Mbe molecular beam epitaxy beam source furnace and thin film growth method thereof

Abstract

This invention discloses an MBE (molecular beam epitaxy) source furnace and its thin film growth method, belonging to the field of semiconductor thin film preparation technology. The source furnace includes a gas mass flow controller, a plasma initiation device, a plasma generation cavity, and a plasma termination device connected sequentially along the beam direction. The outer wall of the plasma generation cavity is surrounded by a cross-distributed RF electrode system for efficient and uniform gas ionization. The plasma termination device is a double-layer batching disk structure used to shape the plasma into a uniform molecular beam. The method for growing thin films using this source furnace includes steps such as substrate heating, introduction of reactive gas, RF excitation to generate plasma, and beam application to the substrate for thin film growth. This invention effectively solves the problems of low ionization efficiency and non-uniform beam flow in traditional plasma sources, and is particularly suitable for the preparation of high-quality nitride and oxide semiconductor single-crystal thin films.

Description

Technical Field

[0001] This invention relates to the field of semiconductor thin film preparation, and in particular to an MBE molecular beam epitaxy source furnace and its thin film growth method. Background Technology

[0002] Molecular beam epitaxy (MBE) is an advanced technology for growing single-crystal thin films. The beam source furnace is one of the most important core components of a molecular beam epitaxy system. Its main function is to heat the required material to a sufficiently high temperature to generate a high-purity molecular beam with a specific beam angle distribution, stable beam flow rate, and rapid control response. The material molecules are then transported from the furnace to the substrate surface through thermal evaporation (or sublimation) via the molecular beam, thereby achieving thin film growth.

[0003] Wide bandgap semiconductor materials, represented by oxynitrides, have become a hot topic in research and application worldwide, and MBE (Metal-Oxide-Based Enzyme) is an important technology for growing oxynitride semiconductor materials. A particularly prominent problem currently facing MBE is how to provide N and O atoms. To solve this problem, researchers have developed a method to ionize N2 and O2 into active N and O atoms to provide nitrogen and oxygen sources.

[0004] Currently, radio frequency (RF) inductively coupled plasma (ICP) sources are commonly used as active nitrogen or oxygen sources in PA-MBE. However, traditional RF plasma beam source furnaces have some inherent technical bottlenecks in practical applications: 1) Limited ionization efficiency: Some gas molecules are extracted before being fully ionized, resulting in insufficient supply of active atoms, which affects the film growth rate and crystal quality.

[0005] 2) Poor beam uniformity: The plasma is not evenly distributed in the cavity and the outlet design is simple (such as a single straight tube or a single hole), which leads to the uneven spatial distribution of the active particle beam sprayed onto the substrate surface, directly causing uneven thickness and composition of the epitaxial film.

[0006] Therefore, developing an MBE beam source furnace with higher ionization efficiency and capable of generating highly uniform molecular beams, along with its supporting growth process, is of great significance for improving the material properties and device uniformity of wide bandgap semiconductor thin films. Summary of the Invention

[0007] The purpose of this invention is to provide an MBE molecular beam epitaxy source furnace and its thin film growth method to solve the problems in the background art.

[0008] To achieve the above objectives, the present invention provides an MBE molecular beam epitaxy beam source furnace, comprising a gas mass flow controller (MFC), a plasma mounting initiator, a plasma generating cavity, and a plasma end-effector connected sequentially along the beam direction. The outer wall of the plasma generating cavity is surrounded by an RF electrode system. The plasma mounting initiator has a conical structure, and the plasma end-effector has a double-layer batching disk structure.

[0009] Preferably, the plasma generating chamber is further provided with an outer shell, which is sealed to the vacuum chamber of the MBE system via connectors and flanges.

[0010] Preferably, the plasma installation initiator is embedded inside the inlet of the plasma generating chamber to uniformly divert and introduce the reaction gas into the generating chamber. The plasma installation initiator includes an initiation chamber and a gas hole for gas diversion opened on the upper wall of the initiation chamber. The outside of the gas hole is connected to the gas inlet chamber of the plasma generating chamber.

