JP7439660B2 - Method for forming silicon nitride passivation film and method for manufacturing semiconductor device - Google Patents

Description

The present disclosure relates to a method for forming a silicon nitride passivation film, a method for manufacturing a semiconductor device, and a semiconductor device.

Patent Document 1 discloses a technology related to manufacturing a high electron mobility transistor (HEMT) as a semiconductor device using a nitride semiconductor. The HEMT described in Patent Document 1 includes a nitride semiconductor layer on a substrate such as SiC, and a silicon nitride passivation film formed on the surface of the nitride semiconductor layer. The silicon nitride passivation film is formed by a low pressure CVD method.

Japanese Patent Application Publication No. 2018-181885

In recent years, semiconductor devices using nitride semiconductors such as GaN-based semiconductors have been developed. In semiconductor devices, an insulating silicon compound film is provided to protect (passivate) the outermost surface of the semiconductor, but in the case of semiconductor devices using nitride semiconductors, a silicon nitride (SiN) film, which is the same nitride, is used. Often used. When forming a silicon nitride film on a nitride semiconductor, a film forming method using plasma (plasma CVD, electron cyclotron resonance (ECR) sputtering, etc.) is used to form the film at a relatively low temperature. is used. Therefore, damage caused by the plasma is formed on the surface of the nitride semiconductor.

On the other hand, when forming a silicon nitride film on a silicon semiconductor, a low pressure (LP) CVD method is used. According to the low-pressure CVD method, a high-quality film can be formed by increasing the film-forming temperature instead of lowering the film-forming pressure. Therefore, according to the low pressure CVD method, damage caused by plasma to the silicon semiconductor, which is the underlying crystal, is suppressed. The inventor believes that if the low pressure CVD method can also be used for silicon nitride films on nitride semiconductors, it is possible to suitably form silicon nitride films without damaging the surface of the nitride semiconductors due to plasma. Ta.

When a silicon nitride film is formed by low pressure CVD, an oxide film is formed by exposing the surface of the epitaxial wafer to the atmosphere during a waiting period after epitaxial growth. In addition, in a high-temperature environment, the presence of moisture and oxygen due to degassing within the device causes an oxide layer (for example, a gallium oxide layer, an aluminum oxide layer, or an indium oxide layer) to be formed on the surface of the nitride semiconductor. It is also possible that Oxygen from the oxide film and oxide layer at the interface between the nitride semiconductor and the silicon nitride film thus formed may deteriorate the operating characteristics of the semiconductor device.

Note that in the general low-pressure CVD technology that has matured by forming a silicon nitride film on a silicon semiconductor, consideration regarding oxygen at the interface between the silicon semiconductor and the silicon nitride film has not been required. This is because when forming a silicon nitride film on a silicon semiconductor, if an oxide film is formed on the surface of the silicon semiconductor, the oxide film on the surface can be removed by a simple method due to the characteristics of the silicon material. Another reason is that the oxide film hardly affects the electrical characteristics.

According to the inventor's findings, when applying the low pressure CVD method to forming a silicon nitride film on a nitride semiconductor, unless the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride film is reduced, such interface Oxygen affects the operating characteristics of semiconductor devices. For example, if a poor quality oxide film is formed by degassing near the interface where the electric field at the gate edge is concentrated, leakage current increases and electrical characteristics deteriorate.

However, in the general low-pressure CVD method, it is difficult to completely eliminate the desorption gas (here, moisture and oxygen) in order to reduce the amount of oxygen at the interface due to the mechanism of the device. This is because there is degas from the epitaxial wafer itself, which is the film-forming sample. Furthermore, it is difficult to manage all the members of the film forming chamber inside the apparatus so as not to expose them to the atmosphere.

The present disclosure has been made in view of such problems, and when forming a silicon nitride passivation film on a nitride semiconductor using a low pressure CVD method, the nitride semiconductor and the silicon nitride passivation film are It is an object of the present invention to provide a method for forming a silicon nitride passivation film that can reduce the amount of oxygen at an interface, a method for manufacturing a semiconductor device, and a semiconductor device that can suppress deterioration of electrical characteristics.

In order to solve the above-mentioned problems, a method for forming a silicon nitride passivation film according to an embodiment is a method for forming a silicon nitride passivation film in contact with a nitride semiconductor layer, the method comprising: forming a silicon nitride passivation film on a substrate including a nitride semiconductor layer; a step of replacing the atmosphere in the reactor in which is placed with an atmosphere containing NH ₃ or an atmosphere containing H ₂ ; a temperature raising step of raising the temperature in the reactor to a first temperature; A cleaning process in which the inside of the reactor is kept at a first temperature and the atmosphere inside the reactor is an atmosphere containing NH ₃ or H ₂ for 3 minutes or more, and the temperature inside the reactor is lower than the first temperature. A silicon nitride passivation film is formed by supplying dichlorosilane (SiH ₂ Cl ₂ ) into the reactor while the temperature is lowered to a second temperature and the pressure inside the reactor is set to a second pressure of 30 Pa or less. A step of forming a film.

Further, a semiconductor device according to another embodiment includes a substrate and a plurality of nitride semiconductor layers each made of a nitride semiconductor, and covers a semiconductor stack provided on the substrate and a surface of the semiconductor stack. A silicon nitride passivation film and oxygen atoms present in an interface oxygen amount of 0.16×10 ¹⁵ atoms/cm ² or less at an interface between the silicon nitride passivation film and the semiconductor stack.

According to the method for forming a silicon nitride passivation film and the method for manufacturing a semiconductor device according to the present disclosure, when forming a silicon nitride passivation film on a nitride semiconductor using a low pressure CVD method, the nitride semiconductor and the silicon nitride passivation film are The amount of oxygen at the interface with the film can be reduced. Further, according to the semiconductor device according to the present disclosure, deterioration of electrical characteristics can be suppressed.

FIG. 1 is a side view showing a silicon nitride passivation film formed by the film forming method according to the first embodiment. FIG. 2 is a flowchart showing a method for forming a silicon nitride passivation film according to the first embodiment. FIG. 3 is a diagram showing the temperature in the reactor and the procedure for supplying gas when growing a silicon nitride passivation film. FIG. 4 is a flowchart showing a method for forming a silicon nitride passivation film according to Comparative Example 1. FIG. 5 is a diagram showing the temperature inside the reactor and the procedure for supplying gas when growing a silicon nitride passivation film according to Comparative Example 1. FIG. 6 is a flowchart showing a method for forming a silicon nitride passivation film according to Comparative Example 2. FIG. 7 is a diagram showing the temperature inside the reactor and the procedure for supplying gas when growing a silicon nitride passivation film according to Comparative Example 2. FIG. 8 is a graph showing the relationship between temperature increasing conditions and interfacial oxygen amount. FIG. 9 is a graph showing the relationship between cleaning time and interfacial oxygen amount. FIG. 10 is a graph showing the relationship between cleaning pressure and interfacial oxygen amount. FIG. 11 is a graph showing the relationship between cleaning temperature and interfacial oxygen amount. FIG. 12 is a diagram showing each step of the manufacturing method according to the second embodiment. FIG. 13 is a diagram showing each step of the manufacturing method according to the second embodiment. FIG. 14 is a diagram showing each step of the manufacturing method according to the second embodiment. FIG. 15 is a graph showing the results of ToF-SIMS analysis after the silicon nitride passivation film was formed. FIG. 16 is a flowchart showing a method for forming a silicon nitride passivation film according to the third embodiment. FIG. 17 is a graph showing pressure changes in the reactor when growing a silicon nitride passivation film according to Comparative Example 1. FIG. 18 is a graph showing pressure changes in the reactor when growing a silicon nitride passivation film according to the third embodiment. FIG. 19 is a flowchart showing a method for forming a silicon nitride passivation film according to the fourth embodiment. FIG. 20 is a diagram showing the temperature in the reactor and the procedure for supplying gas when growing a silicon nitride passivation film. FIG. 21 is a graph showing the relationship between the H ₂ partial pressure of the atmospheric gas and the amount of interfacial oxygen. FIG. 22 is a graph showing the relationship between the pressure inside the reactor and the amount of interfacial oxygen. FIG. 23 is a graph showing the relationship between cleaning time and interfacial oxygen amount. FIG. 24 is a graph showing the relationship between cleaning time and interfacial oxygen amount.

