Methods for forming silicon nitride passivation films, methods for manufacturing semiconductor devices, and semiconductor devices.

Abstract

This application relates to a method for forming a silicon nitride passivation film, a method for manufacturing a semiconductor device, and a semiconductor device. The method for forming a silicon nitride passivation film on a nitride semiconductor layer includes the following steps: introducing a substrate containing the nitride semiconductor layer into a reactor; replacing the atmosphere in the reactor from air to an ammonia (NH3) atmosphere or a hydrogen (H2) atmosphere; raising the temperature in the reactor to a first temperature; maintaining the temperature in the reactor at the first temperature and maintaining the atmosphere in the reactor as an NH3 or H2 atmosphere for at least three minutes; lowering the temperature in the reactor to a second temperature below the first temperature; and forming the silicon nitride passivation film by supplying dichlorosilane (SiH2Cl2) into the reactor at a first pressure below 100 Pa.

Description

[0001] Cross-reference to related applications

[0002] The disclosures, including the description, drawings, and abstract, of Japanese Patent Application No. 2019-161282 filed on September 4, 2019, and Japanese Patent Application No. 2020-115056 filed on July 2, 2020, are incorporated herein by reference in their entirety. Background Technology

[0003] This disclosure relates to a method for forming a silicon nitride passivation film, a method for manufacturing a semiconductor device, and a semiconductor device.

[0004] Japanese Patent Application Publication No. 2018-181885 discloses a technology related to the production of high electron mobility transistors (HEMTs) as semiconductor devices using nitride semiconductors. The HEMT described in Japanese Patent Application Publication No. 2018-181885 comprises a nitride semiconductor layer on a substrate such as SiC (silicon carbide), and a silicon nitride passivation film formed on the surface of said nitride semiconductor layer. The silicon nitride passivation film is formed by a low-pressure chemical vapor deposition (CVD) method. Summary of the Invention

[0005] Recently, semiconductor devices using gallium nitride (GaN)-based semiconductors and nitride semiconductors have been developed. While insulating silicon compound films are used to protect and passivate the outermost surface of semiconductor devices, silicon nitride (SiN) films, which are also nitrides, are often used in the case of semiconductor devices using nitride semiconductors. When forming a silicon nitride film on a nitride semiconductor, plasma-based film deposition methods (plasma CVD, electron cyclotron resonance (ECR) sputtering, etc.) are used to form the film at relatively low temperatures. Therefore, plasma-induced damage is formed on the surface of the nitride semiconductor.

[0006] On the other hand, low-pressure (LP) CVD is used when forming silicon nitride films on silicon semiconductors. According to LP CVD, high-quality films can be formed by increasing the deposition temperature rather than decreasing the deposition pressure. Therefore, damage to the underlying silicon semiconductor caused by plasma is suppressed according to LP CVD. The inventors believe that if LP CVD can also be used in silicon nitride films on nitride semiconductors without causing damage to the surface of the nitride semiconductor due to plasma, then silicon nitride films can be appropriately formed.

[0007] When silicon nitride films are formed using LP CVD, an oxide film is formed by exposing the surface of the epitaxial wafer to the atmosphere during the resting period after epitaxial growth. Alternatively, it is conceivable that an oxide layer (e.g., gallium oxide, aluminum oxide, indium oxide, etc.) can be formed on the surface of the nitride semiconductor due to degassing in a reactor at high temperatures, in the presence of moisture and oxygen. Oxygen from the oxide film and oxide layer at the interface between the nitride semiconductor and the thus obtained silicon nitride film can degrade the operating characteristics of the semiconductor device.

[0008] Incidentally, in the mature and conventional LP CVD technology used to form silicon nitride films on silicon semiconductors, it is not necessary to consider oxygen at the interface between the silicon semiconductor and the silicon nitride film. This is because, due to the properties of silicon, the oxide film formed on the surface of the silicon semiconductor can be removed by a simple method when forming a silicon nitride film on the silicon semiconductor.

[0009] According to the inventors' findings, when the LP CVD method is applied to the deposition of silicon nitride films on nitride semiconductors, if the interfacial oxygen content at the interface between the nitride semiconductor and the silicon nitride film is not reduced, the oxygen at the interface will affect the operating characteristics of the semiconductor device. For example, near the interface where the electric field is concentrated at the gate edge, degassing can form a poor-quality oxide film, leading to increased leakage current and deteriorated electrical characteristics.

[0010] However, in conventional LP CVD, from the perspective of apparatus mechanism, it is difficult to completely eliminate desorbed gases (here, moisture and oxygen) in order to reduce the oxygen content at the interface. This is because degassing occurs from the epitaxial wafer itself, which serves as the film-forming sample. Furthermore, it is difficult to control all components of the film-forming reactor inside the apparatus to prevent them from being exposed to air.

[0011] This disclosure addresses these issues by providing a method for forming a silicon nitride passivation film that reduces oxygen levels at the interface between the semiconductor nitride and the passivation film when forming a silicon nitride passivation film on a nitride semiconductor using LP CVD. Furthermore, a method for manufacturing a semiconductor device using the silicon nitride passivation film is provided. Finally, a semiconductor device capable of suppressing electrical characteristic degradation is also provided.

[0012] To address the aforementioned problems, a method for forming a silicon nitride passivation film according to one embodiment includes a method for forming a silicon nitride passivation film on a nitride semiconductor layer, comprising the following steps: introducing a substrate containing a nitride semiconductor layer into a reactor; replacing the atmosphere in the reactor from air to an ammonia (NH3) atmosphere or a hydrogen (H2) atmosphere; raising the temperature in the reactor to a first temperature; maintaining the temperature in the reactor at the first temperature and maintaining the atmosphere in the reactor as an NH3 atmosphere or H2 atmosphere for more than three minutes; lowering the temperature in the reactor to a second temperature below the first temperature; and forming the silicon nitride passivation film by supplying dichlorosilane (SiH2Cl2) into the reactor at a first pressure below 100 Pa.

[0013] Furthermore, a semiconductor device according to another embodiment includes: a substrate; a semiconductor stack formed on the substrate, the semiconductor stack comprising a plurality of nitride semiconductor layers; a silicon nitride passivation film covering the surface of the semiconductor stack; and oxygen atoms present at the interface between the silicon nitride passivation film and the semiconductor stack, wherein the interfacial oxygen content of the oxygen atoms is 0.6 × 10⁻⁶. 15 atoms / cm 2 the following.

[0014] According to the method for forming a silicon nitride passivation film and the method for manufacturing a semiconductor device disclosed herein, when a silicon nitride passivation film is formed on a nitride semiconductor by LP CVD, the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride passivation film can be reduced. Furthermore, according to the semiconductor device disclosed herein, a decline in electrical properties can be suppressed. Attached Figure Description

[0015] The foregoing and other objects, aspects, and advantages will be better understood from the following detailed description of embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0016] Figure 1 This is a side view showing a silicon nitride passivation film formed by the film formation method according to the first embodiment.

[0017] Figure 2 This is a flowchart illustrating a method for forming a silicon nitride passivation film according to a first embodiment.

[0018] Figure 3 This is a diagram showing the control procedure for temperature and gas supply within the reactor during the formation of the silicon nitride passivation film.

[0019] Figure 4 This is a flowchart illustrating the method for forming a silicon nitride passivation film according to Comparative Example 1.

