Silicon nitride thin film, method for manufacturing the same, and solar cell

Abstract

The application discloses a silicon nitride film and a preparation method thereof and a solar cell, and the preparation method of the silicon nitride film comprises the following steps: introducing a process gas into a process chamber and loading radio frequency power to the process chamber to generate plasma, the process gas comprises silicon elements and nitrogen elements, and the ratio of the silicon elements and the nitrogen elements ranges from 2 to 4; and a silicon nitride film is deposited on a substrate in the process chamber. The application can achieve the purpose of forming a silicon nitride film with an ultra-high refractive index on the surface of the substrate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor, in particular to a silicon nitride film, a preparation method thereof and a solar cell. BACKGROUND

[0002] The increasing energy crisis makes solar energy, which is convenient, green and sustainable, the focus of current research. Solar photovoltaic cells are a kind of photovoltaic semiconductor sheet that can generate electricity directly under a certain solar irradiance by using the photovoltaic effect. According to the different materials used, solar photovoltaic cells can be divided into single crystal silicon solar cells, polycrystalline silicon solar cells, amorphous silicon solar cells and compound solar cells. Among them, crystalline silicon solar cells are highly regarded due to their high power generation efficiency and low large-scale manufacturing cost.

[0003] The traditional crystalline silicon solar cell structure is shown in FIG. 1. Figure 1 The SiNx (silicon nitride) film can reduce the reverse leakage current of the cell due to its unique passivation characteristics, and has good oxidation resistance. In addition, the SiNx film can play a key role in reducing the reflectivity of the solar cell. Studies have shown that the reflectivity of the solar cell surface can be reduced from 19% to 25% to about 10% after the SiNx film is added, thereby greatly increasing the light energy absorption efficiency. In recent years, some researchers have proposed that by changing the single-layer SiNx film to a multi-layer SiNx film system with gradually changing refractive index, the reflectivity of the solar cell can be further reduced. However, the highest refractive index of the SiNx film reported so far is only 2.7, which is difficult to effectively reduce the reflectivity of the solar band (300-2500 nm). Therefore, how to realize the ultra-high refractive index of the silicon nitride film is a key problem that needs to be solved in the photovoltaic industry.

[0004] Some schemes have studied the deposition of high-refractive-index SiNx film. The specific process flow is as follows: first, insert the original P-type polycrystalline silicon wafer into the graphite boat and place it in the PECVD (Plasma Enhanced Chemical Vapor Deposition) chamber, vacuumize, adjust the butterfly valve to maintain constant pressure, and then perform process deposition. The corresponding process formula is shown in Table 1. The refractive index of the SiNx film formed by this scheme is 2.68 (632.8 nm), which is still relatively low and difficult to effectively reduce the reflectivity of the solar photovoltaic cell for the solar band.

[0005] Table 1

[0006] SUMMARY

[0007] In view of this, the application provides a silicon nitride film, a preparation method thereof and a solar cell to solve the problem of low refractive index of a conventional SiNx film.

[0008] The application provides a preparation method of a silicon nitride film, comprising:

[0009] Process gas containing silicon and nitrogen is introduced into the process chamber and radio frequency power is loaded into the process chamber to generate plasma, and the ratio of the silicon and the nitrogen ranges from 2 to 4;

[0010] A silicon nitride film is deposited on a substrate in the process chamber.

[0011] Optionally, the process gas comprises a silicon-containing gas, a nitrogen-containing gas and a carrier gas, and the carrier gas does not contain nitrogen.

[0012] Optionally, the flow rate of the carrier gas is 20-30 times that of the silicon-containing gas or the nitrogen-containing gas.

[0013] Optionally, the temperature of the process chamber is constant, and the corresponding temperature ranges from 280 to 330℃.

[0014] Optionally, the pressure of the process chamber is set to 4300-4800 mTorr.

[0015] Optionally, the carrier gas comprises at least one of He and Ar.

[0016] Optionally, the silicon-containing gas comprises SiH4, and the nitrogen-containing gas comprises NH3.

[0017] Optionally, the flow rate of the SiH4 ranges from 190 to 230 sccm, the flow rate of the NH3 ranges from 70 to 100 sccm, and the flow rate of the carrier gas ranges from 5000 to 7000 sccm.

[0018] Optionally, the radio frequency power ranges from 400 to 450 W, and the duration of the deposition process is greater than or equal to 20 s.

[0019] The application also provides a silicon nitride film prepared by any one of the above preparation methods.

[0020] Optionally, the uniformity of the silicon nitride film ranges from 0.8% to 1.3%.

