Non-volatile memory with protection diode

Abstract: To protect the thin tunnel oxide layer (8a) interposed between the floating gate region (6) of memory cells (1) and the substrate (2) and subject to in-process damage when the wafer is subjected to radiation, provision is made for a diode (D) connected between the control gate region (7) of the cells and the substrate (1). The diode (D) defines a conductive path which, when normal operating voltage is applied to the control gate regions (7), is turned off and has no effect on normal operation of the memory (15), and which is turned on to permit the passage of charges between the control gate region (7) and the substrate (2) when the control gate potential exceeds normal operating potential but is less than the breakdown voltage of the tunnel oxide (8a) divided by the coupling factor of the control and floating gate regions of the cells. The diode (D) is appropriately formed prior to patterning the control gate regions of the cells.

Description: The present invention relates to a nonvolatile EPROM, EEPROM or FLASH-EEPROM memory with tunnel oxide protection. Though applicable to all the above types of memory, the present invention is particularly advantageous in the case of flash-EEPROM memories to which special reference is made in the following description. Flash-EEPROM memories are Electrically Erasable Programmable Read Only Memories (EEPROMs) which combine the high density and low cost of EPROMs with the advantage of electrical erasability, and have recently become the most attractive of nonvolatile memories for their potential application in solid state disks for portable computers. Though various flash memory concepts have been developed, most flash memories employ a very thin (8-12 nm) oxide layer through which each memory cell may be programmed and erased by Fowler-Nordheim tunneling, and which is therefore known as the "tunnel oxide." The following description deals with the most commonly used flash-EEPROM cell structure, the double-poly single-transistor cell, though the present invention applies to any cell structure employing tunnel oxide. Double-poly single-transistor flash cells present a structure closely resembling that of standard EPROM cells, and comprise an NMOS transistor with a polysilicon floating gate region capacitively coupled to a second polysilicon layer defining a control gate. The main difference between these two types of cell lies in the thickness of the oxide between the floating gate region and the substrate, which is much thinner in the case of flash cells. For the sake of clarity, a description will now be given of a flash-EEPROM cell with reference to the cross section shown in Fig.1. Number 1 in Fig.1 indicates a flash-EPROM cell formed, in this case, in a P type substrate 2 housing N+ type source and drain regions 3 and 4 separated by a channel region 5 formed by the substrate itself. Over substrate 2, aligned with channel 5, there are provided a floating gate region 6, fully embedded in an oxide layer 8, and a control gate region 7. Floating gate region 6 is separated from the substrate by a thin oxide layer 8a; control gate region 7 is separated from floating gate region 6 by a dielectric layer 8b; and source region 3 presents a graded junction with the substrate, that is, is formed in two parts: a heavily doped (N+) part 3a facing the larger surface 10 of the substrate; and a deeper, lightly doped (N-) part 3b surrounding part 3a on the sides not facing surface 10. The yield and reliability of flash memories are known to be closely related to the quality of the tunnel oxide, which quality not only depends on pre-oxidation cleaning and the oxidation process itself, but is also strongly affected by the post-oxidation process steps. Of all the process steps, the most critical are those which expose the wafer to radiation damage, namely, ion implantation, plasma etching, sputtering and plasma enhanced chemical vapour deposition. Currently used flash-EEPROM memory processes present a large number of such steps following growth of the tunnel oxide. The model generally accepted for in-process radiation is the so-called "antenna effect" whereby charges are trapped in conductive layers and may result in a critical increase in the potential of the layers. To explain the phenomenon, it should be borne in mind that ion implantation and plasma processes involve the collision of charge particles (either electrons or ions) with the wafer surface, so that conductive polysilicon layers insulated from the silicon substrate may be charged by capturing the charge particles. If the polysilicon layer is not patterned, it acts as an electrostatic shield. In fact, the charge is distributed evenly over the entire area of the wafer, thus generating a low electric field. Also because, in the case of ion implantation, the beam is localized and the overall charge density is low. What is more, the metal grips holding the wafer may act as discharge lines for at least partly removing the captured charge particles, so that