Abstract:
Array of electrically programmable non-volatile memory cells, each cell comprising a stacked-gate MOS transistor having a lower gate electrode, an upper gate electrode coupled to a row of the array, a first electrode associated with a column of the array and a second electrode separated from the first electrode by a channel region underlying said lower gate electrode, the first electrode, the second electrode and the channel region being formed in a layer of semiconductor material of a first conductivity type and having a second conductivity type, comprising at least one ROM memory cell which identically to the electrically programmable non-volatile memory cells comprises a stacked-gate MOS transistor and is associated with a respective row and a respective column of the array, the ROM cell including means for allowing or not allowing the electrical separation between said respective column and the second electrode of the ROM cell, if the ROM cell must store a first logic state or, respectively, a second logic state.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns an array of electrically programmable non-volatile semiconductor memory cells comprising ROM (Read Only Memory) memory cells. 
     2. Discussion of the Related Art 
     Generally, in semiconductor devices it is often necessary to store permanently some information, that can be necessary for operating the device or for identifying the same. 
     This is true in particular for electrically programmable non-volatile semiconductor memory devices, such as EPROM, Flash EEPROM and EEPROM memories. In such devices said information (e.g., for identifying the device) is stored in one or more rows of EPROM, Flash EEPROM or EEPROM memory cells, respectively, belonging to the array of memory cells of the device. 
     However, it is known that EPROM, Flash EEPROM and EEPROM memory cells include floating-gate MOS transistors, and the information is stored by means of charge trapped in the floating gate. The information might thus be lost for several reasons, in particular due to a loss of charge by the floating gates of the memory cells. 
     In order to permanently store the information, ROM memory cells are to be used. Conventionally, a ROM memory cell is formed by a MOS transistor, e.g. with an N-type channel, having N-type drain and source regions formed in a P-type substrate or well and spaced apart. The portion of P-type substrate or well between the source and drain regions forms a channel region, and a gate electrode is placed above the channel region with the interposition of a thin gate oxide layer. 
     Programming of a ROM memory cell is made during the manufacturing thereof, either by means of dedicated process steps or by means of a structure suitable for making the cell non-conductive. This can be made, for example, by implanting a dopant in the channel region, so as to vary the threshold voltage of the cell. 
     The interposition of ROM memory cells in an EPROM, Flash EEPROM or EEPROM memory device for storing information that might get lost is however disadvantageous under several respects. 
     Firstly, it is necessary to provide, in the normal manufacturing process flow for the EPROM, Flash EEPROM or EEPROM memories, additional process steps and photolithographic masks dedicated for forming the ROM cells. This causes an increase in the manufacturing costs. 
     Secondly, the ROM memory cells cannot be integrated in the same array of EPROM, Flash EEPROM or EEPROM memory cells, because having a different layout they would cause irregularities. It is thus necessary to provide suitable small arrays of ROM memory cells. This however causes an increase in the overall area of the memory device, and thus an increase of the manufacturing costs. 
     Thirdly, addressing and sensing of the ROM cells require dedicated circuits normally different from those already provided for addressing and sensing EPROM, Flash EEPROM or EEPROM memory cells, and this has the consequence of an increase in the overall area of the memory device and in the complexity of the design. 
     All this makes it disadvantageous to provide, in an electrically programmable non-volatile memory device, a small array of ROM cells for permanently storing identifying information. 
     SUMMARY OF THE INVENTION 
     In view of the state of the art described, it is an object of the present invention to provide an array of electrically programmable non-volatile memory cells integrating at the same time ROM memory cells, without additional costs due to dedicated process steps or an increase in the overall area, and allowing for using, for addressing and sensing the ROM cells, substantially the same addressing and sensing circuits already provided for the electrically programmable non-volatile memory cells. 
