Manufacturing method of semiconductor memory device using insulating film as charge storage layer

A manufacturing method of a semiconductor memory device includes forming a first gate electrode having a charge storage layer, a block layer, and a control gate electrode on a first region of a semiconductor substrate, forming a second gate electrode on a second region of the semiconductor substrate, forming a protective insulating film on a side surface of the block layer, exposing the first region while covering the second region on the semiconductor substrate with a photoresist, using the photoresist, the first gate electrode, and the protective insulating film as masks to implant an impurity into the first region of the semiconductor substrate, and removing the photoresist by wet etching which uses a mixed solution containing H2SO4 and H2O2. The protective insulating film having an etching selective ratio of 1:100 or above with respect to the photoresist under wet etching conditions using the mixed solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2007-216032, filed Aug. 22, 2007, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manufacturing method of a semiconductor memory device. More particularly, the present invention relates to a nonvolatile memory using an insulating film as a charge storage layer.

2. Description of the Related Art

As a structure of a memory cell transistor in a nonvolatile memory, a metal oxide nitride oxide semiconductor (MONOS) type structure has been conventionally known. The MONOS type structure is a structure having: a charge storage layer (e.g., an insulating film) formed on a silicon substrate through an insulating film; an insulating film which is formed on the charge storage layer and has a higher dielectric constant than the charge storage layer (which will be referred to as a block layer hereinafter); and a control gate electrode formed on the block layer (see, e.g., “Charge Trapping Memory Cell of TANOS (Si-Oxide-SiN—Al2O3—TaN) Structure Compatible to Conventional NAND Flash Memory,” IEEE NVSMW 2006. 21stVolume, Issue 2006 pp. 54-55).

The nonvolatile memory requires peripheral transistors constituting, e.g., a power supply generation circuit or a decoder circuit besides the memory cell transistors. An impurity that is implanted into the silicon substrate and energy that is used to implant the impurity vary depending on the types of these transistors. Therefore, the memory cell transistor is protected by, e.g., a photoresist when, e.g., forming the peripheral transistors. Therefore, the memory cell transistor is exposed to wet etching that is performed to remove, e.g., a photoresist more than once.

During this wet etching, not only the photoresist but also the block layer of the memory cell transistor may be simultaneously etched in some cases. As a result, the memory cell transistor having the etched block layer has a problem of degradation in characteristics of the nonvolatile memory, e.g., a reduction in a data writing speed or erasing speed.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a manufacturing method of a semiconductor memory device comprising:

forming a first gate electrode having a charge storage layer, a block layer, and a control gate electrode on a first region of a semiconductor substrate through a first gate insulating film;

forming a second gate electrode on a second region of the semiconductor substrate through a second gate insulating film;

forming a protective insulating film on a side surface of the block layer;

applying a photoresist to an upper side of the semiconductor substrate to expose the first region while covering the second region;

using the photoresist, the first gate electrode, and the protective insulating film as masks to implant an impurity into the first region of the semiconductor substrate; and

removing the photoresist by wet etching which uses a mixed solution containing H2SO4and H2O2,

wherein the protective insulating film has an etching selective ratio of 1:100 or above with respect to the photoresist under wet etching conditions using the mixed solution.

According to a second aspect of the present invention, there is provided a manufacturing method of a semiconductor memory device comprising:

forming a plurality of first gate electrodes each having a charge storage layer, a block layer, and a control gate electrode on a first region of a semiconductor substrate through a first gate insulating film;

forming a plurality of second gate electrodes on a second region of the semiconductor substrate through a second gate insulating film;

forming a protective insulating film on at least side surfaces of the block layers of the plurality of first gate electrodes and side surfaces of the plurality of second gate electrodes;

applying a first photoresist to an upper side of the semiconductor substrate to expose the first region while covering the second region;

using the first photoresist, the plurality of first gate electrodes, and the protective insulating film as masks to implant an impurity into the first region of the semiconductor substrate;

removing the first photoresist by wet etching which uses a mixed solution containing H2SO4and H2O2;

applying a second photoresist to an upper side of the semiconductor substrate to expose the second region while covering the first region;

using the second photoresist, the plurality of second gate electrodes, and the protective insulating film as masks to implant an impurity into the second region of the semiconductor substrate; and

removing the second photoresist by wet etching which uses a mixed solution containing H2SO4and H2O2,

wherein the protective insulating film has an etching selective ratio of 1:100 or above with respect to the first and second photoresists under wet etching conditions using the mixed solution.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that the drawings are schematic ones and the dimension ratios shown therein are different from the actual ones. The dimensions vary from drawing to drawing and so do the ratios of dimensions. The following embodiments are directed to a device and a method for embodying the technical concept of the present invention and the technical concept does not specify the material, shape, structure or configuration of components of the present invention. Various changes and modifications can be made to the technical concept without departing from the scope of the claimed invention.

First Embodiment

A manufacturing method of a semiconductor memory device according to a first embodiment of the present invention will now be explained with reference toFIG. 1. It is to be noted that a NAND type flash memory will be explained as an example in this embodiment.

As shown in the drawing, a NAND type flash memory includes a memory cell array1, a power supply voltage generation circuit2, a row decoder3, and a column decoder. First, the memory cell array1will be explained.

As shown in the drawing, the memory cell array1includes a plurality of NAND cells5. Each of the NAND cells5includes a plurality of (e.g., 16) nonvolatile memory cell transistors MT connected in series, and selective transistors ST1and ST2. Each memory cell transistor MT includes a MONOS type structure having a charge storage layer (e.g., an insulating film) formed on a semiconductor substrate through a gate insulating film, an insulating film (a block layer) that is formed on the insulating film and that has a higher dielectric constant than the charge storage layer, and a control gate electrode formed on the block. It is to be noted that the number of the memory cell transistors MT may be eight, 32, 64, 128, or 256 without being restricted to 16, and this number is not limited. Further, the memory cell transistors MT adjacent to each other share a source region and a drain region. That is, the respective memory cell transistors MT are arranged in such a manner that current paths can be connected in series between the selective transistors ST1and ST2. Furthermore, a drain region on one end side of the memory cell transistors MT connected in series is connected with a source region of the selective transistor ST1, and a source region on the other end side of the same is connected with a drain region of the selective transistor ST2.

