JP7628911B2 - Semiconductor device manufacturing method - Google Patents

Description

The technology disclosed in this specification relates to a method for manufacturing a semiconductor device.

Patent Document 1 discloses a method for annealing a gallium nitride semiconductor substrate. In this method, first, a protective film made of aluminum nitride is formed on the surface of the gallium nitride semiconductor substrate. The gallium nitride semiconductor substrate is then annealed at a temperature of 1200°C or less. This method can suppress deterioration of the surface of the gallium nitride semiconductor substrate during annealing. Patent Document 1 also describes that even with this method, if the annealing temperature exceeds 1200°C, the crystal quality of the gallium nitride semiconductor substrate is impaired.

Japanese Patent Application Publication No. 08-186332

This specification proposes a technique for annealing a gallium nitride semiconductor substrate at a temperature higher than 1200°C while suppressing degradation of the gallium nitride semiconductor substrate.

The method for manufacturing a semiconductor device disclosed in this specification includes the steps of forming a protective film made of aluminum nitride having at least one of an amorphous structure and a granular crystal structure on the surface of a gallium nitride semiconductor substrate, and annealing the gallium nitride semiconductor substrate whose surface is covered with the protective film at a temperature higher than 1200°C.

The gallium nitride semiconductor substrate is a semiconductor layer whose main component is gallium nitride. For example, the gallium nitride semiconductor substrate may be made of gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), or indium aluminum gallium nitride (InAlGaN).

The inventors of the present application conducted an experiment in which various aluminum nitride films (i.e., protective films) with different structures were formed on the surfaces of multiple gallium nitride semiconductor substrates, and then each gallium nitride semiconductor substrate was annealed at a temperature higher than 1200°C. As a result, it was found that in a gallium nitride semiconductor substrate on which a protective film having a columnar crystal structure (polycrystalline in which each crystal grain extends in a columnar shape from the surface of the gallium nitride semiconductor substrate) was formed, significant deterioration occurred on the surface of the gallium nitride semiconductor substrate below the protective film. It was also found that in a gallium nitride semiconductor substrate on which a protective film having an amorphous and granular crystal structure (polycrystalline in which each crystal grain is granular) was formed, deterioration of the surface of the gallium nitride semiconductor substrate below the protective film was suppressed.

In the above-described manufacturing method, a protective film made of aluminum nitride having at least one of an amorphous structure and a granular crystal structure is formed on the surface of the gallium nitride semiconductor substrate. Therefore, when the gallium nitride semiconductor substrate is subsequently annealed at a temperature higher than 1200°C, deterioration of the surface of the gallium nitride semiconductor substrate below the protective film is suppressed. As described above, according to this manufacturing method, the gallium nitride semiconductor substrate can be annealed at a temperature higher than 1200°C while suppressing deterioration of the gallium nitride semiconductor substrate.

1A to 1C are cross-sectional views showing a method for manufacturing a semiconductor device. 1A to 1C are cross-sectional views showing a method for manufacturing a semiconductor device. 1A to 1C are cross-sectional views showing a method for manufacturing a semiconductor device. FIG. 1 is a configuration diagram showing a sputtering apparatus. 1A to 1C are cross-sectional views showing a method for manufacturing a semiconductor device. FIG. 3 is a cross-sectional view showing a pit in a protective film. FIG. 2 is a cross-sectional view showing a pit extending into the gallium nitride semiconductor substrate. 11 is a table showing the results of an experiment in which the flow rate ratio R is changed in the manufacturing method of Example 1. 13 is a graph showing the ratio of constituent elements in the protective film 18 of sample A. 13 is a graph showing the ratio of constituent elements in the protective film 18 of sample C. 13 is an image showing a cross section of the protective film 18 of sample A. 13 is an image showing a cross section of the protective film 18 of sample C. 6 is a graph showing the relationship between the flow rate ratio R, the pit density, and the pit area ratio in Example 1. 13 is a table showing the results of an experiment in which the flow rate ratio R is changed in the manufacturing method of Example 2. 10 is a graph showing the relationship between the flow rate ratio R and the pit area ratio in Example 2.

In one example of the manufacturing method disclosed in this specification, at least a portion of the protective film may be made of aluminum nitride having an amorphous structure.

