Abstract:
A nonvolatile semiconductor memory device capable of readily distinctively forming transistors in a peripheral circuit part and a transistor in a memory cell part while minimizing the number of times of high-temperature heat treatment are obtained. In the peripheral circuit part, at least one of a first transistor and a second transistor has a lower conductive layer having the same perpendicular structure as a floating gate, an intermediate insulator film including an insulator film of the same perpendicular structure as an inter-gate isolation film and an upper conductive layer of the same perpendicular structure as a conductive layer of a control gate in ascending order on a gate insulator film thereof, and the intermediate insulator film includes a conduction part electrically connecting the upper conductive layer and the lower conductive layer with each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a nonvolatile semiconductor memory device, and more specifically, it relates to a nonvolatile semiconductor memory device capable of readily distinctively forming a transistor in a memory cell part and transistors in a peripheral circuit part while reducing the number of times of high-temperature heat treatment. 
     2. Description of the Background Art 
     Referring to FIG. 34, a conventional nonvolatile semiconductor memory device is divided into a memory cell part R 1  and a peripheral circuit part R 2  located in the periphery thereof. A memory cell transistor  150  is arranged on the memory cell part R 1  while two types of transistors  161  and  162  are arranged on the peripheral circuit part R 2 . The difference between the two types of transistors  161  and  162  resides in difference between the thicknesses of gate insulator films  127  and  137 , as described later. 
     FIG. 34 shows the memory transistor  150  of the memory cell part R 1  in two sections along bit and word lines respectively. An n-conductivity type bottom well  103  is provided at a part of the bottom of a silicon substrate  101  formed in the memory cell part R 1  isolated from the peripheral circuit part R 2  by an element isolation zone  102 . A p-conductivity type well  105  is formed at the n-conductivity type bottom well  103 . The memory cell transistor  150  has source and drain regions  108   a  and  180   b  formed in the p-conductivity type well  105  and a gate insulator film  106  arranged on the silicon substrate  101 . A floating gate  107  enclosed with an insulating region  109  is arranged on the gate insulator film  106 . An inter-gate isolation film  110  consisting of a three-layer insulator film including a silicon oxide film, a silicon nitride film and a silicon oxide film is formed on the floating gate  107 . A control gate  113  is arranged on the inter-gate isolation film  110 . A layer  114  of WSi and an insulator film  115  are arranged on the control gate  113 . 
     An n-conductivity type well  104  and a p-conductivity type well  105  are provided on the peripheral circuit part R 2 . The two types of transistors  161  and  162  are provided in each of the wells  104  and  105 . The transistor  161  has a gate oxide film  127 , and the transistor  162  has a gate oxide film  137  having a larger thickness than the gate oxide film  127 . Conductive layers  113  of the same perpendicular structure as the control gate  113 , WSi films  114  and insulator films  115  are provided on the gate oxide films  127  and  137  respectively. In the peripheral circuit part R 2 , the transistors  161  and  162  include low-concentration impurity regions  116  and  117  provided on the silicon substrate  101  and high-concentration impurity regions  119  and  120  formed by implanting an impurity through masks defined by side wall spacers provided on the side surfaces of gate electrodes. Plug wires  125  conductive with wires  126  arranged on an interlayer dielectric film  124  are connected to the high-concentration impurity regions  119  and  120 . 
     A method of fabricating the conventional nonvolatile semiconductor memory device is now described with reference to FIGS. 35 to  43 . 
     First, the element isolation zone  102  is formed on the main surface of the p-conductivity type silicon substrate  101  having &lt;100&gt;crystal orientation (see FIG.  35 ). Then, a resist pattern is formed on the main surface of the silicon substrate  101  as a mask for ion-implanting phosphorus into the memory cell part R 1  with acceleration energy of 3 MeV and density of 1.0E13, for example, thereby forming the n-conductivity type bottom well region  103 , and the resist pattern is removed. In the following description, processing of removing a resist film is not described. 
     Then, phosphorus is ion-implanted into the region of the peripheral circuit part R 2  to be formed with p-conductivity type MOS (metal oxide semiconductor) transistors with acceleration energy of 1.2 MeV and density of 1.0E13, for example, through a resist pattern serving as a mask. Further, phosphorus for channel cutting and boron for counter doping are ion-implanted into the same region with 700 keV and 3.0 E 12 and with 20keV and 1.5 E12 respectively, for example. The n-conductivity type well region  104  is formed by this ion implantation (see FIG.  35 ). 
     Then, the p-conductivity type well regions  105  are formed in the region of the peripheral circuit part R 2  to be formed with n-conductivity type MOS transistors and a region of the memory cell part R 2  to be formed with a memory cell through a resist pattern serving as a mask in the following three stages (a), (b) and (c) (see FIG.  35 ): (a) Boron is ion-implanted with acceleration energy of 700 keV and density of about 1.0E13, for example. (b) Boron for p-channel cutting is ion-implanted with acceleration energy of 270 keV and density of 3.5E12, for example. (c) Boron for channel doping is ion-implanted with acceleration energy of 50 keV and density of 1.2E12, for example. 
     Thereafter a silicon oxide film  106  of about 10 nm in thickness is formed on the main surface of the silicon substrate  101  by thermal oxidation. Then, a phosphorus-doped polycrystalline silicon film  107  of about 200 nm in thickness is formed. Thereafter a resist pattern is formed on the overall main surface of the silicon substrate  101  by photolithography. This resist pattern is employed as a mask for patterning the phosphorus-doped polycrystalline silicon film  107  thereby forming the floating gate  107  on the region to be formed with the memory transistor  150 . 
     Then, arsenic is ion-implanted into the region of the silicon substrate  101  to be formed with the memory cell with acceleration energy of 35 keV and density of about 3.0E15, for example, through a resist pattern serving as a mask for forming n-conductivity type impurity diffusion regions as the source and drain regions  108   a  and  108   b . Thereafter a silicon oxide film  109  of 800 nm in thickness is deposited on the silicon substrate  101  by low-pressure CVD (chemical vapor deposition). The overall surface of this silicon oxide film  109  is etched thereby exposing the surface of the phosphorus-doped polycrystalline silicon film  107  (see FIG.  35 ). 
     Then, the three-layer insulator film  110  is formed on the main surface of the silicon substrate  101 . In formation of the three-layer insulator film  110 , a silicon oxide film of 5 nm in thickness is first formed by thermal oxidation. Then, a silicon nitride film of 10 nm in thickness is formed thereon by low-pressure CVD. Further, another silicon oxide film of 5 nm in thickness is formed thereon by low-pressure CVD, thereby defining the three-layer insulator film  110 . 
     Thereafter a resist pattern is formed on the silicon substrate  101  by photolithography. This resist pattern is employed for patterning the three-layer insulator film  110 , the phosphorus-doped polycrystalline silicon film  107  and the gate oxide film  106  on the peripheral circuit part R 2 , as shown in FIG.  35 . 
