Circuit device for providing a hierarchical row decoding in semiconductor memory devices

Abstract: The invention relates to a circuit device for carrying out a hierarchic form of row decoding in semiconductor memory devices of the non-volatile type and comprising at least one matrix of memory cells (5) with sectors (3,4) organized into columns, wherein each sector has a specific group of local word lines (LWL) individually connected to a main word line (MWL) running through all of the matrix sectors which have rows in common. The device comprises a first transistor (M1) of the PMOS type having its conduction terminals connected, the one to the main word line (MWL) and the other to the local word line (LWL), and a second transistor (M3) of the NMOS type having its conduction terminals connected, the one to the local word line (LWL) and the other to a reference voltage (GND).

Description: Field of the Invention This invention relates to a circuit device for carrying out a hierarchic form of row decoding in semiconductor memory devices. The invention relates to a device as above which comprises at least one matrix of memory cells having sectors organized into columns, wherein each sector has a specific group of local word lines individually connected to a main word line running through all of the matrix sectors which have rows in common. As is well known, a memory matrix architecture most widely utilized in the construction of semiconductor integrated, non-volatile memory devices is the NOR type. In this type of architecture, memory cells belonging to one matrix row have their gate terminals in common, while memory cells belonging to one matrix column have their drain terminals in common. The source terminals are, on the other hand, shared by all the cells of one matrix sector. A portion of a NOR matrix is shown in Figure 1 by way of example. To univocally situate a given memory location, it is sufficient to identify a given row and given column: one memory cells is to be found at their intersection. As said before, in non-volatile memories, a memory cell comprises a floating-gate transistor which also has drain and source conduction terminals. Shown in Figures 2A, 2B and 2C are the values of bias voltages to be applied to the terminals of a memory cell during read, write and erase operations, respectively. Prior Art A pre-requisite of non-volatile memories of the Flash EEPROM type is that the information stored therein should be erased as groups or packages of bits. The erase operation is the single operation that involves biasing of the source terminal, and since all the cells have this terminal in common, they can be written into and read from in an independent manner but must be erased simultaneously. Particularly with Flash memories, the erase operation is performed by sectors, in the sense that all the cells that run to the same source line must be erased simultaneously. Within a non-volatile memory matrix, the sectors can be organized either into rows or columns. In a row type of organization, the size of a sector is given by the number of rows that it contains. The architecture of the storage device is designed to suit the number and size of the sectors, in order to optimize the circuit area consumption, as well as the device performance and reliability. Since a single bit line to be shared by all 1 of the matrix sectors would be impossible to provide due to a problem known as "drain stress", each sector is arranged to include a specific group of columns referred to as the "local bit lines". Local bit lines are individually connected, via a pass transistor, to a main metallization connection referred to as the "main bit line". Each sector is assigned a local group of pass transistors which are only turned on in the addressed sector, so that the cells of the other matrix sectors need not be affected by drain stress. Shown schematically in Figure 3 is a conventional architecture for a non-volatile memory matrix wherein the sectors are organized into rows. Also shown therein are the various row decoders associated with each sector. Such an architecture is high in circuit space requirements because it entails the provision of a decoder for each sector, and of local column decoders to avoid the drain stress phenomenon. Furthermore, it should be noted that the rows of the memory matrix are physically in the form of polysilicon strips interconnecting all the gate terminals of the cells in one row. From an electrical standpoint, each polysilicon strip may be viewed as a distributed RC network. For example, even when a relatively small number of cells such as 1024 is assumed, the time constant associated with one row in the matrix and defined by the RC network would be approximately 10 nsec. This value represents the time interval to be allowed for an electric signal to go through a matrix row, and it affects the memory access time directly which, as can be appreciated, should be kept as short as possible. Due to the high cell integration density in the integrated storage circuit, the local bit lines and main bit line are formed by a process which results in two different metallization layers or levels being deposited, and unless a third metallization level is provided, the polysilicon row cannot be short-circuited and the row charging time be reduced. Another prior technical solution provides non-volatile memory matrices which are organized into columns. In this case, the matrix rows are shared in common by all sectors, and the sector size is determined by the number of columns. Figure 4 is a schematic view of a portion of a memory matrix organized into columns. With this type of architecture, the parasitic capacitance of each bit line can be kept quite low, which is beneficial to the circuit portion involved in reading the memory contents. In addition, row decoding will be shared by several matrix sectors, which affords savings in circuit space. Where a matrix is fabricated by a technological process providing two metallization levels, one level is used for forming the bit