JP2012234984A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce gate leakage current while reducing current collapse and increasing drain current.SOLUTION: A semiconductor device comprises: a substrate 1; semiconductor layers 2 and 3 each formed on the substrate 1 and including a nitride of a Group 3 element; a source electrode 5, a gate electrode 7, and a drain electrode 6 formed on the semiconductor layers 2 and 3; a first protective film 8 formed on the semiconductor layers 2 and 3 in contact with a lower part of the gate electrode 7 and the semiconductor layers 2 and 3, formed apart from the source electrode 5 and the drain electrode 6, and not including silicon; and a second protective film 9 formed on the semiconductor layers 2 and 3 in contact with the semiconductor layers 2 and 3, formed apart from the lower part of the gate electrode 7, having a different composition from the first protective film 8, and including nitrogen.

Description

  The present invention relates to a semiconductor device, and more particularly to a semiconductor device including a nitride semiconductor.

  Group III nitride semiconductors such as gallium nitride (GaN) have a larger forbidden band width and electron saturation rate than silicon (Si) and gallium arsenide (GaAs), and a greater breakdown electric field. For this reason, in recent years, nitride semiconductors are expected to be applied to high-frequency and high-power field effect transistors (FETs).

  A conventional semiconductor device using a nitride semiconductor will be described with reference to FIG. As shown in FIG. 12, a GaN layer 102 and an aluminum gallium nitride (AlGaN) barrier layer 103 are sequentially formed on a substrate 101. A large amount of charge is generated at the heterointerface between the GaN layer 102 and the AlGaN barrier layer 103 due to piezoelectric polarization and spontaneous polarization. Thus, a two-dimensional electron gas (2DEG) is formed in the vicinity of the hetero interface between the GaN layer 102 and the AlGaN barrier layer 103. In the nitride semiconductor FET, this is used as a channel, and here it is referred to as a channel layer 104. On the AlGaN barrier layer 103, a source electrode 105, a drain electrode 106, and a gate electrode 107 are formed. As a material for the gate electrode 107, gold (for example, see Non-Patent Document 1 and the like), nickel (for example, see Non-Patent Document 2 and the like), platinum (for example, Non-Patent Document 2 and the like), which have a large work function with respect to a nitride semiconductor. Non-Patent Document 3 and the like) and palladium (for example, see Non-Patent Document 4 and the like) are used.

In such a conventional semiconductor device, there is a problem that current collapse occurs, which is a phenomenon in which the drain current is greatly reduced when a high voltage is applied. The current collapse is generated when hot electrons accelerated by applying a high voltage are trapped in the level of the nitride semiconductor (see, for example, Non-Patent Document 5). When hot electrons are trapped in the level of the nitride semiconductor, the potential generated by the trapped electrons acts as a virtual gate. This constricts the channel and reduces the maximum drain current. Since the output of the FET and the maximum drain current are proportional, it is desirable to suppress current collapse. Therefore, the surface level of the nitride semiconductor is reduced by passivation of the surface of the nitride semiconductor with a silicon nitride (SiN) film that serves as a surface protection film, and current collapse caused by the surface of the nitride semiconductor is suppressed. it can. As a silicon-based insulating film used as such a surface protective film, a silicon oxide (SiO ₂ ) film is often used in addition to the SiN film. However, when the SiO ₂ film is used, the interface is larger than when the SiN film is used. The level density increases by an order of magnitude or more (see, for example, Non-Patent Document 6). For this reason, a SiN film is generally used as the surface protective film. In addition, as a surface protective film for reducing current collapse, an oxide film such as an aluminum oxide film (see, for example, Non-Patent Document 6) and an aluminum nitride film (see, for example, Non-Patent Document 7). ) Etc. are used.

P. Hacke et al., Appl. Phys. Lett. 63, 2676 (1993) L. Yu et al., Appl. Plys. Lett. 73, 238 (1998) K. Suzue et al., J. Appl. Phys. 80, 4467 (1996) L. Wang. Et al., Appl. Phys. Lett. 68, 1267 (1996) J.A.Mitterender et al., Appl. Phys. Lett. 83, 1650 (2003) H. Hasegawa et al., J. Vac. Sci. Tech. B21, 1844 (2003) J. Hwang et al., Solid-State Electronics 48, 363 (2004)

