Patent Publication Number: US-6984567-B2

Title: Nonvolatile semiconductor memory device and manufacturing method thereof

Description:
This application is a Continuation application of U.S. application Ser. No. 10/301,643 filed Nov. 22, 2002 now U.S. Pat. No. 6,741,501. Priority is claimed based on U.S. application Ser. No. 10/301,643 filed Nov. 22, 2002, which claims the priority date of Japanese Patent Application No. 2001-366870 filed Nov. 30, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is related to semiconductor integrated circuit devices and a manufacturing technique therefor and, in particular, to a method of attaining high integration, high reliability and high performance of a nonvolatile semiconductor memory device. 
     2. Description of Related Art 
     Since flash memory devices functioning similarly to EEPROMs (Electrically Erasable Programmable Read Only Memory) are capable of electrical writing and erasing and are very portable and impact resistant, the demand therefor has recently increased rapidly for use in mobile phones, portable personal computers, digital cameras and in microcomputers in the engine control systems of automobiles. 
     To improve the marketability of such devices, reduction of the memory cell area is essential for reducing the cost thereof and various memory cell systems have been proposed for attaining the same. One such system is a virtual ground type memory cell using a 3-layered polysilicon gate as disclosed, for example, in JP-A No. 200242/1999. 
     The memory cell of the type described above comprises, as shown in  FIG. 60 , a well  601  in a silicon substrate  600 , source and drain diffusion layer regions  605 ,  605 ′ in the well, and three gates including a floating gate  603   b  as a first gate comprising a polysilicon film formed on the well, a control gate  611   a  as a second gate, and a third gate  607   a  for controlling an erasing gate and a split channel. 
     Each of the gates  603   b ,  611   a , and  607   a  comprises a polysilicon film, and are isolated from the well  601  by insulator films  602 ,  606   a ,  606   b ,  608   a , and  610 . The control gates  611   a  are connected in a row to constitute word lines. The source and drain diffusion layer  605  is a virtual ground type having the diffusion layer of an adjacent memory cell in common to reduce the pitch in the direction of the row. The third gates  607   a  are disposed in parallel with the channels and perpendicular to the word lines  611   a.    
     Upon writing, positive voltage independent of each other is applied to the word line  611   a , the drain  605  and the third gate  607   a  while setting the well  601  and the source  605 ′ to 0 V. Thus, hot electrons are generated in a channel at the boundary between the third gate and the floating gate and they are injected into the floating gate  603   b . This increases the threshold voltage of the memory cell. Upon erasing, a positive voltage is applied to the third gate  607   a  while setting the word line  611   a , the source  605 ′, the drain  605  and the well  601  to 0 V. This discharges electrons from the floating gate  603   b  to the third gate  607   a  to lower the threshold voltage. “0” and “1” of information is determined by changing the threshold voltage of the memory cell transistor. 
     Problems arise, however, when to the density is increased in the nonvolatile semiconductor memory device described above. 
     At first, this concerns the reduction in the size of the memory cell. When it is intended to further reduce the size of the split gate type memory cell, it is important to reduce the gate length for the floating gate  603   b  and the third gate  607   a . For this purpose, it is necessary to reduce the thickness of the gate insulator films  602  and  606   a  and improve the punch through resistance. However, no discussion has been made on the thickness of each gate insulator film. 
     Next, there is a problem in improving the reliability. In a flash memory, it has to be ensured that the data is held for more than 10 years after conducting writing/erasing cycles for 100,000 cycles or more. Elimination of data is caused by leakage of electrons accumulated in the floating gate. 
     While there are present plural modes for the cause of the leakage, the present inventors have found that the mode where electrons accumulate in the floating gate and incidentally leak at certain bits is most problematic. Such leakage has a correlation with the thickness of the gate insulator film  602  between the floating gate and the substrate, the so-called tunnel film, and the number of failure bits increases as the thickness of film is reduced. 
     According to the study made by the present inventors, it is necessary to increase the film thickness of the tunnel insulator film  602  to 9 nm or more in order to hold the data for 10 years, particularly in multi-level storage of 2 bits in one memory cell. Accordingly, to minimize the size of these memory devices, the thickness of the gate insulator films  602  and  606   a  must be considered. 
     The third problem is the increase in the number of manufacturing steps. A usual flash memory has a peripheral circuit for applying voltage to the memory cell or conducting logic operations. In the circuit for applying the voltage to the cell, since a high voltage, for example, of 18 V is applied to the word line, a MOS (Metal Oxide Semiconductor) transistor constituting the circuit has a high voltage withstanding structure using a thick gate insulator film of, for example, 25 nm. On the other hand, in the circuit for conducting the logic operation, the applied voltage is, for example, 3 V of an external power source voltage and high-speed operation is required. Therefore, the thickness of the gate insulator film of the MOS constituting the circuit for conducting the logic operation is extremely thin compared with the structure for withstanding high voltage. Thus, improved manufacturing methods are needed to form the two kinds of gate insulator films for the peripheral circuit and the two kinds of gate insulator films for the memory cell to simplify production and reduce the cost thereof. 
     As described above, the development of a new nonvolatile semiconductor memory device and a manufacturing method therefor are desirable with respect to gate insulator films of a split gate type memory cell. 
     A nonvolatile semiconductor memory device of reduced size and improved reliability are desirable. 
     Simplified production steps for producing such nonvolatile semiconductor memory devices are also desirable. 
     SUMMARY OF THE INVENTION 
     In a first preferred embodiment, the nonvolatile semiconductor memory device has a plurality of memory cells each comprising a first conduction type well formed in a silicon substrate, source/drain diffusion layer regions of a second conduction type formed in the well, a channel formed in a direction perpendicular to the diffusion layer region, a floating gate as a first gate formed on the silicon substrate comprising an insulator film, a control gate as a second gate comprising an insulator film disposed on the floating gate, a word line formed by connection of the control gate, and a third gate having a function different from those of the floating gate and the control gate and comprising an insulator film insulative with respect to the silicon substrate, the floating gate and the control gate, wherein the thickness of a gate insulator film for isolating the floating gate from the well is made larger than the thickness of the gate insulator film isolating the third gate from the well. 
     Preferably the third gate controls the split channel. Alternatively, the third gate functions as an erasing gate and to control the split channel. 
     The insulator film between the third gate and the well is formed by the same process step as the gate insulator film disposed on the low voltage area of the peripheral circuit region. 
     Alternatively, the insulator film between the floating gate and the well is preferably formed by the same process step as the gate insulator film disposed on the low voltage area of the peripheral circuit region. 
     The thickness of the insulator film between the floating gate and the third gate is preferably larger than the insulator film thickness between the floating gate and the well. Alternatively, the thickness of the insulator film between the floating gate and the third gate is preferably substantially identical to the insulator film thickness between the floating gate and the well. 
     In this first exemplary embodiment, the insulator film between the floating gate and the third gate preferably comprises a silicon oxide film containing nitrogen. 
