Abstract:
A nonvolatile semiconductor memory includes rewritable nonvolatile memory cell transistors connected in series. The nonvolatile memory cell transistors includes at least two charge storage layers formed on a first insulating film, a control gate shared by two adjacent transistors which are two of the nonvolatile memory cell transistors and which are adjacent to each other, and a second insulating film formed between the at least two charge storage layers and the control gate. A top of the control gate has a flat surface that covers the at least two charge storage layers that correspond to the two adjacent transistors, and the flat surface extends from one of the at least two charge storage layers to the other of the at least two charge storage layers.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of U.S. patent application Ser. No. 09/814,711 filed Mar. 23, 2001, now U.S. Pat. No. 6,577,533 and claims benefit of priority under 35 U.S.C. §119 to Jananese Patent Application No. 2000-89100, filed on Mar. 28, 2000, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a nonvolatile semiconductor memory and a method of manufacturing the same, and particularly relates to a nonvolatile semiconductor memory having a NAND-type memory cell unit structured by connecting a plurality of memory cell transistors in series and a method of manufacturing the same. 
     2. Related Background Art 
     An EEPROM which enables electrical rewrite has been hitherto known as one of semiconductor memories. Especially, a NAND-type EEPROM in which a NAND-type memory cell unit is structured by connecting a plurality of memory cell transistors in series has attracted considerable attention as one capable of high integration. 
     FIG. 41 is a diagram showing an equivalent circuit of the NAND-type memory cell unit, and FIG. 42 is a diagram showing the structure of a memory cell portion of the NAND-type memory cell unit in plan view. This example in FIG. 42 shows the NAND-type memory cell unit in the case where STI (Shallow Trench Isolation) is used for isolating elements. 
     One memory cell transistor MT of the NAND-type EEPROM has an FETMOS structure in which a floating gate FG (a charge storage layer) and a control gate CG are stacked with an insulating film therebetween above a semiconductor substrate, a plurality of memory cell transistors are connected in series with their respective adjoining ones sharing a source/drain to constitute the NAND-type memory cell unit. The NAND-type memory cell units like this are arranged in a matrix form to constitute a memory cell array. 
     A drain D at one end side of the NAND-type memory cell unit is connected to a bit line BL via a select gate transistor ST 31 , while a source at the other end side of the NAND-type memory cell unit is connected to a common source line SL via a select gate transistor ST 32 . The control gate CG of the memory cell transistor MT and gate electrodes of the select gate transistors ST 31  and ST 32  are respectively connected to compose a word line WL and a select gate line in a direction perpendicular to the direction of the bit line BL. 
     As shown in FIG. 42, in the NAND-type EEPROM, one source/drain line formed out of a diffusion layer in a silicon active region is formed for one bit line BL. Namely, one NAND-type memory cell unit is formed for one bit line BL. Assuming that a design rule is F (Feature size), the line/space of the bit line BL is 1 F/1 F, and the line/space of the word line WL is also 1 F/1 F. Therefore, the cell size of one memory cell transistor MT is 2 F×2 F=4 F 2 . Since the select gate transistors ST 31  and ST 32  are provided in one NAND-type memory cell unit, the substantial one cell size is 4 F 2 +α if the sizes of these select gate transistors ST 31  and ST 32  are taken into account as an overhead α. 
     As a widely known example of such a NAND-type EEPROM, there are reports such as “A 3.3 V 32 MB NAND Flash Memory with Incremental Step Pulse Programming Scheme” by K.-D. Suh et al. in IEEE J. Solid-State Circuits, vol. 30, pp. 1149-1156, November 1995 and “A 35 ns Cycle Time 3.3 V Only 32 MB NAND Flash EEPROM” by Y. Iwata et. al. in IEEE J. Solid-State Circuits, vol. 30, pp. 1157-1164, 1995. In these references, the operation of a related NAND-type EEPROM is explained. 
     FIG. 43 is a diagram showing an equivalent circuit of a nonvolatile semiconductor memory having AND-type memory cell units, and FIG. 44 is a diagram showing the structure of a memory cell portion of the AND memory cell unit in plan view. 
     The name of an AND type originates in that its connection mode is the same parallel connection as an NOR type and that its logic mode is inverse to the NOR type. Namely, as shown in FIG. 43, the AND-type memory cell unit has a sub-bit line SBBL and a sub-source line SBSL, and a plurality of memory cell transistors MT are connected in parallel between the sub-bit line SBBL and the sub-source line SBSL. For example, in the case of a 64 Mbits AND-type nonvolatile semiconductor memory, 128 memory cell transistors are connected in parallel in one AND-type memory cell unit. 
     The sub-bit line SBBL is connected to a main bit line MBL via a select gate transistor ST 41 . The sub-source line SBSL is connected to a main source line MSL via a select gate transistor ST 42 . 
     A memory cell array composed of these AND-type memory cell units is characterized by its pseudo contactless structure, in which the main bit line MBL and word lines WL are made hierarchical and the sub-bit line SBBL and the sub-source line SBSL are formed out of a diffusion layer writing/erase into/from the memory cell transistor MT is performed by an FN (Fowler-Nordheim) tunnel current. Specifically, the writing to the memory cell transistor MT is performed by extracting electrons in the floating gate FG to the drain side by the use of the FN tunnel current. The erase from the memory cell transistor MT is performed by injecting electrons from the semiconductor substrate to the floating gate FG by the FN tunnel current on the entire surface of a channel. 
     As shown in FIG. 44, in the AND-type memory cell unit, two lines in total, i.e. the sub-source line SBSL and the sub-bit line SBBL formed of the diffusion layer in the silicon active region, are formed for one main bit line MBL. Hence, the line/space of each of the sub-source line SBSL and the sub-bit line SBBL is 1 F/1 F, and the line/space of the word line WL is also 1 F/1 F. Consequently, the cell size of one memory cell transistor MT is 2 F×4 F=8 F 2 . Moreover, since the select gate transistors ST 41  and ST 42  are provided in one AND memory cell unit, the substantial one cell size is 8 F 2 +α if the sizes of these select gate transistors ST 41  and ST 42  are taken into account as an overhead α. 
     Meanwhile, Japanese Patent Laid-open No. Hei 7-45797 discloses a nonvolatile semiconductor memory in which a vertical NAND memory cell unit is formed in a side wall portion of a trench in order to reduce one cell size. FIG. 45 is a diagram showing a cross section of a memory cell transistor MT portion of the nonvolatile semiconductor memory disclosed in this Japanese Patent Laid-open No. Hei 7-45797. 
     As shown in FIG. 45, in this nonvolatile semiconductor memory, a trench region TC is formed on a semiconductor substrate, and memory cell transistors MT are formed respectively on both side wall portions of this trench region TC. In this case, the floating gates FG are formed along a side wall on the inside of the trench region TC, and source/drains SD are formed as a diffusion layer along a side wall of the trench region TC of the semiconductor substrate. Namely, in this NAND-type memory cell unit, a plurality of memory cell transistors MT are each formed along the side wall of the trench region TC, and thus a source/drain current flows along the side wall of the trench region TC. The bit line BL is formed via an interlayer dielectric for each NAND-type memory cell unit. The line/space of this bit line BL is 1 F/1 F. 