The control gate electrode of each memory cell transistor MT in the same row is connected with one of word lines WL0to WL15in common, and the gate electrode of the selective transistor ST1or ST2in the same column is connected with a selective gate line SGD or SGS in common, respectively. It is to be noted that the word lines WL0to WL15may be simply referred to as word lines WL hereinafter to simplify the explanation. Furthermore, a drain region of the selective transistor ST1in the same column is connected with one of bit lines BL0to BLn (n is a natural number) in common. The bit lines BL0to BLn may be likewise generically referred to as bit lines BL hereinafter when these lines are not discriminated. A source region of the selective transistor ST2is connected with a source line SL in common. It is to be noted that both the selective transistors ST1and ST2are not necessarily required, and one of the transistors can suffice if the NAND cell5can be selected.

FIG. 1shows the NAND cell5for one row alone. However, the NAND cells5for a plurality of rows may be provided in the memory cell array. In such a case, the NAND cells5in the same column are connected with the same bit line BL. Moreover, data is collectively written in the plurality of memory cell transistors MT connected with the same word line WL, and this unit will be referred to as a page. Additionally, data in the plurality of NAND cells5in the same row is collectively erased, and this unit will be referred to as a memory block.

The row decoder3selects a row direction of the memory cell array1. That is, the row decoder3selects the word line WL and applies a voltage to the selected word line WL. The column decoder4selects a column direction of the memory cell array1. That is, the column decoder4selects the bit line BL. The power supply voltage generation circuit2generates a voltage and supplies the generated voltage to the row decoder3.

Each of the power supply voltage generation circuit2, the row decoder3, and the column decoder4includes, e.g., a low-breakdown-voltage MOS transistor and a high-breakdown-voltage MOS transistor. The low-breakdown-voltage MOS transistor uses a voltage VDD (e.g., 1.5V) as a power supply voltage. The high-breakdown-voltage MOS transistor uses a voltage VPP (e.g., 20V), which is higher than the power supply voltage (the voltage VDD) of the low-breakdown-voltage MOS transistor as a power supply voltage. To simplify the explanation, a p-channel MOS transistor alone will be explained as the low-breakdown-voltage MOS transistor, an n-channel MOS transistor alone will be explained as the high-breakdown-voltage MOS transistor, and these transistors will be referred to as peripheral transistors PT1and PT2.

A structure of the memory cell array1will now be explained with reference toFIG. 2.FIG. 2shows a cross section of the NAND cell5along a wiring direction of the bit line BL. An n-type well region (an n-well)11is formed in a surface region of a p-type semiconductor substrate (a p-substrate)10. A p-type well region (a p-well)12is formed in a surface region of the n-type well region11. A gate insulating film14is formed on the p-type well region12, and gate electrodes of the memory cell transistors MT are formed on the gate insulating film14. The gate electrode of each memory cell region MT has a MONOS type structure in which an insulating film15formed on the gate insulating film14, an insulating film16that is formed of a material with a higher dielectric constant than a material used for the gate insulating film (e.g., a silicon oxide film) on the insulating film15, and a polysilicon layer17formed on the insulating film16are laminated. A surface of the polysilicon layer17is silicided. The insulating film15functions as a charge storage layer that stores electric charges, the insulating film16serves as a block layer that confines electric charges to the insulating film15, and the polysilicon layer17functions as a control gate electrode. The insulating films15and16and the polysilicon layer17may be referred to as a charge storage layer15, a block layer16, and a control gate electrode17, respectively. Further, a polysilicon layer20that serves as a gate electrode of each of the selective transistors ST1and ST2is formed on the gate insulating film14. A surface of the polysilicon layer20is likewise silicided. The polysilicon layer20may be simply referred to as a gate electrode20hereinafter.

Furthermore, an insulating film18is formed on a sidewall of the gate electrode of each memory cell transistor MT. That is, the insulating film18is formed on side surfaces of the charge storage layer15, the block layer16, and the control gate electrode17. It is to be noted that covering an entire side surface of at least the block layer16with the insulating film18can suffice, and the insulating film18is not necessarily required for the side surfaces of the charge storage layer15and the control gate electrode17. Moreover, the insulating film18is also formed on side surfaces of the gate electrodes20of the selective transistors ST1and ST2.

In the structure shown inFIG. 2, a film thickness of the gate insulating film14placed immediately below the gate electrode of each memory cell transistor MT and the gate electrode20of each selective transistor ST1or ST2is larger than a film thickness of the gate insulating film14placed between the gate electrodes. That is, steps are formed in the gate insulating film14at positions where the film thickness varies, and the insulating film18is also formed on sidewalls of such step portions.

Additionally, an insulating film19is formed on the gate insulating film14between the gate electrodes of the memory cell transistors MT and the selective transistors ST1and ST2. That is, the region between adjacent gate electrodes is filled with the insulating film19. An upper surface of the insulating film19is substantially level with an upper surface of the insulating film18, and a side surface of the insulating film18is completely covered with the insulating film19.

In the memory cell transistors MT, the control gate electrodes17which are adjacent to each other in a direction perpendicular to the bit line BL are connected in common to function as the word line WL. In the selective transistors ST1and ST2, likewise, the gate electrodes20which are adjacent to each other in the direction perpendicular to the bit line BL are connected in common. Further, the gate electrodes20function as either the selective gate line SGS or SGD.

An n+-type impurity diffusion layer13is formed in a surface of the p-type well region12placed between the gate electrodes. This n+-type impurity diffusion layer13is shared by the transistors which are adjacent to each other, and functions as a source (S) region or a drain (D) region. Furthermore, a region between the source region and the drain region adjacent to each other below the gate electrodes serves as a channel region that becomes an electron movement region. The control gate electrode17, the n+-type impurity diffusion layer13, and the channel region form an MOS transistor that functions as the memory cell transistor MT. Moreover, the gate electrode20, the n+-type impurity diffusion layer13, and the channel region form the MOS transistor that functions as the selective transistor ST1or ST2.

An interlayer insulating film21is formed on the p-type semiconductor substrate10to cover the memory cell transistors MT and the selective transistors ST1and ST2. A contact plug CP1that reaches the n+-type impurity diffusion layer (the source region)13of the selective transistor ST2on the source side is formed in the interlayer insulting film21. Further, a metal wiring layer22connected with the contact plug CP1is formed on a surface of the interlayer insulating film21. The metal wiring layer22functions as a part of the source line SL. Furthermore, a contact plug CP2reaching the n+-type impurity diffusion layer (the drain region)13of the selective transistor ST1on the drain side and a metal wiring layer23connected with the contact plug CP2are formed in the interlayer insulating film21. An interlayer insulating film24is formed on the interlayer insulating film21. An interlayer insulating film25is formed on the interlayer insulating film24. A metal wiring layer26is formed on the interlayer insulating film25. The metal wiring layer26functions as the bit line BL. A contact plug CP3having an upper surface that is in contact with the metal wiring layer26and a bottom surface that is in contact with the metal wiring layer23is formed in the interlayer insulating films24and25. An insulating film27is formed on the metal wiring layer26.