     Thereafter silicon oxide films  111  of about 20 nm in thickness are formed on the regions of the peripheral circuit part R 2  to be formed with thick gate insulator films, i.e., to be formed with high withstand voltage transistors. At this time, the silicon nitride film included in the three-layer insulator film  110  prevents the underlayer from thermal oxidation in the memory cell part Rd. Then, resist patterns are formed on the regions of the peripheral circuit part R 2  to be formed with the high withstand voltage transistors and the memory cell part R 1  by photolithography for patterning the silicon oxide films  111  in regions of the peripheral circuit part R 2  to be formed with low withstand voltage transistors (FIG.  36 ). 
     A silicon oxide film  127  of about 10 nm for defining the gate oxide films of the low withstand voltage transistors of the peripheral circuit part R 2  is grown on the silicon substrate  101  by thermal oxidation. At this time, the silicon nitride film included in the three-layer insulator film  110  prevents the underlayer from thermal oxidation in the memory cell part R 1 . On the other hand, a silicon oxide film  137  for defining the gate oxide films of the high withstand voltage transistors of the peripheral circuit part R 2  is larger than 20 nm and smaller than 30 nm in thickness. Then, a phosphorus-doped polycrystalline silicon film  113  of about 200 nm in thickness, a WSi film  114  of about 100 nm in thickness and a silicon oxide film  115  of about 200 nm in thickness are successively deposited in ascending order. Thereafter a resist pattern is formed by photolithography and employed as a mask for patterning the silicon oxide film  115 . The patterned silicon oxide films  115  are employed as masks for patterning the WSi film  114  and the phosphorus-doped polycrystalline silicon film  113  (FIG.  37 ). 
     Thereafter the phosphorus-doped polycrystalline silicon film  113 , the WSi film  114  and the silicon oxide film  115  of about 200 nm in thickness in the memory cell part R 1  of the silicon substrate  101  are employed as masks for patterning the three-layer insulator film  110  and the phosphorus-doped polycrystalline silicon film  107  (FIG.  38 ). 
     Then, phosphorus is ion-implanted into the region of the silicon substrate  101  to be formed with n-conductivity type MOS transistors of the eripheral circuit part R 2  with acceleration energy of 50 keV and density of about 4.0E13 through a resist pattern serving as a mask. Thus, the low-concentration impurity regions  116  of the n-conductivity MOS transistors of the peripheral circuit part R 2  are formed (FIG.  39 ). 
     Then, boron is ion-implanted into the region of the silicon substrate  101  to be formed with p-conductivity type MOS transistors of the peripheral circuit part R 2  with acceleration energy of 50 keV and density of about 1.5E13 through a resist pattern serving as a mask. Thus, the low-concentration impurity regions  117  of the p-conductivity type MOS transistors are formed in the peripheral circuit part R 2  (FIG.  40 ). A silicon oxide film of about  100  nm is formed on the silicon substrate  101  by CVD. Then, side wall spacers  123  are formed by anisotropic etching (FIG.  41 ). 
     Then, arsenic is ion-implanted into the region of the silicon substrate  101  to be formed with the n-conductivity type MOS transistors of the peripheral circuit part R 2  with acceleration energy of 35 keV and density of about 4.0E15, for example, through a resist pattern serving as a mask. 
     Thus, the high-concentration impurity regions  119  of the n-conductivity type MOS transistors are formed (FIG.  42 ). 
     Further, BF 2  is ion-implanted into the region of the silicon substrate  101  to be formed with the p-conductivity type MOS transistors of the peripheral circuit part R 2  with acceleration energy of 20 keV and density of 2.0E15, for example, through a resist pattern serving as a mask. Thus, the high-concentration impurity regions  120  of the p-conductivity type MOS transistors are formed (FIG.  43 ). Thereafter wires are formed through general wire formation. The conventional nonvolatile semiconductor memory device is fabricated through the aforementioned method. 
     In the nonvolatile semiconductor memory device, a high voltage V PP  of about 20 V is generally applied to the control gate  113  while grounding the n-conductivity type diffusion regions  108   a  and  108   b  and the silicon substrate  101  in program formation. Thus, electrons are generated in a channel formed between the n-conductivity type diffusion layers  180   a  and  108   b . These electrons tunnel through an energy barrier formed by the tunnel insulator film  106  and are injected into the floating gate  107 . Consequently, the threshold voltage of the memory cell is increased. 
     In program erasing, a high voltage VPP of about−20 V is generally applied to the control gate  113  while grounding the n-conductivity type diffusion regions  108   a  and  108   b  and the silicon substrate  101 . Tunneling results from this circuit formation, to discharge electrons from the floating gate  107  to the silicon substrate  101 . Consequently, the threshold voltage of the memory cell is reduced. 
     In a read operation of a selected memory transistor, voltages of 3.3 V (Vcg=3.3 V) are applied to the control gate  113  and the drain  108   a  of the n-conductivity type diffusion layer while grounding the source  108   b  of the n-conductivity type diffusion layer and the silicon substrate  101 . Assuming that V thp  &gt;3.3 V &gt;V the , no current flows between the source  108   b  and the drain  108   a  of the memory transistor in a reading state while a current flows in a program erasing state. 
     In reading, the control gate  113  is grounded (Vcg=0 V), a voltage of 3.3 V is applied to the drain  108   a  of the n-conductivity type diffusion region, and the source  108   b  of the n-conductivity type diffusion region and the silicon substrate  101  are grounded in a non-selected transistor. Assuming that V thp  &gt;V the  &gt;0 V, no current flows between the source  108   b  and the drain  108   a  of the memory transistor if the voltage Vcg is 0 V. 
     A current flows between the source  108   b  and the drain  108   a  only in a memory transistor of a program reading state among selected ones, so that information can be detected from each memory cell. 
     Therefore, the peripheral circuit part R 2  of the nonvolatile semiconductor memory device requires two types of transistors, i.e., (1) a low withstand voltage transistor having a thin gate oxide film with high current drivability for increasing the speed for the reading operation and (2) a high withstand voltage transistor having a gate oxide film capable of withstanding a high applied voltage. 
     In the conventional fabrication method, however, the memory cell transistor and the two types of transistors having gate oxide films of different thicknesses in the peripheral circuit part must be formed independently of each other. Therefore, high-temperature heat treatment must be performed a number of times for forming the gate oxide films with a long time. This leads to the following problems: 
     (1) The number of thermal oxidation steps requiring a high cost is increased. 
     (2) Diffusion regions formed by ion implantation are spread due to exposure to high-temperature heat treatment over a long time, to inhibit refinement of semiconductor elements. 
     (3) The number of high-temperature heat treatment steps is so large that stress is applied to a portion of the silicon substrate  101  around the element isolation zone  102  due to difference between the thermal expansion coefficients of the silicon oxide film and the silicon substrate  101  to cause crystal defects in the silicon substrate  101 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a nonvolatile semiconductor memory device and a method of fabricating the same capable of distinctively forming transistors of a peripheral circuit part and a memory cell part while minimizing the number of times of high-temperature heat treatment, particularly capable of readily distinctively forming a high withstand voltage transistor and a low withstand voltage transistor directed to improvement of the operating speed in the peripheral circuit part. 
