Method of manufacturing semiconductor device having oxide films with different thickness

Abstract

After a first gate oxide film is formed on a substrate, a nitride layer is formed by a first oxynitriding process. The first gate oxide film is selectively removed from a thinner film part area of the substrate. A second gate oxide film forming process forms a second gate oxide film in the thinner film part area and a third gate oxide film in a thicker film part area. By executing second oxynitriding process, nitride layers are formed at the thinner and the thicker part areas.

Description

This application is a Continuation in Part of U.S. patent application Ser. No. 10/843,694, filed on May 12, 2004. This application claims priority to prior Japanese Application JP 2003-134265, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a method of manufacturing a semiconductor device, in particular, to a manufacturing method of a semiconductor device including transistors having gate insulating films with different thickness.

There is a known semiconductor device, in which plural kinds of transistors having gate insulating films with different thickness are formed on a common substrate as a combination of a semiconductor memory and peripheral circuits thereof.

A conventional method of manufacturing the semiconductor device of the above type uses an oxynitriding process for a thinner gate insulating film for one of the transistors. That is, nitrogen elements are mainly introduced into the thinner gate insulating film. No or few nitrogen elements are introduced into a thicker gate insulating film for another one of the transistors.

Generally, when thickness of a gate oxide film is 7 nm or more as before, the oxynitriding process is unnecessary. This is because the thicker gate oxide film equal to or more than 7 nm has no problem such as leakage current and boron leakage. Moreover, the oxynitriding process is undesirable when the thickness of the gate oxide film is 5 nm or more because it deteriorates reliability of the gate oxide film.

However, the gate oxide film of the transistor tends to become thin according to demands of miniaturizing, implementing thin design, and saving power consumption of the semiconductor device recently. Thus, importance of the oxynitriding process becomes high to suppress leakage current and to improve operating characteristics of the transistor. Therefore, in a case of manufacturing the semiconductor device including plural kinds of transistors having gate insulating films with different thickness, it becomes important to introduce nitrogen elements into not only the thinner gate insulating film but also the thicker gate insulating film.

SUMMARY OF THE INVENTION

In view of the foregoing and other exemplary problems, drawbacks, and disadvantages of the conventional methods and structures, an exemplary feature of the present invention is to provide a method of manufacturing a semiconductor device capable of introducing nitrogen elements into not only a thinner gate insulating film formed on a substrate but also a thicker gate insulating film formed on the substrate.

Other exemplary features of this invention will become clear as the description proceeds.

According to an exemplary aspect of this invention, a method of manufacturing semiconductor device includes multi-oxidation process for forming oxide films with different thickness on a substrate. The method includes executing an oxide film forming process for forming each of said oxide films on said substrate, and inevitably executing an oxynitriding process for forming nitride layer in each of said oxide films after the oxide film forming process.

According to another exemplary aspect of this invention, a semiconductor device has a substrate with a plurality of regions. The semiconductor device comprises oxide films which are formed in the regions and which have different thickness. Nitride layers are formed at vicinities of interfaces between the oxide films and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F are schematic sectional views for describing a method of manufacturing a related semiconductor device including transistors having gate insulating films with different thickness;

FIGS. 2A-2F are schematic sectional views for describing another method of manufacturing another related semiconductor device including transistors having gate insulating films with different thickness;

FIGS. 3A-3F are schematic sectional views for describing a method of manufacturing a semiconductor device according to a first embodiment of this invention;

FIG. 4 shows oxygen and nitrogen profiles before and after a second oxide film forming process using ISSG or plasma oxidation;

FIG. 5 shows oxygen and nitrogen profiles before and after a second oxynitriding process;

FIGS. 6A-6E are schematic sectional views for describing a method of manufacturing a semiconductor device according to a second embodiment of this invention;

FIGS. 7A-7F are schematic sectional views for describing a method of manufacturing a semiconductor device according to a third embodiment of this invention;

FIG. 8 depicts a semiconductor device according to an exemplary embodiment of this invention;

FIGS. 9A-9E are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention;

FIG. 10 depicts a semiconductor device according to an exemplary embodiment of this invention;

FIGS. 11A-11B are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention;

FIG. 12 depicts a semiconductor device according to an exemplary embodiment of this invention;

FIGS. 13A-13D are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention;

FIG. 14 depicts a semiconductor device according to an exemplary embodiment of this invention;

FIGS. 15A-15B are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention;

FIG. 16 depicts a semiconductor device according to an exemplary embodiment of this invention;

FIGS. 17A-17B are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention;

FIG. 18 depicts a semiconductor device according to an exemplary embodiment of this invention;

FIGS. 19A-19F are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention;

FIG. 20 depicts a semiconductor device according to an exemplary embodiment of this invention;

FIGS. 21A-21B are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention;

FIG. 22 depicts a semiconductor device according to an exemplary embodiment of this invention; and

FIGS. 23A-23B are schematic sectional views for describing a method of manufacturing a semiconductor device according to an exemplary embodiment of this invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Referring to FIGS. 1A to 1F, description will be at first directed to a method of manufacturing a related semiconductor device including transistors having gate insulating films with different thickness. Such a process is disclosed in Unexamined Japanese Patent Publication No. 2000-216257.

At first, as illustrated in FIG. 1A, a silicon substrate 101 is provided and LOCOS (Local Oxidation of Silicon) oxide films 102 are formed in the silicon substrate 101. The LOCOS oxide films 102 define device forming areas including higher and lower voltage system transistor forming areas A-11 and A-12 and isolate them from each other.

Next, as shown in FIG. 1B, a first heat-treating process is executed to the silicon substrate 101 in atmosphere of oxidation seeds 103. The first heat-treating process oxidizes exposed surfaces of the silicon substrate 101 and thereby silicon oxide films 104 are formed on/in the silicon substrate 101.

Next, as shown in FIG. 1C, after a resist film 105 is formed at the higher voltage system transistor forming area A-11, the silicon oxide film 104 of the lower voltage system transistor forming area A-12 is removed by a wet etching process to expose the silicon substrate 101. Then the resist 105 is completely removed from the higher voltage system transistor forming area A-11.

