VOLATILE/NON-VOLATILE DYNAMIC RAM CELL AND SYSTEM

Abstract: The invention is concerned with the problem of reducing the chip area occupied by volatile/nonvolatile dynamic RAM (random access memory) cells. A volatile/non-volatile dynamic RAM cell (30, 80) includes a storage capacitor (32) for volatilely storing binary information during normal RAM operation; an alterable-threshold storage capacitor (33A or 83) for non-volatilely storing the information in non-volatile fashion during poweroff conditions; and an energy barrier (33F or 45) between the two capacitors. Information can be restored to the volatile capacitor either by CCD charge transfer or by charge-pumped operation. The energy barrier facilitates efficient charge pumped restore of information. In one embodiment, the energy barrier is a high concentration substrate surface region (45) having the same conductivity type as the substrate. Alternatively, the alterable-threshold non-volatile capacitor and the energy barrier are provided by a split-gate capacitor (33) which has an alterable threshold non-volatile section (33A) (the non-volatile capacitor) and a non-alterable threshold section (33F) (the energy barrier). The cells may be arranged in an array.

Description: VOLATILE/NON-VOLATILΞ DYNAMIC RAM CELL AND SYSTEM Technical Field This invention relates to volatile/non-volatile dynamic RAM (random access memory) cells of the kind including volatile storage means including a capacitor for storing binary charge information, and non-volatile storage means for selectively storing the charge infor¬ mation in the volatile storage means. Background Art Semiconductor RAM systems generally fall into two categories: static RAM systems and dynamic RAM systems. Static RAM systems typically employ bistable multivibrator or flip-flop circuits in which the stored bit value is determined by the two bistaole states available to the circuit. Since static RAMs employ bistable devices, the bit content of each cell is re¬ tained without refreshing and tne readout of the bit content is nondestructive. In contrast, dynamic RAMs typically employ a cell arrangement in which the stored bit value is determined oy tne presence or absence of a voltage stored on a semiconductor capacitor structure. Dynamic RAMs typically require periodic refreshing of the information stored on the capacitor, although the readout may be either destructive or non-destructive depending upon the cell design. As a general rule, static RAM systems are favored for smaller memory systems, whereas dynamic RAMs provide lower cost per bit for, and are thus favored for larger memory systems of perhaps 32 kilobits or larger. RAMs are generally volatile devices, i.e., the bit: content of tne memory is lost if electric power is removed or lost. Referring to Figs. 1 and 2, there are shown cross-sections of conventional MOS dynamic, volatile RAM cells 10 and 20. RAM cell 20 is similar to and operates identically to cell 10, but has a two-level polysilicon gate 24 instead of the single level gate 14 of cell 10, The dual level gate 24 eliminates the diffusion 9 required in cell 10 and therefore permits size reduction and a corresponding increase in density in cell 20. During tne operation of either cell, the voltage V applied to' the bit line diffusion 8 controls the availability of charge. Capacitors 12 (Fig. 1) and 22 (Fig. 2) are written to a "0" or "1" (charged or uncharged) state by driving V high to enable charge transfer gates 13 and 23 to transfer any charge from diffusion 8 to the capaci¬ tor. During a read operation, the transfer gates are again turned on to transfer the charge from the capacitor to a sense amplifier (not shown) connected to the diffu- sion line 8. As mentioned, RAM cells 10 and 20 are dynamic and volatile, that is, they require periodic refreshing of the information stored on the capacitors 12 and 22, and lose the stored information if electric power is re- moved or lost. Over the last decade or so, various ap¬ proaches have been taken to add back up non-volatile stor¬ age capability to dynamic RAMs. In general, the non¬ volatile backup data storage capability can oe provided by adding non-volatile alterable threshold capacitor or transistor devices to the RAM cell. These cells will be referred to as volatile/non-volatile