Nonvolatile memory device and operating method

Abstract: A nonvolatile memory device (10) comprises first and second transistors (M10, M11) connected between respective first and second terminals and a reference potential terminal (12), the transistors (M10, M11) having first and second floating gates, (G10, G11) respectively, for storing complementary charges. The device (10) further comprises first and second input lines (14, 16) capacitively coupled by thin oxide capacitors to the gates (G10, G11), and means for providing a biasing voltage slightly in excess of the threshold voltage of the transistors (M10, M11) to the input lines (14, 16), such that the device continues to function in the event of a thin oxide failure.

Description: The present invention relates to nonvolatile memory devices. Semiconductor memory chips may be divided into volatile and nonvolatile types. Volatile memory devices are typified by random access memory (RAM) chips which require power in order to maintain data stored therein. In contrast, nonvolatile memory devices such as read only memory (ROM) chips maintain stored data after external power is removed. Data is readily changed in a RAM by a simple write operation. However, ROMs are programmed during manufacture and the stored data can not thereafter be changed. A particular type of nonvolatile memory device which allows data to be altered is an electrically erasable programmable read only memory (EEPROM). An EEPROM cell is a transistor having a gate, drain and source. The gate is referred to as a "floating gate" since it is totally surrounded by an insulator so that it is not electrically connected to anything. The floating gate can be charged with electrons by capacitively coupling an overlying electrode to the floating gate and applying a relatively large voltage to such electrode. Some electrons are able to cross the insulator in response to the applied voltage leaving a charge on the floating gate. A nonvolatile memory cell is known from U.S. Patent No. 4,748,593 wherein a pair of transistors are coupled to form a single memory cell. The paired arrangement allows complementary charges to be stored on the corresponding floating gates which may be read with a differential sense amplifier. The complementary charges improve the long term retention of data as compared to maintaining a charge on a single floating gate. Said US Patent also discloses a margining test wherein test voltages are applied to respective programming lines. Furthermore, the document EP-A-0 293 339 discloses a nonvolatile memory device including an array of "semidouble" memory cells, each formed by a pair of EEPROM cells connected substantially in parallel, and a single select transistor. A sensing circuit includes a reference cell, identical to one of the memory cells. To test whether one of the two elementary cells of a semidouble cell is no longer operative, additional loads may be added to shift the sensing threshold. A problem generally with the known nonvolatile memory cell design is the reliability of the cell. If the insulator surrounding the floating gate should develop a short, the gate will no longer be able to maintain charge. If created during cell manufacture, such shorts can normally be detected by testing. However, cell failure that develops shortly thereafter is more problematic. It is an object of the present invention to provide a nonvolatile memory cell which is highly reliable. Therefore, according to one aspect of the present invention, there is provided a nonvolatile memory device comprising a nonvolatile memory cell, including first and second transistors connected between respective first and second terminals and a reference potential terminal, said transistors having respective first and second floating gates adapted to store complementary charges, and first and second input lines capacitively coupled to said floating gates, characterized by biasing means adapted to supply a bias voltage to said input lines during the post-write period of the memory cell slightly in excess of the intrinsic threshold voltage of said transistors to attain operation of either transistor in its ohmic region should its floating gate develop a short. According to another aspect of the present invention, there is provided a method of operating a nonvolatile memory device having first and second transistors connected between respective first and second terminals and a reference potential terminal, said transistors having first and second floating gates, respectively, and first and second input lines capacitively coupled to said gates and comprising storing complementary charges on said gates, characterized by the step of providing a biasing voltage to said first and second input lines during the post-write period of the memory cell slightly in excess of the intrinsic threshold voltage of said transistors to attain operation of either transistor in its ohmic region should its floating gate develop a short. It will be appreciated that a nonvolatile memory cell according to the invention has the advantage of improved reliability since the provision of the bias voltage enables the cell to continue to function despite a failure in the capacitive coupling. One embodiment of the invention will now be described by way of example, with reference to the accompanying drawing, in which the Figure is a circuit diagram of a nonvolatile memory device according to one form of the present invention. The Figure shows a memory device 10 including NMOS field effect transistors M10 and M11. Transistor M10 is connected between an output