[0011] Preferably, the RF electrode system is connected to an external RF power supply. The RF electrode system includes electrode plates that are uniformly and crosswise distributed around the plasma generating cavity. The electrode plates are radio frequency electrode plates. The radio frequency electrode plates are connected to the RF power supply through a radio frequency matching device. The frequency of the RF power supply is 13.56MHz, which is used to form a uniform strong electric field in the cavity to ionize the gas.

[0012] Preferably, the plasma terminal is connected to the outlet end of the plasma generating cavity and is used to shape the plasma generated in the cavity into a spatially uniform molecular beam. The plasma terminal includes a first orifice plate and a second orifice plate arranged sequentially along the beam direction. The first orifice plate and the second orifice plate are uniformly provided with a plurality of through holes, and the radius of each through hole is 0.1 mm.

[0013] Preferably, the first perforated plate has a planar structure, and the second perforated plate has a conical structure.

[0014] Preferably, the range of the gas mass flow controller is 200 sccm.

[0015] The present invention also provides a method for growing thin films using the above-mentioned MBE molecular beam epitaxy source furnace, comprising the following steps: S1. First, the substrate is cleaned and its surface is pretreated. Then, the beam source furnace is sealed and connected to the vacuum chamber of the MBE system through a flange. The treated substrate is placed in the vacuum chamber of the MBE system and heated to the growth temperature to prepare for epitaxial growth. S2. The reaction gas is introduced into the MBE molecular beam epitaxy source furnace through the gas mass flow controller. The reaction gas enters the plasma generation chamber after being split by the plasma installation initiator. S3. Simultaneously start the power supply connected to the RF electrode system to form a strong electric field in the plasma generation cavity, causing the reactant gas to ionize and generate plasma. S4. The plasma is shaped into a uniform molecular beam through the double-layer batching disk structure of the plasma terminal device and sprayed onto the substrate surface, where thin film epitaxial growth is performed to form a crystalline thin film.

[0016] Preferably, in step S1, the substrate is first cleaned and pretreated on the surface, specifically by sequentially using acetone solution, anhydrous ethanol and deionized water to perform ultrasonic cleaning on the substrate, and then drying the substrate surface. The processed substrate is transferred to the main reaction chamber of the MBE system and preheated at a temperature of 250-300°C for 10-20 minutes.

[0017] Preferably, in step S1, the preset temperature is 500~850℃.

[0018] Preferably, in step S2, the reactant gas is oxygen or nitrogen; when the reactant gas is oxygen, the gas flow rate is 30~100 sccm; when the reactant gas is nitrogen, the gas flow rate is 1.5~3 sccm.

[0019] Preferably, in step S4, when the reactant gas is oxygen, the epitaxial growth process parameters are: growth time of 2 hours, oxygen plasma power of 250W, gallium source temperature of 800~840℃, substrate temperature of 650~700℃, and growth gas pressure of 1.4×10⁻⁶. -3 Pa ~ 2.6 × 10 -3 Pa; When the reactant gas is nitrogen, the epitaxial growth process parameters are: growth time of 2 hours, nitrogen plasma power of 330~450W, and gallium source pressure of 8×10⁻⁶. -6 Pa ~ 1.6 × 10 -5 Pa, substrate temperature 700~850℃, growth gas pressure 2.7×10 -3 Pa ~ 5.3 × 10 -3 Pa.

[0020] Preferably, in step S4, the thickness and quality of the crystalline film are determined by controlling the temperature of the molecular beam and adjusting the beam density sprayed onto the substrate.

[0021] Therefore, the MBE molecular beam epitaxy source furnace and its thin film growth method of the present invention have the following beneficial effects: (1) The socket-type starting component meets the requirement of small inlet area, while ensuring the airtightness and vacuum of the cavity; combined with the cross-distributed RF electrode system, it realizes the uniformity and efficient coupling of the electric field in the large volume cavity, making the radio frequency distribution more uniform and stable, improving the electrolysis efficiency, and thus significantly improving the ionization efficiency of the reaction gas.

