Method of programming a flash EEPROM memory cell optimized for low power consumption and a method for erasing said cell

Abstract: Fast 5V-only programming of a Flash EEPROM cell or array of such cells is disclosed. Use is made of an enhanced source-side injection mechanism. This concept requires a program gate in the field oxide region which serves to capacitively couple a high voltage to the floating gates. Thus a very high injection current is established during programming. This additional program gate, however, increases the cell area considerably. The present disclosure shows a contactless 5V-only Flash EEPROM array configuration that relies on shared program lines in order to minimize the area overhead that is caused by this program gate. Furthermore, a memory array with shared wordlines is presented which further enhances the density achievable.

Description: FIELD OF THE INVENTION The present invention relates to a method for programming a fast-programmable Flash EEPROM (Electrically Erasable Programmable Read-Only Memory) cell or an array of such cells that rely on the Source-Side Injection (SSI) mechanism for programming. Said method is particularly suited for low-power applications. THE PRIOR ART Presently, most Flash memories use Channel Hot Electron Injection (CHEI) at the drain side of the memory cell, or Fowler-Nordheim Tunneling (FNT) for programming. The CHEI mechanism provides a relatively high programming speed (± 10µs) at the expense of a high power consumption (± 1mA/bit) which limits the number of cells that can be programmed simultaneously (so-called page-mode programming) to a maximum of 8 bytes (Y. Miyawaki et al., IEEE J. Solid-State Circuits, vol. 27, p. 583, 1992). Furthermore, in order to allow a further scaling of the transistor dimensions towards 0.5 µm and below, supply voltage scaling from 5V towards 3.3V and below also becomes mandatory. This supply voltage scaling is known to cause considerable degradation of the CHEI efficiency as well as the corresponding programming speed because the high power needed to trigger the CHEI can not be easily supplied on-chip from a high voltage generator or charge pumping circuit. Such additional circuitry increases the complexity and expense of the device. On the other hand, FNT tunneling provides slower programming times (± 100µs) but requires a low power consumption so as to allow larger pages (± 4kbit). This reduces the effective programming time to 1µs/byte (T. Tanaka et al., IEEE J. Solid-State Circuits, vol. 29, p. 1366, 1994). Further improvements are hindered, however, by tunnel-oxide scaling limits and by the very high voltage (± 18V) needed on chip for FNT, both compromising device reliability and process scalability. The recent success of SSI as a viable alternative over FNT and CHEI for Flash programming is mainly due to its unique combination of moderate-to-low power consumption with very high programming speed at moderate voltages. A typical example of such a device relying on SSI for programming is the Applicant's High Injection Metal-Oxide-Semiconductor or HIMOS memory cell (J. Van Houdt et al., 11th IEEE Nonvolatile Semiconductor Memory Workshop, Feb. 1991; J. Van Houdt et al., IEEE Trans. Electron Devices, vol. ED-40, p. 2255, 1993; J. Van Houdt et al., EP501941, published Sept. 1992). Said afore-mentioned EP-A-0501941 discloses a programming method for non-volatile memory cells as indicated in the preamble of claim 1. It is based on applying a high programming voltage to a program gate and a control voltage greater than the threshold voltage of the control gate channel to the control gate. It further describes, a speed-optimized implementation of the HIMOS (High Injection MOS) cell in a 0.7-µm CMOS technology which exhibits a 400 nanoseconds programming time while consuming only a moderate current (± 35 µA/cell) from a 5V supply. This result can be obtained by biasing the device at the maximum gate current, i.e. at a control-gate (CG) voltage (V cg ) of 1.5V. The corresponding cell area is in the order of 15 µm 2 for a 0.7-µm embedded Flash memory technology when implemented in a contactless virtual ground array. However, due to the growing demand for higher densities, also in embedded memory applications like, for example, smart-cards, a continuous increase in array density and the scaling of the supply voltage will become desirable, if not mandatory. This evolution increases the relative impact of bitline voltage drops, and the importance of power consumption during programming. Other references to SSI devices are listed below: U.S. Patent No. 5,338,952, issued August 16, 1994, to Y. Yamauchi. U.S. Patent No. 5,394,360, issued February 28, 1995, to T. Fukumoto. U.S. Patent No. 5,280,446, issued January 18, 1994, to Y.Y. Ma et al. U.S. Patent No. 5,408,115, issued April 18, 1995, to K.T. Chang. U.S. Patent No. 5,284,784, issued February 8, 1994, to M.H. Manley. U.S. Patent No. 5,029,130, issued July 2, 1991, to B. Yeh. In this patent, the non-volatile memory cells do not comprise a program gate, and programming of the non-volatile memory cells is based on applying a high programming voltage to the drain and applying a control voltage equal to the threshold voltage of the control gate channel to the control gate. Statements relating to the use of control voltages smaller than the threshold voltage of the control gate channel are contradictory. In contrast to Applicant's device as described in EP-A-0501941, all these patents describe SSI devices and/or arrays that require complicated processing technologies (Yamauchi, Fukumoto, Ma and Manley use 3 polysilicon layers, while Chang and Yeh use special dielectrics) which presently are cost-effective only for very high density stand-alone memories. Moreover, the corresponding devices described in the above-mentioned patents aim at maximizing the gate injection current in order to obtain the highest possible programming speed. This is typically done by biasing the select gate at a voltage slightly higher than the threshold voltage of this select channel. For the Applicant's device, the select gated is referred to as Control Gate and although the function is identical during programming, this select gate is given several names depending on the specific device, as indicated parenthically in the following discussion. Yamauchi mentions a cell current of 10 µA for a select gate voltage of 2V (Auxiliary Gate, see Fig. 4). Fukumoto mentions a voltage of 1.5V to be applied to the select gate (Third Gate Electrode or Selection Gate, see col. 3, line 39). Ma et al. mention a select gate voltage between 1.5 and 2V (Select Gate 47 in Fig. 4) which corresponds to a cell current of 20 µA (see col. 9, line 22). More specifically, it is stated in the corresponding paper by Ma et al. (Symposium on VLSI Technology Digest of Technical Papers, p. 49, 1994) that "maximum programmed threshold happens at Vsg of 0.6V above Vt(SG)" (col. 2, 1st paragraph), where Vt(SG) represents the threshold voltage of the Select Gate SG. This implies that the 'optimum' value for the select gate voltage is 1.8V (see table 1 in the same paper). Similarly, another publication on the same device (Symposium on VLSI Circuits Digest of Technical Papers, p. 77, 1995) mentions that "a low wordline voltage of 1.8V is used during programming to maximize the hot electron injection rate" (col. 2, 3rd paragraph). Chang mentions a typical select gate voltage of between 1 and 2V which corresponds to a cell current in the range of 10-15 µA (select gate 14 in Fig. la), while Manley prefers a select gate voltage of 1.5V or "a potential just above the threshold voltage of the select transistor (select gate 20 in Fig. 11), since this is the optimum condition for hot-electron injection..." (col. 4, lines 22-24). Finally, Yeh reports that "a positive voltage level in the vicinity of the threshold voltage of the MOS structure defined by the control gate 29 (on the order of approximately 1 Volt), is applied to the control gate 29" (col. 3, lines 46-49). Thus, all above-mentioned patents mention a select-gate voltage between 1 and 2V, which corresponds to the maximum gate current and, hence, maximum programming speed. Since for state-of-the art MOS technologies the threshold voltage of the select channel is definitely smaller than 1V (typically 0.7V for a 0.7-µm technology), all above-mentioned patents still describe a programming scheme which is essentially in the strong inversion or on-state of the MOS structure. This has the advantage that the maximum gate current (and thus the maximum threshold-voltage shift) is obtained and that the gate current is less sensitive to small variations in the select gate voltage (see e.g. EP-A-0501941, Fig. 4 or U.S. Patent No. 5,338,952, issued August 16, 1994 to Y. Yamauchi, Fig. 4). Although in this case the programming cell current is only in the range of 10 - 50 µA, a further reduction of this parameter will greatly improve the conditions for page-mode application and for supply voltage scaling. Indeed, if the supply voltage is scaled towards 3.3V, the injection efficiency may be increased by increasing the bitline voltage internally on-chip, as already suggested in EP-A-0501941 and also in U.S. Patent No. 5,280,446. The power that can be delivered from such an on-chip high voltage generator or charge pump is, however, known to be limited in the order of a few µA's (see e.g. U.S. Patent No. 5,042,009 to R. Kazerounian et al.). It is, therefore, still not straightforward to increase the drain voltage internally on-chip for the devices reported in the prior art. On the other hand, programming a larger number of cells simultaneously (page-mode programming) can reduce the effective programming time per bit considerably. However, the simultaneously programming of e.g. 1024 cells on a common wordline would again require 10-50 mA in the prior art devices, which is unacceptable from the point of view of power consumption and bitline voltage drops, especially if at the same time the drain voltage is to be increased internally on-chip for higher speed performance. In the present application, these problems are solved and additional interesting features are demonstrated by optimizing the programming operation from the point of view of power consumption and page-mode programming speed, in contrast to the prior art which shows optimum solutions only from the point of view of programming speed per cell. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for programming an SSI cell or an array of said cells. According to this invention, the method comprises the steps of: applying a first high voltage to the program gate thereby coupling a second high voltage to the floating gate, said first high voltage being generated on the chip; applying a voltage equal to or higher than the supply voltage of the chip to the drain region; and applying a low voltage to the control gate or to a common wordline connecting the control gates of a number of cells, said low voltage being substantially smaller than the threshold voltage of the control-gate channel and being about the maximum injection efficiency, thereby causing a diffusion current to flow from the source region towards the floating-gate channel while achieving the programming of the cell with low energy dissipation. Said method achieves very fast programming while consuming only a very small amount of power, which paves the way for new applications such as battery-operated systems, page-mode programming for very high data throughput, and which also allows for the bitline voltage to be increased internally on the chip in order to circumvent the efficiency decrease associated with supply voltage scaling. By exploring the SSI mechanism in the subthreshold regime, an optimum value for the CG voltage is found for which the gate current is no longer maximized, but the energy consumed from the power supply is minimized and the injection efficiency during programming is maximized. The programming of a memory cell in this regime, where the gate current is a very steep function of the CG voltage is however still achieved in a few microseconds while consuming only a very small cell current in the range of nano-Amperes. This allows an entire wordline in the array to be programmed simultaneously, even if the supply voltage is scaled down to 3.3V or below. This possibility, combined with a physical programming time per cell which is still very short, realizes an effective programming time in the order of 20 ns/byte for a lMbit device, which corresponds to a maximum programming transfer rate of 50 Mbyte/s. State-of-the-art Flash memories typically show a transfer rate in the order of 20-200 Kbyte/s during programming which is 2 to 3 orders of magnitude slower than in the case of the present invention (see e.g. A. Bergemont, Proceedings of the European Solid-State Device Research Conference, Leuven 1993, p. 577, table 1). Therefore, the programmation method of the present invention with the accompanying programming scheme represents a substantial improvement in terms of high-speed-oriented as well as low-power-oriented applications. It is important to mention here that optimizing the injection efficiency instead of the gate current itself, is not a straightforward solution for next generation Flash memories, since the gate current is much more difficult to control due to the steepness subthreshold characteristic of the MOS device. The present invention allows for low-voltage operation on-chip and furthermore reduces the additional cost of incorporating the resulting Flash array in a CMOS (Complementary Metal-Oxide-Semiconductor) process flow. The latter is particularly important for so-called embedded memory applications where the Flash process modules have to be economically reconciled with an already existing CMOS baseline process for fabricating chips that contain large amounts of digital or analog functions as well as Flash memory, such as e.g. smart cards. The present invention also provides a method of programming a nonvolatile memory cell integrated on a chip in a metal-oxide-semiconductor technology by using a control gate voltage that is less than the threshold voltage of the control channel, and that also results in high injection efficiency. The gate voltage can be less than 1 Volt, or even less than 0.7 Volt. A relatively high voltage is applied to the program gates to couple a voltage to the floating gate, and a voltage equal to or higher than the supply voltage of the chip is applied to the drain region. The diffusion current which flows from the source to the floating gate is in the order of nanoamperes. The supply voltage is typically 5 Volt for a 1.25 µm or 0.7 µm CMOS technology. The present invention also allows for the programming of an array of cells containing a number of rows of cells having interconnected control gates to form a wordline. Again, a relatively high voltage is applied to the program gates to couple a voltage to the floating gate, and a voltage equal to or higher than the supply voltage of the chip is applied to the drain region. Additionally, a low voltage is applied to the wordline, the low voltage being smaller than or equal to the threshold voltage of the control-gate channel This causes a current to flow from the source regions of the cells towards the floating-gate channels of the cells to achieve simultaneous programming of the memory cells on that particular wordline. These objects features, and advantages of the present invention, are apparent in the following Detailed Description of the Preferred Embodiments. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a cross-section of a memory cell and the corresponding current components under Subthreshold Source-Side Injection (S 3 I) conditions. Fig. 2 shows the current components for the set-up illustrated in Fig. 1. As an example, the control-gate voltage V cg is set to 0.5V. Fig. 3 shows the ratio of the gate current and the drain current and the ratio of the substrate current and the drain current, respectively, as a function of the control-gate voltage V cg for the set-up of Fig. 1. Fig. 4 shows the transient programming characteristics for the memory cell of Fig. 1 and for a program-gate voltage V pg of 12V, a drain voltage of 5V, and for different values of the control-gate voltage V cg . Fig. 5 shows the programming time T p and its product with the corresponding drain current T p XI dprog versus the control-gate voltage V cg for a program gate voltage V pg of 12V and a drain voltage V d of 5V. Fig. 6 shows the gate current versus the floating-gate voltage V fg as derived from the transient programming characteristics of Fig. 4, for 12V at the program gate and 5V at the drain, and for different values of the control-gate voltage V cg . Fig. 7 shows the transient programming characteristic for V cg =0.3V taken from Fig. 4 but displayed on a linear time scale. The linearity of the characteristic can be exploited for multilevel storage as indicated by the dotted lines. Fig. 8 shows an exemplary embodiment of the memory cell array for the case of a virtual ground configuration with shared program lines. Fig. 9 illustrates the S 3 I disturb characteristics for a nearly-zero voltage at the control gate, a program gate voltage of 12 Volt, and different drain voltages. Fig. 10 shows an optimized HIMOS cell layout as used in a contactless Virtual Ground Array (VGA) with shared program gates. The floating gates are formed in the first polysilicon layer, while the control gate and the program gate are formed in the second polysilicon layer. Each program gate serves as a coupling capacitor for two different cells. Duplicating the presented layout in the horizontal direction constitutes a program line which connects all program gates of two adjactent rows of cells. Fig. 11 sows an Optimized HIMOS cell layout as used in a contactless VGA with shared program gates and shared control gates. In this case, also the control gates of adjacent cells are shared. Duplicating the presented layout in the horizontal direction constitutes a wordline which connects all control gates of two adjacent rows of cells. Fig. 12 shows an Optimized HIMOS cell layout as used in a contactless common source array. In this case, each column of cells has a separate bitline. Fig. 13 shows a basic virtual ground array with a shared program line. Fig. 14 shows a virtual ground array with shared program gates. The diffusion lines are buried under the field oxide in order to save cell area (the field oxide area is complementary to the active area shown in the figure). Fig. 15 shows a virtual ground array with alternating source/drain configuration. Fig. 16 shows an array layout in the case of alternating source/drain configuration. Fig. 17 shows a virtual ground array with shared program lines and shared wordlines. Fig. 18 shows a contactless NOR array with typical voltages to be applied during programming. The cell being programmed and four bitlines are shown. Dedicated source and bitlines are shared between adjacent columns. Fig. 19 shows a contactless common source array where the bitlines of the respective columns in the array are entirely isolated. In this case, each column has its own separated bitline while the source lines can be shared and physically connected to ground (common source). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Exemplary devices have been fabricated in a 0.7 µm nonvolatile technology embedded in a CMOS technology. These devices have a thin oxide (7-9 nm) under the floating gate. The interpoly dielectric is a polyoxide of thickness 25-30 nm, and the oxide between the control gate and the channel region is 15 nm. The length of the floating gate is 0.7 µm, the length of the control gate is 1.0 µm, and the overall cell size is 13-15 µm 2 . The method as disclosed in the present application can also be applied to a 1.25 µm nonvolatile technology embedded in a CMOS technology or in a 0.5 µm non-volatile technology embedded in a CMOS technology or in a 0.35 µm or smaller length nonvolatile technology embedded in a CMOS technology. The term "supply voltage" of a chip including non-volatile memory cells is defined as the external voltage by which power is added to the chip or the electronic system in which the chip is incorporated. By preference the supply voltage of a chip including non-volatile memory cells is the supply voltage of the CMOS technology in which the nonvolatile memory technology is incorporated. The supply voltage of a CMOS technology is the maximum allowed voltage on gate and drain for the minimum allowed gate length in that technology. For the nonvolatile memory 0.7 µm technology as disclosed EP-A-0501941, the supply voltage of the nonvolatile memory technology is 5 Volt. Any external voltage other than the supply voltage is defined as an external voltage. A voltage that is generated on-chip is meant to be generated by charge pumps incorporated on said chip and is negative or higher than the supply voltage. Such an on-chip generated voltage is not able to deliver a high power. The subthreshold Source-Side Injection (or S 3 I) mechanism In order to further reduce the power consumption during Flash programming, a novel scheme is presented which relies on the appearance of high-efficiency SSI in the subthreshold regime of the memory cell. Fig. 1 shows a symbolic cross-section of a split-gate device which is used to trigger the SSI mechanism, where the Control Gate CG and the Floating Gate FG are indicated, while the external Program Gate PG is left out for clarity. According to EP-A-0501941, this external PG is necessary to couple a high voltage in the order of 8V onto the Floating Gate. Fig. 1 also illustrates the different current components that play a role inside the device. Their relative values at a CG voltage of 0.5V, a drain voltage of 5V and a floating-gate (FG) voltage of 8V are indicated in terms of a number of electrons, travelling inside the device from a low to a higher potential. Fig. 2 shows all these current components (gate current I g , source current I s , drain current I d and substrate current I sub ) as a function of V cg . The gate current has been determined indirectly from a programming characteristic in order to rule out trapping effects by a procedure as described by J. Van Houdt et al. in IEEE Trans. Electron Devices, vol. ED-40, p. 2255, 1993. The above mentioned procedure is necessary since the direct measurement of the gate current yields a very inaccurate result due to the discrepancy