However, gold (Au), nickel (Ni), platinum (Pt), palladium (Pd) or the like having a work function larger than that of a nitride semiconductor is used as a material for the gate electrode, and a SiN film and a SiO ₂ film are used as the surface protective film. When a silicon-based insulating film such as the above is used, there arises a problem that the gate leakage current increases. If the gate leakage current increases, the reliability of the device is impaired, so it is desirable to reduce the gate leakage current. The increase in gate leakage current occurs when the resistance of the silicon-based insulating film in contact with the surface of the semiconductor is reduced or the resistance of a part of the gate electrode in contact with the surface of the semiconductor is reduced. These resistance reductions occur when silicon contained in the silicon-based insulating film reacts with the gate electrode, and the gate electrode or the silicon-based insulating film is silicided. In general, such silicidation occurs when thermal annealing is performed, and is more likely to occur as the annealing temperature increases. When a SiN film is formed on a nitride semiconductor, an insulating film can be formed at a relatively low temperature of about 260 ° C. by using a plasma chemical vapor deposition (P-CVD) method. However, even at this temperature, silicidation of the gate electrode occurs and the gate leakage current increases.

On the other hand, in an FET made of a nitride semiconductor, when an Al-based insulating film such as an aluminum nitride (AlN) film or an aluminum oxide (Al ₂ O ₃ ) film is used as a surface protective film for reducing current collapse. Since the insulating film does not contain Si, silicidation of the gate electrode does not physically occur. When an Al-based insulating film is used, there is no particular alloying between the gate electrode and Al.

However, when passivation is performed using an Al-based insulating film, the gate leakage current does not increase due to silicidation and current collapse does not occur, but the sheet resistance of the semiconductor is higher than when the passivation is performed using a SiN film. The problem of increasing arises. When the SiN film is used as the surface protective film, nitrogen is supplied to the surface of the nitride semiconductor by nitrogen plasma, so that nitrogen vacancies in the nitride semiconductor are reduced. As a result, the sheet resistance is reduced because the interface state density of the nitride semiconductor is reduced. On the other hand, when an Al ₂ O ₃ film is used as a surface protective film, nitrogen is not supplied to the surface of the nitride semiconductor, so that nitrogen vacancies in the nitride semiconductor do not decrease. As a result, it is considered that the sheet resistance increases because the interface state density of the nitride semiconductor does not decrease as compared with the case where passivation is performed by the SiN film. That is, in selecting the surface protective film, it is necessary to pay attention to three points: current collapse, current leakage, and semiconductor sheet resistance.

  For example, when an AlN film is used as a surface protective film, a metal organic chemical vapor deposition (MOCVD) method is used on an AlGaN film formed on the outermost surface at a crystal growth temperature of 1000 ° C. or more. When an AlN film is formed in-situ, so-called in-situ film formation, and a film thickness of about 5 nm is formed, cracks occur due to distortion caused by the difference in lattice constant between the AlGaN film and the AlN film. In order to reduce strain, an AlN film is grown at a temperature of about 500 ° C. to 650 ° C. to form an amorphous AlN film, or thereafter annealed at a temperature of 1000 ° C. or higher. A means of terminating the surface with an AlN film mainly composed of columnar crystals can be considered. However, when these films are used, although the occurrence of current collapse can be suppressed, it has been experimentally verified that the sheet resistance is larger than that in the case where surface protection is performed with a SiN film. In addition, when the AlN film is formed by the sputtering method, the current collapse can be reduced as in the case of forming the AlN film at a low temperature, but the sheet resistance is higher than when the SiN film is used. It has been confirmed. This is because when an AlN film is formed by sputtering, nitrogen is supplied to the surface of the nitride semiconductor, so that nitrogen vacancies can be reduced, but sputtering damage is large and the surface defect density of the nitride semiconductor is increased. It is considered a thing. Therefore, in the selection of the surface protective film for reducing the current collapse of the FET made of nitride semiconductor, a reduction in sheet resistance and a reduction in gate leakage current are in a trade-off relationship.

  In view of the above problems, an object of the present invention is to reduce a current leakage, reduce a sheet resistance, and increase a drain current while reducing a gate leakage current.

  In order to achieve the above object, according to the present invention, a semiconductor device is configured to use an insulating material not containing silicon for a protective film in contact with a lower portion of a gate electrode.

