Method of manufacturing a semiconductor device

A method of manufacturing a semiconductor device includes forming first and second active regions and a field region in a surface of a substrate; forming a first gate insulating film in the first and second active regions; covering the surface of the substrate with a first polycrystalline silicon film; exposing the first gate insulating film on the second active region by forming an aperture in the first polycrystalline silicon film over the second active region; removing the first gate insulating film in the second active region; forming a second gate insulating film which is thicker than the first gate insulating film in the second active region; covering the surface of the substrate with a second polycrystalline silicon film; removing the second polycrystalline silicon film on the first active region until it becomes a predetermined film thickness; and forming gate electrodes on the first and second active regions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a semiconductor device. More specifically, the present invention relates to a method of manufacturing a semiconductor device where MOS type semiconductor elements having gate insulating films with different film thicknesses are formed on the same semiconductor substrate.

2. Background Information

With respect to the field of semiconductor devices, forming semiconductor elements driven by different power supply voltages on the same substrate has become common. In cases of forming MOS type semiconductor elements on the same substrate, since a withstand voltage demanded according to the power supply voltage may vary, a plurality of specifications are needed with respect to film thicknesses of gate insulating films formed on the same semiconductor substrate.

In general, when forming gate insulating films with different film thicknesses, a technique of performing an oxidization process in two or more steps is used. In most cases, the oxidization process is divided into two steps. This technique of oxidization is called double oxidization. By using the technique of double oxidization, a thin gate insulating film for internal circuit elements which require high-speed operation, and a thick gate insulating film for peripheral circuit elements which require a high withstand voltage can be formed on the same semiconductor substrate.

When forming gate insulating films by using the technique of double oxidization, for example, first a gate insulating film having a thickness between a thickness of a thick gate insulating film and a thickness of a thin gate insulating film is uniformly formed on the entire substrate by thermal oxidization. Then, this gate insulating film is removed only from the region where the thin gate insulating film is to be formed. After that, the entire surface of the substrate is oxidized again according to the specification of the thin gate insulating film. At this point, formations of both the thin gate insulating film and the thick gate insulating film are completed. In this re-oxidization process, the regions where the thin gate insulating film and the thick gate insulating film are to be formed are oxidized while the gate insulating film formed in the first oxidization process still remains in the region where the thick gate insulating film is to be formed. Therefore, the overall thickness of the gate insulating film formed in the region where the thick gate insulating film is to be formed should become thicker by the thickness of the gate insulating film formed in the re-oxidization process.

For example, manufacturing methods of semiconductor devices using the double oxidization technique are shown in Japanese Laid Open Patent Publication No. 2-271659, especially, pp. 3-4, and FIG. 1, and Japanese Patent Laid Open Patent Publication No. 4-297063, especially, p. 3, FIGS. 1 and 3. Japanese Laid-Open Patent Publication Nos. 2-271659 and 4-297063 are hereby incorporated by reference.

According to the double oxidization technique used in the manufacturing method of a semiconductor device as disclosed in Publication No. 2-271659 silicon nitride films used as oxidation resist films at the time of forming a field oxide film is further used as oxidation resist films for regions where thin gate insulating films are to be formed when forming a thick gate insulating film by using a LOCOS (Local Oxidation Of Silicon) method.

The manufacturing method of a semiconductor device shown in Japanese Laid-Open Patent Publication No. 4-297063 is similar to that of Publication No. 2-271659. Publication No. 4-297063 provides a modified example of the method in which the silicon nitride film used as the oxidation resist film at the time of forming the field oxide film is further used as the thick gate insulating film.

As mentioned above, in the generally used technique of double oxidizing, first a gate insulating film having a thickness between a thickness of a thick gate insulating film and a thickness of a thin gate insulating film is formed uniformly. For instance, supposing that the thin gate insulating film is 3 nm (nanometer) thick and the thick gate insulating film is 7 nm thick, a gate insulating film of about 5 to 6 nm in thickness would be formed in the first oxidization process. After the gate insulating film is formed, this gate insulating film is etched only at the region where the thin gate insulating film is to be formed. At this time, however, the field oxide film is also etched, and is made thinner. Such thinning of the field oxide film reduces the threshold voltage at which a parasitic MOS transistor turns on, and invites adverse effects with regards to the operation and reliability of a circuit.