The peripheral transistors PT1and PT2constituting each of the power supply voltage generation circuit2, the row decoder3, and the column decoder4will now be explained with reference toFIG. 3.FIG. 3is a cross-sectional view of the peripheral transistors PT1and PT2. As shown in the drawing, the peripheral transistors PT1and PT2are formed on the same p-type semiconductor substrate10as that of the memory cell array1.

An n-type well region (an n-well)31and a p-type well region (a p-well)32are formed in a surface region of the p-type semiconductor substrate10. The n-type well region31and the p-type well region32are surrounded by an element isolating region38and electrically separated from each other. Furthermore, the low-breakdown-voltage peripheral transistor PT1is formed on the n-type well region31, and the high-breakdown-voltage peripheral transistor PT2is formed on the p-type well region32.

That is, a gate insulating film33is formed on the n-type well region31. Moreover, a gate electrode34of the peripheral transistor PT1is formed on the gate insulating film33. Additionally, a gate insulating film37is formed on the p-type well region32, and a gate electrode34of the peripheral transistor PT2is formed on the gate insulating film37. A film thickness of the gate insulating film37of the peripheral transistor PT2is larger than a film thickness of the gate insulating film33of the peripheral transistor PT1. That is because a higher voltage is applied to the peripheral transistor PT2as compared with the peripheral transistor PT1. Further, a step where a film thickness of the gate insulating film33or37immediately below the gate electrode34of the peripheral transistor PT1or PT2becomes larger than a film thickness of the same at any other position is formed on a surface of each of the gate insulating films33and37. Furthermore, an insulating film18is formed on sidewalls of the gate electrodes34of the peripheral transistors PT1and PT2. The insulating film18is also formed on sidewalls of the step portions of the gate insulating films33and37like the insulating film18formed on the gate electrodes of the memory cell transistors MT and the gate electrodes20of the selective transistors ST1and ST2. A side surface of the insulating film18is completely covered with an insulating film19.

A p+-type impurity diffusion layer39that functions as a source region or a drain region of the peripheral transistor PT1is formed in the surface of the n-type well region31. Moreover, an n+-type impurity diffusion layer40that functions as a source region or a drain region of the peripheral transistor PT2is formed in the surface of the p-type well region32. The impurity diffusion layers39and40are connected with non-illustrated contact plugs, and voltages are applied to these layers through the contact plugs. Additionally, a region between the source region and the drain region adjacent to each other immediately below the gate electrode34functions as a channel region that becomes an electron or hole movement region. The gate electrode34, the impurity diffusion layer39or40, and the channel region form an MOS transistor that serves as the peripheral transistor PT1or PT2. It is to be noted that an impurity concentration profile of the impurity diffusion layer39or40in a depth direction of the p-type semiconductor substrate10varies depending on characteristics required for the peripheral transistor PT1or PT2. The impurity concentration profile of the impurity diffusion layer39or40is different from an impurity concentration profile of the n+-type impurity diffusion layer13of the memory cell transistor MT.

An interlayer insulating film21is formed on the p-type semiconductor substrate10to cover the peripheral transistors PT1and PT2that are placed to sandwich the element isolating region38therebetween. An interlayer insulating film24is formed on the interlayer insulating film21. Further, non-contact plugs are buried to pierce the interlayer insulating films21and24.

A manufacturing method of the memory cell array1and the peripheral transistors PT1and PT2will now be explained with reference toFIGS. 4 to 9.FIGS. 4 to 9are cross-sectional views sequentially showing manufacturing steps of the memory cell array1and the peripheral transistors PT1and PT2according to this embodiment. It is to be noted that, in regard to the memory cell array1, the memory cell transistors MT14and MT15and the selective transistor ST1alone are depicted.

As shown inFIG. 4, first, the n-type well region11is formed in the surface of the p-type semiconductor substrate10corresponding to a region where formation of the memory cell array1is planned, and then the p-type well region12is formed in the surface of the n-type well region11. The n-type well region31and the p-type well region32are formed on the surface of the p-type semiconductor substrate10corresponding to regions where formation of the peripheral transistors PT1and PT2is planned. The p-type well regions12and32and the n-type well regions11and31are formed to have appropriate impurity types and concentration profiles in accordance with characteristics required for the respective transistors. The impurity concentration profiles of the p-type well region12, where the memory cell array1is formed, and the well regions31and32, where the peripheral transistors PT1and PT2are formed in a depth direction, are usually different from each other. Further, since the well region where the low-breakdown-voltage peripheral transistor is formed, and the well region where the high-breakdown-voltage peripheral transistor is formed have different breakdown voltages required for the transistors, it is general to form the transistors by using different impurities even though the transistors have the same conductivity type.

Furthermore, the element isolating regions38are formed in the p-type semiconductor substrate10by using, e.g., a shallow trench isolation (STI) technology. As a result, the n-type well regions11and31and the p-type well region32are electrically separated from each other through the element isolating regions38.

Then, the gate electrodes of the memory cell transistors MT, the gate electrodes20of the selective transistors ST1and ST2, and the gate electrodes34of the peripheral transistors PT1and PT2are formed in the region where formation of the memory cell array1is planned and the regions where formation of the peripheral transistors PT1and PT2is planned on the p-type semiconductor substrate10, respectively. That is, first, the gate insulating film14, which is made of, e.g., SiO2, is formed on the n-type well region11including the p-type well region12. The gate insulating film14functions as a tunnel insulating film in each memory cell transistor MT. Subsequently, the insulating films15and16, and the polysilicon layer17are formed on the gate insulating film14. The insulating film15is, e.g., one of an SiN single layer film, an HfAlO single layer film, and a laminated film including a high-dielectric-constant film having a higher dielectric constant than a silicon oxide film. It is to be noted that the high-dielectric-constant film is formed of one of Al2O3, MgO, SrO, BaO, TiO, Ta2O5, BaTiO3, BaZrO, ZrO2, HfO2, Y2O3, ZrSiO, HfSiO, and LaAlO, and may have a lamination layer structure containing these materials. Moreover, the laminated film may have one of a structure where an SiN film, a high-dielectric-constant film, and the SiN film are sequentially laminated on the gate insulating film14, a structure where an HfAlO film, the high-dielectric-constant film, and the SiN film are sequentially laminated on the same, a structure where the SiN film, the high-dielectric-constant film, and the HfAlO film are sequentially laminated on the same, and a structure where the HfAlO film, the high-dielectric-constant film, and the HfAlO film are sequentially laminated on the same. Additionally, the insulating film16is formed of a material having a higher dielectric constant than a material used for the gate insulating film14. Specifically, the material of the insulating film16is either a high-dielectric-constant film having a higher dielectric constant than a silicon oxide film, a laminated film including a silicon oxide film, or a high-dielectric-constant film. The high-dielectric-constant film is formed of one of Al2O3, MgO, SrO, SiN, BaO, TiO, Ta2O5, BaTiO3, BaZrO, ZrO2, HfO2, Y2O3, ZrSiO, HfSiO, and LaAlO, or may have a lamination layer structure containing any of these materials. The laminated film may have one of a structure where the silicon oxide film, the high-dielectric-constant film, and the silicon oxide film are sequentially laminated on the insulating film15, a structure where the silicon oxide film and the high-dielectric-constant film are sequentially laminated on the same, a structure where the high-dielectric-constant film and the silicon oxide film are sequentially laminated on the same, and a structure where the high-dielectric-constant film, the silicon oxide film, and the high-dielectric-constant film are sequentially laminated on the same.