dynamic- RAM cells or simply V/NV RAM cells. A volatile/non-volatile dynamic RAM cell of the. kind specified is known from U.S. Patent No. 4,175,291 which discloses a four gate V/NV dynamic RAM cell in¬ cluding a volatile storage capacitor, an adjacent, alter¬ able threshold, non-volatile storage capacitor, and two charge transfer gates which are located one each on opposite sides of the capacitors. One transfer gate and the volatile capacitor are activated for volatile oper¬ ation; both capacitors are used to effect charge trans¬ fer for V/NV store; and both transfer gates and both OMPl 1PO capacitors are used to effect NV/V restore, also by charge transfer. The known volatile/non-volatile dynamic RAM cell has the disadvantage of occupying a relatively large area when embodied in an integrated circuit chip. Disclosure of the Invention It is an object of the present invention to provide a volatile/non-volatile dynamic RAM cell of the kind specified, wherein the aforementioned disadvantage is alleviated. Therefore, according to the present invention, there is provided a volatile/non-volatile dynamic RAM cell of the kind specified, characterized by barrier means arranged to provide a preselected energy barrier to charge transfer between the volatile storage means and the non-volatile storage means. It will be appreciated that a volatile/non¬ volatile dynamic RAM cell according to the invention has the capability of being implemented utilizing a small number of control gates. A further advantage is that the cell has the versatility when restoring information from the non-volatile storage means to the volatile stor¬ age means, of operating in a charge-coupled charge transfer mode and in a charge pumping mode. Brief Description of the Drawings Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figs. 1 and 2 are simplified cross-sectional representations of conventional volatile random access memory cells,- Fig. 3 is a simplified cross-sectional repre¬ sentation of a volatile/nonvolatile random access memory cell embodying the principles of the present invention; Fig. 4 is a simplified schematic of a portion of a random access memory which utilizes the cell of Fig. 3; OMPI WIPO ^ Fig. 5 is a timing diagram of a nonvolatile store operation for the memory cell of Fig. 3; Fig. 6 is a simplified timing diagram of a CCD mode restore operation for the random access memory cell of Fig. 3; Fig. 7 is a simplified timing diagram of a charge pumping mode restore operation for the cell of Fig. 3; and Fig. 8 is an alternative embodiment of the cell of Fig. 3. Detailed Description of the Preferred Embodiment Fig. 3 is a cross-sectional schematic of an n-channel V/NV dynamic RAM cell 30 which embodies the principles of the present invention. Illustratively, the cell 30 is formed in a p-type silicon semiconductor substrate 6 and includes charge transfer device 21; volatile storage capacitor 32; and "split gate", energy barrier/NV storage capacitor 33, which is described in the immediately succeeding paragraph. Devices 21 and 32 have conductive gate electrodes 23 and 34, respectively, formed on a gate dielectric 7, such as silicon dioxide, of suitable thickness. N-type bit line 8 is formed in the substrate 6 laterally adjacent the transfer device 21 and on the side opposite the volatile capacitor 32. Bit line 8 is used for effecting the charged/no charge condition of this capacitor. Split gate capacitor 33 has an NV alterable threshold-voltage section 33A and a non-alterable (fixed) threshold-voltage (NA) section 33F. Illus- tratively, these different characteristics of the capa¬ citor sections are provided by a dual, silicon dioxide 47-silicon nitride 49 gate dielectric. Preferably, the silicon dioxide 47 of the alterable NV section 33A is very thin, typically 10-60 Angstroms thick, for permit- ting charge transfer into and out of the charge-storing silicon nitride-silicon dioxide gate dielectric to O I thereby alter the threshold voltage of the capacitor section 33A. In contrast, the oxide 48 of the NA sec¬ tion 33F is sufficiently thick, typically about 1,000 Angstroms, to prevent charge transfer and thereby pre- elude alteration of the threshold during normal operation. The oxide gate dielectric 7 associated with fixed thres- - hold devices 21 and 32 is typically 