This configuration makes it possible to more effectively suppress deterioration of the surface of the gallium nitride semiconductor substrate below the protective film during annealing.

In one example of the manufacturing method disclosed in this specification, in the step of forming the protective film, the protective film may be formed by sputtering. In the sputtering, aluminum nitride may be used as a target.

When aluminum nitride is used as the target, the sputtering process may be performed by supplying a sputtering gas into the chamber of the sputtering device, without supplying nitrogen gas into the chamber.

This configuration makes it possible to more effectively suppress deterioration of the surface of the gallium nitride semiconductor substrate below the protective film during annealing.

When aluminum nitride is used as the target, nitrogen gas may be supplied into the chamber of the sputtering device together with the sputtering gas. In this case, the value obtained by dividing the supply amount of the nitrogen gas expressed in sccm by the supply amount of the sputtering gas expressed in sccm may be less than 0.25.

This configuration makes it possible to more effectively suppress deterioration of the surface of the gallium nitride semiconductor substrate below the protective film during annealing.

In one example of the manufacturing method disclosed in this specification, the step of forming the protective film may include forming the protective film by sputtering. In the sputtering, aluminum may be used as a target, and nitrogen gas and a sputtering gas may be supplied into a chamber of a sputtering device.

When aluminum is used as the target, a protective film made of aluminum nitride can be formed by supplying nitrogen gas into the chamber.

When aluminum is used as the target, the value obtained by dividing the supply amount of the nitrogen gas expressed in sccm by the supply amount of the sputtering gas expressed in sccm may be 0.25 or more and 0.43 or less.

This configuration makes it possible to more effectively suppress deterioration of the surface of the gallium nitride semiconductor substrate below the protective film during annealing.

The manufacturing method disclosed in this specification may further include a step of implanting a p-type impurity into the gallium nitride semiconductor substrate before the step of forming the protective film.

With this configuration, p-type impurities can be efficiently activated by annealing at temperatures higher than 1200°C.

A manufacturing method for the semiconductor device of Example 1 will be described. In the manufacturing method of Example 1, as shown in FIG. 1, a drift layer 14 made of GaN is epitaxially grown on a support layer 12 made of GaN. The drift layer 14 is an n-type layer with a low concentration of n-type impurities. Hereinafter, the entire support layer 12 and drift layer 14 are referred to as a gallium nitride semiconductor substrate 10.

Next, an ion implantation process is performed. In the ion implantation process, as shown in FIG. 2, p-type impurities are ion-implanted into the drift layer 14. For example, magnesium can be used as the p-type impurity. Here, a mask layer 80 is formed on a portion of the surface 10a of the gallium nitride semiconductor substrate 10 where the drift layer 14 is exposed, and p-type impurities are selectively implanted into the drift layer 14 through the mask layer 80. This forms an ion implantation layer 16 in the surface portion of the drift layer 14. After the ion implantation is completed, the mask layer 80 is removed.

Next, a protective film forming process is carried out. In the protective film forming process, as shown in FIG. 3, a protective film 18 made of aluminum nitride is formed on the surface 10a of the gallium nitride semiconductor substrate 10. Here, the protective film 18 is formed by sputtering.

FIG. 4 shows a sputtering apparatus 100 for forming the protective film 18. The sputtering apparatus 100 has a chamber 110, a stage 120, and a target 130. The stage 120 and the target 130 are arranged inside the chamber 110. The stage 120 is arranged at the bottom of the chamber 110. The gallium nitride semiconductor substrate 10 can be placed on the stage 120. The target 130 is arranged above the stage 120. The target 130 is arranged at a position facing the gallium nitride semiconductor substrate 10 placed on the stage 120. In the first embodiment, the target 130 is made of aluminum nitride (AlN). Furthermore, the sputtering apparatus 100 has a sputtering gas supply pipe 150, a nitrogen gas supply pipe 160, a gas exhaust pipe 170, and a power source 180. The sputtering gas supply pipe 150 supplies a sputtering gas into the chamber 110. An inert gas such as a rare gas can be used as the sputtering gas. In Example 1, argon (i.e., Ar) is used as the sputtering gas. The nitrogen gas supply pipe 160 supplies nitrogen gas (i.e., _N2 ) into the chamber 110. The gas exhaust pipe 170 exhausts the gas in the chamber 110 to the outside. The power supply 180 applies a voltage (e.g., a DC voltage or a high-frequency voltage) between the stage 120 and the target 130.