     In a second preferred embodiment, the nonvolatile semiconductor memory device comprises (1) a plurality of memory cells each comprising a first conduction type well formed in a silicon substrate, source/drain diffusion layer regions of a second conduction type formed in the well, a channel formed in a direction perpendicular to the diffusion layer region, a floating gate as a first gate comprising an insulator film disposed on the silicon substrate, a control gate as a second gate comprising an insulator film disposed on the floating gate, a word line formed by connection of the control gate, and a third gate of having a function different from the floating gate and the control gate and comprising an insulator film isolating the silicon substrate, the floating gate and the control gate, and (2) a peripheral circuit region for operating the memory cell, wherein the peripheral circuit region comprises MOS transistors of a low voltage system and a high voltage system in which the film thickness of the gate insulator film for the low voltage system MOS transistor is substantially identical i.e., formed in the same process step and comprising part of the same layer as the gate insulator film between the third gate and the well, and the thicknesses of the insulator films are larger in the order of (i) the gate insulator film for the high voltage system MOS transistor, (ii) the gate insulator film between the floating gate and the well, and (iii) the gate insulator film between the third gate and the well. 
     In a third preferred embodiment, the nonvolatile semiconductor memory device comprises (1) a plurality of memory cells each comprising a first conduction type well formed in a silicon substrate, source/drain diffusion layer regions of a second conduction type formed in the well, a channel formed in a direction perpendicular to the diffusion layer region, a floating gate as a first gate comprising an insulator film disposed on the silicon substrate, a control gate as a second gate comprising an insulator film disposed on the floating gate, a word line formed by connection of the control gate, and a third gate having a function different from the floating gate and the control gate and comprising of an insulator film isolating the silicon substrate, the floating gate and the control gate, and (2) a peripheral circuit region for operating the memory cell, wherein the peripheral circuit region comprises MOS transistors of a low voltage system and a high voltage system in which the film thickness of the gate insulator film for the low voltage system MOS transistor is substantially identical i.e., formed in the same process step and comprising part of the same layer as the gate insulator film between the floating gate and the well, and the thicknesses of the insulator films are larger in the order of (i) the gate insulator film for the high voltage system MOS transistor, (ii) the gate insulator film between the floating gate and the well, and (iii) the gate insulator film between the third gate and the well. 
     In the nonvolatile semiconductor memory device of this third preferred embodiment, the memory cell conducts writing by the charging of hot electrons from the channel to the floating gate and erasing by tunnel emission from the floating gate to the well. Alternatively, the memory cell conducts writing by the charging of hot electrons from the channel to the floating gate and erasing by tunnel emission from the floating gate to the third gate. 
     Alternatively, a nonvolatile semiconductor memory device of the present invention preferably comprises a memory cell comprising a first conduction type well formed in a silicon substrate, source/drain diffusion layer regions of a second conduction type formed in the well, a channel formed in a direction perpendicular to the diffusion layer region, a floating gate as a first gate formed on the silicon substrate by way of an insulator film, a control gate as a second gate formed by way of an insulator film to the floating gate, a word line formed by connection of the control gate, and a third gate of a function different from the floating gate and the control gate and formed by way of an insulator film to the silicon substrate, the floating gate and the control gate, wherein the thickness of the gate insulator film for isolating the floating gate from the well is larger compared with the thickness of the gate insulator film for isolating the control gate from the well. 
     In this case, the third gate preferably is an erasing gate. 
     Further, the thickness of the insulator film between the floating gate and the control gate preferably is larger compared with the film thickness between the floating gate and the well. 
     Alternatively, the thickness of the insulator film between the floating gate and the control gate is substantially identical with the thickness between the floating gate and the well. 
     In this preferred example, the insulator film between the floating gate and the third gate is a silicon oxide film containing add nitrogen. 
     In the operation system of this case, writing to the memory cell is conducted by injection of channel hot electrons from the channel to the floating gate and erasing from the memory cell is conducted by tunnel emission from the floating gate to the third gate. 
     Alternatively, the nonvolatile semiconductor memory device of the present invention preferably comprises a memory cell comprising a first conduction type well formed in a silicon substrate, source/drain diffusion layer regions of a second conduction type formed in the well, a channel formed in a direction perpendicular to the diffusion layer region, a floating gate as a first gate formed on the silicon substrate by way of an insulator film, and a control gate as a second gate formed by way of an insulator film to the floating gate, and the channel, wherein the thickness of the gate insulator film for isolating the floating gate from the well preferably is larger compared with the thickness of the gate insulator film for isolating the control gate from the well. 
     In this case, the control gate preferably is a gate for controlling a split channel. 
     Further, writing to the memory cell is conducted by injection of channel hot electrons from the channel to the floating gate and erasing from the memory cell is conducted by tunnel emission from the floating gate to the drain. 
     Further, the gate insulator film between the floating gate and the well and the gate insulator film between the third gate and the well may be formed in the same manufacturing step that produces the gate insulator film of a MOS transistor in a peripheral circuit low voltage portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention is to be described specifically for preferred embodiments with reference to the drawings. Throughout the drawings for explaining the preferred embodiments, those having identical functions carry the same reference numerals, for which duplicate explanations have been omitted, wherein: 
         FIG. 1  is a plan view for a main portion of a substrate showing the nonvolatile semiconductor memory device (flash memory) of a first preferred embodiment of the present invention; 
         FIG. 2  is a circuit diagram showing the constitution of a memory array for the nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 3  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 4  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of the nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 5  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of the nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 6  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of the nonvolatile semiconductor memory device of the first preferred as embodiment of the present invention; 
         FIG. 7  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of the nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 8  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 9  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 10  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 11  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 12  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 13  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 14  is a cross sectional view for a main portion of a substrate showing a nonvolatile semiconductor memory device of the first preferred embodiment of the present invention; 
         FIG. 15  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of a second preferred embodiment of the present invention; 
         FIG. 16  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the second preferred embodiment of the present invention; 
         FIG. 17  is a cross sectional view for a main portion of a substrate showing a nonvolatile semiconductor memory device of the second preferred embodiment of the present invention; 
         FIG. 18  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of a third preferred embodiment of the present invention; 
         FIG. 19  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the third preferred embodiment of the present invention; 
         FIG. 20  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the third preferred embodiment of the present invention; 
         FIG. 21  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the third preferred embodiment of the present invention; 
         FIG. 22  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the third preferred embodiment of the present invention; 
         FIG. 23  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the third preferred embodiment of the present invention; 
         FIG. 24  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the third preferred embodiment of the present invention; 
         FIG. 25  is a cross sectional view for a main portion of a substrate showing a nonvolatile semiconductor memory device of the third preferred embodiment of the present invention; 
         FIG. 26  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of a fourth preferred embodiment of the present invention; 
         FIG. 27  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fourth preferred embodiment of the present invention; 
         FIG. 28  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fourth preferred embodiment of the present invention; 
         FIG. 29  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fourth preferred embodiment of the present invention; 
         FIG. 30  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fourth preferred embodiment of the present invention; 
         FIG. 31  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fourth preferred embodiment of the present invention; 
         FIG. 32  is a cross sectional view for a main portion of a substrate showing a nonvolatile semiconductor memory device of the fourth preferred embodiment of the present invention; 
         FIG. 33  is a cross sectional view for a main portion of a substrate showing a nonvolatile semiconductor memory device of the fourth preferred embodiment of the present invention; 
         FIG. 34  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of a fifth preferred embodiment of the present invention; 
         FIG. 35  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 36  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 37  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 38  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 39  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 40  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 41  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 42  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 43  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 44  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 45  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 46  is a cross sectional view for a main portion of a substrate showing a nonvolatile semiconductor memory device of the fifth preferred embodiment of the present invention; 
         FIG. 47  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of a sixth preferred embodiment of the present invention; 
         FIG. 48  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 49  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 50  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 51  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 52  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 53  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 54  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 55  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 56  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 57  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 58  is a cross sectional view for a main portion of a substrate showing a preferred manufacturing method of a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; 
         FIG. 59  is a cross sectional view for a main portion of a substrate showing a nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention; and 
         FIG. 60  is a cross sectional view for a main portion of a substrate of a nonvolatile semiconductor memory device for showing the effect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements that may be well known. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. The detailed description will be provided herein below with reference to the attached drawings. 