400-700 Angstroms thick. A suitable thickness for the silicon nitride layer 49 is 450-550 Angstroms in both sections 33A and 33F. It will be appreciated by those skilled in the art that the cell 30 can be p-channel. Also, the above thickness values are examples only and can be readily varied to alter performance characteristics. The dual dielectric silicon dioxide-silicon nitride MNOS structure can be used for all the devices 21, 32 and 33, with appropriate adjustment of the thicknesses associated with the devices 21 and 32 to provide normal, non-alter¬ able-threshold MOSFΞT behavior. In general, the cell of the present invention can comprise various, non-volatile conductor-insulator-substrate field effect transistor structures. Examples are CI ^ I^SFΞT, where I-, and I ? are dielectric such as aluminum oxide and silicon oxide (MAOS), or are the aforementioned silicon nitride and silicon oxide (MNOS); or CI j ^I- j SFET, where I-,, I 2 and I-, are silicon oxide (or silicon oxynitride), silicon nitride, and silicon oxide. While the invention is applicable to conventional metal gate technology, the emerging semiconductor (polycrystalline silicon or poly- silicon) gate structure (e.g., SNOS or SONOS) is pre¬ ferred. A SNOS structure is shown in Fig. 3 and re¬ ferred to subsequently. The following terminology will be used. First, device 21 is conventionally termed, and will be referred to herein as "transfer gate". However, it is understood that the term "transfer gate" encompasses a transistor such as 11 (Fig. 1). Secondly, regarding the control voltag ~~ es for the cell 30, Vn+ refers to the voltage applied to the bit line 8 via address line 16; V is the voltage applied to the gate electrode 23 of transfer gate 21 via address line 27; V refers to the voltage applied to the volatile transistor gate elec¬ trode 34 via bias line 38; and Vnv refers to the voltage ' applied to the gate electrode 44 of the split gate capacitor 33 via address line 39. Finally, in reference to tne n-channel cell 30, unless otherwise specified, the supply voltage V is 12v., V and V + ' are 12v. (high) or 0 volts (low), and the substrate bias voltage V βB is -5v. VOLATILE OPERATION During volatile operation of the cell, the voltage V is at ground, Ov. , to block the split gate capacitor 33 from the volatile portion 21-32 of the cell. To volatilely write the cell 30 to the (uncharged) 1 state, the diffusion line 8, transfer gate electrode 23 and capacitor gate electrode 34 are all driven high to thereby drive the capacitor substrate surface poten¬ tial high. Please note, as used here, reference to a "0" capacitor state refers to the presence of sufficient minority charge carriers (electrons for p-type sub¬ strate) to place the capacitor substrate surface at a low potential relative to the high potential gate elec¬ trode so that the capacitor is polarized. A "1" state implies a relative lack of charge, a high substrate surface potential, and an unpolarized capacitor. Next, the transfer gate 23 is driven low to block the capa- citor 32 from the diffusion line 8. The diffusion line 8 may then be driven low. If, instead, the volatile capacitor 32 is to be written to a (charged) 0 state, the same voltage timing sequence applied except that the diffusion line 8 is held low throughout the cycle. As a result, the sub¬ strate surface region of the capacitor 32 is held low. O A To further illustrate volatile operation of the RAM cell 30, consider first a read cycle for RAM array 60 shown in Fig. 4. As those skilled in the art will appreciate, memory cycles normally start with cell read-out and function to refresh the stored information. During a typical volatile read cycle, the row or word addresses are low (V is at ground). Then, the pre- charge clock 0 turns on. The load transistors L, and L~ of sense amplifier 70 are of high impedance; the input transistors I, and I- are of low impedance. Thus, with clocks 0- and 0 2 on, when precharge clock 0.. turns on, nodes 71 and 71' of bistable multivibrator sense amplifier 70 are equalized at ground (approxi¬ mately). Precharge clock j3 also sets the nodes 72A and 72B associated with the dummy capacitors A and B to V K__---1 π J- 1 l VK„-t-,-----., is between the 0 (low) and 1 (high) states, for example, at 6 volts. Next, precharge clock 