Thereafter, each of the polysilicon layer17and the insulating films16and15are patterned into a pattern of the gate electrode of the memory cell transistor MT by using a photolithography technology and anisotropic etching, e.g., a reactive ion etching (RIE) method. At this time at least a part of the gate insulating film14may be etched. As a result, as shown inFIG. 4, the gate electrode of each memory cell transistor MT in which the insulating film15serving as a charge storage layer, the insulating film16functioning as a block layer, and the polysilicon layer17serving as a control gate electrode are sequentially laminated is brought to completion.

The gate electrode20of each selective transistor ST1and ST2is formed by, e.g., the following method. That is, the polysilicon layer20is formed on the gate insulating film14. Then, the polysilicon layer20is patterned into a desired pattern by photolithography and RIE, thereby bringing the gate electrode20to completion.

The gate electrodes34of the peripheral transistors PT1and PT2are also formed by the same method as the gate electrode20of the selective transistor ST1or ST2. That is, after forming the gate insulating films33and37, the polysilicon layers are formed on the gate insulating films33and37. Thereafter, the polysilicon layers are patterned into desired patterns based on the photolithography technology and RIE, thereby bringing the gate electrodes34to completion.

It is to be noted that the gate electrodes of the memory cell transistors MT, the gate electrodes20of the selective transistors ST1and ST2, and the gate electrodes34of the peripheral transistors PT1and PT2may be simultaneously formed. For example, after forming the gate insulating films14,33, and37, the insulating films15and16are formed on the entire surface of the p-type semiconductor substrate10. Then, the insulating films15and16are left in a region where the memory cell transistors MT are planned to be formed but are removed in any other region by photolithography and etching. Subsequently, the polysilicon layer is formed on the entire surface. The polysilicon layer becomes the control gate electrode17and the gate electrodes20and34. Then, the polysilicon layer and the insulating films15and16are etched, thereby bringing the gate electrodes of the respective transistors to completion.

With the above-explained steps, as shown inFIG. 4, the gate electrodes (the MONOS type structure) of the memory cell transistors MT and the gate electrodes20and34of the selective transistors ST1and ST2and the peripheral transistors PT1and PT2are brought to completion. It is to be noted that a film thickness of the gate insulating film33of the low-breakdown-voltage peripheral transistor PT1is not necessarily equal to a film thickness of the gate insulating film14of the memory cell transistor MT. A film thickness of the gate insulating film37of the high-breakdown-voltage peripheral transistor PT2is set to, e.g., approximately 30 nm, which is larger than film thicknesses of the gate insulating films14and33, to provide resistance against the voltage VPP. In particular, in the row decoder3, a high-breakdown-voltage MOS transistor is used for a MOS transistor that is present in a path through which the voltage VPP is applied to the word line WL when, e.g., writing data.

Then, as shown inFIG. 5, the insulating film18is formed on side surfaces and upper surfaces of the gate electrodes of the memory cell transistors MT, side surfaces and upper surfaces of the gate electrodes20of the selective transistors ST1and ST2, side surfaces and upper surfaces of the gate electrodes34of the peripheral transistors PT1and PT2, and the gate insulating films14,33, and37. The insulating film18is, e.g., a silicon oxide film (which may be referred to as an HTO film hereinafter) formed by using a high temperature oxidation (HTO) method (a film forming technique based on a reduced pressure CVD method). As a result, the side surfaces of the block layers16of the memory cell transistors MT are completely covered with the HTO film18. It is to be noted that setting a film forming temperature (a substrate temperature in this example) in the HTO method to, e.g., 750° C. or above is desirable. However, when using a metal material, e.g., TaN as a material for the control gate electrodes17and the gate electrodes20, setting the film forming temperature to a temperature lower than 750° C. is desirable.

Here, a silicon oxide film (which may be simply referred to as a TEOS film hereinafter) formed of a silicon compound containing nitrogen or oxygen as a material, e.g., an SiN film, SiON film, or a Tetra-Ethoxy silane (TEOS) may be used in place of the HTO film as the insulating film18, and using a material which can be readily subjected to film thickness control is preferable. As a material of the insulating film18, a material that can sufficiently ensure an etching selective ratio with respect to mask materials41to43, which will be explained with reference toFIGS. 6,7, and8, can suffice. For example, as the insulating film18, when using a photoresist as the mask materials41to43, a material which is hardly etched under etching conditions at the time of removing the photoresist based on wet etching can suffice. In this example, setting the selective ratio to 1:100 or above is desirable. That is, it is desirable for a speed at which the insulating film18is etched to be equal to or below 1/100 of a speed at which the mask materials41to43are etched. The following explains why. That is, when using a photoresist as the mask materials41to43, a film thickness of the photoresist becomes 1 μm or above. As will be explained later, the film thickness of the insulating film18is in the order of nm. Therefore, the insulating film18is removed together with the photoresist unless the selective ratio of at least 1:100 is provided with respect to the photoresist.