terminal DT (data true) and a reference potential terminal 12 which in a preferred embodiment is ground. Transistor M10 is a three terminal device with source, drain and floating gate G10 electrodes. Its drain is connected to terminal DT and its source to terminal 12. Transistor M11 is connected between an output terminal DF (data false) and a reference potential terminal 12. Transistor M11 is a three terminal device with source, drain and floating gate G11 electrodes. Its drain is connected to terminal DF and its source to terminal 12. As will be described more fully hereinafter, floating gates G10 and G11 store nonvolatile, complementary (high and low) charges. Memory device 10 also includes input lines 14 and 16 which are capacitively coupled to floating gates G10 and G11. Input line 14 is capacitively coupled to floating gate G10 by a relatively small thin oxide capacitor C1 and to floating gate G11 by a relatively large thin oxide capacitor C2. Similarly, input line 16 is capacitively coupled to floating gate G11 by a relatively small thin oxide capacitor C3 and to floating gate G10 by a relatively large thin oxide capacitor C4. In a preferred embodiment, the ratio of capacitances of C2 to C1 and C4 to C3 will be about 9:1. Device 10 also includes a differential sense amplifier with PMOS field effect transistors M8 and M9. Transistor M8 is connected between a power supply terminal VCC and terminal DT. Transistor M9 is connected between power supply terminal VCC and terminal DF. The control electrode or gate of transistor M8 is connected to terminal DF, and the control electrode or gate of transistor M9 is connected to terminal DT. Transistors M13 and M12 are provided for margin testing, as will be described more fully later. Transistor M13 is a PMOS field effect transistor connected in parallel with transistor M8 between power supply terminal VCC and terminal DT. Transistor M12 is also a PMOS field effect transistor which is connected in parallel with transistor M9 between power supply terminal VCC and terminal DF. The control electrode or gate of each of transistors M13 and M12 is connected to a control line 18 for receiving an active low margining signal IM/. Memory device 10 also includes a control circuit 20 for providing and connecting various charging, biasing, and margining voltages to input lines 14 and 16 as will be described more fully later. Control circuit 20 receives VHV (high voltage), EW (enable write), DAT (data), and RDBI (read bias) input signals and provides output signals on lines 14 and 16. In a preferred embodiment, the VHV provides low and high voltages of about.9 and 15 volts, respectively, and the RDBI provides a low voltage of about.9 volts. For margin testing, VHV and RDBI can also be provided with other voltages in excess of.9 volts, as will be discussed later. Data to be written into memory device 10 is received as the DAT signal and inverted by CMOS inverter 22. The inverted value is provided to node 24. Node 24 is connected to line 14 through transistor M3. The programming voltage signal VHV is connected to line 14 through PMOS transistor M6 and to line 16 through PMOS transistor M7. Line 16 is connected to reference potential terminal 12 through NMOS transistors M14A and M14. The read bias signal RDBI is connected to line 16 through NMOS transistors M5 and M5A, and is connected to line 14 through NMOS transistors M4 and M4A. The write enable signal EW is inverted by the CMOS inverter consisting of PMOS transistor M1 and NMOS transistor M2 and the inverted signal provided to the control electrode of transistors M4 and M5. The EW signal is also provided to the control electrode of transistors M3, M2 and M14A. Line 16 is connected to the control electrode of transistor M6. Line 14 is connected to the control electrode of transistors M7 and M14. The control electrodes of transistors M4A and M5A are connected to power supply terminal VCC. In order to store a charge in memory device 10, the data to be stored is provided as the DAT signal, the write enable signal EW is active high and the programming voltage VHV is at its high value, preferably about 15 volts. If a data value "1" is to be written to device 10, inverter 22 provides a "0" (ground potential) to node 24. Transistor M3 is turned on by the EW signal which allows transistor M7 to be turned on. This connects the 15 volt VHV signal to line 16 while 0 volts are provided to line 14. Floating gates G10 and G11 are now charged as the current through capacitor C3, a thin oxide tunneling device, builds a positive charge on gate G11, and the current through capacitor C1, a thin oxide tunneling device, builds a negative charge on gate G10. The charge developed on floating gate G11 will exceed the threshold voltage of transistor M11 and be effective to drive transistor M11 into saturation. In contrast, the charge developed on floating gate G10 will be less than the threshold voltage of transistor M10 and be effective to prevent transistor M10 from conducting. In a similar fashion, a "0" data value to be written to device 10 is provided to node 24 as a "1" (the VCC potential), which in a preferred embodiment is about 5 volts. Transistor M7 is off, but transistors M14 and M14A are turned on by the signal on node 24 and the EW signal, respectively. This provides ground potential to line 16 and also to the gate of transistor M6 which connects the 15 volts of the VHV signal to line 14. Gates G10 and G11 will charge to positive and negative values through capacitors C1 and C3 respectively. After data is stored in device 10, the EW signal goes low