A method for forming a silicon nitride passivation film, a method for manufacturing a semiconductor device, and a specific example of a semiconductor device according to an embodiment of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is indicated by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the following description, the same elements are given the same reference numerals in the description of the drawings, and redundant description will be omitted.

FIG. 1 is a side view showing a silicon nitride passivation film formed by a film forming method according to a first embodiment of the present disclosure. This silicon nitride passivation film 3 is in contact with the surface of the nitride semiconductor layer 5, and is formed on the surface of the nitride semiconductor layer 5 by a low pressure CVD method. The nitride semiconductor layer 5 is a layer grown on a substrate 7 made of, for example, SiC, and is, for example, a GaN layer. Nitride semiconductor layer 5 and substrate 7 constitute epitaxial wafer 9 . Oxygen atoms 11 are present at the interface between epitaxial wafer 9 and silicon nitride passivation film 3 . In this specification, the unit "atom/cm ² " (number of atoms/cm ² ) is used to express the amount of oxygen at this interface (hereinafter referred to as "interfacial oxygen amount"). Note that the interfacial oxygen amount is a value obtained by evaluating the amount of oxygen at the interface between the epitaxial wafer 9 and the silicon nitride passivation film 3 based on the distribution of oxygen concentration in the depth direction by SIMS analysis. Specifically, the SIMS analysis is performed after the silicon nitride passivation film 3 is formed according to the flow of film formation in the low pressure CVD method. At the interface between epitaxial wafer 9 and silicon nitride passivation film 3 shown in FIG. 1, oxygen atoms 11 exist in an amount of interfacial oxygen of, for example, 0.16×10 ¹⁵ atoms/cm ² or less.

FIG. 2 is a flowchart showing a method for forming the silicon nitride passivation film 3 according to this embodiment. First, a nitride semiconductor layer 5 is grown on a substrate 7 using a MOCVD (Metal Organic Chemical Vapor Deposition) method to fabricate an epitaxial wafer 9 (step S1). Next, silicon nitride passivation film 3 is grown on nitride semiconductor layer 5 using a low pressure CVD method (step S2). Details of this step S2 will be explained with reference to FIGS. 2 and 3. FIG. 3 is a diagram showing the temperature in the reactor and the procedure for supplying gas when growing the silicon nitride passivation film 3.

First, the temperature inside the reactor is set to 500° C. or lower (step S21). This temperature may be 400°C or less, or 300°C or less. Moreover, this temperature may be room temperature (25° C.) or higher. In one example, the temperature set in step S21 is 400°C. After the temperature in the reactor is stabilized at the set temperature, the epitaxial wafer 9 including the nitride semiconductor layer 5 is set in a transfer device in an air atmosphere and introduced into the reactor (step S22). Next, by repeatedly performing a vacuum purge and an NH ₃ purge in the reactor (cycle purge), the gas atmosphere in the reactor is replaced with an atmosphere containing NH ₃ (step S23). In one embodiment, an NH ₃ atmosphere is substituted. In addition, in this step S23, the atmospheric atmosphere in the reactor may be replaced with a mixed atmosphere of NH ₃ and N ₂ in which the partial pressure of NH ₃ is 0.1 or more.

After the cycle purge described above is completed, the pressure in the reactor is changed to a second pressure that is higher than the pressure during film formation (first pressure described later) (step S24). This second pressure may be 300 Pa or more, 5 kPa or more, or 10 kPa or more. Further, this second pressure may be equal to or lower than atmospheric pressure (100 kPa). In one example, the second pressure is 3 kPa. Then, while maintaining the pressure inside the reactor at the second pressure, the inside of the reactor is heated to a first temperature higher than 700° C. (step S25, temperature raising step). This first temperature is higher than the film forming temperature (second temperature described later). The first temperature may be 20° C. or more higher than the second temperature. Further, the first temperature may be 800°C or higher or 900°C or lower. In one example, the first temperature is 800°C.

Subsequently, the inside of the reactor is maintained at a first temperature (here, 800° C.), an atmosphere containing NH ₃ (here, NH ₃ atmosphere), and a second pressure (here, 3 kPa) for 3 minutes or more. (Step S26, cleaning step; holding step). In one example, the hold time is 10 minutes. Note that the holding time may be 2 minutes or more. By performing step S26, the inside of the reactor is cleaned. Next, a film forming environment is prepared (temperature lowering step). Specifically, steps S27 and S28 described below are performed. First, the temperature inside the reactor is lowered to a second temperature for film formation (step S27). However, the second temperature is lower than the first temperature. The second temperature may be 700°C or higher. In one example, the second temperature is 780°C. After the temperature in the reactor is stabilized at the second temperature, the pressure in the reactor is reduced to a first pressure of 100 Pa or less (step S28). This first pressure may be 40 Pa or less, or may be 10 Pa or more. In one example, the first pressure is 20 Pa.

Then, with the inside of the reactor at the first pressure (here, 20 Pa), dichlorosilane (SiH ₂ Cl ₂ ) is supplied into the reactor to form the silicon nitride passivation film 3 (step S29). . In this step S29, the NH ₃ flow rate and the dichlorosilane flow rate may be made substantially equal to each other. The NH ₃ flow rate and the dichlorosilane flow rate are both 100 sccm, for example. 1 sccm represents 1 cm ³ /min at 1 atm and 0°C.

After the formation of the silicon nitride passivation film 3 is completed, the source gas is stopped and the temperature inside the reactor is lowered to a predetermined temperature (for example, 700° C.). Then, in order to drive out the chlorine gas in the reactor, a cycle purge with nitrogen gas is performed to dilute the chlorine gas to the detection limit (step S30). After that, the epitaxial wafer 9 is taken out from the reactor (step S31). Through the above steps, silicon nitride passivation film 3 is formed on nitride semiconductor layer 5.