     The nonvolatile semiconductor memory device according to the present invention comprises a memory cell part and a peripheral circuit part located in the periphery of the memory cell part on a semiconductor substrate. The memory cell part includes a memory cell transistor having a floating gate located on a gate insulator film, an inter-gate isolation film located on the floating gate and a control gate located on the inter-gate isolation film. The peripheral circuit part includes a first transistor including a first gate insulator film and a second transistor including a second gate insulator film. In this nonvolatile semiconductor memory device, at least one of the first and second transistors includes a lower conductive layer, an intermediate insulator film and an upper conductive layer located in ascending order on the gate insulator film in contact with each other. The lower conductive layer has the same perpendicular structure as the floating gate, the intermediate insulator film includes an insulator film of the same perpendicular structure as the inter-gate isolation film, and the upper conductive layer has the same perpendicular structure as the conductive layer of the control gate. Further, the intermediate insulator film includes a conduction part electrically connecting the upper conductive layer and the lower conductive layer with each other. 
     According to this structure, the floating gate, and the control gate isolated from each other by the isolation film in the memory cell part can be electrically connected with each other in the peripheral circuit part. In the peripheral circuit part, therefore, either one of the floating gate and the control gate can be employed as a gate electrode. Therefore, (A) a gate portion of the nonvolatile transistor of the memory cell part and gate portions of the transistors of the peripheral circuit part can be simultaneously formed in the same perpendicular structure, for reducing the number of fabrication steps. Further, for example, (B) the number of the fabrication steps can be further reduced by simultaneously forming a gate oxide film of the nonvolatile transistor and a gate oxide film of the first transistor, for example, of the peripheral circuit part in common with the same peripheral structure. Consequently, (B1) thermal oxidation can be suppressed for suppressing crystal defects in the semiconductor substrate. Further, heat history applied to the semiconductor substrate is so reduced that (B2) impurity diffusion regions are not enlarged in size, not to inhibit the nonvolatile semiconductor memory device from miniaturization. 
     In general, the aforementioned first and second transistors are classified into a high withstand voltage transistor and a transistor directed to a high-speed operation with no requirement for high voltage resistance respectively. In other words, first and second gate oxide films are different in thickness from each other. However, the present invention is not necessarily restricted to the aforementioned classification but another classification may alternatively be employed. The aforementioned first and second transistors are formed in each of n- and p-conductivity type wells. 
     The wording “lower conductive layer having the same perpendicular structure as the floating gate” indicates that the same layer as the floating gate is formed on the peripheral circuit part in formation of the floating gate and employed as the lower conductive layer. This also applies to the remaining layers. When two layers have the same perpendicular structure, therefore, the layers are identical in (a) perpendicular size and (b) perpendicular chemical composition distribution to each other. 
     Each of the first and second transistors includes the lower conductive layer of the same perpendicular structure as the floating gate, the intermediate insulator film including the insulator film of the same perpendicular structure as the inter-gate isolation film and the upper conductive layer of the same perpendicular structure as the control gate on the gate insulator film. 
     The first transistor includes the lower conductive layer of the same perpendicular structure as the floating gate, the intermediate insulator film including the insulator film of the same perpendicular structure as the inter-gate isolation film and the upper conductive layer of the same perpendicular structure as the control gate on the first gate insulator film. The second transistor can include a conductive layer of the same perpendicular structure as the control gate on the second gate insulator film. 
     Also according to this structure, functions/effects identical to the above inventive functions/effects (A), (B), (B1) and (B2) can be attained. 
     The method of fabricating a nonvolatile semiconductor memory device according to the present invention comprises steps of forming a lower insulator film on a region of the peripheral circuit part on the semiconductor substrate formed with the second transistor, forming the gate insulator film covering the semiconductor substrate and the lower insulator film, forming a floating conductive layer defining the floating gate on the gate insulator film, forming the inter-gate isolation film on the floating conductive layer, and opening a through hole reaching the floating conductive layer in regions of the inter-gate isolation film formed with the first and second transistors. 
     According to this method, the gate insulator films of the first transistor and the memory transistor can be simultaneously fabricated in common in the same composition. The thickness of the gate insulator film of the second transistor can be increased beyond that of the gate insulator film of the first transistor by the thickness of the lower insulator film. In the aforementioned method, the gate oxide films of the transistors may not be separately formed for the memory cell part and the peripheral circuit part, whereby thermal oxidation is suppressed. Therefore, crystal defects can be suppressed in the semiconductor substrate. Heat treatment history applied to the semiconductor substrate is suppressed, whereby an impurity diffusion region can be inhibited from spreading, not to inhibit refinement of the nonvolatile semiconductor memory device. 
     Another method of fabricating a nonvolatile semiconductor memory device according to the present invention comprises a step of successively stacking a gate insulator film, a floating conductive layer for defining the floating gate on the gate insulator film and the inter-gate isolation film on the conductive layer in common to the memory cell part and the peripheral circuit part. The method further comprises steps of partially removing the successively stacked gate insulator film, floating conductive layer and inter-gate isolation film only in the range of regions formed with the second transistor in plan view for exposing the semiconductor substrate and forming the second gate insulator film having a larger thickness than the gate insulator film only on the region of the second transistor. The method further comprises steps of opening a through hole reaching the floating conductive layer in a region of the inter-gate isolation film formed with the first transistor and forming a control conductive layer defining the control gate to fill up the through hole and cover the second gate insulator film over the memory cell part and the peripheral circuit part. 
     According to this method, the gate oxide films of the first transistor and the memory transistor can be formed through a common step for reducing the number of fabrication steps also when only the first transistor has the insulator film of the same perpendicular structure as the inter-gate isolation film of the memory cell part for the gate electrode and only the upper conductive layer is stacked for the second transistor, thereby reducing the number of steps. 