Subsequently, as shown in FIG. 1D, nitrogen ions 106 are implanted in the areas A-11 and A-12 by an ion implanter (not shown). As a result, an azotized silicon oxide film 107 is formed at the higher voltage system transistor forming area A-11 while a silicon nitride film 108 is formed at the lower voltage system transistor forming area A-12.

Next, as shown in FIG. 1E, a second heat-treating process is made to the silicon substrate 101 in atmosphere of oxidation seeds 109 and thereby a thicker gate oxide film 110 and a thinner gate oxide film 111 are formed at the areas A-11 and A-12, respectively.

Lastly, as shown in FIG. 1F, a polysilicon film 112 is deposited on the upper exposed surface of the silicon substrate 101 with the thicker gate oxide film 110 and the thinner gate oxide film 111.

After that, the polysilicon film 112 is patterned in a predetermined pattern. Then, gate electrodes and source-drain regions are formed on/in the semiconductor substrate 101 to form the semiconductor device. Thus, the semiconductor device including two (or more) kinds of transistors with different thickness of gate insulating film is completed.

Another method of manufacturing another related semiconductor device of the type is described in reference to FIGS. 2A-2F. Such a method is disclosed in Unexamined Japanese Patent Publication No. 2001-53242.

At first, as shown in FIG. 2A, a silicon substrate 201 is provided and device isolation layers 202 are formed in the substrate 201 by a trench isolation method. The device isolation layers 202 define device areas A-21, A-22 and A-23. The device areas A-21, A-22 and A-23 are used for a core area, a SRAM area and a peripheral I/O area, respectively. Furthermore, necessary preprocessing such as ion implantation is performed to the silicon substrate 201 with the device isolation layers 202.

Next, as shown in FIG. 2B, oxide films 203 are formed at the device areas A-21, A-22 and A-23 by a thermal oxidation method using oxygen gas supplied on the silicon substrate 201. Each of the oxide films 203 has thickness, for example, of 4.5 nm.

As shown in FIG. 2C, after only the peripheral I/O area A-23 and vicinity is covered by a resist film 204, the oxide films 203 of the core area A-21 and the SRAM area A-22 are removed by etching. Then the resist 204 is completely removed from the peripheral I/O are A-23 and the vicinity.

Next, a first oxynitriding process is performed to form oxynitride films 205 at the device areas A-21 and A-22. In this event a two-layer film 206 consist of the oxide film and an oxynitride film is formed at the device area A-23. Each of the oxynitride films 205 has a thickness, for example, of 1.6 nm while the two-layer film 206 has a thickness, for example, of 4.8 nm.

Next, as shown in FIG. 2E, after the device areas A-21 and A-23 are covered by resist films 207, the oxynitride film 205 of the device area A-22 is removed by etching. Then the resist films 207 are completely removed from the device areas A-21 and A-23.

After that, a second oxynitriding process is performed to the silicon substrate 201 with the oxynitride film 205 at the device area A-21 and the two-layer film 206 at the device area A-23. The second oxynitriding process uses source gas whose density of nitrogen is lower than that of the source gas used in the first oxynitriding process. Accordingly, as shown in FIG. 2F, an oxynitride film 208, an oxynitride film 209 having nitrogen density lower than that of the oxynitride film 208, and a two layer film 210 are formed at the core area A-21, the SRAM area A-22, and the peripheral I/O area A-23, respectively. For example, the films 208, 209 and 210 have thickness of 2.0 nm, 2.5 nm and 5.0 nm, respectively. The films 208, 209 and 210 are used for gate insulating films of transistors.

In the former of the related methods mentioned above, the oxynitride process (e.g. nitrogen ion implantation) is performed only after the first gate oxide film (104) is formed. Moreover, in the latter of the related methods, the oxynitride processes are used for forming the second and third gate insulating films (208 and 209). At any rate, the oxynitride process(es) is(are) used to introduce nitrogen into the thinner (oxide) film part area(s). Accordingly, the related methods can insufficiently introduce nitrogen into the thicker (oxide) film part area. In addition, each of the related methods cannot form a nitride layer in the vicinity of interface between the substrate and the gate insulating film. This makes it difficult to obtain desirable characteristics of the semiconductor device manufactured by the method.

Referring to FIGS. 3A-3F, the description will proceed to a method of manufacturing a semiconductor device according to a first exemplary embodiment of this invention.

In each of FIGS. 3A-3F, the left hand side shows a thinner film part area A-31 (or a low voltage transistor forming area) while the right hand side shows a thicker film part area A-32 (or a high voltage transistor forming area). Though the thinner film part area A-31 must be isolated from the thicker film part area A-32 by a device isolating region, the device isolating region has no relation with this invention and illustrating thereof is omitted in the present specification and drawings. Other parts, such as gate, source and drain regions, having no relation to this invention are also omitted in the present specification and drawings.

Hereafter, the description will be mainly directed to forming gate oxide films and oxynitriding the gate oxide films. Known processes can be used for other necessary processes in the method of manufacturing the semiconductor device.

As illustrated in FIG. 3A, at first, a semiconductor substrate (e.g. Si substrate) 301 is provided and a first gate oxide film 302 is formed by a first oxide film forming process on the surface of the semiconductor substrate 301. For the first gate oxide film forming process, various processes may be used. For instance, there are a wet, dry or halogen oxidation using a vertical diffusion equipment, an RTO (Rapid Thermal Oxidation), ISSG (In-Situ Steam Generation) or WVG (Water Vapor Generation) using a sheet fed equipment, and a plasma oxidation with a plasma treatment equipment.

Next, a first oxynitriding process is applied to the semiconductor substrate 301 on which the first gate oxide film 302 is formed. As a result, a first nitride layer 303 is formed in the first gate oxide film 302 as illustrated in FIG. 3B. To execute the oxynitriding process, an NO (nitric oxide), N₂O (nitrous oxide) or NH₃ (ammonia) treatment using the vertical diffusion equipment or a sheet fed equipment, or a plasma nitriding using the plasma treatment equipment can be used, for example.

