Voltage regulator for programming non-volatile memory cells

Abstract: The present invention relates to a voltage regulator (1) for the programming of electrically programmable non-volatile memory cells in a cells matrix divided in segments with said regulator being the type comprising an amplifier stage (2) connected and powered between a first (Vpp) and a second reference voltage (GND) and having: a first input terminal (+) connected to a divider (3) of the first reference voltage (Vpp), an output terminal (U) connected to the control terminal of a MOS transistor (Mu) which has a conduction terminal connected to the memory cells through a programming line (6), a second input terminal (-) connected in feedback to said programming line (6), and an input circuit portion (8) made up of active elements and inserted in turn between said reference voltages (Vpp,GND). This portion (8) is active on the connection between the amplifier (2) and the first reference voltage (Vpp) in response to at least one pair of signals (PG,DI) to activate the regulator only when there is at least one cell to be programmed in the segment associated with the regulator. Specifically, the above mentioned input circuit portion (8) is made up of a complementary pair of transistors (Pin,Min) and at least one enablement transistor (M2) with there being applied to the complementary pair a first programming enablement signal (PG) and to the transistor (M2) there being applied a second signal (DI) indicating the presence of a bit to be programmed.

Description: Field of application The present invention relates to a voltage regulator for appropriate biasing of electrically programmable non-volatile memory cells. Specifically the present invention concerns a voltage regulator for programming of electrically programmable non-volatile memory cells of the type comprising an amplifier stage connected and powered between a first and a second reference voltage and having: a first input terminal connected to a divider of said first reference voltage, an output terminal connected to the control terminal of a MOS transistor which has its conduction terminal connected to the memory cells through a programming line, and a second input terminal connected in feedback to said programming line. A voltage regulator of this type in known from EP-A-0 661 717. As known, the electrically programmable non-volatile memory cells are structured in cells matrixes each of which comprises a floating gate MOS transistor with related drain and source regions. The floating gate is provided over the semiconductor substrate and is separated therefrom by a thin layer of gate oxide. A control gate is capacitatively coupled to the floating gate through a dielectric layer. Metal electrodes are provided for contacting drain, source and control gate to be able to apply predetermined voltage values to the memory cell. By appropriately biasing the cell terminals it is possible to vary the quantity of charge present in the floating gate. The operation by means of which charge is stored in the floating gate is termed 'programming' and consists of biasing the drain terminal and the control gate at a predetermined value higher than the potential of the source terminal. Normally a memory device integrated on a semiconductor comprises a very high number of cells of the above mentioned type. The cells are organized in a matrix having rows (termed 'word lines') and columns (termed 'bit lines'). The cells belonging to the same word line have a common electric line which drives the respective control gates while the cells belonging to the same bit line have common drain terminals. Prior art The programming of a single memory cell is strongly influenced by the voltage Vpd present on the drain terminal and that is by the voltage present on the bit line to which it belongs. It is also known that for the flash cells a reduced drain voltage value Vpd involves insufficient or slow cell programming. But an excessive drain voltage value Vpd can release destructive 'snap-back' phenomena or cause partial erasing of the cell, and this phenomenon is known as 'soft-erasing'. The optimal interval for the drain voltage Vpd is rather small with variations on the order of 200 to 300 millivolts with respect to a typical level due to the technology. The memory device must be equipped with a very refined and accurate voltage regulator in supplying the correct voltage to the bit lines during programming. This requirement becomes still more critical when there is a plurality of cells to be programmed simultaneously in parallel such as for example during testing to reduce the time and cost of the operation. In addition, the value of the drain voltage Vpd must be held steady during programming. The variations in this drain voltage are essentially linked to the following phenomena: technological variations linked to the manufacturing process of the various components of the memory device, increase in the threshold voltage of the cells during programming and the resulting reduction in the absorbed current, voltage drop on the power supply line of the programming voltage Vpp, and voltage drop on the selection transistors present on the bit lines. This last effect can be particularly significant in large-capacity memories in which the selection transistors have high series