[0020] Figure 5 This is a diagram showing the control procedure of temperature and gas supply in the reactor when a silicon nitride passivation film is formed according to Comparative Example 1.

[0021] Figure 6 This is a flowchart illustrating the method for forming a silicon nitride passivation film according to Comparative Example 2.

[0022] Figure 7 This is a diagram showing the control procedure of temperature and gas supply in the reactor when a silicon nitride passivation film is formed according to Comparative Example 2.

[0023] Figure 8 This is a graph showing the relationship between heating conditions and interfacial oxygen content.

[0024] Figure 9 This is a graph showing the relationship between cleaning time and interfacial oxygen content.

[0025] Figure 10 This is a graph showing the relationship between cleaning pressure and interfacial oxygen content.

[0026] Figure 11 This is a graph showing the relationship between cleaning temperature and interfacial oxygen content.

[0027] Figure 12A This is a diagram illustrating the steps of the manufacturing method according to the second embodiment.

[0028] Figure 12B This is a diagram illustrating the steps of the manufacturing method according to the second embodiment.

[0029] Figure 12C This is a diagram illustrating the steps of the manufacturing method according to the second embodiment.

[0030] Figure 13A This is a diagram illustrating the steps of the manufacturing method according to the second embodiment.

[0031] Figure 13B This is a diagram illustrating the steps of the manufacturing method according to the second embodiment.

[0032] Figure 14A This is a diagram illustrating the steps of the manufacturing method according to the second embodiment.

[0033] Figure 14B This is a diagram illustrating the steps of the manufacturing method according to the second embodiment.

[0034] Figure 15 This is a graph showing the results of ToF-SIMS analysis of the silicon nitride passivation film after its formation.

[0035] Figure 16This is a flowchart illustrating a method for forming a silicon nitride passivation film according to a third embodiment.

[0036] Figure 17 This is a graph showing the pressure changes inside the reactor when a silicon nitride passivation film is formed according to Comparative Example 1.

[0037] Figure 18 This is a graph showing the pressure changes inside the reactor when a silicon nitride passivation film is formed according to the third embodiment.

[0038] Figure 19 This is a flowchart illustrating a method for forming a silicon nitride passivation film according to the fourth embodiment.

[0039] Figure 20 This is a diagram showing the control procedure for temperature and gas supply within the reactor during the formation of the silicon nitride passivation film.

[0040] Figure 21 This is a graph showing the relationship between H2 partial pressure and interfacial oxygen content.

[0041] Figure 22 This is a graph showing the relationship between pressure inside the reactor and interfacial oxygen content.

[0042] Figure 23 This is a graph showing the relationship between cleaning time and interfacial oxygen content.

[0043] Figure 24 This is a graph showing the relationship between cleaning time and interfacial oxygen content. Detailed Implementation

[0044] First Implementation Method

[0045] The following description, with reference to the accompanying drawings, describes a method for forming a silicon nitride passivation film, a method for manufacturing a semiconductor device, and specific examples of semiconductor devices according to one embodiment of the present disclosure. It should be noted that the present disclosure is not limited to these embodiments, but is indicated by the claims and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, like elements are denoted by like reference numerals in the description of the drawings, and repeated descriptions are omitted.

[0046] Figure 1 This is a side view showing a silicon nitride passivation film formed by a film formation method according to a first embodiment of the present disclosure. The silicon nitride passivation film 3 on the surface of the nitride semiconductor layer 5 is formed by LP CVD. The nitride semiconductor layer 5, such as a GaN layer, is a layer formed on a substrate 7, such as SiC. The nitride semiconductor layer 5 and the substrate 7 constitute an epitaxial wafer 9. Oxygen atoms 11 are present at the interface between the epitaxial wafer 9 and the silicon nitride passivation film 3. In this specification, atoms / cm² are used. 2 (per cm)2 The amount of oxygen at the interface (hereinafter referred to as interfacial oxygen content) is expressed in units of the number of atoms. Incidentally, based on the distribution of oxygen concentration in the depth direction analyzed by SIMS, the interfacial oxygen content is a value used to assess the amount of oxygen at the interface between the epitaxial wafer 9 and the silicon nitride passivation film 3. Specifically, SIMS analysis is performed after a series of processes for forming the silicon nitride passivation film 3 using LP CVD. Figure 1 In the interface between the epitaxial wafer 9 and the silicon nitride passivation film 3 shown, oxygen atoms 11, for example, are distributed at a density of 0.6 × 10⁻⁶. 15 atoms / cm 2 The following interfacial oxygen content is present.

[0047] Figure 2 This is a flowchart illustrating the method for forming a silicon nitride passivation film 3 according to this embodiment. First, a nitride semiconductor layer 5 is formed on a substrate 7 using MOCVD (metal-organic chemical vapor deposition) to fabricate an epitaxial wafer 9 (step S1). Next, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5 using LP CVD (step S2). Details of step S2 are shown in [reference needed]. Figure 2 and Figure 3 Describe it. Figure 3 This is a diagram showing the control procedure of temperature and gas supply in the reactor when the silicon nitride passivation film 3 is formed.

[0048] First, in step S21, the temperature inside the reactor is set to below 500°C. This temperature can be below 400°C or below 300°C. Furthermore, this temperature can be equal to or higher than room temperature (25°C). In one embodiment, the temperature set in step S21 is 400°C. After stabilizing the temperature inside the reactor at the set temperature, an epitaxial wafer 9 containing the nitride semiconductor layer 5 is placed in a carrier in an atmospheric atmosphere and introduced into the reactor (step S22). Next, the atmosphere inside the reactor is replaced with an atmosphere containing NH3 by repeatedly performing vacuum purging and NH3 purging (circular purging) in the reactor (step S23). In one embodiment, it is replaced with an NH3 atmosphere. Incidentally, in step S23, the atmosphere inside the reactor can be replaced with a mixed atmosphere of NH3 and N2, wherein the partial pressure of NH3 is 0.1 or higher.

[0049] After the cyclic purging is completed, the pressure inside the reactor is changed to a second pressure (the first pressure described later) that is higher than the film-forming pressure (step S24). This second pressure can be greater than or equal to 300 Pa, greater than or equal to 5 kPa, or greater than or equal to 10 kPa. Furthermore, this second pressure can be equal to or lower than atmospheric pressure (100 kPa). In one embodiment, the second pressure is 3 kPa. Then, while maintaining the pressure inside the reactor at the second pressure, the temperature inside the reactor is raised to a first temperature higher than 700°C (step S25, heating step). The first temperature is higher than the film-forming temperature (the second temperature (described later)). The first temperature can be at least 20°C higher than the second temperature. Furthermore, the first temperature can be above 800°C and below 900°C. In one example, the first temperature is 800°C.

[0050] Subsequently, the temperature inside the reactor is maintained at a first temperature (here, 800°C), an NH3-containing atmosphere (here, NH3 atmosphere), and a second pressure (here, 3 kPa) for at least 3 minutes (step S26, cleaning step; holding step). In one embodiment, the holding time is 10 minutes. It should be noted that the holding time can be at least 2 minutes. The interior of the reactor is cleaned by performing step S26. Next, the film-forming environment is prepared, i.e., the cooling step. Specifically, the following steps S27 and S28 are performed. First, in step S27, the temperature inside the reactor is reduced to a second temperature for film formation. However, the second temperature is a temperature lower than the first temperature. The second temperature can be 700°C or higher. In one embodiment, the second temperature is 780°C. After stabilizing the temperature inside the reactor to the second temperature, in step S28, the pressure inside the reactor is reduced to a first pressure below 100 Pa. This first pressure can be below 40 Pa and above 10 Pa. In one embodiment, the first pressure is 20 Pa.

