and the deep first-type well layer are both P-doped layers, the second-type well layer, the source terminal and the drain terminal are N-doped layers. Conversely, if the shallow first-type well layer and the deep first-type well layer are both N-doped layers, the second-type well layer, the source terminal and the drain terminal are P-doped layers.  
           [0015]    The stack gate of this invention may comprise of a first dielectric layer, a floating gate layer, a second dielectric layer and a control gate layer. The second dielectric layer, for example, is a three-layered oxide/nitride/oxide (ONO) structure.  
           [0016]    Alternatively, the stack gate of this invention may comprise of a first dielectric layer, a trap layer, a second dielectric layer and a control gate layer. The first dielectric layer and the second dielectric layer can be oxide layers and the trap layer can be a silicon nitride layer. In other words, a three-layered oxide/nitride/oxide (ONO) structure is formed underneath the control gate.  
           [0017]    The source terminal within the substrate comprises of a lightly doped section and a heavily doped section. The lightly doped section is close to the surface of the substrate while the heavily doped section is underneath the lightly doped section in connection with lightly doped section. Furthermore, the heavily doped section passes through the shallow first-type well layer and connects with the second-type well layer.  
           [0018]    The source terminal that passes through the shallow first-type well layer is formed, for example, by performing a one-time ion implantation. By controlling implant depth of the ions, a lower dopant concentration is established near the surface of the substrate while a higher dopant concentration is established close to the junction between the second-type well layer and the shallow first-type well layer.  
           [0019]    Alternatively, the source terminal that passes through the shallow first-type well layer is formed, for example, by performing more than one ion implantation. A first ion implantation is performed to form a lightly doped region near the surface of the substrate. A second ion implantation is performed to form a heavily doped region close to the junction between the second-type well layer and the shallow first-type well layer. The source terminal comprises the lightly doped region and the heavily doped region.  
           [0020]    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 THE DRAWINGS  
       [0021]    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,  
         [0022]    FIGS.  1  to  3  are schematic cross-sectional views showing the steps for producing a conventional non-volatile semiconductor memory cell;  
         [0023]    FIGS.  4  to  6  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a first embodiment of this invention;  
         [0024]    FIGS.  7  to  9  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a second embodiment of this invention;  
         [0025]    FIGS.  10  to  12  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a third embodiment of this invention;  
         [0026]    FIGS.  13  to  15  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a fourth embodiment of this invention; and  
         [0027]    [0027]FIG. 16 shows a short-circuited structure in the drain side of the cell.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    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.  
         [0029]    FIGS.  4  to  6  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a first embodiment of this invention. As shown in FIG. 4, a substrate is provided. The substrate  200  can be a second-type-doped layer, for example. A deep first-type well layer  202 , a second-type well layer  204  and a shallow first-type well layer  206  are sequentially formed over the substrate  200 . The dopants in both the substrate  200  and the second-type well layer  204  are N-type dopants and the dopants in both the deep first-type well layer  202  and the shallow first-type well layer  206  are P-type dopants, for example. In addition, the shallow first-type well layer  206  is close to the surface of the substrate  200 . The second-type well layer  204  is underneath the shallow first-type well layer  206  and the deep first-type well layer  202  is underneath the second-type well layer  204 .  
         [0030]    As shown in FIG. 5, a stack gate  208  is formed over the shallow first-type well layer  206 . The stack gate  208 , for example, includes a first dielectric layer  208   a , a floating gate  208   b , a second dielectric layer  208   c  and a control gate  208   d . The first dielectric layer  208   a  can be, for example, a silicon oxide layer. The second dielectric layer  208   c  can be, for example, an oxide/nitride/oxide (ONO) composite layer. The floating gate  208   b  and the control gate  208   d  can be, for example, polysilicon layers. In the fabrication of the stack gate  208 , the floating gate  208   b  and the control gate  208   d  are patterned differently. Hence, the first dielectric layer  208   a  and the floating gate  208   b  are patterned using a first mask while the second dielectric layer  208   c  and the control gate  208   d  are patterned using a second mask.  
         [0031]    After complete fabrication of stack gate  208 , a source terminal  212  and a drain terminal  210  are formed as shown in FIG. 6. The source terminal  212  and the drain terminal  210  are formed, for example, by conducting an ion implant followed by a drive-in operation. The source terminal  212  and the drain terminal  210  are regions with a high concentration of second-type dopants, for example. The source terminal  212  has a depth great enough to localize the shallow first-type well layer  206  into separate blocks and the two stack gates  208  on each block uses a common drain terminal  210 .  