[0021] Optionally, the refractive index of the silicon nitride film ranges from 3.0 to 3.3.

[0022] The application also provides a solar cell, comprising:

[0023] a first conductive type semiconductor layer;

[0024] a second conductive type semiconductor layer disposed on the first conductive type semiconductor layer;

[0025] any one of the above-mentioned silicon nitride thin films is disposed on a surface of the second conductive type semiconductor layer away from the first conductive type semiconductor layer; and / or is disposed on a surface of the first conductive type semiconductor layer away from the second conductive type semiconductor layer.

[0026] The silicon nitride thin film and the preparation method thereof and the solar cell disclosed in the present application are prepared by introducing a process gas containing silicon and nitrogen into a process chamber, and loading radio frequency power into the process chamber to generate plasma, so as to deposit and form a silicon nitride thin film on a substrate, wherein the ratio of silicon and nitrogen ranges from 2 to 4, and the content of silicon is greater than that of nitrogen, so that silicon can enter the silicon nitride film layer and form clusters with silicon atoms therein, thereby improving the refractive index of the formed silicon nitride thin film. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0028] Figure 1 A schematic diagram of a crystalline silicon solar cell structure in the research process;

[0029] Figure 2 A schematic diagram of a preparation method of a silicon nitride thin film according to an embodiment of the present application;

[0030] Figure 3a 、 Figure 3b and Figure 3c A schematic diagram of a solar photovoltaic cell structure according to an embodiment of the present application. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. In the case of no conflict, each of the following embodiments and technical features can be combined with each other.

[0032] The first aspect of the present application provides a preparation method of a silicon nitride thin film, which is mainly used for preparing a silicon nitride thin film on a substrate surface such as a Si cell substrate. Referring toFigure 2 As shown, the method for preparing the silicon nitride film includes steps S110 to S130.

[0033] S110, a process gas containing silicon and nitrogen elements is introduced into a process chamber and radio frequency power is applied to the process chamber to generate plasma, and the ratio of the silicon element to the nitrogen element ranges from 2 to 4.

[0034] The process chamber can include a PECVD (Plasma Enhanced Chemical Vapor Deposition) chamber or other deposition process chamber. A substrate is placed in the process chamber. The substrate includes a bare silicon wafer of a specified size. The bare silicon wafer can be a single crystal silicon wafer or a polycrystalline silicon wafer. Optionally, the specified size can include any of 4 inches, 6 inches, 8 inches, and 12 inches.

[0035] Optionally, the temperature of the process chamber is constant, and the corresponding temperature range is 280-330℃. This provides a suitable constant temperature environment for the subsequent deposition process, so that the formed silicon nitride film has a high density, thereby further improving the refractive index of the obtained silicon nitride film.

[0036] Optionally, the pressure of the process chamber is set to 4300-4800 mTorr, which is equivalent to 572-638 Pa. The pressure characteristics herein provide a suitable pressure environment in the process chamber. The pressure environment has a certain high pressure characteristic, which can obtain a high plasma density, realize the preparation of a high-density silicon nitride film, and further obtain a super-high refractive index film. However, too high pressure will reduce the mean free path of gas molecules in the PECVD chamber, so that the kinetic energy is insufficient to deposit a high-density film on the substrate. Therefore, the appropriate but not too high pressure herein is conducive to the preparation of a super-high refractive index silicon nitride film and does not adversely affect the silicon nitride film.

[0037] Optionally, before step S110, ethanol can be used to wipe the substrate, and the wiped substrate is placed in the process chamber to improve the cleanliness of the substrate surface and improve the quality of the subsequent process.

[0038] The process gas contains silicon and nitrogen elements, and the ratio of the silicon element to the nitrogen element ranges from 2 to 4. The content of the silicon element is greater than that of the nitrogen element, so that the silicon element can enter the silicon nitride film layer and form clusters with silicon atoms therein, thereby improving the refractive index of the subsequently formed silicon nitride film.

[0039] Optionally, the process gas comprises a silicon-containing gas, a nitrogen-containing gas and a carrier gas; wherein the carrier gas does not contain nitrogen element, and the flow rate ratio of the silicon-containing gas and the nitrogen-containing gas ranges from 2 to 4. The silicon-containing gas and the nitrogen-containing gas are source gases in the deposition process, and the flow rate of the carrier gas is 20-30 times of the flow rate of the silicon-containing gas or the nitrogen-containing gas. The flow rate of the carrier gas is large enough, and the flow rate of the source gas is small enough, so that the deposition rate is not too fast, which is beneficial to control the thickness of the silicon nitride film to be formed, and the low flow rate of the source gas can be uniformly distributed in the process chamber along with the high flow rate of the carrier gas, which can improve the consistency and uniformity of the formed silicon nitride film.

