Abstract:
The memory device includes a memory cell unit of the electrically erasable and programmable non-volatile type including two memory cells respectively connected to two bit lines via two bit line select transistors. The common terminal between the bit line select transistor and the floating-gate transistor of each memory cell of the memory cell unit is connected to the control gate of the floating-gate transistor of the other memory cell of the memory cell unit.

Description:
FIELD OF THE INVENTION 
     The invention relates to memories, in particular to memories of the electrically erasable and programmable non-volatile (EEPROM) type having two cells per bit. 
     BACKGROUND OF THE INVENTION 
     In EEPROM memories, the logic value of a bit stored in a memory cell is represented by the value of the threshold voltage of a floating-gate transistor, which can be modified at will by erasing or programming operations. The programming or erasing of a floating-gate transistor includes injecting electrical charges into the gate of a transistor, or extracting electrical charges therefrom, through the tunnel or Fowler-Nordheim effect via a programming/erase voltage Vpp that may be around 10 to 20 volts. 
     To read such a memory cell, the threshold voltage of the transistor is compared with a reference voltage which, in general, is substantially midway between the negative threshold voltage of a programmed transistor and the positive threshold voltage of an erased transistor. In practice, this comparison is made by applying a read voltage substantially equal to the reference voltage on the gate of the transistor and then observing whether the transistor is in the on-state or the off-state. The on-state or off-state of the transistor is detected by a read circuit commonly referred to by those skilled in the art as a “sense amplifier” connected to the bit line to which the floating-gate transistor is itself connected. During this read process, an erased transistor remains in the off-state since its threshold voltage is above the read voltage. In this way, no current can flow in the bit line and this corresponds to a first logic state of the bit stored in the cell, for example a bit of logic value zero. 
     A programmed transistor is, on the contrary, in the on-state since its threshold voltage is below the read voltage. In this case, a current flows in the bit line and this corresponds by convention to a bit having the opposite logic value, for example the logic value 1. Conventional architectures of EEPROM memory cell units are illustrated in  FIGS. 1 and 2 . More precisely, in  FIG. 1 , the memory cell unit MCU is made up of a memory cell CEL comprising a transistor FGT having a control gate CG and a floating gate FG. The cell CEL is connected to a bit line BL via a bit line select transistor BLST. 
     The cell also includes a control gate select transistor CGST connected between a gate control line GCL and the control gate CG of the floating-gate transistor FGT. The gates of the transistors COST and BLST are connected to a word line WL conventionally extending perpendicular to the bit line BL. The architecture in  FIG. 1  therefore provides one memory cell per bit. To erase the cell, the programming voltage Vpp is applied on the word line WL and on the gate control line GCL, whereas a zero voltage is applied on the terminal GND connected to the source of the transistor FGT, the bit line BL generally being left floating. 
     In writing, the programming voltage is applied on the word line WL while a zero voltage is applied on the line GCL. To write a “1” (for example), the programming voltage Vpp is applied on the bit line BL, whereas this bit line BL is left floating to write a “0”. In both cases, the terminal GND is floating. 
     In general, words of x bits, typically 8 bits, forming in this case eight-bit bytes, are stored in the EEPROM memory. Typically, the storage zone of an eight-bit byte therefore comprises eight memory cell units plus one control gate select transistor CGST (since in general the control gates of the eight memory cells of the eight memory cell units are all connected together and selected from the line GCL) and a ground line. Programming an eight-bit byte is therefore made up of an overall word erase cycle followed by a selective write cycle. 
     To improve the reliability of memory cell units, it is possible to use architectures having two cells per logic bit, as illustrated in  FIG. 2 . The logic state “1” or “0” of a bit, or logic bit, is then represented by two data bits. The purpose of such architectures is to maintain data integrity if one of the paired memory cells is out of action. With such architectures, it is possible to store a binary data bit twice or else, as illustrated in  FIG. 2 , one binary data bit in one of the memory cells and the complementary binary data bit in the other memory cell. 