lines, and the other level is used for short-circuiting the row in order to reduce its parasitic resistance during the charging phase. While being advantageous in several ways, the last-mentioned architecture has a drawback in that, each time that a cell is addressed, all the cells in the same row also become biased and affected by the so-called "gate stress". Consistently with a current trend in Flash storage devices provided with a single supply voltage, the erasing phase is performed by sectors, with the gate terminals of the cells being biased with negative voltages. The negative gate erase mode used for matrices having sectors organized into columns results in an unacceptable amount of gate stress for the memory device. A semiconductor memory device comprising a matrix of memory cells having sectors organized into columns is described in the US patent No. 5,506,816 issued on April 9, 1996. The underlying technical problem of this invention is to provide a circuit device which allows of negative voltage erasing in memory matrices which are organized into columns, thereby overcoming the limitations and drawbacks of prior art solutions for non-volatile memory devices having a single-voltage supply. Summary of the Invention The principle on which the present invention stands is one of performing a row decoding phase of the hierarchic type by means of an additional transistor having its conduction terminals connected, the one to the local word line and the other to a ground reference voltage. Based on this principle, the technical problem is solved by a circuit device as defined in appended Claim 1. The features and advantages of a device according to the invention will be apparent from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 is a schematic view of a portion of a memory matrix of the NOR type. Figures 2A, 2B and 2C show schematically memory cells and typical associated bias voltage values to be applied to the cell terminals during the read, write and erase phases thereof, respectively. Figure 3 shows schematically a prior art architecture for a non-volatile memory matrix having sectors arranged in row form. Figure 4 is a schematic view of a portion of a memory matrix having sectors arranged in column form. Figure 5 is a schematic view of a first embodiment of a device according to this invention for carrying out a hierarchic row decoding. Figure 6 is a schematic view of a portion of a memory matrix, organized into columns and incorporating decoding devices according to the invention. Figure 7 is a schematic view of a portion of a memory matrix, organized into columns and incorporating decoding devices according to the invention, shown during a matrix sector erase phase. Figure 8 is a schematic view of a portion of a memory matrix, organized into columns, according to a modified embodiment of this invention. Figure 9 is a schematic view of the circuit device shown in Figure 5, bringing out components as well as resistive and capacitive loads thereof. Figure 10 shows schematically another modified embodiment of the device of Figure 5. Detailed Description Referring to the drawing Figures, and specifically to the example of Figure 5, generally and schematically shown at 1 is a circuit device which embodies this invention to allow sectors of a non-volatile memory matrix 2 to be erased by a negative voltage. The matrix may comprise memory cells of the Flash EEPROM type, for example. Depicted schematically in Figure 6 is a portion of the matrix 2 which includes first 3 and second 4 sectors. Each of the two sectors 3 and 4 is shown schematically with just a few, designated 5, of the thousands of memory cells that would be incorporated to the actual sectors. The cells 5 are arranged as a plurality of local word lines or rows denoted by the acronym LWL (Local Word Line). The first row is designated LWL<0>, the second row LWL<1>, and so on. Provided parallel with each matrix row is a main word line or row, referenced MWL (Main Word Line), running through all those sectors which have rows in common, such as the sectora 3 and 4. Advantageously, the circuit device of this invention is connected, within each sector, upstream of each local row LWL of the memory matrix. The memory device is supplied a single supply voltage Vdd having a value in the 2.5 to 3.6 Volts range, and is also connected to a second voltage reference GND, such as a signal ground. The device 1 comprises a first transistor M1, of the PMOS type, which has its conduction terminals connected, the one to the main word line MWL and the other to the local word line LWL. The body terminal of the transistor M1 receives a bias signal VPCX, its gate terminal receiving a signal PCH. The gate terminals of all the transistors M1 of the devices 1 associated with the matrix rows of one sector 3 or 4 are all connected together and receive the same voltage signal. The device 1 further comprises a second transistor M2, of the NMOS type, which has its conduction terminals connected, the one to the main word line MWL and the other to the local word line LWL. The body terminal of the transistor M2 receives a negative voltage (approximately -8V) bias signal HVNEG during the erase phase, and its gate terminal receives a signal NCH. The voltage value of HVNEG will be 0V (i.e., GND) throughout the other operating phases. The gate terminals of all the transistors M2 of the devices 1 associated with the matrix rows of one sector 3 or 4 are all connected together and receive the same voltage signal. A transistor M3 of the NMOS type is advantageously connected with its conduction terminals between the local word line LWL and the ground reference GND. The body terminal of the transistor M3 receives a a bias signal HVNEG at a negative or zero voltage, and its gate terminal receives a signal DISCH. The gate terminals of all the transistors M3 of the devices 1 which are associated with the matrix rows of one sector, 3 or 4, are all connected together and receive the same voltage signal. Advantageously, moreover, the NMOS transistors M2 and M3 are formed using a triple-well technology, as explained herein below. The operation of the device 1 according to the invention will now be discussed, where the word line is biased to the voltage value of VPCX. The voltage VPCX is the row decode supply voltage; it will be Vdd during the read phase and a voltage Vpp of about 10V during the program phase. The voltage VPCX can be higher than the supply voltage Vdd, even when the read operation requires a boosted voltage for the word line. The voltage HVNEG is at the ground value GND at all times other than during the erase phase, when it takes a negative value (-8V). 