Semiconductor memory device having non-volatile memory transistors

Abstract: A semiconductor memory device having a redundancy memory cell array is disclosed. UV-PROM's are employed as a programming means in the redundancy control section. A first impurity region (17) is formed in the substrate (1) and the control gate electrode (61) of the UV-PROM is led out through the first impurity region. A second impurity region (76) is formed in the substrate (1) and crosses the first impurity region, and the floating gate electrode (12) of the UV-PROM is covered by a metallic film which is contacted to the second impurity region.

Description: Background of the Invention: The present invention relates to a semiconductor memory device having a plurality of non-volatile memory transistors of electrically writable and erasable by ultra-violet rays (hereinafter called as UV-PROM), and more particularly to a memory device in which a redundancy memory cell array is incorporated. UV-PROM's are widely used in a semiconductor memory device as described in U.S.P.4,527,259 and U.S.P.4,727,221. On the other hand, a semiconductor memory device having a redundancy memory cell array is shown, for example, in U.S.P. 4,281,398 and U.S.P. 4,428,068, in which a main memory cell section, a redundancy memory cell section and a redundancy control section for diciding whether the redundancy memory cell section is to be used or not with respect to received address signals, are provided. As a program means in the redundancy control section by which an information corresponding to a defect line produced in the main memory cell section is set, polysilicon fuses, UV-PROM's, EE-PROM's (electrically erasable PROM), polysilicon layers having a high electrical resistivity, etc. are employed. When UV-PROM's are used as memory cells in the main memory cell section and in the redundancy memory cell section, UV-PROM's are beneficially used in the redundancy control section as the program means in view of the high reliability of the programming and the easiness of manufacturing because they can be formed with the memory cells in the memory sections. However, the UV-PROM's in the redundancy control section must be shielded from ultra-violte rays so that the programmed information stored in the flating gate electrodes in the UV-PROM's in the redundancy control section is not eliminated during erasing operations for the UV-PROM's as the memory cell's in the memory cell sections. Therefore, in the prior art, the UV-PROM in the redundancy control section is covered by a metal layer. However, the metal layer can not be provided at a section where the control gate electrode is led out, and an aperture of the metal layer is inevitably formed there. Consequently, some ultra-violet rays are introduced into the metal cover structure through the aperture, and unfavorably eliminate the information, that is, unfavorably release electrons stored in the floating gate electrode. From EP-A-0 227 549 a semiconductor memory device is known in which the leading outstructure of the control gate which is prolonged to minimize the light insertion through the insulator film under and/or on the control gate wiring is improved. The minimization is achieved by modifying the let out path of the control gate as a certain time passes. However, by this modification no complete shield from light is achieved because this leading out path cannot completely surround the transistor. Summary of the Invention: Accordingly, an object of the present invention is to provide a semiconductor memory device in which an UV-PROM is completely shielded from ultra-violet rays irradiating toward the whole chip area. According to a feature of the present invention, there is provided a semiconductor memory device as defined in the appended claim. Brief Description of the Drawings: Fig. 1 is a layout diagram of one example of a semiconductor memory device having a redundancy memory cell section in a single semiconductor substrate; Fig. 2 is a circuit diagram showing one example of a main memory cell section and a part of a redundancy memory cell section in Fig. 1; Fig. 3 is a circuit diagram showing one example of a part of a redundancy control section; Fig. 4A is a plan view showing a prior art structure, and Figs. 4B and 4C are cross-sectional views taken along lines B-B′ and C-C′ in Fig. 4A as viewed in the direction of arrows, respectively; and Fig. 5A is a plan view showing an embodiment of the present invention, and Figs. 5B and 5C are cross-sectional views taken along lines B-B′ and C-C′ in Fig. 5A as viewed in the direction of arrows, respectively. Description of the Present Invention: Referring to Fig. 1, a memory device in a silicon substrate 1 is disclosed. X-address signals 180 are inputted to an X-address buffer section 160, and from the X-address buffer section 160, X-address buffer signals 181 are sent to an X-decoder section 110. X-decoder signals 182 from the X-decoder section 110 are sent to