between the stress time associated with a DC gate current measurement and the programming time of the cell, as described in the above-mentioned publication (see in particular Fig. 5). The other current components have been measured directly on an equivalent cell with a contact attached to the floating gate in order to accurately control the floating-gate potential. Also for these measurements, the use of state-of-the-art equipment with a very high accuracy and low leakage is mandatory because of the very small current levels to be detected. If these precautions are not taken, the obtained injection efficiency is highly questionable, since this parameter corresponds to the ratio of two almost-zero currents. Because all currents are in the same (small) order of magnitude in the subthreshold regime, the following Kirchoff balance has to be taken rigorously into account: I s + I sub = I d + I g This is illustrated for V cg = 0.5V: for every 14 electrons travelling from the source to the weak-gate-control region between CG and FG, three electron-hole pairs are generated through impact ionization. The three holes are collected at the substrate (or well), while two electrons are injected onto the FG. The remaining electron is collected at the drain which makes the total number of collected drain electrons equal to fifteen. The initial injection efficiency I g /I d and multiplication factor I sub /I d at the start of the programming operation are calculated and plotted as a function of V cg in Fig. 3. An optimum for the injection efficiency is found at V cg = 0.5V or about 0.2V below the threshold voltage of the CG channel. For these conditions, the relative values of the current components are already shown on Fig. 1. It is concluded that 13 % of the consumed drain current of 15 nA is injected on the floating gate, which corresponds to an extremely high injection efficiency of 1/8 at the start of the programming operation. At such low current levels, an entire wordline is easily programmed with one gate pulse, and, moreover, the drain voltage can be easily increased on-chip for scaled-down technologies. This is not possible in previously disclosed memory arrays of the prior art, since the channel current is then still in the order of several tens of µA's, which corresponds to the strong inversion regime. In the present invention, the channel current which is flowing from the source junction towards the floating-gate channel is essentially an extremely small diffusion current since the MOS channel underneath the CG is in the off-state, according to the classical MOS theory as described in the prior art (see e.g. S.M. Sze, Physics of Semiconductor Devices, 2nd Edition, Ed. John Wiley & Sons, 1981). Fig. 4 shows the transient programming characteristics for a PG voltage of 12V, a drain voltage of 5V, and for different CG voltages in the range of 0.1-2V. It is clear that the fastest programming operation is obtained for CG voltages of 1-2V and that the programming speed is rapidly decreasing for lower CG voltages (<1V). However, the data of Fig. 3 show that the injection efficiency is still increasing in this region and is only maximized for a CG voltage as low as 0.5V, the latter value being well below the threshold voltage of the CG channel which is 0.7V in the present case. In fact, measurements on similar memory cells which had a CG threshold voltage of only 0.5V, showed a maximum injection efficiency for a CG voltage of only 0.3V, indicating that the optimum value is systematically about 200mV below this CG threshold voltage. In order to analyze these observations, Fig. 5 shows the programming time which is defined as the time necessary to obtain a threshold-voltage shift from -4V to +2V, and the product of the programming time with the corresponding drain current. This product is minimized at a CG voltage of 0.5V. This can be explained as follows. Since the programming time T P is inversely proportional to the average gate current over the programming window, the product T P x I d is inversely proportional to the average injection efficiency. Moreover, this parameter is a direct measure for the energy E p required to program a single cell at that particular CG voltage: where V supply and Q supply refer to the supply (or bitline) voltage and the charge consumed from the voltage supply, respectively. The last step in Eq. (2) is justified since the drain current is a constant during programming in the case of most SSI devices as explained by J. Van Houdt et al. in IEEE Trans. Electron Devices, vol. ED-40, p. 2255, 1993, and in EP-A-0501941. Applications of S 3 I Extremely low-power, highly linear programming versus high programming transfer rate Figs. 3 to 5 thus imply that although ultra-fast programming is only obtained for CG voltages between 1 and 2V the optimum value in terms of power consumption is a CG voltage below threshold (in this particular case 0.5V). In this case, the 256 cells connected to the same wordline of a 64kbit (256 x 256) device can be programmed simultaneously in 35µs consuming only 4 µA. The ratio of injected floating-gate charge over consumed charge from the bitline is then equal to 7 %. This implies that the average injection efficiency over the programming window is less than a factor of 2 smaller than the initial value, indicating that Subthreshold SSI (S 3 I) almost acts as a current source during programming. Fig. 6 confirms this by showing the calculated gate current versus floating-gate voltage characteristics for different values of the CG voltage. For CG voltages in the high gate current regime (1-2V), these charactristics still show a significant gate-current increase with increasing FG voltage, which implies that the final gate current at the end of a programming operation is considerably smaller than its initial value. However, for small CG voltages (≤0.7V), the gate current becomes more constant as a function of the FG voltage, which implies a current-source-like behaviour: the gate current is almost constant during the entire programming operation. This is an interesting and unique property for nonvolatile memory devices, because an eventual increase in the margins on the programmed threshold level (e.g. due to retention requirements, high temperature operability, disturb conditions, etc.), only marginally influences the programming time. On the other hand, the coupling ratio can be considerably reduced, since the programming speed remains high, as long as the final value of the FG voltage during programming stays above a certain level, indicated as V fgsat in Fig. 6. This is in contrast to tunneling-based and CHEI-based devices which show a large dependence of the programming time on the programmed-Vt margins, and on the gate coupling factor. It is also in contrast to e.g. U.S. Patent No. 5,029,130, where the supply of carriers is counteracted by the programming operation itself, which eventually stops the programming due to a correspondingly steep decrease in gate current (see U.S. Patent No. 5,029,130, col. 3, lines 62-68). Finally, multi-level programming for mass storage becomes very attractive since the obtained threshold-voltage shift is almost proportional to the width of the applied programming pulse. Fig. 7 shows the transient programming characteristic for V cg =0.3V taken from Fig. 4 but displayed on a linear time scale. It is clear that the threshold-voltage shift is almost a linear function of time, which means that a plurality of pulse widths which are a multiple of a