[0022] (2) The double-layer batching disk outlet structure (especially the combination of planar and conical hole plates) can effectively shape the plasma and output a highly uniform and stable molecular beam, laying the foundation for the growth of uniform thin films.

[0023] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the overall structure of the MBE molecular beam epitaxy source furnace according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of the plasma mounting initiator according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the first perforated plate structure of the plasma end device according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the second perforated plate structure of the plasma end device according to an embodiment of the present invention; Figure 5 This is a flowchart illustrating the growth of a crystalline thin film according to an embodiment of the present invention; Figure label: 1. Gas mass flow controller; 2. Plasma mounting initiator; 21. Initiation chamber; 22. Gas port; 3. Plasma generation chamber; 4. RF electrode system; 5. Plasma end effector; 51. First orifice plate; 52. Second orifice plate; 53. Through hole; 6. Housing. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0027] like Figure 1As shown, the present invention provides an MBE molecular beam epitaxy beam source furnace, which is an overall cylindrical structure extending along the horizontal beam direction. The beam source furnace mainly includes: a gas mass flow controller (MFC) 1, a plasma mounting initiator 2, a plasma generation chamber 3, an RF electrode system surrounding the outer wall of the generation chamber 4, a plasma end device 5, and an optional external housing 6.

[0028] The gas mass flow controller 1 is located outside the inlet of the beam source furnace and is used to precisely control the flow rate of the input reaction gas (such as high-purity O2 or N2). In a preferred embodiment, the gas mass flow controller 1 has a range of 200 sccm, providing high control accuracy and rapid response.

[0029] Inside the inlet of the plasma generation chamber 3 of the beam source furnace, another inner cavity is formed. This inner cavity is the plasma installation initiator 2, which is connected to the output of the gas mass flow controller 1 via a vacuum-sealed interface (such as a CF flange or metal seal). The plasma installation initiator 2 has a conical structure, such as... Figure 2 As shown, multiple gas holes 22 for gas diversion are formed on the upper wall of the starting chamber 21. The reactant gas first enters the starting chamber 21, and then is uniformly diverted through the multiple gas holes 22 and introduced into the inlet chamber of the outer plasma generating chamber 3, and then directly enters the plasma generating chamber 3. This design achieves uniform diffusion of gas from point source to surface source, ensuring the initial uniformity of the gas entering the discharge region. At the same time, the compact structure helps to maintain the high vacuum of the system.

[0030] The plasma generating chamber 3 is typically made of high-temperature, high-vacuum compatible materials (such as stainless steel, quartz, or alumina ceramic). In the following embodiment, a quartz plasma generating chamber 3 is used. Its inlet end is connected to the outer gas inlet chamber of the plasma mounting initiator 2, and its outlet end is connected to the plasma terminal component 5. The plasma generating chamber 3 is the core region of the gas ionization reaction.

[0031] RF electrode system 4 Figure 3 As shown, it includes at least one pair (typically made of copper) of radio frequency (RF) electrode plates arranged in a crisscross pattern around the outer wall of the plasma generation cavity 3. The RF electrode plates are connected to an RF power supply with a frequency of 13.56 MHz via an RF matching unit to create a uniform and strong electric field within the cavity for gas ionization. When the RF power supply is activated, alternating current flows through the electrodes, inducing a strong high-frequency electromagnetic field within the internal space of the plasma generation cavity 3. Without increasing the discharge power, the crisscrossing electrode design increases the coupling electric field strength within the discharge cavity, resulting in a more uniform and stable RF distribution and improved electrolysis efficiency. This allows the introduced N2 or O2 gas to be efficiently and uniformly excited and ionized throughout the entire cavity space, forming a high-density plasma.