     More precisely, a memory plane MP having two cells per bit comprises in this case two memory cells CEL 1  and CEL 2  each having a bit line select transistor BLST 1 , BLST 2  and a floating-gate transistor FGT 1 , FGT 2 . A single control gate select transistor CGST is provided for controlling the two control gates CG 1 , CG 2  of the transistors FGT 1 , FGT 2 . With such an architecture, an eight-bit byte therefore typically comprises 16 memory cells plus one control gate select transistor CGST. 
     To be able to program more rapidly, for example, to reduce the programming time by a factor of two, it is possible to provide an individual control gate select transistor per memory cell, so that each memory cell can be individually erased. Thus, it is possible to simultaneously erase certain cells and to write others. With such an architecture, an eight-bit byte therefore comprises 16 memory cells and 16 control gate select transistors. Moreover, each memory cell must have a dedicated ground line, which results in a total of 16 ground lines. 
     Another approach, enabling two cells per bit to be combined and with simultaneous electrical erasing, could include, in the case of an eight-bit byte, reproducing the architecture of  FIG. 2  eight times, which therefore results in 16 memory cells, 8 control gate select transistors and 8 ground lines, since one ground line is dedicated per pair of cells. Thus, whatever the option chosen, storing a digital word with two cells per bit, while being able to erase and write simultaneously, is costly in terms of area. 
     SUMMARY OF THE INVENTION 
     According to one embodiment, a memory device is consequently proposed that makes it possible to combine storage of a bit on two cells per bit with simultaneous erasing and writing of bits of a stored digital word, for example, an eight-bit byte, i.e. with a single programming cycle and with a substantial space saving. 
     According to another embodiment, a memory device is also provided that offers more uniform, simpler and more compact decoding. Thus, according to one aspect, what is provided is a memory device comprising at least one memory plane of the electrically erasable and programmable non-volatile type comprising two memory cells respectively connected to two bit lines via two bit line select transistors. 
     According to a general feature of this aspect, the common terminal between the bit line select transistor and the floating-gate transistor of each memory cell of the memory plane is connected to the control gate of the floating-gate transistor of the other memory cell of the memory cell unit. Thus, the bit line select transistor of one memory cell serves as control gate select transistor for the other memory cell, and vice versa. This therefore dispenses with a specific control gate select transistor, which is present in conventional EEPROM memory cell unit architectures. 
     According to one embodiment, a binary data unit and the complementary binary data unit are stored in such a memory plane, and the two electrodes or terminals of the two floating-gate transistors, for example, their sources, which are opposite the two common terminals, are connected to two separate ground terminals respectively. Moreover, the connection between said common terminal and the control gate of the floating-gate transistor of the other memory cell of the memory plane is advantageously a direct connection, namely without intermediate element. 
     According to one embodiment, the two memory cells are placed facing each other, back to back. The two floating-gate transistors being mutually offset so as to avoid any crossover between the internal interconnects of the memory cell unit. 
     According to one embodiment, to ensure, on the one hand, continuity of a word line produced in polysilicon and, on the other hand, connection between the state transistor (floating-gate transistor) of a memory cell and the corresponding ground line, the two electrodes or terminals of the two floating-gate transistors, which are opposite the two common terminals, are connected to the two separate ground terminals via two auxiliary transistors respectively. Each auxiliary transistor has its source and its drain connected. The gate of one auxiliary transistor associated with one memory cell is connected to the gate of the bit line select transistor of the other memory cell, and vice versa. 
     According to one embodiment, the device further includes a memory latch (or programming latch) connected to the two bit lines and having a structure which is completely symmetrical relative to the two bit lines. In other words, such a completely symmetrical memory latch controls the two bit lines of the paired cells. There is therefore no longer any need to provide memory latches dedicated to controlling the control gates, which latches are present in conventional EEPROM architectures. 
