EPROM erasable by UV radiation having redundant circuit

Abstract: A semiconductor device comprises a semiconductor substrate (21), source (2) and drain (5) regions, a first gate insulating film (28), a floating gate electrode (20), a second gate insulating film (29), a control gate electrode (1), a third insulating (24) film with a thickness substantially equal to that of the first gate insulating film (28), first and second polycrystalline silicon layers (9, 15), a first conductive layer (7), a second conductive (13) layer, a fourth insulating film (25) with a thickness substantially equal to that of the second gate insulating film (29), a third polycrystalline silicon layer (19) to cross the first and second polycrystalline silicon layers (9, 15), a fifth insulating film (27) covering the entire surface and having a contact hole (4) which exposes a part of the source region (2) and a part of the third polycrystalline silicon layer (19), respectively, a shield layer (18) formed on the fifth insulating film (27) and contacting with the parts of the source region (2) and the third polycrystalline silicon layer (19) through the contact hole (4), and third and fourth conductive layers (11, 17) contacting with an led from the first and second polycrystalline silicon layers (9, 15), respectively.

Description: BACKGROUND OF THE INVENTION The present invention relates to a semiconductor device and, more particularly, to a floating-gate EPROM (Electrically Programmable Read Only Memory) whose contents are erasable with UV (Ultra-Violet) radiation (called hereinafter "UV-EPROM"). As a UV-EPROM is increased in a memory capacity, it is equipped with a redundant circuit, similarly to a random access memory of a large memory capacity. The redundant circuit requires storage cells for storing redundant control information used for detecting address data designating a defect memory cell, to interchange the defect memory cell with a redundant memory cell. In the UV-EPROM, a floating-gate PROM (Programable Read Only Memory) cell is widely used as the redundant control information storage cells, which is the same in structure as EPROM memory cells, because it is convenient to electrically program the redundant control information. The redundant control information, however, must not be erased with UV radiation. Therefore, the PROM cell in the redundant circuit is required to be free from UV radiation. For this purpose, a metal shield layer is fabricated to cover and surround the PROM cell. However, the metal shield layer cannot completely cover and surround the PROM cell, because interconnection lines are led from the gate and drain thereof to program the redundant control information therein. The metal shield layer is isolated from the interconnection lines by an insulating film such as a silicon dioxide film, so that UV light propagate through the insulating film to reach the PROM cell. The redundant control information stored in the PROM cell is thereby erased slowly. If only slight UV light reaches the PROM cell in the redundant circuit, it takes a very long period of time to erase the redundant control information. In other words, a floating-gate type PROM cell, which is hardly erasable with UV light, is demanded. For this purpose, a prior art forms the metal shield layer over a wide area along the interconnection lines led from the gate and drain of the PROM cell. As a result, most UV light entering the insulating layer is absorbed during the passing therethrough. Only slight UV light can reach the PROM cell. However, this means that the redundant circuit occupies a large area on a semiconductor chip. From EP-A-0 227 549 is known a semiconductor memory device which employs a diffusion region to lead out the drain region beyond the periphery of the shield layer. For this reason, the total resistance of the drain region is increased to lower an operation speed. Moreover, the contact hole for the shield layer is not formed to cross the gate wiring layer, so that a desired ultraviolet light may reach the transistor through a gap between the shield layer and the gate wiring layer. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an improved UV-EPROM having a redundant circuit. Another object of the present invention is to provide a semiconductor device having a PROM cell which is hardly erasable with UV light without increasing an occupying area. A semiconductor device according to the present invention is described in claim 1. Thus, UV light enters only through the third and fourth insulating films and reaches the floating gate electrode. Since the third and fourth insulating films are substantially equal in thickness to the first and second gate insulating films, respectively, they are very thin. As a result, only slight UV light reaches the floating gate. Thus, an EPROM cell, which is hardly erasable with UV light, is realized without enlarging the occupying area thereof. Since the third and fourth insulating films and first, second and third polycrystalline silicon layers can be formed simultaneously with other films and layers constituting UV-EPROM, respectively, there occurs no increase in manufacturing steps. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which Fig. 1 is a partial plane view representative of an embodiment according to the present invention; Figs. 2A, 2B and 2C are cross-sectional views of the device shown in Fig. 1 along lines A-A′, B-B′ and C-C′, respectively; Figs. 3A to 3G are cross-sectional views representative of manufacturing steps of the device shown in Fig. 1 along line A-A′; Figs. 4A to 4G are cross-sectional views representative of manufacturing steps of the device shown in Fig. 1 along line C-C′; Fig. 5 is a partial plane view representative of another embodiment according to the present invention; and Figs. 6A and 6B are cross-sectional views of the device shown in Fig. 5 along lines D-D′ and E-E′. DETAILED DESCRIPTION OF THE EMBODIMENTS Referring to Fig. 1, there is shown a floating-gate PROM cell for storing redundant control information employed in a UV-EPROM according to an embodiment of the present invention. Cross-sectional views along lines A-A′, B-B′ and C-C′ are shown in Figs. 2A, 2B and 2C, respectively. Since other sections of the UV-EPROM, i.e. EPROM cell array, data programming and read control circuit and so forth, are well known in the art, the device structures thereof are omitted in the drawings. The PROM cell for storing the redundant control information includes a control gate 1 made of polycrystalline silicon, a floating gate 20 made of polycrystalline silicon, an N-type source region 2 and an N-type drain region 5, similarly to each of EPROM cells. The source and drain regions 2 and 5 are formed in a P-type well region 22 which is in turn formed in a P-type substrate 21. The floating gate 20 is formed on a first gate insulating film 28 of silicon dioxide covering the surface of the well region 22 between the source and drain regions 2 and 5. The control gate 1 is formed on a second gate insulating film 29 of silicon dioxide covering the surface of the floating gate 29 and is elongated over a field insulating layer 23 of silicon dioxide to form an elongated portion. The portion of the control gate 1 is connected through a contact hole 6 to a conductive layer 7. This layer 7 is made of refractory metal silicide such as tungsten silicide, molybdenum silicide, etc., in this embodiment. If desired, it may be made of a metal such as aluminum. The layer 7 constitutes of a part of a gate signal line. The drain region 5 is connected through a contact hole 12 to a conductive layer 13 made of the same material as the layer 7. The layer 13 constitutes a part of a drain signal line. An insulating film 26 made of, for example, borophosphosilicate glass intervenes between the field insulating layer 23 and each of the layers 7 and 13. The conductive layers 7 and 13 are in turn connected through contact holes 8 and 14 to polycrystalline silicon layers 9 and 15, respectively. These layers 9 and 15 are spread over two field insulating layer portions 23 and an insulating film 24 formed therebetween. The layer 9 constitutes another part of the gate signal line and the layer 15 constitutes another part of the drain signal line. The insulating film 24 is made of silicon dioxide and has a thickness substantially equal to that of the first gate insulating film 28. The layers 9 and 15 are covered with an insulating film 25 which is made of silicon dioxide and has a thickness substantially equal to that of the second gate insulating film 29. A polycrystalline silicon layer 19 is formed on the insulating films 24 and 25 to cross the layers 9 and 15. The source region 2 is in contact with an N-type layer 3 which is in turn formed in the well region 22 in a ring shape to surround in a plan view the PROM cell, the conductive layers 7 and 13 and parts of the layers 9 and 15. It should be noted that the layer 3 is not formed in surface portions of the well region 22 just under the polycrystalline silicon layers 9, 15 and 19. An insulating layer 27 such as borophosphosilicate glass is deposited over the entire surface, and a contact hole 4 is provided along the layer 3 to expose a part thereof. The contact hole 4 further exposes a part of the layer 19, so that it completely surrounds in a plan view the PROM cell, the conductive layers 7 and 13 and portions of the layers 9 and 15. A shield layer 18 of aluminum is formed on the insulating layer 27 to cover a portion surrounded by the contact hole 4. The shield layer 18 is further in contact with the layer 3 and the polycrystalline silicon layer 19 to form a wall surrounding the PROM cell. The polycrystalline silicon layers 9 and 15 have respective portions which are outside the shield layer 18, these portions are connected through contact holes 10 and 16 