0^ and power clocks J2L and 0 ~ are turned off. A selected row address and the opposite side dummy (for example word A, and dummy A) are turned on by J Vn and information in the associated row storage capacitor 32 and the opposite side dummy capacitor A is available via A-, and dummy A transfer gates 21, respectively, to amplifier nodes 71 and 71'. Power clocks 0- and 0 ' are turned on again to provide amplified information at the nodes 71 and 71'. That is, if the selected volatile storage capacitor 32 is in a charged, 0 state, associated node a is low and the node 72A for the associated dummy capacitor A is higher, at about 6 volts. As a result, as the capacitances asso¬ ciated with cross-connect lines 81 and 32 are charged, line 82 (associated with dummy capacitor A) is charged faster than line 81 (associated with the lower potential of node a) . Consequently, line 82 is charged and turns on transistor I, before I- can be turned on. I-, con¬ nects node 71 to ground, thereby preventing I„ from turning on, and permits node 71' to become fully charged t0 V DD- In short, in response to the low potential, 0 state on the node a of A, or any other even matrix capacitor 32, node 71 is placed at "0" and node 71' is placed at an amplified "1" state (inverted relative to node a). Similarly, if . capacitor 32 or any of the other even matrix capacitors is initially in a "1" state, a "0" ("-1") is applied to node 71' (71). Since information is normally read out from the node 71', to implement read out, the column address is used to turn on transistor 87 to access the information on node 71'. The same type of sequence applies to the storage capacitors of the odd matrix. Thus, information in the odd matrix capacitors is applied without inver¬ sion to the adjacent node 71' and is applied in inverted form to the other node 71. For example, a "1" on node a of B, capacitor 32 ^ results in an amplified "1" at node 71' and a 0 at node 71. Since the data typically is read out from node 71', the information from the odd matrix capacitors is in non-inverted form, while the information from the even matrix capacitors is inverted. Thus, typically the information at node 71' from the even matrix capacitors is inverted to provide non-inver¬ ted data out. Since those skilled in the art will readily implement this technique and since it is not part of the invention, the inversion technique need not be discussed further here. After the read cycle, all clocks return to • their original state, preparatory to the next operating cycle. As the row address transfer gate 21 turns off, the amplified information on node 71 (71') is isolated on the even (odd) storage capacitor 32 and the capacitor is thereby "ref eshed" to its original state. In a refresh only cycle (no read), it is not necessary to turn on column address transistor 87 to access node 71' . Instead, the amplified information available on nodes 71 or 71' is returned to the corres¬ ponding even or odd matrix storage cell, and is isolated OMPI in the cell as the associated row address transistor 32 turns off. ERASE OF NONVOLATILE CAPACITORS Preparatory to storing information from the volatile storage capacitors 32, the alterable threshold voltage, non-volatile (AT/NV) capacitor sections 33A are erased to a low threshold, "0" state (VTO) . Since the NV sections may be erased during normal RAM operation without destroying the data in the volatile RAM capa- citors 32, erasing can be done either during normal RAM operation or immediately upon power down, prior to writing the volatile capacitor data into the nonvolatile capacitor section. Hereafter, to facilitate descrip¬ tion, reference will be made simply to erasing/writing split gate capacitor 33 with the understanding that this reference is to the AT/NV section 33A of the capacitor. The n-channel split gate capacitors 33 are erased by applying a large negative voltage to the gate 44. Preferably the erase voltage is near the breakdown voltage of the silicon nitride-silicon dioxide gate insulator. The erase voltage is applied for a predeter¬ mined time sufficient to move the threshold voltage of the non-volatile capacitors in the negative direction to VTO. It will be appreciated by those skilled in the art that the erase times and erase voltages are interdepen¬ dent, i.e., that