and the programming voltage drops to the same voltage as RDBI, which has a value slightly in excess of the threshold voltage of transistors M10 and M11. In a preferred embodiment this is about .9 volts. This read bias signal (RDBI) is provided to line 16 through transistors M5 and M5A and to line 14 through transistors M4 and M4A. The assertion of VHV to the sane potential as RDBI prevents current from flowing through transistors M6 and M7. If the floating gates G10 and G11 have not developed a short across C1 or C3, the thin oxide tunneling devices, one of transistors M10 or M11 will be on and the other off. Assuming transistor M11 is "on" and transistor M10 is "off" then the DF output terminal will be pulled low to ground 12. This will also turn on transistor M8 which will connect VCC to output terminal DT. Similarly, assuming transistor M10 is "on" and transistor M11 is "off" then the DT output terminal will be pulled to ground 12. This will also turn on transistor M9 which will connect VCC to output terminal DF. In operation, only one of the output terminals DF or DT will normally be used; however, either one will provide a well defined signal. The present invention has particular utility when one of the capacitors C1, C2, C3 or C4 develops a short in its thin oxide so that the respective floating gate G10 or G11 can not hold a charge. Assume first that the complementary charging of gates G10 andG11 charges gate G11 to a positive value, but gate G11 develops a short and can not hold the charge. Since input line 16 has a read bias voltage of .9 volts which is slightly above the threshold voltage of transistor M11, transistor M11 will start to pull terminal DF to ground potential 12. This will turn on transistor M8 bringing terminal DT to VCC, and the DT and DF terminals will provide the correct output signals. Next assume gate G10 is charged high, gate G11 is charged low but G11 develops a short. Transistor M11 will start to conduct as a result of the .9 RDBI voltage applied to its gate G11. However, transistor M10 will be driven into saturation by the high voltage (much greater than .9 volts) on its gate G10, pulling terminal DT low which will turn on transistor M9 hard. Current will flow from VCC to ground through transistors M9 and M11. However, since transistor M9 is saturated and transistor M11 is operating in its ohmic region, most of the voltage drop will occur across transistor M11 thereby keeping output DF near the high value of VCC. This will be high enough to keep transistor M8 off. Again, the DT and DF terminals will provide correct output signals. In order to test the margin of a "0" state at the DT or DF terminal, an active low IM/ signal is provided to the control electrode of transistors M13 and M12. In a preferred embodiment, the high and low values of the IM/ signal are about 5 and 2 volts, respectively. When an active low IM/ signal of 2 volts is applied to the control electrode of transistors M13 and M12, each transistor begins to conduct. Assuming a "0" state at the DF terminal is to be tested, transistor M11 would be conducting so as to pull the DF terminal to ground. As current is conducted from terminal VCC to the DF terminal through transistor M12, transistor M11 should be able to sink some of the additional current. The voltage measured at terminal DF will reflect the current sinking capability of transistor M11 which in turn reflects the strength of the charge on floating gate G11. It will be appreciated that the actual voltage at the DF terminal will not normally be measured in practice. Father, since output terminal DF is typically connected to one or more series connected inverters, the ability of the DF terminal to remain below the threshold voltage of the following inverters will be reflected in the output state of a selected inverter. In a similar manner, the "0" state at the DT terminal can be margined by observing the output of a selected downstream inverter when an active low IM/ signal is provide to line 18. In order to test the margin of a "1" state at the DT or DF terminal, the IM/ signal is returned to its inactive high state of about 5 volts. This turns off transistors M13 and M12. The read bias input signal RDBI and the high voltage signal VHV are increased from their biasing value and maintained at substantially the same intermediate voltage. In a preferred embodiment this will mean raising the voltages from about .9 volts to about 3 volts. This provides 3 volts to input lines 14 and 16 which are now equalized through control circuit 20 (assuming EW is low). By raising the potential on both sides of the now parallel coupling capacitors C1-C4, the relative potential of the floating gates is increased by about the same amount. In practice, raising the potential of lines 14 and 16 from .9 to 3 volts will raise the relative potential of gates G10 and G11 by about 2 volts. Assuming a "1" state at the DF terminal is to be tested, transistor M11 would be off and transistor M9 conducting so as to pull up the DF terminal to the potential of VCC. If the negative charge on floating gate G11 was sufficient when set, an increase of about 2 volts should not drive transistor M11 into a conductive state whereby the voltage at the DF terminal falls below a predetermined threshold value. The determination of whether the voltage at the DF terminal does not exceed this threshold is again made by reading the voltage on the DF terminal or, equivalently, by sampling the output of one of the inverters connected to the output. In a similar manner, the "1" state at the DT terminal can be margined.

