Nonvolatile semiconductor device

Abstract: A nonvolatile semiconductor device according to the present invention comprises a memory cell array (14) having memory cells arranged in matrix, each memory cell being composed of floating gate electrode (5a, 5b, 5c, 5d), control gate electrode (9a, 9b), a source diffusion layer (2), and drain diffusion layer (3a, 3b, 3c, 3d), tunnel oxide films (6a, 6b, 6c, 6d) formed on the side walls of the floating gate electrodes on the side where adjacent memory cells, sharing the control gate electrode, face each other, and an erasure gate electrode (7) formed so as to sandwich the tunnel oxide films between the floating gate electrodes and itself, and electrically connected to the source diffusion layer at nearly an equal distance from the adjacent memory cells sharing the control gate electrode.

Description: This invention relates to improvements in an electrically reprogrammable nonvolatile memory device having a gate electrode structure of two or more layers. A Flash EEPROM consisting of a single transistor/cell, which are replacing ultraviolet erasable EPROM, have been attracting attention these days in the field of nonvolatile semiconductor memory devices. Fig. 1 shows a cell transistor of the Flash EEPROM having a two-layer polycrystalline silicon gate structure. In a p-type silicon substrate 21, a source diffusion layer 22 and a drain diffusion layer 23 are formed. The depth of the source diffusion layer 22 is such that its junction break down voltage is sufficiently larger than a voltage applied to the source diffusion layer 22 during erasure operation. Formed on the p-type silicon substrate 21 between the source diffusion layer 22 and drain diffusion layer 23 is a gate oxide film 24, on which a floating gate electrode 25 is formed. On the floating gate electrode 25, an insulating film 26 is formed, on which a control gate 27 is formed. Operation of the cell transistor of the Flash EEPROM will be explained hereinafter in Fig. 1. As with ultraviolet erasable EPROM, the writing of information is achieved by applying a high voltage to the control gate electrode 27 and drain diffusion layer 23, injecting hot electrons generated by the high electric field in Silicon into the floating gate electrode 25, and storing therein to cause the transistor's threshold voltage value to increase. The erasing of information is done by applying a high voltage (hereinafter, referred to as erasure voltage) required to erase to the source diffusion layer 22, applying the GND potential on the control gate electrode 27. In this condition, an F.N tunneling current flows through the gate oxide film 24, extracting the stored electrons from the floating gate electrode 25 to the source diffusion layer 22. The cell transistors of Flash EEPROM in Fig. 1 are generally constructed in array form and all the cell transistors in the array have the source diffusion layer 22 in common. This enables simultaneous erasure of the information stored in all cell transistors. In addition, the common source diffusion layer 22 makes the cell area almost the same as that for the ultraviolet erasable EPROM. However, to set the erasure voltage at a practical value, e.g., 12.5 V, it is necessary to make the gate oxide film 24 as thin as approximately 100 Å. This raises the probability of more defects developing in the gate oxide film 24, resulting in poorer production yields. The thinner gate oxide film causes gate-induced junction leakage current, together with the F.N tunneling current, to flow towards the substrate 21 and source diffusion layer 22. This prevents erasure of information from being performed sufficiently, interfering with the stable memory operation. Such draw backs can be overcome by providing an internal charge pump circuit that has a very large power capacity. The size of the internal step-up circuit, however, becomes extremely large, making it impossible to obtain a practical chip size. An LSI external power supply could supply the erasure voltage (12.5 V) but this would go against the request to simplify external power supplies into 5 V only. A semiconductor memory device (EP-A-0 282 022) contains a number of memory cells each of which has a floating gate electrode, an erasure gate electrode, and a control gate electrode. An insulating film is interlaid between the erasure gate electrode and the control gate electrode. Further, a silicon oxide film is disposed between side walls of the erasure gate electrode and the floating gate electrode in a sandwich-like manner. Source and drain diffusion layers are formed on the element region surfaces located at both sides of floating gate, respectively. A further non-volatile semiconductor memory (JP-A-60-117 656) has an erasure gate electrically connected to a source of field effect transistor. It is the object of the present invention to provide a non-volatile semiconductor device that is as reliable as that of ultraviolet erasable EPROMs and enables electrical erasure with a single transistor cell. To achieve the object, a non-volatile semiconductor device is provided as described in claim 1. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view for a cell transistor of a conventional nonvolatile semiconductor device; Fig. 2 is a sectional view for a nonvolatile memory device as a first embodiment of the present invention; Fig. 3 is a sectional view taken along line A-A′ in Fig. 2; Fig. 4 is a sectional view taken along line B-B′ in Fig. 2; Fig. 5 is a sectional view taken along line C-C′ in Fig. 2; Fig. 6 is a sectional view of a nonvolatile semiconductor device as a second embodiment of the present invention; Fig. 7 is a sectional view taken on line D-D′of Fig. 6; Fig. 8 is a sectional view taken on line E-E′of Fig. 6; Fig. 9 is a sectional view for a nonvolatile semiconductor device as a third embodiment of the present invention; Fig. 10 is a sectional view taken along line F-F′ of Fig. 9; Fig. 11 is a sectional view taken along line G-G′ of Fig. 9; Fig. 12 is a sectional view taken along line H-H′ of Fig. 9; and Fig. 13 is a sectional view for a nonvolatile memory device as a fourth embodiment of the present invention. Embodiments of the present invention will be explained in detail, referring to the accompanying drawings. Like parts are shown by corresponding reference characters throughout the several views of the drawings. Fig. 2 shows a nonvolatile semiconductor device as a first embodiment according to the present invention. Fig. 3 is a sectional view taken along line A-A′ in Fig. 2. Fig. 4 is a sectional view taken along line B-B′ in Fig. 2. Fig. 5 is a sectional view taken along line C-C′ in Fig. 2. In the surface region of a p-type silicon substrate 1, a source diffusion layer 2 is formed. Also formed in the surface region of the p-type silicon substrate 1 are drain diffusion layers 3a, 3b, 3c, and 3d. On the channel regions between the source diffusion layer 2 and the drain diffusion layers 3a, 3b, 3c, and 3d, gate oxide films 4a, 4b, 4c, and 4d are formed. On the gate oxide films 4a, 4b, 4c, and 4d, floating gate electrodes 5a, 5b, 5c, and 5d are formed so that their end portions, except for those facing the source diffusion layer 2 and drain diffusion layers 3a, 3b, 3c, and 3d, are positioned over a field oxide film 13. The thickness of the field oxide film 13 is made so as to be sufficiently larger than that of the gate oxide films 4a, 4b, 4c, and 4d which put the floating gate electrodes 5a, 5b, 5c, and 5d in an electrically floating state. For example, the ratio of the thickness of the field oxide film 13 to that of the gate oxide films 4a, 4b, 4c, and 4d are set at 10:1 or more. On the floating gate electrodes 5a, 5b, 5c, and 5d, insulating films 8a and 8b are formed. Formed on the insulating films 8a and 8b are control gate electrodes 9a and 9b. The control gate electrode 9a is shared by the floating gate electrodes 5a and 5c, while the control gate electrode 9b is shared by the floating gate electrodes 5b and 5d. The source diffusion layer 2 is shared by memory cells that have the control gate electrodes 9a and 9b in common. On the side where adjacent memory cells, sharing the control gate electrodes 9a and 9b, face each other, tunnel oxide films 6a, 6c, and 6b, 6d are formed on the side walls of the floating gate electrodes 5a, 5c, and 5b, 5d. An erasure gate electrode 7 is formed so as to sandwich the tunnel oxide films 6a and 6c between the floating gate electrodes 5a and 5c and itself, respectively. The erasure gate electrode 7 is shared by adjacent cells having the control gate electrodes 9a and 9b in common, while it is also shared by adjacent cells having the source diffusion layer 2 in common. Furthermore, the erasure gate electrode 7 is placed between the floating gate electrodes 5a and 5c and between the floating gate electrodes 5b and 5d. That is, the erasure gate electrode 7 is shared by four memory cells: two adjacent cells sharing the control gate electrodes 9a and 9b, and the other two adjacent cells sharing the source diffusion layer 2. Four memory cells sharing the erasure gate electrode 7 contains the source diffusion layer 2, drain diffusion layers 3a, 3b, 3c, and 3d, gate oxide films 4a, 4b, 4c, and 4d, floating gate electrodes 5a, 5b, 5c, and 5d, and control gate electrodes 9a and 9b. Those memory cells