    
    
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a nonvolatile semiconductor memory device according to a first embodiment of the present invention; 
     FIG. 2 is an enlarged view of a part A in FIG. 1; 
     FIG. 3 is an enlarged view of a part B in FIG. 1; 
     FIG. 4 is a sectional view showing a stage forming an element isolation film on a silicon substrate for fabricating the nonvolatile semiconductor memory device shown in FIG. 1; 
     FIG. 5 is a sectional view showing a stage forming a bottom well on the bottom of a memory cell part; 
     FIG. 6 is a sectional view showing a stage forming an n-conductivity type well in a region of a peripheral circuit part for forming p-conductivity type transistors; 
     FIG. 7 is a sectional view showing a stage forming p-conductivity type wells in regions of the memory cell part and the peripheral circuit part for forming n-conductivity type transistors; 
     FIG. 8 is a sectional view showing a stage forming lower oxide films of gate oxide films in regions of the peripheral circuit part for forming high withstand voltage transistors; 
     FIG. 9 is a sectional view showing a stage forming a gate oxide film to cover the lower oxide films and the silicon substrate, then forming a conductive layer for defining floating gates over the memory cell part and the peripheral circuit part and patterning the same in the memory cell part; 
     FIG. 10 is a sectional view showing a stage implanting an n-conductivity type impurity into the memory cell part on the silicon substrate for forming source and drain regions of a memory cell transistor; 
     FIG. 11 is a sectional view showing a stage forming a silicon oxide film on the source and drain regions of the memory cell transistor; 
     FIG. 12 is a sectional view showing a stage depositing a three-layer insulator film for defining an inter-gate isolation film over the memory cell part and the peripheral circuit part; 
     FIG. 13 is a sectional view showing a stage forming a phosphorus-doped polycrystalline silicon film and thereafter opening contact holes through the three-layer insulator film and the phosphorus-doped polycrystaliine silicon film on regions formed with the high withstand voltage transistors and low withstand voltage transistors in the peripheral circuit part; 
     FIG. 14 is a sectional view showing a stage depositing the phosphorus-doped polycrystalline silicon film, then successively stacking a WSi film and an insulator film and thereafter patterning gate portions of the transistors of the memory cell part and the peripheral circuit part; 
     FIG. 15 is an enlarged view of a part C in FIG. 14; 
     FIG. 16 is a sectional view showing a stage implanting an n-conductivity type impurity into portions of the silicon substrate for forming the n-conductivity type transistors of the peripheral circuit part; 
     FIG. 17 is a sectional view showing a stage implanting an p-conductivity type impurity into portions of the silicon substrate for forming the p-conductivity type transistors of the peripheral circuit part; 
     FIG. 18 is a sectional view showing a stage forming side wall spacers on the side surfaces of the gate portions of the memory cell part and the peripheral circuit part; FIG. 19 is a sectional view showing a stage implanting an n-conductivity type impurity into source and drain regions of the n-conductivity type transistors through the side wall spacers serving as masks for forming high-concentration n-conductivity type regions; 
     FIG. 20 is a sectional view showing a stage implanting a p-conductivity type impurity into source and drain regions of the p-conductivity type transistors through the side wall spacers serving as masks for forming high-concentration p-conductivity type regions; 
     FIG. 21 is a sectional view showing a stage opening contact holes in a three-layer insulator film and a phosphorus-doped polycrystalline silicon film and thereafter further implanting phosphorus into a floating gate conductive layer of a peripheral circuit part in fabrication of a nonvolatile semiconductor memory device according to a second embodiment of the present invention; 
     FIG. 22 is a sectional view of a nonvolatile semiconductor memory device according to a third embodiment of the present invention; 
     FIG. 23 is an enlarged view of a part D in FIG. 22; 
     FIG. 24 is a sectional view showing a stage forming a gate insulator film on a silicon substrate, thereafter depositing a conductive layer for defining a floating gate and patterning the same in fabrication of the nonvolatile semiconductor memory device shown in FIG. 22; 
     FIG. 25 is a sectional view showing a stage forming n-conductivity type impurity regions for defining source and drain regions of a memory transistor; 
     FIG. 26 is a sectional view showing a stage forming a silicon oxide film on the source and drain regions of the memory transistor; 
     FIG. 27 is a sectional view showing a stage forming a three-layer insulator film for defining an inter-gate isolation film; 
     FIG. 28 is a sectional view showing a stage removing parts of a gate oxide film, a floating gate conductive layer and the three-layer insulator film located on regions for forming high withstand voltage transistors of a peripheral circuit part by etching and exposing the silicon substrate; 
     FIG. 29 is a sectional view showing a stage forming gate oxide films having a larger thickness than the gate oxide film on the exposed silicon substrate; 
     FIG. 30 is a sectional view showing a stage forming contact holes in the three-layer insulator film on regions of the peripheral circuit part for forming low withstand voltage transistors; 
     FIG. 31 is a sectional view showing a stage implanting phosphorus into the floating gate conductive layer on the regions for forming the low withstand voltage transistors and patterning gate portions of the transistors of the memory cell part and the peripheral circuit part; 
     FIG. 32 is an enlarged view of a part E in FIG. 31; 
     FIG. 33 is a sectional view showing a stage opening contact holes in a three-layer insulator film and thereafter further implanting phosphorus into a floating gate conductive layer of a peripheral circuit part in fabrication of a nonvolatile semiconductor memory device according to a fourth embodiment of the present invention; 
     FIG. 34 is a sectional view of a conventional nonvolatile semiconductor memory device (Prior Art); 
     FIG. 35 is a sectional view showing a stage forming a gate oxide film, a floating conductive layer and a three-layer insulator film and thereafter partially removing the gate oxide film, the floating conductive layer and the there-layer insulator film in a peripheral circuit part for exposing a silicon substrate in the peripheral circuit part in fabrication of the nonvolatile semiconductor memory device shown in FIG. 34 (Prior Art); 
     FIG. 36 is a sectional view showing a stage forming thick gate insulator films on regions of the peripheral circuit part for forming high withstand voltage transistors (Prior Art); 
     FIG. 37 is a sectional view showing a stage further forming a gate insulator film only on the peripheral circuit part, depositing a control gate conductive layer, a WSi layer and an insulating layer in ascending order over the peripheral circuit part and a memory cell part and patterning only gate portions of the transistors of the peripheral circuit part (Prior Art); 
     FIG. 38 is a sectional view showing a stage patterning a gate portion of a transistor of the memory cell part (Prior Art); 
     FIG. 39 is a sectional view showing a stage implanting an n-conductivity type impurity into regions of a silicon substrate for defining source and drain regions of n-conductivity type transistors of the peripheral circuit part (Prior Art); 
     FIG. 40 is a sectional view showing a stage implanting a p-conductivity type impurity into regions of the silicon substrate for defining source and drain regions of p-conductivity type transistors of the peripheral circuit part (Prior Art); 
     FIG. 41 is a sectional view showing a stage forming side wall spacers on the side surfaces of gate portions of the memory cell part and the peripheral circuit part (Prior Art); 
     FIG. 42 is a sectional view showing a stage further implanting an n-conductivity type impurity into the source and drain regions of the n-conductivity type transistors through the side wall spacers serving as masks for forming high-concentration n-conductivity type impurity regions (Prior Art); and 
     FIG. 43 is a sectional view showing a stage further implanting a p-conductivity type impurity into the source and drain regions of the p-conductivity type transistors through the side wall spacers serving as masks for forming high-concentration p-conductivity type impurity regions (Prior Art). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are now described with reference to the drawings. 
     First Embodiment 
     Referring to FIG. 1, a nonvolatile semiconductor memory device according to a first embodiment of the present invention is divided into a memory cell part R 1  and a peripheral circuit part R 2  located in the periphery thereof. A memory cell transistor  50  is arranged on the memory cell part R 1  while two types of transistors  61  and  62  are arranged on an n-conductivity type well  4  and a p-conductivity type well  5  of the peripheral circuit part R 2  respectively. The two types of transistors  61  and  62  are different in thickness of gate insulator films from each other, as described later. 