The film thickness of the insulating film18is, e.g., approximately 2 to 10 nm. The reason why the minimum value of the film thickness is 2 nm is that a limit value that enables film thickness control is 2 nm when considering an incubation time. It is to be noted that incubation time means the time required to form an initial layer that becomes a “nucleus” of growth at an initial stage of film growth. Further, the reason why the maximum value of the film thickness is 10 nm is as follows. That is, each interval between the gate electrodes of the memory cell transistors MT adjacent to each other has been reduced to 30 to 40 nm or below because of a demand for high integration. For example, if each interval between the gate electrodes adjacent to each other is 25 nm and a film thickness of the insulating film18is 10 nm, a frontage width for later-explained ion implantation at the time of forming source/drain regions is 5 nm. That is, a distance (a width) between the insulating films18adjacent to each other is 5 nm, and an impurity is implanted into the p-type well region12through this region having the width of 5 nm. At this time, it can be considered that the minimum width necessary of the region for ion implantation is 5 nm. Therefore, the maximum value of the film thickness of the insulating film18is 10 nm.

Then, as shown inFIG. 6, a mask material, e.g., a photoresist is formed on the p-type semiconductor substrate10, and the photoresist in the region where the memory cell array1is planned to be formed is removed based on photolithography. Furthermore, in a state that a region where the peripheral transistors PT1and PT2are planned to be formed is masked with a photoresist41, the n+-type impurity diffusion layer13that functions as source/drain regions of the memory cell transistors MT and the selective transistors ST1and ST2is formed. That is, an n-type impurity is ion-implanted into portions between the gate electrodes of the memory cell transistors MT adjacent to each other, between the gate electrodes of the memory cell transistor MT and the selective transistor ST1or ST2, between the gate electrodes20of the selective transistors ST1and ST1adjacent to each other, and between the gate electrodes20of the selective transistors ST2and ST2adjacent to each other. At this time, the gate electrodes of the memory cell transistors MT, the gate electrodes20of the selective transistors ST1and ST2, and the insulating film18function as masks for ion implantation. After ion implantation, the photoresist41covering the peripheral transistors PT1and PT2is incinerated by, e.g., ashing, and further subjected to wet etching which uses, e.g., a mixed solution containing H2SO4and H2O2(which will be referred to as an SH liquid hereinafter). In this wet etching, the block layer16of each memory cell transistor MT is protected by the insulating film18. In other words, wet etching is carried out under conditions where the photoresist41is sufficiently etched and the insulating film18is not substantially etched.

Then, as shown inFIG. 7, a mask material, e.g., a photoresist is formed on the p-type semiconductor substrate10, and the photoresist in a region where the peripheral transistor PT2is planned to be formed is removed based on photolithography. Moreover, in a state that regions where the memory cell transistors MT, the selective transistors ST1and ST2, and the peripheral transistor PT1are planned to be formed are masked with a photoresist42, the n+-type impurity diffusion layer40that functions as source/drain regions of the peripheral transistor PT2is formed. That is, an n-type impurity is ion-implanted into a surface region of the p-type well region32. At this time, the gate electrode34of the peripheral transistor PT2and the insulating film18function as masks for ion implantation. After ion implantation, the photoresist42is incinerated and subjected to wet etching which uses the SH liquid, thereby delaminating the photoresist42. In this wet etching, likewise, the block layer16of each memory cell transistor MT is protected by the insulating film18. That is, wet etching is carried out under conditions that the photoresist42is sufficiently etched and the insulating film18is not substantially etched.

Then, as shown inFIG. 8, a mask material, e.g., a photoresist is formed on the p-type semiconductor substrate10, and the photoresist in a region where the peripheral transistor PT1is planned to be formed is removed based on the photolithography technology. Additionally, in a state that regions where the memory cell transistors MT, the selective transistors ST1and ST2, and the peripheral transistor PT2are planned to be formed are masked with a photoresist (a mask material)43, the p+-type impurity diffusion layer39that functions as source/drain regions of the peripheral transistor PT1is formed. That is, a p-type impurity is ion-implanted into a surface region of the n-type well region31. At this time, the gate electrode34of the peripheral transistor PT1and the insulating film18function as masks for ion implantation. After ion implantation, the photoresist43is incinerated and subjected to wet etching which uses the SH liquid, thereby delaminating the photoresist43. In this wet etching, likewise, the block layer16of each memory cell transistor MT is protected by the insulating film18. That is, wet etching is carried out under conditions that the photoresist43is sufficiently etched and the insulating film18is not substantially etched.

Further, as shown inFIG. 9, the insulating film18on the upper surfaces of the gate electrodes of the memory cell transistors MT, the upper surfaces of the gate electrodes20of the selective transistors ST1and ST2, the upper surfaces of the gate electrodes34of the peripheral transistors PT1and PT2, and the upper surfaces of the gate insulating films14,33, and37is removed. Thereafter, the insulating film19is formed on the p-type semiconductor substrate10to cover the gate electrodes of the memory cell transistors MT, the gate electrodes20of the selective transistors ST1and ST2, and the gate electrodes34of the peripheral transistors PT1and PT2. Furthermore, the insulating film19is left on sidewall portions alone of the gate electrodes of the memory cell transistors MT, the gate electrodes20of the selective transistors ST1and ST2, and the gate electrodes34of the peripheral transistors PT1and PT2while exposing the upper surfaces of the control gate electrodes17and the gate electrodes20and34by etching back the insulating film19based on, e.g., RIE.

It is to be noted that, inFIG. 9, each region between the gate electrodes of the memory cell transistors MT adjacent to each other and each region between the gate electrode of the memory cell transistor MT and the gate electrode20of the selective transistor ST1or ST2are completely filled with the insulating film19. However, these regions do not have to be completely filled, and the surface of the gate insulating film14may be partially exposed.

Subsequently, after forming a metal layer on the control gate electrodes17and the gate electrodes20and34, a heat treatment is performed to form a metal silicide layer on the respective upper surfaces of the control gate electrodes17, the gate electrodes20, and the gate electrodes34.

With the above-explained steps, the memory cell transistors MT, the selective transistors ST1and ST2, and the peripheral transistors PT1and PT2are brought to completion. Subsequently, the interlayer insulating film21that covers the respective transistors is formed on the p-type semiconductor substrate10, and the contact plugs CP1and CP2and the metal wiring layers22and23are formed in the interlayer insulating film21. Furthermore, the interlayer insulating films24and25are formed on the interlayer insulating film21, and the contact plug CP3reaching the metal wiring layer23is formed in the interlayer insulating films24and25. Then, the metal wiring layer26that is in contact with the contact plug CP3is formed on the interlayer insulating film25, and the interlayer insulating film27is formed on the metal wiring layer26, thereby bringing the structure depicted inFIG. 2to completion.

According to the structure of this embodiment, the following effects (1) and (2) can be obtained.

(1) Operation characteristics of the semiconductor memory device can be improved.