Here, the NO or N₂O treatment tends to form the first nitride layer 303 at vicinity of an interface between the first gate oxide film 302 and the semiconductor substrate 301. Moreover, the NH₃ treatment tends to form the first nitride layer 303 both at the vicinity of an upper surface of the first gate oxide film 302 and at the vicinity of the interface between the first gate oxide film 302 and the semiconductor substrate 301. Furthermore, the plasma nitriding tends to form the first nitride layer 303 at the vicinity of the upper surface of the first gate oxide film 302.

Next, a resist film for an etching mask is deposited on the upper surface of the first gate oxide film 302. Then the resist film is selectively removed from the thinner film part area A-31 by etching to leave the part thereof at the thicker film part area A-32 as illustrated in FIG. 3C. That is, the remaining part of the resist film forms the etching mask 304 at the thicker film part area A-32.

Next, the first gate oxide film 302 of the thinner film part area A-31 is removed by a wet etching method using diluted or buffered hydrofluoric acid or a dry etching method. In this event, the first nitride layer 303 of the thinner film part area A-31 is partly removed together with the first gate oxide film 302. As a result, the first nitride layer 303 is divided into a second nitride layer 303A of the thinner film part area A-31 and a third nitride layer 303B of the thicker film part area A-32. Then, the etching mask 304 is completely removed to expose the first oxide film 302 of the thicker film part area A-32 as illustrated in FIG. 3D.

Subsequently, a second oxide film forming process which may be similar to or different from the first oxide film forming process is executed to the semiconductor substrate 301 of FIG. 3D. As a result, as shown in FIG. 3E, a second gate oxide film 305A is formed on the second nitride layer 303A of the thinner film part area A-31. At the same time, a third gate oxide film 305B (including the first oxide film 302) is formed at the thicker film part area A-32.

Here, the third nitride layer 303B (which is maldistributed at the vicinity of interface between the substrate 301 and the third gate oxide film 305B) migrates to the inner part of the third gate oxide film 305B accordingly as the third gate oxide film 305B increases the thickness thereof when the above mentioned oxide film forming methods are used for the second oxide film forming process except the ISSG and the plasma oxidation.

To the contrary, when the ISSG or the plasma oxidation is used for the second oxide film forming process, the third nitride layer 303B remains at the vicinity of interface between the substrate 301 and the third gate oxide film 305B as shown in FIG. 4 regardless of increase of the thickness of the third gate oxide film 305B (and/or 302). This is because the ISSG and the plasma oxidation are strong oxidizing methods and cause the oxidative reaction even in the nitride film. By each of the ISSG and the plasma oxidation, the oxidative reaction is advanced at a surface side of the nitride layer previous to at an interface side between the oxide film and the substrate. Thus, the ISSG and the plasma oxidation can execute additional oxidative reaction without losing shape of a nitride profile of a sample having a nitride layer at vicinity of interface between an oxide film and a substrate. In other words, the ISSG and the plasma oxidation can substantially keep the nitride profile formed by previous process(es). Therefore, the ISSG and the plasma oxidation are very useful for a manufacturing process of a semiconductor device whose electronic characteristics of the vicinity of the interface between the oxide film and the substrate are important.

Next, a second oxynitriding process, which may be similar to or different from the first oxynitriding process, is performed to the semiconductor substrate 301 with the second and the third gate oxide film 305A and 305B. Hereby, as shown in FIG. 3F, fourth and fifth nitride layers 306A and 306B are formed at the thinner and thicker film part areas A-31 and A-32, respectively. An amount of nitrogen elements and distribution profile in each nitride layer (306A, 306B) depends on the etching process for partially (selectively) etching the first gate oxide film 302, thickness of the gate oxide film (305A, 305B), treatment condition of the second oxynitriding process and so on.

FIG. 5 shows an example of changing oxide and nitride profiles in the NO treatment as the second oxynitriding process. As understood from FIG. 5, the amount of nitrogen in the nitride layer can be increased with hardly changing the position of the nitride layer. This means that it is possible to replenish new nitrogen elements by the second oxynitriding process when the nitrogen elements doped by the first oxynitriding process are missed by the second oxide film forming process.

According to this exemplary embodiment, the oxide films (305A and 305B) having different thickness can be formed in the thinner and the thicker film part area A-31 and A-32 respectively, while the nitride films (306A and 306B) having enough nitrogen elements can be formed in the thinner and the thicker film part areas A-31 and A-32, respectively.

For instance, nitrogen density of 3-5% can be introduced into the vicinity of the interface between the oxide film and the semiconductor substrate in both of the thinner and the thicker film part areas A-31 and A-32, if the first NO (nitric oxide) treatment using NO (2L) of 100% is executed for about 30 seconds at 1050° C. with the sheet-fed equipment after the oxide film of thickness 5.0 nm is formed as the first gate oxide film, and the second NO treatment using NO (2L) of 100% is executed for about 30 seconds at 1050° C. with the sheet-fed equipment after the oxide film of thickness 3.0 nm is formed as the second gate oxide film.

Generally, if the thickness of the oxide film is equal to or less than 5 nm, it is not a considerable problem that the reliability of the oxidation film is decreased by the introduction of nitrogen. Moreover, because the oxide film forming methods described above can form the oxide film with high reliability, it is hard that introduction of the nitrogen decreases the reliability of the oxide film formed by those methods.

According to the exemplary embodiment, the amount of nitrogen element introduced into the thinner film part area A-31 and the thicker film part area A-32 can be independently controlled. For instance, to introduce nitrogen into the thicker film part area A-32 chiefly, the amount of the introduction of nitrogen by the second oxynitriding process only has to be decreased. Oppositely, to introduce nitrogen into the thinner film part area A-31 chiefly, the amount of the introduction of nitrogen by the first oxynitriding process only has to be decreased. Additionally, the amount of the introduction of nitrogen is controlled by changing treatment time of the oxynitoriding process, gas pressure, and/or treatment temperature.