basic time step can provide almost equally spaced threshold voltage levels or charge levels on the FG if applied to the PG with a fixed amplitude. Certainly the pulses could be arranged to all be of the same duration, and have different amplitudes; however, due to the linear characteristics, the preferred embodiment utilizes pulses of nearly equal amplitude and different durations. This can be utilized for efficient multilevel storage of data in the corresponding memory array: if two intermediate charge levels are added to the conventional two (programmed and erased level), 2 bits of information can be stored inside one memory cell which doubles the capacity of the memory array for the same silicon area. This principle is, however, generally applicable to 2 n charge levels corresponding to n bits of information. Although this multilevel application is already interesting for the case of maximum gate current, it becomes much easier to control in the case of subthreshold SSI since the linearity is much better. Another feature of the invention is that the very small channel current that is needed to program a memory cell by S 3 I, also implies very small internal voltage drops inside the memory array. In fact, the very small current will cause the potential at the split point to be equal to the externally applied drain voltage with a very high accuracy. Therefore, the entire drain voltage is available for the hot-electron generation process, since no significant voltage drop is appearing inside the FG channel. This is in strong contrast to e.g. the case of U.S. Patent No. 5,029,130, issued July 2, 1991, to B. Yeh, where the potential drop, which is available for hot-electron generation, is only a fraction of the applied drain voltage. Evidently, the same argument holds also for the voltage drop across the bitline that connects the drain junctions of the cells that share a common column: it is clear that a smaller channel current is beneficial for the effective drain voltage, especially in contactless arrays such as those used for the HIMOS cell, as described later herein, since these arrays suffer from a relatively high parasitic bitline resistance. This is due to the buried junction concept which avoids contacts but also inhibits silicidation of the entire bitline for improving the bitline sheet resistance. Moreover, when scaling the voltage down to 3.3V and below, an eventual loss in bitline voltage due to parasitics will have an even larger influence on the hot-electron injection efficiency. Finally, the uniformity of the programmed threshold levels is improving if the voltage drops can be cancelled out, since then all cells will be programmed by the same bitline voltage, independently of their position along the bitline and their distance from the nearest contact to metal. These arguments clearly illustrate the major importance of a substantial decrease in consumed current from the supply voltage, for reasons of power reduction as well as for reasons of scalability. Another feature of the invention is the ability to optimize the programming transfer rate of the memory array, i.e. the number of bytes that can be transferred per second towards the array during programming. Due to the above-described low-power programming mechanism, a number of cells that are on the same wordline can be programmed simultaneously, thereby reducing the effective programming time per byte in high-speed applications. This allows a higher transfer of the data to be programmed, provided that registers are provided on-chip to collect these data before programming. The design of such registers are well-known to anyone skilled in the art. Considering again the case of a CG voltage of 0.5V, it has already been mentioned that the cell is then programmed in 35 µs. If an entire wordline of a square lMbit memory, which consists of 1,024 memory cells, is programmed under these conditions, the effective programming time becomes 35 µs/128 bytes or 270 ns/byte. The corresponding current consumption from the bitline is in the order of 15 µA. This result can, however, be significantly improved by trading energy consumption for speed: although Eq. (2) has illustrated that the optimum CG voltage for energy consumption is 0.5V, a higher CG voltage may give a much higher programming transfer rate. Assuming a CG voltage of 0.7V (which is the threshold voltage of the CG channel) instead, the considered wordline of the same 1 Mbit memory can be programmed in only 3 µs, while consuming less than 1 µA/cell. This already implies an effective programming time (or transfer rate) of 25 ns/byte for a total current consumption of less than 1 mA. In other words, increasing the CG voltage to a value well above the optimum for low-power operation (which is about 200mV below the CG threshold voltage as shown on Fig. 5), can provide a considerable gain in effective programming speed without compromising the power consumption and the corresponding voltage drops. A similar calculation for memory devices that have a CG threshold voltage of only 0.5V shows that 16 ns/byte can be obtained for the same example if V cg ≈ Vt(CG). Also in that case, current consumption is limited below 1 mA. If in a particular application, minimization of power consumption is not that important after all, one may even consider further increasing the CG voltage to approximately 1V. In that case, the data from Fig. 5 provide a 7 ns/byte transfer rate which requires about 8 mA for a 1024 x 1024 array. It should be noted that programming of only 1 byte utilizing the conventional CHEI mechanism also consumes about 8 mA (Y. Miyawaki et al., IEEEJ. Solid-State Circuits, vol. 27, p. 583, 1992). In view of the previous paragraph, it is important to note that 8 mA of current consumption is only distributed over 8 bitlines in the CHEI case, while the above-mentioned S 3 I conditions correspond to 8 mA distributed over 1024 bitlines which largely relaxes the constraints on bitline voltage drops with respect to the CHEI case. This is a primary reason why the conventional CHEI devices require a NOR configuration with 1/2 metal-diffusion contact per cell in order to reduce the bitline voltage drop. It can be concluded that: (1) a CG voltage which is about 200mV below the threshold voltage of the CG channel provides the smallest possible energy consumption, which is most interesting for battery-operated and wireless applications, (2) a CG voltage in the vicinity of the CG threshold voltage can provide the highest possible data transfer rate in high-speed applications, while still requiring a relatively small power consumption. Application to a memory array Fig. 8 shows a typical embodiment of a memory array utilizing this programming mechanism. It will be apparent, however, that the following procedure is also applicable to other embodiments. By sharing the program gates of two adjacent rows of memory cells in one single horizontal program line, and by simultaneously sharing the diffusion lines 8 (connected to the source and the drain junctions of the cells) between adjacent columns, a very compact array is achieved which allows the cell size to be reduced to 15 µm 2 in a 0.7-µm double polysilicon CMOS process. For programming the cells in such an array according to the present invention, a high voltage that is generated on-chip from a charge pump (typically 10-12V) is applied to the program line 6, a moderate voltage, which is either equal to the external supply voltage (typically 3.3 or 5V), or is generated on-chip