According to the structure of this embodiment, the insulating film18that completely covers at least the side surfaces of the block layers16is formed on the sidewalls of the gate electrodes of the memory cell transistors MT. Therefore, in a manufacturing process, the side surfaces of the block layers16can be prevented from being etched, thus improving operation characteristics of the NAND type flash memory. This effect will be explained in detail hereinafter.

As memory cells conventionally used in a nonvolatile memory, e.g., a flash memory, two types, having different regions where electric charges are held, are known as typical memory cells. The first type has a structure where a charge storage layer formed of a conductive material is surrounded by an insulating film and data is held by changing a threshold voltage of each memory cell transistor based on a quantity of electric charges stored in this conductive material (which will be referred to as a floating gate (FG) type hereinafter). The second type has a structure where a threshold voltage of each memory cell transistor is changed based on a quantity of electric charges trapped in a charge storage layer (a trap film) formed of an insulator (a MONOS type).

The FG type structure has been conventionally widely used since the MONOS type structure is inferior in the point of charge holding characteristics, which is important as a function of a nonvolatile memory. However, as the size of a memory cell transistor has been reduced, as a result of both pursuit towards high integration and advancements in lithography technology, the following problems have become prominent in the FG type structure.

The first problem is a fluctuation in threshold values of memory cell transistors caused due to capacitance coupling between memory cell transistors adjacent to each other. In recent years, realization of multiple values by narrowing a threshold value distribution that is used to discriminate data in memory cell transistors has advanced in order to increase a storage capacity (a data amount) of a single memory cell transistor. Therefore, a margin of a shift in a threshold value of the memory cell transistor is reduced as compared with that of a binary value, and stored data is apt to be erroneously read due to capacitance coupling between the memory cell transistors adjacent to each other. Furthermore, since a coupling ratio must be maintained, a film thickness of each layer forming the FG type structure must be maintained to some extent. Therefore, when a distance between the memory cell transistors adjacent to each other is narrowed due to miniaturization, capacitance coupling between gate electrodes adjacent to each other is increased in inverse proportion to the distance between the memory cell transistors. It is to be noted that the coupling ratio is CR=CIPD/(CIPD+COX). However, COXis a gate insulating film capacitance between a charge storage layer and a semiconductor substrate, and CIPDis an insulating film capacitance between the charge storage layer and a control gate electrode.

The second problem is that, when a defect occurs even in a part of the insulating film surrounding the charge storage layer in the FG type structure, electric charges held in the charge storage layer flow out through this defect and data that should be held is lost. In the MONOS type structure, since electric charges are stored in the trap film itself, the same problem as that in the FG type structure does not occur.

Based on the above-explained problems, a memory cell transistor having the MONOS type structure that may possibly overcome drawbacks in the FG type structure has again attracted attention. However, the MONOS type structure likewise has the following problems.

As explained in the foregoing embodiment, not only the memory cell transistors MT but also the peripheral transistors PT1and PT2are formed on the p-type semiconductor substrate10. Each of the peripheral transistors PT1and PT2is a transistor used in, e.g., the power supply voltage generation circuit2, the row decoder3, and the column decoder4. Moreover, various kinds of operating voltage values, current values, and threshold values different from those in the memory cell transistors MT are required in the peripheral transistors PT1and PT2. In order to realize this, in the peripheral transistors PT1and PT2, the type of impurity, energy when implanting the impurity, an amount of the impurity, and others at the time of, e.g., forming source/drain regions vary depending on necessary characteristics.

In particular, an impurity that is implanted to improve a breakdown voltage of the transistor or an impurity implanted to suppress a short channel effect is implanted into the silicon substrate after processing each gate electrode. The type of this impurity and implantation energy vary in accordance with the type of each transistor. Therefore, regions other than those where the impurity is implanted must be masked with a photoresist by using the photolithography technology in accordance with the number of types of the transistor. Therefore, after gate processing, application and delamination of the photoresist are repeated every time the impurity is implanted. Delamination of the photoresist is carried out by incineration (ashing) and wet etching after performing exposure.

At this time, an etchant (the SH liquid) that is generally utilized to delaminate the photoresist is used to etch not only the photoresist but also the block layer (e.g., Al2O3)16. Therefore, the gate electrode, especially the block layer16of each memory cell transistor MT is etched every time etching for delamination of the photoresist is carried out. This state will now be explained with reference toFIG. 10.

FIG. 10is a cross-sectional view of an MOS transistor having a MONOS type structure. As shown in the drawing, the MONOS type structure includes a charge storage layer (an SiN film)55formed on a gate insulating film54, a block film (Al2O3)56formed on the charge storage layer55, and a control gate electrode57formed on the block film56. When this structure is exposed to wet etching which uses the SH liquid, a side surface of each block film56is etched and the side surface has a shape curved toward the inner side of each gate electrode, as shown in the drawing.

For example, in a case where a width of each gate electrode (a length in a lateral direction inFIG. 10) is 40 nm or below, when an etching amount of the block film56exceeds 5 nm, a width of the block film56becomes 30 nm or below. The etching amount of the block film56based on wet etching is determined based on a length and the number of times of wet etching, and it is fixed even if a dimension of the gate electrode is reduced. Therefore, when the dimension of the gate electrode will be reduced in the future, there may occur a problem that a percentage of the width of the block film56with respect to the dimension of the gate electrode is decreased and the control gate electrode57falls off the block film56. Additionally, when the block film56is etched, capacitance coupling between the control gate electrode57and the charge storage layer55is reduced, and the data writing speed and data erasing speed are lowered.

Further, when an insulating film59is formed on a sidewall of each gate electrode, a gap58may be generated between the block film56and the insulating film59. Generation of this uncontrollable gap58may become a factor of fluctuations in characteristics in the respective MOS transistors.

As explained above, as a means for preventing the block film56from being etched, there is a method of forming the block film56and then performing annealing at a high temperature of, e.g., 1000° C. or above. When this annealing is carried out, resistance of the block film56against etching can be improved. However, avoiding annealing at a high temperature is desirable in the light of thermal history. That is because annealing at a high temperature may vary a concentration profile of an impurity already implanted in a well and a channel.

In this regard, according to the structure of this embodiment, for example, as shown inFIG. 5, forming the insulating film18can solve the above-explained problem. That is, in this embodiment, the insulating film18is formed on the sidewall of the block layer16. As a result, in the manufacturing steps shown inFIGS. 6 to 8, wet etching for removing the photoresists41to43is carried out in a state where the block layer16is protected by the insulating film18. Therefore, the block layer16can be prevented from being etched without over 1000 degrees centigrade annealing the block layer16.

