Flash-EPROM with enhanced immunity from soft-programming of reference cells

Abstract: The probability of soft-programming of the reference cells of a FLASH-EPROM memory may be excluded by having a decoupling transistor of a type of conductivity opposite to that of the cells functionally connected between the gate of each reference cell and the respective row line. Moreover the elimination of the electrical stresses to which the reference cells are subjected during the repeated programming cycles of the memory cells, increases the stability of the respective reference values of threshold and current level provided by the reference cells, thus increasing the reliability of the device.

Description: FIELD OF THE INVENTION The present invention relates to a FLASH-EPROM memory matrix or more concisely to a FLASH memory with an enhanced immunity against soft-programming phenomena in the reference lines. BACKGROUND OF THE INVENTION FLASH memories are rapidly finding numerous new fields of application. Their success is primarily due to the relatively small dimension of the unit cell, which is about equal to that of a traditional EPROM cell, coupled to the ability of being electrically erased. These characteristics ideally place FLASH memories between traditional EPROM memories (less expensive) and the EEPROM memories (much more expensive) from which they differ by being electrically erasable although the erasing may be carried out exclusively on the whole memory matrix, while an EEPROM memory may be erased byte by byte. On the other hand, a typical EEPROM unit cell is about four times larger than a FLASH cell. It is easily understood why the FLASH memories have conquered large sectors of the market where it is important to have nonvolatile, large-capacity memories which may be electrically erased but whose cost is relatively moderate. One of the industrial sectors which is particularly interested in this type of memories is the automotive sector where on-board reprogramming of the memories is an indispensable requirement, e.g. for modifying the controlling parameters of a motor from time to time, or for recording the operating history of a motor between services, and the like. EP-A-0224125-A2 discloses a sense amplifier for reading a non-volatile memory employing reference cells comprising the state of a selected matrix cell. Intrinsically a FLASH memory may develop specific programming disturbances, which make this type of memory very delicate from the point of view of the design of the circuitry for managing the memory matrix. The erratic programming phenomena known as "soft-programming" and a similar erratic erasing or "soft-erasing" to which the cells may frequently undergo, create remarkable programming problems for this type of memories from the point of view of these recurrent physical error mechanisms. The organization in a matrix of rows and columns of the cells of a FLASH memory is schematically depicted in Fig. 1. By soft-programming it is intended an eventual unwanted programming which a virgin cell (C) may undergo when other cells (A) disposed on the same row of the virgin cell (C) are being programmed. If the total number of cells arranged on the same row is very large and if all the cells of the row are programmed in succession, the repeated electrical stresses induced by high programming voltages (12,5 V on the gate of the cell to be programmed) to which the virgin cell (C) is repeatedly subjected may lead to an unwanted programming thereof. With reference to Fig. 1, by soft-erasing it is intended the possibility that a programmed cell (B, D) may improperly be brought back to a condition of a virgin cell because of the repeated electrical stresses induced by high erasing voltages (12,5 V on the gate for the (B) cell, 6,5 V on the drain of the (D) cell) to which the programmed cell is subjected when the programming of a cell which is on the same row or column of said B and D cells is carried out. If the number of cells, rows and columns of the matrix is large, this error mechanism becomes even more relevant than that due to soft-programming because it has a more relevant overall effect than soft-programming . In both cases, the physical mechanism through which soft-programming or soft-erasing takes place is the socalled Fowler-Nordheim current effect. It has also been observed that the unit cells of the memory more subject to soft-programming are the cells of the reference columns (reference hit lines) of the matrix, which provide a reference of the threshold and current levels for reading by means of the sense-amplifier of the managing circuitry of the memory, the data stored therein. Moreover the repeated electrical stresses to which the reference cells are subjected have a negative influence on the stability in time of the relative threshold and current levels provided by the cells, thus often causing problems of reliability of the reading circuitry of these memories. OBJECTIVE AND SUMMARY OF THE INVENTION The object of the present invention is indicated in claim 1. It provides a FLASH-EPROM memory which is substantially immune to the problem represented by soft-programming or by soft-erasing of the reference cells of the memory matrix and wherein the reference values of threshold and of current level provided by the reference unit cells are substantially stable in time also after a large number of cycles (write-erase cycles) undergone by the memory. These objectives and associated advantages are achieved by the FLASH-EPROM memory object of the present invention which comprises a "decoupling" transistor functionally connected between the gate of the reference cells and the relative row-line and wherein the body region of the decoupling transistor is connected to the supply rail and the gate of the decoupling transistor is connected to a real ground node of the circuit. The "decoupling" transistor effectively limits the voltage which is applied to the gate structure of a respective reference cell during the programming of the cells belonging to the same row of the matrix, thus eliminating any significant electrical stress on the reference cell structure and preserving their condition of virginity and increasing the stability of the relative threshold and current level values even after a large number of write-erase cycles undergone by the memory. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 represents, as already described, a matrix organization scheme of a FLASH-EPROM memory useful for illustrating in general terms, the soft-programming and soft-erasing mechanisms. Figure 2 is a circuit diagram representing a portion of a FLASH-EPROM memory matrix made in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT The fragmentary circuit diagram of Fig. 2 illustrates a FLASH-EPROM memory matrix made in accordance with the present invention, wherein a plurality of unit memory cells (1R, 1.1, 1.2, 1.3, 2R, 2.1, 2.2, 2.3, ...), integrated on a semiconducting substrate, are arranged in rows and columns. Each cell is essentially a floating gate, three terminal device (transistor); namely having a drain, a control gate (which may be physically formed over a first isolated (floating) gate of the device) and a source terminal. As typical in these memory arrays, the sources of all the cells are connected in common, normally to a virtual ground node, at least during the reading of data stored in the memory and during the programming of the cells and to which virtual ground node a certain voltage may be applied when erasing the cells. Each memory cell may be individually addressed through a first order of selectable byte- or word-lines (row-lines) which connect the gates of the memory cells disposed along a same row of the matrix, respectively, and a second order of selectable lines (bit-lines), commonly orthogonal to the former lines, which connect the drains of all the memory cells arranged along a same column, respectively. The unit cells arranged along a first column of the matrix (1R, 2R, ...) are used as reference cells and customarily they are virgin cells. Their function is that of establishing a reference threshold and current level to be used for comparison by the attendant circuitry for reading the data stored in the memory (sense-amplifiers). Commonly in a FLASH-EPROM memory matrix, a relatively high programming voltage, commonly 12 V, is reapeatidly applied on the gate of the reference cells every time a certain cell of the relative row is programmed. These repeated electrical stresses often lead to a soft-programming of the reference cell itself. In the FLASH-EPROM of the invention, depicted in Fig. 2, this eventuality is practically excluded by adding a "decoupling" transistor (TD1, TD2, TD3, ...) between the gate of each reference cell and the relative row-line of the matrix. In the example shown in Fig. 2, the unit cells are n-channel, floating-gate, superficial field effect transistors and in this case the decoupling transistors: TD1, TD2, ..., are transistors of opposite conductivity, i.e. p-channel transistors, of the enhancement type. Each decoupling transistor has a source connected to the control gate of the respective reference cell, a drain connected through a resistive path (r) to the respective row-line and a gate connected to the real ground node of the matrix and a body region connected to the supply rail (Vcc) of the matrix. The decoupling transistor is capable of limiting the voltage which is applied to the gate of the reference cell to a value of about 5 V instead of the full value of the programming voltage of about 12 V. In this way, the electrical stress which is induced on the reference cells at every programming of a memory cell disposed on the same row of the particular reference cell, is negligeable and practically eliminated, thus preventing any possibility of soft-programming of the reference cell. A further advantage is represented by an increased stability in time of the threshold and current level values provided by the reference cells, also after a large number of write-erase cycles undergone by the FLASH memory because of the practically negligeable electric stresses to which the reference cells are exposed. In this way, the reliability of the reading circuitry of the memory is greatly increased. The resistance (r) of the electrical connection path between the drain region of each decoupling transistor (TD) and the respective row-line is sufficiently high to maintain a proper voltage on the row-line and to limit the drain/bulk leakage current.