         [0032]    The source terminal  212  is formed, for example, by performing an ion implantation and controlling the implantation depth to a region close to the junction between the shallow first-type well layer  206  and the second-type well layer  204 . Thereafter, a drive-in operation is performed to form the source terminal  212 . By controlling the depth of implantation, the source terminal  212  has a dopant concentration that varies with depth. The region closer to the surface of the shallow first-type well layer  206  has a lower concentration of dopants and the region closer to the junction between the shallow first-type well layer  206  and the second-type well layer  204  has a higher concentration of dopants. Such distribution of dopants inside the source terminal  212  reduces the amount of disturbances during memory programming.  
         [0033]    In the first embodiment, the shallow first-type well layer  206  replaces the conventional channel-doped region  106  and the P-well layer  114  (shown in FIG. 3). Since a highly uniform distribution of dopants underneath the stack gates  208  can be created inside the shallow first-type well layer  206  by controlling the implantation process, problem arising from non-uniformity of dopant concentration is eliminated. Furthermore, in this embodiment, the shallow first-type well layer  206  is formed before creating a concentration gradient of dopants with depth so that the shallow first-type well layer  206  and the source terminal  212  are localized. Hence, bridging between neighboring memory cell due to drive-in is prevented.  
         [0034]    The first-type dopants and the second type dopants, for example, may be P-type dopants and N-type dopants respectively. In the illustration provided by the second embodiment, both the source terminal and drain terminal are N-doped regions. However, the actual configuration should not be limited as such because identical function will result if the first-type dopants are N-type dopants and the second-type dopants are P-type dopants.  
         [0035]    FIGS.  7  to  9  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a second embodiment of this invention. As shown in FIG. 7, a substrate is provided. The substrate  200  can be a second-type-doped layer, for example. A deep first-type well layer  202 , a second-type well layer  204  and a shallow first-type well layer  206  are sequentially formed over the substrate  200 . The dopants in both the substrate  200  and the second-type well layer  204  are N-type dopants and the dopants in both the deep first-type well layer  202  and the shallow first-type well layer  206  are P-type dopants, for example. In addition, the shallow first-type well layer  206  is close to the surface of the substrate  200 . The second-type well layer  204  is underneath the shallow first-type well layer  206  and the deep first-type well layer  202  is underneath the second-type well layer  204 .  
         [0036]    As shown in FIG. 8, a stack gate  208  is formed over the shallow first-type well layer  206 . The stack gate  208 , for example, includes a first dielectric layer  208   a , a floating gate  208   b , a second dielectric layer  208   c  and a control gate  208   d . The first dielectric layer  208   a  can be, for example, a silicon oxide layer. The second dielectric layer  208   c  can be, for example, an oxide/nitride/oxide (ONO) composite layer. The floating gate  208   b  and the control gate  208   d  can be, for example, polysilicon layers. In the fabrication of the stack gate  208 , the floating gate  208   b  and the control gate  208   d  are patterned differently. Hence, the first dielectric layer  208   a  and the floating gate  208   b  are patterned using a first mask while the second dielectric layer  208   c  and the control gate  208   d  are patterned using a second mask.  
         [0037]    After complete fabrication of stack gate  208 , a source terminal  212  and a drain terminal are formed as shown in FIG. 9. The source terminal  212  and the drain terminal  210  are formed, for example, by performing an ion implant followed by a drive-in operation. The source terminal  212  and the drain terminal  210  are regions with a high concentration of second-type dopants, for example. The source terminal  212  has a depth great enough to localize the shallow first-type well layer  206  into separate blocks and the two stack gates  208  on each block uses a common drain terminal  210 .  
         [0038]    The source terminal  212  can be formed, for example, by performing a number of ion implantation. To form the source terminal  212 , two implant operations are carried out in sequence. In the first ion implant operation, implant depth is controlled to a region close to the surface of the shallow first-type well layer  206 . In the second ion implant operation, implant depth is controlled to a region close to the junction between the shallow first-type well layer  206  and the second-type well layer  204 . Thereafter, a drive-in operation is performed to form the source terminal  212  that includes a lightly doped region  212   a  and a heavily doped region  212   b  inside the substrate  200 .  
         [0039]    In the second embodiment, ions are implanted into the substrate to a different depth level in two separate implant operations to form the lightly doped region  212   a  and the heavily doped region  212   b . The distribution of dopants in the lightly doped region  212   a  and the heavily doped region  212   b  inside the source terminal  212  also reduces the amount of disturbances in the memory cell during memory programming.  
         [0040]    The first-type dopants and the second type dopants, for example, may be P-type dopants and N-type dopants respectively. In the illustration provided by the second embodiment, both the source terminal and drain terminal are N-doped regions. However, the actual configuration should not be limited as such because identical function will result if the first-type dopants are N-type dopants and the second-type dopants are P-type dopants.  