In the protective film forming process, first, as shown in FIG. 4, the gallium nitride semiconductor substrate 10 is placed on the stage 120. Here, the gallium nitride semiconductor substrate 10 is placed in a direction in which the surface 10a is exposed. Next, a vacuum pump (not shown) is operated to exhaust the gas in the chamber 110 through the gas exhaust pipe 170, and the pressure in the chamber 110 is reduced. Next, sputtering gas is supplied from the sputtering gas supply pipe 150 into the chamber 110. Also, if necessary, nitrogen gas is supplied from the nitrogen gas supply pipe 160 into the chamber 110 together with the sputtering gas. Next, a voltage is applied between the stage 120 and the target 130 by the power source 180. Then, the sputtering gas becomes plasma, and the atoms of the sputtering gas (i.e., Ar) collide with the target 130. Then, aluminum nitride atoms fly out of the target 130, and the aluminum nitride atoms that fly out are deposited on the surface 10a of the gallium nitride semiconductor substrate 10. Therefore, a protective film 18 made of aluminum nitride is formed on the surface 10a.

Next, an annealing step is performed. In the annealing step, the gallium nitride semiconductor substrate 10 with the protective film 18 formed on the surface 10a is heated to a temperature higher than 1200°C. For example, the gallium nitride semiconductor substrate 10 is heated at a temperature of 1400°C or higher. Also, for example, the gallium nitride semiconductor substrate 10 may be annealed under an atmosphere of ammonia (i.e., NH ₃ ). When the gallium nitride semiconductor substrate 10 is heated, the p-type impurities in the ion implantation layer 16 are activated. As a result, as shown in FIG. 5, a p-type layer 20 is formed in the surface layer portion of the drift layer 14. Note that, when the gallium nitride semiconductor substrate 10 is heated at a temperature of 1400°C or higher, a p-type layer 20 with a high activation rate of the p-type impurities in the ion implantation layer 16 and a high carrier concentration can be formed.

After the p-type layer 20 is formed in the annealing process, other necessary semiconductor layers are formed, and electrodes and insulating films that contact the gallium nitride semiconductor substrate 10 are formed to complete the semiconductor device. The semiconductor device to be manufactured may be a switching element (e.g., a field effect transistor, a high electron mobility transistor, etc.), a diode, or other element.

In the annealing process, since the surface 10a of the gallium nitride semiconductor substrate 10 is covered with the protective film 18, decomposition of the gallium nitride semiconductor substrate 10 at the surface 10a is suppressed. However, if pits 30 (voids extending in the thickness direction) are present in the protective film 18 as shown in FIG. 6, the gallium nitride semiconductor substrate 10 is decomposed below the pits 30 during the annealing process. In this case, as shown in FIG. 7, the pits 30 expand and reach the surface 10a of the gallium nitride semiconductor substrate 10. That is, the pits 30 are formed on the surface 10a of the gallium nitride semiconductor substrate 10. When the pits 30 are formed on the surface 10a, the surface condition of the gallium nitride semiconductor substrate 10 deteriorates. The relationship between the conditions in the protective film formation process and the surface condition of the gallium nitride semiconductor substrate 10 after the annealing process is described below.

The quality of the protective film 18 varies depending on the ratio of the amount of nitrogen gas supplied in the sputtering process (hereinafter referred to as flow rate FN (unit: sccm)) to the amount of sputtering gas supplied (hereinafter referred to as flow rate FS (unit: sccm)). Hereinafter, the ratio of flow rate FN to flow rate FS is expressed as flow rate ratio R. Flow rate ratio R is a value obtained by dividing flow rate FN by flow rate FS. Figure 8 shows the results of an experiment in which protective film 18 was formed by changing flow rate ratio R. Samples A to D shown in Figure 8 are samples in which protective film 18 was formed under the condition of flow rate ratio R shown in Figure 8. Each image in the middle row of Figure 8 shows the surface of protective film 18 before the annealing process. No abnormalities were observed on the surface of protective film 18 in any of samples A to D before the annealing process.