a main memory cell section 100 to select one of word lines corresponding to the X-address signals. Also, Y-address signals 184 are inputted to a Y-address buffer section 150, and from the Y-address buffer section 150, Y-address buffer signals 185 are sent to a Y-decoder section 140. Y-decoder signals 186 from the Y-decoder section 140 are sent to a sense amplifier 120 to select one of switching transistor connecting a selected digit line corresponding the Y-address signals. Output signals 187 from the memory array are sent to the sense amplifier section 120, and from the section 120, signals 188 are sent to an output circuit section 130 from which output signals 189 are outputted. Further, a redundancy memory cell section 101, a redundancy X-decoder section 111 and a redundancy control section 170 are incorporated in a single chip, that is, in the substrate 1. From the X-address buffer section 160, X-address buffer signals 191 which are the same signals as the signals 181 are sent to the redundancy control section 170, and from the section 170, control signals 192 and 193 are sent to the X-decoder section 110 and to the redundancy X-decoder section 111, respectively. When a defect memory cell exist in a column line 90 in the main memory cell section 100 and the X-address signals 180 are of selecting the line 90, the operation of the X-decoder section 110 is stopped by the control signal 192 and the redundancy X-decoder section 111 is operated by the control signal 193 to select one of word lines in the redundancy memory cell section 101 by redundancy X-decoder signals 194. Namely, the defected word line 90 and the decoder circuit 91 connected to the line 90 is electrically replaced to one of word lines in the redundancy memory cell section and a decoder circuit in the redundancy decoder section 111 connected to the selected word line in the redundancy memory cell. Reversely, when any defect memory cell is not provided in a selected word line in the main memory cell section 100, the selected word line and a decoder circuit in the X-decoder section 110 are operated, and the redundancy memory cell section 101 and the redundancy X-decoder section 111 are not operated. Although the redundancy means for word lines including sections 170, 111, 101 is only disclosed in Fig. 1, the same redundancy means for digit lines can be added to or be replaced to the redundancy means for word lines. Referring to Fig. 2, in the main memory cell section 100, 256 word lines 60 (3 lines are exemplified) and 256 digit lines 70 (3 lines are exemplified) are provided, and a plurality of UV-PROM's 80 (9 PROM'S are exemplified) are arranged in a matrix form and connected to respective word and digit lines. With respect to the redundancy memory cell section 101, 2 to 3 word lines 60 and 256 digit lines 70 are provided, and the same UV-PROM's 80 are arranged and connected to the respective word and digit lines. Each of UV-PROM's includes a pair of N-type source and N-type drain regions 81, 82 formed in the P-type silicon substrate 1, one region being connected to a digit line 70 and the other being connected to a ground potential line, a floating gate electrode 83 and a control gate electrode 84 connected to a word line 60. In the writing operation, electrons are injected into the floating gate electrodes of the selected UV-PROM's by an avalanche injection method in which a PN junction break down is produced between the N-type region 81 and the P-type substrate 1 with applying positive voltage to the control gate electrode 84, or by a channel injection method in which a high positive voltage is applied to the N-type region 81 to induce a high electric field between regions 81, 82 with applying a positive voltage to the control gate electrode 84. In the erasing operation, ultra-violet rays are irradiated to the whole surface of the substrate 1 so that the electrons stored in the floating gate electrodes of the selected UV-PROM's in the main memory cell section and the redundancy memory section are released from the floating gate electrodes. Referring to Fig. 3, a plurality of circuits 50 are arranged, and each circuit includes enhancement type MOS transistors 30, 31, 32, a depletion type MOS transistor 33, and an UV-PROM 40 which is covered by a metal layer 41. The UV-PROM 40 includes N-type source, drain regions 81, 82, the floating gate electrode 83 and the control gate electrode 84 which are the same components as those in UV-PROM 80 in memory sections 100, 101. Therefore, the UV-PROM's 40 and 80 are simultaneously manufactured each other, and electrons are injected into the floating gate electrodes of the selected UV-PROM's by the avalanche injection method or the channel injection method mentioned above. The X-address buffer signals 191 (A₁ ... A k ) are inputted to the gate electrodes of the transistors 32 in respective circuits 50. In accordance to the defected word line 90 in the main memory cell section 100 (Fig. 1), the UV-PROM's 40 