Claims: A FLASH-EPROM memory matrix having families of selectable word-lines and bit-lines, respectively, and comprising at least a column of individually selectable reference cells (1R,2R, ...), characterized by comprising at least a field effect transistor (TD1,TD2, ...) of a type of conductivity opposite to the type of conductivity of the cells (1.1,1.2, ..., 2.1,2.2, ...) of said memory, functionally connected between a control gate of each of said reference cells (1R,2R, ...) and a respective word-line (row line), said transistor being capable of limiting the voltage which is applied to the control gate of said reference cell during the programming of memory cells arranged in the same row of cells as said reference cell. The FLASH-EPROM memory according to claim 1, wherein said cells (1.1,1.2, ...) are n-channel and said transistors (TD1,TD2, ...) functionally connected between the gate structure of each of said reference cells and the respective word-line are p-channel. The FLASH-EPROM memory according to claim 1, wherein said transistor (TD1,TD2, ...) has a source (S) connected to the control gate of a respective reference cell (1R,2R, ...), a drain (D) connected through a resistive path (r) to said respective word-line (row line), a gate (G) connected to a ground node of the memory matrix and a body (B) connected to a supply rail of the memory.

Inventors: NATALE VIRGINIA, PETROSINO GIANLUCA, SCARRA FLAVIO, NATALE, VIRGINIA, PETROSINO, GIANLUCA, SCARRA', FLAVIO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.P.A.
Application year: 1991
Main classification: G11C16
IPC codes: G11C16, G11C17