  Specifically, a semiconductor device according to the present invention includes a substrate, a semiconductor layer made of a group III nitride formed on the substrate, and a source electrode, a gate electrode, and a drain electrode formed on the semiconductor layer, respectively. A first protective film not including silicon formed on the semiconductor layer so as to be in contact with the lower part of the gate electrode and the semiconductor layer and to be separated from the source electrode and the drain electrode; and on the semiconductor layer, the semiconductor The second protective film is formed so as to be in contact with the layer and spaced apart from the lower portion of the gate electrode, and has a composition different from that of the first protective film and contains nitrogen.

  According to the semiconductor device of the present invention, the first protective film not containing silicon formed on the semiconductor layer so as to be in contact with the lower portions of the semiconductor layer and the gate electrode and to be separated from the source electrode and the drain electrode; A second protective film is formed on the semiconductor layer so as to be in contact with the semiconductor layer and to be separated from the lower portion of the gate electrode, and has a composition different from that of the first protective film and contains nitrogen. For this reason, the gate electrode is not silicided, the gate electrode and the semiconductor layer under the gate electrode can maintain a good Schottky junction, and an increase in gate leakage current due to passivation can be prevented. Further, current collapse can be reduced by passivation using the first protective film and the second protective film. That is, since the portions other than the periphery of the lower portion of the gate electrode are passivated by the second protective film containing nitrogen, nitrogen vacancies on the surface of the semiconductor layer can be reduced. As a result, the interface state density of the semiconductor layer can be lowered, current collapse can be reduced, and a semiconductor device with a small gate leakage current and a large drain current can be obtained.

  In the semiconductor device according to the present invention, the semiconductor layer includes a first semiconductor layer formed on the substrate and a second semiconductor layer formed on the first semiconductor layer. The band gap of the layer is preferably larger than the band gap of the first semiconductor layer.

  In the semiconductor device according to the present invention, the first protective film is preferably made of aluminum nitride or aluminum oxide.

  In the semiconductor device according to the present invention, the second protective film is preferably made of silicon nitride.

  The semiconductor device according to the present invention preferably further includes a third protective film formed on the second protective film.

  In this case, it is preferable that the gate electrode is in contact with the third protective film.

  In this case, the third protective film is preferably made of silicon nitride or silicon oxide.

  According to the semiconductor device of the present invention, it is possible to reduce the gate collapse current while reducing the current collapse and increasing the drain current.

1 is a cross-sectional view showing a semiconductor device according to a first embodiment of the present invention. (A)-(d) is sectional drawing which shows the manufacturing method of the semiconductor device which concerns on the 1st Embodiment of this invention in process order. (A)-(c) is sectional drawing which shows the manufacturing method of the semiconductor device which concerns on the 1st Embodiment of this invention in process order. It is sectional drawing which shows the semiconductor device which concerns on the 2nd Embodiment of this invention. (A)-(c) is sectional drawing which shows the manufacturing method of the semiconductor device which concerns on the 2nd Embodiment of this invention in process order. It is sectional drawing which shows the semiconductor device which concerns on the 3rd Embodiment of this invention. (A)-(d) is sectional drawing which shows the manufacturing method of the semiconductor device which concerns on the 3rd Embodiment of this invention in process order. (A)-(c) is sectional drawing which shows the manufacturing method of the semiconductor device which concerns on the 3rd Embodiment of this invention in process order. It is sectional drawing which shows the semiconductor device which concerns on the 4th Embodiment of this invention. (A)-(c) is sectional drawing which shows the manufacturing method of the semiconductor device which concerns on the 4th Embodiment of this invention in process order. (A) And (b) is sectional drawing which shows the manufacturing method of the semiconductor device which concerns on the 4th Embodiment of this invention in process order. It is sectional drawing which shows the conventional semiconductor device.

(First embodiment)
A semiconductor device according to a first embodiment of the present invention will be described with reference to FIG.

As shown in FIG. 1, a GaN layer having a thickness of about 2 μm as a first semiconductor layer is formed on a substrate 1 made of, for example, silicon (Si), gallium nitride (GaN), sapphire, or silicon carbide (SiC). 2 is formed. On the GaN layer 2, the second semiconductor layer, for example, aluminum gallium nitride (Al _0.3 Ga _0.7 N) having a band gap larger than that of the GaN layer 2 and having a film thickness of about 25 nm is used. A barrier layer 3 is formed. In the vicinity of the interface between the GaN layer 2 and the AlGaN barrier layer 3, a large amount of charge is generated by, for example, piezoelectric polarization and spontaneous polarization, thereby forming a channel layer 4 made of a two-dimensional electron gas (2DEG). . A source electrode 5 and a drain electrode 6 are formed on the AlGaN barrier layer 3, and a gate electrode 7 is formed between the source electrode 5 and the drain electrode 6.