The effects obtained by the method for forming a silicon nitride passivation film according to the present embodiment described above will be explained in comparison with Comparative Example 1 and Comparative Example 2. First, Comparative Example 1 will be explained. FIG. 4 is a flowchart showing a method for forming a silicon nitride passivation film according to Comparative Example 1. FIG. 5 is a diagram showing the temperature inside the reactor and the procedure for supplying gas when growing a silicon nitride passivation film according to Comparative Example 1. In Comparative Example 1, after step S1, silicon nitride passivation film 3 is grown on nitride semiconductor layer 5 using a low pressure CVD method (step S4). Specifically, first, the temperature inside the reactor is set to 700°C (step S41). After the temperature in the reactor is stabilized at 700° C., the epitaxial wafer 9 including the nitride semiconductor layer 5 is set on a transfer device in an air atmosphere and introduced into the reactor (step S42). Next, by repeatedly performing a vacuum purge and an N ₂ purge in the reactor, the atmospheric atmosphere in the reactor is replaced with an N ₂ atmosphere (step S43).

After that, the pressure inside the reactor is reduced to, for example, 20 Pa, which is about the same as the film forming pressure (step S44). Then, while maintaining the pressure inside the reactor, the temperature inside the reactor is raised to a film forming temperature (for example, 800° C.) (step S45). Then, the process waits until the temperature in the reactor becomes stable (step S46). Once the temperature inside the reactor is stabilized, the atmosphere inside the reactor is changed from N ₂ atmosphere to NH ₃ atmosphere, which is a film forming gas, by evacuation and purging of NH ₃ gas (step S47). At this time, the pressure inside the reactor is set to 20 Pa, which is the film forming pressure. The flow rate of NH ₃ gas is 100 sccm. Then, once the pressure of the NH ₃ atmosphere is stabilized, dichlorosilane, which is a silicon-based raw material gas, is supplied into the reactor to form the silicon nitride passivation film 3 (step S48). The dichlorosilane flow rate is 100 sccm.

After the formation of the silicon nitride passivation film 3 is completed, the source gas is stopped and the temperature inside the reactor is lowered to a predetermined temperature (for example, 700° C.). Then, in order to drive out the chlorine gas in the reactor, a cycle purge with nitrogen gas is performed to dilute the chlorine gas to the detection limit (step S49). After that, the epitaxial wafer 9 is taken out from the reactor (step S50). Through the above steps, silicon nitride passivation film 3 is formed on nitride semiconductor layer 5.

Next, Comparative Example 2 will be explained. FIG. 6 is a flowchart showing a method for forming a silicon nitride passivation film according to Comparative Example 2. FIG. 7 is a diagram showing the temperature inside the reactor and the procedure for supplying gas when growing a silicon nitride passivation film according to Comparative Example 2. In Comparative Example 2, after step S1, silicon nitride passivation film 3 is grown on nitride semiconductor layer 5 using a low pressure CVD method (step S6). Specifically, first, the temperature inside the reactor is set to 400° C. (step S61). After the temperature in the reactor is stabilized at 400° C., the epitaxial wafer 9 including the nitride semiconductor layer 5 is set in a transfer device in an air atmosphere and introduced into the reactor (step S62). Next, by repeatedly performing a vacuum purge and an NH ₃ purge in the reactor, the atmospheric atmosphere in the reactor is replaced with an NH ₃ atmosphere (step S63).

After that, the pressure inside the reactor is changed to 3 kPa (step S64). Then, while maintaining the pressure inside the reactor, the temperature inside the reactor is raised to a film forming temperature (for example, 800° C.) (step S65). Then, when the temperature inside the reactor becomes stable, the pressure inside the reactor is reduced to 20 Pa, which is the film forming pressure (step S66). Then, dichlorosilane, which is a silicon-based raw material gas, is supplied into the reactor to form the silicon nitride passivation film 3 (step S67). The dichlorosilane flow rate is 100 sccm.

After the formation of the silicon nitride passivation film 3 is completed, the source gas is stopped and the temperature inside the reactor is lowered to a predetermined temperature (for example, 700° C.). Then, in order to expel the chlorine gas in the reactor, a cycle purge with nitrogen gas is performed to dilute the chlorine gas to the detection limit (step S68). After that, the epitaxial wafer 9 is taken out from the reactor (step S69). Through the above steps, silicon nitride passivation film 3 is formed on nitride semiconductor layer 5.

In step S25 of this embodiment shown in FIGS. 2 and 3, unlike step S45 of Comparative Example 1 shown in FIGS. 4 and 5, the atmosphere in the reactor is set to be an atmosphere containing NH ₃ and The temperature is raised while the internal pressure is set to a second pressure that is higher than the first pressure. Table 1 and FIG. 8 show the relationship between temperature increase conditions and interfacial oxygen content. Table 1 shows the amount of interfacial oxygen (nitride semiconductor layer 5 and nitride The results of comparing the amount of oxygen at the interface with the silicon passivation film 3 are shown. FIG. 8 is a graph plotting the comparison results in Table 1. In the graph of FIG. 8, the horizontal axis is the pressure as the temperature increasing condition, and the vertical axis is the interfacial oxygen amount. Note that the interfacial oxygen amount shown below is a value obtained by examining the epitaxial wafer 9 taken out from the reactor by SIMS analysis.

In obtaining the comparison results shown in Table 1, the temperature at which the epitaxial wafer 9 was introduced into the reactor was 400°C. In addition, after raising the temperature inside the reactor under the conditions shown in Table 1 above, the temperature inside the reactor was set to 800°C, the atmosphere inside the reactor was N ₂ atmosphere, and the pressure inside the reactor was set to 30 Pa for 10 minutes. By holding it, the inside of the reactor was cleaned. Further, dichlorosilane was supplied into the reactor at a temperature of 700° C. and a pressure of 30 Pa to form the silicon nitride passivation film 3.

Referring to Table 1, it can be seen that the amount of interfacial oxygen is reduced when the atmosphere in the reactor is NH ₃ atmosphere than when it is N ₂ atmosphere. Furthermore, it can be seen that the amount of interfacial oxygen is particularly reduced when the pressure inside the reactor is 300 Pa or more (especially when it is 3000 Pa or more). In obtaining the following comparison results (Tables 2 to 4), the conditions for raising the temperature inside the reactor were that the atmosphere inside the reactor was an NH ₃ atmosphere and the pressure inside the reactor was 3000 Pa.

In the comparison results shown in Table 1 above, by setting the atmospheric gas to NH ₃ as the temperature raising condition, hydrogen (H ₂ ) is generated by thermal decomposition, and this hydrogen causes oxygen atoms contained in the oxide film on the surface of the epitaxial wafer 9 to be generated. It is thought that 11 exhibits a reducing action and is removed. It is also believed that this reduction action also removes the oxygen atoms 11 contained in the moisture and oxygen adhering to the interior of the reactor. In particular, when the pressure inside the reactor is relatively high (for example, 300 Pa or more), the amount of hydrogen generated is relatively large, so the amount of oxygen atoms 11 removed by the reduction action also increases.