     First Preferred Embodiment 
     Referring to  FIG. 1  to  FIG. 14 , the constitution, operation method and manufacturing method of a nonvolatile semiconductor memory device (flash memory) of a first preferred embodiment of the present invention are described below.  FIG. 1  is a plan view for a main portion of a flash memory, and  FIG. 3  to  FIG. 14  are cross sectional views along line A–A′ of  FIG. 1  illustrating the production steps of the flash memory of this first preferred embodiment. Further,  FIG. 2  is a circuit diagram showing the constitution of memory array in which memory cells are arranged in a matrix in said first preferred embodiment of the present invention. 
     First, the structure of the flash memory cell is described below. As shown in  FIG. 1  and  FIG. 14  which is a cross sectional view along A–A′ in  FIG. 1 , the memory cell comprises a source/drain diffusion layer  105  in a p-type well (semiconductor region)  101  formed in a silicon substrate  100 , a floating gate  107   b  as a first gate, a control gate as a second gate (word line)  110   a , and a third gate  103   a . One memory cell is formed, for example, in a region surrounded with a solid fat line of  FIG. 1 . The control gate  110   a  for the memory cells M by a predetermined number are connected in the direction of a row (direction X) to form word lines (WL 0 −WL n-1  in  FIG. 2 ). 
     The floating gate  107   b  and the p-well  101  are isolated from each other by a gate insulator film  106   a , the third gate  103   a  and the p-well  101  are isolated from each other by a gate insulator film  102 , the floating gate  107   b  and the third gate  103   a  are isolated from each other by an insulator film  106   b , the floating gate  107   b  and the word line (control gate)  110   a  are isolated from each other by an insulator film (ONO film)  109   a , and the third gate  103   a  and the word line (control gate)  110   a  are isolated from each other by a silicon oxide films  104   b  and  109   a.    
     The source/drain diffusion layer  105  is disposed perpendicular to the word line (control gate)  110   a  and present as a local source line and a local data line for connecting the source/drain of the memory cells in the direction of a column (direction Y). That is, this nonvolatile semiconductor memory device comprises a so-called contactless type memory cell array having no contact hole on every memory cell. A channel is formed in a direction perpendicular to the source/drain diffusion layer  105  (direction X). 
     The lateral sides of the third gate  103   a  extending in the direction Y are present opposing to the lateral side of the end faces of the floating gate  107   b  extending in the direction Y by way of the insulator film  106   b , respectively, (refer to  FIG. 1 ). The floating gate  107   b  is disposed in the gaps defined by the plurality of third gates  103   a  present in a perpendicular direction (direction Y) to the word line (control gate)  110   a  and the channel. Further, the floating gates  107   b  are present in symmetry with the third gate  103   a  and the third gates  103   a  are present in symmetry with the floating gate  107   b.    
     In this preferred embodiment, a pair of diffusion layers  105  forming the source/drain are in an asymmetric positional relation to the pattern of the floating gate  107   b  in such an offset structure that one of the diffusion layers does not overlap with the floating gate  107   b . Further, the third gate  103   a  and the diffusion layer  105  are present such that respective portions of them overlap with each other. 
     Next the writing/erasing operation of this preferred embodiment is described with reference to  FIG. 2 . 
     When writing is conducted by selecting the memory cell M in  FIG. 2 , a high positive voltage, for example, at about 12 V is applied to a word line WLm, a low voltage at 2 V is applied to a third gate AGe and, a voltage at about 4 V is applied to a drain DLm. Source DLm- 1  and the well are held at 0 V. Thus, a channel is formed in the well below the third gate  103   a  and hot electrons are generated in the channel at the floating gate end on the side of the source to inject electrons into the floating gate. Thus, the third gate  103   a  functions as a gate for controlling the channel present therebelow. 
     According to this memory cell, generation and injection efficiency of hot electrons is increased compared with existent NOR type flash memories and writing in a region of small channel current is enabled. Accordingly, a number of memory cells of Kbytes order or larger can be written in parallel by an internal power source having the same extent of current supply performance as usual. 
     Upon erasure, a high negative voltage, for example −18 V is applied to the word line WLm. In this case, the third gates AGe and AGo, all the source/drain diffusion layers DL and the well are held at 0 V. Thus, electrons accumulated in the floating gate are emitted into the well by a tunnel phenomenon. 
     The memory cell in this preferred embodiment is different from the memory cell, for example, shown in  FIG. 60  in that the thickness of the gate insulator film  106   a  between the floating gate  107   b  and the p-well  101 , that is, a so-called tunnel insulator film, is made larger compared with the gate insulator film  102  between the third gate  103   a  and the well  101 . 
     Since the thickness of the tunnel insulator film  106   a  is made larger, it is possible to suppress charge retention failure caused by incidental leakage of electrons accumulated in the floating gate at specified bits. Further, since the thickness of the gate insulator film  102  for the third gate  103   a  is reduced, the punch through resistance of MISFET (Metal Insulator Semiconductor Field Effect Transistor) (referred to herein as “MOS transistor”) constituted by the third gate  103   a  is improved, thereby enabling a decrease in the gate length of the third gate. Accordingly, improved reliability and refinement for the memory cell are compatible with each other. 
     According to this preferred embodiment, since the thickness of the gate insulator film  102  for the third gate  103   a  is reduced, the channel current can be increased to improve the access time of the memory cell. 
     A preferred method of manufacturing the memory cell of this preferred embodiment is described below with reference to  FIG. 1  to  FIG. 14 . 
     At first, as shown in  FIG. 3 , a p-well  101  is formed on a silicon substrate  100  and then a gate insulator film  102  of 7.5 nm thickness for isolating a third gate from the well is formed by a known thermal oxidation process. Successively, as shown in  FIG. 4 , a phosphorus doped polysilicon film  103  and a silicon oxide film  104  as a third gate are deposited. Then, as shown in  FIG. 5 , the silicon oxide film  104  and the polysilicon film  103  are patterned by known lithography and dry etching technology. As a result, the silicon oxide film and the polysilicon film are formed as  104   a  and  103   a  (third gate), respectively. 
     Subsequently, as shown in  FIG. 6 , a diffusion layer  105  as a source/drain of a memory cell is formed by implanting arsenic ions by a tilted ion implanting process. 
     Then, as shown in  FIG. 7 , a 9 nm thermal oxidation film for isolating the floating gate from the substrate, a so-called tunnel insulator film  106   a  is formed on the silicon substrate (p-well  101 ) by a known thermal oxidation process. In this step, a thermal oxidation film (insulator film)  106   b  of about 20 nm thickness is formed on the side wall of the third gate  103   a  to enable isolation between the floating gate and the third gate. The thickness of the thermal oxidation film  106   b  is larger compared with  106   a , because phosphorus is doped on the polysilicon film as the material for the third gate  103   a  and accelerated oxidation is caused by phosphorus. 
