Abstract:
A memory structure that combines multiple embedded flash memory. The flash memory can be used, e.g., as air replacement cells or back up memory, or additional memory cells. In one aspect, the flash memory cells are stacked on top of the flash memory cells and the flash memory cells share a gate layer. In another aspect, pairs of stacked flash memory cells are stacked on top of each other with each pair isolated by an isolation oxide. In another aspect, pairs of stacked flash memory cells are stacked on top of each other in an un-isolated configuration.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates generally to memory structures, and more particularly to the design, fabrication, and use of memory structures that comprises embedded flash memory. 
     2. Background of the Invention 
     Many conventional memory devices use flash memory cells. Many conventional flash memory cells use floating gate technology to store one or more bits of information in the floating gate when a program voltage is applied. The operation of floating gate flash memory devices is well known and will not be discussed herein for the sake of brevity. More recently, floating gate technology has been displaced by the use of other technologies that can be scaled to meet increasing memory density demands. For example, SONOS technology has become more prevalent in many applications. In a SONOS cell, the cell comprises a silicon layer (S), an oxide layer (O), a nitride layer (N), another oxide layer (O), and another silicon layer (S). Appropriate programming voltages applied to the SONOS stack causes a bit of data, or a charge, to be stored in the nitride layer. By applying the appropriate read voltages to a SONOS cell, it can be determined whether the cell has been programmed. 
     While there have been advancements in conventional memory cell design, such as the development of SONOS flash memory, new applications are constantly driving new memory requirements that cannot necessarily be met by the use of conventional memory structures. 
     As such, demands are likely to continue, and even increase, in the future, it is important to develop new techniques for memory structure design and fabrication. One such technique comprises stacked thin-film memory cells. Thin-film deposition is any technique for depositing a thin film of material into a substrate or onto previously deposited layers. “thin” is a relative term, but most such deposition techniques allow layer thickness to be controlled within a few hundred nanometers, and some allow one layer of atoms to be deposited at a time. Thus, thin-film structures can be used to reduce the overall size and allow increased density, e.g., by stacking thin-film structures. Unfortunately, the thin-film devices are not necessarily as reliable as devices constructed using more conventional deposition techniques. Thus, the applicability of thin-film structures in memory applications is limited due to their inherent unreliability. 
     SUMMARY 
     A memory cell structure comprises stacked memory cells. The stacked memory cells can be flash memory cells, where one of the stacked flash memory cells is a thin-film flash memory cell. 
     In one aspect, the memory cell structure can be used as a four bit memory cell structure, allowing the memory cell structure to achieve increased density compared to conventional memory cell structures. 
     In another aspect, the stack flash memory cell can be used as a main memory cell, while the other flash memory cell can be used for redundancy or error correction. 
     In another aspect, a plurality of memory cells can be stacked to achieve further memory density. 
     These and other features, aspects, and embodiments of the invention are described below in the section entitled “Detailed Description.” 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Features, aspects, and embodiments of the inventions are described in conjunction with the attached drawings, in which: 
         FIG. 1  is a diagram illustrating an example memory structure that combines multiple flash memory cells in accordance with one embodiment of the systems and methods described herein; 
         FIG. 2  is a diagram illustrating an example memory structure in accordance with the systems and methods described herein; 
         FIG. 3  is a diagram illustrating an example memory structure in accordance with the systems and methods described herein; and 
         FIGS. 4A-4D  illustrate an example method for fabricating a memory structure comprising flash memory cells in accordance with one of the embodiment of the. systems and methods described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     The systems and methods described below are directed to memory cell structures that combine multiple flash memory cells. In the embodiments described, the flash memory cells are generally SONOS cells; however, this should not necessarily be seen as limiting the systems and methods described herein to the use of SONOS cells. It will be clear, that other, and future, flash cell structures can be used with the systems and methods described herein in order to achieve the benefits described. Further, certain specific embodiments of memory structures combining multiple flash cells are described below; however, the specific embodiments described should not be seen as limiting the systems and methods described herein to any particular architecture or design. It would be clear that other combinations, stacking, and arrangements of flash memory cells are possible. 