Further, in step S26 of the present embodiment shown in FIGS. 2 and 3, unlike Comparative Example 2 shown in FIGS. 6 and 7, the inside of the reactor is cleaned. Specifically, the temperature inside the reactor is set to a first temperature higher than the film-forming temperature (second temperature), the atmosphere inside the reactor is set to an atmosphere containing NH ₃ atmosphere, and the pressure inside the reactor is set to a first temperature higher than the film-forming temperature (second temperature). The pressure is maintained at a second pressure higher than the first pressure for 3 minutes or more. In the following, the holding time when performing cleaning (hereinafter also referred to as "cleaning time"), the type of atmospheric gas in the reactor, the temperature in the reactor (hereinafter also referred to as "cleaning temperature"), and The pressure within the reactor (hereinafter also referred to as "cleaning pressure") is referred to as a cleaning condition. Table 2 and FIG. 9 show the relationship between cleaning time and interfacial oxygen amount. Table 2 shows the results of comparing the amount of interfacial oxygen when the film was formed under each cleaning condition by changing the holding time as the cleaning condition. FIG. 9 is a graph plotting the comparison results in Table 2. In the graph of FIG. 9, the horizontal axis is the cleaning time, and the vertical axis is the interfacial oxygen amount. Note that in Table 2, the time was set to 0 minutes in the case where cleaning was not performed. In addition, the cleaning conditions other than the holding time were the same. Specifically, the atmospheric gas was NH ₃ , the temperature was 800° C., and the pressure was 300 Pa.

In obtaining the comparison results shown in Table 2, conditions other than cleaning conditions were as follows. First, the temperature at which the epitaxial wafer 9 was introduced into the reactor was set to 400°C. Furthermore, as described above, the conditions for raising the temperature inside the reactor were that the atmosphere inside the reactor was an NH ₃ atmosphere and the pressure inside the reactor was 3000 Pa. Further, dichlorosilane was supplied into the reactor at a temperature lower than that during cleaning and at a pressure of 30 Pa to form the silicon nitride passivation film 3. Note that conditions other than the cleaning conditions were the same when obtaining the comparison results shown below.

Referring to Table 2 and FIG. 9, the amount of interfacial oxygen is lower when the reactor is cleaned than when the reactor is not cleaned (that is, when the time in Table 2 is 0 minutes). It can be seen that this is reduced. It can also be seen that the amount of interfacial oxygen is particularly reduced when the inside of the reactor is cleaned for 3 minutes or more, and is further reduced when it is 10 minutes or more.

Next, when cleaning the inside of the reactor for 10 minutes, we formed a film by changing any of the cleaning conditions other than the holding time, and compared the amount of interfacial oxygen when the film was formed under each cleaning condition. Explain the results. Table 3 shows the results of comparing the amount of interfacial oxygen when the film was formed under each cleaning condition by changing the type of atmospheric gas and the cleaning pressure as cleaning conditions. FIG. 10 is a graph plotting the comparison results in Table 3, and shows the relationship between cleaning pressure and interfacial oxygen amount. In the graph of FIG. 10, the horizontal axis is the cleaning pressure, and the vertical axis is the interfacial oxygen amount. Here, a common temperature was used as the cleaning condition. Specifically, the temperature was set to 800°C. Referring to Table 3 and FIG. 10, it can be seen that the amount of interfacial oxygen is reduced more when the atmospheric gas in the reactor is NH ₃ than when it is N ₂ . Furthermore, it can be seen that when the atmospheric gas is NH ₃ , the amount of interfacial oxygen is particularly reduced when the pressure is 300 Pa or more.

Table 4 shows the results of comparing the amount of interfacial oxygen when the film was formed under each cleaning condition by changing the temperature as the cleaning condition. FIG. 11 is a graph plotting the comparison results in Table 4, and shows the relationship between cleaning temperature and interfacial oxygen amount. In the graph of FIG. 11, the horizontal axis is the cleaning temperature, and the vertical axis is the interfacial oxygen amount. Here, cleaning conditions other than temperature were the same. Specifically, the atmospheric gas was NH ₃ and the pressure was 3000 Pa. Referring to Table 4, it can be seen that the amount of interfacial oxygen is particularly reduced when the temperature is 750° C. or higher.

From the above results, cleaning inside the reactor can be carried out by maintaining the temperature inside the reactor at 750°C or higher, the atmosphere inside the reactor at NH ₃ atmosphere, and the pressure inside the reactor at 300 Pa or higher for 3 minutes. It has been found that when carried out, the amount of interfacial oxygen is particularly reduced.

As in the present embodiment, compared to Comparative Examples 1 and 2, by exposing the epitaxial wafer 9 before film formation for 3 minutes or more in an environment containing NH ₃ and a temperature higher than the film formation temperature. , the following effects (1) and (2) can be expected.
(1) Oxygen atoms 11 contained in the oxide film on the surface of the nitride semiconductor layer 5 are reduced and removed.
(2) Oxygen atoms 11 contained in moisture and oxygen, which are the causes of desorption gas adhering to the inside of the reactor, are also reduced, and the formation of a new oxide film inside the reactor is suppressed.

Due to the synergistic effect of (1) and (2) above, the amount of interfacial oxygen is sufficiently reduced, so that gate leakage current is reduced. Therefore, the electrical characteristics of the semiconductor device can be improved. In particular, when the amount of interfacial oxygen is 0.6×10 ¹⁵ atom/cm ² or less (0.6×10 ¹⁵ cm ^-2 or less in an atmospheric pressure environment), the gate leakage current becomes dominant. This suppresses deterioration of the electrical characteristics of the semiconductor device.

By the way, if the surface of the epitaxial wafer is exposed to the atmosphere, there is a concern that in addition to the above-mentioned oxygen, carbon, fluorine, etc. (hereinafter simply referred to as "carbon etc.") may be incorporated into the epitaxial wafer. These carbon and the like behave as impurities that form deep acceptor levels in nitride semiconductors. Therefore, if carbon or the like is incorporated into a nitride semiconductor, it will directly lead to problems such as an increase in leakage current and deterioration of collapse characteristics of the semiconductor device.

According to the inventor, it is difficult to imagine that fluorine is present in a larger amount in the reactor than carbon and oxygen. However, after epitaxial growth and before film formation, cleaning treatment using IPA (isopropyl alcohol) is performed and film formation is performed using low-pressure CVD, but 1 fluorine is not present on the surface of the nitride semiconductor layer 5 (for example, a GaN layer). Detected in amounts below ×10 ¹¹ atoms/cm ² . Note that this value is four orders of magnitude smaller than the amount of oxygen atoms 11. Since the processing performed in the reactor includes processing using a hydrofluoric acid (HF) solution such as RCA cleaning, a small amount of fluorine is present in the atmosphere and may adhere to the surface of the epitaxial wafer 9. It is thought that then. When working in a reactor, it is difficult to avoid adhesion of fluorine to the surface of the epitaxial wafer 9.

On the other hand, due to the synergistic effect of the above (1) and (2), carbon and fluorine on the surface of the epitaxial wafer 9 are also removed. This is because H ₂ is generated by an atmosphere containing NH ₃ . Specifically, the remaining fluorine (F) reacts with hydrogen atoms (H) in the atmospheric gas to form hydrofluoric acid (HF) and is vaporized. Carbon (C) reacts with hydrogen atoms (H) in the atmospheric gas to form methane (CH ₄ ) and is vaporized. By removing carbon and fluorine at the interface between nitride semiconductor layer 5 and silicon nitride passivation film 3 in this manner, the problems caused by residual impurities on the surface of epitaxial wafer 9 as described above are reduced. This reduces gate leakage current and improves the electrical characteristics of the semiconductor device.