[0040] Optionally, the carrier gas comprises at least one of He (helium) and Ar (argon). Here, Ar or He is selected as the carrier gas, so that the carrier gas does not participate in the chemical reaction in the deposition process. Compared with N2 (nitrogen) which can form additional N ions in the process and inhibit the formation of SiN products, the carrier gas used herein can improve the density of the formed film, thereby improving the refractive index of the formed silicon nitride film. Further, the thermal conductivity of He and / or Ar is strong, which can timely conduct the heat on the surface of the formed silicon nitride film, avoid heat accumulation on the surface of the silicon nitride film, and further improve the quality of the formed silicon nitride film.

[0041] Optionally, the silicon-containing gas comprises SiH4 (monosilane), and the nitrogen-containing gas comprises NH3 (ammonia). Optionally, the flow rate of SiH4 ranges from 190 to 230 sccm, for example, the flow rate of SiH4 can be 190 sccm, 200 sccm, 210 sccm or 230 sccm, etc. Optionally, the flow rate of NH3 ranges from 70 to 100 sccm, for example, the flow rate of NH3 can be 70 sccm, 80 sccm, 90 sccm or 100 sccm, etc. The flow rate of the carrier gas ranges from 5000 to 7000 sccm, for example, the flow rate of the carrier gas can be 5000 sccm, 6000 sccm or 7000 sccm. Here, the flow rate of the reaction source gas such as SiH4 and NH3 is relatively small, which is beneficial to control the thickness of the silicon nitride film to be formed, and the flow rate of the carrier gas is relatively high, which can make the reaction source gas sufficiently diffuse to every corner of the process chamber, so that a high-uniformity silicon nitride film can be obtained.

[0042] S120, depositing and forming a silicon nitride film on the substrate in the process chamber.

[0043] Optionally, the radio frequency power ranges from 400 to 450 W, and the frequency of the radio frequency power source can be 13.56 MHz.

[0044] The duration of the deposition process can be determined according to the thickness of the silicon nitride film to be formed. Alternatively, the duration of the deposition process is greater than or equal to 20 s, so that the formed silicon nitride film has a suitable thickness.

[0045] The above method for preparing a silicon nitride film includes the steps of: introducing a process gas containing silicon and nitrogen into a process chamber, and applying radio frequency power to the process chamber to generate plasma, so as to deposit and form a silicon nitride film on a substrate. The ratio of silicon to nitrogen is in the range of 2-4, and the content of silicon is greater than that of nitrogen. Silicon can enter the silicon nitride film and form clusters with silicon atoms in the film, thereby increasing the refractive index of the formed silicon nitride film.

[0046] In one embodiment, after placing the substrate to be processed in the process chamber, the method for preparing a silicon nitride film further includes: evacuating the process chamber to a pressure of 15-20 mTorr, so as to provide a suitable vacuum environment in the process chamber before introducing the silicon-containing gas, the nitrogen-containing gas and the carrier gas. The number of particles in the process chamber in the vacuum environment has little effect on the chemical reaction in the subsequent deposition process, and can ensure the stability of the subsequent deposition process.

[0047] The inventors have found that, in a PECVD chamber, the higher the temperature, the more intense the movement of gas molecules, and the lower the gas density at a constant pressure. Therefore, the density of SiH4 and other reaction gases at low temperature is higher than that at high temperature, and the product is more compact. The refractive index of the film is a manifestation of the compactness of the film layer, and the more compact the product, the higher the refractive index. However, if the temperature is too low, the gas molecules cannot completely dissociate into plasma, and the refractive index will still decrease. Based on the above findings, in one example, the temperature in the PECVD chamber is in the range of 280-330°C and is in a constant temperature state, which can provide a suitable constant temperature environment for the subsequent PECVD process. Performing the PECVD process in the constant temperature environment can make the silicon nitride film have a high compactness, thereby further improving the refractive index of the obtained silicon nitride film.

[0048] The inventors have also conducted experimental analysis on the silicon nitride film prepared by the method provided in the present application. The process recipe used in the experiment can be referred to Table 2, and the experimental results can be referred to Table 3.