     In order to attain higher integration, however, it is necessary to lay two source-drain lines formed in silicon active regions within a 2 F bit line pitch and to effectively reduce the memory cell size by half. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the aforesaid problems. An object of the present invention is to three-dimensionally make an NAND-type EEPROM and lay two source/drain lines in silicon active regions within 2 F bit line pitch, that is, to provide two NAND-type memory cell units for one bit line, and thereby to provide a nonvolatile semiconductor memory capable of reducing the memory cell size by half, resulting in a reduction in bit cost and to provide a method of manufacturing the same. 
     In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a nonvolatile semiconductor memory includes a memory cell array having a plurality of memory cell units, one of which has a plurality of rewritable nonvolatile memory cell transistors connected in series, the nonvolatile memory cell transistors having a first insulating film formed on a substrate; a charge storage layer formed on the first insulating film; a control gate shared with two adjacent transistors which are two of the nonvolatile memory cell transistors and which are adjacent to each other; and a second insulating film formed between the charge storage layer and the control gate, both a bottom of the charge storage layer and a bottom of the second insulating film being on a face which is substantially parallel with a surface of the substrate. 
     wherein the NAND-type memory cell units are formed in pairs along both side wall portions of a trench formed in a substrate, 
     two of the nonvolatile memory cell transistors which face each other on the side wall portions of the trench share the one control gate which is formed to extend in a depth direction of the trench, and 
     the control gate is formed to fill a space formed by an insulating film which covers the two charge storage layers facing each other on the side wall portions of the trench, and electrically connected to a word line which extends continuously. 
     In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a nonvolatile semiconductor memory comprising a plurality of NAND-type memory cell units, each of which includes: 
     a NAND-type memory cell column having a plurality of rewritable nonvolatile memory cell transistors connected in series, each of which has a charge storage layer and a control gate; 
     a bit line side switching portion connected between the memory cell column and a bit line; and 
     a source line side switching portion connected between the memory cell column and a source line, 
     wherein the NAND-type memory cell units are formed in pairs along both side wall portions of a trench formed in a substrate, and 
     wherein two of NAND-type memory cell units of the pair are respectively connected to the identical bit line via the bit line switch. 
     According to a further aspect of the present invention, a method of manufacturing a nonvolatile semiconductor memory including a memory cell array having a plurality of NAND-type memory cell units, each of which has a plurality of rewritable nonvolatile memory cell transistors connected in series, each of which has a charge storage layer and a control gate, the method comprising the steps of: 
     forming a trench in a substrate; 
     forming first insulating films on both side wall portions of the trench; 
     forming a pair of the charge storage layers on surface sides of the first insulating films formed on both the side wall portions of the trench; 
     forming a second insulating film so as to cover the pair of charge storage layers on surface sides of the pair of charge storage layers formed on both the side wall portions of the trench; 
     forming the control gate shared by the pair of charge storage layers so as to fill a space formed by the second insulating film; and 
     forming a word line which is electrically connected to the control gate and extends continuously. 
     According to a still further aspect of the present invention, a method of manufacturing a nonvolatile semiconductor memory including a memory cell array having a plurality of NAND-type memory cell units, each of which has a plurality of rewritable nonvolatile memory cell transistors connected in series, each of which has a charge storage layer and a control gate, the method comprising the steps of: 
     forming a trench in a substrate; 
     forming first insulating films for memory cell transistors on both side wall portions of the trench; 
     forming first insulating films for select gate transistors on both side wall portions of the trench; 
     forming a pair of the charge storage layers on surface sides of the first insulating films for the memory cell transistors formed on both the side wall portions of the trench; 
     forming a pair of first gate electrodes on surface sides of the first insulating films for the select gate transistors formed on both the side wall portions of the trench; 
     forming a second insulating film so as to cover the pair of charge storage layers on surface sides of the pair of charge storage layers formed on both the side wall portions of the trench; 
     forming the control gate shared by the pair of charge storage layers so as to fill a space formed by the second insulating film; 
     forming a second gate electrode shared by the pair of first gate electrodes so as to fill a space between the pair of first gate electrodes; 
     forming a word line which is electrically connected to the control gate and extends continuously; and 
     forming a select gate line which is electrically connected to the second gate electrode and extends continuously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an equivalent circuit of a cell array of a NAND-type EEPROM in a nonvolatile semiconductor memory according to the present embodiment; 
     FIG. 2 is a diagram showing the structure of the NAND-type memory cell unit in plan view; 
     FIG. 3A is a schematic diagram showing the structure of a memory cell transistor portion in FIG. 2 in plan view, and FIG. 3B is a diagram showing a cross section thereof taken along the line A-A′; 
     FIG. 4A is a schematic diagram showing the structure of a select gate transistor portion in FIG. 2 in plan view, and FIG. 4B is a diagram showing a cross section thereof taken along the line B-B′; 
     FIG. 5A is a schematic diagram showing the structure of a bit line contact portion in FIG. 2 in plan view, and FIG. 5B is a diagram showing a cross section thereof taken along the line C-C′; 
     FIG. 6A is a schematic diagram showing the structure of a source line contact portion in FIG. 2 in plan view, and FIG. 6B is a diagram showing a cross section thereof taken along the line D-D′; 
     FIG. 7A is a schematic diagram showing a state of a manufacturing process of the memory cell transistor portion in FIG. 2 in plan view, and FIG. 7B is a diagram showing a cross section thereof taken along the line A-A′ (First state); 
     FIG. 8A is a schematic diagram showing a state of a manufacturing process of the select gate transistor portion in FIG. 2 in plan view, and FIG. 8B is a diagram showing a cross section thereof taken along the line B-B′ (First state); 
     FIG. 9A is a schematic diagram showing a state of a manufacturing process of the bit line contact portion in FIG. 2 in plan view, and FIG. 9B is a diagram showing a cross section thereof taken along the line C-C′ (First state); 
     FIG. 10A is a schematic diagram showing a state of a manufacturing process of the source line contact portion in FIG. 2 in plan view, and FIG. 10B is a diagram showing a cross section thereof taken along the line D-D′ (First state); 
     FIG. 11A is a schematic diagram showing a state of the manufacturing process of the memory cell transistor portion in FIG. 2 in plan view, and FIG. 11B is a diagram showing a cross section thereof taken along the line A-A′ (Second state); 