         [0041]    FIGS.  10  to  12  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a third embodiment of this invention. The third embodiment is very similar in many aspects to the first embodiment of this invention. One major difference lies in the stack gate  308  above the substrate  200 . The stack gate  308  comprises of a first dielectric layer  308   a , a trap layer  308   b , a second dielectric layer  308   c  and a control gate layer  308   d . The first dielectric layer  308   a , the trap layer  308   b , the second dielectric layer  308   c  and the control gate layer  308   d  are sequentially formed on the shallow first-type well layer  206 . The first dielectric layer  308   b  and the second dielectric layer  308   c  are, for example, silicon oxide layers. The trap layer  308   b  is, for example, a silicon nitride layer so that the first dielectric layer  308   a , the trap layer  308   b  and the second dielectric layer  308   c  together form an oxide/nitride/oxide (ONO) composite layer. The control gate layer  208   d  is, for example, a polysilicon layer. In the process of fabricating the stack gate  308 , since the trap layer  308   b  is a non-conductive layer, the first dielectric layer  308   a , the trap layer  308   b , the second dielectric layer  308   c  and the control gate  308   d  are patterned together. In other words, the first dielectric layer  308   a , the trap layer  308   b , the second dielectric layer  308   c  and the control gate layer  308   d  are formed together using a single mask.  
         [0042]    The modification of stack gate  308  structure facilitates a further drop in the operating voltage and power and simplifies the manufacturing process. In addition, the shallow first-type well layer  206  together with the source terminal  212  structure with a depth long enough to localize the shallow first-type well layer  212  further boost the reliability of each memory cell.  
         [0043]    FIGS.  13  to  15  are schematic cross-sectional views showing the steps for producing a non-volatile semiconductor memory cell according to a fourth embodiment of this invention. The fourth embodiment is very similar in many aspects to the second embodiment of this invention. One major different lies in the stack gate  308  above the substrate  200 . The stack gate  308  comprises of a first dielectric layer  308   a , a trap layer  308   b , a second dielectric layer  308   c  and a control gate layer  308   d . The first dielectric layer  308   a , the trap layer  308   b , the second dielectric layer  308   c  and the control gate layer  308   d  are sequentially formed on the shallow first-type well layer  206 . The first dielectric layer  308   b  and the second dielectric layer  308   c  are, for example, silicon oxide layers. The trap layer  308   b  is, for example, a silicon nitride layer so that the first dielectric layer  308   a , the trap layer  308   b  and the second dielectric layer  308   c  together form an oxide/nitride/oxide (ONO) composite layer. The control gate layer  208   d  is, for example, a polysilicon layer. In the process of fabricating the stack gate  308 , since the trap layer  308   b  is a non-conductive layer, the first dielectric layer  308   a , the trap layer  308   b , the second dielectric layer  308   c  and the control gate  308   d  are patterned together. In other words, the first dielectric layer  308   a , the trap layer  308   b , the second dielectric layer  308   c  and the control gate layer  308   d  are formed together using a single mask.  
         [0044]    The modification of stack gate  308  structure facilitates a further drop in the operating voltage and power and simplifies the manufacturing process. In addition, the shallow first-type well layer  206  together with the source terminal  212  consisting of the lightly doped region  212   a  and the heavily doped region  212   b  further improves the reliability of each memory cell.  
         [0045]    In addition, the cell structure can be further modified as shown in Fg.  16 . FIG. 16 shows a short-circuited structure in the drain side of the cell. As shown, the drain terminal  210  and the shallow first type well layer  206  are short together as a bit line structure. There are several method to form the bit line structure. For example, the drain terminal  210  and the shallow first type well layer  206  shorted by a metal contact  310  that penetrates through a junction  312  between the drain terminal  210  and the shallow first type well layer  206 . Alternatively, the drain terminal  210  and the shallow first type well layer  206  can be shorted by a metal contact  310  across an exposed surface of the drain terminal  210  and the shallow first type well layer  206 .  
         [0046]    In summary, the non-volatile semiconductor memory cell structure and manufacturing method have the following advantages:  
         [0047]    1. The P-well layer and the channel-doped region in a conventional cell structure are integrated together to form a shallow P-well layer so that non-uniformity of the P-doped layer underneath the stack gate is prevented.  
         [0048]    2. The shallow P-well layer used in this invention prevents conduction problem associated with a conventional P-well structure.  
         [0049]    3. By replacing the steps for producing the P-well layer and the channel-doped region of a conventional cell, the manufacturing process is simplified.  
         [0050]    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.