According to the methods of manufacturing semiconductor devices given in the aforementioned Japanese Laid-Open Patent publications, in order to prevent the field oxide film from thinning due to the etching of the gate insulating film, a process of removing the gate insulating film is not included. However, the methods include a process of removing an underlay oxide film which is placed directly under the silicon nitride film for the purpose of stress relief. Since the underlay oxide film is about 20 nm thick, the field oxide film would become about 20 to 30 nm thinner by this etching process. If a bulk substrate having a semiconductor layer with sufficient thickness is used, thinning of the field oxide film by the thickness of about 20 to 30 nm does not pose any particular problem, because the field oxide film can be thickly formed to become several hundred nm in thickness. However, if an SOI (Silicon On Insulator) substrate or an SOS (Silicon On Sapphire) substrate is to be used, thinning of the field oxide film by the thickness of about 20 to 30 nm will pose a serious problem, because both the SOI substrate and the SOS substrate, where a semiconductor layer of either substrate is about 40 to 50 nm thick, have a field oxide film which is only 80 to 100 nm thick.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to resolve the above-described problems, and to provide an improved method of manufacturing a semiconductor device. More specifically, it is an object of the present invention to provide a method of manufacturing a semiconductor device where MOS type semiconductor elements having gate insulting films with different film thicknesses are formed on the same semiconductor substrate.

In accordance with a first aspect of the present invention, a method of manufacturing a semiconductor device is provided. The method includes: preparing a support substrate; forming first and second active regions and a field region on or in a surface of the support substrate; forming a first gate insulating film in the first and second active regions; covering the entire surface of the support substrate with a first polycrystalline silicon film; exposing the first gate insulating film on the second active region by forming an aperture in the first polycrystalline silicon film over the second active region; removing the first gate insulating film in the second active region; forming a second gate insulating film that is thicker than the first gate insulating film in the second active region; covering the entire surface of the support substrate with a second polycrystalline silicon film; removing the second polycrystalline silicon film on the first active region until it reaches a predetermined film thickness; and forming gate electrodes on the first and second active regions, respectively.

In accordance with another aspect of the present invention, a method of manufacturing a semiconductor device is provided. The method includes: preparing a support substrate; covering a surface of the support substrate with a first insulating film; covering a surface of the first insulating film with a second insulating film; removing the second insulating film from a region other than first and second predetermined regions; forming a field region at the region other than the first and second predetermined regions by subjecting the support substrate to thermal oxidation; covering the entire surface of the support substrate with a third insulating film; covering a surface of the third insulating film with a fourth insulating film; removing the third and fourth insulating films on the first and second predetermined regions until the second insulating film is exposed; removing the first and second insulating films on the first and second predetermined regions; forming a first gate insulating film in the first predetermined region; forming a second gate insulating film that is thicker than the first gate insulating film in the second active region; and forming gate electrodes on the first and second predetermined regions, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, preferred embodiments of the present invention will be described in detail.

(1) First Embodiment

In a first preferred embodiment of the present invention, a case in which a thin gate oxide film is removed before a thick gate oxide film is formed will be explained as an example. Moreover, in this case, a polycrystalline silicon film is used as an oxidation resist film at the time of forming the thick gate oxide film.

FIGS. 1ato1gare cross-sectional views provided to explain a method of manufacturing a semiconductor device according to the first embodiment of the present invention.

As shown inFIG. 1a, in the method of manufacturing a semiconductor device according to the first embodiment of present invention, first a silicon oxide film and a silicon nitride film (not shown), both used as a buffer of which are formed one by one on a silicon support substrate1. Next, using a known LOCOS method or the like, a field oxide film2is formed for element separation. Herewith, a field region, which is the region where the field oxide film2is formed, and active regions3and4are formed. In later processes, an MOS type semiconductor element with a thin gate insulating film is formed in the active region3and an MOS type semiconductor element with a thick gate insulating film is formed in the active region4. In addition, the silicon support substrate1is not limited to a bulk substrate. For example, it is also possible to use an SOI substrate100as shown inFIG. 2a, and an SOS substrate101as shown inFIG. 2b. InFIG. 2a,100arepresents a silicon substrate,100brepresents a buried oxide film, and100crepresents a semiconductor layer. InFIG. 2b,101arepresents a sapphire substrate and101brepresents a semiconductor layer.