one can be increased or decreased, respectively, by decreasing or increasing the other. STORE Upon power down, and prior to the loss of the biasing voltage on the volatile storage capacitor 32, the split gate capacitor 33 is turned on to receive the charge associated with the volatile storage capacitor 32. That is, and referring to Figs. 3 and 5, at store time t„ (which is just after the onset of power down at t, and prior to V n on the volatile capacitor 32 going to ground at t_) , V is driven to a large positive write OMPI voltage, V . With the split gate high and the volatile capacitor gate 34 going low, any charge asso¬ ciated with the volatile capacitor is transferred to the substrate of the AT/NV capacitor section 33A. If the volatile capacitor 32 was in the charged, 0 state at the onset of power down, charges are - transferred to the relatively deep potential well beneath the AT/NV section 33A of the split capacitor 33, de¬ creasing the depth of the potential well and the voltage drop across it. This produces a relatively high elec¬ trical field in the gate dielectric and shifts the thres¬ hold voltage of the AT/NV section 33A to the more posi¬ tive, VTl state. See TABLE I, state No. 3. OMPI . TABLE I SEQUENCE OF CAPACITOR STATES CCD Restore (Charge Storage (State After State After Restore) Initialization) Conventional theory is that the charges tunnel into the gate dielectric 47-49 and are stored there, primarily proximate the silicon nitride-silicon dioxide interface. If the volatile capacitor 32 was in the un- charged 1 state, there are insufficient charge carriers to collapse the potential well and shift the threshold voltage of the AT/NV section 33A. The capacitor 33 re¬ mains in the erased, VTO threshold state. See TABLE I, state No. 3. Those skilled in the art will understand that the .applied voltage V = V W RΪ TE must ■ e sufficient to fully write the individual cells 30 before all cells are charged by thermally-generated recombination of elec¬ tron-hole p c airs. That is, the com *p • letion of t.W.,R„.I,.„T,_E, at t. must occur within a few milliseconds. For the exem¬ plary parameters for cell 30, V wRI m E = 20v. completes the writing in about one millisecond, as discussed below. It is desired to have complete transfer of charge from volatile capacitor 32 to non-volatile capa¬ citor 33 to write the non-volatile capacitor strongly and thereby facilitate the sense amplifier 70 distin¬ guishing between VTO and VTl. It is thus necessary that non-volatile capacitor 33 have sufficiently high capa- citance to store essentially all the charge available from the volatile capacitor 32. This charge storage requirement is in conflict with the aim of minimizing surface area of the'non-volatile capacitor to maximize device cell density. These conflicting performance and density requirements are both optimized by using an AT/NV capa¬ citor section 33A which has a surface area that is just large enough to set the charge of section 33A approxi¬ mately equal to the charge of the volatile storage capacitor 32. A charge on the volatile capacitor Q = V x C is available for transfer to the nonvolatile capacitor. O j . For charge equality and complete transfer the following condition, expressed in terms of capacitance, must be met: Q.v = V v x C v = Qnv = V_nv nv (1 ) In terms of the respective capacitance per unit area, C, and capacitor surface areas, A, capa¬ citance is given by C = C'A and equation (1) becomes: V nv (C 'nv x A nv ) = V v (C 'v x V' < 2 ) ' The capacitance per unit area, C , pro¬ vided by the thin, oxide-nitride dielectric of the AT/NV capacitor 33A is nearly three times that, C , of the thick oxide dielectric of the volatile capa¬ citor 32. Also, the write voltage (V r W 7 K D 1 Tr l rι b c , = 20v.) ■ on capacitor 22 is approximately twice the voltage ( (VV v == 1122vv..)) aappppjlied to the volatile capacitor 32. Thus, typically Vnv = VWRITE = 2Vv' (3 ') and C' nv = 3C' V . (4) Substituting (3) and (4) into (2) gives: 2V y x 3C' V x A nv -= v x C « v x A v , or A nv = 5 ' (5) Thus, the charge associated with capacitors 32 and 33 is equal (for the given dielectrics and thicknesses and conditions of relative capacitance per unit area and relative write voltages) when the surface area of the OMPI /., WIPO .. * nonvolatile