  As the material of the gate electrode 7, for example, gold (Au), nickel (Ni), or palladium (Pd) is used. The source electrode 5 and the drain electrode 6 are made of, for example, a multilayer film or an alloy of titanium (Ti) and Al (aluminum).

  In the present embodiment, the gate length, which is the length of the portion of the gate electrode 7 in contact with the AlGaN barrier layer 3 along the straight line connecting the source electrode 5 and the drain electrode 6, is about 1 μm. The length between the source electrode 6 and the gate electrode 7 is about 3 μm, and the length between the drain electrode 6 and the gate electrode 7 is about 1 μm.

On the AlGaN barrier layer 3, a first protective film 8 made of, for example, aluminum nitride (AlN) is formed so as to be in contact with the lower portion of the gate electrode 7 and the AlGaN barrier layer 3. Here, the first protective film 8 may be an aluminum oxide (Al ₂ O ₃ ) film. A second protective film 9 made of, for example, silicon nitride (SiN) is formed on the AlGaN barrier layer 3 so as to be in contact with the first protective film 8 and cover the source electrode 5 and the drain electrode 6. Yes. That is, the second protective film 9 is formed so as to be separated from the lower portion of the gate electrode 7.

  Here, the thickness of the second protective film 9 (from the surface of the source electrode 5 and the drain electrode 6 to the surface of the second protective film 9) is about 100 nm. In the portion of the first protective film 8 in contact with the AlGaN barrier layer 3, the length in the gate length direction is about 0.3 μm on both the source electrode 5 side and the drain electrode 6 side. By doing so, the AlGaN barrier layer 3 can be protected by the second protective film 9 while reliably blocking the current leak by the first protective film 8, so that the current leak can be suppressed and the current collapse can be reduced.

  According to the semiconductor device of the first embodiment of the present invention, since the first protective film 8 not containing Si is in contact with the lower portion of the gate electrode 7, silicidation of the gate electrode 7 can be prevented. Further, since the second protective film 9 containing nitrogen is in contact with the AlGaN barrier layer 3, nitrogen vacancies on the surface of the AlGaN barrier layer 3 can be reduced by the nitrogen of the second protective film 9. As a result, the current collapse of the FET can be reduced.

  Next, a method for manufacturing a semiconductor device according to the first embodiment of the present invention will be described with reference to FIGS.

First, as shown in FIG. 2A, a GaN layer 2 having a thickness of about 2 μm is formed on a substrate 1 made of Si, GaN, sapphire, or SiC by, for example, metal organic chemical vapor deposition (MOCVD). The AlGaN barrier layer 3 made of Al _0.3 Ga _0.7 N and having a thickness of about 25 nm is sequentially formed. At this time, a channel layer 4 made of 2DEG is formed in the vicinity of the interface between the GaN layer 2 and the AlGaN barrier layer 3.

  Next, as shown in FIG. 2B, a source electrode 5 and a drain electrode 6 are formed on the AlGaN barrier layer 3, respectively.

  Next, as shown in FIG. 2C, SiN is formed on the AlGaN barrier layer 3 so as to cover the source electrode 5 and the drain electrode 6 by, for example, plasma chemical vapor deposition (P-CVD). The second protective film 9 having a thickness of about 100 nm (from the surface of the source electrode 5 and the drain electrode 6 to the surface of the second protective film 9) is formed.

  Next, as shown in FIG. 2D, an opening 7A is formed in the second protective film 9 by performing resist patterning and dry etching, for example. Here, the opening 7 </ b> A is formed between the source electrode 5 and the drain electrode 6.

  Next, as shown in FIG. 3A, for example, a first film having a thickness of about 50 nm made of AlN is formed on the AlGaN barrier layer 3 so as to cover the second protective film 9 by sputtering or MOCVD. 1 A protective film 8 is formed. Note that when the MOCVD method is used, the crystal growth temperature is preferably about 500 ° C. to 650 ° C.

  Next, as shown in FIG. 3B, the first protective film 8 is left only on the side wall of the opening 7A by uniformly dry-etching the thickness of the first protective film 8. .

  Next, as shown in FIG. 3C, the gate electrode 7 is formed so as to fill the opening 7A.