As mentioned above, because the amounts of the nitrogen elements in the nitride layers formed in the thinner and the thicker film part areas can be adjusted in the method of this exemplary embodiment, prevention of missing B (boron) and reduction of current leakage in the thinner film part area A-31 and improvement of characteristic regarding interface between the oxide film and the substrate in the thicker film part area A-32 can be both achieved.

Referring to FIGS. 6A to 6E, the description will be made about a second exemplary embodiment of this invention. In each of FIGS. 6A to 6E, first, second and third device areas A-61, A-62 and A-63 are arranged from the left side to the right side.

At first, like the first exemplary embodiment, the first oxide film forming process and the first oxynitriding process are executed to a semiconductor substrate 601. As a result, as shown in FIG. 6A, a first gate oxide film 602 is formed on the semiconductor substrate 601 while a first nitride layer 603 is formed in the first gate oxide film 602.

Next, a first resist mask 604 is formed by means of the known method on the third device area A-63. By the use of the first resist mask 604, the first gate oxide film 602 of the first and the second device areas A-61 and A-62 is etched as shown in FIG. 6B. At this time, the first nitride layer 603 is divided into a second nitride layer 603A at the first and the second device areas A-61 and A-62 and a third nitride layer 603B at the third device area A-63.

After the first resist mask 604 is removed from the third device area A-63, the second oxide film forming process and the second oxynitriding process are executed to form a second gate oxide film 605A and a fourth nitride layer 606A at the first and the second device areas A-61 and A-62 and a third gate oxide film 605B and a fifth nitride layer 606B at the third device area A-63 as shown in FIG. 6C.

Next, a second resist mask(s) 607 is formed at the first and the third device area A-63. The second oxide film 605A of the second device area A-62 is etched by the use of the resist mask 607. Then, the fourth nitride layer 606A of the second device area A-62 is changed into a sixth nitride film 606C as shown in FIG. 6D.

After the resist mask 707 is removed from the first and the third device areas A-61 and A-63, the third oxide film forming process and the third oxynitriding process are executed. Consequently, as shown in FIG. 6E, first, second and third gate oxide films 608A, 608B and 608C are formed in the first, the second and the third device areas A-61, A-62 and A-63 respectively. Furthermore, first, second and third final nitride layers 609A, 609B and 609C are formed in the first, the second and the third device areas A-61, A-62 and A-63 respectively.

As mentioned above, according to this exemplary embodiment, three gate oxide films different from one another in thickness can be formed. Furthermore, the final nitride layers different from one another in amount of doped nitrogen elements can be formed in the interfaces between the gate oxide films and the substrate. In other words, according to the exemplary embodiment, it is possible to make three elemental devices, such as transistors, having different (gate) oxide films in thickness and different amounts of nitrogen elements in the nitride layers at the first, the second and the third device areas of the common substrate.

Additionally, the methods used in the first embodiment can be used for the oxide film forming process, the oxynitriding process and the etching process of the second embodiment.

This invention is used for manufacturing four or more elements having different gate oxide films in thickness on a common substrate.

Though the explanation is made for manufacturing the three elemental devices different from one another in thickness of the oxide film on the substrate, this invention can be used for manufacturing four or more elemental devices different from one another in thickness of the gate oxide film on a substrate.

Referring to FIGS. 7A to 7F, the description will be made about a method of manufacturing a semiconductor device according to a third exemplary embodiment. FIGS. 7A to 7F are different from FIGS. 3A to 3F in arrangement of device area. In each of FIGS. 7A to 7F, a right hand side shows a thinner film part area A-71 while a left hand side shows a thicker film part area A-72.

At first, as shown in FIG. 7A, a semiconductor substrate 701 is provided and dealt with a first oxide film forming process to form a first gate oxide film 702.

Next, a first oxynitriding process is executed to the semiconductor substrate 701 with the first gate oxide film 702 to form a first nitride layer 703 in the vicinity of an interface between the semiconductor 701 and the first gate oxide film 702 as shown in FIG. 7B. The first nitride layer 703 is formed so that a lot of nitrogen elements are doped compared with the case of the first exemplary embodiment.

After an etching resist mask 704 is formed in the thinner film part area A-71 as illustrated in FIG. 7C, the first oxide film 702 of the thicker film part area A-72 is selectively removed as shown in FIG. 7D. In this event, the first nitride layer 703 is divided into second and third nitride layers 703A and 703B. Then, the resist mask 704 is completely removed from the thinner film part area A-71.

Next a second oxide film forming process is executed to form a second gate oxide film 705A as shown in FIG. 7E. In this event, the first oxide film 702 of the thinner film part area A-71 is changed into a third oxide film 705B. The third oxide film 705B is slightly thicker than the first oxide film 702 and thinner than the second gate oxide film 705A. This is because introduction of a large amount of the nitrogen elements reduces an oxidation rate of the semiconductor substrate 701.

After that, execution of a second oxynitiriding process forms fourth and fifth nitride layers 706A and 706B in the thicker and the thinner film part areas A-72 and A-71, respectively, as shown in FIG. 7F.

As mentioned above, according to the exemplary embodiment, the oxide films with different thickness can be formed at the thinner and the thicker part areas of the semiconductor substrate. Furthermore, the nitride layers with sufficient nitrogen elements can be formed by the exemplary embodiment. In addition, a single layer film formed by the second gate oxide film forming process and the subsequent oxynitriding process can be assigned to the thicker film part area which needs high reliability in its oxide film while a double layer film formed by two oxide film forming processes can be assigned to the thinner film part area which needs prevention of boron leakage and reduction of current leakage rather than the high reliability in its oxide film.

The method according to this exemplary embodiment can be used for manufacturing three or more elements with different gate oxide films in thickness on a common substrate.

Referring to FIG. 8, a semiconductor device has a silicon substrate 801. The silicon substrate 801 has thin and thick film regions, which are isolated by oxide films 802. The thin and the thick film regions are shown on the left and the right sides of FIG. 8, respectively. Gate insulating films 803-1 and 804-1 having different thicknesses are formed on the silicon substrate 801. Each of the gate insulating films 803-1 and 804-1 has a two layer structure including an oxide layer 805a (or 805b) including no nitrogen atoms and a nitride layer 806a (or 806b) disposed on the oxide layer 805a (or 805b). The gate insulating films 803-1 and 804-1 may be different from each other in number of nitrogen atoms per unit area. The nitride layer 806a (or 806b) may be smaller than the oxide layer 805a (or 805b) in thickness.