     Then, as shown in  FIG. 8 , a phosphorus-doped polysilicon film  107  as the floating gate is deposited. In this step, the thickness of the polysilicon film  107  is set to such a value as to not fill the gap between the stacked films of the third gate  103   a  and the silicon oxide film  104   a.    
     Successively, as shown in  FIG. 9 , a flowable organic material  108 , for example, a resist material for lithography or an anti-reflection film, is coated such that the gap between the stacked films of the third gate  103   a  and the silicon oxide film  104   a  is completely filled. 
     Then, as shown in  FIG. 10 , the flowable organic material  108  is etched back by a dry etching process to leave the material only in the gap between the stacked films of the third gate  103   a  and the silicon oxide film  104   a . As a result, the flowable organic material  108  is formed as  108   a . Subsequently, the polysilicon film  107  is etched back using the organic material  108   a  as a mask. As a result, the polysilicon film  107   a  remains between the stacked films of the third gate  103   a  and the silicon oxide film  104   a  to form a polysilicon film  107   a . Further, the organic material  108   a  is formed as  108   b.    
     Then as shown in  FIG. 12 , the organic material  108   b  is removed by a known ashing process. Then, as shown in  FIG. 13 , a stacked film of silicon-oxide film/silicon nitride film/silicon oxide film for isolating the floating gate from the word line, a so-called ONO film  109 , is formed at a thickness of 13 nm by being converted from the oxide film on the silicon substrate. 
     Then, as shown in  FIG. 14 , a stacked film of a polysilicon film and a tungsten silicide film  110 , a so-called polyside film, is deposited. Then, it is patterned by known lithography and dry etching technology so as to extend in the direction X shown in  FIG. 1  to form word lines (control gate). As a result, the polyside film  110  is formed as  110   a  (word line, control gate). 
     Further, the ONO film  109  and the polysilicon film  107   a  are successively etched using the word lines (control gate)  110   a  as a mask to complete the floating gate. Thus, the polysilicon film  107   a  is formed as  107   b  (floating gate), the ONO film  109  is formed as  109   a  and the silicon oxide film  104   a  is formed as a silicon oxide film  104   b.    
     Subsequently, although not illustrated in the drawing, after forming an interlayer insulator film on the silicon substrate, contact holes reaching the word line (control gate)  110   a , the source/drain diffusion layer  105 , the p-well  101  and the third gate  103   a  are formed and, successively, a metal film is deposited and patterned into wirings to complete the memory cell. 
     In the memory cell of this preferred embodiment in which the thickness of the gate insulator film  106   a  between the floating gate  107   b  and the p-well  101  is made larger compared with the gate insulator film  102  between the third gate  103   a  and the p-well  101 , when compared with the case in which the thickness of the gate insulator film  106   a  is made equal with or smaller than that of the gate insulator film  102 , the charge retention characteristic of the memory cell was comparable therewith after many writing/easing cycles but punch through did not occur even when the gate length of the third gate was decreased and stable operation was possible. Further, a larger channel current was obtained to improve the access time of the nonvolatile semiconductor memory device. 
     As described above, in this preferred first exemplary embodiment of the nonvolatile semiconductor memory device of the present invention, memory cell area has been reduced while maintaining reliability after many writing/erasing cycles and access time has been improved. 
     Second Preferred Embodiment 
     The, constitution, operation method and manufacturing method of a nonvolatile semiconductor memory device (flash memory) of a second preferred embodiment of the present invention are described below with reference to  FIGS. 15 to 17 . This embodiment is different from the first preferred embodiment in that the gate insulator film between the floating gate  107   b  and the p-well  101  is preferably formed by a low pressure chemical vapor deposition process (LPCVD) instead of the thermal oxidation process. The plan view for the main portion of the flash memory, the cross sectional structure of the memory cell after completion, and the constitution of the memory array are identical with those of the first preferred embodiment, which are omitted here. 
     A preferred manufacturing method of this memory cell of the second preferred embodiment is described below. 
     At first, by the same method as in Embodiment 1 which has been explained with reference to  FIGS. 3 to 6 , a p-well  101 , a gate insulator film  102 , a third gate  103   a , a silicon oxide film  104   a  and a source/drain diffusion layer  105  are formed successively on the main surface of a silicon substrate  100  ( FIG. 15 ). 
     Then, as shown in  FIG. 16 , a silicon oxide film  111  of 11 nm is formed for isolating the floating gate from the silicon substrate and the floating gate from the third gate. The forming method is as described below. At first, a silicon oxide film is deposited to 11 nm by a low pressure chemical vapor deposition process using mono-silane and di-nitrogen sub-oxide as the starting material. Then, this specimen is heat treated in an ammonia atmosphere to introduce nitrogen into the silicon oxide film  111 . Then, the specimen is annealed in wet oxidation. 
     Micro defects and electron traps in the silicon oxide film  111  are decreased by this step. Since the low pressure chemical vapor deposition process is used, the thickness of the silicon oxide film  111  deposited on the upper surface and the side wall of the stacked film of the third gate  103   a  and the silicon oxide film  104   a , and on the surface of the p-well  101 , are substantially identical. Accordingly, the thickness of the silicon oxide film  111  between the side wall of the third gate  103   a  and the floating gate  107   b  is substantially identical with the thickness of the silicon oxide film (gate insulator film)  111  between the floating gate  107   b  and the p-well  101  (refer to  FIG. 17 ). 
     Then, as shown in  FIG. 17 , a floating gate  107   b , an ONO film  109   a , and a word line (control gate)  110   a  are formed by the same method as in Embodiment 1, which has been explained with reference to  FIGS. 7 to 10 , to complete a memory cell. 
     The memory cell formed according to this second preferred embodiment, like the first preferred embodiment, caused no punch-through even when the gate length of the third gate was reduced and can operate stably. Further, the channel current is increased to improve the access time of the nonvolatile semiconductor memory device. 
     Further, in this second preferred embodiment, the thickness of the insulator film between the floating gate  107   b  and the sidewall of the third gate  103   a  can be decreased compared with that of Embodiment 1. Therefore, erasing operation can be conducted by applying voltage at about −13 V to the selected word line, at about 3 V to the third gate and at 0 V to the source/drain and the well, to conduct electron emission from the floating gate to the third gate. Thus, the internal operation voltage can be lowered and the area of the peripheral circuit region can be decreased compared with the first preferred embodiment. 
     As described above, in this second preferred embodiment of the nonvolatile semiconductor memory device of the present invention, memory cell area has been reduced while maintaining reliability after many writing/erasing cycles, access time has been improved, and internal operation voltage has been lowered. 
     Third Preferred Embodiment 
     In this embodiment, different from the first and second preferred embodiments, the present invention is applied to a split gate type memory cell having no third gate. The constitution, operation method and manufacturing method of a nonvolatile semiconductor memory device (flash memory) of a third preferred embodiment of the present invention are described below with reference to  FIGS. 18 to 25 . 
     At first, the structure of a flash memory cell is described. As shown in  FIG. 25 , this memory cell comprises a source  209 , a drain  207 , a floating gate  203   a  and a control gate  205   a  formed in a p-well  201  formed to a silicon substrate  200 . 
     The floating gate  203   a  and the p-well  201  are isolated from each other by a gate insulator film  202 , the control gate  205   a  and the p-well  201  are isolated from each other by a gate insulator film  204   a , and the floating gate  203   a  and the control gate  205   a  are isolated from each other by an insulator film  204   b , respectively. 
