Patent Application
20230279581
The present invention relates to a method for producing a nitride semiconductor wafer and a nitride semiconductor wafer.
Nitride semiconductors such as GaN and AlN are expected to be applied as semiconductor devices for high-frequency applications, since these make the fabrication of high electron mobility transistors (HEMTs) using two-dimensional electron gas possible. In addition, the above-mentioned nitride semiconductor is also a piezoelectric material with excellent mechanical properties, and is expected to be used in high-frequency filters for communication, sensors, energy harvesters, and the like.
However, it is difficult to produce wafers of these nitride semiconductors. For industrial applications, nitride semiconductor thin films formed by vapor phase growth on sapphire, SiC, and silicon substrates are used. In particular, the production of nitride semiconductor thin film by vapor phase growth on silicon substrates is considered promising because it allows the use of large-diameter substrates compared to sapphire and SiC, resulting in high productivity and an advantage in terms of heat dissipation properties. Patent Document 1 discloses use of a high resistance silicon wafer as a support substrate for a high-frequency communication device.
In addition, in high-frequency devices, in order to improve high-frequency characteristics, it is necessary to reduce parasitic capacitance of the device, its support substrate, and peripheral packages. It is considered that the using of a silicon substrate with a low oxygen concentration and a high resistance as a support substrate or a package for the purpose of the reduction of the parasitic capacity can improve the characteristics and have an advantage in terms of cost.
On the other hand, device manufacturing process includes processes such as epitaxial growth on the substrate, heat treatment, and bonding. There is a risk of plastic deformation resulting from the stress caused in the substrate due to differences in lattice constant and thermal expansion coefficient between different materials. When plastic deformation occurs, the wafer is greatly distorted and cannot return to its original shape. There is a possibility that warpage abnormality or bonding failure may occur.
In particular, a silicon substrate with a high resistivity and a low oxygen concentration has a low mechanical strength because it contains an extremely small amount of impurities such as dopants and oxygen. Therefore, when a nitride semiconductor is epitaxially grown, warpage increases and plastic deformation tend to occur resulting from stress due to difference in lattice constant and thermal expansion coefficient, so stress relaxation is performed by growth conditions and relaxation layers. Patent Document 2 discloses a gallium nitride-based compound semiconductor substrate in which periodic-deposition layers are formed by laminating a plurality of composite layers on a silicon single crystal substrate in order to reduce the warpage of the substrate.
Further, Patent Document 3 discloses a GaN/Si substrate using a silicon substrate with a high resistivity of 5000 Ω·cm, but the amount of warpage increases as the oxygen concentration decreases. However, it is desirable that the amount of warpage is 50 μm or less for use in a general device-manufacturing process, and it cannot be said that sufficient countermeasures have been taken.
The present invention has been made to solve the above problems, and the object of the present invention is to provide a method for producing a nitride semiconductor wafer in which plastic deformation and warpage are suppressed even in the case a silicon single crystal substrate with high resistivity and low oxygen concentration, especially ultra-low oxygen concentration, which is promising as a support substrate for high-frequency devices, is used.
The present invention has been made to achieve the above objects, and provides a method for producing a nitride semiconductor wafer in which a nitride semiconductor thin film is grown on a silicon single crystal substrate by vapor phase growth, comprising, by using a silicon single crystal substrate having a resistivity of 1000 Ω·cm or more, an oxygen concentration of less than 1×1017 atoms/cm3 and a thickness of 1000 μm or more, growing the nitride semiconductor thin film on the silicon single crystal substrate by vapor phase growth.
According to such a method for manufacturing a nitride semiconductor wafer, even when using a silicon single crystal substrate with low mechanical strength, high resistivity, and an ultra-low oxygen concentration of less than 1×1017 atoms/cm3, a nitride semiconductor wafer in which plastic deformation and warpage are suppressed can be produced.
At this time, after the nitride semiconductor thin film is grown by vapor phase growth, surface of the silicon single crystal substrate opposite to the surface on which the nitride semiconductor thin film is grown is polished to thin the silicon single crystal substrate.