[0049] Table 2

[0050]

[0051] Table 3

[0052]

[0053] The experimental results shown in Table 3 represent that the thickness of the silicon nitride film obtained in the present experiment is 3198 Å, the uniformity (using mean algorithm, 9 points) is 0.98%, and the refractive index at 632.8 nm is 3.124. It can be seen that the silicon nitride film realizes high uniformity and high refractive index which is much higher than the prior art. Therefore, the silicon nitride film preparation method provided in the present application has greater advantages than the traditional method.

[0054] In the above silicon nitride film preparation method, the process gas containing silicon and nitrogen elements is introduced into the process chamber, and the radio frequency power is loaded into the process chamber to generate plasma to deposit and form a silicon nitride film on the substrate. The ratio of silicon and nitrogen elements is in the range of 2-4, so that the content of silicon elements is greater than that of nitrogen elements. The silicon elements can enter the silicon nitride film layer and form clusters with silicon atoms therein, thereby improving the refractive index of the formed silicon nitride film.

[0055] Specifically, the temperature in the process chamber is in a constant temperature state in the range of 280-330℃, which corresponds to a suitable constant temperature environment for the deposition process. The deposition process is performed in this constant temperature environment, which can make the silicon nitride film have a higher density, thereby improving the refractive index of the obtained silicon nitride film. The flow rate of the carrier gas is 20-30 times that of the silicon-containing gas or nitrogen-containing gas, so that the flow rate of the carrier gas is large enough and the flow rate of the source gas is small enough, so that the deposition rate is not too fast, which is beneficial to control the thickness of the silicon nitride film to be formed. The low flow rate of the source gas can be uniformly distributed in the process chamber, which can improve the consistency and uniformity of the formed silicon nitride film. Ar or He is selected as the carrier gas, so that the carrier gas does not participate in the chemical reaction in the deposition process, which can improve the density of the formed film, thereby improving the refractive index of the formed silicon nitride film. In addition, such carrier gas has high thermal conductivity, which can timely conduct the heat on the surface of the formed silicon nitride film, avoid heat accumulation on the surface of the silicon nitride film, and further improve the quality of the formed silicon nitride film. The pressure in the process chamber is in the range of 4300-4800 mTorr, which provides a suitable pressure environment in the process chamber. The corresponding high pressure feature can obtain high plasma density, realize the preparation of high-density silicon nitride film, and further obtain ultra-high refractive index of the film, without the adverse effects such as insufficient particle motion energy in the process chamber to deposit high-density film on the substrate.

[0056] It can be seen that the above silicon nitride film preparation method sets the chamber temperature, process gas flow rate, carrier gas type and pressure in the process chamber of the deposition process, so that the deposition process can obtain a silicon nitride film with high uniformity and ultra-high refractive index.

[0057] The second aspect of the present application also provides a silicon nitride film, which is prepared by the silicon nitride film preparation method in any of the above embodiments, and has the advantages of high uniformity and high refractive index.

[0058] Optionally, the uniformity of the silicon nitride film ranges from 0.8% to 1.3%.

[0059] Optionally, the refractive index of the silicon nitride film ranges from 3.0 to 3.3.

[0060] The above silicon nitride film is prepared by the silicon nitride film preparation method in any of the above embodiments, and has all the beneficial effects of the silicon nitride film preparation method in any of the above embodiments, which will not be repeated here.

[0061] The third aspect of the present application also provides a solar cell, as shown in Figures 3a to 3c The solar cell comprises:

[0062] a first conductive type semiconductor layer 211;

[0063] a second conductive type semiconductor layer 212, which is arranged on the first conductive type semiconductor layer 211;

[0064] a silicon nitride film 213 according to any of the above embodiments, which is arranged on at least one surface of one of the first conductive type semiconductor layer 211 and the second conductive type semiconductor layer 212 away from the other. For example, as shown in Figure 3a the silicon nitride film 213 can be arranged on a surface of the second conductive type semiconductor layer 212 away from the first conductive type semiconductor layer 211; for another example, as shown in Figure 3b the silicon nitride film 213 can be arranged on a surface of the first conductive type semiconductor layer 211 away from the second conductive type semiconductor layer 212; for another example, as shown in Figure 3c the silicon nitride film 213 can be arranged on a surface of the second conductive type semiconductor layer 212 away from the first conductive type semiconductor layer 211, and on a surface of the first conductive type semiconductor layer 211 away from the second conductive type semiconductor layer 212, respectively.

