NONVOLATILE MEMORY CELL

Abstract: A nonvolatile memory cell (1) utilizes a pair of cross-coupled, capacitively complementary, centrally disposed floating gate electrodes (11, 12). The cell (1) is written directly by the provision of complementary signals on a pair of program lines (22, 23), which lines are capacitively coupled to the floating gate electrodes (11, 12). The data state of the nonvolatile memory cell (1) is sensed by conduction in two bit lines (16, 17), the conductive states of the lines being determined by the charge transferred onto the two floating gate electrodes (11, 12) during the simultaneous but complementary programming of such electrodes. The advantages of the nonvolatile memory cell include high speed read sensing, write without a prior erase operation, single polysilicon fabrication capability, and memory margining capabilities.

Description: NONVOLATILE MEMORY CELL Technical Field This invention relates to nonvolatile memory cells. Background Art Integrated circuit nonvolatile memory cells which store binary data (0 or 1) for extended periods of time are generally well-known. Contemporary and representative designs involve the use of MNOS or floating gate structures which are programmed by the tunneling of charged through dielectrics, the conduction of charge through dielectrics, or the injection of high energy electrons through dielectrics under the influence of electric fields. Heretofore, the cell designs were primarily influenced by the continuing pursuit of smaller dimensions with the intent of maximizing the density of nonvolatile memory arrays and minimizing chip size. A nonvolatile memory cell is known from IEEE Journal of Solid-State Circuits, Vol. SC-20, No. 2, April 1985, pages 603-608, R. Cuppens et al: "An EEPROM for Microprocessors and Custom Logic". The known nonvolatile memory cell includes a transistor having a polysilicon floating gate which has a relatively large capacitive coupling between the floating gate and a control gate and a relatively small capacitive coupling between the floating gate and the transistor drain region. The control gate is formed by a diffusion region in the semiconductor substrate. The known nonvolatile memory cell has the advantage that only a single polysilicon layer is utilized in the manufacture of the cell. However, the cell has the disadvantage that the readout speed is slow. Disclosure of the Invention It is an object of the present invention to provide a nonvolatile memory cell having a relatively high readout speed. Therefore, according to the present invention, there is provided a nonvolatile memory cell formed in a semiconductor substrate, characterized by: a first conductive pat -in said semiconductor substrate, between a first bit line and a common control electrode, including therein a serial connection of a first access transistor operatively connected to a word line and a first floating gate sense transistor; a second conductive path in said semiconductor substrate, between a second bit line and said common control electrode, including therein a serial connection of a second access transistor operatively connected to said word line, and a second floating gate sense transistor; first and second electrodes respectively connected to first and second program lines; a first floating gate electrode adapted to control said first floating gate sense transistor, having a first relatively small capacitive coupling to said first electrode and a second relatively large capacitive coupling to said second electrode; and a second floating gate electrode adapted to control said second floating gate sense transistor, having a first relatively small capacitive coupling to said second electrode and a second relatively large capacitive coupling to said first electrode. It will be appreciated that in a nonvolatile memory cell according to the invention, the provision of first and second bit lines serving as readout lines enables a sense amplifier connected to said first and second bit lines to be differentially driven, whereby a high ' sensing speed is attainable. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic embodiment of a high speed direct write differential nonvolatile memory cell. Fig. 2 is an embodying layout for a high speed direct write differential nonvolatile memory cell. Fig. 3 is a schematic illustrating the placement of a high speed nonvolatile memory cell within an array having a differential nonvolatile data sensing means. Best More for Carrying Out the Invention The present invention is illustrated by the embodiment schematically depicted in Fig. 1 of the drawings. The high speed, direct write differential nonvolatile memory cell 1 uses as a row select common word line 2, which line ohmically connects the gate electrodes of cell access field effect transistors (FETs) 3, 4, 6 and 7. Floating gate sense FETs 8 and 9 are connected in electrical series with cell access transistors 3 and 7, respectively, and are individually responsive to the charge held on respective floating gate electrodes 11 and 12. The polarity of the charge transferred onto floating gate electrodes 11 and 12 is inherently complementary, to conductively bias one of the floating gate sense transistors 8 or 9 at a substantially higher level than the differentially situated complement. An addressing of cell access field effect transistors 3 and 7 by a select signal on word line 2 enables the conductive states of floating gate sense transistors 8 and 9 to be sensed on bit lines number one and number two using a high speed sense amplifier. The nonvolatile data state stored in memory cell 1 is read out at relatively high speed because the sense amplifier connected to bit lines number one and number two is