[0051] Then, dichlorosilane (SiH2Cl2) is supplied to the reactor at a first pressure (20 Pa here) to form a silicon nitride passivation film 3 (step S29). In step S29, the NH3 flow rate and the dichlorosilane flow rate can be substantially equal to each other. For example, both the NH3 flow rate and the dichlorosilane flow rate are 100 sccm. 1 sccm means 1 cm per minute at 0°C and 1 atm. 3 Traffic.

[0052] After the formation of the silicon nitride passivation film 3 is completed, the supply of raw material gas is stopped, and the temperature inside the reactor is lowered to a predetermined temperature, such as 700°C. Then, in order to remove the chlorine gas from the reactor, nitrogen gas is circulated and purged, and the chlorine gas is diluted to the detection limit (step S30). Subsequently, the epitaxial wafer 9 is removed from the reactor (step S31). Through the above steps, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5.

[0053] Compared with Comparative Example 1 and Comparative Example 2, the effects obtained by the silicon nitride passivation film formation method according to this embodiment described above are described. First, Comparative Example 1 is described. Figure 4 This is a flowchart illustrating the method for forming a silicon nitride passivation film according to Comparative Example 1. Figure 5 This diagram illustrates the control procedure for temperature and gas supply within the reactor during the formation of a silicon nitride passivation film according to Comparative Example 1. In Comparative Example 1, after step S1, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5 using the LP CVD method (step S4). Specifically, firstly, the temperature within the reactor is set to 700°C (step S41). After the temperature within the reactor stabilizes at 700°C, the epitaxial wafer 9 containing the nitride semiconductor layer 5 is placed in a carrier in an atmospheric atmosphere and introduced into the reactor (step S42). Next, the air atmosphere within the reactor is replaced with an N2 atmosphere by repeatedly performing vacuum purging and N2 purging within the reactor (step S43).

[0054] Subsequently, in step S44, the pressure inside the reactor is reduced to, for example, 20 Pa, which is equal to the film-forming pressure. Then, while maintaining the pressure inside the reactor, the temperature inside the reactor is increased to the film-forming temperature (e.g., 800°C) (step S45). In step S46, the temperature inside the reactor is allowed to stabilize. In step S47, when the temperature inside the reactor stabilizes, in order to change the atmosphere inside the reactor from an N2 atmosphere to an NH3 atmosphere as the raw material gas, the atmosphere is made into a vacuum with extremely low pressure, and the NH3 gas is purged. At this time, the pressure inside the reactor is set to the film-forming pressure of 20 Pa. The flow rate of NH3 is 100 sccm. Then, in step S48, when the pressure of the NH3 atmosphere stabilizes, dichlorosilane as the silicon-based raw material gas is supplied to the reactor to form a silicon nitride passivation film 3. The flow rate of dichlorosilane is 100 sccm.

[0055] After the formation of the silicon nitride passivation film 3 is completed, the supply of raw material gas is stopped, and the temperature inside the reactor is lowered to a predetermined temperature, such as 700°C. Then, in order to remove the chlorine gas from the reactor, nitrogen gas is circulated and purged, and the chlorine gas is diluted to the detection limit (step S49). Subsequently, in step S50, the epitaxial wafer 9 is removed from the reactor. Through the above steps, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5.

[0056] Next, we will describe Comparative Example 2. Figure 6 This is a flowchart illustrating the method for forming a silicon nitride passivation film according to Comparative Example 2. Figure 7 This diagram illustrates the control procedure for temperature and gas supply within the reactor during the formation of the silicon nitride passivation film according to Comparative Example 2. In Comparative Example 2, after step S1, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5 using the LP CVD method (step S6). Specifically, firstly, in step S61, the temperature within the reactor is set to 400°C. After the temperature within the reactor stabilizes at 400°C, the epitaxial wafer 9 containing the nitride semiconductor layer 5 is placed in a carrier in an atmospheric atmosphere and introduced into the reactor (step S62). Next, the atmosphere within the reactor is replaced with an NH3 atmosphere by repeatedly performing vacuum purging and NH3 purging within the reactor (step S63).

[0057] Subsequently, in step S64, the pressure inside the reactor is changed to 3 kPa. Then, while maintaining the same pressure inside the reactor, the temperature inside the reactor is increased to the film formation temperature (e.g., 800°C) (step S65). Then, when the temperature inside the reactor stabilizes, in step S66, the pressure inside the reactor is reduced to the film formation pressure of 20 Pa. Then, in step S67, dichlorosilane, as a silicon-based raw material gas, is supplied to the reactor to form a silicon nitride passivation film 3. The dichlorosilane flow rate is 100 sccm.

[0058] After the formation of the silicon nitride passivation film 3 is completed, the supply of raw material gas is stopped, and the temperature inside the reactor is lowered to a predetermined temperature, such as 700°C. Then, in order to remove the chlorine gas from the reactor, nitrogen gas is purged and diluted to the detection limit (step S68). Subsequently, in step S69, the epitaxial wafer 9 is removed from the reactor. Through the above steps, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5.

[0059] exist Figure 2 and Figure 3 In step S25 of this embodiment shown, with Figure 4 and Figure 5 The step S45 of Comparative Example 1 shown is different; the temperature is increased while the atmosphere inside the reactor is an atmosphere containing NH3 and the pressure inside the reactor is a second pressure greater than the first pressure. Table 1 and Figure 8 The relationship between heating conditions and interfacial oxygen content is shown in Table 1. Table 1 shows the relationship between heating conditions (type and pressure of the atmosphere gas) and interfacial oxygen content (amount of oxygen at the interface between the nitride semiconductor layer 5 and the silicon nitride passivation film 3). Figure 8 This is a graph showing the results of Table 1. Figure 8In the graph, the horizontal axis represents the pressure required for heating, and the vertical axis represents the interfacial oxygen content. Incidentally, the interfacial oxygen content shown below was obtained through SIMS analysis of the epitaxial wafer 9 removed from the reactor.

[0060] Table 1

[0061]

[0062] To obtain the results in Table 1, the temperature was set to 400°C when the epitaxial wafer 9 was introduced into the reactor. Furthermore, after raising the temperature inside the reactor under the conditions in Table 1, the interior of the reactor was cleaned by maintaining the temperature at 800°C, maintaining the atmosphere inside the reactor as N2, and maintaining the pressure inside the reactor at 30 Pa for 10 minutes. Additionally, dichlorosilane was supplied to the reactor at 700°C, and a silicon nitride passivation film 3 was formed in the reactor at a pressure of 30 Pa.

[0063] Referring to Table 1, it can be seen that the interfacial oxygen content is lower in the NH3 atmosphere compared to the N2 atmosphere inside the reactor. Furthermore, it should be understood that the interfacial oxygen content decreases significantly when the pressure inside the reactor is above 300 Pa, especially above 3000 Pa. To obtain the following results (Tables 2 to 4), the atmosphere inside the reactor was set to NH3, and the pressure inside the reactor was set to 3000 Pa as a condition for increasing the temperature inside the reactor.

