Methods to reduce background doping concentration in silicon carbide epitaxy

Abstract

This invention discloses a method for reducing the background doping concentration in silicon carbide epitaxy. First, the silicon carbide reaction chamber is pretreated by baking it for 1-2 hours under high vacuum, high temperature, and low pressure conditions to remove air and moisture, effectively improving the cleanliness of the chamber. Then, before growing the epitaxial layer, ethylene gas is introduced to regulate the atmosphere of the reaction chamber, creating a carbon-rich environment that discourages the doping of impurity gases onto the epitaxial layer. Finally, the epitaxial layer is grown using chemical vapor deposition under high temperature and low pressure conditions, employing ethylene (C2H4) gas as the carbon source and trichlorosilane (SiHCl3) gas as the silicon source. By controlling the carbon-silicon flow ratio to a carbon-rich condition, the growth rate is accelerated during growth, thereby reducing the background doping concentration. Simultaneously, the carbon-rich condition also reduces the doping efficiency.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and in particular to a method for effectively reducing the background doping concentration of silicon carbide epitaxy. Background Technology

[0002] Silicon carbide (SiC), as a wide bandgap semiconductor material, has the characteristics of high critical breakdown electric field, high thermal conductivity and high electron saturation migration rate. These unique advantages enable it to be used in high-power, high-temperature, high-voltage and radiation-resistant applications.

[0003] The fabrication of silicon carbide (SiC) power electronic devices requires single-layer or multi-layer SiC epitaxial materials with specific doping concentrations and thicknesses. Currently, SiC epitaxial materials are mainly prepared using chemical vapor deposition (CVD). Background doping is a critical parameter in the SiC epitaxial process, as its level determines the doping concentration range, resistivity, thermal conductivity, and other properties of the epitaxial layer, and also affects the device's breakdown voltage. The sources of background doping impurities are primarily oxygen, water molecules, and nitrogen adsorbed on the inner wall of the reactor and the surface of graphite components within the reactor. During the epitaxial process, the gases adsorbed on the reactor inner wall are slowly released into the reaction chamber; nitrogen is the primary gas affecting the background doping concentration.

[0004] For high-voltage bipolar devices, thin, lightly doped epitaxial films are essential for manufacturing high-voltage SiC devices, while also requiring sufficiently low carrier concentrations. For example, to manufacture a 20kV SiC bipolar device, the epitaxial film thickness must be greater than 200µm, and the carrier concentration must be close to 1×10⁻⁶. 14 / cm 3 This requires that the background doping concentration of silicon carbide epitaxy be sufficiently low. Epitaxial materials with low background doping concentration can greatly reduce the reverse bias electric field of high voltage devices. If the background doping concentration is too high, the epitaxial material cannot obtain a low carrier concentration.

[0005] However, the background doping concentration of silicon carbide epitaxy is currently generally around 5 × 10⁻⁶. 13 / cm 3 ~1×10 14 / cm 3 However, ultra-high voltage device materials require a lower background doping concentration, and existing methods cannot achieve such a low background doping concentration, thus failing to meet the fabrication requirements of ultra-high voltage devices.

[0006] Therefore, it is necessary to provide a method that can effectively reduce the background doping concentration of silicon carbide epitaxy in order to solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to provide a method that can effectively reduce the background doping concentration of silicon carbide epitaxy.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: a method for reducing the background doping concentration of silicon carbide epitaxy, comprising the following steps:

[0009] (1) The silicon carbide reaction chamber is pretreated by baking under high vacuum, high temperature and low pressure conditions for 1-2 hours to remove air and moisture from the reaction chamber;

[0010] (2) Select a suitable silicon carbide substrate, and use the robotic arm of the epitaxial reaction chamber system to automatically transfer the silicon carbide substrate into the interior of the reaction chamber and place it on the graphite base; wherein, the graphite base has good heat transfer effect and plays the role of quickly transferring heat to the silicon carbide substrate.