     A portion of the control gate  205   a  overlaps to a channel portion on the silicon substrate while the remaining portion thereof overlaps while riding over the floating gate  203   a . The source  209  overlaps with the control gate  205   a  and the drain  207  overlaps with the floating gate  203   a  by way of the gate insulator film  204   a  or  202 , respectively. 
     As described above, this memory cell is a so-called split gate type memory cell in which a transistor controlled by the control gate  205   a  and the transistor controlled by the floating gate  203   a  are connected in series. 
     Upon writing, voltage is applied at about 2 V to the control gate  205   a  and 12 V to the drain  207 , and the p-well  201  and the source  209  are kept at 0 V. Thus, hot electrons are generated in the channel on the floating gate  203   a  at the source end and electrons are injected into the floating gate  203   a.    
     Upon erasing, voltage is applied at 12 V to the drain  207  and at such a level as not deteriorating to the gate insulator film  204   a , for example, 4 V to the control gate  205   a , and the source  209  and the p-well  201  are kept at 0 V. Thus, electrons accumulated in the floating gate  203   a  are emitted by the tunnel phenomenon into the drain  207 . 
     The reason for adopting such an erasing method is described below. That is, since the thickness of the gate insulator film  204   a  between the control gate  205   a  and the p-well  201  is reduced, a high voltage can not be applied between them and the voltage applied to the control gate  205   a  has to be restricted, for example, to about 4 V as described above. Accordingly, upon erasing, a method of applying a high potential (12 V) on the drain  207  to extract electrons toward the drain  207  has to be taken. 
     Also in the memory cell of this preferred embodiment, different from the memory cell shown in  FIG. 60 , the thickness of the gate insulator film  202  between the floating gate  203   a  and the p-well  201 , a so-called tunnel insulator film, is made larger compared with the gate insulator film  204   a  between the control gate  205   a  and the p-well  201 . 
     By increasing the thickness of the tunnel insulator film  202 , it is possible to suppress charge retention failure caused by incidental leakage of electrons accumulated in the floating gate into the substrate at specified bits. Further, since the thickness of the gate insulator film  204   a  of the control gate  205   a  is reduced, punch-through resistance of the MOS transistor constituted with the control gate  205   a  is improved to reduce the gate length of the control gate  205   a . Accordingly, improved reliability and the refinement of the memory cell are compatible with each other. Further, the channel current can be increased to improve the access time of the memory cell. 
     A preferred method of manufacturing the memory cell of this third preferred embodiment is described with reference to  FIGS. 18 to 25 . 
     At first, as shown in  FIG. 18 , after forming a p-well  201  on a silicon substrate  200 , a gate insulator film  202  of 9 nm thickness for isolating the floating gate from the well is formed by a known thermal oxidation process and, successively, a phosphorus-doped polysilicon film  203  as the floating gate is deposited. Then, as shown in  FIG. 19 , the polysilicon film  203  is patterned by known lithograph and dry etching technology. As a result, the polysilicon film  203  is formed as  203   a  (floating gate). 
     Further, as shown in  FIG. 20 , after patterning the gate insulator film  202 , a thermal oxidation film (gate insulator film)  204   a  for isolating the control gate from the substrate is formed to 7.5 nm thickness on the silicon substrate by a known thermal oxidation process. In this step, a thermally oxidized film  204   b  of about 20 nm is formed on the sidewall and on the floating gate  203   a  to enable isolation between the floating gate and the control gate. The thickness of the thermally oxidized film  204   b  is larger than that of  204   a , because phosphorus is doped in the polysilicon film as the material for the floating gate  203   a  and phosphorus caused accelerated oxidation. 
     Successively, as shown in  FIG. 21 , a stacked film of a phosphorus-doped polysilicon film and a tungsten silicide film as the control gate  205 , a so-called a polyside film, is deposited. Then, as shown in  FIG. 22 , the polyside film,  205  is patterned by known lithography and dry etching technology. As a result, the polyside film  205  is formed as  205   a  (control gate). As illustrated, the control gate  205   a  extends from about the center of the polysilicon film  203   a  to a position above the silicon substrate (p-well  201 ). 
     Subsequently, as shown in  FIG. 23 , a photo-resist pattern  206  is formed on the silicon substrate, phosphorus ions are ion implanted by an ion implantation process and a heat treatment is applied to form the drain region  207  of the memory cell. 
     Successively, as shown in  FIG. 24 , a photo-resist pattern  208  is formed, and arsenic ions are implanted by an ion implanting process to form a source region  209  of the memory cell. Then, the photo-resist pattern  208  is removed ( FIG. 25 ). 
     Subsequently, although not illustrated in the drawing, after forming an interlayer insulator film on the silicon substrate, a contact hole extending in the control gate  205   a , the source region  209 , the drain region  207  and the p-well  201  is formed and, successively, a metal film is deposited and patterned into wirings to complete the memory cell. 
     In this memory cell of the third preferred embodiment in which the thickness of the gate insulator film  202  between the floating gate  203   a  and the p-well  201  is made larger compared with the gate insulator film  204   a  between the control gate  205   a  and the p-well  201 , when compared with the case in which the thickness of the gate insulator film  202  is made equal with or smaller than that of the gate insulator film  204   a , the charge retention characteristic of the memory cell was comparable therewith after many writing/erasing cycles but punch-through did not occur even when the gate length of the control gate  205   a  was decreased and stable operation was possible. Further, a larger channel current was obtained to improve the access time of the nonvolatile semiconductor memory device. 
     As described above, in this third preferred embodiment of the nonvolatile semiconductor memory device of the present invention, memory cell area has been reduced while maintaining reliability after many writing/erasing cycles and access time has been improved. 
     Fourth Preferred Embodiment 
     In this fourth preferred embodiment, the invention is applied to a split gate type memory cell having an erasing gate as a third gate. The constitution, the operation method and the manufacturing method of a nonvolatile semiconductor memory device (flash memory) of the fourth preferred embodiment of the present invention are described below with reference to  FIGS. 26 to 33 . 
     As shown in  FIG. 32  and  FIG. 33 , which is a cross sectional view taken along X–X′ in  FIG. 32 , the memory cell comprises a source/drain diffusion layer  303  in a p-well  301  formed on the main surface of a silicon substrate  300 , a floating gate  305   b  as a first gate, a control gate  307   a  as a second gate, and an erasing gate  309   a  as a third gate. 
     The control gate  307   a  for each of the memory cells is connected in the direction of a row to form a word line. The floating gate  305  and the p-well  301  are isolated from each other by a gate insulator film  304   a , the floating gate  305   b  and the word line (control gate)  307   a  are isolated from each other by an insulator film  306   b , and the erasing gate  309   a  and the word line (control gate)  307   a  are isolated from each other by an insulator film  308 . 
     Further, as shown in  FIG. 33 , the floating gate  305   b  and the erasing gate  309   a  are isolated from each other by an insulator film  311 . Further, the erasing gate  309   a  and the p-well  301  are also isolated from each other by an insulator film  312 . The insulator film  312  is buried in the p-well  301 . 
     The source/drain diffusion  303  is disposed perpendicular to the word line (control gate)  307   a  and is present as a local source line and a local data line connecting the source/drain of the memory cell in the columnar direction. That is, the nonvolatile semiconductor memory device of this fourth preferred embodiment comprises a so-called contactless array having no contact hole on every memory cell. 