Figure 9 shows the concentration distribution of the constituent elements in the depth direction of the protective film 18 of sample A. Figure 10 shows the concentration distribution of the constituent elements in the depth direction of the protective film 18 of sample C. As shown in Figure 10, the protective film 18 of sample C contains nitrogen atoms and aluminum atoms in approximately the same ratio. In contrast, as shown in Figure 9, the protective film 18 of sample A contains aluminum atoms in a significantly higher ratio than nitrogen atoms. That is, the protective film 18 of sample A contains a higher ratio of aluminum atoms than the protective film 18 of sample C. In this way, by changing the flow rate ratio R in the sputtering process, the ratio of the constituent elements in the protective film 18 formed can be changed.

Figures 11 and 12 are images of the cross section of the protective film 18 taken with a transmission electron microscope (TEM) before the annealing process. Figure 11 is an image of the protective film 18 of sample A, and Figure 12 is an image of the protective film 18 of sample C.

As described above, the protective film 18 of sample C contains nitrogen atoms and aluminum atoms in approximately the same ratio. Therefore, as shown in FIG. 12, the protective film 18 of sample C has a polycrystalline structure. As shown in FIG. 12, in the protective film 18 of sample C, each crystal grain extends in a columnar shape along a direction approximately perpendicular to the surface 10a of the gallium nitride semiconductor substrate 10. That is, the protective film 18 of sample C has a columnar crystal structure. Also, as shown in FIG. 12, in sample C, pits 30 (white parts) extending in the depth direction from the surface of the protective film 18 were confirmed. The depth of the pits 30 is approximately 122 nm. Thus, in sample C, the protective film 18 is composed of columnar crystals, and therefore a large number of pits 30 exist in the protective film 18. Although not shown, the protective film 18 of sample D is also composed of columnar crystals, and a large number of pits 30 exist in the protective film 18.

As described above, the proportion of aluminum atoms is much higher than the proportion of nitrogen atoms in the protective film 18 of sample A. Therefore, as shown in FIG. 11, in sample A, almost the entire protective film 18 has an amorphous structure. In sample A, the entire protective film 18 is almost homogeneous, and no pits 30 are observed in the protective film 18. In addition, although not shown, in sample B, a portion of the protective film 18 is composed of granular crystals, and the other portion of the protective film 18 is composed of amorphous. In sample B, almost no pits 30 are observed in the protective film 18.

The images in the lower part of FIG. 8 show the surfaces of the gallium nitride semiconductor substrate 10 and the protective film 18 after the annealing process is performed at 1400° C. In each image in the lower part, the left part shows the surface of the protective film 18, and the right part shows the surface 10a of the gallium nitride semiconductor substrate 10 exposed by removing the protective film 18. In each image showing the surface 10a, the black parts are pits 30. FIG. 13 shows the results of calculating the pit density and pit area ratio based on each image in the lower part of FIG. 8. The pit density is the density of the pits 30 on the surface 10a, and the pit area ratio is the ratio of the area of the pits 30 to the surface 10a. As shown in FIGS. 8 and 13, the lower the flow rate ratio R, the lower the pit density and pit area ratio.

As described above, in samples A and B in which the protective film 18 was formed under conditions of a low flow rate ratio R, the protective film 18 has an amorphous or granular crystal structure, and the number of pits 30 in the protective film 18 is small. Therefore, in samples A and B, the number of pits 30 formed on the surface 10a of the gallium nitride semiconductor substrate 10 in the annealing process is small. Thus, in the protective film formation process of Example 1, the lower the flow rate ratio R, the lower the crystallinity of the protective film 18, and the fewer the pits 30 in the protective film 18. Therefore, the lower the flow rate ratio R, the fewer the pits 30 formed on the surface 10a in the annealing process. As is clear from FIG. 13, in Example 1, by setting the flow rate ratio R to less than 0.25, the protective film 18 has at least one of an amorphous and granular crystal structure, and the occurrence of pits 30 in the gallium nitride semiconductor substrate 10 can be suppressed. In particular, by setting the flow rate ratio R to 0.04 or less, the occurrence of pits 30 can be more effectively suppressed. In this way, when the protective film 18 has at least one of an amorphous and granular crystal structure, a single protective film 18 can suppress deterioration of the gallium nitride semiconductor substrate 10 during an annealing process at temperatures higher than 1200°C.