resistance. There is thus clearly the need to provide a drain voltage regulator allowing for all these phenomena. The prior art already proposes different solutions for facing these problems. For example a typical solution proposed by the prior art provides a regulator whose output voltage is a little higher than that desired on the bit line. The difference in voltage depends instant by instant on the current which actually traverses the selection transistors of the bit line. A regulator of this type is provided by an adaptive biasing technique and through a positive feedback structure. An example of this prior art solution is described in European patent application No. 93830545.5 (EP-A-0 661 717) filed by this same applicant. Although advantageous in some ways, this prior art solution performs its control action on the entire collection of cells and does not allow provision of high parallelism of programmability which could then accelerate the testing phase of the memory device. The technical problem underlying the present invention is to conceive a voltage regulator having structural and functional characteristics such as to allow holding constant the drain voltage Vpd in the parallel programming phase of a plurality of memory cells to accelerate the testing phases of the memory device and overcome the shortcomings pointed out with reference to the current solutions of the prior art proposed. Summary of the invention The technical problem is solved by a voltage regulator of the type indicated above and defined in the characterizing part of the annexed claim 1. The characteristics and advantages of the method in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. Brief description of the drawings In the drawings: FIG. 1 shows a diagrammatic view of the voltage regulator provided in accordance with the present invention, FIG. 2 shows another diagrammatic view in greater circuit detail of the regulator of FIG. 1, FIG. 3 shows a diagrammatic view of a voltage divider associated with the regulator of FIG. 1, FIG. 4 shows a diagrammatic view in greater detail of the divider of FIG. 3, and FIG. 5 shows a diagrammatic view of the complete circuit details of the voltage regulator in accordance with the present invention. Detailed description With particular reference to the example of FIG. 1 reference number 1 indicates as a whole and diagramatically a voltage regulator for biasing in an appropriate manner the terminals of electrically programmable and erasable non-volatile memory cells, for example flash EPROM cells. The cells are organized in a matrix not shown because conventional but of the type having rows - termed word lines - and columns - termed bit lines. The cells belonging to the same bit line have in common the respective drain terminals. The regulator 1 is designed specifically but not exclusively for a flash memory device with cells matrix having dimensions quantifiable in 16 megabytes divided in 16 1Mbit segments each with 16-bit memory words. The topography of the memory device integrated on a semiconductor provides two distinct portions of matrix segments separated by a sort of central corridor in which is housed selection and decoding circuitry as well as an assembly of sense amplifiers for reading and checking the contents of the cells. Advantageously an assembly of electrical connections BUS extends along said central corridor to allow passage of data and/or electrical signals away from and towards the above mentioned circuitry. There is provided a regulator for each segment of the memory matrix. The various regulators have the same structure and the structure of only one of them is described below. The regulator 1 is powered between a first Vpp and a second reference voltage GND. The first reference Vpp is a programming voltage of approximately 12V to be applied in the programming phase to the word line of the cells matrix. The second reference voltage is a signal ground GND. The regulator 1 incorporates high voltage transistors. The regulator 1 comprises an appropriately fedback operational amplifier 2. The amplifier 2 has a first noninverting input (+) on which is applied a reference voltage VPD-rif taken from a divider 3 of the programming voltage Vpp. The divider 3 comprises at least two resistances R1 and R2 of the integrated type and is provided opposite a pad 4, that is to say a contact terminal of the integrated memory device on which is directly applied the voltage value Vpp. In this manner the divider 3 takes voltage directly from the power supply Vpp. Advantageously, as shown in FIG. 3, the divider 3 is enabled by a signal PG for enablement of the programming phase. Specifically, as shown in FIG. 4, the divider 3 comprises also a pair of PMOS transistors P1 and P2 connected in series between the first resistance R1 and the terminal 4. These transistors P1 and P2 are in diode configuration and the first P1 of these receives on its control terminal a signal PGn which is the logical negation of the programming start signal PG. A third PMOS transistor P3 is connected between the second resistance R2 and ground GND. Even this transistor P3 is in diode configuration and has its control terminal grounded. The amplifier 2 of the regulator 1 has an output U connected to an output switch 5 comprising a complementary pair of transistors Pu and Mu. On the