[0011] (3) Hydrogen is introduced into the reaction chamber as a carrier gas, and the pressure of the reaction chamber is controlled at 50 mbar. The temperature inside the reaction chamber is slowly increased. When the temperature inside the reaction chamber reaches 1600°, ethylene gas is introduced and maintained for 20 minutes, and then the ethylene gas is turned off. Then, the temperature inside the reaction chamber is slowly increased to the temperature required for epitaxial growth. Specifically, the introduction of ethylene gas in this heating step is mainly used to adjust the atmosphere of the reaction chamber before epitaxial growth. Ethylene gas decomposes into carbon atoms at high temperature, so that the reaction chamber is in a carbon-biased atmosphere, which is not conducive to the doping of impurity gases onto the epitaxial layer.

[0012] (4) The temperature of the reaction chamber is maintained at the temperature required for epitaxial growth, and the pressure in the reaction chamber is maintained at 50 mbar. Ethylene and trichlorosilane are introduced as growth gases, and the carbon-silicon flow ratio is controlled under carbon-rich conditions. Epitaxial layer growth is carried out by chemical vapor deposition for 1 hour. After the growth is completed, hydrogen is retained and ethylene and trichlorosilane gases are turned off. Among them, controlling the carbon-silicon flow ratio under carbon-rich conditions can reduce the doping efficiency.

[0013] (5) After the epitaxial layer growth is completed and the wafer is obtained, the reaction chamber is automatically cooled. When the temperature of the reaction chamber slowly drops to 900°, the robotic arm of the epitaxial reaction chamber system automatically transfers the wafer to the wafer pick-up area.

[0014] Preferably, in one embodiment of the method for reducing the background doping concentration of silicon carbide epitaxy according to the present invention, step (1) specifically includes:

[0015] (11) Raise the temperature of the reaction chamber to between 1200° and 1600°, while introducing 100 liters of hydrogen gas, and control the pressure of the reaction chamber between 100 mbar and 200 mbar, and maintain the temperature and pressure for 30 minutes; specifically, hydrogen gas plays the role of heat transfer, and this step removes the air and moisture in the reaction chamber by raising the temperature and introducing hydrogen gas (H2);

[0016] (12) Reduce the temperature of the reaction chamber to about 1100°, then turn off the hydrogen gas, and use the mechanical vacuum pump of the epitaxial reaction chamber system to pump the pressure in the reaction chamber to between 1 mbar and 5 mbar.

[0017] (13) The molecular pump of the epitaxial reaction chamber system was activated to further pump the pressure inside the reaction chamber to 2 × 10⁻⁶. - 5 mbar ~ 5×10 -5 Between mbar and maintain this pressure state for 1 to 2 hours;

[0018] (14) The temperature of the reaction chamber is slowly reduced to 900°C, and 30 liters of hydrogen gas is introduced at the same time to release the air, nitrogen and moisture remaining in the graphite fittings in the reaction chamber. Then the gas in the reaction chamber is extracted to achieve a higher level of cleanliness in the reaction chamber.

[0019] In one embodiment of the method for reducing the background doping concentration of silicon carbide epitaxy according to the present invention, in step (2), a 6-inch N-type silicon carbide substrate biased 4° in the <11-20> direction is selected.

[0020] In one embodiment of the method for reducing the background doping concentration of silicon carbide epitaxy according to the present invention, the flow rate of hydrogen gas introduced in step (3) is 150 L / min, the flow rate of ethylene gas introduced is 7-15 ccm / min, and the temperature required for epitaxial growth is 1680°.

[0021] In one embodiment of the method for reducing the background doping concentration of silicon carbide epitaxy according to the present invention, in step (4), ethylene gas is introduced at a flow rate of 180-250 ccm / min and trichlorosilane gas is introduced at a flow rate of 450-560 ccm / min, and the carbon-silicon flow rate ratio is controlled between 1.4 and 1.6. Such a flow rate ratio makes the reaction chamber carbon-rich. The carbon-rich condition has two effects: first, it reduces the doping efficiency, and second, it accelerates the growth rate. In the present invention, the growth rate of the epitaxial layer can reach 100 micrometers per hour. Compared with the existing growth rates which are generally below 60 micrometers per hour, the higher growth rate of the present invention also plays a role in reducing the doping efficiency.