     Upon writing, a voltage is applied at about 12 V to the control gate  307   a , at about 4 V to the drain ( 303 ), and the p-well  301  and the source ( 303 ) are kept at 0 V. Thus, hot electrons are generated in the channel on the drain end and electrons are injected into the floating gate  305   b.    
     Upon erasing, a voltage at about 10 V is applied to the erasing gate  309   a , while the control gate  307   a , the source  303  and the p-well  301  are kept at 0 V. Thus, electrons accumulated in the floating gate  305   b  are discharged by the tunnel phenomenon into the erasing gate  309   a.    
     Also in this preferred embodiment, different from the memory cell shown in  FIG. 60 , the thickness of the insulator film  304   a  between the floating gate  305   b  and the p-well  301 , a so-called tunnel insulator film, is made greater compared with the gate insulator film  306   a  between the control gate  307   a  and the p-well  301 . 
     By increasing the thickness of the tunnel insulator film  304   a , it is possible to suppress charge retention failure caused by incidental leakage of electrons accumulated in the floating gate into the substrate at specified bits. 
     Further, since the thickness of the gate insulator film  306   a  of the control gate  307   a  is reduced, punch-through resistance of the MOS transistor constituted with the control gate  307   a  can be improved to reduce the gate length of the control gate  307   a.    
     Accordingly, improved reliability and the refinement of the memory cell are compatible with each other. Further, the channel current can be increased to improve the access time of the memory cell. 
     A preferred method of manufacturing the memory cell of the fourth preferred embodiment is described below with reference to  FIGS. 26 to 33 . 
     At first, as shown in  FIG. 26 , a p-well  301  is formed on a silicon substrate  300 . Successively, as shown in  FIG. 27 , after forming a thin oxide film  302  on the surface of the p-well  301 , arsenic ions are implanted by an ion implantation process to form a diffusion layer  303  as source/drain of the memory cell. 
     Then, as shown in  FIG. 28 , a gate insulator film  304  of 9 nm for isolating the floating gate from the well, and a phosphorus-doped polysilicon film  305  as a floating gate are formed successively by a known thermal oxidation process. 
     Then, as shown in  FIG. 29 , the polysilicon film  305  and the gate insulator film  304  are patterned by known lithography and dry etching technology. As a result, the polysilicon film  305  is formed as  305   a  and the gate insulator film  304  is formed as  304   a.    
     Then, as shown in  FIG. 30 , by a known thermal oxidation process, a thermal oxidation film  306   a  for isolating the control gate from the substrate is formed to 7 nm thickness on the silicon substrate (p-well  301 ) by a known thermal oxidation process. In this case, a thermal oxidation film  306   b  of about 20 nm thickness is formed on the side wall and on the upper surface of the polysilicon film  305   a  to isolate the floating gate and the control gate from each other. The thickness of the thermal oxidation film  306   b  is larger compared with  306   a  because phosphorus is doped in the polysilicon film  305  as the material for the floating gate  305   b , and phosphorus caused accelerated oxidation. 
     Then, as shown in  FIG. 31 , a polysilicon film  307  is deposited, which is patterned by known lithography and dry etching technology to form word lines (control gate). As a result, the polysilicon film  307  is formed as  307   a  (word line, control gate) (refer to  FIG. 33 ). 
     Further, the thermal oxidation film  306   b  and the polysilicon film  305   a  are etched successively using the word line (control gate)  307   a  as the mask to complete a floating gate. Thus, the thermal oxidation film is formed as  306   b  and the polysilicon film  305   a  is formed as  305   b  (floating gate) (refer to  FIG. 33 ). 
     Then, as shown in  FIGS. 32 and 33 , insulator films  308  and the  311  for isolating the control gate from the erasing gate, and the floating gate from the erasing gate, and a polyside film  309  as the erasing gate are formed successively, and the polyside film  309  is patterned to form an erasing gate  309   a  ( FIG. 33 ). 
     Then, although not illustrated in the drawing, after forming an interlayer insulator film on the silicon substrate, a contact hole extending in the word line (control gate)  307   a , the source/drain diffusion layer  303 , the p-well  301 , and the erasing gate  309  is formed and, successively, a metal film is deposited which is patterned and wired to compete a memory cell. 
     In this memory cell of the fourth preferred embodiment in which the thickness of the gate insulator film  304   a  between the floating gate  305   b  and the p-well  301  is made larger compared with the gate insulator film  306   a  between the control gate  307   a  and the p-well  301 , when compared with the case in which the thickness of the gate insulator film  304   a  is made equal with or smaller than that of the gate insulator film  306   a , the charge retention characteristic of the memory cell was comparable therewith after many writing/erasing cycles but punch-through did not occur, even when the gate length of the control gate was decreased and stable operation was possible. Further, a larger channel current was obtained to improve the access time of the nonvolatile semiconductor memory device. 
     As described above, in this fourth preferred embodiment of the nonvolatile semiconductor memory device of the present invention, memory cell area has been reduced while maintaining reliability after many writing/erasing cycles and access time has been improved. 
     In this preferred embodiment, while the gate insulator film  304  or the like for isolating the floating gate from the p-well preferably was formed by a thermal oxidation process, it may also be formed by low pressure chemical vapor deposition process as in preferred Embodiment 2, and the same effect can also be obtained by using a nitrogen-added oxide film. 
     Fifth Preferred Embodiment 
     In this preferred embodiment, described is an example of a simplified production method by simultaneously forming a gate insulator film of a memory cell and a gate insulator film of MOS transistors for a low voltage system peripheral circuit of a nonvolatile semiconductor memory device. The constitution and the manufacturing method of a nonvolatile semiconductor memory device of this fifth preferred embodiment of the present invention are described below with reference to  FIGS. 34 to 46 . 
     As shown in  FIG. 46 , the nonvolatile semiconductor memory device of this preferred embodiment comprises a memory cell region in which a plurality of memory cells for storing information are arranged in a matrix and a peripheral circuit region in which a plurality of MOS transistors (MISFET) are arranged for selecting bits for conducting writing, erasing or reading and constituting a peripheral circuit for generating a voltage necessary in the chip. 
     The peripheral circuit region is divided into a low voltage portion in which only a relatively low voltage, for example, of 3.3 V of a power source voltage is applied and a high voltage portion in which a high voltage necessary for writing, for example, of 18 V is applied. 
     Each of the low voltage portion and the high voltage portion comprises, as shown in  FIG. 46 , a plurality of NMOS transistors (Qn 1 , Qn 2 ) and PMOS transistors (Qp 1 , Qp 2 ) formed on the p-wells  404   b ,  404   c  and the n-wells  405   a ,  405   b.    
     The memory cell formed in the memory cell region is a flash memory as has been described above with respect to the first preferred embodiment and is formed on a p-well  404   a.    
       FIG. 34  to  FIG. 46  are cross sectional views parallel with the word lines (control gate)  415   a  of the memory cell and perpendicular to the word lines  409   c  of the peripheral circuit MOS transistors. 
     A preferred manufacturing method of the nonvolatile semiconductor memory device of this preferred embodiment is described below with reference to  FIGS. 34 to 46 . 
     First, as shown in  FIG. 34 , a shallow trench isolation region  402  for isolating each of the memory cells and the peripheral circuit MOS transistors is formed to a p-Si substrate  401  of face orientation ( 100 ). Then, P-well regions  404   a ,  404   b ,  404   c  and N-well regions  405   a ,  405   b , as well as isolation regions  403  between the wells, are formed by an ion implantation process. 