The semiconductor device according to the exemplary embodiment depicted in FIG. 8 may be manufactured by the process depicted in FIGS. 9A-9E. First, a silicon substrate 901 is prepared and a first oxidizing process is executed to form a gate oxide layer 902 on a silicon substrate 901 (FIG. 9A). Next, a first nitriding process, such as plasma nitriding, is executed to form a nitride layer 903 at a surface of the gate oxide layer 902 (FIG. 9B). Then, an etching process using a photo resist is executed to partially remove the nitride layer 903 and the gate oxide layer 902 from a thin film region 904 of the silicon substrate 901 (FIG. 9C).

Next, a second oxidizing process, such as ISSG or plasma oxidizing, is executed to form a gate oxide layer 905 on an exposed surface of the silicon substrate 901 and a surface oxide layer 906 at a surface of the nitride layer 903 (FIG. 9D). Finally, a second nitriding process, such as plasma nitriding, is executed to form a nitride layer 907 at a surface of the gate oxide layer 905. The second nitriding process simultaneously nitrides the surface oxide film 906 to form a unified nitride layer 908 integrated with the nitride layer 903 (FIG. 9E).

In FIGS. 9A-9E, the elements designated by the reference numerals of 901, 902, 905, 907 and 908 correspond to those designated by the reference numerals of 801, 805b, 805a, 806a and 806b of FIG. 8, respectively.

The gate insulating films 803-1 and 804-1 have the two-layer structure including the oxide layer and the nitride layer in common. Therefore, they have equal reliability and allow matching characteristics of transistors formed in the thin and the thick film regions.

The nitride layers 806a and 806b suppress the diffusion of dopants doped in gate electrodes into the silicon substrate 801. In addition, good controllability of dopant profile in the silicon semiconductor 801 is obtained as a relatively small heat treatment is executed to form the two-layer structure.

Referring to FIG. 10, a semiconductor device is similar to that of FIG. 8 except for upper oxide layers 1001a and 1001b on the nitride layers 806a and 806b. Hereinafter the oxide layers 805a and 805b are referred as lower oxide layers to be distinguished from the upper oxide layers 1001a and 1001b. The upper oxide layer 1001a forms a gate insulating film 803-2 together with the lower oxide layer 805a and the nitride layer 806a. Similarly, the upper oxide layer 1001b forms a gate insulating film 804-2 together with the lower oxide layer 805b and the nitride layer 806b. The gate insulating films 803-2 and 804-2 are different from each other in thickness.

Each of the oxide layers 805a, 805b, 1001a and 1001b may include nitrogen atoms whose density is lower than that of the nitride layer 806a or 806b adjacent thereto. The gate insulating films 803-2 and 804-2 may be different from each other in number of nitrogen atoms per unit area. The lower oxide layer 805a (or 805b) may be different from the upper oxide layer 1001a (or 1001b) in thickness. Furthermore, when the lower oxide layer 805a is thinner/thicker than the upper oxide layer 1001a, the lower oxide layer 805b may be thicker/thinner than the upper oxide layer 1001b.

The semiconductor device according to the exemplary embodiment depicted in FIG. 10 may be manufactured by the process depicted in FIGS. 1A-11B. The steps mentioned above with referring to FIGS. 9A-9E are executed to obtain a product depicted in FIG. 11A (or FIG. 9E). Next, a third oxidizing process, such as ISSG or plasma oxidizing, is executed to form oxide layers 1101 and 1102 at the surfaces of the nitride layers 907 and 908 (FIG. 11B).

In FIG. 11B, the elements designated by the reference numerals of 901, 902, 905, 907, 908, 1101 and 1102 correspond to those designated by the reference numerals of 801, 805b, 805a, 806a, 806b, 1001a and 101b of FIG. 10, respectively.

The gate insulating films 803-2 and 804-2 have the three layer structure including the lower and the upper oxide layers and the nitride layer between the upper and the lower oxide layers in common. Therefore, they have equal reliability and allow matching characteristics of transistors formed in the thin and the thick film regions.

The oxide layers 805a and 805b are in contact with the silicon substrate 801. The upper oxide layers 1001a and 1001b are formed in contact with the gate electrodes (not shown in FIG. 10). The nitride layers 806a and 806b are between the oxide layers. Therefore, the gate insulating films 803-2 and 804-2 show high reliability in comparison with a case that the silicon substrate 801 and/or the gate electrode are/is in contact with nitride layer(s) of a gate insulating film.

The nitride layers 806a and 806b suppress diffusion of dopants doped in the gate electrode into the silicon substrate 801. The oxide layers 805a and 1001a (or 805b and 1001b) disposed at both sides of the nitride layer 806a (or 806b) reduces internal stress caused by the nitride layer 806a (or 806b) and thereby a leakage current is suppressed. The nitride layer 806a (or 806b) causes large internal stress in the gate insulating film 803-2 and 804-2.

Referring to FIG. 12, gate insulating films 803-3 and 804-3 are different from each other in thicknesses. Each of the gate insulating films 803-3 and 804-3 has a two-layer structure including a nitride layer 1201a (or 1201b) and an oxide layer 1202a (or 1202b) disposed on the nitride layer 1201a (or 1201b). Each of the oxide layers 1202a and 1202b may include nitrogen atoms whose density is smaller than that of the adjacent nitride layer 1201a or 1201b. The gate insulating films 803-3 and 804-3 may be different from each other in number of nitrogen atoms per unit area. The nitride layer 1201a (or 1201b) may be smaller than the oxide layer 1202a (or 1202b) in thickness.

The semiconductor device according to the embodiment depicted in FIG. 12 may be manufactured by the process depicted in FIGS. 13A-13D. First, a silicon substrate 1301 is prepared and a first oxidizing process is executed to form a gate oxide layer 1302 on a silicon substrate 1301 (FIG. 13A). Next, an oxynitriding process, such as NO or N₂O treatment, is executed to form a nitride layer 1303 between the silicon substrate 1301 and the gate oxide layer 1302 (FIG. 13B).