[0022] Compared with existing technologies, the method for reducing the background doping concentration of silicon carbide epitaxy in this invention first pre-treats the silicon carbide reaction chamber by baking it for 1-2 hours under high vacuum, high temperature, and low pressure conditions to remove air and moisture, effectively improving the cleanliness of the reaction chamber. Secondly, ethylene (C2H4) gas is introduced before growing the epitaxial layer to regulate the atmosphere of the reaction chamber, creating a carbon-rich environment that discourages impurity gases from doping onto the epitaxial layer. Finally, the epitaxial layer is grown using chemical vapor deposition under high temperature and low pressure conditions, with ethylene (C2H4) gas as the carbon source and trichlorosilane (SiHCl3) gas as the silicon source. By controlling a high carbon-silicon flow ratio, the reaction chamber is kept under carbon-rich conditions, thereby accelerating the growth rate and reducing the background doping concentration. The carbon-rich conditions also reduce doping efficiency. Attached Figure Description

[0023] Figure 1 This is a flowchart of the method for reducing the background doping concentration of silicon carbide epitaxy according to the present invention.

[0024] Figure 2 yes Figure 1 The sub-flowchart of step S01.

[0025] Figure 3 This is a background doping concentration map measured after epitaxial growth using the method of reducing the background doping concentration of silicon carbide epitaxy according to the present invention.

[0026] Figure 4 This is a background doping concentration map measured after epitaxial growth using existing methods. Detailed Implementation

[0027] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which similar element reference numerals represent similar elements. It should be noted that the directional descriptions involved in the present invention, such as up, down, left, right, front, and rear, indicating directions or positional relationships, are based on the directions or positional relationships shown in the drawings and are only for the convenience of describing the technical solutions of this application or / and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. The terms "first," "second," etc., described are only used to distinguish technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the sequential relationship of the indicated technical features.

[0028] The method for reducing the background doping concentration in silicon carbide epitaxy provided by this invention is applicable to epitaxial reaction chamber systems for growing epitaxial layers. The epitaxial reaction chamber system mainly consists of several parts, including an inlet device, an exhaust device, a reaction chamber, a cooling cavity, and a robotic arm. A graphite substrate is provided inside the reaction chamber, and the robotic arm is responsible for transferring the substrate onto the graphite substrate inside the reaction chamber and transferring the epitaxially grown wafer to the wafer pick-up area. The structure and working principle of each part of the epitaxial reaction chamber system involved in this invention are conventional in the art and therefore will not be described in detail.

[0029] Please refer to the following first. Figure 1 As shown, the method for reducing the background doping concentration of silicon carbide epitaxy provided by the present invention specifically includes the following steps:

[0030] S01. The silicon carbide reaction chamber is pretreated by baking under high vacuum, high temperature and low pressure conditions for 1-2 hours to remove air and moisture from the reaction chamber.

[0031] In the method of the present invention, a pretreatment step for the reaction chamber is specifically added to fully remove air and moisture from the reaction chamber, thereby effectively improving the cleanliness of the reaction chamber.

[0032] S02. Select a suitable silicon carbide substrate, and use the robotic arm of the epitaxial reaction chamber system to automatically transfer the silicon carbide substrate into the interior of the reaction chamber and place it on the graphite base; wherein, the graphite base has good heat transfer effect, which plays the role of quickly transferring heat to the silicon carbide substrate.

[0033] In a preferred embodiment of this invention, a 6-inch N-type silicon carbide substrate offset at 4° in the <11-20> direction is selected. In SiC epitaxial growth, to ensure the epitaxial material effectively inherits the substrate's stacking sequence and guarantees crystal quality, substrates cut along the c-axis at an offset of <11-20> are currently widely used. The significance of this offset cutting lies in introducing atomic-level steps on the substrate surface. During epitaxy, adsorbed atoms tend to nucleate and grow at these steps, ensuring the epitaxy process proceeds in a step-flow pattern. Currently, the commonly used SiC substrate offset angles are 4° and 8°. The step density on the surface of 4° and 8° offset SiC substrates differs, resulting in different hydrogen etching rates and silicon vapor equilibrium partial pressures above the mesa. Therefore, the required in-line etching processes differ. Understandably, this invention is not limited to the aforementioned silicon carbide substrate; other types of silicon carbide substrates can be selected according to production needs.