differentially driven, in contrast to a reference level responsive sense amplifier which must await a bit line charge decay to detect the cell data. Floating gate electrodes 11 and 12 of cell 1 are capacitively cross-coupled to source/drain diffusion electrode regions 13 and 14 of field effect transistors 4 and 6. In this respect, note that floating gate electrode 11 is coupled by relatively small capacitor C3 to source/drain electrode region 13, and by relatively large capacitor C2 to complementary operable source/drain electrode region 14. Correspondingly, floating gate electrode 12 is coupled through relatively small capacitor C4 to source/drain electrode region 14 and by relatively ' large capacitor Cl to source/drain electrode region 13. Electrode regions 13 and 14 are separated from complementary operable program lines number one (reference numeral 22) and number two (reference numeral 23) by respective cell access FETs 4 and 6, which as noted earlier are responsive to the signal on word line 2. The circuit symmetry conveyed by the schematic in Fig. 1 is more vividly apparent upon considering a representative layout of high speed direct write differential nonvolatile memory cell 1, as depicted in Fig. 2. The layout in Fig. 2 depicts one representative cell from an array configuration ' composed of multiple nonvolatile memory cells, fabricated " as an integrated circuit using a silicon semiconductor substrate and contemporary single polysilicon manufacturing techniques. As shown, complementary bit lines number one and number two, referenced as n + doped diffused regions 16 and 17, are connected through cell access field effect transistors 3 and 7 to further n + diffusions 18 and 19 in the p~ doped silicon substrate. Preferably, this is implemented using p-wells in an n-type substrate. Floating gate sense transistors 8 and 9 are interposed respectively between diffused regions 18 and 19 and common control electrode 21. Together these patterns define commonly addressed parallel bit line paths on opposite sides of the cell. The interior structure of the cell is comprised of program lines number one and number two,' referenced as n + diffusions 22 and 23, which upon passing through corresponding cell access field effect transistors 4 and 6 continue as conductive diffusion regions 13 and 14. Substrate diffusions 13 and 14 project in parallel through the cell interior, generally defined by dielectric fabrication mask pattern 27. As preferably embodied, word line electrode 2 and floating gate electrodes 11 and 12 are formed from a conductively doped polysilicon layer, following selective processing to create the FET gate silicon dioxides (oxides) and the thin capacitor and charge transfer oxides. Typically, field effect transistors 3, 4, 6, 7, 8 and 9 utilize a gate oxide approximately 40 nanometers thick, while the thin oxide dielectric layers in regions C1-C4 have a nominal thickness of 9 nanometers, the latter selected to match the needs of both capacitive coupling and charge transfer. Within the region defined by dielectric fabrication mask pattern 27 polysilicon floating gate electrode 11 overlaps diffused electrode 13 in a relatively small area, to create a comparatively small capacitive coupling C3, and overlaps diffused electrode 14 in a relatively large area, to form a comparatively large capacitive coupling C2. Polysilicon floating gate electrode 12 has a complementary arrangement to that of electrode 11, in that electrode 12 overlaps diffusion 14 to form a relatively small capacitive coupling C4 and then extends over diffusion 13 to form a relatively large capacitive coupling Cl. The magnitudes of the capacitors C1-C4 are defined in conventional manner by the areas of overlap and the thicknesses of the respective thin dielectric layers. The movement of the charge to and from floating gate electrodes 11 and 12 occurs in capacitive coupling regions C3 and C4 of the respective electrodes. The thin dielectric in these regions is preferably composed of thermally grown silicon dioxide, so that charge transfer occurs in the regions C3 and C4 by Fowler-Nordhei tunneling. If, in the alternative, the thin dielectrics are composed of a silicon nitride dielectric layer, or a thin silicon oxynitride composite dielectric layer, charge transfer would occur by Poole-Frenkel conduction or a combination of such conduction with tunneling. The choice of the dielectric material for regions C1-C4 is influenced by a number of factors not central to this invention, such as capacitor area requirements, dielectric coefficients, cell retention requirements and cell endurance characteristics, which relationships are commonly known and considered by practitioners in the art. Table A summarizes the operating conditions suitable for the direct writing of the exemplary high speed nonvolatile memory cell depicted in the Figs. 1 and 2. TABLE A Operating Conditions Program/ Program/ Connection Write "0" Write "1" Read Margin Bit Line -9V(0V) -9V(0V) Sense Sense Number One Bit Line -9V(0V) -9V(0V) Sense Sense Number Two Word Line +5V(+15V) +5V(+15V) +5V +5V Prog. Line +5V(+15V) -9V(0V) +5V(0V) 0V(Test V) Number One Prog. Line -9V(0V) +5V(+15V) +5V(0V) TestV(0 V) Number Two P-Well -9V(0V) -9V(0V) 0V 0V Common Control -9V(0V) -9V(0V) 0V 0V ' Electrode To understand and appreciate the implications of the operating conditions set forth in Table A, each will be considered with reference to the schematics depicted in Figs. 1 and 2. The programming or writing of differential nonvolatile memory cell 1 to retain a binary "0" state is accomplished by providing a +5 volt select signal to word line 2; providing a +5 volt signal to the program line number one, connection 22; and providing a -9 