By polishing and thinning the silicon single crystal substrate on the back surface after vapor phase growth in this way, plastic deformation during high-temperature growth can be prevented, and wafers having small warpage can be obtained even if the substrate is processed to a thickness suitable for device manufacturing after cooling.
At this time, it provides a nitride semiconductor wafer having a nitride semiconductor thin film on a silicon single crystal substrate, wherein the silicon single crystal substrate has a resistivity of 1000 Ω·cm or more, an oxygen concentration of less than 1×1017 atoms/cm3, and a thickness of 1000 μm or more.
According to such a nitride semiconductor wafer, it becomes that plastic deformation and warpage are suppressed even when a silicon single crystal substrate having low mechanical strength, high resistivity and ultra-low oxygen concentration is used as the support substrate.
As described above, according to the nitride semiconductor wafer and the method for producing the same according to the present invention, even a silicon single crystal substrate with a high resistivity and an ultra-low oxygen concentration, which is extremely promising as a support substrate for a high frequency device, a nitride semiconductor wafer in which plastic deformation and warpage are suppressed can be provided.
The present invention will be described in detail below, but the present invention is not limited to these.
As described above, there has been a demand for nitride semiconductor wafers and methods of producing the same of which plastic deformation and warpage are suppressed even when using a silicon single crystal substrate with high resistivity and low oxygen concentration, which is promising as a support substrate for high frequency devices but has low mechanical strength.
Silicon single crystals having the low oxygen concentration, in particular, ultra-low oxygen concentration of less than 1×1017 atoms/cm3, and even 0.5×1017 atoms/cm3 or less, and high resistivity can reliably reduce parasitic capacitance, and improve the high-frequency characteristics of the high-frequency devices. Therefore, such silicon single crystals is highly expected to be applied as semiconductor devices that will be required for high-frequency applications with higher-performance in the future.
However, in case of ultra-low oxygen, the amount of oxygen impurities is particularly low, so the mechanical strength is extremely low, and warpage and plastic deformation are likely to become extremely large during epitaxial growth.
Silicon single crystal substrates for high frequency devices are required to have a high resistivity of 1000 Ω·cm or more. Further, by setting the oxygen concentration to 7×1017 atoms/cm3 or less, particularly less than 1×1017 atoms/cm3, the effect of thermal donors on resistivity can be completely suppressed. However, when the resistivity is high and the oxygen concentration is extremely low, the Young's modulus when dislocations occur is lower than that of a normal low-resistivity substrate, and the substrate is extremely susceptible to plastic deformation. Under this circumstance, the inventors of the present invention have found that plastic deformation can be suppressed by using a silicon single crystal substrate having a wafer thickness of 1000 μm or more.
That is, as a result of extensive studies on the above problems, the inventors of the present invention have found by a method for producing a nitride semiconductor wafer in which a nitride semiconductor thin film is grown on a silicon single crystal substrate by vapor phase growth, including, by using a silicon single crystal substrate having a resistivity of 1000 Ω·cm or more, an oxygen concentration of less than 1×1017 atoms/cm3 and a thickness of 1000 μm or more, growing the nitride semiconductor thin film on the silicon single crystal substrate by vapor phase growth, a nitride semiconductor wafer in which plastic deformation and warpage are suppressed even when using a silicon single crystal substrate with a high resistivity and an ultra-low oxygen concentration can be produced and have completed the present invention.
Furthermore, the inventors of the present invention also have found that plastic deformation and warpage are suppressed by a nitride semiconductor wafer having a nitride semiconductor thin film on a silicon single crystal substrate, wherein the silicon single crystal substrate has a resistivity of 1000 Ω·cm or more, an oxygen concentration of less than 1×1017 atoms/cm3, and a thickness of 1000 μm or more.
Description will be made below with reference to the drawings.