Next, as shown inFIG. 1b, thin gate oxide films5are formed on the active region3and the active region4by a known thermal oxidization technique. The thin gate oxidization film5is preferably about 3 nm thick. Then, a polycrystalline silicon film6(first polycrystalline film) is formed by depositing polycrystalline silicon on the entire surface using a CVD (Chemical Vapor Deposition) method or the like.

Next, a photoresist (not shown) is applied on the polycrystalline silicon film6. Further, through processes of exposure and development, a resist pattern having an aperture in the upper part of the active region4is formed on the polycrystalline silicon film6. Next, the polycrystalline film6exposed at the aperture is removed by etching while using the resist pattern as a mask. Then, the thin gate oxide film5in the active region4is removed using hydrofluorinated (HF) etchant. By this process, the surface of the silicon substrate1in the active region4is exposed as shown inFIG. 1c. In the etching process of the thin gate oxide film5, the surface of the field oxide film2not covered with the polycrystalline silicon film6is etched away for about 4.5 to 6 nm (1.5 times to twice the thickness of the thin gate oxide film5which is 3 nm thick), and thus the field oxide film2is made thinner. However, in the usual bulk substrate, since the field oxide film2is about 500 to 1000 nm thick, thinning of 4.5 to 6 nm is so small that the effects thereof can be mostly disregarded. Likewise, in the SOI substrate or the SOS substrate, since the field oxide films2as shown inFIGS. 2aand2bare about 80 to 100 nm thick, thinning of 4.5 to 6 nm is so small that it can be mostly disregarded.

Next, as shown inFIG. 1d, a thick gate oxide film7is formed in the active region4by the known thermal oxidization technique. The thick gate oxidization film7is preferably about 7 nm thick. During the thermal oxidization process to form the thick gate oxide film7, the polycrystalline silicon film6works as an oxidation resist mask in the active region3where the thin gate oxide film5is already being formed. Accordingly, the thin gate oxide film5will not become any thicker by the re-oxidization.

Next, a polycrystalline silicon film8(second polycrystalline film) is formed by depositing polycrystalline silicon on the entire surface using the CVD method or the like. By this process, a film thickness of the polycrystalline silicon film8on the active region3is formed to be the film thickness of the polycrystalline silicon film8on the active region4, as shown inFIG. 1e.

Next, as shown inFIG. 1f, using a known lithography technique and etching technique, the polycrystalline silicon film8on the active region3is removed until it reaches to a predetermined film thickness.

Next, as shown inFIG. 1g, using the known lithography and etching techniques, a gate electrode9is formed on the active region3, and a gate electrode10is formed on the active region4. After this, MOS type semiconductor elements are formed (not show) using a known method.

Although this embodiment shows a method of forming two kinds of gate oxide films, the present invention can also be applied in manufacturing a semiconductor device having gate oxide films with three or more different film thicknesses.

Operational Effect

According to the method of manufacturing a semiconductor device with respect to the first embodiment of the present invention, in forming gate oxide films with different film thicknesses, the thin gate oxide film5is formed uniformly before the thick gate oxide film7is formed. After that, the thin gate oxide film5in the active region4where the thick gate oxide film7is to be formed is removed, and then the thick gate oxide film7is formed in the active region4. Therefore, the amount of outage of the field oxide film2at the time of removing the gate oxide film can be reduced. Specifically, when the film thickness of the thin gate oxide film5is set to 3 nm, the amount of outage is approximately 1% with respect to the bulk substrate where the field oxide film is about 500 to 1000 nm thick. Also in the SOI substrate or the SOS substrate where the field oxide film is about 80 to 100 nm thick, the amount of outage is below 10%.

Furthermore, since the polycrystalline silicon being a material of the gate electrodes is used as an oxidation resist mask when forming the thick oxide film7, it is possible to form the gate oxide films with different thicknesses and the gate electrodes at the same time.