little danger exists for the dielectric layer underlying the polysilicon. If, on the other hand, the layer is patterned and comprises "islands", i.e. areas insulated conductively from the rest of the layer and separated from the substrate by a thin oxide layer, as in the case of gate regions, the layer is charged to a potential which depends on the collecting area and its capacity versus the substrate, i.e. the area of the thin oxide. If the ratio between the collecting area and the area of the thin oxide is unfavourable (high), the gate region may easily reach a potential higher than the breakdown voltage of the thin oxide, thus resulting in oxide breakdown - in turn resulting in "zero time" device failure, i.e. before it is even used - or in oxide damage with the formation of traps in the oxide itself and which in turn results in "latent" device failure and impaired reliability. The above model also applies to flash cells. In fact, in certain situations, the tunnel oxide may be damaged due to capacitive coupling of the control and floating gate regions, and the lower dielectric resistance of the tunnel oxide in relation to the dielectric layer separating the control and floating gates. The likelihood of this occurring is even greater, in view of the extensive collecting area defined by the polysilicon strips forming the control gates, and the small area of the tunnel oxide of each cell. Consequently, the tunnel oxide may be broken or damaged in the event the control gate region is charged to a potential greater than the breakdown voltage of the tunnel oxide divided by the coupling factor of the control gate. At present, the only way of minimizing the "antenna effect" as described above is to modify the process by exposing the wafer, in the case of high-current implantation, to an electron "shower" for compensating the positive charge produced by the ion beam, or by optimizing the plasma etching process for reducing radiation damage. Such solutions, however, complicate the process and are difficult to control. In Patent Abstracts of Japan, vol. 16, No. 50 (JP-A-32 50494) a voltage limiting structure for a memory device including FAMOS cells is disclosed; the voltage limiting structure including a diode arranged between a boost line (for biasing the FAMOS cells through selection transistors) and ground. No structure is taught to avoid the antenna effect due to in-process radiation. US-A-4,928,157 describes a protection diode structure for MOS transistors; GB-A-2 118 797 describes a nonvolatile memory circuit including diodes interposed between the source terminals of the cells and a common ground line to allow biasing of the control gates of the cells to negative voltages. None of these documents discloses a structure for protecting against the antenna effect. It is an object of the present invention to prevent post-oxidation in-process radiation damage of the tunnel oxide while at the same time eliminating the disadvantages of known solutions. According to the present invention, there is provided a nonvolatile EPROM, EEPROM or FLASH-EEPROM memory with tunnel oxide protection, as claimed in Claim 1. The present invention also relates to a method of fabricating a nonvolatile EPROM, EEPROM or FLASH-EEPROM memory with tunnel oxide protection, as claimed in Claim 8. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Fig.1 shows a cross section of a known flash-EEPROM memory cell; Fig.2 shows an equivalent electric diagram of a flash-EEPROM memory array in accordance with the present invention; Fig.3 shows a cross section along line III-III in Fig.1 of the memory according to the present invention; Fig.s 4-9 show cross sections as in Fig.3 at various steps in the fabrication of the memory. Number 15 in Fig.2 indicates a flash-EEPROM memory array comprising a number of cells 1, each having the structure shown in the Fig.1 section. In known manner, cells 1 are arranged in lines and columns, and present control gate regions connected to word lines WL0-WL2 defining the lines of the array; drain regions connected to bit lines BL0-BL4 defining the columns of the array; and source regions connected to source lines SL (parallel to the word lines). According to the present invention, a diode is inserted between the gate region of cells 1 and substrate 2, for discharging the charges which would otherwise result in a dangerous increase in the potential of the control gate region. In particular (Fig.2), each word line WL0, WL1, ... is connected to the cathode of a diode D, the anode of which is connected to substrate 2 shown by the ground symbol in the diagram. Diodes D may be implemented as shown in the Fig.3 cross section (perpendicular to the Fig.1 section), in which, for the sake of simplicity, the same numbering system is used as in Fig.s 1 and 2. Fig.3 is a cross section of the last cell 1 of one of the lines of array 15, next to respective line diode D, and shows P type substrate 2; channel region 5 defined by two thick field oxide portions 20, one of which separates cell 1 from diode