The reason why the silicon single crystal substrate has a resistivity of 1000 Ω·cm or more is to achieve a level required as a substrate for high-frequency devices. Since the resistivity of the silicon single crystal substrate is preferable as high as possible, the upper limit is not particularly limited and may be infinite. As for the oxygen concentration, if it exceeds 7×1017 atoms/cm3 (JEIDA), the influence of thermal donors on resistivity cannot be ignored, so 7×1017 atoms/cm3 (JEIDA) is set as the upper limit. In addition, in order to completely eliminate the influence of resistivity due to thermal donors, to reliably reduce parasitic capacitance, and improve the high-frequency characteristics of high-frequency devices, and then make the wafer applicable for semiconductor devices which are required for future high-frequency applications with higher performances, an oxygen concentration is set low, in particular, an ultra-low oxygen concentration of less than 1×1017 atoms/cm3, even 0.5×1017 atoms/cm3 or less. This can almost completely eliminate the influence of oxygen concentration. The oxygen concentration is preferable as low as possible, and although the lower limit is not particularly limited, it can be substantially 0 like a FZ single crystal.
On the other hand, by setting the thickness of the silicon single crystal substrate to 1000 μm or more, it is possible to prevent plastic deformation during growth of the nitride semiconductor and reduce warpage. Although the upper limit of the thickness of the silicon single crystal substrate is not particularly limited, it is preferably about 1500 μm or less. If the thickness is within such a range, it is possible to suppress an increase in cost, to perform epitaxial growth of the nitride semiconductor thin film more stably, and to polish the silicon single crystal substrate in a post-process making it thinner as described later, the decrease of productivity can be suppressed.
Further, as shown in
In addition, a trap-rich layer that reduces the lifetime of carriers may be formed on the surface of the silicon single crystal substrate 12 (the interface with the intermediate layer 14 in
A device layer 16 made of a nitride semiconductor thin film is provided on the silicon single crystal substrate 12 (on the intermediate layer 14 when the intermediate layer 14 is formed on the silicon single crystal substrate 12). As the nitride semiconductor of the device layer, for example, GaN, AlN, InN, AlGaN, InGaN, AlInN, etc. can be used.
As an application example of the nitride semiconductor wafer according to the present invention,
Gallium nitride (GaN) has lattice constant difference of 17% and thermal expansion coefficient difference of 116% from Si (111) single crystal, and stresses are applied to the thin film and substrate during growth at high temperature. In addition, since the wafer is heated to 1000° C. or higher during growth, when stress is applied to the wafer, it does not undergo brittle fracture but exhibits ductility, generates dislocations, and undergoes plastic deformation.
Therefore, according to the present invention, by using a silicon single crystal substrate 12 having a thickness of 1000 μm or more, it is possible to prevent dislocation propagation in the silicon single crystal substrate 12 and prevent plastic deformation. By preventing plastic deformation, abnormal warpage can be reduced, and the yield of producing the nitride semiconductor wafer 10 can be improved. In addition, since the silicon single crystal substrate 12 can withstand stress, the film thickness of the nitride semiconductor thin film that becomes the device layer 16 can be increased by vapor phase growth, and the degree of freedom in designing the device is improved.
Next, a method for producing a nitride semiconductor wafer according to the present invention will be described. Referring to
A method for producing such a silicon single crystal substrate having a thickness of 1000 μm or more is not particularly limited, and it can be produced by a known method. By slicing, polishing, cleaning, etc., a silicon ingot formed by the CZ method or the FZ method, a silicon single crystal substrate having a thickness of 1000 μm or more may be produced, or a silicon single crystal substrate having a thickness of 1000 μm or more may be formed by forming a silicon epitaxial growth layer on a silicon single crystal substrate having a thickness of less than 1000 μm.
An intermediate layer 14 may be formed on the silicon single crystal substrate 12 before the growth of the nitride semiconductor thin film that becomes the device layer 16, and the nitride semiconductor thin film that becomes the device layer 16 may be grown on the intermediate layer 14.