As used herein, the following directional terms “forward, rearward, above, downward, vertical, horizontal, below, and transverse” as well as any other similar directional terms refer to those directions of a device equipped with the present invention. Accordingly, these terms, as utilized to describe the present invention should be interpreted relative to a device equipped with the present invention.

ALTERNATE EMBODIMENTS

(2) Second Embodiment

In a second preferred embodiment of the present invention, a field oxide film is formed using the LOCOS method, and then the film thickness of the field oxide film is augmented using the CVD method. Moreover, the surface is planarized using an SOG (Spin on Glass) method to eliminate level differences on the surface.

FIGS. 3ato3gare cross-sectional views provided to explain a method of manufacturing a semiconductor device according to the second embodiment of the present invention.

As shown inFIG. 3a, in the method of manufacturing a semiconductor device according to the second embodiment of the present invention, at first, a silicon oxide film11is formed on the silicon support substrate1by thermal oxidation. Then a silicon nitride film12is formed on the silicon oxide film11by depositing silicon nitride using the CVD method. The silicon oxide film11performs as an underlay oxide film, and the silicon nitride film12performs as an oxidation resist mask at the time of forming a field oxide film, which is described later. In addition, the silicon support substrate1is not limited to a bulk substrate. For example, it is also possible to use an SOI substrate100as shown inFIG. 4a, and an SOS substrate101as shown inFIG. 4b. InFIG. 4a,100arepresents a silicon substrate,100brepresents a buried oxide film, and100crepresents a semiconductor layer. InFIG. 4b,101arepresents a sapphire substrate and101brepresents a semiconductor layer.

Next, a photoresist (not show) is applied on the silicon nitride film12, and through processes of exposure and development, a resist pattern having an aperture in the upper part of a field region is formed on the silicon nitride film12. Next, the silicon nitride film12is removed by etching while using the resist pattern as a mask. By this process, the surface of the silicon oxide film11in the field region is exposed as shown inFIG. 3b. In addition, although the silicon oxide film11in the field region preferably remains in this embodiment, it may be removed as well.

Next, the photoresist is removed, and then the silicon support substrate1is thermally oxidized through the silicon oxide film11. By this process, a field oxide film2is formed in the field region as shown inFIG. 3c. At the same time, an active region3and an active region4are formed in the silicon support substrate1as shown inFIG. 3c.

Next, as shown inFIG. 3d, a silicon oxide film13is formed by depositing silicon oxide on the entire surface using the CVD method, and a silicon oxide film14is formed on the silicon oxide film13using the SOG method. The purpose of forming the silicon oxide film14is to eliminate level differences on the surface of the silicon oxide film13.

Next, as shown inFIG. 3e, the silicon oxide film13and the silicon oxide film14are etched back until the surface of the silicon nitride film12in the active region3and the active region4is exposed. In this etching back process, the silicon oxide film14must be completely removed. The reason is that an SOG film is a liquid solution melting and combining with silica (SiO2) into a solvent such as alcohol or the like, and it contains comparatively much moisture. For this reason, if the etching back of the SOG film is not completely finished, the remainder of the SOG film may become a cause of erosion or the like.

Next, as shown inFIG. 3f, the silicon nitride film12and the silicon oxide film11on the active region3and the active region4are selectively removed one by one by etching. By this process, a thick field oxide film15, which is constituted from the field oxide film2formed by the thermal oxidation and the silicon oxide film13formed by the CVD method, is formed.

Next, as shown inFIG. 3g, a thin gate oxide film5is formed in the active region3and a thick gate oxide film7is formed in the active region4, by using a conventional double oxidization technique. In other words, first a gate oxide film having a thickness between a thickness of the thin gate oxide film5and a thickness of the thick gate oxide film7is formed uniformly, and through a process of partially removing the gate oxide film, the two kinds of gate oxide films are formed. In addition, the gate oxide film is formed by thermal oxidization, plasma oxidization or radical oxidization. Then, using the known lithography technique and etching technique, a gate electrode9is formed on the active region3, and a gate electrode10is formed on the active region4

After this, MOS type semiconductor elements are formed (not show) using a known method.

Although this embodiment shows a method of forming two kinds of gate oxide films, the present invention can also be applied in manufacturing a semiconductor device having gate oxide films with three or more different film thicknesses.