     FIG. 12A is a schematic diagram showing a state of the manufacturing process of the select gate transistor portion in FIG. 2 in plan view, and FIG. 12B is a diagram showing a cross section thereof taken along the line B-B′ (Second state); 
     FIG. 13A is a schematic diagram showing a state of the manufacturing process of the bit line contact portion in FIG. 2 in plan view, and FIG. 13B is a diagram showing a cross section thereof taken along the line C-C′ (Second state); 
     FIG. 14A is a schematic diagram showing a state of the manufacturing process of the source line contact portion in FIG. 2 in plan view, and FIG. 14B is a diagram showing a cross section thereof taken along the line D-D′ (Second state); 
     FIG. 15A is a schematic diagram showing a state of the manufacturing process of the memory cell transistor portion in FIG. 2 in plan view, and FIG. 15B is a diagram showing a cross section thereof taken along the line A-A′ (Third state); 
     FIG. 16A is a schematic diagram showing a state of the manufacturing process of the select gate transistor portion in FIG. 2 in plan view, and FIG. 16B is a diagram showing a cross section thereof taken along the line B-B′ (Third state); 
     FIG. 17A is a schematic diagram showing a state of the manufacturing process of the bit line contact portion in FIG. 2 in plan view, and FIG. 17B is a diagram showing a cross section thereof taken along the line C-C′ (Third state); 
     FIG. 18A is a schematic diagram showing a state of the manufacturing process of the source line contact portion in FIG. 2 in plan view, and FIG. 18B is a diagram showing a cross section thereof taken along the line D-D′ (Third state); 
     FIG. 19A is a schematic diagram showing a state of the manufacturing process of the memory cell transistor portion in FIG. 2 in plan view, and FIG. 19B is a diagram showing a cross section thereof taken along the line A-A′ (Fourth state); 
     FIG. 20A is a schematic diagram showing a state of the manufacturing process of the select gate transistor portion in FIG. 2 in plan view, and FIG. 20B is a diagram showing a cross section thereof taken along the line B-B′ (Fourth state); 
     FIG. 21A is a schematic diagram showing a state of the manufacturing process of the bit line contact portion in FIG. 2 in plan view, and FIG. 21B is a diagram showing a cross section thereof taken along the line C-C′ (Fourth state); 
     FIG. 22A is a schematic diagram showing a state of the manufacturing process of the source line contact portion in FIG. 2 in plan view, and FIG. 22B is a diagram showing a cross section thereof taken along the line D-D′ (Fourth state); 
     FIG. 23A is a schematic diagram showing a state of the manufacturing process of the memory cell transistor portion in FIG. 2 in plan view, and FIG. 23B is a diagram showing a cross section thereof taken along the line A-A′ (Fifth state); 
     FIG. 24A is a schematic diagram showing a state of the manufacturing process of the select gate transistor portion in FIG. 2 in plan view, and FIG. 24B is a diagram showing a cross section thereof taken along the line B-B′ (Fifth state); 
     FIG. 25A is a schematic diagram showing a state of the manufacturing process of the bit line contact portion in FIG. 2 in plan view, and FIG. 25B is a diagram showing a cross section thereof taken along the line C-C′ (Fifth state); 
     FIG. 26A is a schematic diagram showing a state of the manufacturing process of the source line contact portion in FIG. 2 in plan view, and FIG. 26B is a diagram showing a cross section thereof taken along the line D-D′ (Fifth state); 
     FIG. 27A is a schematic diagram showing a state of the manufacturing process of the memory cell transistor portion in FIG. 2 in plan view, and FIG. 27B is a diagram showing a cross section thereof taken along the line A-A′ (Sixth state); 
     FIG. 28A is a schematic diagram showing a state of the manufacturing process of the select gate transistor portion in FIG. 2 in plan view, and FIG. 28B is a diagram showing a cross section thereof taken along the line B-B′ (Sixth state); 
     FIG. 29A is a schematic diagram showing a state of the manufacturing process of the bit line contact portion in FIG. 2 in plan view, and FIG. 29B is a diagram showing a cross section thereof taken along the line C-C′ (Sixth state); 
     FIG. 30A is a schematic diagram showing a state of the manufacturing process of the source line contact portion in FIG. 2 in plan view, and FIG. 30B is a diagram showing a cross section thereof taken along the line D-D′ (Sixth state); 
     FIG. 31A is a schematic diagram showing a state of the manufacturing process of the memory cell transistor portion in FIG. 2 in plan view, and FIG. 31B is a diagram showing a cross section thereof taken along the line A-A′ (Seventh state); 
     FIG. 32A is a schematic diagram showing a state of the manufacturing process of the select gate transistor portion in FIG. 2 in plan view, and FIG. 32B is a diagram showing a cross section thereof taken along the line B-B′ (Seventh state); 
     FIG. 33A is a schematic diagram showing a state of the manufacturing process of the bit line contact portion in FIG. 2 in plan view, and FIG. 33B is a diagram showing a cross section thereof taken along the line C-C′ (Seventh state); 
     FIG. 34A is a schematic diagram showing a state of the manufacturing process of the source line contact portion in FIG. 2 in plan view, and FIG. 34B is a diagram showing a cross section thereof taken along the line D-D′ (Seventh state); 
     FIG. 35 is a diagram showing voltage relationship when the nonvolatile semiconductor memory according to this embodiment is operated; 
     FIG. 36 is a timing chart showing voltage relationship during an actual write period in a write operation (when a channel region is preliminarily charged from the bit line side); 
     FIG. 37 is a timing chart showing voltage relationship during an actual write period in a write operation (when the channel region is preliminarily charged from the source line side); 
     FIG. 38 is a circuit diagram when the NAND-type memory cell unit shown in FIG. 1 is modified and two select gate transistors are provided on the source line side; 
     FIG. 39 is a circuit diagram when the NAND-type memory cell unit shown in FIG. 1 is modified, two select gate transistors are provided on the bit line side, and two select gate transistors are provided also on the source lined side; 
     FIG. 40 is a circuit diagram when the NAND-type memory cell unit shown in FIG. 1 is modified, one select gate transistor is provided on the bit line side, and one select gate transistor is provided also on the source lined side; 
     FIG. 41 is an equivalent circuit diagram showing the interconnection of a related NAND-type memory cell unit; 
     FIG. 42 is a schematic diagram showing a memory cell transistor portion of the related NAND-type memory cell unit in plan view; 
     FIG. 43 is an equivalent circuit diagram showing the interconnection of a related AND cell; 
     FIG. 44 is a schematic view showing a memory cell transistor portion of the related AND cell in plan view; and 
     FIG. 45 is a sectional diagram of a memory cell transistor portion of a related nonvolatile semiconductor memory in which NAND-type memory cell units are formed on both side wall portions of its trench region. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a diagram showing an equivalent circuit of a vertical NAND-type EEPROM cell array according to an embodiment of the present invention. First, the interconnection of a memory cell array of a nonvolatile semiconductor memory according to this embodiment will be explained based on FIG.  1 . 
     As shown in FIG. 1, two NAND-type memory cell units ND 1  and ND 2  are provided for one bit line BL in the nonvolatile semiconductor memory according to this embodiment. The NAND-type memory cell unit ND 1  is structured by connecting select gate transistors ST 1  and ST 3 ,  16  memory cell transistors MT, and a select gate transistor ST 5  in series. Similarly, the NAND-type memory cell unit ND 2  is structured by connecting select gate transistors ST 2  and ST 4 ,  16  memory cell transistors MT, and a select gate transistor ST 6  in series. 