     According to the present invention, these and other objects are achieved by means of an array of electrically programmable non-volatile memory cells, each cell comprising a stacked-gate MOS transistor having a lower gate electrode, an upper gate electrode coupled to a row of the array, a first electrode associated with a column of the array and a second electrode separated from the first electrode by a channel region underlying said lower gate electrode, the first electrode, the second electrode and the channel region being formed in a layer of semiconductor material of a first conductivity type and having a second conductivity type, including at least one ROM memory cell which identically to the electrically programmable non-volatile memory cells comprises a stacked-gate MOS transistor and is associated with a respective row and a respective column of the array, the ROM cell comprising means for allowing or not allowing the electrical separation between said respective column and the second electrode of the ROM cell, if the ROM cell must store a first logic state or, respectively, a second logic state. 
     As a result of the present invention, it is possible to provide arrays of electrically programmable non-volatile memory cells also integrating ROM memory cells. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will be made apparent by the following detailed description of some embodiments thereof, illustrated as non-limiting examples in the annexed drawings, wherein: 
     FIG. 1, referring to a first embodiment of the present invention, is a top-plan view of a portion of a memory array comprising EEPROM memory cells and ROM memory cells; 
     FIG. 2 is a sectional view along line II—II in FIG. 1; 
     FIG. 3 is a sectional view along line III—III in FIG. 1; 
     FIG. 4, referring to a second embodiment of the present invention, is a top-plan view of a portion of a memory array comprising again EEPROM memory cells and ROM memory cells; 
     FIG. 5 is a sectional view along line V—V in FIG. 4; 
     FIG. 6 is a sectional view along line VI—VI in FIG. 4; 
     FIG. 7, referring to a third embodiment of the present invention, is a top-plan view of a portion of a memory array comprising again EEPROM memory cells and ROM memory cells; 
     FIG. 8 is a sectional view along line VI—VI in FIG. 7; 
     FIG. 9 is a sectional view along line IX—IX in FIG. 7; 
     FIGS. 10 and 11 show sectional views similar to those shown in FIGS. 8 and 9, referring to an alternative embodiment of the third embodiment of the invention; 
     FIG. 12, referring to a fourth embodiment of the present invention, is a top-plan view of a portion of a memory array comprising again EEPROM memory cells and ROM memory cells; 
     FIG. 13 is a sectional view along line XIII—XIII in FIG. 12; 
     FIG. 14 is a sectional view along line XIV—XIV in FIG. 12; 
     FIGS. 15 and 16 show sectional views similar to those shown in FIGS. 13 and 14, referring to an alternative embodiment of the fourth embodiment of the invention; and 
     FIGS. 17 and 18 show sectional views similar to those shown in FIGS. 2 and 3, referring to an alternative embodiment of the first embodiment of the invention, comprising Flash EEPROM memory cells and ROM memory cells. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows in top-plan view and partially sectioned along several plates a portion of an array of memory cells comprising EEPROM semiconductor memory cells and ROM memory cells. 
     Conventionally, the semiconductor memory array comprises memory cells arranged in rows (word lines) WL and columns (bit lines) BL 1 , BL 2 , . . . ,BL 8 . The portion of array shown in FIG. 1 comprises eight ROM memory cells  2  and eight EEPROM memory cells  1  arranged symmetrically with respect to line A—A. 
     Referring to FIG. 2, each EEPROM memory cell  1  comprises a stacked-gate MOSFET  3  and a selection transistor  4 . For each stacked-gate MOSFET  3 , in a P-type substrate or well  5  a source region  6  and a spaced-apart drain region  7  are implanted. A further N-type region  8  is implanted between the drain region  7  and a part of substrate or well  5  comprised between regions  6  and  7 . The portion of substrate or well  5  comprised between regions  6  and  7  forms a channel region, and a lower gate electrode  9  is disposed above the channel and part of the drain region  7  with the interposition of a gate oxide layer  10  (typically of thickness comprised between 150 and 300 Angstroms), the latter having a thinner portion  11  (tunnel oxide of thickness between 70 and 120 Angstroms) at the further region  8  for allowing the passage of electrons by tunnel effect from the drain to the lower gate electrode and vice-versa, when information is stored in the memory cell  1 . 
     An upper, control gate electrode  12  is insulatively disposed above the lower gate electrode  9 , which is electrically floating. 