[0064] In the results in Table 1 above, it is believed that an NH3 atmosphere was used as the heating condition, and hydrogen (H2) was generated through thermal decomposition. It is also believed that oxygen atoms 11 contained in the oxide film on the surface of the epitaxial wafer 9 were removed by the reduction of hydrogen. Furthermore, it is believed that oxygen atoms 11 contained in moisture and oxygen adhering to the reactor were also removed through this reduction. In particular, when the pressure inside the reactor is relatively high (e.g., when the pressure is above 300 Pa), the amount of oxygen atoms 11 removed by the reduction also increases due to the relatively large amount of hydrogen produced.

[0065] exist Figure 2 and Figure 3 In step S26 of this embodiment shown, with Figure 6 and Figure 7Unlike Comparative Example 2 shown, cleaning was performed in a reactor. Specifically, the temperature inside the reactor was maintained at a first temperature higher than the film-forming temperature (second temperature), the atmosphere inside the reactor was maintained as an atmosphere containing NH3, and the pressure inside the reactor was maintained at a second pressure higher than the pressure at the time of film formation (first pressure) for at least three minutes. Hereinafter, the retention time (hereinafter referred to as "cleaning time"), the type of atmosphere gas inside the reactor, the temperature inside the reactor (hereinafter referred to as "cleaning temperature"), and the pressure inside the reactor (hereinafter referred to as "cleaning pressure") are referred to as cleaning conditions. Table 2 and Figure 9 The relationship between cleaning time and interfacial oxygen content is shown. Table 2 shows the results of interfacial oxygen content when the cleaning time, as a cleaning condition, is changed. Figure 9 This is a graph showing the results of Table 2. Figure 9 In the graph, the horizontal axis represents cleaning time, and the vertical axis represents interfacial oxygen content. When no cleaning is performed, the time is set to 0 minutes in Table 2. Additionally, cleaning conditions other than cleaning time are used. Specifically, an NH3 atmosphere is used at 800°C and 300 Pa.

[0066] Table 2

[0067]

[0068] To obtain the results shown in Table 2, the conditions other than the cleaning conditions were as follows. First, the temperature at which the epitaxial wafer 9 was introduced into the reactor was set to 400°C. As described above, to increase the temperature in the reactor, the atmosphere inside the reactor was set to an NH3 atmosphere, and the pressure inside the reactor was set to 3000 Pa. A silicon nitride passivation film 3 was formed by supplying dichlorosilane into the reactor. The film formation temperature was lower than the cleaning temperature. And the film formation pressure in the reactor was 30 Pa. When the results shown below were obtained, all conditions except the cleaning conditions were the same.

[0069] Refer to Table 2 and Figure 9 As can be seen, the interfacial oxygen content decreases when the reactor is cleaned, compared to when no cleaning is performed (i.e., when the time in Table 2 is 0 minutes). Furthermore, the interfacial oxygen content decreases significantly when cleaning is performed for more than 3 minutes, and further decreases are observed when cleaning time exceeds 10 minutes.

[0070] Next, the results of interfacial oxygen content are described for cases where the reactor is cleaned for 10 minutes and any cleaning conditions other than the cleaning time are changed to form a film, and for films formed under each cleaning condition. Table 3 shows the results of interfacial oxygen content when films are formed under various cleaning conditions by changing the type of atmosphere gas and the cleaning pressure, which are the cleaning conditions. Figure 10This is a graph plotting the results of Table 3, showing the relationship between cleaning pressure and interfacial oxygen content. Figure 10 In the graph, the horizontal axis represents cleaning pressure, and the vertical axis represents interfacial oxygen content. Here, the temperature used as the cleaning condition is set to a common value. Specifically, the temperature is 800°C. Refer to Table 3 and... Figure 10 It can be seen that the interfacial oxygen content is further reduced when the atmosphere gas in the reactor is NH3 compared to when the atmosphere gas in the reactor is N2. Furthermore, it should be understood that the interfacial oxygen content is significantly reduced when the atmosphere gas is NH3 and the pressure is above 300 Pa.

[0071] Table 3

[0072]

[0073] Table 4 shows the results of interfacial oxygen content under various cleaning conditions by changing the temperature as a cleaning condition. Figure 11 This is a graph plotting the results of Table 4, showing the relationship between cleaning temperature and interfacial oxygen content. Figure 11 In the graph, the horizontal axis represents the cleaning temperature, and the vertical axis represents the interfacial oxygen content. Here, the cleaning conditions other than temperature are used similarly. Specifically, the atmosphere is NH3 and the pressure is 3000 Pa. Referring to Table 4, it can be seen that the interfacial oxygen content decreases significantly when the temperature is above 750°C.

[0074] Table 4

[0075]

[0076] Based on the above results, it was found that when the interior of the reactor was cleaned by maintaining the temperature above 750°C, the atmosphere inside the reactor as NH3, and the pressure inside the reactor above 300Pa for 3 minutes, the interfacial oxygen content was significantly reduced.

[0077] As in this embodiment, by exposing the epitaxial wafer 9 to an atmosphere with a temperature higher than the film formation temperature and an atmosphere containing NH3 for more than 3 minutes before film formation, the following effects (1) and (2) can be expected.

[0078] (1) The oxygen atoms 11 contained in the oxide film on the surface of the nitride semiconductor layer 5 were removed by reduction.

[0079] (2) This causes the oxygen atoms 11 contained in the moisture and oxygen in the desorbed gas attached to the reactor to be reduced, and inhibits the formation of new oxide films in the reactor.

[0080] Due to the synergistic effect of (1) and (2) above, the interfacial oxygen content is sufficiently reduced and the gate leakage current is reduced. Therefore, the electrical characteristics of the semiconductor device can be improved. In particular, when the interfacial oxygen content is 0.6 × 10⁻⁶... 15 atoms / cm 2 The following applies when the interfacial oxygen content is 0.6 × 10⁻⁶ under atmospheric pressure. 15 atoms / cm 2 When the following conditions are met, the gate leakage current is significantly reduced, and the electrical characteristics of the semiconductor device are prevented from deteriorating.

[0081] Incidentally, when the surface of an epitaxial wafer is exposed to the atmosphere, in addition to oxygen as mentioned above, carbon, fluorine, etc. (hereinafter referred to as "carbon, etc.") can be introduced into the epitaxial wafer. These carbons act as impurities, forming deep acceptor levels in nitride semiconductors. Therefore, when carbon, etc., is incorporated into nitride semiconductors, for example, an increase in leakage current in the semiconductor device is directly related to problems such as degradation of collapse characteristics.

[0082] Here, according to the inventors, fluorine is hardly considered to be present in large quantities in the reactor compared to carbon and oxygen. However, after epitaxial growth and before film formation, a cleaning process using IPA (isopropanol) is performed, and the nitride semiconductor layer 5 (e.g., a GaN layer) formed by LP CVD contains fluorine on its surface, with a detected amount of 1 × 10⁻⁶. 11 atoms / cm 2 The following should be noted: This value is four orders of magnitude smaller than the amount of oxygen atoms (11). Since the processes performed in the reactor include, for example, RCA cleaning using solutions based on hydrofluoric acid (HF), it is assumed that some fluorine remains in the atmosphere and adheres to the surface of the epitaxial wafer 9. When operating in the reactor, it is difficult to avoid fluorine adhering to the surface of the epitaxial wafer 9.