to conductive layers 11 and 17, respectively. These layer 11 and 17 are made of the same material as the conductive layer 7 and constitute remaining parts of the gate and drain signal lines respectively. Therefore, the control gate 1 and drain region 5 of the PROM cell are led out externally from the shield layer 18 by the gate and drain signal lines (7, 9 and 11) and (13, 15 and 17), respectively. In the above device structure, the shield layer 18 prevents most UV light from entering and reaching the PROM cell. Only through the insulating films 24 and 25, UV light enters the PROM cell and reaches the floating gate 20 thereof. Since the insulating films 24 and 25 have respective thickness substantially equal to the first and second gate insulating films 28 and 29, UV light passing through the insulating films 24 and 25 is restricted to a slight amount. Thus, a PROM cell, which is hardly erasable with UV light, is realized. Since the control gate 1 and the drain 5 of the PROM cell are led out from the shield layer 18 without short-circuiting to each other, the redundant control information can be electrically programmed into the PROM cell. Next, description will be made on manufacturing step of the above device with reference to Figs. 3A to 3G and 4A to 4G. As shown in Figs. 3A and 4A, a P-type well region 22 is formed in a surface portion of a P-type silicon substrate 21, followed by carrying out the selective oxidation of silicon. As a result, field insulating films 23 of silicon dioxide are selectively formed. Thereafter, thermal oxidation is performed to form insulating films 24 of silicon dioxide. The film 24 of the left side shown in Fig. 3A is utilized as a first gate insulating film later. A polycrystalline silicon layer is deposited over the entire surface by CVD (Chemival Vapor Deposition) technique, followed by patterning to form polycrystalline silicon layers 30, 15 and 9. The layer 30 is used as a floating gate. As shown in Figs. 3B and 4B, thermal oxidation is carried out to cover the layer 30, 15 and 9 with an insulating film 31 of silicon dioxide. The film 31 covering the layer 30 in Fig. 2B is used as a second gate insulating film. A polycrystalline silicon layer 31 is thereafter deposited over the entire surface by the CVD. As shown in Figs. 3C and 4C, mask layers 33 such as photoresist are selectively formed. Selective etching is then carried out to form a floating gate 20 and a control gate 1 of the PROM cell. Simultaneously, respective floating and control gates of EPROM cells redundant EPROM cells (not shown) are formed. As shown in Figs. 3D and 4D, the mask layers 33 are removed and new mask layers 34 such as photoresist are then formed. The mask layers 34 covers the PROM cell and the EPROM cells. Selective etching is then carried out to from a polycrystalline silicon layer 19 crossing the layers 15 and 9 via the insulating film 25. Simultaneously, respective gate electrode of transistors (not shown) constituting peripheral circuit such as a data programming and reading circuit, an address decoder circuit and so forth are formed. As shown in Figs. 3E and 4E, the mask layers 34 are removed and thermal oxidation is then performed to cover the exposed surfaces of respective polycrystalline layers 1, 20 and 19. Thereafter, the ion implantation of N-type impurities such as arsenic is carried out to form a drain region 5, a source region 2 and an N-type layer 3. Subsequently, an insulating layer 36 such as borophosphosilicate glass is deposited over the entire surface by means of CVD. As shown in Figs. 3F and 4F, contact holes 6, 8, 10, 12, 14 and 16 are selectively formed to expose a part of the control gate 1, a part of the drain region 5, one end portion of the layer 9, the other end portion of the layer 9, one end portion of the layer 15 and the other end portion of the layer 15, respectively. Thereafter, a conductive layer made of refractory metal silicide or aluminum are deposited over the entire surface, followed by selective etching to form the conductive layers 7, 11, 13 and 17. Subsequently, an insulating layer 37 such as borophosphosilicate glass is deposited over the entire surface. As shown in Figs. 3G and 4G, a contact hole 4 is formed along the layers 3 and 19 to expose respective parts thereof. Simultaneously, respective parts of source and drain regions of the EPROM cells and redundant EPROM cells are exposed by respective contact holes (not shown). Thereafter, an aluminum layer is deposited over the entire surface, followed by selective etching to form a shield layer 18 also operating as a source electrode of the PROM cell. Simultaneously, respective source and drain electrodes of the EPROM cells are formed. As a result, the device shown in Fig. 1 is obtained. Since the polycrystalline layers 9, 15 and 19 absorbs UV light, only slight UV light passes through the insulating films 24 and 25. The PROM cell thus stores the redundant