     In series to each MOSFET  3  a selection transistor  4  is connected having an N-type source region coincident with the drain region  7  of MOSFET  3 , and an N-type drain region  13  formed in the P-type substrate or well  5 . The region of the substrate or well  5  comprised between the drain region  13  and the source region  7  of transistor  4  forms a channel region, over which two polysilicon electrodes  14 ,  15  are disposed, one over the other with the interposition of different layers of oxide, for forming the gate of transistor  4 . 
     Conventionally, for the selection transistors  4  the two polysilicon electrodes  14 ,  15  belong to respective lines formed from a first and a second levels of polysilicon, and said lines are electrically short-circuited in a region of the array not shown in FIG.  1 . 
     The lower gate electrode  9  of MOSFETs  3  of the cells  1  is formed by definition of the first level of polysilicon, while the upper, control gate electrode  12  of MOSFET  3  of the cells  1  is formed by a respective line formed from the second level of polysilicon, such a line forming a word line WL. 
     Each ROM memory cell  2  comprises, identically to the EEPROM cells  1 , a stacked-gate MOSFET  3 ′ and, in series thereto, a selection transistor  4 ′. The ROM memory cells  2  and the EEPROM memory cells  1  are in fact formed by means of the same process steps and have identical structure. 
     Differently from the EEPROM memory cell  1 , a further N-type region  80  is implanted between the drain region  7 ′ and the source region  6 ′ of the stacked-gate MOSFET  3 ′, with the function of short-circuiting regions  6 ′ and  7 ′. Preferably, region  80  is formed by means of the same implant used to form region  8  for the EEPROM memory cells  1 . 
     Over the whole surface of the memory array a dielectric layer  16  is then formed, in which openings  17  are formed at the drains  13  of the selection transistors  4  of the EEPROM cells  1 . 
     In this first embodiment of the invention, if it is desired to program a “1” logic state into the ROM cell  2 , then further openings  18  at the drains  13 ′ of the selection transistors  4 ′ of the ROM cells  2  (FIG.  2 ). When instead it is desired to program a “0” as shown in FIG. 3, no openings  18  will be formed. Then a level of metallization is deposited for forming metal lines  19  contacting through openings  17  and, where provided, openings  18  the drain diffusions  13  and  13 ′ of the respective selection transistors  4  and  4 ′. 
     Lines  19  form the bit lines BL 1 , BL 2 , . . . ,BL 8 . 
     As known, reading of the EEPROM cell  1  is performed bringing the respective word line WL to a voltage VREAD of e.g. 2 V, and turning the respective selection transistor  4  on. 
     For reading a ROM cell  2 , the respective word line WL can be brought to voltage VREAD or also kept at ground (it is not necessary to form the channel in the MOSFET  3 ′, because region  80  is provided), and the respective selection transistor  4 ′ is selected; if the contact between the respective bit line BL 1 , BL 2 , . . . ,BL 8  and drain  13 ′ of the selection transistor is provided, then the ROM cell  2  sink current from the bit line; if differently the contact is not provided, then the drain of the selection transistor  4 ′ is disconnected from the bit line and no current will flow in the latter. 
     Another embodiment of the present invention is shown in FIGS. 4-6. In this embodiment, differently from the previous one, apertres  17  and  18  in the dielectric layer  16  are always provided. 
     Programming of the ROM cell  2  is obtained by providing or not providing the N-type region  80  which short-circuits the source  6 ′ and the drain  7 ′ of MOSFET  3 ′ of the ROM cell  2 . 
     As shown in FIG. 5, when region  80  is provided, the source and drain regions  6 ′ and  7 ′ of MOSFET  3 ′ are short-circuited, so that when transistor  4 ′ is activated a current will flow through the bit line. In this case the ROM memory cell  2  stores a “1”. If, differently, region  80  is not provided, as shown in FIG. 6, when the ROM cell  2  is read turning the selection transistor  4 ′ on and keeping the respective word line at ground (or generally at a voltage lower than the threshold voltage of MOSFET  3 ′), no channel will form in MOSFET  3 ′ so that no current flows in the ROM cell, and the latter is read as a “0”. 