[0083] On the other hand, through the synergistic effect of (1) and (2), carbon and fluorine on the surface of the epitaxial wafer 9 are also removed. This is because H2 is generated by an atmosphere containing NH3. Specifically, residual fluorine (F) reacts with hydrogen atoms (H) in the atmospheric gas to form hydrofluoric acid (HF) and evaporates. Carbon (C) reacts with hydrogen atoms (H) in the atmospheric gas to form methane (CH4) and evaporates. Therefore, by removing carbon and fluorine at the interface between the nitride semiconductor layer 5 and the silicon nitride passivation film 3, the problems caused by residual impurities on the surface of the epitaxial wafer 9 as described above are reduced. As a result, the gate leakage current is reduced, and the electrical characteristics of the semiconductor device can be improved.

[0084] Second Implementation Method

[0085] Next, as a second embodiment including the method of forming a silicon nitride passivation film 3 according to the first embodiment, a method for manufacturing a semiconductor device comprising a nitride semiconductor as the main constituent material will be described. Figures 12A to 14B This diagram illustrates the steps of the manufacturing method according to this embodiment. This embodiment exemplifies GaN-HEMT as a semiconductor device.

[0086] First, such as Figure 12A As shown, a stacked structure 20 (semiconductor stacked structure, semiconductor stacked portion) comprising multiple nitride semiconductor layers is formed on a substrate 10 using MOCVD. The substrate 10 is, for example, a SiC substrate having a (0001) main surface, and the stacking direction of the stacked 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 capping layer 18 formed sequentially from the substrate 10 side. The nucleation layer 12 is, for example, an AlN layer with a thickness of tens of nm. The electron transport 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 capping layer 18 is, for example, an n-type GaN layer with a thickness of 5 nm.

[0087] At this point, an oxygen film forms on the surface of the capping layer 18 by exposure to the atmosphere. When this oxygen film is studied by SIMS analysis, the oxygen atoms have a density of approximately 0.6 × 10⁻⁶. 15 atoms / cm 2 The interfacial oxygen content is present.

[0088] Next, as Figure 12B As shown, a silicon nitride passivation film (SiN film) 26 is formed on the upper surface of the stacked structure 20 by LP CVD. At this time, the film formation method of the silicon nitride passivation film 3 according to the first embodiment is applied. The film formation temperature is, for example, 800°C, and the film is formed using NH3 gas and dichlorosilane (SiH2Cl2) gas as feed gases. At this time, when the oxide film present at the interface between the SiN film 26 and the stacked structure 20 is analyzed by ToF-SIMS (Time-of-Flight Second Ion Mass Spectrometry), oxygen atoms are present as silicon oxynitride.

[0089] Specifically, the surface of the epitaxially grown nitride semiconductor is oxidized due to atmospheric exposure. However, after the SiN film 26 is formed, the oxygen in the oxynitride semiconductor migrates to the silicon nitride side and becomes silicon oxynitride. Figure 15 This is a graph showing the results of ToF-SIMS analysis after the formation of the silicon nitride passivation film. Figure 15 In the graph, the horizontal axis represents depth and the vertical axis represents secondary ion intensity. However, the left vertical axis is an order of magnitude larger than the right vertical axis. Figure 15The diagram shows that after the formation of the silicon nitride passivation film, gallium oxide on gallium nitride can be transformed into silicon oxynitride.

[0090] It should be noted that ToF-SIMS analysis is an analytical method used to analyze the mass of secondary ions emitted by irradiation with a pulsed beam, which serves as a primary ion beam. In ToF-SIMS analysis, a small ion beam dose (1 × 10⁻⁶) can be used. 12 cm -2 The following describes the detection of information about elements and molecules present on the outermost surface of a sample. Examples of primary ion beams used in ToF-SIMS analysis include ion beams with relatively heavy masses (Bi, Au, etc.) and Ga ion beams, which can be easily fabricated into thin beams.

[0091] Next, as Figure 12C As shown, photoresist 50 is coated on SiN film 26. An opening 50a is formed in the photoresist 50 using photolithography. An opening is then formed in SiN film 26 and capping layer 18 using photoresist 50 as a mask via reactive ion etching (RIE). Subsequently, the photoresist 50 is removed.

[0092] Next, as Figure 13A As shown, another photoresist 51 is applied to the SiN film 26. An opening 51a of the photoresist 51 is formed in the opening of the SiN film 26 using photolithography. A source electrode 22 and a drain electrode 24 are formed on the electron supply layer 16 through the opening of the SiN film 26 using vapor deposition. Both the source electrode 22 and the drain electrode 24 have Ti and Al films. 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 can be a Ta film. Metal 23 is deposited on the photoresist 51. Subsequently, the metal 23 on the photoresist 51 is removed by removing the photoresist 51. Thus, the source electrode 22 and the drain electrode 24 in contact with the electron supply layer 16 are formed. For example, the source electrode 22 and the drain electrode 24 are alloyed with the electron supply layer 16 by heat treatment at 400°C. If the temperature is above 550°C, the contact resistance is further reduced to a low resistance.

[0093] Subsequently, as Figure 13B As shown, another photoresist 52 is applied to the multilayer structure 20. An opening 52a is formed in the photoresist 52 using photolithography. Using the photoresist 52 as a mask, an opening is formed in the SiN film 26 via RIE. Subsequently, the photoresist 52 is removed.

[0094] Subsequently, photoresist is applied onto the stacked structure 20. Openings serving as gate electrode patterns are formed in the photoresist using photolithography. This is achieved through deposition methods such as… Figure 14AAs shown, a gate electrode 28 is formed on the capping layer 18. The gate electrode 28 has a Ni film and an Au film from the side of the stacked structure 20. The thickness of the Ni film is, for example, 50 nm, and the thickness of the Au film is, for example, 400 nm. As an evaporation method, various methods such as EB evaporation, sputtering evaporation, and resistance heating evaporation are used. The metal deposited on the photoresist is removed together with the photoresist.

[0095] Subsequently, as Figure 14B As shown, an insulating film 30 is formed on the SiN film 26 by, for example, PE CVD (plasma-enhanced chemical vapor deposition), and the insulating film 30 covers the gate electrode 28. 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 with buffered hydrofluoric acid to expose the source electrode 22 and the drain electrode 24. Through the above steps, HEMT 1A is fabricated.

[0096] like Figure 14B As shown, the fabricated HEMT 1A includes a substrate 10 and a stacked structure 20 disposed on the substrate 10, the stacked structure 20 including multiple nitride semiconductor layers. The stacked structure 20 includes a nucleation layer 12, an electron transport layer 14, an electron supply layer 16, and a capping layer 18 sequentially formed from the substrate 10 side. The nucleation layer 12, electron transport layer 14, electron supply layer 16, and capping layer 18 are each composed of nitride semiconductors. The HEMT 1A also 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 capping layer 18. In the fabricated HEMT 1A, the interface between the SiN film 26 and the capping layer 18 contains oxygen atoms present as silicon oxynitride, and the interface oxygen content is 0.6 × 10⁻⁶. 15 atoms / cm 2 The SiN film 26 has multiple openings. In these openings, the stacked structure 20 from the SiN film 26 (electron supply layer 16 or capping layer 18) is exposed.