     According to one embodiment, the device comprises a matrix memory plane comprising several memory cell units, all the bit lines being formed by first lines extending along a first direction and the gates of the bit line select transistors of the memory cell units are connected to second lines, usually called “word lines”, extending along a second direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent from the detailed description of entirely non-limiting embodiments and the appended drawings in which: 
         FIGS. 1 and 2 , already described, are schematic diagrams illustrating conventional architectures of EEPROM memory cell units according to the prior art; 
         FIGS. 3 and 4  are schematic diagrams illustrating two embodiments of a memory cell unit according to the invention; 
         FIG. 5  is a schematic diagram illustrating one example of a lay-out of a memory plane according to the invention; and 
         FIG. 6  is a schematic diagram illustrating one embodiment of a bit line memory latch according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 3 , the reference MCU denotes a differential EEPROM memory cell unit, i.e. one intended for storing a binary data unit and its complement, of the type having two cells per bit. More precisely, this memory cell unit MCU comprises a first memory cell CEL 1  and a second memory cell CEL 2 . 
     The first memory cell CEL 1  comprises a state transistor or floating-gate transistor FGT 1  and a bit line select transistor BLST 1 , enabling the cell CEL 1  to be connected to the bit line BL+. Likewise, the second memory cell CEL 2  comprises a floating-gate state transistor FGT 2  connected to the bit line BL− via a bit line select transistor BLST 2 . 
     The source of the bit line select transistor BLST 1  also forms the drain of the transistor FGT 1  and this common terminal, referenced S 1 , is connected to the control gate CG 2  of the transistor FGT 2  via an interconnect CX 12 . Likewise, the common terminal S 2  (the source of the transistor BLST 2  and the drain of the transistor FGT 2 ) is connected to the control gate CG 1  of transistor FGT 1  via an interconnect CX 21 . 
     Since the memory cell unit is differential, the other two electrodes or terminals of the two floating-gate transistors, in this case their sources, are connected to two separate ground terminals GND+ and GND− respectively. Finally, the gates of the two bit line select transistors BLST 1  and BLST 2  are connected to a word line WL. It may therefore be seen that, with such an architecture, the bit line select transistor of one cell serves as a control gate select transistor for the other cell, and vice versa. This therefore results in a substantial space saving. 
     Moreover, assuming for example that a programmed logic “0” bit corresponds to the programming of two binary data units (1,0) and that a programmed logic “1” bit corresponds to the programming of two binary data units (0,1), the following procedures are carried out to write a logic “0” bit, (corresponding to erasure of the memory cell unit): the bit line BL− is generally left floating; a programming voltage Vpp is applied on the word line WL and on the bit line BL+, while a zero voltage is applied on the ground terminal GND−; and the ground terminal GND+ is left floating. 
     To write a logic “1” bit, the programming voltage Vpp is applied on the word line WL and on the bit line BL−, while the zero voltage is applied on the bit line BL+ and on the ground terminal GND+; the ground terminal GND− is left floating. It may therefore be seen that a memory cell unit can receive a write or erase stimulus. Thus, in the case of storing a word comprising several bits on several memory cell units MCU respectively, the memory unit cell units may be individually written and erased. In other words, it is possible, simultaneously, i.e. during a single programming cycle, to program certain memory cell units with a word and to erase others. 
     To read a memory cell unit MCU, a zero voltage is applied to their ground terminals GND+ and GND−. Thus, storing an eight-bit byte requires 16 memory cells including, of course, 16 bit line select transistors, but no additional specific control gate select transistor. 
     In the embodiment shown in  FIG. 4 , the two memory cells CEL 1  and CEL 2  are placed facing each other, back to back, and the two floating-gate transistors FGT 1  and FGT 2  are mutually offset so as to avoid any crossover between the internal interconnects CX 12  and CX 21  of the memory cell unit MCU. This makes it easier to produce the lay-out of a memory plane MP, one representative example of which is illustrated in  FIG. 5 . In this lay-out, the memory plane PM comprises horizontal polysilicon lines forming word lines WL i +, WL i − and vertical metallizations forming the bit lines BL j + and BL j −. 