1) A first case of MWL=VPCX and LWL=VPCX will be analysed: we are within the addressed sector. The gate terminal of the transistor M1 receives a signal PCH at the ground value GND, and the transistor M1 ensures full transfer of the positive voltages, from the main word line MWL to the local word line LWL. The body terminal of the transistor M1 is at the highest of the available potentials, i.e. at the value of VPCX.The transistor M3 is in the off state, to avoid bonding the local word line LWL to ground.The presence of the transistor M2 is here wholly uninfluential, since if on, it would assist in charging the local word line LWL up to a voltage equal to its gate-threshold voltage; if off, its contribution would be nil.Thus, by turning on the transistor M2, the effectiveness of the local word line LWL charging phase is enhanced. 2) A second case of MWL=VPCX and LWL=GND will be analysed: we are now without the addressed sector. The transistor M1 should be in the off state, and accordingly, the signal PCH brought to the value of VPCX. The transistor M2 also should be in the off state, not to drain charge away from the main word line MWL. Consequently, the third transistor should be on, to ensure bonding the line LWK to ground. It will be sufficient, for the purpose, that the signal DISCH be biased to Vdd. 3) A third case of WL=GND and LWL=GND will now be analysed: under these conditions, the state of -the transistor M1 is uninfluential because this is a P-channel transistor, unable to transfer the ground potential unless its gate terminal is brought to a negative voltage value. Therefore, to bond the line LWL to ground, the transistors M2 and M3 can be acted upon to bias their gate terminals to Vdd. The three operative conditions outlined herein above are summarized schematically in Figure 6, where the necessary bias levels are shown, both within and without the addressed sector. The gate terminal of the transistor M1 is driven by a logic signal PCH which is made to work between the voltage levels GND and VPCX. The gate terminals of the transistors M2 and M3 are driven by respective logic signals NCH and DISCH, made to work between the voltage levels GND and Vdd. It is interesting to observe that three control signals per sector will suffice. The erasing phase at a negative voltage will now be discussed. In this case, all of the word lines MWL are biased to a voltage HVNEG of -8V. A) The sector is being erased: the state of the transistor M1 is uninfluential because the voltage to be passed on'through the local word line LWL is negative. The gate terminal of the transistor M1 is, therefore, brought to the same value as the supply voltage Vdd. Concurrently therewith, the transistors M2 are turned on to admit negative voltage and bias the local word line LWL with the negative voltage HVNEG. The transistor M3 is turned off to prevent charge from being transferred to the negative voltage source from the ground GND. This transistor M3 is turned off by biasing the signal DISCH of the sector being erased to the value HVNEG, as shown in Figure 7. B) The sector is not being erased: the transistors M1 and M2 are in the off state because PCH=VPCX and NCH=HVNEG. The transistor M3, on the other hand, will be activated not to allow the local word lines LWL to float. For the purpose, the gate of the transistor M3 is delivered the signal DISCH=Vdd. The local word lines LWL of the unaddressed sectors are all biased to ground potential. For erasing, VPCX is generally equal to Vdd or some lower voltage, in order to monomize the stress on the final inverter of the row decode. Since the transistors M2 and M3 are seeing a negative voltage across them in the various conditions of operation, they are suitably formed using a triple-well technology. Figure 7 shows the various bias levels, both for the sector which are being erased and the other sectors which are undergoing no erasing procedure. A major advantage of the inventive solution is a reduced number of control signals for managing all the operations in which a given sector is involved. The signals PCH, NCH and DISCH are shared by all local word lines of one sector. The above scheme may be modified by omitting the transistor M2, whose single function is that of transferring the ground voltage level from the unselected main line MWL to the local line LWL. It is in fact necessary to have the gate terminal of the transistor M1 biased below the ground value (by at least one P-channel threshold and associated body). It should be noted, however, that with single supply memories, no negative voltages will usually be available and these must be produced by specially provided circuitry. It can be appreciated from the foregoing that the device of this invention would also allow a hierarchic type of row decoding to be carried out. In this respect, Figure 8 shows schematically a modified embodiment of the invention. In this modification, plural lines 11 branch off a single main word line MWL, referenced 10, and run parallel to the local word lines LWL. In this way, it is as if no main line MWL for each local line LWL were provided, so that a metallization connect at row pitch is no longer needed. This solution is highly advantageous in terms of the main word line MWL charging time, since it decreases the value of the coupling parasitic capacitance between adjacent word-line metallizations. Concurrently therewith, the likelihood of short-circuits occurring