(Second embodiment)
Next, as a second embodiment, a method for manufacturing a semiconductor device mainly composed of a nitride semiconductor will be described, including a method for forming the silicon nitride passivation film 3 according to the first embodiment. 12 to 14 are diagrams showing each step of the manufacturing method according to this embodiment. In this embodiment, a GaN-HEMT is exemplified as a semiconductor device.

First, as shown in FIG. 12A, a stacked structure 20 (semiconductor stacked structure, semiconductor stacked part) including a plurality of nitride semiconductor layers is grown on a substrate 10 using the MOCVD method. The substrate 10 is, for example, a SiC substrate having a (0001) principal surface, and the lamination direction of the laminated structure 20 is, for example, the [0001] direction. The stacked structure 20 includes a nucleation layer 12, an electron transport layer 14, an electron supply layer 16, and a cap layer 18, which are formed in this order from the substrate 10 side. The nucleation layer 12 is, for example, an AlN layer with a thickness of several tens of nanometers. The electron transit layer 14 is, for example, an undoped GaN layer with a thickness of 1000 nm. The electron supply layer 16 is, for example, an n-type AlGaN layer with a thickness of 20 nm. The cap layer 18 is, for example, an n-type GaN layer with a thickness of 5 nm.

At this time, an oxygen film is formed on the surface of the cap layer 18 due to exposure to the atmosphere. When this oxygen film is investigated by SIMS analysis, oxygen atoms are present in an interfacial oxygen amount of about 0.6×10 ¹⁵ atoms/cm ² .

Next, as shown in FIG. 12(b), a silicon nitride passivation film (SiN film) 26 in contact with the upper surface of the stacked structure 20 is formed using a low pressure CVD method. At this time, the method for forming the silicon nitride passivation film 3 according to the first embodiment is applied. The film forming temperature is, for example, 800°C. Moreover, NH ₃ gas and dichlorosilane (SiH ₂ Cl ₂ ) are used as source gases. At this time, when the above-mentioned oxide film existing at the interface between the SiN film 26 and the laminated structure 20 is investigated by ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) analysis, it is found that oxygen atoms exist as silicon oxynitride. There is.

Specifically, the surface of the nitride semiconductor after epitaxial growth is oxidized due to exposure to the atmosphere. However, after the SiN film 26 is formed, oxygen in the oxidized nitride semiconductor moves to the silicon nitride side and becomes silicon oxynitride. FIG. 15 is a graph showing the results of ToF-SIMS analysis after the silicon nitride passivation film was formed. In the graph of FIG. 15, the horizontal axis is depth, and the vertical axis is secondary ion velocity. However, the left vertical axis is one digit larger than the right vertical axis. It can be seen from FIG. 15 that gallium oxide on gallium nitride becomes silicon oxynitride after the silicon nitride passivation film is formed.

Note that ToF-SIMS analysis is an analysis method in which secondary ions released by irradiation with a pulsed beam as a primary ion beam are subjected to mass spectrometry. In ToF-SIMS analysis, information on elements and molecules present on the outermost surface of a sample can be detected with a small ion beam irradiation dose (1×10 ¹² cm ⁻² or less). In ToF-SIMS analysis, primary ion beams used include, for example, relatively heavy ion beams (Bi, Au, etc.), Ga ion beams that can be easily made into fine beams, and the like.

Subsequently, as shown in FIG. 12(c), a photoresist 50 is applied on the SiN film 26. An opening 50a is formed in the photoresist 50 by photolithography. Openings are formed in the SiN film 26 and the cap layer 18 by reactive ion etching (RIE) using the photoresist 50 as a mask. After that, the photoresist 50 is removed.

Subsequently, as shown in FIG. 13(a), another photoresist 51 is applied on the SiN film 26. An opening 51a of the photoresist 51 is formed over the opening of the SiN film 26 by photolithography. A source electrode 22 and a drain electrode 24 that contact the electron supply layer 16 through the opening in the SiN film 26 are formed using a vapor deposition method. Both the source electrode 22 and the drain electrode 24 have a Ti film and an Al film. The thickness of the Ti film is, for example, 30 nm, and the thickness of the Al film is, for example, 300 nm. The Ti film may be a Ta film. Metal 23 is deposited on photoresist 51 . Thereafter, by removing the photoresist 51, the metal 23 on the photoresist 51 is removed. As a result, a source electrode 22 and a drain electrode 24 in contact with the electron supply layer 16 are formed. For example, the source electrode 22, the drain electrode 24, and the electron supply layer 16 are alloyed by heat treatment at 400°C. When the temperature is 550° C. or higher, it further contributes to lowering the contact resistance.

Subsequently, as shown in FIG. 13(b), another photoresist 52 is applied on the laminated structure 20. An opening 52a is formed in the photoresist 52 by photolithography. An opening is formed in the SiN film 26 by RIE using the photoresist 52 as a mask. After that, the photoresist 52 is removed.

Subsequently, a photoresist is applied onto the laminated structure 20. An opening that will become a gate electrode pattern is formed in the photoresist by photolithography. Using a vapor deposition method, a gate electrode 28 is formed in contact with the cap layer 18, as shown in FIG. 14(a). The gate electrode 28 includes a Ni film and an Au film from the stacked structure 20 side. The thickness of the Ni film is, for example, 50 nm, and the thickness of the Au film is, for example, 400 nm. Various methods are used as the vapor deposition method, such as EB vapor deposition, sputter vapor deposition, and resistance heating vapor deposition. The metal deposited on the photoresist is removed along with the photoresist.

Subsequently, as shown in FIG. 14(b), an insulating film 30 is formed on the SiN film 26 by, for example, PECVD, and the gate electrode 28 is covered with this insulating film 30. The insulating film 30 is, for example, a SiN film with a thickness of 500 nm. An opening 30a is formed in the insulating film 30 by etching using buffered hydrofluoric acid, and the source electrode 22 and drain electrode 24 are exposed. HEMT1A is produced through the above steps.

As shown in FIG. 14(b), the fabricated HEMT 1A includes a substrate 10 and a stacked structure 20 provided on the substrate 10 and including a plurality of nitride semiconductor layers. The laminated structure 20 includes a nucleation layer 12, an electron transport layer 14, an electron supply layer 16, and a cap layer 18 formed in this order from the substrate 10 side. The nucleation layer 12, the electron transit layer 14, the electron supply layer 16, and the cap layer 18 are each made of a nitride semiconductor. The HEMT 1A further includes a SiN film 26. The SiN film 26 covers the surface of the stacked structure 20. Specifically, the SiN film 26 covers the surface of the cap layer 18. In the fabricated HEMT 1A, oxygen atoms exist as silicon oxynitride at the interface between the SiN film 26 and the cap layer 18 with an interfacial oxygen amount of 0.6×10 ¹⁵ atoms/cm ² or less. The SiN film 26 has a plurality of openings. In these openings, the stacked structure 20 (electron supply layer 16 or cap layer 18) is exposed from the SiN film 26.

Further, the HEMT 1A further includes a source electrode 22, a drain electrode 24, a gate electrode 28, and an insulating film 30. The source electrode 22 and the drain electrode 24 are arranged in order along the surface of the substrate 10. The source electrode 22, the drain electrode 24, and the gate electrode 28 each cover the opening in the SiN film 26. Further, the gate electrode 28 is provided between the source electrode 22 and the drain electrode 24 on the stacked structure 20. The insulating film 30 is a protective film that covers the gate electrode 28.