[0034] S03. Introduce hydrogen as a carrier gas into the reaction chamber and control the pressure of the reaction chamber at 50 mbar, so that the temperature in the reaction chamber rises slowly. When the temperature in the reaction chamber reaches 1600°, introduce ethylene gas and maintain it for 20 minutes, then turn off the ethylene gas; then, slowly raise the temperature in the reaction chamber to the temperature required for epitaxial growth.

[0035] Specifically, this step is the heating stage of the reaction chamber. The preferred flow rate of hydrogen (H2) is 150 L / min, and the preferred flow rate of ethylene (C2H4) gas is 7-15 ccm / min. However, these flow rates are not limited to the aforementioned rates and can be adjusted flexibly as needed. When ethylene (C2H4) gas is introduced, it decomposes into carbon atoms at high temperature, creating a slightly carbonaceous atmosphere in the reaction chamber. This is primarily used to regulate the atmosphere of the reaction chamber before epitaxial growth, as a slightly carbonaceous atmosphere is unfavorable for the doping of impurity gases onto the epitaxial layer.

[0036] In a preferred embodiment of the present invention, the temperature required for epitaxial growth in this step is 1680°C. It is understood that the temperature is not limited to this, and a suitable growth temperature can be set as needed.

[0037] S04. Maintain the temperature of the reaction chamber at the temperature required for epitaxial growth, and maintain the pressure in the reaction chamber at 50 mbar. Introduce growth gases ethylene and trichlorosilane, and control the carbon-silicon flow ratio under carbon-rich conditions. Use chemical vapor deposition to grow the epitaxial layer for 1 hour. After growth, retain hydrogen gas and shut off the ethylene and trichlorosilane gases.

[0038] In a preferred embodiment of this invention, the temperature of the reaction chamber is maintained at 1680°C and the pressure at 50 mbar. Ethylene (C₂H₄) growth gas is then introduced at a flow rate of 180-250 ccm / min, and trichlorosilane (SiHCl₃) gas is introduced at a flow rate of 450-560 ccm / min. Of course, the flow rates of ethylene (C₂H₄) gas and trichlorosilane (SiHCl₃) gas can be adjusted as needed. Furthermore, in this invention, the carbon-silicon flow ratio is preferably controlled between 1.4 and 1.6 to create a carbon-rich environment within the reaction chamber. Compared to existing methods with carbon-silicon flow ratios between 1.0 and 1.2, the higher carbon-silicon flow ratio of this invention creates carbon-rich conditions that serve two purposes: firstly, reducing doping efficiency, and secondly, accelerating the growth rate. In this invention, the epitaxial layer growth rate can reach 100 micrometers per hour. Compared to existing growth rates that are generally below 60 micrometers per hour, the higher growth rate of this invention also helps to reduce doping efficiency.

[0039] S05. After the epitaxial layer growth is completed and the wafer is obtained, the reaction chamber is automatically cooled. When the temperature of the reaction chamber slowly drops to 900°, the robotic arm of the epitaxial reaction chamber system automatically transfers the wafer to the wafer pick-up area.

[0040] The following is combined with Figure 1-2 As shown, in one embodiment of the method for reducing the background doping concentration of silicon carbide epitaxy according to the present invention, step S01 specifically includes the following steps:

[0041] S11. Raise the temperature of the reaction chamber to between 1200° and 1600°, while simultaneously introducing 100 liters of hydrogen gas, and control the pressure of the reaction chamber between 100 mbar and 200 mbar, maintaining the temperature and pressure for 30 minutes.

[0042] Specifically, since hydrogen (H2) plays a role in heat transfer, this step involves heating and introducing hydrogen (H2), maintaining the temperature and pressure for a certain period of time, thereby initially removing the air and moisture from the reaction chamber.