     Then, as shown in  FIG. 35 , a silicon oxide film  406  as a gate insulator film of the high voltage portion in the peripheral circuit region is formed to about 23 nm by a thermal oxidation process. Then, as shown in  FIG. 36 , a photo-resist pattern  407  is formed and the silicon oxide film  406  is left only in the high voltage portion of the peripheral circuit region by a wet etching process. As a result, the silicon oxide film  406  is formed as  406   a.    
     Then, as shown in  FIG. 37 , a thermally oxidized film  408 , having a thickness of 7.5 nm, as a gate insulator film of the peripheral MOS transistors and the insulator film for isolating the third gate from the well of the memory cell is formed to the low voltage portion in the peripheral circuit region and the memory cell region by a thermal oxidation process. 
     In this step, the thickness of the thermally oxidized film for the high voltage portion in the peripheral circuit region is 25 nm. That is, the thickness of the silicon oxide film  406   a  is increased and formed as  406   b  (gate insulator film of the high voltage portion). 
     Then, as shown in  FIG. 38 , a polysilicon film  409  as the electrode of the third gate and of the peripheral MOS transistors and the third gate of the memory cell and a silicon oxide film  410  are deposited successively. 
     Successively, as shown in  FIG. 39 , the silicon oxide film  410  and the polysilicon film  409  are patterned by using lithographic and dry etching technology. As a result, the silicon oxide film  410  and the polysilicon film  409  in the memory cell region are formed as  410   a  and  409   a , respectively. In this step, it is patterned such that the silicon oxide film  410  and the polysilicon film  409  of the peripheral circuit region are not etched but left as  410   b  and  409   b.    
     Then, tilted ion implantation is applied in the same manner as in the first preferred embodiment to form a source/drain diffusion layer region  411  of the memory cell. 
     Then, as shown in  FIG. 40 , an insulator film  412  for isolating the floating gate from the well and for isolating the floating gate from the third gate is formed by a thermal oxidation process. In this step, the thickness of the oxide film on the well is 9.0 nm. 
     Then, as shown in  FIG. 41 , a polysilicon film  413  as a floating gate is deposited and the polysilicon film  413  is patterned by an etching back process using a fluidizable organic material described in preferred Embodiment 1 as a mask. As a result, the polysilicon film  413  is formed as  413   a  ( FIG. 42 ). 
     Then, as shown in  FIG. 43 , a stacked film of a silicon oxide film/silicon nitride film/silicon oxide film, a so-called ONO film,  414  for isolating the floating gate from the word line, as well as a polyside film  415  as a word line are deposited successively. 
     Then, as shown in  FIG. 44 , they are patterned by known lithography and dry etching technology to form word lines (control gate). As a result, the polyside film  415  is formed as  415   a  (word line). 
     Further, the ONO film  414  and the polysilicon film  413   a  are patterned using the word line  415   a  as a mask to complete the floating gate. That is, the ONO film  414  and the polysilicon film  413   a  are formed as  414   a  and  413   b  (floating gate), respectively. 
     Then, as shown in  FIG. 45 , the silicon oxide film  410   b  and the polysilicon film  409   b  in the peripheral circuit portion are patterned by lithography and dry etching technology to form a gate electrode of the peripheral circuit MOS transistor. That is, the silicon oxide film  410   b  and the polysilicon film  409   b  are formed as  410   c  and  409   c , respectively (gate electrode, gate line). 
     Then, as shown in  FIG. 46 , source/drain regions  416   a ,  416   b ,  417   a  and  417   b  of the peripheral circuit MOS transistors are formed by an ion implantation process. 
     Then, although not illustrated, after depositing an interlayer insulator film on the Si substrate, a contact hole extending in the word line  415   a , the gate electrodes  410   c  of the peripheral MOS transistors (Qn 1 , Qn 2 , Qp 1 , Qp 2 ), and the source/drain regions ( 416   a ,  416   b ,  417   a ,  417   b ) is formed, then, a metal film is deposited and patterned into an electrode to complete a nonvolatile semiconductor memory device. 
     In this preferred embodiment, the gate insulator film  403  for the third gate of the memory cell and the gate insulator film  408  for the MOS transistors of the peripheral circuit low voltage portion are formed by the same process step. 
     Accordingly, four kinds of gate insulator films including the tunnel insulator film of the memory cell can be formed with three kinds of films. Accordingly, the number of production steps can be reduced compared with the case of forming each of the gate insulator films independently. 
     As described above, by forming the gate insulator film  408  of the third gate to a thickness less than that of the insulator film  412  in this fifth exemplary embodiment of the nonvolatile semiconductor memory device of the present invention, memory cell area has been reduced while maintaining reliability after many writing/erasing cycles and access time has been improved. 
     Sixth Preferred Embodiment 
     In this preferred embodiment, a description is made of another example of simplifying the production steps by simultaneously forming a gate insulator film for the memory cell and a gate insulator film of the MOS transistors of the low voltage system peripheral circuit in the nonvolatile semiconductor memory device. This is different from the fifth preferred embodiment in that the gate insulator film of the MOS transistors of the low voltage system peripheral circuit and the gate insulator film between the floating gate and the well of the memory cell, that is, a so-called tunnel insulator film, are formed in common. A preferred constitution and method of manufacturing the nonvolatile semiconductor memory device of the sixth preferred embodiment of the present invention are described below. 
     As shown in  FIG. 59 , the nonvolatile semiconductor memory device of this preferred embodiment comprises a memory cell region in which a plurality of memory cells for storing information are arranged in a matrix, and a peripheral circuit region in which a plurality of MOS transistors are arranged for selecting bits to conduct writing, erasing or reading and constituting a peripheral circuit for generating a voltage necessary in the chip. 
     The peripheral circuit region is divided into a low voltage portion in which only a relatively low voltage, for example, of 3.3 V such as a power source voltage is applied, and a high voltage portion in which a high voltage necessary for writing, for example, of 18 V is applied. 
     Each of the low voltage portion and the high voltage portion comprises, as shown in  FIG. 59 , a plurality of NMOS transistors (Qn 1 , Qn 2 ) and PMOS transistor (Qp 1  Qp 2 ) formed on P-wells  504   b ,  504   c  and N-wells  505   a ,  505   b.    
     The memory cell formed in the memory cell region is a flash memory as has been described above regarding preferred Embodiment 1 and is formed on the P-well  504   a.    
       FIGS. 47 to 59  are cross sectional views parallel with the word line  516   a  of the memory cell and perpendicular to the gate line  513   c  of the peripheral circuit MOS transistors. 
     A preferred method of manufacturing the nonvolatile semiconductor memory device of this preferred embodiment is described below with reference to  FIGS. 47 to 59 . 
     At first, as shown in  FIG. 47 , a shallow trench isolation region  502  for isolating each of memory cells and peripheral circuit MOS transistors is formed to a p-Si substrate  501  of face orientation ( 100 ). Then, P-well regions  504   a ,  504   b ,  504   c  and N-well regions  505   a    505   b , as well as isolation regions  503  between the wells are formed by an ion implantation process. 
     Then, as shown in  FIG. 48 , a silicon oxide film  506  as a gate insulator film as the third gate of the memory cell is formed to 7.5 nm thickness by a thermal oxidation process. 