     The drain sides of the select gate transistors ST 1  and ST 2  are jointly connected to the bit line BL. The select gate transistors ST 1  and ST 4  are depletion-type (normally-on type) MOS transistors, and the other select gate transistors ST 2 , ST 3 , ST 5 , and ST 6  are enhancement-type (normally-off type) MOS transistors. The source sides of the select gate transistors ST 5  and ST 6  are connected to a common source line SL. 
     Gate electrodes of the select gate transistors ST 1  and ST 2  of the NAND-type memory cell units ND 1  and ND 2  are jointly connected to constitute a select gate line SSL 1 . Gate electrodes of the select gate transistors ST 3  and ST 4  of the NAND-type memory cell units ND 1  and ND 2  are jointly connected to constitute a select gate line SSL 2 . Control gates of the 16 memory cell transistors MT of the NAND-type memory cell units ND 1  and ND 2  are jointly connected respectively to constitute word lines WL 0  to WL 15 . Gate electrodes of the select gate transistors ST 5  and ST 6  of the NAND-type memory cell units ND 1  and ND 2  are jointly connected to constitute a select gate line GSL. 
     A bit line side switching portion in this embodiment is composed of the select gate transistors ST 1 , ST 2 , ST 3 , and ST 4 , whereby either one NAND-type memory cell unit is selected out of a pair of NAND-type memory cell units ND 1  and ND 2 . Moreover, a source line side switching portion in this embodiment is composed of the select gate transistors ST 5  and ST 6 . 
     In this embodiment, a plurality of such NAND-type memory cell units ND 1  and DN 2  as shown in FIG. 1 are arranged in an array form to compose one memory cell array. 
     Next, the structure of the NAND-type EEPROM cell array according to this embodiment will be explained based on FIG. 2 to FIG.  6 . 
     FIG. 2 is a diagram schematically showing the NAND-type EEPROM cell array according to this embodiment in plan view, FIG. 3A is a diagram showing a word line portion in FIG. 2, and FIG. 3B is a diagram showing a cross section thereof taken along the line A-A′. FIG. 4A is a diagram schematically showing a selected word line portion in FIG. 2 in plan view, and FIG. 4B is a diagram showing a cross section thereof taken along the line B-B′. FIG. 5A is a diagram schematically showing a bit line contact portion in FIG. 2 in plan view, and FIG. 5B is a diagram showing a cross section thereof taken along the line C-C′. FIG. 6A is a diagram schematically showing a source line contact portion in FIG. 2 in plan view, and FIG. 6B is a diagram showing a cross section thereof taken along the line D-D′. 
     As shown in FIG. 2 to FIG. 6, on both side walls of one trench region  14 , the NAND-type memory cell units ND 1  and ND 2  are formed respectively. Especially as shown in FIG.  2  and FIG. 3, the memory cell transistor MT of the NAND-type memory cell unit ND 1  and the memory cell transistor MT of the NAND-type memory cell unit ND 2  are formed so as to face each other in the one trench region  14 . 
     More specifically, each of the two memory cell transistors MT and MT formed to face each other includes a thin oxide film  17  formed on a side wall, a floating gate FG, and an inter-poly insulating film  19 . This inter-poly insulating film  19  is formed out of an ONO film, for example, in this embodiment. Further, these two memory cell transistors MT and MT formed to face each other include one common control gate CG. Furthermore, in place of the floating gate FG, a nitride film or the like film which traps a charge is also available. 
     This control gate CG is formed to extend substantially perpendicularly to the depth direction of the trench region  14 , and connected to a polysilicon  22 . This polysilicon  22  is formed to extend in the horizontal direction of the trench region  14 , and connected to the continuously formed word lines WL 0  to WL 15  (the word line WL 0  in FIG.  3 ). Incidentally, the control gate CG and the word lines WL 0  to WL 15  (the word line WL 0  in FIG. 3) may be electrically connected directly without this polysilicon  22  therebetween. 
     Moreover, especially as shown in FIG.  2  and FIG. 4, the select gate transistor STI of the NAND-type memory cell unit ND 1  and the select gate transistor ST 2  of the NAND-type memory cell unit ND 2  are formed so as to face each other in one trench region  14 . 
     More specifically, the two select gate transistors ST 1  and ST 2  formed to face each other each include the thin oxide film  17  formed on the side wall, and a gate electrode GE. The select gate transistor ST 1  is, however, a depletion-type MOS transistor, and thus an N-type impurity region  16  is formed in a memory cell P-type well  13  in a side wall portion of the trench region  14 , that is, in a portion composing a channel. Such two select gate transistors ST 1  and ST 2  formed to face each other are connected to the common polysilicon  22 . 
     This polysilicon  22  is formed to extend substantially perpendicularly to the depth direction of the trench region  14 , and connected to the continuously formed select gate line SSL 1 . 
     Especially as shown in FIG.  2  and FIG. 5, these select gate transistors ST 1  and ST 2  are each connected to one plug-shaped metal layer  29  via a contact region  28 , and this plug-shaped metal layer  29  is connected to the bit line BL. 
     Likewise with these select gate transistors ST 1  and ST 2 , as shown in FIG. 2, the select gate transistor ST 3  of the NAND-type memory cell unit ND 1  and the select gate transistor ST 4  of the NAND-type memory cell unit ND 2  are formed so as to face each other in one trench region  14 . As for these select gate transistors ST 3  and ST 4 , the select gate transistor ST 4  is a depletion-type MOS transistor, and hence the N-type impurity region  16  is formed in a portion composing a channel. Except for this point, the structures of the select gate transistors ST 3  and ST 4  are the same as those of the aforesaid select gate transistors ST 3  and ST 4 . 
     Similarly to these select gate transistors ST 1 , ST 2 , ST 3 , and ST 4 , as shown in FIG. 2, the select gate transistor ST 5  of the NAND-type memory cell unit ND 1  and the select gate transistor ST 6  of the NAND-type memory cell unit ND 2  are formed so as to face each other in one trench region  14 . The structures of these select gate transistors ST 5  and ST 6  are the same as those of the aforesaid enhancement-type select gate transistors ST 2  and ST 3 . 
     Especially, as shown in FIG.  2  and FIG. 6, these select gate transistors ST 5  and ST 6  are each connected to the continuously formed common source line SL via the contact region  28 . 
     Next, a method of manufacturing the nonvolatile semiconductor memory according to this embodiment will be explained based on FIG. 7 to FIG.  34 . 
     Out of these Figures, FIG. 7 to FIG. 10, FIG. 11 to FIG. 14, FIG. 15 to FIG. 18, FIG. 19 to FIG. 22, FIG. 23 to FIG. 26, FIG. 27 to FIG. 30, and FIG. 31 to FIG. 34 are diagrams each for explaining a state of a manufacturing process of the nonvolatile semiconductor memory divided into four sections. The diagrams showing these four sections correspond to the aforesaid FIG. 3 to FIG. 6, respectively. 