from a charge pump (typically 4-5V if 3.3V is the external supply voltage), is applied to the bitline 8, and a low voltage, less than or equal to the threshold voltage of the CG channel, is applied to the wordline 10. The latter voltage has to be optimized in terms of the considered application; e.g. for battery-powered applications the consumed energy is the main issue and the optimum is typically 200mV below the threshold voltage of the CG channel, as explained above (see Eq. (2)). Reliability aspects The feasibility of this mechanism for application in low-power Flash memory and the corresponding reliability relies on an accurate control of the S 3 I current. Therefore, the CG voltage is to be generated on-chip by an appropriate circuit that is almost insensitive to process variations. A relatively easy solution, which is generally known in the prior art, could consist of a resistive divider comprising ten identical resistors close to one another which divide the supply voltage into ten equal parts. For a 5V supply this would result in multiples of 500mV, the first one being the optimum value for low-power programming. Since the impact of eventual processing variations (from chip to chip, wafer to wafer, etc.) is identical for these ten resistors, such a simple divider circuit is fairly insensitive to these variations. On the other hand, if the supply voltage of 5V shows a variation of ± 10 %, the 500mV node of the divider will only show the same relative variation, i.e. 500mV ± 50mV, which is acceptable for providing sufficient design margins. These and other circuits (such as bipolar references) are well-known in the state of the art and can be directly used for an accurate generation of the low CG voltage on-chip. Secondly, the uniformity of the threshold voltage of the CG channel is preferably well-controlled over the wafer, since the S 3 I current is essentially a function of the difference between the applied CG voltage and this threshold voltage. This requirement can be easily fulfilled in the case of the HIMOS cell, since the CG channel is identical to the n-type MOS transistor of the corresponding CMOS process. This MOS device has to be well-controlled also for other pur-poses, such that a very sharp distribution is usually obtained. A typical value for a wafer fabricated in Applicant's 0.7-µm technology is 0.660 ± 0.005V, which provides sufficient controlability. A last issue that has to be addressed, is the novel gate disturb effect that could arise from this highly efficient injection mechanism under nearly-zero CG voltage conditions. Indeed, due to the efficient injection continuing even at very low gate voltages (±0.3V), the cells in Fig 8 that are sharing its program line with the device(s) being programmed could also exhibit an (unwanted) threshold-voltage shift. The corresponding disturb characteristic for the worst case condition of a CG threshold voltage of only 0.5V is shown on Fig. 9. It is found that the disturb effect -althourgh different from zero- is still very acceptable since the programming time at a CG voltage of 0.5V is only a few µs in this case. The total disturb time which is given as the product of this programming time and the number of words connected to the same program line is always short enough in order not to program a particular row unintentionally. On the other hand, it has been checked through circuit simulation that a proper decoder design as known to anyone skilled in the art is able to guarantee that the CG voltage on the unselected wordlines will always stay close to the ground potential in a practical situation, even in transient regime. In fact, from the data presented in Figs. 4 and 5, it can be easily derived that a difference in CG voltage of only 200mV suffices in order to establish the necessary margin between programming and disturb immunity. However, indirectly, other gate disturb mechanisms initiated by the high Program Gate voltage, such as tunneling from the substrate to the FG in the erased state and interpoly leakage from the FG to the PG in the programmed state, can be of relatively high importance due to the increase of the physical programming time in the low-power regime. Although several solutions to this problem have already been suggested in the paper by Van Houdt et al. published in the Proceedings of the European Solid-State Device Research Conference 1995, p. 553, the problem could impose additional restrictions on the technology (minimum tunnel oxide thickness), array density (number of words on a wordline) etc. Again, the solution depends on the particular application: e.g. because of the very low power consumption during programming, all cells on the same wordline can be programmed simultaneously (so-called page-mode programming) resulting in a very short disturb time which is equal to the programming time of the cell. Moreover, if the sharing of the program line is no longer maintained, the disturb effect will even disappear completely. In these applications where maximum programming data throughput is envisaged, the disturb effect is almost completely cancelled out 'automatically'. In applications that do not aim at the page-mode option, the problem of the increased disturb effects can be controlled by increasing the CG voltage e.g. to 0.7V or above depending on the required array density. The latter solution illustrates once more in an exemplary manner that the optimum value of the CG voltage in practice depends on a variety of factors, which relative importance depends on the envisaged application and/or the technological boundary conditions. Therefore, it can be concluded from this detailed analysis that adequate disturb margins are feasible for the S 3 I programming scheme. As one of ordinary skill in the art can recognize, the foregoing methods can be executed on the memory cell array configurations as detailed hereafter or on other flash memory cells or array configurations. Several illustrative array embodiments are described below, which use similar cell layouts and the same basic cell operating modes. Depending on the particular array configuration, different features are however obtained from the point of view of array operation. Therefore, the basic cell layouts are first discussed and thereafter, different array configurations and their typical features are presented. Figure 10 shows a HIMOS cell layout as used in a contactless array with shared program gates, The shape of the wordline has been designed in order to minimize the parasitic coupling coefficient between the control gate and the floating gate. On the other hand, the program gates of two adjacent wordlines are merged into one program line, which is also running horizontally across the array. Since the source and drain junctions of the different cells are connected to vertical diffusion lines which run under the polysilicon wordlines and program lines, these diffusion lines are connected to a metal strap (for example, every 16 or 32 bits). This explains the term "contactless array". Fig. 11 shows a HIMOS cell layout as used in a contactless VGA with shared program gates and shared control gates. Fig. 12 shows a HIMOS cell layout as used in a contactless common source array. The read-out operation requires a small voltage at the drain junction (typically 2V) and a small voltage at the control gate (typically 3V) while the program gate is kept grounded. Depending on the particular array organization, it may, however, be necessary to apply a positive or negative voltage