D; thin tunnel oxide layer 8a; a polycrystalline silicon layer 21 defining floating gate region 6; a layer 22 of dielectric material defining oxide portion 8b; a polycrystalline silicon layer 23 and silicide (WSi 2 ) layer 24 defining the control gate region 7 of cell 1 and a word line WL; and a thick oxide layer 25 defining, together with dielectric layer 22, the outer portion of oxide 8 in Fig.1. As shown in Fig.3, substrate 2 also houses an N+ type region 29 surrounded by field oxide regions 20 and facing substrate surface 10 where it electrically contacts polysilicon and silicide layers 23 and 24 defining the word line. To connect region 29 electrically to control gate regions 7 of the cells on the same line, layers 23 and 24 are extended further as compared with known memories. N+ type region 29 forms a PN junction with substrate 2, and so defines the cathode of diode D, the anode of which is defined by the substrate. In the fabrication process, diodes D must be formed before patterning the control gate region using the respective gate mask, and the process parameters must be so selected that the breakdown voltage of diodes D is properly tuned to the operating conditions of the cells, so as to be effective when required, but without interfering with normal operation of the memory array. More specifically, the breakdown voltage of the diodes must be higher than the maximum voltage applied to the control gate during programming, and less than the breakdown voltage of the tunnel oxide divided by the coupling factor of the control gate. By way of example, Fig.s 4-9 show a number of process steps for forming memory array 15 including protection diodes D. The process presents the same steps as in the standard process up to growth of the tunnel oxide of the memory cells, as shown in Fig.4, which shows P type substrate 2, field oxide regions 20 and tunnel oxide 8a. The active area mask must of course be modified slightly to provide for the active areas of diodes D. At this point, the first polysilicon layer 21 is deposited, and, as in known processes, is then patterned. In this case in particular, layer 21 is also removed from the active area in which diode D is to be formed, to give the structure shown in Fig.5. Dielectric layer 22 is then deposited and patterned so as to also remove it from the diode area. In this step, thin oxide layer 8a over the active area of the diode is etched and removed, as shown in Fig.6. This is followed by gate oxidation of the peripheral transistors (not shown) of the control circuits of the memory array, causing the formation, on the active region of the diode, of an oxide layer 32 (Fig.7) which is then removed using a mask 33 covering the whole of the memory array and the circuit transistors (Fig.8). The second polysilicon layer 23 is then deposited and doped in the usual manner, as shown by arrows 34, and the doping species employed also penetrate inside substrate 2 at the active areas of the diodes to produce regions 29 (Fig.9). At this point, diodes D are complete, and may begin operating for removing any excess charges from polysilicon layer 23. The rest of the process comprises the usual steps (not shown) of depositing silicide layer 24 (Fig.3), patterning the gate regions of the circuit transistors and the cells, forming the drain and source regions, and forming the protective layers and connections. The diodes therefore provide for preventing the control gate region of each cell from reaching a potential which may damage the tunnel oxide. Before such a potential is reached, in fact, the diodes become conductive and discharge any excess charges, which may result in an increase in potential in relation to the substrate, thus ensuring a safe maximum potential of the control gate regions. By virtue of the diodes being formed prior to patterning the polysilicon layer forming the control gate regions together with the silicide layer, the diodes are effective prior to performing the critical process steps which may result in an increase in potential of the polysilicon layer. The solution according to the present invention thus provides for solving the problem of damage or even breakage of the thin tunnel oxide in an extremely straightforward manner. Moreover, the increase in the area of the memory as a whole, due to provision of diodes D, is negligible, by virtue of one diode being sufficient for all the cells connected to the same word line. As regards the fabrication process, the diodes may be formed using only one additional mask, if the process does not allow of buried contacts; and, as already stated, the diodes may be fabricated simultaneously with the memory array using well known, highly controllable process steps. To those skilled in the art it will be clear that changes may be made to the memory as described and illustrated herein without, however, departing from the scope of the present invention. In particular, the solution described may, as stated, also be applied to other types of memory cell providing they present a thin tunnel oxide layer.