Claims: A nonvolatile memory device comprising a nonvolatile memory cell, including first and second transistors (M10, M11) connected between respective first and second terminals and a reference potential terminal (12), said transistors (M10, M11) having respective first and second floating gates (G10, G11) adapted to store complementary charges, and first and second input lines (14, 16) capacitively coupled to said floating gates (G10, G11), characterized by biasing means (RDBI, 20) adapted to supply a bias voltage to said input lines (14, 16) during the post-write period of the memory cell slightly in excess of the intrinsic threshold voltage of said transistors (M10, M11) to attain operation of either transistor in its ohmic region should its floating gate (G10, G11) develop a short. A nonvolatile memory device according to Claim 1, characterized in that said biasing means includes a biasing voltage source (RDBI) coupled to said input lines (14, 16) by a control circuit (20) coupled to said first and second input lines (14, 16). A nonvolatile memory device according to Claim 2, characterized by charging means (22, M6, M7, M14, M14A) adapted to charge said first and second gates (G10, G11) with complementary charges such that one of said transistors (M10, M11) is driven to a saturated condition and the other of said transistors (M10, M11) is in a nonconducting condition. A nonvolatile memory device according to Claim 3, characterized in that said charging means includes relatively small thin oxide capacitors (C1, C3) connected between said first input line (14) and said first gate (G10) and between said second input line (16) and said second gate (G11), respectively, relatively large thin oxide capacitors (C2, C4) connected between said first input line (14) and said second gate (G11) and between said second input line (16) and said first gate (G10), respectively, and switching means (M6, M7, M14, M14A) adapted to selectively connect relatively high and low voltages to said first and second input lines (14, 16) to charge said floating gates (G10, G11) with said complementary charges. A nonvolatile memory device according to any one of the preceding claims, characterized by third and fourth transistors (M8, M9) connected between a power supply terminal and said first and second terminals, respectively, the control electrode of said third transistor (M8) being connected to said second terminal and the control electrode of said fourth transistor (M9) being connected to said first terminal. A nonvolatile memory device according to Claim 5, characterized by fifth and sixth transistors (M13, M12) connected in parallel with said third and fourth transistors (M8, M9), the control electrodes of said fifth and sixth transistors (M13, M12) being adapted to receive a margining signal. A nonvolatile memory device according to Claim 6, characterized in that said biasing means (RDBI, 20) is adapted to supply a voltage to said first and second input lines (14, 16) which is greater than said bias voltage to raise the relative potential of said floating gates (G10, G11). A method of operating a nonvolatile memory device (10) having first and second transistors (M10, M11) connected between respective first and second terminals and a reference potential terminal, (12), said transistors (M10, M11) having first and second floating gates (G10, G11), respectively, and first and second input lines (14, 16) capacitively coupled to said gates and comprising storing complementary charges on said gates (G10, G11), characterized by the step of providing a biasing voltage to said first and second input lines (14, 16) during the post-write period of the memory cell slightly in excess of the intrinsic threshold voltage of said transistors (M10, M11) to attain operation of either transistor in its ohmic region should its floating gate (G10, G11) develop a short. A method according to Claim 8, characterized in that said storing step includes: providing differential charging voltages to said first and second input lines (14, 16) respectively.

Inventors: STANCHAK CARL MICHAEL, TURI RAYMOND ALEXANDER, YAKURA JAMES PETER, STANCHAK, CARL MICHAEL, TURI, RAYMOND ALEXANDER, YAKURA, JAMES PETER
Applicants: HYUNDAI ELECTRONICS AMERICA, NCR INT INC, SYMBIOS LOGIC INC, NCR INTERNATIONAL, INC., SYMBIOS LOGIC INC.
Application year: 1990
Main classification: G11C14
IPC codes: G11C14, G11C16