between metallization lines is reduced, with evident benefits from the standpoint of production of the device on an industrial scale. Finally, row decoding also benefits from it: in fact, it is no longer necessary for each decoding driver circuit of the main word line MWL, and its associated control circuitry, to be provided at word line pitch, but only at the pitch of two or more word lines. This reflects in space savings, and most notably in an ability to carry out row decodings of greater complexity than is conventional, or effective to lower power consumption for reading, for example. The last-mentioned aspect is of primary importance to memories which utilize voltage-boosted word lines for reading. In Figure 8, a scheme is depicted for a four-level hierarchic row decoding, that is, wherein four local lines LWL correspond to each main line MWL. This solution involves an increased number of control signals for each sector. In this case, there are four discrete signals, namely: four signals NCH<0>, NCH<1>, NCH<2>, NCH<3>, four signals PCH, and four signals DISCH. The circuit design as proposed by this invention effectively reduces the word line charging time. The main word line MWL is formed from a metallization layer, which makes its actual parasitic resistance trivial. But the value of the parasitic resistance is not negligible, and is due both to the coupling to underlying layers and the coupling to the metallizations of the main line MWL which run alongside it. However, the last-mentioned contribution can be greatly reduced by the hierarchic decoding structure of this invention. The local line LWL, on the other hand, is formed from a polysilicon layer, and from the standpoint of its parasitic components can be represented by a distributed RC network as illustrated by the block 12 in Figure 9. The usual approach to reducing the time constant associated with the word line charging is one of short-circuiting the polysilicon row to a metallization layer. By so doing, however, the metallizations must be designed at word line pitch, thereby increasing the parasitic capacitance, referenced Cmain in Figure 9, and requiring enhanced lithographic definition of the metallization. With the inventive solution, the charging problem is solved as shown in Figure 10. In fact, a P-channel charge transistor M4 is provided which has its conduction terminals connected between the main word line MWL and the local word line LWL. The provision of this transistor is effective to reduce the charging time for the local line LWL, allowing it to be biased across its ends. Of course, the transistors M1, M2 and M4 should be dimensioned such that their turn-on resistance will not affect the charging transient of the local word line LWL. The device of this invention allows the memory erasing to be effected by applying negative voltages to the gate terminals of the cells, even where the sectors are organized into columns. This is achieved with a reduced space occupation by the decoding circuits compared with a row-based organization. In addition, this device allows of a hierarchic type of row decoding, affording considerable advantages in terms of space occupation, row decoding design, and parasitic capacitance reduction. Finally, the charging time for the matrix row can be shortened without resorting to metallization strapping. Changes and modifications may be made unto the device of this invention as defined in the following claims.

Claims: A circuit device for carrying out a hierarchic form of row decoding in semiconductor memory devices of the non-volatile type and comprising at least one matrix of memory cells (5) with sectors (3,4) organized into columns, wherein each sector has a specific group of local word lines (LWL) individually connected to a main word line (MWL) running through all of the matrix sectors which have rows in common, comprising: a first transistor (M1) of the PMOS type having its conduction terminals connected, the one to the main word line (MWL) and the other to the local word line (LWL), the gate terminals of all the first transistors (M1) associated with the matrix rows in one sector (3,4) being connected together and receiving a same first voltage signal (PCH); characterised in that it further comprises a second transistor (M3) of the NMOS type having its conduction terminals connected, the one to the local word line (LWL) and the other to a reference voltage (GND), the gate terminals of all the second transistors (M3) associated with the matrix rows in one sector (3,4) being connected together and receiving a same second voltage signal (DISCH) different from said first voltage signal (PCM). A device according to Claim 1, characterized in that it comprises an additional transistor (M2) of the NMOS type having its conduction terminals connected, the one to the main word line (MWL) and the other to the local word line (LWL). A device according to Claim 2, characterized in that said second transistor (M3) and said additional transistor (M2) are formed using triple-well technology. A device according to Claim 2, characterized in that said second (M3) and additional transistors (M2) have respective body terminals which are applied a negative voltage signal (HVNEG) during the phase of erasing the contents of the memory cells. A device according to Claim 2, characterized in that the gate terminals of all the additional transistors (M2) associated with the matrix rows in one sector (3,4) are connected together and receive a same third voltage signal (NCH). A device according to Claim 2, characterized in that it further comprises a charge transistor (M4) having its conduction terminals connected between the main word line (MWL) and the local word line (LWL) to reduce the charging time for the local word line (LWL). A device according to Claim 6, characterized in that said charge transistor (M4) is a P-channel transistor.

Inventors: CAMPARDO GIOVANNI, MICHELONI RINO, CAMPARDO, GIOVANNI, MICHELONI, RINO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: G11C8
IPC codes: G11C16, G11C8