Then, an etching process using a photo resist is executed to partially remove the oxide layer 1302 from a thin film region 1304 of the silicon substrate 1301 (FIG. 13C). Next, a second oxidizing process, such as ISSG or plasma oxidizing, is executed to form a gate oxide layer 1305 on an exposed surface of the nitride layer 1303 (FIG. 13D). The second oxidizing process keeps a position of the nitride layer 1303 on the interface of the silicon substrate 1301.

In FIG. 13D, the elements designated by the reference numerals of 1301, 1302, 1303 and 1305 correspond to those designated by the reference numerals of 801, 1202b, 1201b and 1202a of FIG. 12, respectively.

The gate insulating films 803-3 and 804-3 have the two-layer structure including the oxide layer and the nitride layer in common. Therefore, they have equal reliability and allow matching characteristics of transistors formed in the thin and the thick film regions.

The structure that the nitride layer include in the gate insulating film is in contact with the silicon substrate enlarges a range of feasible threshold voltage of transistors having the gate insulating film. Therefore, the nitride layer 1201a (or 1201b) allows manufacturing transistors having a high threshold voltage easily.

Referring to FIG. 14, gate insulating films 803-4 and 804-4 are different from each other in thicknesses and in structure. The gate insulating film 803-4 has a two-layer structure including a nitride layer 1401 and an oxide layer 1402 disposed on the nitride layer 1401. On the other hand, the gate insulating film 804-4 has a three-layer structure including a pair of oxide layers 1403 and 1404 and a nitride layer 1405 disposed between the oxide layers 1403 and 1404.

The oxide layer 1402 may include nitrogen atoms whose density is lower than that of the nitride layer 1401. At least one of the oxide layers 1403 and 1404 may include nitrogen atoms whose density is lower than that of the nitride layer 1405. The gate insulating films 803-4 and 804-4 may be different from each other in number of nitrogen atoms per unit area. In the gate insulating film 803-4, the oxide layer 1402 may be disposed on the nitride layer 1401 that is disposed on the substrate 801.

The semiconductor device according to the exemplary embodiment depicted in FIG. 14 may be manufactured by the process depicted in FIGS. 15A-15B. The steps mentioned above with referring to FIGS. 9A-9D are executed to obtain a partly finished product depicted in FIG. 15A (or FIG. 9D).

Next, an oxynitriding process, such as NO or N₂O treatment, is executed to form a nitride layer 1501 at interface between the silicon substrate 901 and the oxide layer 905 (FIG. 15B). In the thick layer region, the nitride layer 903 obstructs the nitrogen atoms which are added by the oxynitriding process and moves toward the silicon substrate 901. Consequently, the nitride layer 903 increases the thickness thereof.

In FIG. 15B, the elements designated by the reference numerals of 901, 902, 903, 905, 906 and 1501 correspond to those designated by the reference numerals of 801, 1403, 1405, 1402, 1404 and 1401 of FIG. 14, respectively.

The structure that the nitride layer include in the gate insulating film is in contact with the silicon substrate enlarges a range of feasible threshold voltage of a transistor having the gate insulating film. Therefore, the nitride layer 1501 allows manufacturing transistors having a high threshold voltage easily even if the gate insulating film is thin.

Referring to FIG. 16, gate insulating films 803-5 and 804-5 are different from each other in thicknesses and in structure. The gate insulating film 803-5 has a three-layer structure including a pair of nitride layers 1601 and 1602 and an oxide layer 1603 between the nitride layers 1601 and 1602. On the other hand, the gate insulating film 804-5 has a four layer structure including two oxide films 1604 and 1605 and two nitride films 1606 and 1607 which are alternately disposed. In other words, the gate insulating film 804-5 has the three-layer structure on the oxide film 1604.

The oxide layer 1603 may include nitrogen atoms whose density is lower than that of each of the nitride layers 1601 and 1602. Each of the oxide layers 1604 and 1605 may include nitrogen atoms whose density is lower than that of each of the nitride layers 1606 and 1607. The gate insulating films 803-5 and 804-5 may be different from each other in number of nitrogen atoms per unit area.

The semiconductor device according to the exemplary embodiment depicted in FIG. 16 may be manufactured by the process depicted in FIGS. 17A-17B. The steps mentioned above with referring to FIGS. 9A-9D and 15B are executed to obtain a partly finished product depicted in FIG. 17A (or FIG. 15B). Next, an additional nitriding process, such as plasma nitriding, is executed to form nitride layers 1701 and 1702 at surfaces of the oxide layers 905 and 906, respectively (FIG. 17B).

In FIG. 17B, the elements designated by the reference numerals of 901, 902, 903, 905, 906, 1501, 1701 and 1702 correspond to those designated by the reference numerals of 801, 1604, 1606, 1603, 1601, 1602 and 1607 of FIG. 16, respectively.

Because the gate insulating film 803-5 (or 804-5) includes two nitride layers 1601 and 1602 (or 1606 and 1607), each nitride layer can be thin in comparison with a case that a gate insulating film includes one nitride layer. Therefore, the gate insulating film 803-5 (or 804-5) can be reduced its internal stress caused by the nitride layers 1601 and 1602 (or 1606 and 1607) and thereby a leak current can be reduced.

The structure that the nitride layer included in the gate insulating film is in contact with the silicon substrate enlarges a range of feasible threshold voltage of transistors having the gate insulating film. Therefore, the nitride layer 1601 allows manufacturing a transistor having a high threshold voltage easily even if the gate insulating film is thin.