     First, as shown in FIG. 7 to FIG. 10, a memory cell N-type well  12  is formed on a semiconductor substrate  11  which is a P-type silicon substrate. Then, the memory cell P-type well  13  is formed in the memory cell N-type well  12 . An oxide film  151  is formed on the surface of the memory cell P-type well  13 . This oxide film  151  is formed, for example, by CVD (Chemical Vapor Deposition). Thereafter, the trench region  14  is formed in the memory cell P-type well  13 . This trench region  14  is formed, for example, by patterning a photoresist and performing RIE (Reactive Ion Etching). Hence, the width of the trench region  14  is 1 F, and the distance between the trench regions  14  is also 1 F. 
     Subsequently, an oxide film  152  is deposited at the bottom of the trench region  14 . Namely, after an oxide film is deposited by the CVC while the photoresist remains, the photoresist is removed, thereby forming the oxide film  152  at the bottom of the trench region  14 . The oxide film  151  and the oxide film  152 , however, may be formed in the same process. In this case, it is recommended that the aforesaid process of forming the oxide film  151  be omitted, and that by depositing an oxide film by the CVD after the photoresist which has been used for forming the trench region  14  is removed, the oxide film  15   1  and the oxide film  15   2  be formed at the same time. 
     Then, especially as shown in FIG. 8, arsenic (As) and phosphorus (P) are selectively ion-implanted into gate regions of the select gate transistors ST 1  and ST 4  to form the N-type impurity regions  16 . Specifically, the photoresist is applied over the entire surface before ion implantation, and photoresist openings are formed in the gate region portions of the select gate transistors ST 1  and ST 4 . Subsequently, ion implantation is performed through the photo resist openings with an inclination of +7 degrees vertical to the semiconductor substrate  11 , whereby the ion implantation can be selectively performed for only one side wall of the trench region  14 . Similarly, by performing ion implantation through the photoresist openings with an inclination of −7 degrees vertical to the semiconductor substrate  11 , the ion implantation can be selectively performed for only the other side wall of the trench region  14 . The aforesaid formation of the N-type impurity regions  16  can make threshold voltages of the select gate transistors ST 1  and ST 4  negative and depletion mode. 
     Thereafter, as shown in FIG. 11 to FIG. 14, the thin oxide film  17  is formed on each of side walls of the trench region  14 . This thin oxide film  17  is formed, for example, by the CVD. Therefore, the oxide film formed by the CVD is also formed on the oxide film  15   1  and the oxide film  15   2 , but the illustration thereof is omitted since it is thin. 
     Subsequently, a polysilicon  18  to which impurities are doped is deposited on each of side wall portions of the trench region  14 . Specifically, after being deposited overall on the bottom, the side walls, and the top of the trench region  14  by the CVD, the polysilicon is etched overall by RIE. Thus, the polysilicon deposited on the side wall portion of the trench region  14  remains, resulting in the formation of the polysilicon  18 . Namely, this polysilicon  18  is formed by a self-aligned side wall leaving technology. The polysilicon  18  left at the side wall portion constitutes the floating gate FG or the gate electrode GE later. 
     Then, as shown in FIG. 15 to FIG. 18, the inter-poly insulating film  19  is deposited overall at the bottom, the side walls, and the top of the trench region  14 . In this embodiment, this inter-poly insulating film  19  is formed out of an ONO (Oxide-Nitride-Oxide) film. For example, this ONO film is formed by forming a lower oxide film by thermal oxidation, forming a nitride film by LP-CVD (Low Pressure CVD), and then forming an upper oxide film by thermal oxidation. Subsequently, a polysilicon  20 , to which impurities are doped, are deposited over the entire surface. In place of this polysilicon  20 , however, a metal such as tungsten may be deposited. This polysilicon  20  constitutes the control gate CG later. 
     Thereafter, as shown in FIG. 19 to FIG. 22, only a memory cell transistor region is covered with a protective film  21 . Namely, as shown in FIG. 2, after a protective film is formed overall, the protective film in the regions of the select gate transistors ST 1  to ST 6  is removed to form the protective film  21  in the memory cell transistor region. Then, with the protective film  21  as a mask, the inter-poly insulating film  19  and the polysilicon  20  formed in regions other than the memory cell transistor region are removed. 
     Then, as shown in FIG. 23 to FIG. 26, the protective film  21  is removed, and the polysilicon  22 , to which impurities are doped, are deposited over the entire surface. Subsequently, the polysilicon  20  and the polysilicon  22  are patterned at the same time and separated in the direction of the bit line BL. On this occasion, in the memory cell transistor region, especially as shown in FIG. 23, the inter-poly insulating film  19  is also patterned simultaneously and separated in the direction of the bit line BL. Specifically, a photoresist having slits in the direction of the bit line BL is formed at the upper portion of the oxide film  151 , and the RIE is performed with this photoresist as a mask. Thereby, in the memory cell transistor region, especially as shown in FIG. 23, the inter-poly insulating film  19 , the polysilicon  20 , and the polysilicon  22  are respectively separated in the direction of the bit line. As a result, in the memory cell transistor region, the polysilicon  22  is deposited directly on the polysilicon  20 , and they have electrically the same node. Further, in the select gate transistor regions which are regions other than the memory cell transistor region, especially as shown in FIG. 24 to FIG. 26, the polysilicon  22  is separated in the direction of the bit line. As a result, in the regions other than the memory cell transistor region, the polysilicon  22  is deposited directly on the polysilicon  18  without the inter-poly insulating film  19  therebetween, and they have electrically the same node. Incidentally, in place of the polysilicon  22 , a metal such as tungsten may be deposited. 
     This patterning, however, can be also performed self-aligningly. In this case, the oxide film  15   1  is formed rather thickly and etched over the entire surface by the RIE. Thus, the inter-poly insulating film  19 , the polysilicon  20 , and the polysilicon  22  may be left self-aligningly for the trench region  14 . In this case, one photoresist mask can be omitted, and a mask alignment tolerance MRG (See FIG. 23) becomes unnecessary. 
     As shown in FIG. 27 to FIG. 30, an interlayer dielectric  23  is formed overall so as to fill spaces between the polysilicons  22 . Subsequently, by depositing and patterning a metal layer, the word lines WL 0  to WL 15  and the select gate lines SSL 1 , SSL 2 , and GSL are formed. Before this patterning, the metal layer may be flattened by CMP (Chemical Mechanical Polishing). Moreover, the metal layer may be tungsten silicide (WSi), aluminum (Al), or polysilicon (Poly-Si). 