  According to the method of manufacturing a semiconductor device according to the first embodiment of the present invention, the lower portion of the gate electrode 7 does not contact the second protective film containing Si at the portion in contact with the AlGaN barrier layer 3. Can be prevented. Thereby, gate leakage current can be reduced. Further, since the second protective film 9 containing nitrogen is formed so as to be in contact with the AlGaN barrier layer 3, the nitrogen of the second protective film 9 reduces the nitrogen vacancies on the surface of the AlGaN barrier layer 3. Collapse can be reduced. In the present embodiment, when the first protective film 8 is formed by the sputtering method, the AlGaN barrier layer 3 is covered by the second protective film 9 except for the opening 7B. Can be reduced. On the other hand, when the first protective film 8 is formed by the MOCVD method, cracks may occur in the first protective film 8 when crystal growth is performed at a crystal growth temperature of 1000 ° C. or higher. When grown, the AlN film becomes amorphous, so that the influence of cracks and the like can be reduced.

(Second Embodiment)
A semiconductor device according to a second embodiment of the present invention will be described with reference to FIG. In the present embodiment, description of the same parts as those of the first embodiment will be omitted, and only different parts will be described. The semiconductor device according to the present embodiment is different from the first embodiment in the position where the first protective film and the second protective film are formed. Furthermore, the semiconductor device according to the present embodiment has a third protective film.

Specifically, as shown in FIG. 4, a source electrode 5 and a drain electrode 6 are formed on the AlGaN barrier layer 3, and a gate electrode 7 is formed between the source electrode 5 and the drain electrode 6. A second protective film 9 is formed on the barrier layer 3 so as to be separated from the gate electrode 7 and cover the source electrode 5 and the drain electrode 6. A first protective film 8 is formed on the AlGaN barrier layer 3 so as to be in contact with the lower portion of the gate electrode 7 and the AlGaN barrier layer 3 and to cover the second protective film 9. A third protective film 10 made of, for example, SiO ₂ is formed on the first protective film 8 so that the gate electrode 7 is in contact with the portion other than the lower portion thereof. Note that the third protective film 10 may be a SiN film.

  Here, the film thickness of the first protective film 8 is about 50 nm, and the film thickness of the second protective film 9 (from the surface of the source electrode 5 and the drain electrode 6 to the surface of the second protective film 9) is about 100 nm. . In this embodiment, the gate length is about 1 μm, the length between the source electrode 5 and the gate electrode 7 is about 3 μm, and the length between the gate electrode 7 and the drain electrode 6 is about 3 μm. The thickness of the third protective film 10 is about 100 nm.

In the semiconductor device according to the second embodiment of the present invention, the gate electrode 7 is not silicided because the first protective film 8 not containing Si is in contact with the lower portion of the gate electrode 7. Further, since the second protective film 9 containing nitrogen is in contact with the AlGaN barrier layer 3, nitrogen vacancies on the surface of the AlGaN barrier layer 3 can be reduced by nitrogen of the second protective film. As a result, the current collapse of the FET can be reduced. Further, the first protective film 8 made of, for example, AlN is formed by, for example, a sputtering method or the like. However, since the crystallinity is not sufficiently good in this way, an alkaline solution such as water, hydrofluoric acid, and a developing solution is used. Easily soluble in etc. Although the first protective film 8 is improved in crystallinity and difficult to dissolve by performing thermal annealing or the like, it is more desirable to protect the surface after undergoing steps such as development and washing. The same applies when Al ₂ O ₃ is used for the first protective film 8 instead of AlN. Therefore, in the present embodiment, the first protective film 8 can be protected by forming the third protective film 10 made of, for example, SiO ₂ on the first protective film 8.

  Next, a method for manufacturing a semiconductor device according to the second embodiment of the present invention will be described with reference to FIG. In the present embodiment, the steps from FIG. 2A to FIG. 3A of the first embodiment are the same, and the description thereof will be omitted.

As shown in FIG. 5A, after forming the first protective film 8, a third protective film 10 made of, for example, SiO ₂ is formed on the first protective film 8.

  Next, as illustrated in FIG. 5B, openings 7 </ b> B are formed in the third protective film 10 and the first protective film 8 by performing resist patterning and dry etching, for example. Here, the opening 7 </ b> B is formed between the source electrode 5 and the drain electrode 6.

  Next, as shown in FIG. 5C, the gate electrode 7 is formed so as to fill the opening 7B.