are selected, and electrons are injected into the floating gate electrodes of one group of UV-PROM's 40 through respective transistors 30 so that when X-address signals 180 which select the defect word line 90 are inputted, the proper control signals 192, 193 can be outputted from the redundancy control section 170 (Fig. 1). The electrical writing operations and the ultra-violet rays erasing operations are repeated in the main and redundancy memory cell sections 100, 101. However, the defect word line 90 once detected does not vary its position. Therefore, the the UV-PROM's 40 in the redundancy control section 170 are covered by metal layers 41 as indicated in dotted line in Fig. 3 so that electrons injected and stored in the floating gate are not easily erased during the erasing operations by ultra-violet rays in the memory sections 100, 101. Referring to Figs. 4A to 4C, a conventional UV-PROM 40 in the redundancy control section 170 is shown. A thick field silicon oxide layer 2 partially embedded is selectively formed in a major surface 22 of a P-type silicon substrate 1, and an N⁺-type drain region 8 and an N⁺-type source region 9 are formed. A first gate insulating film 18 of silicon oxide is formed on a channel region 20 between the source and drain regions 9, 8, and a floating gate electrode 12 of doped polysilicon is formed on the first gate insulating film 18 and extends on the field insulating layer 2 at both sides by 1 to 2 µm (Figs. 4A, 4C). On the floating gate electrode 12, a second gate insulating film 19 of silicon oxide is formed, and a control gate electrode 11 of doped polysilicon is formed on the second gate insulating film 19. A first N-type impurity region 17 is formed in the substrate 1, contacted to the drain region 8 and led out by a metal wiring layer 13 of a first level aluminum layer which is contacted to an N⁺-type contact region 6 of the N-type region 17 through a contact hole 15 formed in inter-ply insulating films 3, 3′. A P⁺-type impurity region 10 having higher impurity concentration than the substrate 1 and the first N-type impurity region 17, formed in the substrate, surrounding the source and drain regions 9, 8 and crossing the first N-type impurity region 17. The control gate electrode 11 extends on a part of the inter-ply insulating film 3 above the field insulating layer 2, crosses over the P⁺-type impurity region 10 and be led out outwardly (Fig. 4C). A contact hole 14 indicated by right-upward hatching in Fig. 4A is formed in the inter-ply insulating films 3, 3′ to expose the surface of the P⁺-type impurity region 10. As shown in Figs. 4A, 4C, at a section where the control gate electrode 11 is leading out,the contact hole 14 cannot be formed. A metal layer 4 of a second aluminum layer is formed over the floating and control gate electrodes via the insulating film 3′ and contacted to the P⁺-type impurity region 10 through the contact hole 14. The metal layer 4 is connected to the source region 9 through a contact hole 16, and a passivation insulating film 5 is entirely provided. Owing to the metal layer 4, unfavorable erasing of the information, for example of electrons stored in the floating gate electrode 12 by ultra-violet rays irradiation during the repeated erasing operations on the memory sections can be prevented in some extent. However, the metal layer 4 cannot be formed at the section where the control gate electrode 11 is led out, and an aperture 21 is inevitably provided at the side portion of the metal layer construction. Therefore, some ultra-violet rays enter in the metal layer structure through the aperture 21, travel in the structure by reflecting between the back surface, that is, the inner surface of metal layer 4 and the substrate; and finally reach to the floating gate electrode. Consequently, electrons stored in the floating gate electrode 12 are unfavorably released by the stray ultra-violet rays. To avoid this advantage, the length ℓ between the edge of the floating gate electrode and the aperture must be long with a straight plan shape or a zigzag plan shape and a metal layer is provided along the shape. However, in a practical memory device, a long length ℓ of 120 µm or more is necessary to eliminate the influence by the stray ultra-violet rays, obstructing to realize a high integrated density of the memory device. Referring to Figs. 5A to 5C, an UV-PROM 40 in an embodiment of the present invention installed in the redundancy control circuit will be explained. In Figs. 5A to 5C, the same components as those in Figs. 4A to 4C are indicated by the same reference numerals. A second N-type impurity region 76 having impurity concentration of 10¹⁶/cm³ and 5 µm width and 20 µm length is formed in the P-type silicon substrate 1 of 10¹⁵/cm³ impurity concentration, and N⁺-type contact regions 74 and 78 of high impurity concentration are provided in the end sections of the second N-type impurity region 76. Contact holes 75 and 79 are formed in a first level