     Specifically, when the metal layer is patterned, a photoresist having slits in the directions of word lines WL 0  to WL 15  is formed. By etching the metal layer by the RIE with this photoresist as a mask, the word lines WL 0  to WL 15  and the select gate lines SSL 1 , SSL 2 , and GSL are formed. Further, with this photoresist as a mask, the interlayer dielectric  23 , the polyhsilicon  22 , the polysilicon  20 , the inter-poly insulating film  19 , the polysilicon  18 , and the thin oxide film  17  are sequentially etched by the RIE. As a result, these films are separated in the directions of the word lines WL 0  to WL 15 , and particularly as shown in FIG. 27, the polysilicon  18  constitutes the floating gate FG of the memory cell transistor MT, and the plysilicon  20  constitutes the control gate CG of the memory cell transistor MT. Especially as shown in FIG. 28 to FIG. 30, the polysilicon  18  constitutes the gate electrode GE of each of the select gate transistors ST 1  to ST 6 . 
     Next, as shown in FIG. 31 to FIG. 34, a source/drain region  25  of each of the memory cell transistors MT and the select gate transistors ST 1  to ST 6  is formed on each of a side wall potion of the memory cell P-type well  13 . Specifically, a photoresist is applied over the entire surface, and photoresist openings are formed in the side wall portion of each of the memory cell transistors MT and the select gate transistors ST 1  to ST 6 . Then, with this photoresist, the word lines WL 0  to WL 15 , and the select gate lines SSL 1 , SSL 2 , and GSL, ion implantation is performed with an inclination of +7 degrees vertical to the semiconductor substrate  11 , whereby the source/drain region  25  is formed in a left portion in FIG. 31 to FIG. 34 of the trench region  14 . Subsequently, by performing ion implantation with an inclination of −7 degrees vertical to the semiconductor substrate  11 , the source/drain region  25  is formed in a right portion in FIG. 31 to FIG. 34 of the trench region  14 . In these cases, for example, N-type impurities such as arsenic (As) and phosphorus (P) are ion-implanted. 
     Thereafter, an interlayer dielectric  26  is deposited over the entire surface, and openings  27  are formed in the interlayer dielectric  26  in the contact regions of the bit line and the source line. To make a contact region  28  of each of the bit lines BL and the common source line SL less resistant, the N-type impurities such as arsenic (As) and phosphorus (P) are diffused again on both side walls of the trench region  14 . 
     Then, as shown in FIG. 3 to FIG. 6, by filling a metal layer in the shape of a plug into the opening  27  formed in each of the contact region portions of the bit lines BL and the common source line SL, the metal layer  29  shown in FIG. 5 is formed and the common source line SL shown in FIG. 6 is formed. As this metal layer, for example, tungsten (W) is used. Subsequently, an interlayer dielectric  30  is deposited over the entire surface and openings  31  are formed in this interlayer dielectric  30 . The openings  31  are formed in the bit line contact regions and source line shunt regions (not illustrated). 
     A metal layer is then formed on the interlayer dielectric  30  and patterned, thereby forming the bit lines BL and source lines (not illustrated). Although the source lines are not illustrated, they are formed parallel to the bit lines BL at the intervals of a plurality of columns, for example, 64 columns. Finally, by covering the whole with a protective film  33 , the nonvolatile semiconductor memory is obtained. 
     Next, the operation of the nonvolatile semiconductor memory according to this embodiment will be explained. As shown in FIG. 1, in this embodiment, one bit line BL is shared by two NAND-type memory cell units ND 1  and ND 2 , and hence it is necessary to select either one of the NAND-type memory cell unit ND 1  or ND 2  by the use of the select gate transistors ST 1  to ST 4  on the occasion of reading and writing. Except for this point, the operation of the nonvolatile semiconductor memory according to this embodiment is basically the same as the operation of an ordinary nonvolatile semiconductor memory. Based on FIG. 35, the operation of this nonvolatile semiconductor memory will be explained below, divided into an erase operation, a read operation, and a write operation. 
     (Erase Operation) 
     In the NAND-type EEPROM, the erase operation is performed in each block as a unit. One block is composed of the memory cell transistors MT to which the word lines WL 0  to WL  15  are respectively connected in common. In other words, the erase operation is performed for the memory cell transistors MT of a plurality of NAND-type memory cell units in one block all at once. 
     Namely, as shown in FIG. 35, the word lines WL 0  to WL 15  in a selected block are set at a ground electric potential. On this occasion, the word lines WL 0  to WL 15  in a non-selected block are set in a floating state. Then, an erase pulse of 21 V and 3 ms is applied to the memory cell P-type well  13  (a bulk). Consequently, in the selected block, an erase voltage of 21 V is added between the bulk and the word lines WL 0  to WL 15  so that electrons in the floating gates FG flow into the memory cell P-type well  13  side by an FN tunnel current. Hence, a threshold voltage of each of the memory cell transistors MT is about −3 V. 
     Since excessive erase is insignificant in the NAND-type EEPROM, erase is performed deeply to abut −3 V with one erase pulse in the memory cell transistor MT. On the other hand, the non-selected block is not affected by the erase pulse because of capacitance coupling of the word lines WL 0  to WL 15  in the floating state and the memory cell P-type well  13  to which the erase voltage of 21 V is applied. Although the word lines WL 0  to WL 15  in the floating state have junction capacitance and wiring capacitance of various kinds, the capacitance between the word lines WL 0  to WL 15  and the memory cell P-type well  13  is conspicuously large so as to occupy a greater part of the total capacitance. Accordingly, the FN tunnel current is prevented from flowing in the non-selected block. In erase verification, it is determined whether or not a threshold voltage of each of all the memory cell transistors MT reaches −1 V or less. 
     (Read Operation) 
     The read operation is performed in each page as a unit. One page is a range connected to one of the word lines WL 0  to WL  15  in one block. Accordingly, in the read operation, cell data on the memory cell transistors MT of one page are simultaneously transferred to a latch circuit in a page buffer and successively read out. 
     It should be mentioned that the operation of an EPROM provided with the select gate transistors ST 1  to ST 4  on the bit line BL side is described in “A High Density EPROM Cell and Array” by R. Stewart et al. in Symp. VLSI Circuits Dig. Tech. Papers, pp. 89-90, June 1987. 
     Namely, the bit line BL is set temporarily at 0 V, the select gate line SSL 1  is set at 0 V, the select gate line SSL 2  is set as 4.5 V, and the select gate line GSL is set at 4.5 V. Thereby, in FIG. 1, the select gate transistor ST 3  is in an ON-state, and the select gate transistor ST 2  is in an OFF-state. As a result, the NAND-type memory cell unit ND 1  side is selected, and the NAND-type memory cell unit ND 2  side is not selected. Contrary to this, when the NAND-type memory cell unit ND 2  side is selected, it is recommended that the select gate line SSL 1  be set at 4.5 V and the select gate line SSL 2  be set at 0 V. 