capacitor section 33A is about one-sixth - the surface area of the volatile capacitor. For the exemplary silicon substrate and gate dielectric, and a surface area of about 90 square micro- meters and 15 square micrometers for the volatile and nonvolatile capacitors, respectively, V-^-,,-^ of about 20 - volts will complete charge transfer and shift the thres¬ hold voltage of the AT/NV capacitor section 33A about 5 volts in t„ ITE = 1 millisecond. RESTORE 1. CCD Mode The CCD mode for restoring data to the volatile capacitor 32 is summarized in Fig. 6. Referring also to Fig. 3, the cell 30 is first initialized by transferring negative charge to the non-volatile capacitor 33. This is done at restore time t, by taking V to ground to make charge available at the bit line diffusion 8 and by driving gate V high, with V high, to transfer charge to the volatile capacitor 32. At restore t~, the -gate 44 of capacitor 33 is taken to a voltage Vnv = v n R n EA..D which is between the two threshold voltages, VTl and VTO, of the AT/NV section 33A. If the threshold is VTO, a portion of the charge, Q v , associated with the vola¬ tile capacitor 32 will be transported to the AT/NV sec- tion 33A of capacitor 33. If the threshold is VTl (written) no depletion layer is induced in the AT/NV capacitor section 33A and no charge is stored therein. Then, commencing at restore t,., V and V are driven low to isolate the charge on the AT/NV capacitor 33A. See the Fig. 6 diagram for AT/NV charge Q NV . See also TABLE I, state No. 3. Now the data is restored to the volatile capacitor 32. At restore t., V is again driven high and, at t^, V is driven low to transfer any charge stored in AT/NV section 33A to the volatile capacitor 32. (It does not matter what state V is in, because - QR OM WI tne bit line 8 is isolated. ) Since REA on the capa¬ citor 33 is only a portion of V„ RITE (for example, 0.5 V RITE^ on ly that corresponding portion of the initial volatile capacitor charge Q v (for the example, about 0.5Q) is stored on the non-volatile capacitor 33 and returned to the volatile capacitor 32. Thus, during the interval " t.-tg, the initial 0 or 1 state of the volatile capacitor 32 is partially restored, inverted, as 1 or 0. Next, at tg, the sense amplifier 70 completes the restore of the capacitor to the full 0 or 1 state. For this purpose, and referring also to Fig. 4, ¥-.„„ is set to a value which is between the relatively high and relatively low voltages on capacitor 32 node a associated with the partially restored capacitor 1 and 0 states, respectively. Typically, " V is approximately 6 volts. If, as an example, at t fi row Al capacitor 32 has been restored inverted to the relatively uncharged, high potential 1 state, the voltage at node a associated with Al capacitor 32 is greater than the V „ p on node 72A. The charge Q v on volatile capacitor 32 and the substrate surface potential relative to V pp are both shown in Fig. 6. Referring to Fig. 4, the higher voltage on node 71 turns on I 2 , clamping node 71' to ground and allowing V = 12 volts to restore node 71 and Al capacitor 32 fully to the 1 state. Conversely, if Al capacitor 32 has been "re¬ stored" to a partially charged, low potential 0 state at t g , V Ep on dummy A node 72A is greater than the voltage on Al capacitor 32 node a. The relatively high voltage on node 71* turns on transistor I,, clamping node 71 ' to ground and completing the restore of Al capacitor 32 to the fully charged 0 state. In short, during the period between tg and t^, sense amplifier 70 completes the full inverted restore of the 0 or 1 state. At η the inversion capability of conventional state-of-the-art RAM systems is used to perform bit OMPI inversion. Typically, data can be inverted at the rate of one clock time per bit or approximately 150 nano¬ seconds. Thus, and referring also to TABLE I, the volatile 0 or 1 data in capacitor 32 at power down is restored in non-inverted fashion at t g and normal vola¬ tile RAM operation can commence. As mentioned previous- • ly, the non-volatile capacitor 33 can be erased during normal RAM operation or immediately upon the next power down situation. 