output U is produced a signal VPD-reg which is the result of the regulation performed by the circuit 1. The transistors Pu and Mu are connected between the power supply Vpp and a programming line 6 connected to the bit line of the memory matrix. The line 6 is connected also to the second inverting input (-) of the amplifier 2 to provide a feedback loop. The output U of the amplifier 2 is connected to the control terminal of the transistor Mu which is the natural NMOS type with body terminal grounded and source terminal connected to the line 6. The other transistor Pu of the switch 5 is the PMOS type and displays its body terminal connected to the power supply Vpp and its control terminal connected to a circuit node A driven by an input switch 7 and by a pull-up transistor Pin. In particular, the node A is connected to the power supply Vdd through the transistor Pin of the PMOS type which has its source and body terminals connected to the power supplyVpp and its drain terminal connected to the node A. On the control terminal of the transistor Pin is applied the programming signal PG. This signal is also applied to the control terminal of a transistor NMOS Min connected to the node A and forming a complementary pair with the above transistor Pin. Two other NMOS transistors M1 and M2 are connected in series with the transistor Min towards ground. The transistors Min, M1 and M2 form virtually the above mentioned input switch 7. Basically, the regulator 1 comprises an input circuit portion 8 made up of active elements Pin, Min, M1, M2 and inserted in turn between said reference voltages Vpp, GND. The input portion 8 made up of the transistors Pin, Min, M1 and M2 is the pseudo-CMOS type because the CMOS structure of the transistors Pin, Min is enabled by the high voltage signal PG while the remaining pair of NMOS transistors is enabled by 3V or 5V low voltage control terminals. The first transistor M1 of the NMOS pair M1, M2 receives on its control terminal a signal L/R for enablement of the programming of a left or right segment of the memory cells matrix. The presence of this transistor M1 is fully optional. The second transistor M2 receives on its control terminal a signal DATA-bus<X> which indicates the presence of a bit to be programmed. This signal is indicated below by the shorter symbol DI. The input circuit portion 8 is active on the connection between the amplifier 2 and the first reference voltage Vpp enabled by the signals PG, DI and possibly L/R to activate the regulator only when there is at least one cell to be programmed in the segment associated with the regulator. For this purpose the structure of the regulator 1 comprises additional transistors as specified below. A power supply transistor P4 is connected between the power supply Vpp and the amplifier 2. This transistor is the PMOS type with control terminal connected to the node A. The input portion 8 is active on this transistor P4 to activate or deactivate the regulator. The body terminal of the transistor P4 is in common with the body terminal of an enablement transistor P5 inserted between the power supply Vpp and the control terminal of the transistor Pu. These body terminals are connected to the power supply Vpp. The control terminal of the transistor P5 is connected to the output U of the amplifier 2. This transistor operates essentially as a static and resistive pull-up. Lastly, a protection transistor M3 of the NMOS type connects the output U to ground. In parallel with this transistor there is a capacitor C. This capacitor is designed to erase any overshoots when the dynamics of the output U are too fast. The control terminal of the transistor M3 is connected to the node A. In FIG. 5 is shown the circuit detail of the regulator 1 in accordance with the present invention. Even the structure of the amplifier 2 is shown in detail in this FIG. 5. However, those skilled in the art can appreciate the fact that the amplifier 2 is the conventional type and is made up of a differential cell of MOS transistors polarized by a current mirror. There is now discussed briefly the operation of the regulator 1 in accordance with the present invention. When the signal PG which controls the starting of the programming phase is at a low logical value the node A moves to a high logical value. The node A represents virtually the output of the CMOS input inverter made up of the transistors Pin and Min. On the control terminal of the transistor P4 is applied a high signal and the transistors P4 and P5 are extinguished. Under these conditions the amplifier 2 is extinguished while the transistor M3 is on and grounds the output U of the amplifier 2. Consequently the signal VPD-reg has ground value, that is to say 0V, and the line 6 will not see current since even the transistors Pu and Mu are completely extinguished. It is important to note that when the output U is grounded the amplifier 2 is not powered and does not absorb current. But when the signal PG has high logical value the regulator 1 could be active to regulate the drain voltage VPD-ref on the bit line of the memory cells but the actual starting of the regulator depends on the presence or absence of a bit to be programmed. Therefore only if the logical signal DI has high logical value the transistor M2 is enabled to start and allow the node A to take on a