       FIG. 1  is a diagram illustrating an example memory cell structure  100  that combines multiple flash memory cells in accordance with one embodiment of the systems and methods described herein. As can be seen, memory cell structure  100  comprises a first flash memory cell layer  102  and a second flash memory cell layer  104 . In this example, first flash memory cell layer  102  comprises a silicon-substrate (Si-substrate) flash memory cell  116 . Si-substrate flash memory cell  116  comprises a poly-silicon gate  106  above silicon-substrate  108 , separated by an ONO layer  136 . ONO layer  136  is formed from oxide layer  110 , nitride layer  112 , and oxide layer  114 . Thus, in the embodiment of  FIG. 1 , Si-substrate flash memory cell  116  is a SONOS flash memory cell. Si-substrate flash memory cell  116  also comprises a source  118  and drain  120  constructed, e.g., via implantation. 
     Second flash memory cell layer  104  also comprise a flash memory cell  130 . In the embodiment of  FIG. 1 , cell  130  is deposited on top of Si-substrate cell  116  using thin-film deposition techniques, i.e., cell  130  is thin-film structure. Poly-silicon layer  106  can act as the gate for cell  130  as illustrated. poly-silicon-substrate  122  can then form the substrate for cell  122  and can comprise drain and source regions  132  and  134  formed, e.g., through implantation techniques. Poly-silicon gate  106  can be separated from substrate  122  via ONO layer  138 , which comprises oxide layer  124 , nitride layer  126  and oxide layer  128 . Thus, cell  130  is also a SONOS flash memory cell. But unlike cell  116 , cell  130  is a thin-film structure. 
     As can be seen, cells  116  and  130  share poly-silicon gate  106 . In one embodiment, gate  106  can be an N-type poly-silicon layer. Such a construction, i.e., comprising a co-used poly-silicon line, can be preferred in order to reduce the size and complexity of memory structure  100 . As will be illustrated below, however, other embodiments can be constructed without the use of co-used poly-silicon lines. 
     As is well known, each SONOS cell  116  and  130  can be configured to store two bits. Thus, memory cell structure  100  can be used to achieve a compact 4-bit cell, and therefore greater memory density. Greater increase in the density can be achieved by stacking a plurality of cells, e.g., a plurality of thin-film, SONOS flash memory cells, on top of Si-substrate cell  116 . 
     In certain embodiments, Si-substrate cell  116  can be used as a redundancy cell or as an error correction cell. Cell  116  can also be used as a high performance memory cell, due to its greater reliability as compared to thin-film cells. Thus, not only can memory cell structure  100  be used to achieve greater density than conventional memory structures, it can also be used to achieve higher performance and greater reliability as compared to conventional stacked memory devices that, e.g., make use strictly of thin-film structures. 
     As mentioned above, greater density can be achieved by stacking a plurality of memory cell structures on top of a Si-substrate cell structure. For example,  FIG. 2  is a diagram illustrating an example memory cell structure  200  that comprises a plurality of stacked memory cells in accordance with the systems and methods described herein. Memory cell structure  200  comprises four flash memory cell layers  202 ,  204 ,  206 , and  208 , respectively. Flash memory cell layer  208  is a Si-substrate layer as illustrated in  FIG. 2 . Thus, flash memory cell layer  208  comprises a Si-substrate flash memory cell  210  that comprises a poly-silicon gate layer  254  separated from a Si-substrate  240  via an ONO layer  238 . Flash memory cell  210  further comprises a source Region  218  and drain Region  220 . 
     Flash memory cell layer  206  is stacked on top of flash memory cell layer  208  and comprises a flash memory cell  212 . As can be seen, flash memory cell  212  shares poly-silicon gate layer  254  with flash memory cell  210 . The substrate of flash memory cell  212  is formed from poly Si-substrate  242  and is separated from poly-silicon gate layer  254  by ONO layer  236 . Source and drain regions,  222  and  224  respectively, cannot be formed in poly Si-substrate layer  242  as shown. Thus, flash memory cell  212  can be deposited on top of Si-substrate flash memory cell  210 , e.g., using thin film deposition techniques. 
     Flash cell structure  200  can further comprise a flash memory cell layer  204  that is stacked above flash memory cell layers  206  and  208 . Flash memory cell layer  204  can comprise a flash memory cell  214 . Flash memory cell  214  can comprise a gate formed from poly-silicon gate layer  256 , which can be separated from a poly Si-substrate  244  via ONO layer  234 . Flash memory cell  214  can further comprise a source and drain region,  226  and  228  respectively, implanted in poly Si-substrate  244 . As can be seen, flash memory cell layer  204  can be isolated from flash memory cell layers  206  and  208  via an isolation oxide layer  250 . It will be apparent, however, that other embodiments of a flash memory cell structure configured in accordance with the systems and methods described herein can comprise a flash memory cell layer  204  that shares a poly Si-substrate from which the source and drain regions of flash memory Cells  214  and  212  can be formed. 