[0097] HEMT 1A also includes a source electrode 22, a drain electrode 24, a gate electrode 28, and an insulating film 30. The source electrode 22 and drain electrode 24 are arranged sequentially along the plane of the substrate 10. The source electrode 22, drain electrode 24, and gate electrode 28 each cover an opening in the SiN film 26. Furthermore, the gate electrode 28 is disposed between the source electrode 22 and the drain electrode 24 on the stacked structure 20. The insulating film 30 is a protective film covering the gate electrode 28.

[0098] According to the semiconductor device manufacturing method described above, similar to the method described in the first embodiment, by forming the SiN film 26 using the LP CVD method, the amount of oxygen at the interface between the nitride semiconductor layer (capping layer 18) and the SiN film 26 can be reduced.

[0099] Furthermore, HEMT 1A includes a substrate 10 and a stacked structure 20 disposed on the substrate 10. The stacked structure 20 includes multiple nitride semiconductor layers (nucleation layer 12, electron transport layer 14, electron supply layer 16, and capping layer 18), each composed of a nitride semiconductor. Additionally, HEMT 1A includes a SiN film 26 covering the surface of the stacked structure 20, and an interfacial oxygen content of 0.6 × 10⁻⁶ at the interface between the SiN film 26 and the stacked structure 20. 15 atoms / cm 2 The following oxygen atoms. According to HEMT 1A, due to the reduction of interfacial oxygen content to 0.6 × 10⁻⁶. 15 atoms / cm 2 Therefore, the gate leakage current is significantly reduced, and the degradation of the electrical characteristics of the semiconductor device is suppressed.

[0100] Third Implementation Method

[0101] Next, the film-forming method according to the third embodiment will be described. Figure 1 The method for the silicon nitride passivation film 3 shown. Figure 16 This is a flowchart illustrating a method for forming a silicon nitride passivation film 3 according to a third embodiment. The film formation method according to the third embodiment differs from the film formation method according to the first embodiment in that step S7 is performed instead of step S2, and is otherwise identical to the film formation method according to the first embodiment. That is, in the third embodiment, after step S1, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5 by performing step S7.

[0102] In step S7, steps S71 to S77 are executed sequentially. Steps S71 to S77 are identical to steps S21 to S27 in step S2. After stabilizing the temperature inside the reactor to a second temperature in step S77, the pressure inside the reactor is gradually changed in step S78 to reduce it to a first pressure below 30 Pa. In step S78, as in S47 of Comparative Example 1, the pressure inside the reactor is reduced without evacuation. Specifically, the flow rate of the input gas into the reactor and the exhaust valve are controlled by an APC (Automatic Pressure Controller). As a result, the pressure inside the reactor is adjusted to a first pressure, which is the pressure required for film formation, while maintaining the desired gas atmosphere without making the pressure extremely low. In one embodiment, the first pressure is 20 Pa.

[0103] Then, steps S79 to S81 are executed sequentially. Steps S79 to S81 are the same as steps S29 to S31 in step S2. Through the above steps, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5.

[0104] The method for forming the silicon nitride passivation film 3 according to the third embodiment described above can achieve the same effect as the method for forming the silicon nitride passivation film 3 according to the first embodiment. Specifically, when the silicon nitride passivation film is formed on the nitride semiconductor by LP CVD, the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride passivation film can be reduced due to step S76.

[0105] Next, compared with Comparative Example 1, the further effects obtained by the method for forming the silicon nitride passivation film 3 according to the third embodiment will be described. Figure 17 This is a graph showing the pressure changes inside the reactor when a silicon nitride passivation film was formed according to Comparative Example 1. Figure 17 In the graph, the horizontal axis represents time, and the vertical axis represents pressure. In Comparative Example 1, during the atmosphere in the displacement reactor before film formation, the high-temperature atmosphere was accompanied by a change from high pressure to low pressure (step S47). Therefore, from the viewpoint of suppressing the extraction of nitrogen atoms from the nitride semiconductor surface, minimizing the time spent maintaining the high-temperature atmosphere at low pressure seems important. Figure 17 As shown, to minimize the time required to maintain the high-temperature atmosphere, the reactor is first evacuated and the feed gas is introduced. The pressure is then regulated. However, LP CVD equipment uses quartz components in the reactor, and due to the large size of the equipment, degassing (the re-release of residues adhering to the inner wall of the reactor into the reactor) occurs due to the evacuation, raising concerns about a potential increase in interfacial oxygen content.

[0106] On the other hand, in the third embodiment, when the atmosphere inside the reaction furnace is replaced before film formation (step S28), the pressure inside the furnace is gradually changed (reduced) to the pressure at the time of film formation, instead of being evacuated as in Comparative Example 1. Figure 18 This is a graph showing the pressure changes inside the reactor when the silicon nitride passivation film 3 is formed according to the third embodiment. Figure 18 In the graph, the horizontal axis represents time, and the vertical axis represents pressure. In the third embodiment, the temperature inside the reactor is reduced in the cooling step (step S77) immediately preceding step S78, and the pressure inside the reactor is reduced in step S78 under the control of a device such as an APC. Therefore, as Figure 18 As shown, the decompression time is comparable to that of Comparative Example 1, and the extreme drop in equilibrium vapor pressure due to the vacuum atmosphere is suppressed, thereby suppressing the extraction of nitrogen atoms. As a result, while suppressing the surface roughness of the nitride semiconductor layer 5 (GaN layer), the increase in oxygen concentration can be suppressed by reducing degassing.

[0107] Table 5 shows the results of interfacial oxygen content during film formation under different pressure conditions. Table 5 also shows that, in the pressure control of the third embodiment, the time used for evacuation was 0 minutes (the atmosphere inside the reactor did not decrease to a very low pressure) (see...). Figure 18 Furthermore, Table 5 shows that in the pressure control of Comparative Example 1, the time used for evacuation was 1 to 10 minutes (during which the atmosphere inside the reactor was reduced to a very low pressure) (see Table 5). Figure 17 As can be seen from Table 5, by keeping the vacuuming time to less than 5 minutes (ideally less than 5 minutes if the atmosphere inside the reactor is reduced to a very low pressure), the interfacial oxygen content can be reduced to 0.6 × 10⁻⁶. 15 atoms / cm 2 The following is a summary of the findings. Furthermore, Table 5 shows that when the vacuum pressure is maintained above 5 Pa, the interfacial oxygen content can be reduced to 0.6 × 10⁻⁶. 15 atoms / cm 2 In other words, if the atmosphere inside the reactor is not reduced to a very low pressure, the interfacial oxygen content is low. In short, from the viewpoint of improving the electrical characteristics of the aforementioned semiconductor devices, in order to reduce the interfacial oxygen content to 0.6 × 10⁻⁶... 15 atoms / cm 2 The following conditions must be met: the vacuuming time must be less than 5 minutes, or the vacuuming pressure must be maintained above 5 Pa.

[0108] Table 5

[0109]

[0110] Fourth Implementation Method

[0111] Next, the film-forming method according to the fourth embodiment will be described. Figure 1 The method for the silicon nitride passivation film 3 shown. Figure 19 This is a flowchart illustrating a method for forming a silicon nitride passivation film 3 according to a fourth embodiment. The film formation method according to the fourth embodiment differs from the film formation method according to the first embodiment in that step S9 is performed instead of step S2, and is otherwise identical to the film formation method according to the first embodiment. That is, in the fourth embodiment, after step S1, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5 by performing step S9. Figure 20 This is a diagram showing the control procedure of temperature and gas supply in the reactor when the silicon nitride passivation film 3 is formed.