Operational Effect

According to the method of manufacturing a semiconductor device with respect to the second embodiment of the present invention, by augmenting the film thickness of the field oxide film2, which is formed using the LOCOS method, by using the CVD method in advance, the level difference between the element separation region and the two active regions, where one is to form the thin gate oxide film and the other is to form the thick gate oxide film, can be set to be large. Therefore, even if the conventional double oxidization technique, which is already a completed technique is applied, the amount of outage in this case does not pose a serious problem. Thus, this method is especially effective with respect to the SOI substrate, and the SOS substrate which cannot form a thick field oxide film.

In a third embodiment, a field oxide film is formed using the LOCOS method, and then the film thickness of the field oxide film is augmented using the CVD method as with the second embodiment of the present invention. In this embodiment, the surface is planarized by a BPSG (Boron Phosphorous Silicon Glass) and level differences of the surface are eliminated.

A method of manufacturing a semiconductor device according to the third embodiment of the present invention can be explained by referring to the cross-sectional viewsFIGS. 3ato3gas referenced in the second embodiment.

However, in the third embodiment of the present invention, the silicon oxide film14in the second embodiment shown inFIG. 3dis replaced with a BPSG film16. In other words, in this embodiment, the silicon oxide film13is formed on the entire surface using the CVD method, and then the BPSG film16is deposited on the silicon oxide film13using the CVD method. Then, the BPSG film16is subjected to reflow and the level differences of the surface are eliminated.

The SOG film as used in the second embodiment has to be removed completely by etching back or the like. As opposed to this, by using the BPSG film, which does not contain moisture, restrictions on the process can be reduced. Moreover, since the BPSG film has a reflow characteristic of liquidizing at comparatively low temperature, it is also useful as a material of the film for planarization.

In addition, in this embodiment, the silicon support substrate1is not limited to a bulk substrate. For example, it is also possible to use the SOI substrate100as shown inFIG. 4a, and the SOS substrate101as shown inFIG. 4b.

Operational Effect

According to the method of manufacturing a semiconductor device with respect to the third embodiment of the present invention, by augmenting the film thickness of the field oxide film2, which is formed using the LOCOS method, by using the CVD method in advance, the level difference between the element separation region and the two active regions, where one is to form the thin gate oxide film and the other is to form the thick gate oxide film, can be set to be large. Therefore, even if the conventional double oxidization which is already a completed technique is applied, the amount of outage in this case does not pose a serious problem. Thus, this method is especially effective with respect to the SOI substrate and the SOS substrate that cannot form a thick field oxide film. Furthermore, by using the BPSG film as the material of the film for planarization, the problem, such as erosion resulting from the remainder of the SPG film not being etched back, as is noted in the case of using the SOG film, does not arise, and thereby the restrictions on the process can be reduced.

As mentioned above, in accordance with the present invention, in a case of forming gate insulating films with different thicknesses, first a relatively thin first gate insulating film is formed. Then, the first gate insulating film is removed only from a region where a relatively thick gate insulating film is to be formed. After that, re-oxidization is performed to form a thick second gate insulating film. Since the first gate insulating film to be removed is thin, thinning (outage) of the field oxide film occurring at the time of removing the gate insulating film, which has posed problems in the conventional technique of double oxidization, can be reduced. Furthermore, in the process of re-oxidization to form the thick second gate insulating film, by having a polycrystalline silicon film used as an oxidation resist film for the thin first gate insulating film, it is possible to form the gate electrodes at the same time.

Furthermore, in accordance with another aspect of the present invention, a field oxide film is made thicker by previously having insulating films being laminated. Therefore, even if thinning (outage) of the field oxide film arises at the time of removing the gate insulating film, the influence can be reduced. This invention is effective especially in manufacturing semiconductor devices which use substrates such as the SOI substrate and SOS substrate that cannot form thick field oxide films.

This application claims priority to Japanese Patent Application No. 2004-171265. The entire disclosure of Japanese Patent Application No. 2004-171265 is hereby incorporated herein by reference.

The terms of degree such as “substantially,” “about,” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least±5% of the modified term if this deviation would not negate the meaning of the word it modifies.