The manufacturing method of Example 2 differs from the manufacturing method of Example 1 in that the target 130 used in the protective film formation process is aluminum. The other configurations of the manufacturing method of Example 2 are the same as those of Example 1. In the protective film formation process of Example 2, aluminum atoms are ejected from the target 130, and the ejected aluminum atoms are deposited on the surface 10a of the gallium nitride semiconductor substrate 10. In addition, the aluminum atoms deposited on the surface 10a react with nitrogen gas to become aluminum nitride. Therefore, a protective film 18 made of aluminum nitride is formed on the surface 10a of the gallium nitride semiconductor substrate 10.

Figure 14 shows the experimental results of forming protective film 18 by changing the flow rate ratio R in the protective film formation process of Example 2. Samples E to I shown in Figure 14 are samples in which protective film 18 was formed under the flow rate ratio R conditions shown in Figure 14. Each image in the middle row of Figure 14 shows the surface of protective film 18 before the annealing process. Before the annealing process, no abnormalities were found on the surface of protective film 18 in any of samples E to I.

The images in the lower part of FIG. 14 show the gallium nitride semiconductor substrate 10 and the protective film 18 after the annealing process is performed at 1400° C. In each image in the lower part, the left part shows the surface of the protective film 18, and the right part shows the surface 10a of the gallium nitride semiconductor substrate 10 exposed by removing the protective film 18. In each image showing the surface 10a, the black parts are pits 30. FIG. 15 also shows the results of calculating the pit area ratio based on each image in the lower part of FIG. 14. As shown in FIGS. 14 and 15, in Example 2, the pit area ratio increased both when the flow rate ratio R was too low and when the flow rate ratio R was too high. As shown in FIG. 15, in Example 2, by setting the flow rate ratio R to 0.25 or more and 0.43 or less, the protective film 18 has at least one of an amorphous structure and a granular crystal structure, and the occurrence of pits 30 in the gallium nitride semiconductor substrate 10 can be suppressed.

In both Examples 1 and 2, the formation of pits 30 on the surface 10a of the gallium nitride semiconductor substrate 10 can be more effectively suppressed by making the thickness of the protective film 18 100 nm or more.

Although the embodiments have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above. The technical elements described in this specification or drawings demonstrate technical utility either alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings achieves multiple objectives simultaneously, and achieving one of these objectives is itself technically useful.

10: gallium nitride semiconductor substrate, 12: support layer, 14: drift layer, 16: ion implantation layer, 18: protective film, 20: p-type layer, 30: pit

Claims (7)

A method for manufacturing a semiconductor device, comprising:
A step of forming a protective film (18) made of aluminum nitride having a portion made of granular crystals and a portion made of amorphous on a surface (10a) of a gallium nitride semiconductor substrate (10);
annealing the gallium nitride semiconductor substrate, the surface of which is covered with the protective film, at a temperature higher than 1200° C.;
The manufacturing method comprising the steps of: In the step of forming the protective film, the protective film is formed by sputtering,
In the sputtering, aluminum nitride is used as a target.
The method of claim 1 . The method according to claim 2 , wherein in the sputtering, a sputtering gas is supplied into a chamber of a sputtering device, but nitrogen gas is not supplied into the chamber. In the sputtering, nitrogen gas and a sputtering gas are supplied into a chamber of a sputtering apparatus;
the value obtained by dividing the supply amount of the nitrogen gas expressed in sccm by the supply amount of the sputtering gas expressed in sccm is less than 0.25;
The method according to claim 2 . In the step of forming the protective film, the protective film is formed by sputtering,
In the sputtering, aluminum is used as a target, and nitrogen gas and a sputtering gas are supplied into a chamber of a sputtering apparatus.
The method of claim 1 . 6. The manufacturing method according to claim 5 , wherein a value obtained by dividing the supply amount of the nitrogen gas expressed in sccm by the supply amount of the sputtering gas expressed in sccm is 0.25 or more and 0.43 or less. 7. The method according to claim 1, further comprising the step of implanting a p-type impurity into the gallium nitride semiconductor substrate before the step of forming the protective film.

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