     FIG. 1 shows sections of the memory cell transistor  50  of the memory cell part R 1  along a right-side bit line and a left-side word line respectively. An n-conductivity type bottom well  3  is provided on the bottom of a part of a silicon substrate  1  formed with the memory cell part R 1  isolated from the peripheral circuit part R 2  by an element isolation zone  2 , and a p-conductivity type well  5  is formed on the n-conductivity type bottom well  3 . The memory cell transistor  50  has n-conductivity type source and drain regions  28   a  and  28   b  located in the p-conductivity type well  5  and a gate insulator film  7  arranged on the silicon substrate  1 . A floating gate  8  of a conductive layer enclosed with an insulating region  9  is arranged on the gate insulator film  7 . An inter-gate isolation film  10  consisting of a three-layer insulator film including a silicon oxide film, a silicon nitride film and a silicon oxide film is formed on the floating gate  8 . A phosphorus-doped polycrystalline silicon film  11  is arranged on the inter-gate isolation film  10  and a control gate  12  of a conductive layer is arranged on the phosphorus-doped polycrystalline silicon film  11 , while a WSi layer  13  is arranged on the control gate  12  and an insulator film  14  is arranged on the WSi layer  13 . 
     The peripheral circuit part R 2  is provided with the n-conductivity type well  4  and the p-conductivity type well  5  each having the two types of transistors  61  and  62  respectively as hereinabove described. The transistor  61  has a gate oxide film  7 , and the transistor  62  has a gate oxide films  6 , 7  having a larger thickness than the gate oxide film  7 . The transistors  61  and  62  have the same multilayer structures as the memory transistor  50  on the gate oxide films  6  and  7 . In other words, floating gates  8  are arranged on the gate oxide films  6  and  7  and inter-gate isolation films  10  are located on the floating gates  8 , while phosphorus-doped polycrystailine silicon films  11 , control gates  12 , WSi layers  13  and insulator films  14  are successively stacked thereon. 
     FIG. 2 is an enlarged view of a part A shown in FIG.  1 . As shown in FIG. 2, through holes are formed in the inter-gate isolation films  10  for defining conduction parts rendering the floating gates  8  and the control gates  12  conductive. The floating gates  8  and the control gates  12  can be employed as gate electrodes of general transistors due to the conduction parts. FIG. 3 is an enlarged view of a part B shown in FIG.  1 . Comparing FIGS. 2 and 3 with each other, it is understood that the memory transistor  50  of the memory cell part R 1  and the two types of transistors  61  and  62  of the peripheral circuit part R 2  have the same multilayer structures of gate portions formed by the gate oxide films  6  and  7  and the films located thereon. Therefore, the gate electrodes of the transistor  61  and  62  of the peripheral circuit part R 2  can be fabricated at the same timing as the gate portion of the memory transistor  50  including the floating gate  8  and the control gate  12 . 
     In the peripheral circuit part R 2 , the transistors  61  and  62  include low-concentration impurity regions  15  and  16  provided on the silicon substrate  1  and high-concentration impurity regions  18  and  19  formed by implanting an impurity through side wall spacers  17  provided on the side surfaces of the gate electrodes. Plug wires  23  conducting with wires  26  arranged on an interlayer dielectric film  22  are connected to the high-concentration impurity regions  18  and  19 . 
     A method of fabricating the nonvolatile semiconductor memory device shown in FIG. 1 is now described. First, element isolation regions  2  consisting of silicon oxide are formed on the main surface of the p-conductivity type silicon substrate  1  having &lt;100&gt;crystal orientation (FIG.  4 ). Then, phosphorus is ion-implanted into a region of the main surface of the silicon substrate  1  for forming a memory cell with acceleration energy of 3 MeV and density of 1.0E13, for example, through a resist pattern serving as a mask. Thus, the n-conductivity type bottom well  3  is formed and the resist pattern is removed (FIG.  5 ). 
     Then, (a) phosphorus is ion-implanted into the region of the peripheral circuit part R 2  to be formed with p-conductivity type MOS transistors with acceleration energy of 1.2 MeV and density of 1.0E13, for example, through a resist pattern serving as a mask. Further, (b) phosphorus for channel cutting is ion-implanted into the same region with acceleration energy of 700 keV and density of 3.0E12, for example, and (c) boron for counter doping is ion-implanted into the same region with acceleration energy of 20 keV and density of 1.5E12, for example, respectively. Thus, the n-conductivity type well region  4  is formed by such ion implantation (FIG.  6 ). 
     Thereafter the following ion implantation steps (a), (b) and (c) are carried out on a region of the peripheral circuit part R 2  to be formed with n-conductivity type MOS transistors and the region of the memory cell part RI to be formed with the memory transistor  50  through resist patterns serving as masks: (a) Boron is ion-implanted with acceleration energy of 700 keV and density of about 1.0E13, for example. (b) Boron for p-channel cutting is ion-implanted with acceleration energy of 270 keV and density of 3.5E12. (c) Boron for channel doping is ion-implanted with acceleration energy of 50 keV and density of 1.2E12. The p-conductivity type well regions  5  are formed through the aforementioned ion implantation steps (a), (b) and (c) (FIG.  7 ). 
     A silicon oxide film  6  of about 20 nm in thickness is formed on the main surface of the silicon substrate  1  by thermal oxidation. Then, a resist pattern is formed on the silicon oxide film  6  by photolithography, and employed as a mask for removing parts of the silicon oxide film  6  located on the region to be formed with the memory cell transistor  50  and the regions of the peripheral circuit part R 2  to be formed with the low withstand voltage transistors  61  by etching. Therefore, the silicon oxide films  6  of about 20 nm in thickness are arranged only on the regions of the peripheral circuit part R 2  to be formed with the high withstand transistors  62 , as shown in FIG.  8 . 
     Then, the silicon oxide films  7  of 10 nm in thickness are formed on the aforementioned silicon oxide films  6  and the main surface of the silicon substrate  1  by thermal oxidation. Then, the phosphorus-doped polycrystailine silicon films  8  of about 200 nm in thickness are formed on the silicon oxide films  7 . At this time, the silicon oxide films  7  formed on the regions of the peripheral circuit part R 2  to be formed with the low withstand voltage transistors  61  are at least 20 nm and less than 30 nm in thickness. Then, resist patterns are formed on the phosphorus-doped polycrystalline silicon films  8  by photolithography and employed as masks for patterning the phosphorus-doped polycrystalline silicon films  8  (FIG.  9 ). 
     Arsenic is implanted into the region of the memory cell part R 2  to be formed with the memory transistor  50  with acceleration energy of 35 keV and density of 3.0E15, for example, through a resist pattern serving as a mask for forming the n-conductivity type impurity diffusion regions  28   a  and  28   b  (FIG.  10 ). 
     Thereafter a silicon oxide film  9  of 800 nm in thickness is deposited on the silicon substrate  1  by low-pressure CVD. Then, the silicon oxide film  9  is subjected to overall etching, thereby exposing the surfaces of the phosphorus-doped polycrystalline silicon films  8  (FIG.  11 ). 