[0032] The plasma terminal component 5 is connected to the outlet end of the plasma generating cavity 3. For example... Figure 4 As shown, its core is a double-layer batching disk structure, specifically including a first aperture plate 51 and a second aperture plate 52 arranged parallel to each other along the beam direction. The first aperture plate 51 is a planar structure, and the second aperture plate 52 is a conical structure facing downstream of the beam. Both aperture plates are precisely machined with multiple uniform through holes 53 in an array, and the radius of all through holes 53 is preferably 0.1 mm. When the plasma enters the plasma terminal device 5 from the plasma generation cavity 3, it first undergoes spatial sampling and flow equalization through the first aperture plate 51, and then undergoes secondary diffusion and shaping through the conical second aperture plate 52 with a certain angle. This combination of "planar screening + conical diffusion" can efficiently convert the plasma generated in the cavity, which may have uneven spatial distribution, into a spatially uniform molecular beam, which is finally directionally sprayed onto the substrate surface in the MBE growth cavity.

[0033] In the following embodiments, the plasma generating chamber 3 and the above-mentioned components can be encapsulated in an outer housing 6. The housing 6 can be reliably sealed to the ultra-high vacuum growth chamber of the MBE system itself through a connecting flange, ensuring the integration and vacuum integrity of the entire beam source furnace and the MBE system.

[0034] Example 1 This embodiment provides a method for growing crystalline thin films using the above-described beam source furnace. Taking the growth of β-Ga2O3 thin films as an example, the process is as follows: Figure 5 As shown, the specific steps are as follows: S1. Substrate Cleaning and Surface Pretreatment: The substrate was ultrasonically cleaned sequentially using acetone solution, anhydrous ethanol, and deionized water, followed by drying the substrate surface with an N2 gun. Next, after confirming that the rapid sample introduction chamber pressure was consistent with atmospheric pressure, the substrate was transferred to the main reaction chamber through the rapid sample introduction chamber, and the sample stage was adjusted to be parallel to the top. The substrate temperature was increased to 300°C and heated for 15 minutes to remove residual moisture from the substrate surface.

[0035] The assembled beam source furnace is installed onto the growth chamber of the MBE system via a flange; the substrate temperature is raised to 650°C by the heater on the back of the sample stage, ready to begin epitaxial growth.

[0036] S2. Introduce reaction gas: Turn on the gas mass flow controller 1 of the beam source furnace to introduce high-purity oxygen into the system, with the gas flow rate precisely controlled at 50 sccm. After being uniformly split by the plasma installation initiator 2, the oxygen smoothly enters the plasma generation chamber 3.

[0037] S3. Excite oxygen plasma: Turn on the 13.56MHz RF power supply connected to the RF electrode system 4 and set the output power to 250W. The strong electric field causes the oxygen in the cavity to discharge and ionize, generating highly active oxygen atom (O) and oxygen ion plasma.

[0038] S4. Simultaneous Growth of Gallium Oxide Thin Films: While maintaining a stable output oxygen plasma beam, the independent gallium (Ga) beam source furnace in the MBE system is activated to provide a Ga molecular beam. The oxygen plasma beam and the Ga molecular beam interact on the high-temperature substrate surface, causing a reaction. The growth parameters are: precisely controlled growth time of 2 hours, substrate temperature of 650°C, and growth gas pressure of 1.0 × 10⁻⁶. -3 Pa, the Ga source temperature is 830℃ (the Ga source temperature determines the Ga beam current density injected into the substrate; the higher the temperature, the greater the Ga beam current density). Based on the above parameter settings, β-Ga2O3 single crystal thin films with a thickness of 100~500nm, high crystal quality, and uniform thickness can be epitaxially grown on the substrate.

[0039] Comparative Example 1 This comparative example follows the same steps as Example 1, except that the growth parameters in step S4 are modified as follows: growth time is 2 hours, oxygen plasma power is 250W, gallium source temperature is 700°C, substrate temperature is 500°C, and growth gas pressure is 1.0 × 10⁻⁶. -3 Pa.

[0040] Under these parameter settings, the Ga beam current density is low, and the particles lack sufficient energy upon reaching the substrate surface, preventing them from diffusing to the optimal nucleation sites and forming amorphous Ga2O3. The substrate temperature is also low, resulting in insufficient diffusion length of adsorbed atoms, weak migration ability, a rough film surface with island and needle-like structures, poor uniformity, high defect density, and a relatively slow growth rate.