2. Charge-Pumped Mode An alternative, charge-pumped mode for re¬ storing data to the volatile capacitor is shown in Fig. 7. At , the volatile capacitor 32 is initialized to 0 using Vn+, = Ov. and V„n = Vv = 12v., to turn on word address transistor 21 and the volatile capacitor 32 to apply charge from the bit line diffusion 8 to the capa¬ citor 32. At t 2 , when the volatile capacitor 32 has become charged, V is taken low to isolate the charge on the volatile capacitor. At restore t_, charge pumping is initiated on non-volatile capacitor 33. Vnv is alternately ■ * switched positive, not exceeding the written threshold VTl, and negatively to below the erased threshold VTO. If the AT/NV capacitor 33A is at VTO (i.e., was written to VTO at power down as the result of the "1" state of capacitor 32) , a relatively deep potential we ' ll is formed beneath the non-volatile capacitor 33 during the positive swing of each pumping cycle. As the result of this deep potential well, a portion of the charge associated with the initialized volatile capacitor 32 is transferred to the non-volatile capacitor 33. This charge re-dis¬ tribution takes place according to Q v = C-,V-> = C~V 2 , where C-, = C (capacitance of volatile capacitor 32) and C 2 = C γ + C (capacitance of volatile capacitor 32 and O non-volatile capacitor 33), and V 2 • During each pumping cycle when V swings, negative, the charge in the non-volatile capacitor 33 is transferred into the substrate 6. This positive-negative cycle is repeated a number of times, until nearly all charge has been re- ' ' moved from the volatile capacitor 32 and transferred to the substrate at t.. At this time, the volatile capaci¬ tor 32 has been returned without inversion to the previous "1" state. Typically, for the topology as described above, about ten charge pumping cycles leave the volatile capacitor 32 with about 0.10Q or 0.90Q depending upon whether the non-volatile capacitor 33 is at VTO or VTl, respectively. State-of-the-art sense amplifiers can readily sense the resulting difference in capacitor charge. In case the non-volatile capacitor 33 has been written to a relatively high threshold VTl at power down by a (charged) 0 state AT/NV capacitor 33A state, the AT/NV capacitor doesn't invert during the positive voltage swing of the pumping cycle and the shallow potential well associated therewith precludes significant charge transfer*. (Furthermore, when V is switched negative, the tendency is for any minority carriers associated with the non-volatile capacitor 33 to trans¬ fer to the relatively deeper potential well associated with the volatile capacitor 32). As a result, after the charge pumping cycle, the volatile capacitor retains its initialized 0 state. In effect, volatile capacitor 32 has been returned without inversion to its original pre-power down "0" state. For efficient charge-pumped restore, a poten¬ tial barrier serving as an energy barrier is inserted between the volatile capacitor 32 and the non-volatile capacitor 33 so that when the non-volatile capacitor is in the VTl state, charge will not be returned to the volatile capacitor during the negative swing of the charge pumping cycle. This can be accomplished by (1) a diffused or implanted barrier 45 (Fig. 8) which is a high concentration surface region of the same conductivity type as the substrate, or (2) by the non-memory section 33F of the split gate capacitor 33 (Fig. 3). As used herein, both of the above types of barriers provide an increased inversion threshold in the underlying substrate region such that the region inverts along with the AT/NV capacitor 33A substrate surface region during the positive swing of the charge pumping cycle to allow unimpeded transfer of charge from the volatile capacitor 32 to the capacitor 33A, but provides a potential energy barrier which prevents return of electrons to the volatile capacitor during the negative swing of the charge-pumping cycle. The barrier thereby improves the efficiency of charge-pumping. The barrier surface area should be small compared to the surface area of- the volatile capacitor 32 to prevent excessive charge losses during pumping. Because even very small barriers are effective, the practical limits on dimen¬ sions are dictated by the photolithographic resolution of minimum feature size. At this point, the data which existed in the volatile capacitor 32 at power down has been restored without inversion. Normal operation can be resumed. The non-volatile capacitor array can be erased at any time during normal volatile RAM operation or immediately upon power down in preparation for the next store cycle. Thus, there has been described a V/NV RAM cell which utilizes only three gates, uses only slightly more area than a volatile RAM cell, and is a straightforward substitution for conventional volatile RAM cells in memory arrays with virtually no additional circuitry required. fθREA OMPI