     Thereafter, the selected word line WLi in a selected block is set at 0 V, and the non-selected word lines WL 0  to WL 15  (except WLi) is set at 4.5V which is a pass voltage. The threshold voltage after writing of the memory transistors MT (after storing an electric charge) in this embodiment is about +2V, and hence the non-selected memory cell transistors MT in the NAND-type memory cell unit ND 1  function as pass transistors. Meanwhile, the memory transistor MT to which a voltage of 0 V is applied and which is selected is conductive only after erase (when an electric charge has not been stored yet), while it is not conductive after writing (after storing the electric charge). For this reason, after erase (when the electric charge has not been stored yet), the bit line BL forms a pass which is grounded to the common source line SL via the selected memory cell transistor MT. On the other hand, after writing (after storing the electric charge), a pass in an open state is formed without the bit line BL being grounded. 
     Incidentally, in this embodiment, a state after erase (when the electric charge has not been stored yet) in the memory cell transistor MT is taken as “1”, and a state after writing (after storing the electric charge) is taken as “0”. The relation between “1” and “0”, however, may be reverse. 
     Subsequently, a load current of 2 μA is applied to the bit line BL. Since the load current flows into the common source line SL in the bit line BL which reads out data from the NAND-type memory cell unit ND 1  after erase (when the electric charge has not been stored yet), the electric potential of this bit line BL reaches a low level of about 7 V. On the other hand, since the load current does not flow into the common source line SL in the bit line BL which reads out the data from the NAND-type memory cell unit ND 1  after writing (after storing the electric charge), the electric potential of this bit line BL reaches a high level of about 1.8 V. The electric potential of this bit line BL is sensed by the latch circuit and stored. 
     (Write Operation) 
     In the write operation, write data are continuously loaded on a page buffer. “0” is cell data to perform writing and means that an electric charge is stored in the floating gate FG. “1’ is cell data to prohibit writing and means that no electric charge is stored in the floating gate FG. The write operation is repeated until all the cell data “0” are written. 
     This write operation is roughly divided into a write period and a verify period. First, the operation during the write period will be explained based on FIG.  36 . 
     FIG. 36 is a timing chart showing the voltage relationship between signal lines in the write period of the write operation. As shown in FIG. 36, at a point in time T1, the select gate lines SSL 1  and SSL 2  are set at Vcc (=3.5 V), and a bit line BL 0  which writes “0”, and a bit line BL 1  which writes “1” are set at Vcc (=3.5 V). Hence, the select gate transistors ST 1  to ST 4  are in an ON-state, and all channel regions of the NAND memory cell units ND 1  and ND 2  are charged with a spare voltage. 
     Thereafter, at a point in time T2, the select gate lines SSL 1  and SSL 2  are set at Vss (=0 V), and the select gate transistors ST 2  and ST 3  are set in an OFF-state. Then, at a point in time T3, the bit line BL 0  to write “0” is set at Vss (=0 V). At a point in time T4, the select gate line SSL 2  is set at Vcc (=3.5 V). Thereby, the select gate transistor ST 3  is in an ON-state, and only the NAND-type memory cell unit ND 1  is selected since the select gate transistor ST 1  is of a depletion type. 
     Subsequently, at a point in time T5, the selected word line WLi is set at Vpgm (=18 V), and the non-selected word lines WL 0  to WL 15  (except WLi) are set at Vpass (=10 V). As a result, a channel region of the memory cell transistor MT in which “0” is to be written is grounded at the voltage (Vss) of the bit line BL 0 , and an electric charge is stored in the floating gate FG. Meanwhile, a channel region of the memory cell transistor MT in which “1” is to be written is in a high floating state by the voltage (Vcc) of the bit lint BL 1 , and no electric charge is stored in the floating gate FG. Namely, an erase state is maintained. Moreover, a channel region of the memory cell transistor MT in the NAND-type memory cell unit ND 2  which is not selected is also in a high floating state, whereby an existing state is maintained. 
     The state at the point in time T5 lasts till a point in time T6, and at the point in time T6, the select gate line SSL 2  is set at Vss (=0 V), and the word lines WL 0  to WL 15  are set at Vss (=0 V). 
     In the aforesaid operation, a period from the point in time T1 to the point in time T3 is (1) a bit line set up time, which is about 8 μs. A period from the point in time T4 to the point in time T6 is (2) an actual write time, which is about 20 μs. 
     Next, a verify period after writing will be explained. This verify period after writing is composed of a word line discharge time and an actual verify time. The word line discharge time is to discharge a high electric potential of the selected word line WLi and prepare for the input of a succeeding low verify potential, which is about 4 μs. The actual verify time is to check whether a threshold voltage of the memory cell transistor MT in which writing has been performed is written to an extent equal to or more than a target value, that is, whether an electric charge is stored in the floating gate FG. 
     In the verify period after writing, as for the memory cell transistor MT in which writing has been necessarily and sufficiently performed, that is, the memory cell transistor MT in which an electric charge is stored in the floating gate FG to a necessary and sufficient extent, it is required to prevent excessive writing. Therefore, as for the memory cell transistor MT in which writing has been necessarily and sufficiently performed, data held by a latch circuit of cell data in the page buffer are changed from “0” to “1”. Thereby, when writing is performed again with respect to the memory cell transistor MT in which writing is insufficient, the threshold voltage of the memory cell transistor MT which has already reached a necessary and sufficient value is prevented from increasing further. 
     A bias condition in the verify operation is almost the same as that in the aforesaid read operation, but differs therefrom in that the cell data are held in the latch circuit in the page buffer and a voltage of 0.7 V is applied to the selected word line WLi. Under this condition, when the threshold voltage of the memory cell transistor MT in which writing has been performed exceeds 0.7 V, that is, when writing is necessarily and sufficiently performed, data in the latch circuit in the page buffer are changed from “0” to “1”. As for the latch circuit in which “1” is loaded as the cell data for writing, the data in the latch circuit only changes from “0” to “1”, whereby this latch circuit receives no influence in the verify operation. 
     The write operation composed of the aforesaid write period and verify period is repeated until all the data in the latch circuit in the page buffer turn into “1” or until it reaches the maximum write time of 10 cycles. 
     Incidentally, a charge of a spare voltage is given from the bit line BL side to the channel regions of the NAND-type memory cell units ND 1  and ND 2  in the write operation in FIG. 36, while a charge of a spare voltage may be given from the common source line SL side as shown in FIG.  37 . In this case, from a point in time T11 to a point in time T12, the select gate line GSL is at Vcc (=3.5 V), and the select gate transistors ST 5  and ST 6  are in an ON-state. During this period of time, the select gate lines SSL 1  and SSL 2  are maintained at Vss (=0 V), whereby the select gate transistors ST 1  to ST 4  are in an OFF-state. Therefore, the supply and charge of a spare voltage are performed from the common source line SL to the channel regions of the memory cell transistors MT. 
     Next, a bias condition on a write prohibiting voltage to be supplied to a channel of a selected cell will be explained. If the NAND-type memory cell unit ND 1  in FIG. 1 is selected as described above, the select gate transistors ST 1  and ST 2  on the bit line BL side are in a conduction state, the select gate transistor ST 5  on the source line SL side is in a non-conduction state, the bit line BL 0  having the memory cell transistor MT in which writing is performed is set at 0 V, and the bit line BL 1  having the memory cell transistor MT in which writing is prohibited is set at Vcc (=3.5). 