Claims: A nonvolatile EPROM, EEPROM or flash-EEPROM memory (15) comprising an array of memory cells arranged in lines and columns (1) and having control gate regions (7) capable of reaching an operating potential, the control gate regions (7) of the cells on a same line being formed by a semiconductor material line (23); floating gate regions (6); and tunnel oxide layers (8a) separating said floating gate regions from a substrate (2) of semiconductor material; said tunnel oxide layers (8a) having a breakdown potential and said control gate and floating gate regions presenting a coupling factor; characterized in that each said semiconductor material line (23) is in direct electrical contact with respective portions of said semiconductor material substrate (2) defining selectively-opened conductive paths (D) interposed between said control gate regions (7) and a ground region (2), said conductive paths (D) being open for arresting the passage of electric charges when said control gate regions (7) are brought to said operating potential, and being closed for permitting the passage of electric charges between said control gate regions (7) and ground (2) upon said control gate regions reaching potentials higher than said operating potential and lower than said breakdown potential divided by said coupling factor. A memory as claimed in Claim 1, characterized by the fact that said conductive paths comprise diodes (D) interposed between said control gate regions (7) and ground (2). A memory as claimed in Claim 2, characterized by the fact that it comprises one diode (D) for each line of semiconductor material. A memory as claimed in Claim 2 or 3, characterized by the fact that said substrate (2) defines said ground regions. A memory as claimed in Claim 4, wherein said substrate (2) is a P type; characterized by the fact that said substrate defines the anode region of said diodes (D), and houses a number of N type regions (29) defining the cathode regions of said diodes; said N type regions facing the larger surface (10) of said substrate (2), and being electrically connected to said control gate regions (7). A memory as claimed in Claim 5, characterized by the fact that each said line of semiconductor material (23) is in direct electrical contact with a respective said N type region (29). An intermediate structure of a nonvolatile memory as claimed in any one of claims 1 to 5, comprising said substrate of semiconductor material (2); thick oxide regions (20) delimiting active areas of said memory (15); thin tunnel oxide gate portions (8a) overlying said active areas of the memory cells (1); semiconductor material floating gate regions (21) overlying said thin tunnel oxide portions; dielectric material portions (22) overlying said semiconductor material floating gate portions; and a semiconductor material gate region unpatterned layer (23) covering the whole of the larger surface of said structure and reaching, in use, an operating potential, characterized in that said semiconductor material gate region unpatterned layer (23) is in direct electrical contact with respective portions of said semiconductor material substrate (2) defining conductive paths (D) to said substrate (2), the conductive paths being interposed between said unpatterned layer (23) and a ground region (2), and being such as to be open for arresting the passage of electric charges when said control gate regions (7) are brought to said operating potential, and to be closed for permitting the passage of electric charges between said semiconductor material gate region unpatterned layer (23) and ground (2) upon said semiconductor material gate region unpatterned layer (23) reaching potentials higher than said operating potential and lower than said breakdown potential divided by said coupling factor. A method of fabricating a nonvolatile EPROM, EEPROM or flash-EEPROM memory with tunnel oxide protection; said memory (15) comprising an array of memory cells (1) having control gate regions (7) capable of reaching an operating potential, floating gate regions (6), and tunnel oxide layers (8a) separating said floating gate regions from a substrate (2) of semiconductor material; said tunnel oxide layers (8a) having a breakdown potential; and said control gate regions (7) and floating gate regions (6) presenting a coupling factor; the method comprising the steps of: forming thick oxide insulating regions (20) in said substrate (2) of semiconductor material; forming a thin tunnel oxide layer (8a) on selective portions of the larger surface (10) of said substrate; depositing a first layer of semiconductor material (21) for forming said floating gate regions (6); depositing a layer of dielectric material (22); and depositing and patterning a second layer of semiconductor material (23) for forming said control gate regions (7) characterized by the step of forming selectively-opened conductive paths (D) interposed between said control gate regions (7) and a ground region (2) such as to be open for arresting the passage of electric charges when said control gate regions is brought to said operating potential, and being closed for permitting the passage of electric charges between said control gate regions (7) and ground (2) upon said control gate regions reaching potentials higher than said operating potential and lower than said breakdown potential divided by said coupling factor, in that said step of forming said conductive paths (D) is performed prior to said step for patterning said second layer of semiconductor material (23), and in that said second layer of semiconductor material is formed in direct electrical connection with said ground region.

Inventors: CAPPELLETTI PAOLO GIUSEPPE, CAPPELLETTI, PAOLO GIUSEPPE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H01L27
IPC codes: H01L29, H01L27, G11C5, H01L21