to the program gate during the read-out operation, as explained below. The considered operating modes are summarized in Table 1. A typical example of a contactless array is the virtual ground array. The practical application of the virtual ground concept to HIMOS cells is shown in Figure 13. Due to the sharing of the program line between two adjacent rows, the cells are mirrored along the two horizontal symmetry axes in order to produce the array. Along every row the cells are oriented in the same direction (source on the left, drain on the right hand side) and, therefore, every diffusion line is connected to the source junction of the cell on the right hand side, as well as to the drain junction of the cell on the left hand side of the considered diffusion line. This configuration has several consequences: Since every diffusion line serves the function of a source line (connected to the source junctions of cells) as well as that of a bitline (connected to the drain junctions of cells), source lines and bitlines are indistinguishable and none of these lines can be physically connected to the substrate (or ground) potential. This "virtual ground property" implies that all diffusion lines have to be connected to the column decoder which has a small but negative impact on read-out speed in a memory circuit; Due to the sharing of the diffusion lines between neighboring cells a very compact array is obtained which allows relatively high density integration. The cell area for a 0.7 µm technology is in the order of 13 to 15 µm 2 depending on the available interpolysilicon technology; Unintentional reverse programming of the cell which is next to the cell being programmed has to be suppressed sufficiently. This offers no problem in HIMOS arrays because of the highly asymmetrical transistor structure; Column decoding is not straightforward since neighboring cells in the same row can not be read-out at the same time, due to the shared diffusion lines. This problem is solved in the peripheral electronics which control the access to the array; The smallest erase sector that can be defined in the case of Fowler-Nordheim tunneling is a pair of adjacent rows sharing the same program line. All diffusion lines in the considered sector are connected to the supply voltage during erasure; In the case of polyoxide erase, the smallest sector is defined as a wordline, since the erase voltage is then applied to the control gate. When the source and drain junctions are formed before the growth of the field oxide, the spacing between adjacent floating gates in a row is reduced to the minimum polysilicon spacing rule, which realizes a very compact array. This embodiment of the cell is also described in EP-A-0501941. Fig. 14 shows the resulting layout when implementing this cell in a contactless array. This cell has an area of 10 µm 2 in a 0.7 µm technology. A second embodiment of a virtual ground HIMOS array is shown in Fig. 15. In this case, all cells on a particular row are oriented in the same direction but the horizontal orientation of the cells is alternating between adjacent rows. Since the source and drain junctions of the cells switch places between rows, the L-shape of the floating gate can be replaced by rectangular floating gates in order to save area (Fig. 16). The floating gates 44 are formed in the first polysilicon layer, while the control gate 46 and the program gate 48 are formed in the second polysilicon layer. Each program gate serves as a coupling capacitor for two different cells. Duplicating the presented layout in the horizontal direction constitutes a program line 50 which connects all program gates of two adjacent rows of cells. The interconnected control gates form the wordline 52 of the array which is running horizontally over the active transistor area and over the source and drain junctions of the cells sharing the same row. Since the source and drain junctions of the different cells are connected to vertical diffusion lines 54 which run under the polysilicon wordlines 52 and program lines 50, these diffusion lines 54 are connected to a metal strap, for example, every 16 of 32 bits. This explains the term "contactless array". Depending on the particular array configuration, the symmetry axes of the array can be positioned in a number of orientations. This is especially interesting in high density applications. Fig. 17 shows a third embodiment of a virtual ground HIMOS array. The wordlines are shared between two adjacent rows in the array in order to further decrease the cell area. This is possible due to the triple gate structure of the transistor; although two rows of cells are now selected through the same wordline, programming selectivity is still maintained since the considered rows do not share the same program line. In order to obtain programming selectivity on a row of cells, the supply voltage has to be applied to the source junction of the cell that shares its drain junction with the cell being programmed as shown in the figure. The resulting gain in cell area is due to the removal of the spacing rule for the polysilicon wordlines from the cell dimensions as shown in Fig. 19. However, during the read-out operation the program line is grounded and, therefore, read-out selectivity is lost, if the known read-out scheme is maintained. This problem can be resolved by applying a negative (deselect) voltage to the program line which is coupled to the row that is unintentionally selected. Consequently, the cells on this deselected row do not draw any current during the read-out operation. This solution maintains the cell performance from the point of view of programming speed, read-out current and drain disturb immunity, at the expense of additional complexity in the row decoder circuitry. If negative voltages are already used for the erase operation, this overhead can, however, be strongly reduced. Otherwise, the threshold-voltage window can also be defined between two distinct positive values (e.g. +2V and +6V), and a read-out voltage of 4V is then applied to the program line of the selected row. This more traditional read-out scheme allows the different rows to be addressed separately without the need for negative deselect voltages, however, at the expense of a lower cell performance with respect to programming speed and cycling endurance. However, if a negative deselect voltage is applied, a novel soft-erase effect might occur in the case of Fig. 17. While reading the indicated cell, a negative voltage of -5V is applied to the program line of the row that shares its wordline with the cell being read. If any cell on this particular program line would be subject to the drain read-out voltage of 2V as well, a soft-erase situation would occur. Indeed, the presence of the deselect voltage together with the drain read-out voltage would further increase the electric field in the tunnel oxide, thereby causing a small amount of charge being transferred across the tunnel oxide during each read-out cycle on the cell above. In order to minimize this charge leakage, the following conditions have to be fulfilled: a. The cells on a particular wordline should have alternating orientations. b. The cells that share a common wordline should be arranged in such a way that each pair of these cells connected to the same bitline have the opposite orientation along this wordline. c. The cells that share a common program line should be arranged in such a way that each pair of these cells connected to the same bitline have the same