Accordingly, the control gate electrode17can be prevented from falling off, and annealing at a high temperature becomes no longer necessary. Therefore, a process yield of the NAND type flash memory can be improved. For example, an impurity can be prevented from being diffused in the well or the channel. Further, a reduction in capacitance coupling between the control gate electrode17and the charge storage layer15can be suppressed, and a decrease in a data writing speed and a data erasing speed can be also suppressed. Moreover, a gap can be prevented from being generated between the insulating film19and the block layer16, thus suppressing occurrence of fluctuations in characteristics of the memory cell transistors.

Furthermore, in the MONOS type structure, increasing a current flowing through each memory cell transistor MT is desirable. Therefore, it is necessary to increase a capacitance of the lamination layer structure including the block layer16, the charge storage layer15, and the gate insulating film14per unit area as much as possible. In order to realize this increase, it is desirable to reduce an equivalent oxide thickness of the block layer16as much as possible within allowable limits in degradation in charge holding characteristics of the block layer16involved by charge leak from the block layer16side. The equivalent oxide thickness (EOT) means a film thickness obtained when a material is reduced to a silicon oxide film. That is, the EOT can be decreased when a material having a higher dielectric constant than, e.g., a silicon oxide film is used. If, as shown inFIG. 10, the side surface of the block film56is etched and the insulating film59is present in this region, the dielectric constant of the block film56is substantially reduced and the equivalent oxide thickness is increased. However, in this embodiment, for example, as shown inFIG. 9, since the insulating film18protects the surface of the block layer16from being etched, an increase in equivalent oxide thickness of the block layer16can be avoided.

Incidentally, it is desirable to form the insulating film18on not only the sidewalls of the gate electrodes of the memory cell transistors MT but also the sidewalls of the gate electrodes20of the selective transistors ST1and ST2and the sidewalls of the gate electrodes34of the peripheral transistors PT1and PT2like this embodiment. That is because the gate electrodes20and34can be thereby prevented from being subjected to unnecessary wet etching.

(2) Manufacturing accuracy can be improved while facilitating manufacturing steps of the NAND type flash memory.

In the structure according to this embodiment, the respective insulating films18for the memory cell transistors MT, the selective transistors ST1and ST2, and the peripheral transistors PT1and PT2are simultaneously formed. Therefore, the effect (1) can be obtained while suppressing an increase in the number of manufacturing steps to the minimum.

Moreover, the HTO film is used as the insulating film18. Therefore, a film thickness of the insulating film18can be easily controlled. This leads to the following effect. That is, each gate electrode and the insulating film18function as masks for ion implantation when forming the impurity diffusion layers13,39, and40that serve as the source/drain regions. Therefore, manufacturing accuracy of the source/drain regions is dependent on a width of each gate electrode and a film thickness of the insulating film18. In particular, a width of the gate electrode in a recent NAND type flash memory has become very small, and a problem, e.g., the short channel effect becomes prominent when, e.g., a distance between the source region and the drain region is too small.

In this regard, according to the structure of this embodiment, presence of the insulating film18can sufficiently assure the distance between the source region and the drain region as compared with a case where the insulating film18is not present. Therefore, forming the insulating film18enables not only protecting the block layer16but also accurately setting a distance of the impurity diffusion layer13,39, or40from an end portion of the gate electrode. Therefore, characteristics of the NAND type flash memory can be improved.

Second Embodiment

A manufacturing method of a semiconductor memory device according to a second embodiment of the present invention will now be explained. According to this embodiment, in the MONOS type structure of the first embodiment, the charge storage layers15depicted inFIG. 2are connected between the neighboring memory cell transistors MT in common.FIG. 11is a cross-sectional view of memory cell transistors MT in a NAND type flash memory according to this embodiment, and memory cell transistors MT0and MT1are depicted as an example. Of course, other memory cell transistors MT2to MT15have the same structure.

As shown in the drawing, in each memory cell transistor MT according to this embodiment, insulating films15that function as charge storage layers are connected between the neighboring memory cell transistors MT in common with respect to the structure depicted inFIG. 2explained in the first embodiment. Other structures are the same as those in the first embodiment. Therefore, points different from the first embodiment alone will be explained with reference toFIG. 11.

For example, a region where a control gate electrode17is formed will be referred to as A1, and a region where the control gate electrode17is not formed will be referred to as A2. Then, the insulating film15is formed on not only the region A1but also a gate insulating film14in the region A2, and the insulating films15in both the regions A1and A2are connected with each other. However, the insulating film15in the region A2does not substantially function as the charge storage layer, and the insulating film15in the region A1alone serves as the charge storage layer. Additionally, an insulating film19is formed on the insulating film15in the region A2alone.

In the example shown inFIG. 11, a film thickness of the insulating film15varies depending on the region A1and the region A2, and the film thickness in the region A1is larger than the film thickness in the region A2. Therefore, a step is generated on a surface of the insulating film15at a boundary portion between the region A1and the region A2, and an insulating film18is also formed on a sidewall of the insulating film15at this step portion. Other structures are as explained in the first embodiment.

In regard to manufacturing steps of the structure according to this embodiment, it is good enough to stop etching of the insulating film15halfway through a patterning step of each gate electrode inFIG. 4explained in the first embodiment. Other manufacturing steps are the same as those in the first embodiment. Of course, etching may be stopped on an upper surface of the insulating film15. In this case, the film thickness of the insulating film15is the same in both the region A1and the region A2.

As explained in this embodiment, even in the NAND type flash memory in which the plurality of memory cell transistors MT share the charge storage layer15, the effects (1) and (2) explained in the first embodiment can be obtained.

That is, in the structure according to this embodiment, as shown inFIG. 11, the charge storage layers15are connected between the memory cell transistors MT adjacent to each other. In this case, like the first embodiment, each block layer16is protected by the insulating film18. As a result, even if the step using photolithography and wet etching is repeated the necessary number of times, the surface of each block layer16is not etched, and the effects (1) and (2) of the first embodiment can be obtained.

Further, in this embodiment, the charge storage layers15are connected to be shared by the memory cell transistors MT adjacent to each other. However, in the MONOS type structure, since the charge storage layer15is formed of an insulating material, electric charges do not flow into the charge storage layer15of one of the memory cell transistors MT adjacent to each other. As a result, it is considered that the same operation characteristics as those in the first embodiment can be obtained.