[0043] S12. Reduce the temperature of the reaction chamber to about 1100°C, then turn off the hydrogen gas, and use the mechanical vacuum pump of the epitaxial reaction chamber system to pump the pressure in the reaction chamber to between 1 mbar and 5 mbar.

[0044] The purpose of this step is to extract air, moisture, etc. from the reaction chamber and reduce the pressure inside the reaction chamber. The mechanical vacuum pump of the epitaxial reaction chamber system involved in this step is part of the air extraction device of the epitaxial reaction chamber system. Its structure and principle are conventional in this field and will not be described in detail here.

[0045] S13. Activate the molecular pump of the epitaxial reaction chamber system to further pump the pressure inside the reaction chamber to 2 × 10⁻⁶. - 5 mbar ~ 5×10 -5 Between mbar and maintain this pressure state for 1 to 2 hours;

[0046] The purpose of this step is to achieve a lower pressure in the reaction chamber. Furthermore, the molecular pump of the epitaxial reaction chamber system involved in this step is also part of the pumping device of the epitaxial reaction chamber system. Its structure and principle are conventional in the field and will not be described in detail here.

[0047] S14. Slowly lower the temperature of the reaction chamber to 900°C, and at the same time introduce 30 liters of hydrogen gas to release the air, nitrogen, and moisture remaining in the graphite components in the reaction chamber. Then, remove the gas from the reaction chamber.

[0048] More specifically, since gases expand under high temperature and low pressure, after reducing the pressure in the reaction chamber through the above steps S12-S13, hydrogen (H2) is introduced in this step. The expansion of hydrogen (H2) releases the air, nitrogen and moisture remaining in the graphite components in the reaction chamber. Then, the gas in the reaction chamber is removed by evacuation, thereby further removing impurities such as gases and moisture from the reaction chamber, so that the interior of the reaction chamber achieves a higher level of cleanliness.

[0049] The following is combined with Figure 1-4 As shown, the method of reducing the background doping concentration of silicon carbide epitaxy according to the present invention can be used to grow epitaxial layers, and the background doping concentration can reach 2×10⁻⁶. 13 / cm 3 See below for details. Figure 3 As shown, the background doping concentration after epitaxial layer growth using the method of the present invention was detected, and the obtained background doping concentration was 1.4155 × 10⁻⁶. 13 / cm 3 Accordingly, the background doping concentration of the epitaxial layer grown using the existing method was detected, and the background doping concentration was found to be 6.851 × 10⁻⁶. 13 / cm 3 See details Figure 4 As shown in the figure. The above comparison demonstrates that the method of the present invention can indeed effectively reduce the background doping concentration.

[0050] In summary, the method for reducing the background doping concentration in silicon carbide epitaxy according to the present invention first pre-treats the silicon carbide reaction chamber by baking it for 1-2 hours under high vacuum, high temperature, and low pressure conditions to remove air and moisture, effectively improving the cleanliness of the reaction chamber. Secondly, ethylene (C2H4) gas is introduced before epitaxial layer growth to regulate the atmosphere of the reaction chamber, creating a carbon-rich environment that discourages impurity gases from doping onto the epitaxial layer. Finally, the epitaxial layer is grown using chemical vapor deposition under high temperature and low pressure conditions, with ethylene (C2H4) gas as the carbon source and trichlorosilane (SiHCl3) gas as the silicon source. By controlling a high carbon-silicon flow ratio, the reaction chamber is kept under carbon-rich conditions, thereby accelerating the growth rate and reducing the background doping concentration. The carbon-rich conditions also reduce doping efficiency. Therefore, the present invention achieves the goals of accelerating the growth rate and reducing the background doping concentration through pre-treatment of the reaction chamber before epitaxial growth and increasing the carbon-silicon flow ratio during growth.

[0051] The description of the above embodiments has been provided for illustrative and descriptive purposes and is not intended to be exhaustive or limiting of this disclosure. Individual elements or features of particular embodiments are generally not limited to those particular embodiments, but may be interchanged and used in selected embodiments where applicable, even if not specifically shown or described. In many respects, the same elements or features may be changed, and such changes are not considered a departure from this application, and all such modifications are intended to be included within the scope of this application.