[0112] In step S9, steps S91 and S92 are first performed in this order. Steps S91 and S92 are the same as steps S21 and S22 in step S2, respectively. Next, vacuum purging and N2 purging (circular purging) in the reactor are repeated, and the atmosphere is replaced with an N2 atmosphere (step S93). After evacuation, H2-containing gas is introduced into the reactor, and the atmosphere in the reactor is replaced with an H2-containing atmosphere. Here, as the H2-containing gas, a gas mixed with N2 and H2 is filled into the reactor, and the atmosphere in the reactor is set to an N2 and H2 atmosphere. In one embodiment, the H2 partial pressure (H2 / (H2+N2)) is greater than or equal to 0.005 (i.e., 0.5%). Incidentally, the H2 partial pressure can be 5% or more.

[0113] After the cyclic purging is completed, step S94 is executed. Step S94 is the same as step S24 in step S2. In step S95, while maintaining the pressure inside the reactor at the second pressure, the temperature inside the reactor is raised to a third temperature of 700°C or higher. The third temperature is higher than the film formation temperature, i.e., the second temperature. The third temperature can be at least 20°C higher than the second temperature. Furthermore, the third temperature can be above 650°C and below 900°C. In one example, the third temperature is 800°C.

[0114] Subsequently, the interior of the reactor is maintained at a third temperature (here, 800°C) and a second pressure (here, 3 kPa) for at least 3 minutes (step S96, cleaning step). In one embodiment, the maintenance time is 10 minutes. Next, the film-forming environment is prepared, i.e., a cooling step. Specifically, steps S97 and S98 are performed. First, in step S97, the temperature inside the reactor is lowered to a second temperature for film formation. The second temperature is a temperature 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, NH3 gas is supplied to the reactor to replace the atmosphere inside the reactor from an N2+H2 atmosphere to an NH3 atmosphere, and the pressure inside the reactor is adjusted to a third pressure below 100 Pa (step S98). This third pressure can be below 20 Pa or above 10 Pa. In one embodiment, the third pressure is 20 Pa.

[0115] Then, steps S99 through S101 are executed sequentially. Steps S99 to S101 are the same as steps S29 to S31 in step S2. Through the above steps, a silicon nitride passivation film 3 is formed on the nitride semiconductor layer 5.

[0116] The silicon nitride passivation film 3 formation method according to the fourth embodiment described above can achieve the same effects as the formation method according to the first embodiment. Specifically, due to the execution of step S96, when a silicon nitride passivation film is formed on a nitride semiconductor by LP CVD, the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride passivation film can be reduced.

[0117] Tables 6 and 21 show the relationship between the H2 partial pressure of the atmospheric gas and the interfacial oxygen content. Table 6 shows the results of the interfacial oxygen content (oxygen content at the interface between the nitride semiconductor layer 5 and the silicon nitride passivation film 3) for each case of H2 partial pressure change in steps S94 to S98. Figure 21 This is a graph showing the results of Table 6. Figure 21 In the graph, the horizontal axis represents the H2 partial pressure of the atmospheric gas, and the vertical axis represents the oxygen content at the interface.

[0118] Table 6

[0119]

[0120] To obtain the results in Table 6, the temperature in step S92, i.e., the temperature at which the epitaxial wafer 9 is introduced into the reactor, was set to 400°C. Furthermore, the N2+H2 atmosphere was defined as the atmosphere inside the reactor. The pressure inside the reactor in steps S94 and S95 was set to 3000 Pa. In step S96, the interior 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. Additionally, dichlorosilane was supplied to the reactor at a temperature of 700°C and a pressure of 30 Pa to form a silicon nitride passivation film 3. Referring to Table 6, it can be seen that when the H2 partial pressure of the atmosphere gas is 0.005 (0.5%) or higher, the interfacial oxygen content is sufficiently reduced. Furthermore, it can be seen that when the H2 partial pressure of the ambient gas is 0.05 (5%) or higher, the interfacial oxygen content is significantly reduced.

[0121] In the results in Table 6 above, it is believed that, similar to the case where the atmosphere is NH3, oxygen atoms 11 contained in the oxide film on the surface of the epitaxial wafer 9 are removed by hydrogen (H2) reduction under an H2+N2 atmosphere. Furthermore, it is believed that oxygen atoms 11 contained in moisture and oxygen adhering to the reactor are also removed by this reduction. In particular, when the pressure inside the reactor is relatively high (e.g., when the pressure is above 300 Pa), the amount of hydrogen produced is relatively large, thus increasing the amount of oxygen atoms 11 removed by reduction.

[0122] Next, film formation was performed by varying the pressure within the reactor, which served as the heating condition, and the results of the interfacial oxygen content under various conditions were described. Table 7 and Figure 22The relationship between pressure inside the reactor and interfacial oxygen content is shown. Table 7 shows the results of film formation by changing the pressure inside the reactor in steps S94 to S98 and comparing the interfacial oxygen content under each condition. Figure 22 This is a graph showing the results of Table 7. Figure 22 In the graph, the horizontal axis represents the pressure inside the reactor, and the vertical axis represents the interfacial oxygen content. Refer to Table 7 and... Figure 22 It can be seen that when the pressure inside the reactor is above 3000 Pa, the interfacial oxygen content is significantly reduced. Furthermore, it can be understood that when the pressure inside the reactor is above 10000 Pa, the interfacial oxygen content is significantly reduced.

[0123] Table 7

[0124]

[0125] Next, regarding the results of membrane formation by changing the cleaning time as a cleaning condition, a description of the results comparing the interfacial oxygen content under each condition is given. Table 8 and Figure 23 The relationship between cleaning time and interfacial oxygen content is shown. Table 8 shows the results of film formation by varying the cleaning time in step S96 and comparing the interfacial oxygen content under each condition. Figure 23 This is a graph showing the results of Table 8. Figure 23 In the graph, the horizontal axis represents cleaning time, and the vertical axis represents interfacial oxygen content. Refer to Table 8 and... Figure 23 It can be seen that when the cleaning time is set to 2 minutes or more, the interfacial oxygen content decreases to some extent. Furthermore, it should be understood that when the cleaning time is set to 5 minutes or more, the interfacial oxygen content decreases sufficiently. Moreover, it should be understood that when the cleaning time is 10 minutes or more, the interfacial oxygen content decreases significantly.

[0126] Table 8

[0127]

[0128] Next, regarding the results of film formation by changing the atmosphere gas and cleaning temperature, a description of the results comparing the interfacial oxygen content under different conditions is given. Table 9 and Figure 24 The relationship between cleaning time and interfacial oxygen content is shown. Table 9 shows the results of film formation in steps S26 and S96 by changing the atmosphere gas and cleaning temperature, and the results of comparing the interfacial oxygen content under each condition. Figure 24 This is a graph of the results from Table 9. Figure 24 In the graph, the horizontal axis represents the cleaning temperature, and the vertical axis represents the interfacial oxygen content. Refer to Table 9 and... Figure 24 It can be seen that when the atmosphere in the reactor is not NH3 but H2+N2, the lower cleaning temperature reduces the interfacial oxygen content.