  According to the method of manufacturing a semiconductor device according to the second embodiment of the present invention, the lower portion of the gate electrode 7 can be made not to contact the layer containing Si at the portion contacting the AlGaN barrier layer 3. Can be prevented. Thereby, gate leakage current can be reduced. Further, since the second protective film 9 containing nitrogen is formed so as to be in contact with the AlGaN barrier layer 3, the nitrogen of the second protective film 9 reduces the nitrogen vacancies on the surface of the AlGaN barrier layer 3. Collapse can be reduced. Furthermore, the first protective film 8 can be protected by forming the third protective film 10 on the first protective film 8. In the present embodiment, problems due to sputtering damage or cracks in the process of forming the first protective film 8 can be improved for the same reason as in the first embodiment.

(Third embodiment)
A semiconductor device according to a third embodiment of the present invention will be described with reference to FIG. In the present embodiment, description of the same parts as those of the first embodiment will be omitted, and only different parts will be described. The semiconductor device according to the present embodiment is different from the first embodiment in the position where the first protective film and the second protective film are formed.

  Specifically, as shown in FIG. 6, a source electrode 5 and a drain electrode 6 are formed on the AlGaN barrier layer 3, and a gate electrode 7 is formed between the source electrode 5 and the drain electrode 6. A first protective film 8 is formed on the barrier layer 3 so as to be in contact with the lower portion of the gate electrode 7 and the AlGaN barrier layer 3. Further, a second protective film 9 is formed on the AlGaN barrier layer 3 so as to cover a part of the first protective film 8, the source electrode 5 and the drain electrode 6, and the gate electrode 7 and the part other than the lower part thereof are in contact with each other. Has been.

  Here, the film thickness of the first protective film 8 is about 50 nm, and the film thickness of the second protective film 9 (from the surface of the source electrode 5 and the drain electrode 6 to the surface of the second protective film 9) is about 100 nm. . In this embodiment, the gate length is about 1 μm, the length between the source electrode 5 and the gate electrode 7 is about 1 μm, and the length between the gate electrode 7 and the drain electrode 6 is about 3 μm.

  In the semiconductor device according to the third embodiment of the present invention, since the first protective film 8 not containing Si is in contact with the lower portion of the gate electrode 7, silicidation of the gate electrode 7 can be prevented. Further, since the second protective film 9 containing nitrogen is in contact with the AlGaN barrier layer 3, nitrogen vacancies on the surface of the AlGaN barrier layer 3 can be reduced by the nitrogen of the second protective film 9. As a result, the current collapse of the FET can be reduced.

  Next, a method for manufacturing a semiconductor device according to the third embodiment of the present invention will be described with reference to FIGS. In the present embodiment, the steps up to FIG. 2A of the first embodiment are the same, and the description thereof is omitted.

  As shown in FIG. 7A, after the AlGaN barrier layer 3 is formed, a first protective film 8 made of AlN is formed on the AlGaN barrier layer 3 by, for example, MOCVD. In this step, since the AlGaN barrier layer 3 is exposed, it is preferable to use the MOCVD method or the like rather than the sputtering method. Here, as in the first embodiment, the crystal growth temperature is preferably about 500 ° C. to 650 ° C.

  Next, as shown in FIG. 7B, a part of the first protective film 8 is removed by, for example, resist patterning and dry etching.

  Next, as illustrated in FIG. 7C, an opening 7 </ b> C is formed in the first protective film 8 again by performing, for example, resist patterning and dry etching.

  Next, as shown in FIG. 7D, the source electrode 5 and the drain electrode are formed on the AlGaN barrier layer 3 so as to be spaced apart from the first protective film 8 and sandwich the first protective film 8 therebetween. 6 is formed.

  Next, as shown in FIG. 8A, a second protective film 9 is formed on the AlGaN barrier layer 3 so as to cover the source electrode 5, the drain electrode 6 and the first protective film 8. At this time, the opening 7 </ b> C is buried in the second protective film 8.

  Next, as shown in FIG. 8B, an opening 7D is formed in the second protective film 9 by performing resist patterning and dry etching, for example. Here, the opening 7D is formed so as to remove the second protective film between the first protective films 8, that is, to reopen the position where the opening 7C has been formed.

  Next, as shown in FIG. 8C, the gate electrode 7 is formed so as to fill the opening 7D.