According to the method for manufacturing a semiconductor device described above, by forming the SiN film 26 by the low pressure CVD method using the method described in the first embodiment, the nitride semiconductor layer (cap layer 18) and the SiN film The amount of oxygen at the interface with 26 can be reduced.

Further, the HEMT 1A includes a substrate 10 and a plurality of nitride semiconductor layers (a nucleation layer 12, an electron transport layer 14, an electron supply layer 16, and a cap layer 18) each made of a nitride semiconductor. At the interface between the laminated structure 20 provided in the laminated structure 20, the SiN film 26 covering the surface of the laminated structure 20, and the SiN film 26 and the laminated structure 20, an amount of interfacial oxygen of 0.6×10 ¹⁵ atoms/cm ² or less exists. and an oxygen atom. According to this HEMT1A, since the amount of oxygen at the interface of oxygen atoms is reduced to 0.6×10 ¹⁵ atoms/cm ² or less, gate leakage current is significantly reduced, and deterioration of the electrical characteristics of the semiconductor device is suppressed. It will be done.

(Third embodiment)
Next, a method for forming the silicon nitride passivation film 3 shown in FIG. 1 using the film forming method according to the third embodiment will be described. FIG. 16 is a flowchart showing a method for forming a silicon nitride passivation film 3 according to the third embodiment. The film forming method according to the third embodiment differs from the film forming method according to the first embodiment in that step S7 is performed instead of step S2, and is similar to the film forming method according to the first embodiment in other respects. That is, in the third embodiment, the silicon nitride passivation film 3 is grown on the nitride semiconductor layer 5 by performing the step S7 after the step S1.

In step S7, first, steps S71 to S77 are sequentially performed. Note that steps S71 to S77 are the same as steps S21 to S27 in step S2, respectively. After the temperature in the reactor is stabilized at the second temperature in step S77, the pressure in the reactor is gradually changed and reduced to a first pressure of 30 Pa or less (step S78). Note that in step S78, the pressure inside the reactor is reduced without evacuation as performed in S47 of Comparative Example 1. Specifically, the flow rate of gas introduced into the reactor and the exhaust valve are controlled by an APC (Auto Pressure Controller). Thereby, the pressure in the reactor is adjusted to the first pressure, which is the pressure during film formation, while maintaining a desired gas atmosphere without reducing the pressure to an extremely low level. In one example, the first pressure is 20 Pa.

Then, steps S79 to S81 are performed in order. Steps S79 to S81 are the same as steps S29 to S31 in step S2, respectively. Through the above steps, silicon nitride passivation film 3 is formed on nitride semiconductor layer 5.

According to the method for forming the silicon nitride passivation film 3 according to the third embodiment described above, the same effects as the film forming method according to the first embodiment can be obtained. Specifically, since step S76 is performed, the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride passivation film can be reduced when forming the silicon nitride passivation film on the nitride semiconductor using the low pressure CVD method. .

Next, further effects obtained by the method for forming the silicon nitride passivation film 3 according to the third embodiment will be described in comparison with Comparative Example 1. FIG. 17 is a graph showing pressure changes in the reactor when growing a silicon nitride passivation film according to Comparative Example 1. In the graph of FIG. 17, the horizontal axis is time and the vertical axis is pressure. In Comparative Example 1, replacing the atmosphere in the reactor before film formation (step S47) involves changing from high pressure to low pressure in a high temperature atmosphere. Therefore, from the viewpoint of suppressing the loss of nitrogen atoms from the nitride semiconductor surface, in order to minimize the time required to reach a low pressure in a high temperature atmosphere, as shown in FIG. 17, after evacuation, the deposition gas has been introduced to adjust the pressure. However, since low-pressure CVD equipment uses a quartz member inside the reactor, and the equipment is large, degassing (residues attached to the inner walls of the reactor, etc.) may be released into the reactor again as the vacuum is drawn. There is also a concern that the interfacial oxygen concentration may increase.

On the other hand, in the third embodiment, in replacing the atmosphere in the reactor before film formation (step S28), evacuation is not performed as in Comparative Example 1, and the pressure at the time of film formation is The pressure inside the furnace is gradually changed (depressurized). FIG. 18 is a graph showing the pressure change in the reactor when growing the silicon nitride passivation film 3 according to the third embodiment. In the graph of FIG. 18, the horizontal axis is time and the vertical axis is pressure. In the third embodiment, the temperature inside the reactor is lowered in the temperature lowering step (step S77) immediately before step S78, and the pressure inside the reactor is lowered in step S78 under the control of a device such as an APC. The pressure is reduced. As a result, as shown in FIG. 18, the depressurization time can be kept to the same value as the atmosphere replacement time in Comparative Example 1, and an extreme drop in the equilibrium vapor pressure due to the vacuum atmosphere can be suppressed, so that the loss of nitrogen atoms can be reduced. As a result, while maintaining the surface of the nitride semiconductor layer 5 (GaN layer) with reduced surface roughness, degassing can be reduced and increase in oxygen concentration can be reduced.

Table 5 shows the results of film formation under different pressure conditions and comparison of the amount of interfacial oxygen when the film was formed under each condition. In Table 5, the evacuation time is 0 minutes for the case where the pressure was controlled as in the third embodiment (see FIG. 18), and the case where the pressure was controlled as in Comparative Example 1 (see FIG. 17). is shown as a vacuuming time of 1 minute to 10 minutes. Table 5 shows that the amount of interfacial oxygen can be reduced to 0.6×10 ¹⁵ atoms/cm ² or less by reducing the evacuation time to 5 minutes or less. Moreover, from Table 5, it was found that by maintaining the vacuum pressure at 5 Pa or more, the interfacial oxygen amount could be reduced to 0.6×10 ¹⁵ atoms/cm ² or less. In other words, in order to reduce the amount of interfacial oxygen to 0.6×10 ¹⁵ atoms/cm ² or less from the viewpoint of improving the electrical characteristics of the semiconductor device mentioned above, it is necessary to reduce the evacuation time to 5 minutes or less, or to It is important to maintain the pressure at 5 Pa or higher.

(Fourth embodiment)
Next, a method for forming the silicon nitride passivation film 3 shown in FIG. 1 using the film forming method according to the fourth embodiment will be described. FIG. 19 is a flowchart showing a method for forming a silicon nitride passivation film 3 according to the fourth embodiment. The film forming method according to the fourth embodiment differs from the film forming method according to the first embodiment in that step S9 is performed instead of step S2, and is similar to the film forming method according to the first embodiment in other respects. That is, in the fourth embodiment, silicon nitride passivation film 3 is grown on nitride semiconductor layer 5 by performing step S9 after step S1. FIG. 20 is a diagram showing the temperature inside the reactor and the procedure for supplying gas when growing the silicon nitride passivation film 3.