volt signal to program line number two, connection 23, both bit lines, the p-well and common control electrode 21. Under those conditions, transistors 4 and 6 are conductive so as to pass approximately +3 volts to diffusion 13 and -9 volts to diffusion 14. Because capacitive coupling Cl is significantly larger than C4, floating gate electrode 12 is pulled to +3 volts via capacitor Cl. Thereby, the majority of the relative potential of approximately 10 volts (85% coupling of 12 volts) is placed across the dielectric of C4, between first layer polysilicon floating gate electrode 12 and diffusion 14. Note that a similar but polarity reversed relationship occurs for capacitor C3. The presence of approximately 10 volts across the thin oxide, nitride or oxynitride dielectrics in capacitive coupling regions C3 and C4 initiates the transfer of charge, by tunneling for a thin oxide, from the diffusion 14 to floating gate electrode 12 of C4, coincident with a complementary, oppositely directed, tunneling of charge between diffusion 13 and floating gate electrode 11 in the region of C3. Thereby, floating gate electrodes 11 and 12 are oppositely charged in a single program/write "0" operation. Clearly, the floating gates would be similarly but oppositely charged under the program/write "1" operating conditions of Table A. It should be noted that the programming voltages set forth in Table A presume that nonvolatile memory cell is fabricated with n-channel field effect transistors in a p-well CMOS process. The features of the cell are, however, not so limited in that they are equally amendable to other CMOS, NMOS or PMOS implementations. A particularly noteworthy feature evidenced by ' the foregoing description of a program/write cycle is the provision of a complementary nonvolatile write without a preceding erase operation, which erase cycle characterizes most conventional nonvolatile memory cells. This direct write capability minimizes the time associated with the programming/writing of a nonvolatile state into each cell, which time for this embodiment entails approximately 10 milliseconds. Reference is again made to Table A, this time for a consideration of the signals used to read the binary state stored in the nonvolatile memory cell. As prescribed, the binary bit stored in nonvolatile cell 1 is detected by sensing the relative conductive states of the paths defined between bit lines number one and number two, and common control electrode 21, having interposed therebetween respective cell access FETs 3 and 7 and floating gate sense FETs 8 and 9. Preferably, from Table A, the reading of the data state in the cell involves the concurrent application of +5 volts to word line 2, to program line number one and to program line number two, and a detection of the relative conductive difference in the bit lines number one and number two. The intrinsic threshold voltage of floating gate sense transistors 8 and 9 is approximately 1 volt. When programmed/written according to the operating condition set forth in Table A the complementary pair of sense FETs have thresholds of approximately +4 and -2 volts. The threshold voltage difference is reflected in conduction current differential for bit lines number one and number two. Differential conduction of floating gate sense transistors 8 and 9, relating to the voltage differential charge on floating gate electrodes 11 and 12, can be detected on bit lines number one and number two using a differential responsive sense amplifier, such as amplifier 33 as embodied in the cell array circuit of Fig. 3. Referring to Fig. 3, bit lines number one and number two are pull-up biased through load connected field effect transistors 28 and 29, and are coupled by way of column select field effect transistors 31 and 32 to the opposite sides of voltage responsive sense amplifier 33. The binary state stored in nonvolatile memory cell 1 is represented by the voltage on data out line 34. It should be understood that the differential character of the present cell consumes somewhat greater chip area, and consequently reduces function density, in comparison to that attainable using a non- differential configuration. However, the differential arrangement provides a nonvolatile cell with distinctly superior readout speed. In contrast to conventional nonvolatile cells, the output state of the present cell can be detected almost immediately, in that sensing is differentially responsive. The delays associated with transient decays when comparing to a fixed reference voltage are obviated. Futhermore, the differential configuration effectively doubles the memory cell window, which window margin is used by differential detection to accentuate speed. The voltages shown by parentheses in Table A represent alternate arrangements. For example, the +5 volt bias voltages coupled floating gate electrodes 11 and 12 from program lines number one and number two during the read operation are preferred but not mandatory conditions. Such biasing of the two floating gate electrodes by +5 volts during reading will shift the operating ranges of floating gate sense transistors 8 and 9 to regions of higher gain, so as to further accentuate the effective difference represented by the complementary charged states of the two floating gate electrodes. The high speed direct write differential nonvolatile memory cell of the present invention also provides the user with a memory margining capability, which capability is suitable for product acceptance testing or for data verification. The margining conditions are also set forth in Table A. For margin testing, bit lines number one and number two are connected to the sense amplifier while the test voltages are applied individually through