[0041] Example 2 This embodiment takes the growth of GaN thin films as an example, and the specific steps are as follows: S1. Substrate Cleaning and Surface Pretreatment: The substrate was ultrasonically cleaned sequentially using acetone solution, anhydrous ethanol, and deionized water, followed by drying the substrate surface with an N2 gun. Next, after confirming that the rapid sample introduction chamber pressure was consistent with atmospheric pressure, the substrate was transferred to the main reaction chamber through the rapid sample introduction chamber, and the sample stage was adjusted to be parallel to the top. The substrate temperature was increased to 300°C and heated for 15 minutes to remove residual moisture from the substrate surface.

[0042] The assembled beam source furnace is installed onto the growth chamber of the MBE system via a flange; the substrate temperature is raised to 760°C by the heater on the back of the sample stage, ready to begin epitaxial growth.

[0043] S2. Introduce reaction gas: Turn on the gas mass flow controller 1 of the beam source furnace to introduce high-purity nitrogen into the system, with the gas flow rate precisely controlled at 1.5 sccm. After being uniformly split by the plasma installation initiator 2, the nitrogen smoothly enters the plasma generation chamber 3.

[0044] S3. Excite nitrogen plasma: Turn on the 13.56MHz RF power supply connected to the RF electrode system 4 and set the output power to 330W. The strong electric field causes the nitrogen gas in the cavity to discharge and ionize, generating highly active nitrogen atom and nitrogen ion plasma.

[0045] S4. Simultaneous GaN Thin Film Growth: While maintaining a stable nitrogen plasma beam output, an independent gallium (Ga) beam source furnace within the MBE system is activated to provide a Ga molecular beam. The nitrogen plasma beam and the Ga molecular beam interact on the high-temperature substrate surface, resulting in a reaction. Growth parameters are: precisely controlled growth time of 2 hours, substrate temperature of 760°C, and growth gas pressure of 2.7 × 10⁻⁶. - 3 Pa, Ga source pressure is 8 × 10 -6 Pa. Based on the above parameter settings, GaN single crystal thin films with a thickness of 10~100nm, high crystal quality, and uniform thickness can be epitaxially grown on the substrate.

[0046] Comparative Example 2 This comparative example follows the same steps as Example 2, except that the growth parameters in step S4 are modified as follows: growth time is 2 hours, nitrogen plasma power is 280 W, and gallium source pressure is 1 × 10⁻⁶. -7 Pa, substrate temperature 600℃, growth gas pressure 1×10⁻⁶ -3 Pa.

[0047] Under these conditions, the Ga flux (BEP) is too low, nitrogen atoms are relatively excessive, and there are insufficient Ga atoms migrating to the surface, which cannot be successfully spread into a flat film, resulting in rough, polycrystalline or columnar three-dimensional growth, making it difficult to obtain a smooth single-crystal film.

[0048] Low substrate temperature disrupts the thermodynamic and kinetic conditions for epitaxial growth, causing the grown film to lose its single-crystal properties and become a pile of useless material full of defects, impurities, and rough surfaces. Insufficient growth pressure and inadequate active nitrogen beam intensity prevent Ga atoms reaching the substrate from finding enough active N atoms to combine with. Excess Ga atoms accumulate on the surface, forming liquid Ga metal spheres due to Ga's low melting point (~30℃), severely affecting parameters such as film surface smoothness and defect density.

[0049] The above embodiments and comparative examples verify that the method protected by the present invention can achieve efficient and uniform ionization of the reaction gas and output a highly uniform beam of active particles, thereby providing a reliable process basis for growing large-area, high-performance semiconductor single-crystal thin films.