inter-ply insulating film 63 of silicon oxide to expose parts of the contact regions 74, 78. A metallic wiring layer 77 of a first level aluminum layer having 0.5 µm thickness is connected to the N⁺-type contact region 78 through the contact hole 79 and extends on the first level inter-ply insulating film 63, and a metallic wiring layer 73 made of the first level aluminum layer is connected to the N⁺-type contact region 74 through the contact hole 75 at one end portion, extends on the insulating film 63 and connected to a control gate electrode 61 of doped polysilicon at the other end through a contact hole 72 provided in a second level inter-ply insulating film 77 of silicon oxide. Therefore, the control gate electrode 61 formed on the second gate insulating film 19 is led out through the metallic layer 73, the second N-type impurity region 76 and the metallic layer 77. A P⁺-type impurity region 62 having a rectangular ring-like shape of 30 µm x 30 µm and having an impurity concentration higher than the P-type substrate 1 and than the first and second N-type impurity regions 17, 76 is formed in the substrate 1, surrounds completely the source, drain regions 9, 8 and crosses the first and second N-type impurity regions 17 and 76. A contact hole 65 is provided in the first level inter-ply insulating film, 63 to expose the P⁺-type regions 62 at the entire length thereof such that the exposing contact hole surrounds completely the source and drain regions as indicated in right-upward hatching in Fig. 5A. A first metal layer 64 made of the first level aluminum layer is formed on the insulating film 63, having a higher level than the control gate electrode 61 and contacted to the P⁺-type region 62 through the contact hole 65 at the full length such that the first metal layer 64 surrounds completely the floating gate electrode 12 and the control gate electrode 61. The first metal layer 64 is connected at the portion 64′ thereof to the source region 9 through a contact hole 69 provided in the insulating film 63. A contact hole 68 is provided in a third level inter-ply insulating film 66 of silicon oxide to expose the part of the first metal layer 64 at the entire length such that the exposing contact hole 68 has a ring-like shape as indicated by left-upward hatching in Fig. 5A. A second metal layer 67 of a second level aluminum layer of 1 µm thickness is formed on the insulating film 66, covers above the floating and control gate electrodes entirely and contacted to the first metal layer 64 at the entire contact hole 68. According to the present invention, the floating gate electrode 12 is completely sealed by the first and second metal layers 64, 67 and an aperture as in Fig. 4 is not provided. Therefore, during erasing operations for memory sections, ultra-violet rays irradiating toward the entire substrate never be reached to the floating gate electrode 12. Consequently, the informations stored in the floating gate electrode of the UV-PROM in the redundancy control section can keep the state beyond a long time with a high integrated circuit structure.

Claims: A semiconductor memory device comprising: a semiconductor substrate (1) of a first conductivity type; a non-volatile memory cell formed on said substrate (1) with a transistor, said transistor having a floating gate electrode (12) and a control gate electrode (61) formed on a channel region between a source (9) and a drain (8) region; a first means for leading out said control gate electrode (61); a second means for leading out said drain electrode, said second means including a first impurity region (17) of a second conductivity type opposite to said first conductivity type formed in said substrate (1), said first impurity region (17) having a first portion electrically connected with said drain region and a second portion (6) electrically connected to a first wiring layer (13); a third impurity region (62) of said first conductivity type formed in said substrate (1) and said first impurity region (17), said third impurity region (62) completely surrounding said transistor; and a metallic film (64, 67) completely covering said floating gate electrode (12) and said control gate electrode (61) and forming a contact (65) to said third impurity region (62); characterized in that said first means includes a second impurity region (76) of said second conductivity type formed in said substrate, said second impurity region having a first part (74) electrically connected with said control gate electrode (61) and a second part electrically connected to a second wiring layer (77), said third impurity region (62) crossing said second region (76) between said first and second part; and that the contact (65) of said metallic film (64, 67) to said third impurity region (62) completely surrounds said transistor.

Inventors: IWASHITA SHINICHI, IWASHITA, SHINICHI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1988
Main classification: H01L29
IPC codes: H01L21, H01L27, H01L29