are arranged in matrix to form a memory cell array 14. The erasure gate electrode 7 can be formed in a self-aligning manner between the floating gate electrodes 5a and 5b and between the floating gate electrodes 5c and 5d. A part of the erasure gate electrode 7 is extended over the source diffusion layer 2 and electrically connected to the latter at an nearly equal distance from the adjacent cells sharing the control gate electrodes 9a and 9b. The location at which the erasure gate electrode 7 and source diffusion layer 2 are connected can be determined on the basis of the maximum potential applied to the erasure gate electrode 7. That is, as shown in Fig. 3, the location is selected so that the edge of a depletion layer developed in the source diffusion layer 2 (indicated by broken line X) does not touch on reach the floating gate edges (5a through 5d) adjacent to the source junction at the channel regions. The source diffusion layer 2 contains a low-concentration region 2a and a high-concentration region 2b. The low-concentration region 2a is formed in a portion adjoining the channel region, while the high-concentration region 2b is formed so that the erasure gate electrode 7 includes at least a portion at which the erasure gate electrode 7 connects with the source diffusion layer 2. It is desirable that the low-concentration region 2a is set at a concentration on the order of approximately 10¹⁸ /cm³ and the high-concentration region 2b is set at a concentration one order or more higher than that of the low-concentration region 2a. A CVD oxide film 10a and 10b is formed to cover the floating gate electrodes 5a, 5b, 5c, and 5d and control gate electrodes 9a and 9b. Over the surface, a BPSG film 11 is formed which has contact holes above the drain diffusion layers 3a, 3b, 3c, and 3d. Through the contact holes, bit lines 12a and 12b are formed so as to electrically connect with the drain diffusion layers 3a, 3b, 3c, and 3d. The floating gate electrodes 5a, 5b, 5c, and 5d and control gate electrodes 9a and 9b can be formed using polysilicon, silicide, or a stacked structure of them, whereas the tunnel oxide films 6a, 6b, 6c, and 6d can be made up of polysilicon oxide film. The tunnel oxide films 6a, 6b, 6c, and 6d are formed well on the side walls of the floating gate electrodes 5a, 5b, 5c, and 5d by, for example, thermal oxidation or chemical vapor deposition. The erasure gate electrode 7 can be composed of, for example, polysilicon. In the first embodiment, applying a suitable potential to the erasure gate electrode 7 causes charges in the floating gates 5a, 5b, 5c, and 5d to extract to the erasure gate 7 through the tunnel oxide films 6a, 6b, 6c, and 6d. This allows the nonvolatile semiconductor device to electrically reprogram information. In this configuration, the tunnel oxide films 6a, 6b, 6c, and 6d are formed between the erasure gate electrode 7 and the side walls of the floating gate electrodes 5a, 5b, 5c, and 5d, which makes it possible to form the tunnel oxide films 6a, 6b, 6c, and 6d and the gate oxide films 4a, 4b, 4c, and 4d separately. This allows optimization of the tunnel oxide films 6a, 6b, 6c, and 6d according to erasure voltage. Since thinner gate oxide films 4a, 4b, 4c, and 4d are not needed for improvements in the erasure characteristics, the junction and surface breakdown voltages of the source diffusion layer 2 can be made sufficiently high. Further, no effect of the concentration in the source diffusion layer 2 adjoining the channel on the erasure characteristics sufficiently suppresses junction leakage current caused by a band-to-band tunneling phenomenon. Namely, use of a high voltage (e.g., 12.5 V) produced by the internal charge pump circuit as the erasure voltage enables operation of the nonvolatile semiconductor device from a single power supply (e.g., 5 V). While in the first embodiment, the erasure gate electrode 7 is electrically connected to the source diffusion layer 2, it may be electrically connected to the drain diffusion layers 3a, 3b, 3c, and 3d. In this case, the concentration in the drain diffusion layer 2 adjacent to the channel can be optimized based on the writing characteristics as with the first embodiment. Fig. 6 illustrates a nonvolatile semiconductor device as a second embodiment of the present invention. Fig. 7 is a sectional view taken along line D-D′ in Fig. 6 and Fig. 8 is a sectional view taken on line E-E′ of Fig. 6. In the surface region of a p-type silicon substrate 1, a source diffusion layer 2, and drain diffusion layers 3a, 3b, 3c, and 3d are formed. Above the channel regions between the source diffusion layer 2 and drain diffusion layers 3a, 