[0050] Therefore, this invention provides a molecular beam epitaxy (MBE) source furnace and its thin film growth method, overcoming the problems of low ionization efficiency and poor uniformity of the generated active particle beam in existing MBE plasma source furnaces. It offers a novel MBE source furnace with a reasonable structural design, high ionization efficiency, and uniform and stable beam flow. The overall structure is compact, and the functions of each component are clearly defined and highly coordinated. The design of the initiator balances uniform gas intake with cavity airtightness; the external shell and flange connection ensure high vacuum compatibility and reliable sealing with the MBE system. This source furnace and its supporting method are particularly suitable for the MBE growth of high-quality, uniform nitride and oxide semiconductor single-crystal thin films, offering excellent controllability.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A molecular beam epitaxy (MBE) source furnace, characterized in that: The device includes a gas mass flow controller, a plasma mounting initiator, a plasma generating cavity, and a plasma end-effector connected sequentially along the beam direction. The outer wall of the plasma generating cavity is surrounded by an RF electrode system. The plasma mounting initiator has a conical structure, and the plasma end-effector has a double-layer batching disk structure.

2. The MBE molecular beam epitaxy source furnace according to claim 1, characterized in that: The plasma installation initiator is embedded inside the inlet of the plasma generating cavity. The plasma installation initiator includes an initiation cavity and gas holes for gas diversion opened on the upper wall of the initiation cavity.

3. The MBE molecular beam epitaxy source furnace according to claim 2, characterized in that: The RF electrode system is connected to an external RF power supply, and the RF electrode system includes electrode plates that are uniformly and crosswise distributed around the plasma generating cavity.

4. The MBE molecular beam epitaxy source furnace according to claim 3, characterized in that: The plasma terminal is connected to the outlet end of the plasma generating cavity. The plasma terminal includes a first orifice plate and a second orifice plate arranged sequentially along the beam direction. Multiple through holes are uniformly opened on the first orifice plate and the radius of each through hole is 0.1 mm.

5. The MBE molecular beam epitaxy source furnace according to claim 4, characterized in that: The first perforated plate has a planar structure, and the second perforated plate has a conical structure.

6. The MBE molecular beam epitaxy source furnace according to claim 5, characterized in that: The range of the gas mass flow controller is 200 sccm.

7. A method for growing thin films using an MBE molecular beam epitaxy source furnace as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. First, the substrate is cleaned and pretreated. Then, the beam source furnace is sealed and connected to the vacuum chamber of the MBE system through a flange. The treated substrate is placed in the vacuum chamber of the MBE system and heated to the growth temperature. S2. The reaction gas is introduced into the MBE molecular beam epitaxy source furnace through the gas mass flow controller. The reaction gas enters the plasma generation chamber after being split by the plasma installation initiator. S3. Simultaneously start the power supply connected to the RF electrode system to form a strong electric field in the plasma generation cavity, causing the reactant gas to ionize and generate plasma. S4. The plasma is shaped into a uniform molecular beam by the double-layer batching disk structure of the plasma terminal device, and sprayed onto the substrate surface, where thin film epitaxial growth is performed to form a crystalline thin film.

8. The method according to claim 7, characterized in that: In S1, the growth temperature is 500~850℃.

9. The method according to claim 8, characterized in that: In step S2, the reactant gas is oxygen or nitrogen; when the reactant gas is oxygen, the gas flow rate is 30~100 sccm; when the reactant gas is nitrogen, the gas flow rate is 1.5~3 sccm.

10. The method according to claim 9, characterized in that: In step S4, when the reactant gas is oxygen, the epitaxial growth process parameters are: growth time of 2 hours, oxygen plasma power of 250W, gallium source temperature of 800~840℃, substrate temperature of 650~700℃, and growth gas pressure of 1.4×10⁻⁶. -3 Pa ~ 2.6 × 10 -3 Pa; When the reactant gas is nitrogen, the epitaxial growth process parameters are: growth time of 2 hours, nitrogen plasma power of 330~450W, and gallium source pressure of 8×10⁻⁶. -6 Pa ~ 1.6 × 10 -5 Pa, substrate temperature 700~850℃, growth gas pressure 2.7×10 -3 Pa ~ 5.3 × 10 -3 Pa.

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