  According to the method for manufacturing a semiconductor device according to the third embodiment of the present invention, the lower portion of the gate electrode 7 can be prevented from coming into contact with the layer containing Si at the portion in contact with the AlGaN barrier layer 3. Can be prevented. Thereby, gate leakage current can be reduced. Further, since the second protective film 9 containing nitrogen is formed so as to be in contact with the AlGaN barrier layer 3, the nitrogen of the second protective film 9 reduces the nitrogen vacancies on the surface of the AlGaN barrier layer 3. Collapse can be reduced. In addition, since the opening 7C is formed in the first protective film 8 before the opening 7D is formed, the gate length can be controlled more easily. Moreover, the problem due to sputtering damage or cracks in the process of forming the first protective film 8 can be improved for the same reason as in the first embodiment.

(Fourth embodiment)
A semiconductor device according to a fourth embodiment of the present invention will be described with reference to FIG. In the present embodiment, description of the same parts as those of the first embodiment will be omitted, and only different parts will be described. The semiconductor device according to the present embodiment is different from the first embodiment in the position where the first protective film and the second protective film are formed.

  Specifically, as shown in FIG. 9, a source electrode 5 and a drain electrode 6 are formed on the AlGaN barrier layer 3, and a gate electrode 7 is formed between the source electrode 5 and the drain electrode 6. A first protective film 8 is formed on the barrier layer 3 so as to be in contact with the gate electrode 7 and to cover the gate electrode 7. A second protective film 9 is formed on the AlGaN barrier layer 3 so as to cover the source electrode 5, the drain electrode 6 and the first protective film 8.

  Here, the film thickness of the first protective film 8 is about 50 nm, and the film thickness of the second protective film 9 (from the surface of the first protective film 8 on the gate electrode 7 to the surface of the second protective film 9) is as follows. About 100 nm. In this embodiment, the gate length is about 1 μm, the length between the source electrode 5 and the gate electrode 7 is about 1 μm, and the length between the gate electrode 7 and the drain electrode 6 is about 3 μm.

  According to the semiconductor device of the fourth embodiment of the present invention, since the first protective film 8 not containing Si is in contact with the entire surface of the gate electrode 7, silicidation of the gate electrode 7 can be prevented. Further, since the second protective film 9 containing nitrogen is in contact with the AlGaN barrier layer 3, nitrogen vacancies on the surface of the AlGaN barrier layer 3 can be reduced by the nitrogen of the second protective film 9. As a result, the current collapse of the FET can be reduced.

  Next, a method for manufacturing a semiconductor device according to the fourth embodiment of the present invention will be described with reference to FIGS. In the present embodiment, the steps up to FIG. 2A of the first embodiment are the same, and the description thereof is omitted.

  As shown in FIG. 10A, after the AlGaN barrier layer 3 is formed, the gate electrode 7 is formed on the AlGaN barrier layer 3.

  Next, as shown in FIG. 10B, a first protective film 8 made of AlN is formed on the AlGaN barrier layer 3 so as to cover the gate electrode 7 by, for example, MOCVD. In this step, since the AlGaN barrier layer 3 is exposed except the region where the gate electrode 7 is formed, it is preferable to use the MOCVD method or the like rather than the sputtering method. Here, as in the first embodiment, the crystal growth temperature is preferably about 500 ° C. to 650 ° C.

  Next, as shown in FIG. 10C, for example, resist patterning and dry etching are performed to leave only the portion of the first protective film 8 covering the gate electrode 7 and remove the other.

  Next, as shown in FIG. 11A, the source electrode 5 and the drain electrode 6 are formed on the AlGaN barrier layer 3 at positions where the gate electrode 7 and the first protective film 8 are sandwiched, respectively.

  Next, as shown in FIG. 11B, a second protective film 9 is formed on the AlGaN barrier layer 3 so as to cover the source electrode 5, the drain electrode 6, and the first protective film 8.

  According to the method of manufacturing a semiconductor device according to the fourth embodiment of the present invention, the lower part of the gate electrode 7 can be prevented from coming into contact with the layer containing Si at the part in contact with the AlGaN barrier layer 3. Can be prevented. Thereby, gate leakage current can be reduced. Further, since the second protective film 9 containing nitrogen is formed so as to be in contact with the AlGaN barrier layer 3, the nitrogen of the second protective film 9 reduces the nitrogen vacancies on the surface of the AlGaN barrier layer 3. Collapse can be reduced. Moreover, the problem due to sputtering damage or cracks in the process of forming the first protective film 8 can be improved for the same reason as in the first embodiment.