control information for an extremely long time. Referring to Fig. 5, there is shown a PROM cell according to another embodiment of the present invention. Cross-sectional views of this cell along lines D-D′ and E-E′ of Fig. 5 are shown in Figs. 6A and 6B, respectively. In these drawings, the same constituents as those shown in Figs. 1 to 4 are denoted by the same reference numerals to omit further description thereof. In this embodiment, the control gate 1 is elonged to be connected through the contact hole 8 to the polycrystalline silicon layer 9. Therefore, the conductive layer 7 shown in Fig. 1 is omitted. The conductive layer 13 shown in Fig. 1 is replaced by a polycrystalline silicon layer 131 which is formed simultaneously with the layer 19. The conductive layers 11 and 17 shown in Fig. 1 are also replaced respectively by polycrystalline silicon layers 111 and 171 which are also formed simultaneously with the layer 19. Since this embodiment requires no conductive layers 7, 13, 11 and 17, the insulating layer 26 (36) is omitted. Also in this embodiment, only slight UV light enters the PROM cell, the redundant control information is stored for an extremely long time. The present invention is not limited to the above embodiment, but may be modified and changed. For example, material of the respective insulating films and conductive layers can be changed to other material. The well region 22 may be omitted. A P-channel type PROM cell may be fabricated.

Claims: A semiconductor device having a transistor formed in a semiconductor substrate (21, 22) of one conductivity type and covered with a shield layer (18), characterized by comprising a strip shape region (3) of an opposite conductivity type selectively formed in said semiconductor substrate, said strip shape region (3) having first and second end portions, a first insulating film (24) formed on a surface of said semiconductor substrate between said first and second end portions of said strip shape region (3), said strip shape region (3) and said first insulating film (24) surrounding in a plan view a portion of said semiconductor substrate, said transistor being formed in said portion of said semiconductor substrate and having a source region (2) of said opposite conductivity type in contact with a part of said strip shape region (3), a drain region (5) of said opposite conductivity type formed in said portion of said semiconductor substrate, a second insulating film (28) formed on a surface of said portion of said semiconductor substrate between said source and drain regions with a thickness substantially equal to the thickness of said first insulating film (24), a floating gate electrode (20) formed on said second insulating film (28), a third insulating film (29) formed on said floating gate (20) and a control gate electrode (1) formed on said third insulating film (29), a first conductive layer (9) crossing said first insulating film (24), a second conductive layer (15) crossing said first insulating film (24), a fourth insulating film (25) formed on said first and second conductive layers (9, 15) with a thickness substantially equal to the thickness of said third insulating film (29), a third conductive layer (7, an elongated portion of control gate 1) interconnecting said control gate electrode (1) and said first conductive layer (9), a fourth conductive layer (13, 131) interconnecting said drain region (5) and said second conductive layer (15), a fifth conductive layer (19) formed on said first and fourth insulating films (24, 25) to cross said first and second conductive layers (9, 15), a fifth insulating film (26, 27) covering said strip shape region (3), said transistor and said first to fifth conductive layers, and a contact hole (4) continuously formed in said fifth insulating film (26, 27) along said strip shape region (3) and said fifth conductive layer (19), said contact hole (4) surrounding in plan view said portion of said semiconductor susbstrate, said shield layer (18) being formed on said fifth insulating film (26, 27) to cover said transistor and being in contact with said strip shape region (3) and said fifth conductive layer (19) through said contact hole (4). The semiconductor device as claimed in claim 1, characterized in that each of said floating and control gate electrodes (20, 1) and said first, second and fifth conductive layers (9, 15, 19) is made of polycrystalline silicon. The semiconductor device as claimed in claim 1 or 2, characterized by further comprising sixth and seventh conductive layers (11, 17, 111, 171) connected respectively to said first and second conductive layers (9, 15), at least on portion of each of said sixth and seventh conductive layers is not covered by said shield layer (18).

Inventors: YAMAGATA YASUSHI C O NEC CORPO, YAMAGATA, YASUSHI C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1989
Main classification: H01L29
IPC codes: H01L27, H01L29, H01L21