Claims: CLAIMS : 1. A volatile/non-volatile dynamic RAM cell (30 or 80) including volatile storage means including a capacitor (32) for storing binary charge information, and non-volatile storage means (33 or 83) for selectively- storing the charge information in the volatile storage means, characterized by barrier means (33F or 45) ar¬ ranged to provide a preselected energy barrier to charge transfer between the volatile storage means and the non¬ volatile storage means. 2» A volatile/non-volatile dynamic RAM cell according to claim 1, characterized in that said non¬ volatile storage means and said barrier means include, respectively, a non-volatile alterable-threshold section (33A) and a non-alterable threshold section (33F) of a split gate capacitor (33). 3. A volatile/non-volatile dynamic RAM cell according to claim 2, characterized in that said split gate capacitor (33) is MNOS or SNOS. 4. A volatile/non-volatile dynamic RAM cell (30 or 80) according to claim 1, characterized by: a semiconductor substrate (6) having majority carriers of a given conductivity type, said capacitor being formed in said substrate; and charge supply means (8) for selectively supplying charge to said capacitor (32), said non-volatile storage means including an alterable thres¬ hold formed in said substrate (6) and arranged to selectively receive, and store said binary charge infor- mation from said volatile storage means, and restore to said volatile storage means said binary charge infor¬ mation utilizing a charge pumping restore operation. 5. A volatile/non-volatile dynamic RAM cell according to claim 4, characterized in that said energy O PI /.„ WIPO -^ 5. (concluded ) barrier means includes a substrate impurity region (45) having a higher concentration of majority carriers of said given conductivity type than said substrate (6). 6. . A volatile/non-volatile dynamic RAM cell according to claim 2, characterized in that said split gate capacitor (33) is of the type CI-,I 2 S, wherein C is a gate electrode (44), I, is a first dielectric material (49) is a second dielectric material (47) and S is a semiconductor substrate (6). 7. A volatile/non-volatile dynamic RAM cell according to claim 6, characterized in that said first dielectric material (49) is silicon nitride and said second dielectric material (47) is silicon dioxide. 8. A volatile/non-volatile dynamic RAM cell according to claim 2, characterized in that said split gate capacitor (33) is of the type CI,I 2 I 3 S, wherein C is a conductive gate electrode (44); I,, I„, are gate insulators, and S is a semiconductor substrate (6). 9. A volatile/non-volatile dynamic random access memory system including a plurality of volatile/ non-volatile dynamic RAM cells according to any one of the preceding claims, characterized by amplifier means (60) including an output node (71') # said amplifier means being connected to said memory cells and being re¬ sponsive to a voltage stored on said capacitor (32) in¬ cluded in said volatile storage means, to generate a signal representing said voltage at said output node; and accessing means (87) for accessing the signal infor¬ mation at said output node. 10. A random access memory system according to claim 9, characterized in that said amplifier means in¬ cludes a bistable multivibrator (70) having a pair of OMPI 10 . ( concluded ) nodes (71, 71*); in that said plurality of memory cells includes two arrays (A and B), the first array . (A) being connected to one of said nodes, and the second array (B) being connected to the second of said nodes for controlling the binary state of said multivibrator (70); and in that said accessing means (87) is connected to one (71') of said nodes. O PI

Inventors: VELTHOVEN A
Applicants: NCR CO, NCR CORP
Application year: 1981
Main classification: G11C13
IPC codes: G11C11, G11C14, G11C16, H01L27