     Assuming in this example that the memory cell unit MCU of  FIG. 4  is that found at the intersection of the word lines WL i +, WL i − and the bit lines BL 1 + and BL 1 −. Thus, it may be seen, that because of the slightly offset back-to-back arrangement of the memory cells, the common termainl S 1  (the source of the transistor BLST 1  and the drain of the transistor FGT 1 ) is easily connected to the control gate of the transistor FGT 2  via two contacts VA 1  and VA 2  mutually connected by a metallization ML 1  located here at metal level  1 . Likewise, the common terminal between the transistor FGT 2  and the transistor BLST 2  is connected to the control gate of the transistor FGT 1  via two contacts VA 3  and VA 4  mutually connected by a metallization ML 2 , again located at metal level  1 . 
     Moreover, to ensure continuity of the polysilicon word lines WL i + and WL i − and also connection between the corresponding floating gate transistor and the ground lines GND 1 + and GND 1 −, provision is made (see  FIG. 4 ) for the two electrodes of the two floating-gate transistors FGT 1  and FGT 2 , which are opposite said common terminals S 1  and S 2 , to be connected to the two ground terminals GND+ and GND− via two auxiliary transistors AXT 1  and AXT 2 , respectively, each auxiliary transistor having its source XS 1 , XS 2  and its drain XD 1 , XD 2  respectively connected. Moreover, the gate of an auxiliary transistor associated with a memory cell is connected to the gate of the bit line select transistor of the other memory cell, and vice versa. 
       FIG. 5  shows that the short-circuiting of the source XS 1  of the auxiliary transistor AXT 1  with its drain XD 1  is achieved by a specific semiconductor implantation IMP 1  into the semiconductor substrate. The same applies to the specific implantation IMP 2  carried out in the substrate, which enables the source XS 2  and the drain XD 2  of the auxiliary transistor AXT 2  to be short-circuited. 
     Moreover, the source XS 1  of the transistor AXT 1  is connected to the ground line GND 1 + via a contact V 7  and a via V 8  that are connected by a metallization ML 3  located at metal level  1 . Likewise, the source XS 2  of the transistor AXT 2  is connected to the dedicated ground line GND 1 − through a contact V 9  and a via V 10  that are connected by a metallization ML 4 , which again is located at metal level  1 . The ground lines GND 1 + and GND 1 − here are in fact metallizations located at metal level  2 . Finally, the drain of the bit line select transistor BLST 1  is connected to the bit line BL 1 + located at metal level  2  by a connection V 5  comprising a contact, a metallization portion located at metal level  1  and a via, whereas the drain of the transistor BLST 2  is connected to the bit line BL 1 − located at metal level  2  by a connection V 6  comprising a contact, a metallization portion located at metal level  1  and a via. 
     Each bit line of each column is connected to a common programming line via a programming latch ML ( FIG. 6 ). Each programming latch ML has two inputs, namely Data and Data−, for receiving the binary data units that have to be recorded in a memory cell unit, and also a column select signal COL. 
     The central part and the right-hand part of  FIG. 6  illustrate a conventional architecture of a memory latch ML. The left-hand part LHP, which is circled, illustrates a modification of the latch according to the invention. This figure therefore shows that the left-hand part LHP, which comprises a transistor connecting the programming voltage Vpp to the bit line BL+ with a transistor, is strictly identical to the right-hand part of the latch. The architecture of this latch ML is therefore completely symmetrical relative to the two bit lines BL+ and BL−. Its structure is considerably simplified compared with a structure of the prior art and the memory latches specifically associated with the control gate select transistors of conventional memory cell architectures no longer exist since these specific control gate select transistors no longer exist in the memory cell units according to the invention.