Referring to FIG. 18, gate insulating films 803-6 and 804-6 are different from each other in thicknesses and in structure. The gate insulating film 803-6 has a three-layer structure like the gate insulating film 803-2 of FIG. 10. The gate insulating film 803-6 includes oxide layers 1801 and 1802 and a nitride layer 1803 disposed between the oxide layers 1801 and 1802. The gate insulating film 804-6 has a four-layer structure including nitride layers 1804 and 1805 and oxide layers 1806 and 1807 which are alternately disposed. Each of the oxide layers 1801 and 1802 may include nitrogen atoms whose density is smaller than that of the nitride layer 1803. Each of the oxide layers 1806 and 1807 may also include nitrogen atoms whose density is smaller than that of each of the nitride layers 1804 an 1805. The gate insulating films 803-6 and 804-6 may be different from each other in number of nitrogen atoms per unit area.

The semiconductor device according to the exemplary embodiment depicted in FIG. 18 may be manufactured by the process depicted in FIGS. 19A-19F. First, a silicon substrate 1901 is prepared and a first oxidizing process is executed to form a gate oxide layer 1902 on a silicon substrate 1901 (FIG. 19A).

Next, an oxynitriding process, such as a NO or N₂O treatment, is executed to form a nitride layer 1903 between the silicon substrate 1901 and the gate oxide layer 1902 (FIG. 19B). Then, an etching process using a photo resist is executed to partially remove the oxide layer 1902 and the nitride layer 1903 from a thin film region 1904 of the silicon substrate 1901 (FIG. 19C).

Next, a second oxidizing process, such as ISSG or plasma oxidizing, is executed to form a gate oxide layer 1905 in the thin layer region 1904 (FIG. 19D). The second oxidizing process keeps a position of the nitride layer 1903 of a thick film region (at a right hand side of FIG. 19D) on the interface of the silicon substrate 1901. A nitriding process, such as a plasma nitriding, is then executed to form nitride films 1906 and 1907 on the oxide layers 1905 and 1902, respectively (FIG. 19E). Finally, a third oxidizing process, such as ISSG or plasma oxidizing, is executed to form oxide layers 1908 and 1909 on the nitride films 1906 and 1907, respectively (FIG. 19F).

In FIG. 19F, the elements designated by the reference numerals of 1901, 1902, 1903, 1905, 1906, 1907, 1908 and 1909 correspond to those designated by the reference numerals of 801, 1806, 1804, 1801, 1803, 1805, 1802, and 1807 of FIG. 18, respectively.

Because the gate insulating film 804-6 includes two nitride layers 1804 and 1805, each nitride layer can be thin in comparison with a single nitride layer structure case that a gate insulating film includes one nitride layer. Therefore, the gate insulating film 804-6 can reduce internal stress caused by the nitride layers 1804 and 1805 and thereby a leakage current can be reduced.

Referring to FIG. 20, gate insulating films 803-7 and 804-7 are different from each other in thicknesses. Each of the gate insulating films 803-7 and 804-7 has a four layer structure including oxide layers 2001a and 2002a (or 2001b and 2002b) and nitride layers 2003a and 2004a (or 2003b and 2004b) which are alternately disposed.

Each of the oxide layers 2001a and 2002a may include nitrogen atoms whose density is lower than that of each of the nitride layers 2003a and 2004a. Similarly, each of the oxide layers 2001b and 2002b may include nitrogen atoms whose density is lower than that of each of the nitride layers 2003b and 2004b. The gate insulating films 803-7 and 804-7 may be different from each other in number of nitrogen atoms per unit area.

The semiconductor device according to the exemplary embodiment depicted in FIG. 20 may be manufactured by the process depicted in FIGS. 21A-21B. The steps mentioned above with referring to FIGS. 9A-9E and 11B are executed to obtain a partly finished product depicted in FIG. 21A (or FIG. 11B). Next, a third nitriding process, such as plasma nitriding, is executed to form nitride layers 2101 and 2102 at surfaces of the oxide layers 1101 and 1102, respectively (FIG. 21B).

In FIG. 21B, the elements designated by the reference numerals of 901, 902, 905, 907, 908, 1101, 1102, 2101 and 2102 correspond to those designated by the reference numerals of 801, 2001b, 2001a, 2003a, 2003b, 2002a, 2002b, 2004a and 2004b of FIG. 20, respectively.

Because the gate insulating film 803-7 (or 804-7) includes two nitride layers 2003a and 2003b, each nitride layer can be thin in comparison with the case that a gate insulating film includes one nitride layer. Therefore, the gate insulating film 803-7 (or 804-7) can reduce internal stress caused by the nitride layers 2003a and 2004a (or 2003b and 2004b) and thereby a leak current can be reduced.

Referring to FIG. 22, gate insulating films 803-8 and 804-8 are different from each other in thicknesses and in structure. The gate insulating film 803-8 has a four-layer structure like the gate insulating film 803-7 of FIG. 20. The gate insulating film 803-8 includes oxide layers 2201 and 2202 and nitride layers 2203 and 2204 which are alternately disposed. The gate insulating film 804-8 has a five-layer structure including nitride layers 2205, 2206 and 2207 and oxide layers 2208 and 2209 which are alternately disposed. Each of the oxide layers 2201 and 2202 may include nitrogen atoms whose density is smaller than that of each of the nitride layers 2202 and 2204. Each of the oxide layers 2208 and 2209 may include nitrogen atoms whose density is smaller than that of each of the nitride layers 2205, 2206 and 2207. The gate insulating films 803-8 and 804-8 may be different from each other in number of nitrogen atoms per unit area. Furthermore, the gate insulating films 803-8 and 804-8 may be different from each other in number of nitrogen layers.

The semiconductor device according to the exemplary embodiment depicted in FIG. 22 may be manufactured by the process depicted in FIGS. 23A-23B. The steps mentioned above with referring to FIGS. 19A-19F are executed to obtain a partly finished product depicted in FIG. 23A (or FIG. 19F). Next, an additional nitriding process, such as a plasma nitriding, is executed to form nitride films 2301 and 2302 on the oxide layers 1908 and 1909, respectively (FIG. 23B).

In FIG. 23B, the elements designated by the reference numerals of 1901, 1902, 1903, 1905, 1906, 1907, 1908, 1909, 2301 and 2302 correspond to those designated by the reference numerals of 801, 2208, 2205, 2201, 2203, 2206, 2202, 2209, 2204 and 2207 of FIG. 22, respectively.