[0065] Optionally, the first conductive type semiconductor layer can include any one of a P-type semiconductor layer and an N-type semiconductor layer, and the second conductive type semiconductor layer includes the other of the P-type semiconductor layer and the N-type semiconductor layer, for example, the first conductive type semiconductor layer is a P-type semiconductor layer, and the second conductive type semiconductor layer is an N-type semiconductor layer.

[0066] The solar cell includes the silicon nitride film prepared by the method described in any of the above embodiments, and has all the beneficial effects of the method described in any of the above embodiments, which will not be repeated here.

[0067] The silicon nitride film and the method for preparing the same, and the solar cell are described in detail above, and the principles and implementation manners of the present application are described by using specific examples. It should be noted that the description of each embodiment in the present application has its own focus, and the parts not described or recorded in a certain embodiment can be referred to the related description of other embodiments.

[0068] Although the present application has been shown and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art based on the foregoing description and accompanying drawings. The present application includes all such modifications and alterations and is limited only by the scope of the following claims. In particular, with respect to the various functions performed by the above described components, the terms used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the component (e.g., that is functionally equivalent), whether or not the structure is structurally identical to the structure shown in the exemplary implementations of the present application described herein.

[0069] That is, the above description is only an embodiment of the present application, and does not limit the patent scope of the present application, and any equivalent structure or equivalent process transformation using the content of the present application specification and drawings, such as the mutual combination of technical features between embodiments, or direct or indirect application in other related technical fields, is also included in the patent protection scope of the present application.

[0070] In addition, although the terms first and second, etc. may be used in the description of the present application to describe various information, these information should not be limited to these terms. These terms are only used to distinguish information of the same type from each other. The terms "first", "second" cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise specifically limited.

[0071] In this application, the use of the singular includes the plural unless specifically stated otherwise. In this application, the use of "or" means "and / or" unless stated otherwise. In this application, the use of "an" includes "one or more". In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at least a third" unless context clearly states otherwise. In this application, the use of "another" means "at least a second" or "at

Claims

1. A method for preparing a silicon nitride thin film, characterized in that, include: A process gas is introduced into the process chamber and radio frequency power is applied to the process chamber to generate plasma. The process gas contains silicon and nitrogen elements, and the ratio of silicon to nitrogen elements is in the range of 2-4. This allows the silicon elements to enter the silicon nitride film layer and form clusters with the silicon atoms therein, thereby increasing the refractive index of the resulting silicon nitride film. A silicon nitride thin film is deposited on the substrate within the process chamber; The process gases include silicon-containing gases, nitrogen-containing gases, and carrier gases, wherein the carrier gases do not contain nitrogen.

2. The method for preparing silicon nitride thin films according to claim 1, characterized in that, The flow rate of the carrier gas is 20-30 times that of silicon-containing gas or nitrogen-containing gas.

3. The method for preparing silicon nitride thin films according to claim 1, characterized in that, The temperature of the process chamber is constant, with a corresponding temperature range of 280-330℃.

4. The method for preparing silicon nitride thin films according to claim 1, characterized in that, The pressure of the process chamber is set to 4300~4800 mTorr.

5. The method for preparing silicon nitride thin films according to claim 1, characterized in that, The carrier gas includes at least one of He and Ar.

6. The method for preparing silicon nitride thin films according to claim 1, characterized in that, The silicon-containing gas includes SiH4, and the nitrogen-containing gas includes NH3.

7. The method for preparing silicon nitride thin films according to claim 6, characterized in that, The flow rate of SiH4 is in the range of 190~230 sccm, the flow rate of NH3 is in the range of 70~100 sccm, and the flow rate of the carrier gas is in the range of 5000~7000 sccm.

8. The method for preparing silicon nitride thin films according to claim 1, characterized in that, The radio frequency power ranges from 400 to 450W, and the deposition process duration is greater than or equal to 20 seconds.

9. A silicon nitride thin film, characterized in that, The silicon nitride thin film is prepared using the silicon nitride thin film preparation method according to any one of claims 1 to 8.

10. The silicon nitride thin film according to claim 9, characterized in that, The uniformity of the silicon nitride film is in the range of 0.8% to 1.3%; and / or, The refractive index of the silicon nitride thin film is in the range of 3.0 to 3.

3.

11. A solar cell, characterized in that, include: First conductivity type semiconductor layer; A second conductivity type semiconductor layer is disposed on the first conductivity type semiconductor layer; The silicon nitride thin film of claim 9 or 10 is disposed on the surface of the second conductivity type semiconductor layer away from the first conductivity type semiconductor layer; and / or disposed on the surface of the first conductivity type semiconductor layer away from the second conductivity type semiconductor layer.

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