the program lines. Preferably, memory margining is done with positive voltages. This avoids the need for negatively biasing the substrate or p-well to prevent inadvertent forward biasing of pn junctions. The differential operation of the cell facilitates memory margin testing by virtue of - li ¬ the symmetry of structure and operation. Because the test voltage used to bias the selected program line causes a cell response which is indicative of a threshold voltage difference between the two floating gate sense transistors, rather than an absolute voltage, multiple cells in an array can first be programmed to a data "0" state and tested using one program line for margining, followed by a reprogramming to the opposite "1" state and similar margining using one other program line. Thereby, both the "0" and "1" states are measured while avoiding the 'use of negative voltages. This margining approach can also be used to evaluate the performance of a memory cell after an extended period of time following a program/write cycle. For example, the sensing of thresholds via one and then the other of the program lines can be used to test retention or to "peek" at the state of the memory cell in actual use. However, it should recognize that "peeking" will subject the cell to limited programming conditions by virtue of the voltages impressed across the charge transfer dielectrics. As is no doubt apparent from considering Figs. 2 and 3, direct write differential nonvolatile memory cell 1 is by virtue of its functional and structural symmetry very suitable for implementation in arrays. In such applications it is foreseeable that the benefits of the high operating speed, the structural and functional symmetry, and the single ■ polysilicon fabrication capability will lead to applications beyond semicustom integrated circuit products. In considering such broader, array type applications, another beneficial aspect of the cell may be exercised. When suitable for the array as a whole, cell access transistors 4 and 6 may be operationally grouped by commonly connecting gate electrodes of multiple cells to provide a block programming capability. The direct write differential nonvolatile memory cell configuration described herein thereby provides not only accentuated read rate capability for an EEPROM cell, but also exhibits a direct write capability, memory window margining capability, symmetry as to both structure and functions, and amenability to being fabricated using a relatively conventional single polysilicon process technology. Furthermore, the cell is configured to allow the partition of the relatively high programming voltage, so that individual transistor voltages are limited to magnitudes which can be handled by very large scale integrated field effect devices of conventional and advanced design.

Claims: CLAIMS 1. A nonvolatile memory cell formed in a semiconductor substrate, characterized by: a first conductive path -(18) in said semiconductor substrate, between a first bit line (16) and a common control electrode (21), including therein a serial connection of a first access transistor (3) operatively connected to a word line (2), and a first floating gate sense transistor (8); a second conductive path (19) in said semiconductor substrate, between a second bit line (17) and said common control electrode (21), including therein a serial connection of a second access transistor (7) operatively connected to said word line (2) and a second floating gate sense transistor (9); first and second electrodes (13, 14) respectively connected to first and second program lines (22, 23); a first floating gate electrode (11) adapted to control said first floating gate sense transistor (8), having a first relatively small capacitive coupling to said first electrode (13) and a second relatively large capacitive coupling to said second electrode (14); and a second floating gate electrode (12) adapted to control said second floating gate sense transistor (9), having a first relatively small capacitive coupling to said second electrode (14) and a second relatively large capacitive coupling to said first electrode (13). 2. A nonvolatile memory cell according to claim 1, characterized by third and fourth access transistors (4, 6), respectively disposed between said first and second electrodes (13, 14) and said first and second program lines (22, 23). 3. A nonvolatile memory cell according to claim 2, characterized in that said third and fourth access transistors (4, 6) are connected to be responsive to a signal on said word line (2). 4. A nonvolatile memory cell according to claim 1 or claim 3, characterized in that charge is transferred between said first floating gate electrode (11) and said first electrode (13), and between said second floating gate electrode (12) and said second electrode (14) . 5. A nonvolatile memory cell according to claim 4, characterized in that a commonly formed layer of dielectric material (27) provides the dielectric for said first and second capacitive couplings of said first and second floating gate electrodes (11, 12). 6. A nonvolatile memory cell according to claim 4, characterized in that a commonly formed layer of conductive polysilicon creates said first and second floating gate electrodes (11, 12) and said word line (2). 7. A nonvolatile memory cell according to claim 5, characterized in that said layer of dielectric material (27) is formed from oxide or nitride of silicon.

Inventors: LOCKWOOD GEORGE CORBIN, TOPICH JAMES ANTHONY, TURI RAYMOND ALEXANDER, LOCKWOOD, GEORGE, CORBIN, TOPICH, JAMES, ANTHONY (DECEASED), TURI, RAYMOND, ALEXANDER
Applicants: NCR CO, NCR CORPORATION
Application year: 1987
Main classification: G11C17
IPC codes: G11C14, G11C16, G11C17