However, in this embodiment, when forming source/drain regions of each memory cell transistor MT, an impurity is implanted into not only the gate insulating film14but also the p-type well region12through the charge storage layer15. Then, it can be considered that an ion seed is diffused at the ion implantation step and forming the source/drain regions as designed becomes difficult. Furthermore, it is also considered that electric charges are trapped in the charge storage layer15in the region A2, which degrade characteristics of each memory cell transistor MT in some cases. Therefore, when there is the above-explained concern, adopting the structure explained in the first embodiment is desirable.

Third Embodiment

A manufacturing method of a semiconductor memory device according to a third embodiment of the present invention will now be explained. In this embodiment, the MONOS type structure explained in the first embodiment is applied to a NOR type flash memory.

FIG. 12is a block diagram showing a NOR type flash memory according to this embodiment. As shown in the drawing, the NOR type flash memory according to this embodiment includes a memory cell array1, a power supply voltage generation circuit2, a row decoder3, and a column decoder4. Since the power supply voltage generation circuit2, the row decoder3, and the column decoder4are the same as those explained in the first embodiment, an explanation thereof will be omitted, and the memory cell array1will be described below.

As shown in the drawing, the memory cell array1includes a plurality of memory cell transistors MT arranged in a matrix. Each memory cell transistor MT includes a MONOS type structure having a charge storage layer (e.g., an insulating film) formed on a semiconductor substrate through a gate insulating film, an insulating film which has a higher dielectric constant than the charge storage layer and is formed on this insulating film, and a control gate electrode formed on this insulating film having a high dielectric constant. That is, this memory cell transistor MT has the same structure as the memory cell transistor MT explained in the first or second embodiment. A drain region of each memory cell transistor MT provided in the same column is connected with one of bit lines BL0to BLn in common, and the control gate electrode of each of the memory cell transistors provided in the same row is connected with one of word lines WL0to WLm in common. Moreover, a source region of each memory cell transistor MT is connected with a source line SL in common. That is, this memory cell array1has a structure in which the memory cell transistors MT explained in the first embodiment are arranged in a matrix and the bit line BL and the word line BL are connected in accordance with each memory cell transistor MT.

In this NOR type flash memory, the effects (1) and (2) explained in the first embodiment can be likewise obtained.

As explained above, in the case of the semiconductor memory device according to the first to third embodiments, the insulating film, e.g., an HTO film that can assure a selective ratio with respect to a photoresist at the time of wet etching is formed on a sidewall of the block layer of each memory cell transistor having the MONOS type structure. Therefore, since the block layer can be protected by the insulating film during wet etching, the block layer can be prevented from being etched.

Therefore, it is possible to prevent each control gate electrode in the MONOS type structure from falling off or prevent the equivalent oxide thickness of each block layer from being increased, thereby improving characteristics of the semiconductor memory device.

It is to be noted that the example where the film thickness of the insulating film18is 2 nm or above has been explained in the foregoing embodiments. However, providing the insulating film18on the side surface of each block layer can suffice, and the insulating film18can function as a protective film for the block layer even if the film thickness of the insulating film18is less than 2 nm.

Additionally, the example where the HTO film, the SiN film, the SiON film, or the TEOS film is used as a material of the insulating film18has been explained in the foregoing embodiments. However, in the light of operation characteristics of the semiconductor memory device, using the HTO film or the TEOS film having a lower dielectric constant is desirable. In particular, in the NAND type flash memory, realization of high integration has been demanded in recent years, and miniaturization of wiring lines including transistors has advanced with this demand. For example, a gate electrode is formed using minimum processing dimensions, and a width of a gate electrode interval is made as small as possible. Therefore, an influence of a parasitic capacitance between gate electrodes prominently appears, and characteristics as a flash memory are reduced due to this influence. Thus, using an HTO film or TEOS film as a material of the insulating film18enables reducing the parasitic capacitance. As a result, interference between the memory cell transistors MT adjacent to each other can be suppressed, thus improving operation characteristics of the flash memory. Furthermore, as a material of the insulating film18, a material having a selective ratio of 1:100 or above with respect to the mask materials41to43at the time of wet etching of the mask materials41to43can suffice. Therefore, when a material other than the photoresist is used as each of the mask materials41to43, it is good enough to appropriately select a material of the insulating film18in accordance with this material.

The insulating film18can be a side-wall oxide film formed by use of the Atomic Layer Deposition (ALD) method, which enables film formation at a temperature in the range of 350 to 550 degrees.

The example where polysilicon is used as a material for the control gate electrode17and the gate electrodes20and34has been explained in the foregoing embodiments. However, the present invention is not restricted to polysilicon, and a metal or TaN may be used as the material, for example. However, since TaN has a higher resistance than polysilicon, using a metal laminated film containing, e.g., polysilicon, a metal, or TaN is desirable. To be more specific, the control gate electrode17and the gate electrodes20and34are not limited to the polysilicon layers described above; they may be formed of a stacked film made up of a metal nitride film and a polyslicon film, a metal nitride film, a metal film, or the like. It should be noted that the metal film is made of W, for example, and the metal nitride film is made of WN, TaN or TiN, for example.

Additionally, the gate electrode of each of the selective transistors ST1and ST2may have the MONOS type structure, like the memory cell transistor MT, as shown inFIG. 13.FIG. 13is a cross-sectional view of a NAND type flash memory according to a modification of the first embodiment.

As shown in the drawing, a gate electrode of each of selective transistors ST1and ST2has a lamination layer structure including a charge storage layer15, a block layer16, and a control gate electrode17. Further, a side surface of the block layer16is covered with an insulating film18. In this structure, injecting electric charges into the charge storage layer15enables setting a threshold voltage of each of the selective transistors ST1and ST2to an optimum value. Of course, in this structure, the charge storage layers15adjacent to each other may be likewise connected with each other as shown in, e.g.,FIG. 11.

Furthermore, in this embodiment, the example where the insulating film18is formed on not only the side surface of the block layer16but also the side surfaces of the charge storage layer15and the control gate electrode17has been explained. However, in the light of protection of the block layer16, providing the insulating film18on the side surface of the block layer16alone is sufficient. However, when the insulating film18is also formed on the side surfaces of the charge storage layer15and the control gate electrode17, these films can be protected from wet etching.

Moreover, in the foregoing embodiments, the NAND type flash memory and the NOR type flash memory have been explained as examples. However, for instance, the present invention can be also applied to a 3Tr-NAND type flash memory having a structure where the number of memory cell transistors is one in the NAND type flash memory. Additionally, the present invention can be also applied to a 2Tr type flash memory having a structure where the selective transistor ST1on the drain region side is eliminated in the 3Tr-NAND type flash memory, and it can be extensively applied to general nonvolatile memories of a laminated gate structure.