Because the gate insulating film 804-8 includes three nitride layers 2205, 2206 and 2207, each nitride layer can be thin in comparison with the case that the gate insulating film includes two nitride layers. Therefore, the gate insulating film 804-8 can further reduce internal stress caused by the nitride layers 2205, 2207 and 2208 and thereby a leak current can be reduced.

In addition, an increase of the number of nitride layers in the gate insulating film improves the ability of the suppressing diffusion of the dopants from the gate electrode into the silicon substrate 801.

Though each of the exemplary embodiments mentioned above includes two regions having different thicknesses, this invention may be applied to a device which includes three or more regions having different thickness.

The gate insulating films may be used for MOSFETs.

While this invention has thus far been described in conjunction with the exemplary embodiments thereof, it will readily be possible for those skilled in the art to put this invention into practice in various other manners.

While the invention has been described in terms of several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

Further, it is noted that, Applicants' intent is to encompass equivalents of all claim elements, even if amended later during prosecution.

Claims

1. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein each of said gate insulating films includes a two-layer structure comprising an oxide layer and a nitride layer disposed on said oxide layer.

2. A semiconductor device as claimed in claim 1, wherein said gate insulating films are different from each other in number of nitrogen atoms per unit area.

3. A semiconductor device as claimed in claim 1, wherein said nitride layer is thinner than said oxide layer in one of said gate insulating films.

4. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein each of said gate insulating films includes a three layer structure including lower and upper oxide layers and a nitride layer disposed between said lower and said upper oxide layers.

5. A semiconductor device as claimed in claim 4, wherein one of said lower and said upper oxide layers includes nitrogen atoms whose density is lower than that of said nitride layer.

6. A semiconductor device as claimed in claim 4, wherein said gate insulating films are different from each other in number of nitrogen atoms per unit area.

7. A semiconductor device as claimed in claim 4, wherein said lower and said upper oxide layers have different thicknesses in one of said gate insulating films.

8. A semiconductor device as claimed in claim 4, wherein said lower oxide layer is thinner than said upper oxide layer in one of said gate insulating films, and said lower oxide layer is thicker than said upper oxide layer in the other of said gate insulating films.

9. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein each of said gate insulating films includes a two-layer structure comprising a nitride layer and an oxide layer disposed on said nitride layer.

10. A semiconductor device as claimed in claim 9, wherein said oxide layer includes nitrogen atoms whose density is lower than that of said nitride layer.

11. A semiconductor device as claimed in claim 9, wherein one of said gate insulating films is different from the other in number of nitrogen atoms per unit area.

12. A semiconductor device as claimed in claim 9, wherein said nitride layer is thinner than said oxide layer in one of said gate insulating films.

13. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein one of said gate insulating films includes a two layer structure comprising a first nitride layer and a first oxide layer, and

wherein the other of said gate insulating films includes a three layer structure comprising second and third oxide layers and a second nitride layer disposed between said second and said third oxide layers.

14. A semiconductor device as claimed in claim 13, wherein each of said first, said second and said third oxide layers includes nitrogen atoms whose density is lower than that of an adjacent one of said first and said second nitride layers.

15. A semiconductor device as claimed in claim 13, wherein one of said gate insulating films is different from the other in number of nitrogen atoms per unit area.

16. A semiconductor device as claimed in claim 13, wherein said first oxide layer is disposed on said first nitride layer.

17. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein one of said gate insulating films includes a three layer structure comprising first and second nitride layers and an oxide layer disposed between said first and said second nitride layers.

18. A semiconductor device as claimed in claim 17, wherein said oxide layer includes nitrogen atoms whose density is lower than that of each of said first and said second nitride layers.

19. A semiconductor device as claimed in claim 17, wherein the other of said gate insulating films includes at least one nitride layer, and

wherein said gate insulating films are different from each other in number of nitrogen atoms per unit area.

20. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein one of said gate insulating films includes a four layer structure comprises a lower nitride layer, a lower oxide layer disposed on said lower nitride layer, an upper nitride layer disposed on said lower oxide layer and an upper oxide layer disposed on said upper nitride layer.

21. A semiconductor device as claimed in claim 20, wherein each of said lower and said upper oxide layers includes nitrogen atoms whose density is lower than that of each of said lower and said upper nitride layers.

22. A semiconductor device as claimed in claim 20, wherein the other of said gate insulating films includes a nitride layer, and

wherein said gate insulating films are different from each other in number of nitrogen atoms per unit area.

23. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein one of said gate insulating films includes a four layer structure comprising a lower oxide layer, a lower nitride layer disposed on said lower oxide layer, an upper oxide layer disposed on said lower nitride layer and an upper nitride layer disposed on said upper oxide layer.

24. A semiconductor device as claimed in claim 23, wherein each of said lower and said upper oxide layers includes nitrogen atoms whose density is lower than that of each of said lower and said upper nitride layers.

25. A semiconductor device as claimed in claim 23, wherein the other of said gate insulating films includes a nitride layer, and

wherein said gate insulating films are different from each other in number of nitrogen atoms per unit area.

26. A semiconductor device, comprising:

a semiconductor substrate; and

two gate insulating films having different thicknesses on said semiconductor substrate,

wherein one of said gate insulating films includes a five-layer structure comprising three nitride layers and two oxide layers which are alternately disposed.

27. A semiconductor device as claimed in claim 26, wherein each of said oxide layers includes nitrogen atoms whose density is lower than that of each of said nitride layers.

28. A semiconductor device as claimed in claim 26, wherein the other of said gate insulating films includes a nitride layer, and

wherein said gate insulating films are different from each other in number of nitrogen atoms per unit area.

29. A semiconductor device as claimed in claim 26, wherein the other of said gate insulating films includes a nitride layer, and

wherein said gate insulating films are different from each other in number of nitrogen layers.

30. A semiconductor device having two gate insulating film different from each other in thickness, produced by a process comprising:

executing a first oxide process for forming a first oxide layer;

executing a nitriding process for forming a nitride layer;

executing an etching process for partially etching the first oxide layer; and

executing a second oxide process for forming a second oxide layer.