Furthermore, a trap-rich layer may be formed on the surface of the silicon single crystal substrate 12 (the interface with the intermediate layer 14 in
On the silicon single crystal substrate 12 (on the intermediate layer 14 when the intermediate layer 14 is formed on the silicon single crystal substrate 12), a device layer 16 made of a nitride semiconductor thin film is produced by vapor phase growth such as MOVPE (metal organic chemical vapor phase epitaxy) method or sputtering method. The nitride semiconductor thin film can be 1-10 μm thick and can be designed for the device.
In the high electron mobility transistor (HEMT) structure shown in
As described above, plastic deformation during high-temperature growth can be prevented by making the silicon single crystal substrate thicker than 1000 μm. A wafer with small warpage can be obtained, even if after the nitride semiconductor thin film is grown by vapor phase growth, and after cooling, the substrate is thinned to have a suitable thickness for device production by polishing the surface of the silicon single crystal substrate opposite to the surface on which the nitride semiconductor thin film is grown, or the like. In this case, even if the silicon single crystal substrate is thinned, plastic deformation does not occur in the substrate, so that a nitride semiconductor wafer having a device layer on a thinned silicon single crystal substrate without plastic deformation can be obtained.
The present invention will be specifically described below with reference to Examples and Reference Examples, but these are not intended to limit the scope of the present invention.
A silicon single crystal substrate having a diameter of 150 mm, an axial orientation of <111>, a resistivity of 1000 Ω·cm, an oxygen concentration of 5×1017 atoms/cm3, and a thickness of 1000 μm was provided. An epitaxial layer of a nitride semiconductor thin film having a total thickness of 2.8 μm was grown on the provided silicon single crystal substrate at growth temperature of 1200° C. in a MOVPE furnace.
A nitride semiconductor wafer was produced under the same conditions as in Reference Example 1, except that thickness of the silicon single crystal substrate was 675 μm.
In Reference Example 2, the curvature changed significantly during the epitaxial growth. Moreover, plastic deformation occurred during the epitaxial growth, and the bow after growth was as large as −233.2 μm, resulting in a failure.
From Reference Examples 1 and 2, it was found that if the thickness of the silicon single crystal substrate 12 with an oxygen concentration of 5×1017 atoms/cm3 was 1000 μm or more, the occurrence of plastic deformation could be suppressed and the warpage could be reduced.
A nitride semiconductor was epitaxially grown under the same conditions as in Reference Example 1, except that a thickness of the silicon single crystal substrate was 1200 μm, but plastic deformation did not occur in the substrate as in Reference Example 1.
A nitride semiconductor wafer was produced under the same conditions as in Reference Example 1, except that an oxygen concentration of the silicon single crystal substrate was 1×1017 atoms/cm3.
As a result, no plastic deformation occurred during epitaxial growth, and the warpage after growth was 20.0 μm.
A nitride semiconductor wafer was produced under the same conditions as in Reference Example 1, except that an oxygen concentration of the silicon single crystal substrate was 0.8×1017 atoms/cm3.
As a result, no plastic deformation occurred during epitaxial growth, and the warpage after growth was 32.0 μm.
A nitride semiconductor wafer was produced under the same conditions as in Reference Example 1, except that the silicon single crystal substrate had an oxygen concentration of 0.5×1017 atoms/cm3 and a thickness of 1200 μm.
As a result, no plastic deformation occurred during epitaxial growth, and the warpage after growth was 22.0 μm.
As described above, in the examples, even if the oxygen concentration of the silicon single crystal substrate was set to ultra-low oxygen concentration of less than 1×1017 atoms/cm3, plastic deformation did not occur, and the warpage after growth could be 50 μm or less, which was significantly smaller than that of Reference Example 2.
Therefore, the nitride semiconductor wafer of the present invention can be applied to semiconductor devices that is required for feature high-frequency applications with higher-performance.
The present invention is not limited to the above embodiments. The above-described embodiments are just examples, and any examples that substantially have the same configuration and demonstrate the same functions and effects as those in the technical concept disclosed in the claims of the present invention are included in the technical scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-137796 | Aug 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/014837 | 4/8/2021 | WO |