     Flash memory cell layer  204  can be used to further increase the density of flash memory cell structure  200  as compared to conventional flash memory cell structures. Flash memory cell structure  200  can further comprise another flash memory cell layer  202  stacked on top of flash memory cell layer  204  as illustrated. Thus, the density of flash memory cell structure  200  can be increased even further. 
     Flash memory cell layer  202  can comprise a flash memory cell  216  that comprises a gate formed from poly-silicon gate layer  256 , which can be separated from poly Si-substrate  248  by ONO layer  232 . Flash memory cell  216  can also comprise source and drain regions,  230  and  231  respectively, implanted in poly Si-substrate layer  248 . An isolation oxide layer  252  can be deposited on top of flash memory cell layer  202  as illustrated. 
     Each of the flash memory cell layers  202 ,  204 , and  206  can be deposited on top of Si-substrate flash memory cell layer  208  using, e.g., thin film deposition techniques. Thus, flash memory Cells  212 ,  214  and  216  can be thin film flash memory Cells. Further, poly-silicon gate layers  256  and  254  can be shared as shown in order to decrease the size and complexity of memory cell structure  200 . It will be clear, however, that another embodiments of flash memory cell layers can be formed that do not share or that do not use of any co-used poly silicon lines, such as poly silicon lines  254  and  256 . It will also be apparent that each of the flash memory Cells  210 ,  212 ,  214  and  216  are SONOS flash memory Cells. 
     The flash memory cell structure  200  can be used to form two 4-bit memory Cells or one 8-bit memory cell depending on the embodiment. One 4-bit memory cell can be formed from flash memory cell  216  and  214 , while another 4-bit memory cell can be formed from flash memory Cells  212  and  210 . Depending on the embodiment, one such 4-bit memory cell can be used as a main memory cell, while the other can be used as a redundancy cell or as an error correction cell. Alternatively, certain of the cells, e.g., Cells  216  and  212 , can be as memory cells, while the other cells, e.g., Cells  214  and  210  are used as redundancy cells or as error correction cells. 
     As mentioned, isolation oxide layer  250  is not necessarily required and can actually be eliminated depending on the embodiment.  FIG. 3  is a diagram illustrating a memory cell structure  300  that also comprises four flash memory cell layers  302 ,  304 ,  306 ,  308 , but which excludes an isolation layer such as isolation oxide layer  250 . Thus, memory cell structure  300  can comprise a Si-substrate flash memory cell layer  308  that includes a flash memory cell  310 . Flash memory cell  310  can comprise a gate formed from poly silicon gate layer  334  separated from Si-substrate  338  by ONO layer  348 . Flash memory cell  310  can also comprise source  318  and drain  320  regions implanted in Si-substrate  338 . memory cell structure  300  can further comprise a flash memory cell layer  306  that includes a flash memory cell  312  deposited on top of Si-substrate flash memory cell layer  308  as illustrated. Flash memory cell layer  306  can comprise a flash memory cell  312  that comprises a gate formed by co-used poly silicon gate layer  334 , which can be separated from poly Si-substrate  340  by ONO layer  352 . Flash memory cell  312  also comprises source and drain regions  322  and  324 , respectively. 
     Flash memory cell structure  300  can also comprise flash memory cell layer  304  deposited on top of flash memory cell layer  306 . Flash memory cell layer  304  can comprise a flash memory cell  314  as illustrated. Flash memory cell  314  can include a gate formed from poly silicon gate layer  336 , which can be separated from co-used poly Si-substrate  340  by ONO layer  346 . Flash memory cell  314  can also comprise source and drain regions  326  and  328 , respectively, implanted in co-used poly Si-substrate  340 . 
     Flash memory cell  300  can further comprise a flash memory cell layer  302  deposited on top of flash memory cell layer  304  as illustrated. Flash memory cell layer  302  can comprise a flash memory cell  316  that includes a gate formed from co-used poly silicon gate layer  336 , which is separated from poly Si-substrate  342  by ONO layer  344 . Source and drain regions,  330  and  332  respectively, can be implanted in by Si-substrate layer  342 . An isolation oxide layer  350  can be deposited on poly Si-substrate  342 . 