     Then, a three-layer insulator film  10  is formed on the overall main surface of the silicon substrate  1  by stacking three layers (FIG.  12 ). In formation of this three-layer insulator film  10 , a silicon oxide film of 5 nm in thickness is first formed by thermal oxidation. Then, a silicon nitride film of 10 nm is formed on the silicon oxide film by low-pressure CVD, and another silicon oxide film of 5 nm in thickness is formed on the silicon nitride film by low-pressure CVD. 
     Thereafter the phosphorus-doped polycrystalline silicon film  11  is formed on the three-layer insulator film  10 . Then, a resist pattern is formed on the overall main surface of the silicon substrate  1 , for patterning the phosphorus-doped polycrystalline silicon film  11  and the three-layer insulator film  10  on the peripheral circuit part R 2 . Contact holes  25  are opened in the regions of the peripheral circuit part R 2  to be formed with the high- and low-withstand voltage transistors due to this patterning, as shown in FIG.  13 . 
     A natural oxide film adhering to the phosphorus-doped polycrystalline silicon film  11  is removed with an HF solution or the like, followed by formation of a phosphorus-doped polycrystalline silicon film  12  of 200 nm in thickness. Then, a WSi film  13  of about 100 nm in thickness is formed on the phosphorus-doped polycrystalline silicon film  12 , followed by formation of a silicon oxide film  14  of about 200 nm in thickness. Thereafter a resist pattern is formed by photolithography and employed as a mask for patterning the silicon oxide film  14 . Then, the silicon oxide film  14  is employed as a mask for successively patterning the WSi film  13 , the phosphorus-doped polycrystailine silicon films  12  and  11 , the three-layer insulator film  10  and the phosphorus-doped polycrystalline silicon film  8  (FIG.  14 ). 
     FIG. 15 is an enlarged view of the part C shown in FIG.  14 . The phosphorus-doped polycrystalline silicon film  12  of the control gate is electrically connected to the floating gate  8  through the contact hole  25  formed in the three-layer insulator film  10  and the phosphorus-doped polycrystalline silicon film  11 . 
     Then, phosphorus is ion-implanted into the region of the peripheral circuit part R 2  to be formed with the n-conductivity MOS transistors with acceleration energy of 50 keV and density of about 4.0E13, for example, through a resist pattern serving as a mask, for forming the low-concentration impurity regions  15  of the n-conductivity type MOS transistors (FIG.  16 ). Then, boron is ion-implanted into the region of the peripheral circuit part R 2  to be formed with the p-conductivity type MOS transistors with acceleration energy of 50 keV and density of about 1.5E13, for example, through a resist pattern serving as a mask for forming the low-concentration impurity regions  16  of the p-conductivity type MOS transistors (FIG.  17 ). 
     Thereafter a silicon oxide film is formed by CVD, and the side wall spacers  17  are formed by anisotropically etching the silicon oxide film (FIG.  18 ). Then, arsenic is ion-implanted into the region of the peripheral circuit part R 2  to be formed with the n-conductivity type MOS transistors with acceleration energy of 35 keV and density of about 4.0E15, for example, through a resist pattern serving as a mask, for forming the high-concentration impurity regions  18  of the n-conductivity type MOS transistors (FIG.  19 ). Then, BF 2  is ion-implanted into the region of the peripheral circuit part R 2  to be formed with the p-conductivity type MOS transistors with acceleration energy of 20 keV and density of about 2.0E15, for example, through a resist pattern serving as a mask, for forming the high-density impurity regions  19  of the p-conductivity type MOS transistors (FIG.  20 ). 
     The aforementioned nonvolatile semiconductor memory device renders the floating gate (FG)  8  and the control gate (CG)  12 , isolated by the inter-gate isolation film consisting of the three-layer insulator film  10  in the memory cell part R 1 , conductive in the peripheral circuit part R 2  as general gate electrodes. Therefore, the gate insulator films  7  of the nonvolatile transistor  50  of the memory cell part R 1  and the low withstand voltage transistors  61  of the peripheral circuit part R 2  can be fabricated at the same time. Further, the gate portions of the nonvolatile transistor  50  of the memory cell part R 1  and the transistors  61  and  62  of the peripheral circuit part R 2  can be fabricated in parallel at the same time. Consequently, the number of processing steps can be reduced for inhibiting the semiconductor substrate  1  from crystal defects resulting from thermal oxidation. Further, the impurity regions can be suppressed from enlargement by reducing the time of heat treatment applied to the memory cell part R 1 , not to inhibit refinement of the memory cell part R 1 . 
     Further, the thin phosphorus-doped polycrystalline silicon films  11  are deposited on the three-layer insulator films  10  so that natural oxide films adhering to the phosphorus-doped polycrystalline silicon films  11  can be removed with an HF solution, for example, after opening the contact holes  25  through the phosphorus-doped polycrystalline silicon films  11  and the three-layer insulator films  10 . Therefore, floating conductive layers and control conductive layers can be reliably rendered conductive in the peripheral circuit part R 2 . 
     Second Embodiment 
     A nonvolatile semiconductor memory device according to a second embodiment of the present invention is identical in structure to the nonvolatile semiconductor memory device shown in FIG.  1 . The feature of the nonvolatile semiconductor memory device according to the second embodiment resides in that the phosphorus concentration of a phosphorus-doped polycrystalline silicon film  8  in a peripheral circuit part R 2  is higher than that in the first embodiment. 
     A method of fabricating the nonvolatile semiconductor memory device according to the second embodiment is basically identical to the fabrication method according to the first embodiment. Steps similar to those shown in FIGS. 1 to  13  with reference to the first embodiment are employed as such. After contact holes  25  are formed in a three-layer insulator film  10  and a phosphorus-doped polycrystalline silicon film  11  as shown in FIG. 13, phosphorus is ion-implanted into the peripheral circuit part R 2  through a resist pattern serving as a mask. Therefore, the phosphorus-doped polycrystalline silicon film  8  of the peripheral circuit part R 2  has higher phosphorus concentration than a phosphorus-doped silicon film  8  for defining a floating gate in a memory cell part R 1 . Subsequent fabrication steps are identical to those shown in FIGS. 14 to  20  with reference to the first embodiment. 
     The aforementioned nonvolatile semiconductor memory device according to the second embodiment has the following function/effect, in addition to the function/effect of the nonvolatile semiconductor memory device according to the first embodiment: In general, the impurity concentration of the phosphorus-doped polycrystalline silicon film  11  is decided in response to the characteristics of a memory cell transistor  50  of the memory cell part R 1  and set to about 4E20/cm 3 , for example. In the peripheral circuit part R 2 , however, the phosphorus-doped polycrystalline silicon films  11  for defining gate electrodes of transistors preferably have higher concentration than that in the memory cell part R 1 , in order to suppress depletion of the gate materials. According to this embodiment, only a single ion implantation step may be added to the processing steps according to the first embodiment. Consequently, gate materials different in impurity concentration from each other can be obtained for the transistors of the memory cell part R 1  and the peripheral circuit part R 2  through simple steps. 