[0052] Exemplary embodiments are provided to make this application thorough and to fully convey its scope to those skilled in the art. Numerous details, such as examples of specific parts, apparatus, and methods, are set forth in order to provide a thorough understanding of embodiments of this application. It will be apparent to those skilled in the art that specific details are not required, and that the exemplary embodiments may be implemented in many different forms, neither of which should be construed as limiting the scope of this application. In some exemplary embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.

[0053] The above-disclosed embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, any equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method of reducing the background doping concentration of silicon carbide epitaxy, characterized by, The method comprises the following steps: (1) Pre-treating the silicon carbide reaction chamber by baking it under high vacuum, high temperature and low pressure for 1-2 hours to discharge air and moisture in the reaction chamber; (2) Selecting a suitable silicon carbide substrate and automatically transferring the substrate to the interior of the reaction chamber by a mechanical arm device of the epitaxial reaction chamber system and placing the substrate on a graphite base; (3) Introducing hydrogen as a carrier gas into the reaction chamber and controlling the pressure of the reaction chamber at 50 mbar, slowly increasing the temperature in the reaction chamber, introducing ethylene gas when the temperature in the reaction chamber reaches 1600° and maintaining the ethylene gas for 20 minutes, then turning off the ethylene gas, and then slowly increasing the temperature in the reaction chamber to a temperature required for epitaxial growth; (4) Maintaining the temperature of the reaction chamber at a temperature required for epitaxial growth and the pressure in the reaction chamber at 50 mbar, introducing growth gases ethylene and trichlorosilane, controlling the carbon-silicon flow ratio under carbon-rich conditions, and performing epitaxial layer growth by chemical vapor deposition, the growth time being 1 hour, retaining hydrogen after the growth is completed, and turning off the ethylene and trichlorosilane gases; (5) After the epitaxial layer growth is completed, automatically cooling the reaction chamber, automatically transferring the wafer to a wafer taking area by the mechanical arm device of the epitaxial reaction chamber system when the temperature of the reaction chamber slowly decreases to 900°; The step (1) comprises: (11) Increasing the temperature of the reaction chamber to 1200°-1600°, introducing 100 liters of hydrogen at the same time, and controlling the pressure of the reaction chamber at 100 mbar-200 mbar, maintaining the temperature and pressure for 30 minutes; (12) Decreasing the temperature of the reaction chamber to about 1100°, then turning off the hydrogen, and using a mechanical vacuum pump of the epitaxial reaction chamber system to pump the pressure in the reaction chamber to 1 mbar-5 mbar; (13) A molecular pump of the epitaxial reactor system is activated to further pump the pressure in the reactor to between 2 x 10 -5 mbar and 5 x 10 -5 mbar and maintain this pressure for 1 to 2 hours; (14) Slowly decreasing the temperature of the reaction chamber to 900°, introducing 30 liters of hydrogen at the same time, and releasing air, nitrogen and moisture remaining in the graphite fittings in the reaction chamber, then pumping away the gas in the reaction chamber.

2. The method of claim 1, wherein the carbonization is performed at a temperature of about 1,300°C to about 1,400°C. In the step (2), a 6-inch N-type silicon carbide substrate deviated by 4° in the <11-20> direction is selected.

3. The method of claim 1, wherein the carbonization is performed at a temperature of about 1400°C to about 1500°C. In the step (3), the flow rate of the introduced hydrogen is 150 L / min, the flow rate of the introduced ethylene gas is 7-15 ccm / min, and the temperature required for epitaxial growth is 1680°.

4. The method of claim 1, wherein the silicon carbide epitaxial background doping concentration is reduced by: In the step (4), the flow rate of the introduced ethylene gas is 180-250 ccm / min, the flow rate of the introduced trichlorosilane gas is 450-560 ccm / min, and the carbon-silicon flow ratio is controlled at 1.4-1.6.

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