     It is important to underline that in this second embodiment, in order to read the ROM cells it is necessary to keep the respective word lines at a voltage lower than the threshold voltage of MOSFETs  3 ′, otherwise also the ROM cells without the region  80  would become conductive. 
     An advantage of this embodiment with respect to the previous one is due to the impossibility of determining the code stored in the ROM cells  2  by way of a simple visual inspection of the memory array, because all the contacts to the ROM cell are present. 
     Another embodiment of the present invention is shown in FIGS. 7-9. 
     As shown in FIG. 8, in this embodiment both the contact with a respective bit line, through a respective opening  18  in the dielectric layer  16 , and the short-circuit between source  6 ′ and drain  7 ′ through a respective region  80  are always provided for each ROM cell  2 . 
     When it is desired to store a state “0” in a ROM cell  2 , the latter is formed such that in the drain region  7 ′ of the respective stacked-gate MOSFET  3 ′ (drain region which is normally formed by means of an LDD, Light Doped Drain implant) a lightly doped P-type is formed by implanting a P-type dopant with a concentration sufficient to compensate the N-type LDD implant used for forming the drain region  7 ′ of the stacked-gate MOSFET  3 ′. 
     The P−region  20  forms an electrical separation region between the MOSFET  3 ′ and the selection transistor  4 ′ of the ROM cell  2 . During reading of the ROM cell  2 , no current will flow and the ROM cell  2  is thus non-conductive, independently of the voltage of the respective word line. 
     Alternatively, as shown in FIGS. 10 and 11, the same function of the P−region  20  which separates MOSFET  3 ′ from the selection transistor  4 ′ can be achieved selectively preventing (where it is desired to program a “0” state) the N-type LDD implant provided for forming the drain region  7 ′ of MOSFET  3 ′. In this case region  20  is substituted by a portion of the substrate or well  5 , and MOSFET  3 ′ is electrically separated from the selection transistor  4 ′ because the drain of the former is not linked to the channel of the latter (the selection transistor  4 ′ not having the source diffusion). 
     Another embodiment of the present invention is shown in FIGS. 12-14. 
     As in the previous embodiment, both the contact with a respective bit line, through a respective opening  18  in the dielectric layer  16 , and the short-circuit between source  6 ′ and drain  7 ′ through a respective region  80  are always provided for each ROM cell  2 . 
     Storing the “0” state in a ROM cell  2  is achieved by means of an implant of a P-type dopant for forming a P−region  21  located in a portion of the drain  13  of the selection transistor  4 ′. 
     Also in this embodiment, region  21  forms an electrical separation region between the selection transistor  4 ′ and the bit line of the ROM cell  2 , so that during reading of the ROM cell  2  no current will flow in the latter, the ROM cell  2  being thus non-conductive. 
     Similarly to the previous embodiment, as shown in FIGS. 13 and 14, the separation region  21  can be substituted by a portion of the substrate or well  5 , selectively preventing the implant used for forming the drain region  13 ′ of the selection transistor  4 ′, at least to the opening  18 . 
     The separation regions  20  and  21  can also be provided simultaneously in the same ROM cell; the result is always the programming of a “0” state. The same holds true for the absence of the N-type LDD implants that form the drain and source of the selection transistor  4 ′. 
     It is also possible to combine the previous embodiments, so that the identification of the stored code is made more difficult. 
     The lower gate electrode of the stacked-gate MOSFETs  3 ′ of the ROM cells can be electrically floating, just as the lower gate electrode  9  of the stacked-gate MOSFETs  3  of the EEPROM memory cells. Alternatively, the lower gate electrode of the stacked-gate MOSFETs  3 ′ can be short-circuited to the upper gate electrode thereof. The short-circuit can be provided outside the area of the memory cell array; in this case the lower gate electrodes of all the stacked-gate MOSFETs  3 ′ of a same row are not separated, but form instead a continuous stripe. Otherwise, the short-circuit can be obtained directly in each memory cell; in this case, the lower gate electrodes of all the stacked-gate MOSFETs  3 ′ are separated from each other, just as they are in the EEPROM memory cells. 