     The aforementioned nonvolatile semiconductor memory device can be fabricated as follows: First, the nonvolatile semiconductor memory device is fabricated along a procedure identical to that shown in FIGS. 1 to  10  with reference to the first embodiment. Referring to FIG. 10, a phosphorus-doped polycrystalline silicon film  11  is formed on a three-layer insulator film  10 , and through holes reaching the phosphorus-doped polycrystalline silicon films  8  are formed on transistor forming regions of the peripheral circuit part R 2 . 
     According to this embodiment, phosphorus is thereafter implanted into only the peripheral circuit part R 2  excluding the memory cell part R 1 , as shown in FIG.  19 . The phosphorus-doped polycrystalline silicon films  8  of the peripheral circuit part R 2  exhibit higher phosphorus concentration than that in the memory cell part R 1  due to this implantation of phosphorus into the peripheral circuit part R 2 . 
     Subsequent fabrication steps are carried out along the processing steps shown in FIGS. 11 to  18  with reference to the first embodiment. 
     The aforementioned nonvolatile semiconductor memory device can attain the following advantage, in addition to that obtained in the nonvolatile semiconductor memory device according to the first embodiment: 
     In general, the impurity concentration of the phosphorus-doped polycrystalline silicon films  11  is decided in response to the characteristics of the memory cell transistor  50  of the memory cell part R 1  and set to about 4E20/cm 2 , for example. In the peripheral circuit part R 2 , however, the phosphorus-doped polycrystalline silicon films  11  for defining gate electrodes of the transistors preferably have higher concentration than that in the memory cell part R 1 , in order to suppress depletion of the gate materials. According to this embodiment, only a single ion implantation step may be added to the processing steps according to the first embodiment for changing the impurity concentration values of the gate electrodes in the memory cell part R 1  and the peripheral circuit part R 2 . 
     Third Embodiment 
     Referring to FIG. 22, high withstand voltage transistors  62  and low withstand voltage transistors  61  in a peripheral circuit part R 2  are different from each other not only in the thicknesses of gate insulator films but also in multilayer structures in a nonvolatile semiconductor memory device according to a third embodiment of the present invention. FIG. 23 is an enlarged view of a part D of the peripheral circuit part R 2  shown in FIG.  22 . Referring to FIG. 23, a gate insulator film  37  of the high withstand voltage transistor  62  is formed by a single layer having a larger thickness than a gate insulator film  7  of the low withstand voltage transistor  61 . 
     A gate portion of the low withstand voltage transistor  61  has the same multilayer structure as that of a memory transistor. However, a contact hole  25  is opened in an inter-gate isolation film  10  and filled up with a conductive layer of a control gate  12 , which is electrically connected with a floating gate  8 . The floating gate layer  8  is formed on a gate oxide film  7  while the control gate layer  12 , a WSi layer  13  and an insulator film  14  are arranged thereon similarly to the memory transistor except the conducting part. 
     In the high withstand voltage transistor  62 , on the other hand, a control gate layer  12 , a WSi layer  13  and an insulator film  14  are arranged on a gate oxide films  37 . The high withstand voltage transistor  62  includes no inter-gate isolation film. 
     A method of fabricating the nonvolatile semiconductor memory device according to the third embodiment shown in FIG. 22 is now described. First, element isolation regions  2  consisting of silicon oxide are formed on the main surface of a p-conductivity type silicon substrate  1  having &lt;100&gt;crystal orientation (refer to FIG.  4 ). Then, phosphorus is ion-implanted into a region of the main surface of the silicon substrate  1  for forming a memory cell with acceleration energy of 3 MeV and density of 1.0E13, for example, through a resist pattern serving as a mask, for forming a n-conductivity type bottom well  3  (refer to FIG.  5 ). 
     Then, (a) phosphorus is ion-implanted into a region of the peripheral circuit part R 2  to be formed with p-conductivity type MOS transistors with acceleration energy of 1.2 MeV and density of 1.0E13, for example, through a resist pattern serving as a mask. Further, (b) phosphorus for channel cutting is ion-implanted into the same region with acceleration energy of 700 keV and density of 3.0E12, for example, and (b) boron for counter doping is ion-implanted into the same region with acceleration energy of 20 keV and density of 1.5E12 respectively. Thus, an n-conductivity type well region  4  is formed by such ion implantation (refer to FIG.  6 ). 
     Thereafter (a) boron is ion-implanted into a region of the peripheral circuit part R 2  to be formed with n-conductivity type MOS transistors and the region of the memory cell part R 1  to be formed with the memory transistor with acceleration energy of 700 keV and density of about 1.0E13, for example, through a resist pattern serving as a mask. (b) Boron for p-channel cutting is ion-implanted with acceleration energy of 270 keV and density of 3.5E12. Further, (c) boron for channel doping is ion-implanted with acceleration energy of 50 keV and density of 1.2E12. P-conductivity type well regions  5  are formed through the aforementioned ion implantation steps (a), (b) and (c) (refer to FIG.  7 ). 
     Then, a silicon oxide film  7  of 10 nm in thickness is formed on the main surface of the silicon substrate  1  by thermal oxidation. Then, a phosphorus-doped polycrystalline silicon film  8  of about 200 nm in thickness is formed on the silicon oxide film  7 . Then, a resist pattern is formed on the phosphorus-doped polycrystalline silicon film  8  by photolithography for patterning the phosphorus-doped polycrystalline silicon film  8  through the resist pattern serving as a mask (FIG.  24 ). 
     Arsenic is implanted into the region of the memory cell part R 1  to be formed with the memory transistor with acceleration energy of 35 keV and density of 3.0E15, for example, through a resist pattern serving as a mask for forming n-conductivity type impurity regions  28   a  and  28   b  (FIG.  25 ). 
     Thereafter a silicon oxide film  9  of 800 nm in thickness is deposited on the silicon substrate  1  by low-pressure CVD and subjected to overall etching, thereby exposing the surfaces of the phosphorus-doped polycrystalline silicon films  8  (FIG.  26 ). 
     Then, a silicon oxide film of 5 nm in thickness is formed on the overall main surface of the silicon substrate  1  by thermal oxidation. Then, a silicon nitride film of 10 nm in thickness is formed thereon by low-pressure CVD. A silicon oxide film of 5 nm in thickness is formed thereon by low-pressure CVD, thereby forming a three-layer insulator film  10  (FIG.  27 ). 
     Thereafter a resist pattern is formed on the three-layer insulator film  10  and employed as a mask for removing parts of the three-layer insulator film  10  and the phosphorus-doped polycrystalline silicon film  8  from regions of the peripheral circuit part R 2  to be formed with high withstand voltage transistors. 