orientation along this program line. For this "disturb-immune" array organization, the deselect voltage is never present simultaneously with the drain read-out voltage. Under these circumstances, more than 10 years soft-erase lifetime can be guaranteed without any problem ever for tunnel oxides of only 8nm. The resulting minimum cell area is 11 µm 2 in a 0.7 µm technology. Another embodiment of a contactless array for the HIMOS cell is given in Fig. 18. In this case, every diffusion line is either a dedicated bitline or a dedicated source line. This configuration is very similar to the conventional NOR configuration, but the diffusion lines are now shared between cells in a row in contrast with the conventional NOR scheme where sharing is established between cells in the same column. A problem with this array is the fact that additional voltages are necessary in order to obtain selectivity during programming. Indeed, the cell that shares the bitline of the cell being programmed on the same row will also be programmed unless the supply voltage is also applied to its source junction (Fig. 18). If this correction is included the problem is resolved since the next cell on the same row is biased in the reverse way, which suppresses the enhanced hot-electron injection mechanism. A consequence of this configuration is, again, that adjacent cells in the same row cannot be programmed or read-out at the same time. The byte organization thus has to be adapted by a proper decoding scheme. Fig. 19 shows a contactless common source array, where the bitlines of the respective columns in the array are entirely isolated from each other. The main advantage of this configuration is the fact that the source lines can be grounded through a substrate (or well) contact, e.g. every 16 or 32 bits. In that case, the source potential always stays very close to the ground potential which ensures a high read-out speed, even in large memory arrays. Another advantage is the straightforward decoder design: the control gate forms the wordline, each column of cells connects to a separate bitline, and source decoding becomes redundant. The main disadvantage is the larger cell area (typically 20-25 µm 2 in a 0.7 µm CMOS technology) which is due to the bitline isolation. Preferred embodiments of the present invention have been described herein. It is to be understood, however, that changes and modifications can be made without departing from the scope of the present invention. The scope of the present invention is defined by the following claims, to be interpreted in light of the foregoing specification. Typical operating voltages for the HIMOS cell in the write mode, the different erase modes and the read-out mode. Source Drain Control gate Program gate Substrate or p-well Write 0 3.3/5 1/1.5 12 0 Tunnel erase - 3.3/5 0 -12 0 Tunnel erase (alternative) - 3.3/5 -7 -7 0 Channel erase 3.3/5 3.3/5 0 -12 3.3/5 Channel erase (alternative) 3.3/5 3.3/5 -7 -7 3.3/5 Polyoxide erase - 0 12 0 0 Read-out 0 2 3 ≥0 0

Claims: A method of programming at least one non-volatile memory cell integrated on a chip in a metal-oxide-semiconductor technology, said cell comprising a semiconductor substrate including a source region, a drain region and a channel therebetween, a floating gate (FG) extending over a portion of said drain region, and over a portion of said channel, defining the floating-gate channel, a control gate (CG) extending over another portion of the channel region, thereby defining the control-gate channel, and over a portion of the source region, and a program gate (PG) capacitively coupled through a dielectric layer to said floating gate (FG), said method comprising the steps of: applying a first high voltage (V pg ) to said program gate (PG) thereby coupling a second high voltage (V fg ) to said floating gate (FG), said first high voltage (V pg ) being generated on said chip; applying a voltage (V d ) equal to or higher than the supply voltage of said chip to said drain region; and applying a low voltage (V cg ) to said control gate (CG), thereby causing a current to flow from said source region towards said floating-gate channel while achieving the programming of said cell with low energy dissipation, characterised in that said low voltage (V cg ) is smaller than the threshold voltage of the control-gate channel and is about the maximum injection efficiency. The method according to claim 1, wherein said low voltage is substantially lower than the threshold voltage of the control-gate channel. The method according to claim 2, wherein said current is a diffusion current in the order of nano-Amperes. The method according to claim 1 or 2, wherein an array of non-volatile memory cells is programmed, said array comprising a number of rows of said cells. The method according to claim 4, wherein said control gates (CG) of said cells on the same row are connected to a common wordline (10), and wherein said low voltage is applied to said wordline (10), said low voltage being smaller than the threshold voltage of the control-gate of said wordline (10). The method according to claim 1, wherein : the memory cell integrated on the chip is established, the on-chip generated high voltage (V pg ) is applied with an essentially fixed amplitude and with a predetermined pulse width, selected from a plurality of predetermined pulse widths, to said program gate (PG), said predetermined pulse widths determining charge levels. The method according to claim 6, wherein the amplitude of each one of said plurality of predetermined high voltage pulses is nearly identical to the amplitude of each other one of said plurality of pulses, and wherein the duration of each one of said plurality of predetermined high voltage pulses is different from the duration of each other one of said plurality of pulses. The method according to claim 7 wherein said duration of each of said plurality of pulses is a multiple of a predetermined fundamental time duration. The method of claim 8, wherein the selection of said high voltage pulse from said plurality of predetermined pulses is based upon a plurality of information bits. The method of claim 9 wherein each one of said plurality of pulses corresponds to a block of information bits to be stored as a charge level in said cell. The method according to any one of the preceding claims, wherein said voltage (V d ) applied to said drain region is a voltage being generated on said chip and that is higher than said supply voltage. The method according to any one of the preceding claims, wherein said supply voltage is 5 Volt and said metal-oxide-semiconductor technology is a 1.25 µm CMOS technology. The method according to any one of the preceding claims, wherein said supply voltage is 5 Volt and said metal-oxide-semiconductor technology is a 0.7 µm CMOS technology. The method according to any one of the preceding claims, wherein said low voltage (V cg ) is lower than 1 Volt. The method according to any one of the preceding claims, wherein said low voltage (V cg ) is lower than 0.7 Volt. The method according to any one of the preceding claims, wherein said chip furthermore comprises building blocks with logic functionality.

Inventors: DEFERM LUDO, GROESENEKEN GUIDO, HASPESLAGH LUC, MAES HERMAN, VAN HOUDT JAN, DEFERM, LUDO, GROESENEKEN, GUIDO, HASPESLAGH, LUC, MAES, HERMAN, VAN HOUDT, JAN
Applicants: IMEC INTER UNI MICRO ELECTR, INTERUNIVERSITAIR MICROELEKTRONICA CENTRUM VZW
Application year: 1996
Main classification: G11C16
IPC codes: G11C16, H01L27, G11C5