     Then, as shown in  FIG. 49 , a polysilicon film  507  and a silicon nitride film  508  as the third gate electrode in the peripheral MOS transistors and the memory cell are deposited successively. 
     Successively, as shown in  FIG. 50 , the silicon nitride film  508  and the polysilicon film  507  are patterned using lithography and dry etching technology. As a result, the silicon nitride film  508  and the polysilicon film  507  are formed as  508   a  and  507   a , respectively. In this step, the silicon nitride film  508  and the polysilicon film  507  in the peripheral circuit region are removed by etching. 
     Then, tilted ion implantation like in preferred Embodiment 1 is conducted to form a source/drain diffusion layer region  509  of the memory cell. 
     Then, as shown in  FIG. 51 , a silicon oxide film  510  as a gate insulator film for the high voltage portion in the peripheral circuit region is formed on the p-Si substrate to about 23 nm thickness by the combination of a thermal oxidation process and a CVD process. 
     Then, as shown in  FIG. 52 , a photo-resist pattern  511  is formed and the silicon oxide film is left only in the high voltage portion of the peripheral circuit region by a wet etching process. As a result, the silicon oxide film  510  is formed as  510   a.    
     Then, as shown in  FIG. 53 , a gate insulator film  512  for the low voltage portion in the peripheral circuit region, and as an insulator film for isolating the floating gate from the well and isolating the floating gate from the third gate in the memory cell, is formed to 9 nm thickness by a thermal oxidation process. 
     In this step, the thickness of the gate insulator film for the high voltage portion in the peripheral circuit region is 25 nm. That is, the silicon oxide film  510   a  is formed as  510   b  (gate insulator film). 
     Then, as shown in  FIGS. 54 and 55 , after depositing a polysilicon film  513  as a floating gate, a silicon oxide film  514  is deposited and the silicon oxide film  514  is left in the peripheral circuit region by using lithography and dry etching technology ( FIG. 55 ). 
     Then, as shown in  FIG. 56 , the polysilicon film  513  is patterned by an etching back method using an organic material having flowability described above regarding the preferred Embodiment 1. As a result, the polysilicon film  513  is formed as  513   a . Then, the silicon oxide pattern  514  is removed by wet etching. The polysilicon film exposed to the peripheral circuit region is defined as  513   b.    
     Then, as shown in  FIG. 57 , a stacked film of silicon oxide film/silicon nitride film/silicon oxide film for isolating a floating gate from a word line, a so-called ONO film,  515  and a polyside film  516 , as a word line, are deposited successively. 
     Then, as shown in  FIG. 58 , they are patterned by known lithography and dry etching technology to form word lines (control gate). That is, the polyside film  516  is formed as  516   a  (word line). Further, the ONO film  515  and the polysilicon pattern  513   a  are patterned by using the word line  516   a  as a mask to complete the floating gate. That is, the ONO film  515  and the polysilicon film  513   a  are formed as  515   a  and  513   b  (floating gate), respectively. 
     Simultaneously, the gate electrode for the peripheral MOS transistor is also formed. That is, the polyside film  516 , ONO film  515  and the polysilicon film  513   b  for the peripheral circuit region are formed as  516   b ,  515   b  and  513   c , respectively. 
     Then, as shown in  FIG. 59 , source/drain regions  517   a ,  517   b ,  518   a ,  518   b  of the peripheral circuit MOS transistors are formed by an ion implantation process. 
     Then, although not illustrated in the drawing, after depositing an interlayer insulator film on the Si substrate, a contact hole extending in the word line (control gate), gate electrode  513   c  of the peripheral MOS transistor, and the source/drain region are formed to the interlayer insulator film, and then a metal film is deposited and patterned into an electrode to complete a nonvolatile semiconductor memory device. 
     In this preferred embodiment, the gate insulator film  512  for the floating gate of the memory cell and the gate insulator film  512  for the MOS transistor of the peripheral circuit low voltage portion are formed by the same process step. 
     Accordingly, four kinds of gate insulator films including the tunnel insulator film of the memory cell can be formed with three kinds of films as in preferred Embodiment 5. Accordingly, the number of production steps is reduced compared with the case of forming each of the gate insulator films individually. 
     Further, as described with respect to preferred Embodiment 1, by forming the gate insulator film  506  of the third gate to a thickness less than that of the insulator film  512  for the floating gate, refinement and insurance of reliability of the memory cell are compatible with each other. Further, improvement is obtained in the access time of the memory cell. 
     In the preferred Embodiments 1 to 6, while description has been made of an n-channel type memory cell in which an n-type diffusion layer is formed in the p-well as an example, the same effect can be obtained in a p-channel type memory cell in which the well is an n-type and the diffusion layer is a p-type. 
     Further, in preferred Embodiments 1 to 6, while description has been made of a thermally oxidized film as a gate insulator film for the third gate or the control gate of the memory cell as an example, it is possible to further decrease the gate length and increase the reading current by using a film including a silicon nitride film or a material of high dielectric constant. 
     Further, in any of the preferred embodiments of the present invention, at least two states of electrons accumulated in the floating gate are necessary upon writing and it may be applied to a so-called multi-level memory in which 4 or more states are formed and 2 or more bits of data are stored in one memory cell. 
     In a general multi-level memory, even when the amount of electrons accumulated in the floating gate was controlled at high accuracy and the threshold voltage distribution at each level is compressed, there was a problem that the difference increased between the state of the lowest threshold voltage and the state of the highest threshold voltage compared with the binary level memory. Accordingly, in the Fowler-Nordheim type writing, it resulted in a problem that the writing rate was lowered or writing voltage increased. 
     However, the present invention is extremely effective for multi-level memories, since both writing and erasing can be conducted at the low voltage of about 13 V thereby increasing the writing rate. 
     The present invention has been described specifically with reference to the preferred embodiments, but it will be apparent that this invention is not restricted to the preferred embodiments described above but can be modified within a range not departing the gist thereof. 
     For example, this invention may be applied to a one-chip microcomputer (semiconductor device) comprising a memory cell array portion having a nonvolatile semiconductor memory device. 
     The effects obtained by typical inventions among those disclosed in the present application are to be explained simply as below. 
     Since the thickness of the gate insulator film between the floating gate and the substrate (well) is made larger than the gate insulator film between the third gate or the control gate and the substrate, the memory cell area of the nonvolatile semiconductor memory device can be reduced. 
     Further, reliability after many writing/erasing cycles in the nonvolatile semiconductor memory device of the present invention is improved. 
     Further, operation speed of the nonvolatile semiconductor memory device of the present invention is improved. 
     Further, since the gate insulator film between the floating gate and the substrate, and the gate insulator film between the third gate and the substrate are formed by the same process step as that of the gate insulator film for the MOS transistors in the lower voltage portion of the peripheral circuit, the production steps of the nonvolatile semiconductor memory device of the present invention have been simplified. 
     Nothing in the above description is meant to limit the present invention to any specific materials, geometry, or orientation of parts. Many part/orientation substitutions are contemplated within the scope of the present invention. The embodiments described herein were presented by way of example only and should not be used to limit the scope of the invention. 
     Although the invention has been described in terms of particular embodiments in an application, one of ordinary skill in the art, in light of the teachings herein, can generate additional embodiments and modifications without departing from the spirit of, or exceeding the scope of, the claimed invention. Accordingly, it is understood that the drawings and the descriptions herein are proffered by way of example only to facilitate comprehension of the invention and should not be construed to limit the scope thereof.