     Since the bit line BL 0  having the memory cell transistor MT in which writing is performed is set at 0 V, each channel of its NAND-type memory cell unit ND 1  has a ground potential. Since the bit line BL 1  having the memory cell transistor MT in which writing is prohibited is set at Vcc (=3.5 V), the channels of its NAND-type memory cell unit ND 1  are preliminarily charged. When a write voltage Vpgm is inputted to the selected word line WLi and a pass voltage Vpass (=10 V) is inputted to the non-selected word lines WL 0  to WL 15  (except WLi), a capacitance of channel is automatically increased in voltage by series coupling of capacitances via the word lines WL 0  to WL 15 , the floating gate FG, the channel, and the memory cell P-type well. As described above, the channel potential of the NAND-type memory cell unit ND 1  in which writing is prohibited in the selected block is determined by capacitance coupling of the word lines and the channels. Accordingly, in order to sufficiently increase a write prohibiting potential, it is important to perform sufficient initial charge of the channels and to increase a capacitance coupling ratio among the channels of the word lines WL 0  to WL 15 . 
     A coupling ratio B among the word lines WL 0  to WL 15  is calculated as follows: 
     
       
           B−Cox /( Cox+Cj )  
       
     
     where Cox is a total of gate capacitances between the word lines WL 0  to WL 15  and the channels and Cj is a total of junction capacitances of sources and drains of the memory cell transistors MT. A channel capacitance of the NAND-type memory cell unit ND 1  is a sum of the total Cox of gate capacitances and the total Cj of junction capacitances. Moreover, other capacitances such as an overlap capacitance of sources in the select gate transistors ST 1 , ST 3 , and ST 5 , and capacitances between the bit line BL, and the source line SL and the drains are very small compared with the whole channel capacitances, and hence neglected here. 
     As described above, according to the nonvolatile semiconductor memory in this embodiment, the NAND-type EEPROM is three-dimensionally made, whereby the size of a cell can be reduced by half, resulting in a reduction in bit cost. Namely, since two NAND-type memory cell units ND 1  and ND 2  are disposed in one bit line pitch 2 F, the cell size can be reduced by half. 
     More specifically, as shown in FIG. 42, the substantial memory cell size of the related NAND-type EEPROM is 4 F 2 +α obtained by adding a bit line contact, a source contact, and the select gate transistors ST 1  and ST 2  to a bit line BL pitch 2 F×a word line WLi pitch 2 F=4 F 2 . This 4 F 2 +α is about 5 F 2 . As against this, in the NAND-type EEPROM according to this embodiment, as shown in FIG. 2, its substantial memory cell size is 2 F 2 +α obtained by adding a bit line contact, a source contact, and the select gate transistors ST 1  and ST 6  to a bit line BL pitch F×a word line WLi pitch 2 F=2 F 2 . This 2 F 2 +α is about 2.5 F 2 . Thus, the substantial memory cell size can be reduced by half compared with the related one. 
     As shown in FIG. 3B, the control gate CG and the polysilicon  22  are formed out of a polysilicon with a relatively high resistivity, and the word lines WL 0  to WL 15  are formed out of a metal layer with a resitivity lower than the polysilicon. Consequently, the resistance of the word lines WL 0  to WL 15  can be reduced while a coupling ratio of the channel region to the floating gate FG in the memory cell transistor MT is kept large. The aforesaid reduction in the resistance of the word lines WL 0  to WL 15  makes it possible to secure the high-speed property of the operation of the nonvolatile semiconductor memory. 
     Further, as shown in FIG.  2  and FIG. 4, in the select gate transistors ST 1  to ST 6 , the inter-poly insulating film  19  is removed. Therefore, the gate electrode GE and the polysilicon  22  can be electrically connected directly, whereby it becomes unnecessary to form a shunt which electrically connects the gate electrode GE and each of the select gate lines SSL 1  and SSL 2  which has been hitherto necessary. As a result, an unnecessary shunt region can be omitted, leading to a reduction in manufacturing cost. 
     It should be mentioned that the present invention is not limited to the aforesaid embodiment and can be modified variously. For example, although two select gate transistors ST 1  and ST 3  (ST 2  and ST 4 ) as the bit line side switching portion are provided on the bit line BL side of the NAND-type memory cell unit ND 1  (ND 2 ) and one select gate transistor ST 5  (ST 6 ) as a source line side switching portion is provided on the common source line SL side as shown in FIG. 1 in the aforesaid embodiment, the numbers of select gate transistors in these bit line side switching portion and source line side switching portion may be reversed. Namely, as shown in FIG. 38, it is suitable to provide one select gate transistor ST 10  (ST 11 ) as the bit line side switching portion on the bit line BL side of the NAND-type memory cell unit ND 1  (ND 2 ) and to provide two select gate transistors ST 12  and ST 14  (ST 13  and ST 15 ) as the source line side switching portion on the source line SL side. In this case, it is recommended that the select gate transistor ST 12  (ST 15 ) be of a depletion type, and that the select gate transistor ST 10  (ST 11 ) and the select gate transistor ST 14  (ST 13 ) be of a enhancement type. In this case, a schematic diagram showing a cross section of the select gate transistors ST 12  and ST 13  are like FIG.  4 . 
     Furthermore, as shown in FIG. 39, it is also suitable to provide two select gate transistors ST 1  and ST 3  (ST 2  and ST 4 ) as the bit line side switching portion and to provide two select gate transistors ST 12  and ST 14  (ST 13  and ST 15 ) as the source line side switching portion. 
     Moreover, as shown in FIG. 40, if the disposition of the bit lines BL permits, it is possible to provide only enhancement-type select gate transistors ST 20  (ST 21 ) and ST 5  (ST 6 ) respectively in the bit line side switching portion and the source line side switching portion in the vertical NAND-type memory cell unit ND 1  (ND 2 ) without using the depletion-type select gate transistor as described above. 
     When 4 G (giga) or 16 Gbits NAND-type EEPROM is designed by a 0.1 μm rule, in FIG.  2  and FIG. 3, the width of the trench region  14  is 0.1 μm, thin oxide films  17  with a thickness of 8 nm are formed on both side wall portions inside the trench region  14 , two floating gates FG 10 nm in thickness are formed on inner sides of both the thin oxide films  17 , the inter-poly insulating film  19  with a thickness of 20 nm is formed to cover the floating gates FG, and the one control gate CG 24 nm in thickness is filled therein, thereby composing a pair of memory cell transistors MT and MT. When a design rule finer than the 0.1 μm rule is used, the thin oxide film  17 , the floating gate FG, the inter-poly insulating film  19 , and the control gate CG are respectively made thinner in moderation. 
     As explained above, according to the aforesaid embodiment, two NAND-type memory cell units are connected to one bit line, and hence the two NAND-type memory cell units can be formed within a bit line pitch of 2 F, resulting in a reduction in the size of the nonvolatile semiconductor memory.