low logical value corresponding to the negated signal PGn. Under these conditions the transistor P4 is lit and powers the amplifier 2. The transistor M3 is extinguished and the regulator 1 is active to power the programming line 6 with a current flowing through the complementary output pair Pu and Mu. The feedback loop connected to the inverting input of the amplifier 2 allows stabilization of the value of VPD-reg depending on the current absorbed by the line 6. The regulator in accordance with the present invention achieves numerous advantages the first of which is certainly the fact that the differential amplifier is activated only if a cell is actually to be programmed. In addition, the regulation is performed locally for each segment of the matrix. In this manner the regulation is independent of any voltage drops of the power supply Vpp and is also independent of the location of the various circuit portions to be programmed. The circuit area occupied by the regulators associated with each segment is entirely irrelevant because in the memory device with double metal technology they find housing in free and utilizable circuit portions. Regulation of the drain voltage is optimal in case of parallel programming and thus ensures feasibility without complex compensation systems. The programming parallelism made possible by the regulator in accordance with the present invention is very useful in testing of the cells matrix. The parallelism can be pushed considerably with a resulting reduction in selection times. Lastly, the deactivation of the regulator is provided by programming path interdiction transistors made possible by the particular structure of the regulator. The presence of a capacitor of adequate value on the output of the regulator protects from overshoots both in the transitory phase and in regulation regime to ensure stability of the regulator circuit. Modifications and variations falling within the scope of the following claims can be made to the regulator in accordance with the present invention.

Claims: Voltage regulator (1) for the programming of electrically programmable non-volatile memory cells in a cells matrix divided in segments with said regulator being the type comprising an amplifier stage (2) connected and powered between a first (Vpp) and a second reference voltage (GND) and having: a first input terminal (+) connected to a divider (3) of the first reference voltage (Vpp), an output terminal (U) connected to the control terminal of a MOS transistor (Mu) which has a conduction terminal connected to the memory cells through a programming line (6), and a second input terminal (-) connected in feedback to said programming line (6) and characterized in that it comprises an input circuit portion (8) made up of active elements and inserted in turn between said reference voltages (Vpp,GND) with said portion controlling the connection between the amplifier stage (2) and the first reference voltage (Vpp) in response to at least one pair of signals (PG,DI) to activate the regulator only when there is at least one cell to be programmed in the segment associated with the regulator. Regulator in accordance with claim 1 and characterized in that said input circuit portion (8) is made up of a complementary pair of transistors (Pin,Min) and of at least one enablement transistor (M2) with there being applied to the complementary pair a first programming enablement signal (PG) and to said at least one transistor there being applied a second signal (DI) indicating the presence of a bit to be programmed. Regulator in accordance with claim 1 and characterized in that it comprises a protective capacitor (C) connected between the output (U) of the amplifier (2) and said second reference voltage (GND) Regulator in accordance with claim 2 and characterized in that it comprises a pull-down transistor (M3) connected between the output (U) of the amplifier (2) and said second reference voltage (GND) and having its control terminal connected to an output node (A) of said complementary pair (Pin,Min). Regulator in accordance with claim 2 and characterized in that it comprises another transistor (M1) connected in series with said at least one transistor (M2) and to which is applied another enablement signal (L/R). Regulator in accordance with claim 1 and characterized in that said circuit portion (8) is active on the control terminal of a power transistor (P4) inserted between the first reference power supply voltage (Vpp) and the amplifier (2). Regulator in accordance with claim 6 and characterized in that said power transistor (P4) is a PMOS in diode configuration. Regulator in accordance with claim 4 and characterized in that it comprises a static and resistive pull-up transistor (P5) inserted between the first reference voltage (Vpp) and said output node (A) with the control terminal of said pull-up transistor (P5) being connected to the output (U) of the amplifier (2). Regulator in accordance with claim 5 and characterized in that said at least one transistor (M2) and said additional transistor (M1) are NMOS. Regulator in accordance with claim 1 and characterized in that there is provided a regulator for each segment of the cells matrix.

Inventors: FONTANA MARCO, PASCUCCI LUIGI, FONTANA, MARCO, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C5
IPC codes: G11C16