In step S9, first, steps S91 and S92 are performed in order. Note that steps S91 and S92 are the same as steps S21 and S22 in step S2, respectively. Next, vacuum purge and N ₂ purge in the reactor are repeated (cycle purge) to replace the atmospheric atmosphere with N ₂ atmosphere (step S93). After the final evacuation, the reactor is filled with a gas containing H ₂ to replace the atmosphere inside the reactor with an atmosphere containing H ₂ . Here, a mixed gas of N ₂ +H ₂ is filled into the reactor as a gas containing H ₂ , and the atmosphere inside the reactor is set to be an N ₂ +H ₂ atmosphere. In one example, the H ₂ partial pressure (H ₂ /(H ₂ +N ₂ )) is greater than or equal to 0.005 (ie, 0.5%). Note that the H ₂ partial pressure may be 5% or more.

After the cycle purge described above is completed, step S94 is performed. Step S94 is similar to step S24 in step S2. Then, while maintaining the pressure inside the reactor at the second pressure, the inside of the reactor is heated to a third temperature of 700° C. or higher (step S95, temperature raising step). This third temperature is higher than the film forming temperature (that is, the second temperature). The third temperature may be 20° C. or more higher than the second temperature. Further, the third temperature may be 650°C or higher or 900°C or lower. In one example, the third temperature is 800°C.

Subsequently, the inside of the reactor is maintained at a third temperature (here, 800° C.) and a second pressure (here, 3 kPa) for 3 minutes or more (step S96, cleaning step). In one example, the hold time is 10 minutes. Next, a film forming environment is prepared (temperature lowering step). Specifically, steps S97 and S98 described below are performed. First, the temperature inside the reactor is lowered to a second temperature for film formation (step S97). The second temperature is lower than the third temperature. In one example, the second temperature is 700°C. After the temperature inside the reactor stabilizes at the second temperature, NH ₃ gas is supplied into the reactor to switch the atmosphere inside the reactor from N ₂ + H ₂ atmosphere to NH ₃ atmosphere, and at the same time, the pressure inside the reactor is reduced. is adjusted to a third pressure of 100 Pa or less (step S98). This third pressure may be 20 Pa or less, or may be 10 Pa or more. In one example, the third pressure is 20Pa.

Then, steps S99 to S101 are performed in order. Steps S99 to S101 are the same as steps S29 to S31 in step S2, respectively. Through the above steps, silicon nitride passivation film 3 is formed on nitride semiconductor layer 5.

According to the method of forming the silicon nitride passivation film 3 according to the fourth embodiment described above, the same effects as the film forming method according to the first embodiment can be obtained. Specifically, since step S96 is performed, the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride passivation film can be reduced when forming the silicon nitride passivation film on the nitride semiconductor using the low pressure CVD method. .

Table 6 and FIG. 21 show the relationship between the H ₂ partial pressure of the atmospheric gas and the amount of interfacial oxygen. Table 6 compares the amount of interfacial oxygen (the amount of oxygen at the interface between nitride semiconductor layer 5 and silicon nitride passivation film 3) in each case by changing the H ₂ partial pressure from step S94 to step S98. The results are shown below. FIG. 21 is a graph plotting the results of Table 6. In the graph of FIG. 21, the horizontal axis is the H ₂ partial pressure of the atmospheric gas, and the vertical axis is the interfacial oxygen amount.

In obtaining the comparison results shown in Table 6, the temperature in step S92 (that is, the temperature when introducing the epitaxial wafer 9 into the reactor) was set to 400°C. Further, the atmosphere inside the reactor was set to be N ₂ +H ₂ atmosphere. The pressure inside the reactor in steps S94 to S95 was set to 3000 Pa. Further, in step S96, the inside of the reactor was cleaned by maintaining the temperature inside the reactor at 800° C. and the pressure inside the reactor at 3000 Pa for 10 minutes. Further, dichlorosilane was supplied into the reactor at a temperature of 700° C. and a pressure of 30 Pa to form the silicon nitride passivation film 3. Referring to Table 6, it can be seen that the amount of interfacial oxygen is sufficiently reduced when the H ₂ partial pressure of the atmospheric gas is 0.005 (0.5)% or more. It is also seen that the amount of interfacial oxygen is particularly reduced when the H ₂ partial pressure of the atmospheric gas is 0.05 (5%) or more.

In the comparison results shown in Table 6 above, by setting the atmospheric gas to H ₂ +N ₂ atmosphere, hydrogen (H ₂ ) is included in the oxide film on the surface of the epitaxial wafer 9, similar to when the atmospheric gas is NH ₃ atmosphere. It is thought that the oxygen atoms 11 that are removed exhibit a reducing action and are removed. It is also believed that this reduction action also removes the oxygen atoms 11 contained in the moisture and oxygen adhering to the interior of the reactor. In particular, when the pressure inside the reactor is relatively high (for example, 300 Pa or more), the amount of hydrogen generated is relatively large, so the amount of oxygen atoms 11 removed by the reduction action also increases.

Next, the results of forming a film by changing the pressure inside the reactor as a temperature raising condition and comparing the amount of interfacial oxygen in each case will be described. Table 7 and FIG. 22 show the relationship between the pressure inside the reactor and the amount of interfacial oxygen. Table 7 shows the results of forming a film by changing the pressure in the reactor from step S94 to step S98, and comparing the amount of interfacial oxygen in each case. FIG. 22 is a graph plotting the results of Table 7. In the graph of FIG. 22, the horizontal axis is the pressure inside the reactor, and the vertical axis is the interfacial oxygen amount. Referring to Table 7 and FIG. 22, it can be seen that the amount of interfacial oxygen is sufficiently reduced when the pressure inside the reactor is set to 3000 Pa or more. It is also seen that the amount of interfacial oxygen is particularly reduced when the pressure inside the reactor is set to 10,000 Pa or more.

Next, the results of forming a film while changing the cleaning time as the cleaning condition and comparing the amount of interfacial oxygen in each case will be described. Table 8 and FIG. 23 show the relationship between cleaning time and interfacial oxygen amount. Table 8 shows the results of forming a film by changing the cleaning time in step S96 and comparing the amount of interfacial oxygen in each case. FIG. 23 is a diagram plotting the results of Table 8. In the graph of FIG. 23, the horizontal axis is the cleaning time, and the vertical axis is the interfacial oxygen amount. Referring to Table 8 and FIG. 23, it can be seen that the amount of interfacial oxygen is reduced to some extent when the cleaning time is set to 2 minutes or more. It is also seen that the amount of interfacial oxygen is sufficiently reduced when the cleaning time is 5 minutes or more. Furthermore, it can be seen that the amount of interfacial oxygen is particularly reduced when the cleaning time is 10 minutes or longer.

Next, the results of forming a film while changing the atmospheric gas and cleaning temperature and comparing the amount of interfacial oxygen in each case will be described. Table 9 and FIG. 24 show the relationship between cleaning time and interfacial oxygen amount. Table 9 shows the results of forming films by changing the atmospheric gas and cleaning temperature in steps S26 and S96, and comparing the amount of interfacial oxygen in each case. FIG. 24 is a diagram plotting the results of Table 9. In the graph of FIG. 24, the horizontal axis is the cleaning temperature, and the vertical axis is the interfacial oxygen amount. Referring to Table 9 and FIG. 24, it can be seen that the lower cleaning temperature reduces the amount of interfacial oxygen when the atmospheric gas in the reactor is H ₂ +N ₂ than when it is NH ₃ .