     In the present description reference has been made to ROM memory cells integrated in an array of double-polysilicon-level EEPROM memory cells; however it is straightforward to realize that it is possible to integrate ROM memory cells in arrays of single-polysilicon-level EEPROM memory cells. Additionally, the present invention can straightforwardly apply to the integration of ROM memory cells in arrays of EPROM or Flash-EEPROM memory cells, forming the ROM cells by means of the same process steps used for fabricating the EPROM or Flash-EEPROM memory cells. 
     For example, FIGS. 17 and 18 show an alternative of the first embodiment previously described, referring to an array of Flash-EEPROM memory cells  100  with ROM memory cells  200  having the same structure as the Flash-EEPROM cells  100 . Each Flash-EEPROM cell  100  comprises a stacked-gate MOSFET, wherein in a P-type substrate or well  5  spaced-apart N-type source and drain regions  60 ,  70  are implanted. The portion of substrate or well  5  comprised between regions  60  and  70  forms a channel region, and a lower gate electrode  90  is disposed above the channel region, with the interposition of a gate oxide layer  110 . 
     An upper control gate electrode  120  is insulatively disposed over the lower gate electrode  90 , which is electrically floating. 
     Each ROM memory cell  200  comprises, identically to the Flash-EEPROM cells  100 , a stacked-gate MOSFET comprising a source region  61  separated from a drain region  71  by a channel region, and a lower gate electrode  91  disposed over the channel region with the interposition of a gate oxide layer  111 . 
     An upper control gate electrode  121  is disposed above the lower gate electrode  91 . Over the whole surface of the memory array a dielectric layer  160  is successively formed, wherein openings  170  in correspondence of the drains  70  of the Flash-EEPROM cells  100  are formed. 
     Similarly to the first embodiment previously described, if it is desired to program in the ROM cell  200  a “1” logic state, in the dielectric layer  160  further openings  180  are formed in correspondence of the drains  71  of the ROM cells  200  (FIG.  17 ). If instead it is desired to program a “0” state as shown in FIG. 18, no openings  180  are formed. 
     If similarly to the Flash EEPROM cells the lower gate electrode  91  of the ROM cells is electrically floating, then in order to prevent the ROM memory cells  200  which have to store a “1” state (i.e., those for which the contact  180  with a respective bit line is provided) from becoming permanently conductive even when not addressed due to trapping of positive charges in the floating lower gate electrode  91 , a situation that could take place due to the electrical erasure of the array of Flash-EEPROM cells  100  (electrical erasure is a bulk operation affecting all the cells of the array) and that would cause a constant current flow through the bit line preventing the correct reading of the other memory cells connected to the bit line, it is preferred to form the gate oxide  111  of the ROM cell  200  thicker than that of the gate oxide  110  of the memory cell  100 . For example, the gate oxide  111  of the ROM cell  200  can be made 200 Angstroms thick, while the gate oxide  110  of the Flash-EEPROM cell  100  approximately 100 Angstroms thick. 
     Another possibility is to control the voltages applied to the ROM cell during erasure of the Flash-EEPROM cells; in this case it is not necessary to form gate oxides with different thickness for the Flash-EEPROM cells and the ROM cells. 
     As known, one technique for electrically erasing the flash-EEPROM cells  100  provides for bringing the voltage of source electrodes  60  to approximately 12V, while keeping the control gates (word lines) at ground; another technique provides for biasing the source electrodes at approximately 4V and bringing the control gates to approximately −8V. For preventing the ROM cells  200  form achieving a negative threshold voltage, it sufficient that the control gates (word lines) of the ROM cells are kept at a voltage lower than 12V, or alternatively that the control gates are kept at ground, so that the electric field in the gate oxide is sufficiently low not to trigger the tunnel effect. 
     A third possibility provides for electrically short-circuiting the two polysilicon electrodes  91 ,  121  formed by lines in the first and second polysilicon levels, for example in a region external to the array of memory cells. In this way the lower gate electrode  91  of the ROM cells in not left floating, but is electrically short-circuited to the upper gate electrode  90 . 
     These solutions assure that the ROM cells  200  do not become depletion MOS transistors. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.