  In the first to fourth embodiments, before forming the GaN layer 2 on the substrate 1, depending on the type of the substrate 1, an AlN buffer having a thickness of, for example, about 10 nm to 100 nm is formed on the substrate 1. A layer or a GaN buffer layer may be formed. In this way, the crystallinity of the GaN layer 2 can be improved. The crystal growth of the AlN buffer layer or the GaN buffer layer may be performed at a lower temperature (500 ° C. to 700 ° C.) than the crystal growth temperature (about 1000 ° C.) of the GaN layer 2 and the AlGaN barrier layer 3. Moreover, the buffer layer here may be a so-called superlattice buffer layer in which, for example, a GaN layer having a thickness of about several nm and an AlN layer having a thickness of about several nm are alternately formed.

  In the first to fourth embodiments, the GaN layer 2 and the AlGaN barrier layer 3 are formed. However, the AlGaN barrier layer 3 is not formed, and the source electrode 5 and the drain electrode 6 are directly formed on the GaN layer 2. In addition, the gate electrode 7 may be formed. Even such a configuration has the above-described effects.

  Moreover, the confinement of carriers in the channel layer 4 may be increased by forming an AlGaN layer having a band gap larger than that of the GaN layer 2 so as to be in contact with the GaN layer 2 on the substrate 1 side of the GaN layer 2. . Even such a configuration has the above-described effects.

For example, an In _0.1 Ga _0.9 N layer may be used instead of the GaN layer 2, and a GaN layer or an InAlGaN layer may be used instead of the AlGaN layer 3. As a combination of the two semiconductor layers forming the channel layer 4, the Al composition and the Ga composition of the semiconductor layer are set so that the band gap of the semiconductor layer formed thereon is larger than the semiconductor layer on the substrate side. And the In composition may be determined. Further, the thickness of the semiconductor layer, the gate length, the distance between the source electrode 5 and the drain electrode 6 and the distance between the source electrode 5 and the gate electrode 7 may be appropriately changed according to the specifications of the FET. Further, the arrangement of the source electrode 5 and the drain electrode 6 with respect to the gate electrode 7 may be symmetric or asymmetric.

  The source electrode 5 and the drain electrode 6 can be used without any particular limitation as long as they are in ohmic contact with the AlGaN barrier layer 3. The gate electrode 7 can be used without any particular limitation as long as it is in Schottky contact with the AlGaN barrier layer 3.

  The film thickness of the 1st protective film 8 and the film thickness of the 2nd protective film 9 can be suitably changed according to the specification of FET.

  The semiconductor device according to the present invention can reduce gate collapse current while reducing current collapse and increasing drain current, and is particularly useful for a semiconductor device including a nitride semiconductor.

1 Substrate 2 GaN layer (first semiconductor layer)
3 AlGaN barrier layer (second semiconductor layer)
4 channel layer 5 source electrode 6 drain electrode 7 gate electrodes 7A to 7D opening 8 first protective film 9 second protective film 10 third protective film

Claims (7)

A substrate,
A semiconductor layer made of group III nitride formed on the substrate;
A source electrode, a gate electrode and a drain electrode respectively formed on the semiconductor layer;
A first protective film not including silicon formed on the semiconductor layer so as to be in contact with a lower portion of the gate electrode and the semiconductor layer and to be separated from the source electrode and the drain electrode;
A second protective film formed on the semiconductor layer so as to be in contact with the semiconductor layer and to be separated from a lower portion of the gate electrode, and having a composition different from that of the first protective film and containing nitrogen; A featured semiconductor device. The semiconductor layer includes a first semiconductor layer formed on the substrate, and a second semiconductor layer formed on the first semiconductor layer,
The semiconductor device according to claim 1, wherein a band gap of the second semiconductor layer is larger than a band gap of the first semiconductor layer.   The semiconductor device according to claim 1, wherein the first protective film is made of aluminum nitride or aluminum oxide.   The semiconductor device according to claim 1, wherein the second protective film is made of silicon nitride.   5. The semiconductor device according to claim 1, further comprising a third protective film formed on the first protective film. 6.   The semiconductor device according to claim 5, wherein the gate electrode is in contact with the third protective film.   The semiconductor device according to claim 5, wherein the third protective film is made of silicon nitride or silicon oxide.

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