As described above, when the atmospheric gas is NH ₃ , hydrogen (H ₂ ) is generated by thermal decomposition, and this hydrogen causes the oxygen atoms 11 contained in the oxide film on the surface of the epitaxial wafer 9 to exhibit a reducing action. It is thought that it will be removed. In other words, by thermally decomposing NH ₃ to generate H ₂ , an effect of reducing the amount of interfacial oxygen can be obtained. However, if the cleaning temperature is low, the amount of H ₂ generated when NH ₃ is thermally decomposed will be correspondingly small, so the above-mentioned reduction effect will be small. On the other hand, when the atmospheric gas is an atmosphere containing H ₂ , H ₂ is directly provided, so even when the atmospheric gas is NH ₃ , the cleaning temperature is lower than that when the atmospheric gas is NH ₃ . A reduction effect by H ₂ equivalent to that obtained can be obtained. In particular, when forming a film on a GaN layer which is a compound semiconductor, it is desirable that the film forming temperature be low from the viewpoint of nitrogen removal from the surface of the epitaxial wafer 9. Therefore, the method for forming silicon nitride passivation film 3 according to the fourth embodiment is more suitable when nitride semiconductor layer 5 is a GaN layer. Further, when the cleaning temperature is 700° C. in an H ₂ +N ₂ atmosphere, the amount of interfacial oxygen is sufficiently reduced even if the film forming temperature and the cleaning temperature are the same. Therefore, it can be seen that a cleaning process using NH ₃ gas at a temperature higher than the film forming temperature is not necessarily necessary.

Further, according to the method for forming the silicon nitride passivation film 3 according to the fourth embodiment, in steps S93 to S95, oxidation due to degas in the reactor is suppressed from a state where the temperature in the reactor is relatively low. At the same time, an oxidation-reduction effect is obtained on the surface of the epitaxial wafer 9. Therefore, when forming a silicon nitride passivation film on a nitride semiconductor using the low pressure CVD method, the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride passivation film can be further reduced.

The method for forming a silicon nitride passivation film, the method for manufacturing a semiconductor device, and the semiconductor device according to the present disclosure are not limited to the embodiments described above, and various other modifications are possible. For example, the embodiments described above may be combined with each other depending on the desired purpose and effect. Further, in the second embodiment, a HEMT is shown as an example of a semiconductor device, but the semiconductor device manufacturing method and semiconductor device according to the present disclosure can be applied to various nitride semiconductor devices other than the HEMT.

(Additional note)
As understood from the embodiments described above, this specification includes disclosure of the following aspects.

A method for forming a silicon nitride passivation film according to one embodiment of the present disclosure is a method for forming a silicon nitride passivation film in contact with a nitride semiconductor layer, the method comprising: forming a silicon nitride passivation film in a reactor containing a substrate including the nitride semiconductor layer. At least one of changing the atmosphere inside the reactor and changing the pressure inside the reactor is carried out while maintaining the pressure inside the reactor at 5 Pa or more while the temperature inside the reactor is 700° C. or higher. and a step of supplying dichlorosilane (SiH ₂ Cl ₂ ) into the reactor to form the silicon nitride passivation film.

A method for forming a silicon nitride passivation film according to one embodiment of the present disclosure is a method for forming a silicon nitride passivation film in contact with a nitride semiconductor layer, the method comprising: forming a silicon nitride passivation film in a reactor containing a substrate including the nitride semiconductor layer. Changing the atmosphere in the reactor and changing the pressure in the reactor while the temperature in the reactor is 700 ° C. or higher, the time for the pressure in the reactor to become 5 Pa or less is within 5 minutes. and a step of supplying dichlorosilane (SiH ₂ Cl ₂ ) into the reactor to form the silicon nitride passivation film.

1A...HEMT (semiconductor device), 3...Silicon nitride passivation film, 5...Nitride semiconductor layer, 7...Substrate, 9...Epitaxial wafer, 11...Oxygen atom, 10...Substrate, 12...Nucleation layer, 14...Electron transport layer, 16... electron supply layer, 18... cap layer, 20... stacked structure (semiconductor stacked structure, semiconductor stacked part), 22... source electrode, 23... metal, 24... drain electrode, 26... SiN film, 28... gate electrode , 30... Insulating film, 50, 51, 52... Photoresist.

Claims (11)

A method for forming a silicon nitride passivation film in contact with a nitride semiconductor layer, the method comprising:
replacing the atmosphere in the reactor containing the substrate containing the nitride semiconductor layer with an atmosphere containing NH ₃ or an atmosphere containing H ₂ ;
a temperature raising step of raising the temperature in the reactor to a first temperature;
a cleaning step of maintaining the temperature in the reactor at the first temperature and the atmosphere in the reactor in an atmosphere containing the NH ₃ or the H ₂ for 3 minutes or more;
a temperature lowering step of lowering the temperature in the reactor to a second temperature lower than the first temperature;
supplying dichlorosilane (SiH ₂ Cl ₂ ) into the reactor while the pressure in the reactor is set to a first pressure of 100 Pa or less to form the silicon nitride passivation film;
A method for forming a silicon nitride passivation film, including: 2. The method for forming a silicon nitride passivation film according to claim 1, wherein in the cleaning step, the pressure in the reactor is maintained at a second pressure that is higher than the first pressure. 3. The method for forming a silicon nitride passivation film according to claim 2, wherein the second pressure is 300 Pa or more. The second temperature is 700°C or higher,
The method for forming a silicon nitride passivation film according to claim 1, wherein the first temperature is at least 20° C. higher than the second temperature. The method for forming a silicon nitride passivation film according to any one of claims 1 to 4, wherein the first temperature is 750°C or higher and 900°C or lower. The method for forming a silicon nitride passivation film according to any one of claims 1 to 5, wherein in the temperature lowering step, the pressure in the reactor is reduced to the first pressure. A method for manufacturing a semiconductor device whose main constituent material is a nitride semiconductor, the method comprising:
growing a semiconductor stacked structure including the nitride semiconductor layer on the substrate;
forming a silicon nitride passivation film in contact with the semiconductor stacked structure using the method according to any one of claims 1 to 6;
forming an opening in the silicon nitride passivation film and forming an electrode in contact with the semiconductor stacked structure through the opening;
A method for manufacturing a semiconductor device, including: A method for forming a silicon nitride passivation film in contact with a nitride semiconductor layer, the method comprising:
replacing the atmosphere in the reactor containing the substrate containing the nitride semiconductor layer with an atmosphere containing _H2 ;
a temperature raising step of raising the temperature in the reactor to a first temperature;
maintaining the temperature in the reactor at the first temperature;
a temperature lowering step of lowering the temperature in the reactor to a second temperature that is lower than or the same as the first temperature;
supplying dichlorosilane (SiH ₂ Cl ₂ ) into the reactor while the pressure in the reactor is set to a first pressure of 100 Pa or less to form the silicon nitride passivation film;
A method for forming a silicon nitride passivation film, including: 9. The method for forming a silicon nitride passivation film according to claim 8 , wherein in the holding step, the H ₂ partial pressure in the reactor is set to 0.5% or more. The method for forming a silicon nitride passivation film according to claim 8 or 9 , wherein in the holding step, the pressure in the reactor is set to 300 Pa or more. Forming the silicon nitride passivation film according to any one of claims 8 to 10 , wherein in the holding step, the temperature in the reactor is held at the first temperature for 2 minutes or more. Method.

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