     Then, silicon oxide films  37  of about 20 nm in thickness are formed on the silicon substrate  1  by thermal oxidation, as shown in FIG.  29 . The silicon oxide films  37  define gate insulator films of the high withstand voltage transistors. In formation of the silicon oxide films  37 , the silicon nitride film included in the three-layer insulator film  10  prevents the memory cell part R 1  and surface parts of the silicon substrate  1  in regions of low withstand voltage transistors of the peripheral circuit part R 2  from thermal oxidation. 
     Then, contact holes  25  are opened to reach the phosphorus-doped polycrystalline silicon films  12  in the three-layer insulator films  10  on the regions of the peripheral circuit part R 2  to be formed with the low withstand voltage transistors (FIG.  30 ). 
     Then, a phosphorus-doped polycrystalline silicon film  12  of 200 nm in thickness is formed on the three-layer insulator films  10  and the silicon oxide films  37  to fill up the contact holes  25 . Then, a WSi film  13  of about 100 nm in thickness is formed on the phosphorus-doped polycrystalline silicon film  12 , followed by formation of a silicon oxide film  14  of about 200nm in thickness. Thereafter a resist pattern is formed by photolithography and employed as a mask for patterning the silicon oxide film  14 . The patterned silicon oxide films  14  are employed as masks for successively patterning the WSi film  13 , the phosphorus-doped polycrystalline silicon film  12 , the phosphorus-doped polycrystalline silicon film  11 , the three-layer insulator films  10  and the phosphorus-doped polycrystalline silicon films  8  (FIG.  31 ). 
     FIG. 32 is an enlarged view of a part E shown in FIG.  31 . The phosphorus-doped polycrystalline silicon film  12  defining the control gate is electrically connected to the floating gate  8  through the contact hole  25  formed in the three-layer insulator film  10 . 
     Subsequent fabrication steps are carried out along the fabrication steps shown in FIGS. 16 to  20  with reference to the first embodiment. 
     In the nonvolatile semiconductor memory device according to this embodiment, an inter-gate isolation film isolates a floating gate and a control gate of the memory cell transistor in the memory cell part R 1 . In each low withstand voltage transistor of the peripheral circuit part R 2 , however, two conductive layers corresponding to a floating gate and a control gate are electrically connected with each other. In the low withstand voltage transistor, therefore, both of the conductive layers corresponding to the floating gate and the control gate respectively can be employed as gate electrodes. In the high withstand voltage transistor of the peripheral circuit part R 2 , a conductive layer corresponding to a control gate is employed for a gate electrode. Therefore, gate oxide films  7  of the memory transistor and the low withstand voltage transistor can be rendered in common. Consequently, an oxidation step can be immediately eliminated. Further, crystal defects caused in the silicon substrate  1  in thermal oxidation can be suppressed. In addition, the time of thermal oxidation applied to the memory cell part R 1  is reduced, not to hinder refinement of the memory cell. 
     Fourth Embodiment 
     A nonvolatile semiconductor memory device according to a fourth embodiment of the present invention is identical in structure to the nonvolatile semiconductor memory device shown in FIG.  22 . The feature of this embodiment resides in that the phosphorus concentration of phosphorus-doped polycrystalline silicon films  8  in a peripheral circuit part R 2  is higher than that in the third embodiment. 
     A method of fabricating the nonvolatile semiconductor memory device according to the fourth embodiment is basically identical to the fabrication method according to the third embodiment. Steps identical to those shown in FIGS. 4 to  7  with reference to the first embodiment and those shown in FIGS. 24 to  31  with reference to the third embodiment are employed as such. 
     A silicon oxide film  7  of 10 nm in thickness is formed on the main surface of a silicon substrate  1  by thermal oxidation, as shown in FIG.  7 . Then, a phosphorus-doped polycrystalline silicon film  8  of about 200 nm in thickness is formed on the silicon oxide film  7 . Then, a resist pattern is formed on the phosphorus-doped polycrystalline silicon film  8  by photolithography and employed as a mask for patterning the phosphorus-doped polycrystalline silicon film  8  (refer to FIG.  24 ). 
     Arsenic is implanted into a region of a memory cell part R 1  to be formed with a memory transistor with acceleration energy of 35 keV and density of 3.0E15, for example, through a resist pattern serving as a mask for forming n-conductivity type impurity regions  28   a  and  28   b  (refer to FIG.  25 ). 
     Thereafter a silicon oxide film  9  of 800 nm in thickness is deposited on the silicon substrate  1  by low-pressure CVD. Then, this silicon oxide film  9  is subjected to overall etching, thereby exposing the surface of the phosphorus-doped polycrystalline silicon film  8  (refer to FIG.  26 ). 
     Then, a silicon oxide film of 5 nm in thickness is formed on the overall main surface of the silicon substrate  1  by thermal oxidation. A silicon nitride film of 10 nm in thickness is formed thereon by low-pressure CVD. A silicon oxide film of 5 nm in thickness is formed thereon by low-pressure CVD, thereby forming a three-layer insulator film  10  (refer FIG.  27 ). 
     Thereafter a resist pattern is formed on the three-layer insulator film  10  and employed as a mask for removing parts of the three-layer insulator film  10  and the phosphorus-doped polycrystalline silicon film  8  from regions of the peripheral circuit part R 2  formed with high withstand voltage transistors (refer to FIG.  28 ). 
     Then, silicon oxide films  37  of about 20 nm in thickness are formed on the silicon substrate  1  by thermal oxidation (refer to FIG.  29 ). The silicon oxide films  37  define gate insulator films of the high withstand voltage transistors. In formation of the silicon oxide films  37 , the silicon nitride film included in the three-layer insulator film  10  prevents the memory cell part R 1  and surface parts of the silicon substrate  1  in regions of low withstand voltage transistors of the peripheral circuit part R 2  from thermal oxidation. 
     Then, contact holes  25  are opened to reach the phosphorus-doped polycrystalline silicon films  12  in the three-layer insulator films  10  on the regions of the peripheral circuit part R 2  to be formed with the low withstand voltage transistors (refer to FIG.  30 ). 
     Thereafter phosphorus is ion-implanted into regions of the peripheral circuit part R 2  to be formed with low withstand voltage transistors, as shown in FIG.  33 . The impurity concentration of a phosphorus-doped polycrystalline silicon film is decided in response to characteristics required to a memory transistor. This impurity concentration, about 4 E20/cm 3 , for example, is at a low level as the impurity concentration of a gate electrode treated as a conductive layer. Gate electrodes of the transistors of the peripheral circuit part R 2  preferably have higher concentration, in order to suppress depletion of the gate electrodes. The impurity concentration of the gate electrodes of the low withstand voltage transistors of the peripheral circuit part R 2  can be rendered higher than that of the gate electrode of the memory transistor by merely adding this ion implantation step. 
     The gate electrodes of the high withstand voltage transistors of the peripheral circuit part R 2 , identical to a conductive layer corresponding to a control gate of the memory transistor essentially having high impurity concentration, contain an impurity required to the gate electrodes of the transistors of the peripheral circuit part R 2  with no particular requirement for processing of increasing the impurity concentration. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.