     Thus, flash memory cell structure  300  makes use of co-used poly silicon lines  334  and  336  as well as co-used poly Si-substrates  340  and  342 . Use of such co-used poly silicon lines and substrates can decrease the size and complexity of a memory cell structure configured in accordance with the systems and methods described herein, and can therefore be preferable; however, as mentioned, and illustrated in  FIG. 2 , other embodiments in which certain flash memory cell layers are isolated from certain other flash memory Cells are also possible. 
     Generally, by stacking multiple SONOS cells in this way increasing memory density demands can be met. Multiple stacked flash memory cells, such as stacked SONOS cells generally provide a greater number of bits of storage for a given area. 
       FIGS. 1-3  illustrates specific implementations of a memory structure that comprises multiple flash memory cells in accordance with the systems and methods described herein. It will be clear, however, that the systems and methods described herein are not limited solely to the implementations illustrated in  FIGS. 1-3 . For example, other implementations can use co-used poly-silicon lines or not use co-used poly-silicon lines in ways not illustrated by the embodiments of  FIGS. 1-3 . 
     Depending on the embodiment, the bottom flash memory can be used, e.g., as an air replacement cell, or a memory storage cell. The use of the flash memory cell will be dependent on the specific implementation for a flash structure configured in accordance with the systems and methods described herein. Thus, the specific requirements of a particular implementation will dictate how the flash memory cells are used. 
       FIGS. 4A-4D  illustrate an example method for fabricating a memory structure comprising flash memory cells in accordance with one of the embodiment of the systems and methods described herein. The process can begin in  FIG. 4A  with a deposition of an ONO layer  402  on top of silicon-substrate  406 . Next, a photo resist  404  can be deposited on top of ONO layer  402  as illustrated. In the next step, photo resist  404  can be photo defined. Electron implantation can then be used to define the source  408  and drain  410  within silicon-substrate  406 . 
     Next, as illustrated in  FIG. 4B , the photo resist layer  404  from  FIG. 4A  can be removed and the next poly-silicon layer, in this case N-type layer  412 , can be deposited. It will be understood that poly-silicon layer  412  can be deposited in areas defined by the photo definitions described above. Next, photo resist layer  414  can then be deposited on top of poly-silicon layer  412  and photo resist layer  414  can be photo defined in the next step. Poly-silicon layer  412  can then be poly etched as required in the next step. 
     In the next step,  FIG. 4C , photo resist layer  414  can be removed. This step can be followed by the deposition of an oxide layer, the oxide layer can then be etched back in the next step and this can be followed by the deposition of an ONO layer  416 . Next, P-type poly-silicon layer  418  can be deposited and photo resist layer  420  can be deposited above poly-silicon layer  418  as illustrated. Photo resist layer  420  can then be photo defined. Electron implantation can then be used to define the source  422  and drain  424  within poly-silicon-substrate  418 . 
     Next, as illustrated in  FIG. 4D , photo resist layer  422  of  FIG. 4C  can be removed and oxide layer  426  can be deposited on top of the structure as shown.  FIG. 4D  also illustrates storage of four bits within an example memory structure as signified by the circles number  1 ,  2 ,  3 , and  4 . The bits can be stored as localized areas of charge within the nitride layers. The nitride layers do not conduct; therefore electrons that “jump” over the potential barrier of the oxide layer as they travel from source to drain and become trapped. As the electrons travel from source to drain they gain energy, therefore, it is most likely that the electrons will “jump” over the potential barrier of the oxide layer near the drain. This is why, for example, circles  2  and  4  are located near the drains. Note that to program the bits represented by circles  1  and  3  the functions of the source and drain are reversed, e.g. voltages are applied so that electrons flow in the reverse direction. When a bit is programmed, e.g., charge is stored in the localized are indicated by the circles, it can indicate, for example a logic “0” while a lack of charge can, for example, indicate a logic “1”. In this way bits of data can be stored in the memory device. 
     The process illustrated by  FIG. 4A-4D  is just one example process for fabricating a memory structure that includes flash memory cells in accordance with the systems and methods described herein. It will be understood that other fabrication processes and techniques can be used in order to achieve a memory structure that includes flash memory cells configured as described herein. 
     While certain embodiments of the inventions have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the inventions should not be limited based on the described embodiments. Rather, the scope of the inventions described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.