[0129] Table 9

[0130]

[0131] As described above, when the atmosphere gas is NH3, it is believed that hydrogen (H2) is generated through thermal decomposition, and oxygen atoms 11 contained in the oxide film on the surface of the epitaxial wafer 9 are removed through hydrogen reduction. In other words, the H2 generated by the thermal decomposition of NH3 reduces the interfacial oxygen content. However, when the cleaning temperature is low, the amount of H2 generated due to the thermal decomposition of NH3 is correspondingly reduced, and therefore the above-mentioned reduction effect is reduced. On the other hand, in an H2 atmosphere, since H2 is directly supplied from the atmosphere, the same H2 reduction effect as in an NH3 atmosphere can be obtained even at a lower cleaning temperature. In particular, from the viewpoint of nitrogen removal from the surface of the epitaxial wafer 9, a low film formation temperature is desirable when forming a film on a GaN layer, which is a compound semiconductor. Therefore, when the nitride semiconductor layer 5 is a GaN layer, the film formation method of the silicon nitride passivation film 3 according to the fourth embodiment is more suitable. Furthermore, when the cleaning temperature is 700°C in an H2+N2 atmosphere, even if the film formation temperature and the cleaning temperature are the same, the interfacial oxygen content is sufficiently reduced. Therefore, in the cleaning process, it can be understood that if an H2 atmosphere is provided, the higher temperature required when supplying raw material gases such as NH3 is unnecessary.

[0132] Furthermore, according to the silicon nitride passivation film 3 formation method of the fourth embodiment, in steps S93 to S95, starting from a relatively low temperature inside the reactor, oxidation generated by degassing in the reactor is suppressed, while redox reaction is obtained on the surface of the epitaxial wafer 9. Therefore, when a silicon nitride passivation film is formed on a nitride semiconductor by LP CVD, the amount of oxygen at the interface between the nitride semiconductor and the silicon nitride passivation film can be further reduced.

[0133] The methods for forming silicon nitride passivation films, the methods for manufacturing semiconductor devices, and the semiconductor devices according to this disclosure are not limited to the embodiments described above, and various other modifications are possible. For example, each of the above embodiments can be combined with each other according to the necessary purposes and effects. Furthermore, the second embodiment has shown an example of a HEMT as a semiconductor device, and the methods for manufacturing semiconductor devices and the semiconductor devices according to this disclosure can be applied to various nitride semiconductor devices other than HEMTs.

[0134] Supplementary Explanation

[0135] As can be understood from the above embodiments, the disclosure includes the aspects shown below.

[0136] According to one embodiment of the present disclosure, the method for forming a silicon nitride passivation film is a method for forming a silicon nitride passivation film on a nitride semiconductor layer, wherein the temperature inside the reactor containing the substrate containing the nitride semiconductor layer is 700°C or higher, the pressure inside the reactor is maintained at 5 Pa or higher, and at least one of changing the atmosphere inside the reactor and changing the pressure inside the reactor is performed, and the silicon nitride passivation film is formed by supplying dichlorosilane (SiH2Cl2) into the reactor.

[0137] A method for forming a silicon nitride passivation film according to the present disclosure is as follows: forming a silicon nitride passivation film on a nitride semiconductor layer at a temperature of 700°C or higher in a reactor containing a nitride semiconductor layer, wherein the pressure in the reactor becomes 5 Pa or less within 5 minutes, performing at least one of the steps of changing the atmosphere in the reactor and changing the pressure in the reactor, and forming the silicon nitride passivation film by supplying dichlorosilane (SiH2Cl2) into the reactor.

Claims

1. A method of forming a silicon nitride passivation film on a nitride semiconductor layer by an LPCVD method, the method comprising the steps of: introducing a substrate including the nitride semiconductor layer into a reaction furnace; replacing an atmosphere in the reaction furnace from air to an ammonia (NH3) atmosphere or a hydrogen (H2) atmosphere; raising a temperature in the reaction furnace to a first temperature; maintaining the temperature in the reaction furnace at the first temperature and maintaining the atmosphere in the reaction furnace as the NH3 atmosphere or the H2 atmosphere for three minutes or more; lowering the temperature in the reaction furnace to a second temperature lower than the first temperature; and forming the silicon nitride passivation film by supplying dichlorosilane (SiH2Cl2) into the reaction furnace at a first pressure of 100 Pa or less in the reaction furnace.

2. The method of forming a silicon nitride passivation film according to claim 1, wherein the step of maintaining the temperature and the atmosphere further maintains a pressure in the reaction furnace at a second pressure greater than the first pressure.

3. The method of forming a silicon nitride passivation film according to claim 2, wherein the second pressure is 300 Pa or more.

4. The method of forming a silicon nitride passivation film according to any one of claims 1 to 3, wherein the second temperature is 700°C or more, and the first temperature is at least 20°C higher than the second temperature.

5. The method of forming a silicon nitride passivation film according to claim 1, wherein the first temperature is 750°C or more and 900°C or less.

6. The method of forming a silicon nitride passivation film according to claim 1, wherein the step of lowering the temperature lowers a pressure in the reaction furnace to the first pressure.

7. A method of manufacturing a semiconductor device using a nitride semiconductor as a main constituent material, the method comprising the steps of: forming a semiconductor stack structure including a nitride semiconductor layer on a substrate; forming a silicon nitride passivation film on the semiconductor stack structure using the method according to claim 1; forming an opening in the silicon nitride passivation film; and forming an electrode on the semiconductor stack structure through the opening.

8. A semiconductor device comprising: a substrate; a semiconductor stack portion formed on the substrate, the semiconductor stack portion including a plurality of nitride semiconductor layers; a silicon nitride passivation film covering a surface of the semiconductor stack portion; and oxygen atoms present at an interface between the silicon nitride passivation film and the semiconductor stack portion; wherein the silicon nitride passivation film is formed by the method of claim 1.

9. The semiconductor device according to claim 8, wherein the oxygen atoms are present in the form of silicon oxynitride Si2N2O.

10. A method of forming a silicon nitride passivation film on a nitride semiconductor layer by an LPCVD method, the method comprising the steps of: introducing a substrate including a nitride semiconductor layer into a reaction furnace; replacing an atmosphere in the reaction furnace from air to a hydrogen (H2) atmosphere; raising a temperature in the reaction furnace to a first temperature; maintaining the temperature in the reaction furnace at the first temperature; ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ wherein the interface oxygen content of the oxygen atoms is 0.6 x 10 15 atoms / cm 2 Hereinafter, ​ ​ ​ ​ ​ ​ ​ reducing the temperature in the reaction furnace to a second temperature lower than or equal to the first temperature; and forming the silicon nitride passivation film by supplying dichlorosilane (SiH2Cl2) into the reaction furnace at a first pressure of 100 Pa or less in the reaction furnace.

11. The method of forming a silicon nitride passivation film according to claim 10, wherein the step of maintaining the temperature further maintains the H2 partial pressure in the reaction furnace at 0.5% or more.

12. The method of forming a silicon nitride passivation film according to claim 10 or 11, wherein the step of maintaining the temperature further maintains the pressure in the reaction furnace at 300 Pa or more.

13. The method of forming a silicon nitride passivation film according to claim 10, wherein the step of maintaining the temperature maintains the temperature in the reaction furnace at the first temperature for two minutes or more.

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