3b, 3c, and 3d, gate oxide films 4a, 4b, 4c, and 4d are formed. On the gate oxide films 4a, 4b, 4c, and 4d close to the drain diffusion layers 3a, 3b, 3c, and 3d, floating gate electrodes 5a, 5b, 5c, and 5d are formed. On the floating gate electrodes 5a, 5b, 5c, and 5d, insulating films 8a and 8b are formed. On the insulating films 8a and 8b and above the gate oxide films 13a, 13b, 13c, and 13d close to the source diffusion layer 2, control gate electrodes 9a and 9b are formed so as to have what is called an offset structure. On the side where the adjacent memory cells, sharing the control gate electrode 9a, face each other, tunnel oxide films 6a and 6c are formed on the side walls of the floating gate electrodes 5a and 5c. An erasure gate electrode 7 is formed so as to sandwich the tunnel oxide films 6a and 6c between the floating gate electrodes 5a and 5c and itself, respectively. A part of the erasure gate electrode 7 is extended to the source region to connect to the source diffusion layer 2 at an nearly equal distance from the adjacent memory cells sharing the control gate electrode 9. The second embodiment has an offset structure consisting of the control gate electrodes 9a and 9b formed above the channel regions close to the source diffusion layer 2. With this arrangement, electrical conduction of the memory cell channels can be prevented even when overerasure occurs during the movement of charges from the floating gate electrodes 5a, 5b, 5c, and 5d to the gate electrode 7. In this configuration, sufficiently thin tunnel oxide films 6a, 6b, 6c, and 6d are formed between the floating gate electrodes 5a, 5b, 5c, and 5d and the erasure gate electrode 7, which offers almost the same effects as in the first embodiment. While in the second embodiment, the erasure gate electrode 7 is electrically connected to the source diffusion layer 2, it may be electrically connected to the drain diffusion layers 3a, 3b, 3c, and 3d. Fig. 9 shows a nonvolatile semiconductor device as a third embodiment of the present invention. Fig. 10 is a sectional view taken along line F-F′ in Fig. 9. Fig. 11 is a sectional view taken on line G-G′ in Fig. 9. Fig. 12 is a sectional view taken on line H-H′ in Fig. 9. In the surface region of a p-type silicon substrate 1, a source diffusion layer 2 and drain diffusion layers 3a, 3b, 3c, and 3d are formed. Above the channel regions between the source diffusion layer 2 and drain diffusion layers 3a, 3b, 3c, and 3d, gate oxide films 4a, 4b, 4c, and 4d are formed. On the gate oxide films 4a, 4b, 4c, and 4d, floating gate electrodes 5a, 5b, 5c, and 5d are formed. Above the gate electrodes 5a, 5b, 5c, and 5d, control gate electrodes 9a and 9b are formed via insulating film 8a and 8b. On the side where the adjacent cells sharing the control gate electrodes 9a and 9b face each other, tunnel oxide films 6a, 6b, 6c, and 6d are formed in a specific region containing the side walls of the floating gate electrodes 5a, 5b, 5c, and 5d. An erasure gate electrode 7 is formed so as to sandwich the tunnel oxide films 6a, 6b, 6c, and 6d between the floating gate electrodes 5a, 5b, 5c, and 5d and itself. The erasure gate electrode 7 is shared by the floating gate electrodes 5a, 5b, 5c, and 5d. A part of the erasure gate electrode 7 is extended over the source diffusion layer 2 and connected to the latter at an equal distance from the adjacent memory cells sharing the control gate electrodes 9a and 9b. In this arrangement, the tunnel oxide films 6a, 6b, 6c, and 6d are formed between the erasure gate electrode 7 and the floating gate electrodes 5a, 5b, 5c, and 5d. A part of the erasure gate electrode 7 is connected to the source diffusion layer 2 at an nearly equal distance from the memory cells sharing the control gate electrodes 9a and 9b. Therefore, this configuration provides similar effects to those described in the first embodiment. While in the third embodiment, the erasure gate electrode 7 is electrically connected to the source diffusion layer 2, it may be electrically connected to the drain diffusion layers 3a, 3b, 3c, and 3d. Fig. 13 shown a nonvolatile semiconductor device as a fourth embodiment of the present invention. The control gate electrode 9a is shared by the floating gate electrodes 5a and 5c, while the control gate electrode 9b is shared by the floating gate electrodes 5b and 5d. The erasure gate electrode 7a is shared by adjacent cells having the drain diffusion layer 3a in common. A part of the erasure gate electrode 7a is extended over the drain diffusion layer 3a and electrically connected to the latter at an nearly equal distance from the adjacent cells sharing the drain diffusion layer 3a. A part of the erasure gate electrode 7b is extended over the drain diffusion layer 3c and electrically connected to the latter at an nearly equal distance from the adjacent cells sharing the drain diffusion layer 3c. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A nonvolatile semiconductor device comprising: a memory cell array (14) having memory cells arranged in matrix, each memory cell being composed of a floating gate electrode (5a, 5b, 5c, 5d), a control gate electrode (9a, 9b), a source diffusion layer (2) and a drain diffusion layer (3a, 3b, 3c, 3d); tunnel oxide films (6a, 6b, 6c, 6d) each formed on the side walls of said floating gate electrode on the side where adjacent cells, sharing said control gate electrode, face each other; and an erasure gate electrode (7) formed so as to sandwich said tunnel oxide films between said floating gate electrodes and itself; characterized in that said erasure gate electrode (7) is electrically connected to said source diffusion layer (2) or said drain diffusion layer at a nearly equal distance from the adjacent memory cell sharing said control gate electrode; and in that said source diffusion layer (2) or drain diffusion layer contacting said erasure gate electrode is composed of a stack of first and second impurity regions of different concentrations, the concentration of the first impurity region (2a) touching a channel region just below said floating gate electrode being lower than that of the second impurity region (2b) just below said erasure gate electrode. The nonvolatile semiconductor device according to claim 1, characterized in that said source diffusion layer (2) is shared by memory cells having said control gate electrode in common. The nonvolatile semiconductor device according to claim 1, characterized in that said tunnel oxide film (6a, 6b, 6c, 6d) is formed in a region containing the side wall of said floating gate electrode on the side where memory cells sharing said control gate electrode face each other. The nonvolatile semiconductor device according to claim 1, characterized in that charges in said floating gate move through said tunnel oxide film to said erasure gate electrode when a specific potential is applied to said erasure gate electrode. The nonvolatile semiconductor device according to claim 1, characterized in that said erasure gate electrode (7) is shared by adjacent memory cells having said control gate electrode in common. The nonvolatile semiconductor device according to claim 1, characterized in that said erasure gate electrode (7) is shared by adjacent memory cells in a row-which have said control gate electrode in common, and by adjacent memory cells in a column which have said source diffusion layer in common. The nonvolatile semiconductor device according to claim 1, characterized in that said floating gate electrode (5a, 5b, 5c, 5d) is made from polysilicon or silicide, or made up of a stacked structure of polysilicon and silicide. The nonvolatile semiconductor device according to claim 1, characterized in that said control gate electrode (9a, 9b) is made from polysilicon or silicide, or made up of a stacked structure of polysilicon and silicide. The nonvolatile semiconductor device according to claim 1, characterized in that said erasure gate electrode (7) is made from polysilicon. The nonvolatile semiconductor device according to claim 1, characterized in that said tunnel oxide film (6a, 6b, 6c, 6d) is made from polysilicon film. The nonvolatile semiconductor device according to claim 1, characterized in that each of said memory cells is composed of: a semiconductor substrate (1); a source diffusion layer (2a, 2b) and drain diffusion layer (3a, 3b, 3c, 3d) formed in the surface region of said semiconductor substrate; channel region between said source diffusion layer and drain diffusion layer; first insulating film (4a, 4b) formed above said channel region; floating gate electrode (5a, 5b, 5c, 5d) formed on said first insulating film and put in an electrically floating state; second insulating film (8a, 8b) formed on said floating gate electrode; and control gate electrode (9a, 9b) formed on said insulating film. The nonvolatile semiconductor device according to claim 11, characterized in that said floating gate electrode (5a, 5b, 5c, 5d) is formed above said channel region close to said drain diffusion layer; and said control gate electrode (9a, 9b) is formed above said channel region close to said source diffusion layer.

Inventors: YOSHIKAWA KUNIYOSHI C O INTELL, YOSHIKAWA, KUNIYOSHI, C/O INTELLECTUAL PROP. DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01L27
IPC codes: H01L27, H01L21, H01L29