The present invention relates to a method for manufacturing silicon carbide in which a silicon carbide crystal is grown by a sublimation method.
Recently, inverter circuits have been commonly used in electric vehicles and electric air-conditioners. This creates demands for semiconductor crystal of silicon carbide (hereinafter may also be referred to as SiC) because of the properties of less power loss and higher breakdown voltage in devices than those using semiconductor Si crystal.
As a typical and practical method for growing a crystal with a high melting point or a crystal that is difficult to grow by liquid phase growth such as SiC, a sublimation method exists. In this method, a solid raw material is sublimated in a container at a high temperature around 2000° C. or higher, and a crystal is grown on a seed crystal located on the opposite side (Patent Document 1). However, SiC crystal growth requires high temperature for the sublimation, and the growth apparatus requires temperature control at the high temperature. Moreover, to stabilize the pressure of the sublimated substance, it is necessary to steadily control the pressure inside the container. Further, SiC crystal growth depends on the sublimation rate, and the growth rate is relatively quite slow in comparison with Czochralski method for Si, LPE manufacturing method for GaAs and so forth, for example. Hence, long time is required for the growth. Fortunately, the developments of control units, computers, personal computers, and so forth nowadays enable long-term steady adjustments of pressure and temperature.
Specifically, a method for growing SiC by sublimation is performed using an apparatus 101 for growing a silicon carbide single crystal as shown in
The growth container 104 is disposed in a vacuum quartz tube or a vacuum chamber, and filled with a gas with low reactivity once. The atmosphere is lower than the atmospheric pressure so as to increase the SiC sublimation rate.
Outside the growth container 104, a heat-insulating material (heat-insulating container) 105 is disposed. At least one hole (upper-portion temperature measurement hole) 106 for measuring the temperature with a pyrometer (temperature-measurement sensor) 107 is provided to a portion of the heat-insulating material 105. The growth container 104 is mainly made of a carbon material and is air permeable, and the pressures inside and outside the container are equal. The silicon carbide raw material 103 is disposed at a lower portion of the growth container 104. The material 103 is solid, and sublimates at high temperature under reduced pressure. The sublimated material grows as the single crystal (grown crystal) 102a on the seed crystal substrate (seed wafer) 102 located on the opposite side.
Here, a conventional method for manufacturing a silicon carbide single crystal by using the above-described manufacturing apparatus will be described with reference to
Meanwhile, SiC single crystal includes cubic, hexagonal crystals, for example. Further, among hexagonal crystals, 4H, 6H, and so forth are known as typical polytypes.
In many cases, single crystal grows on the same type like 4H grows on a 4H type (Patent Document 2).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-191399
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-239465
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2010-126380
However, in a case where a seed substrate having a crystal orientation of a {0001} plane with no off angle (what is called just plane with an off angle of 1° or less) is used, there is a problem that a crystal started to grow is likely to be disordered (crystallites of different polytypes) (see
Moreover, when a SiC hexagonal crystal is grown on a seed substrate having a just plane with no off angle, for example, a seed substrate with a surface having a plane parallel to a hexagonal basal plane, some polytypes of the original seed are often formed during the random two-dimensional nucleation in the initial growth stage; for example, 6H structure is formed in addition to 4H structure (see
Like this example, if 4H structure is mixed with 6H structure as a different polytype, the value as a semiconductor is completely lost. This is because the properties such as breakdown voltage are greatly changed. In addition, cracking may also occur during slicing.
When a substrate having an off angle is used, the growth proceeds by step growth mechanism with relatively less disorder.
As often stated in textbooks of crystal growth, when carbon and silicon sources as raw materials enter a kink, the resultant keeps the 4H structure of the substrate and is hardly disordered.
A hexagonal crystal has three sites: A, B, and C. This is caused by twisting of a bond at the C side relative to Si in the following general formula (1), thereby creating a position or axis where three atoms enter on a hexagonal basal plane. As shown in
Si=C (1)
Thus, in the case of a seed (seed crystal substrate) having an off angle, even if the surface is flat, the growth can be started without modifying the polytype. In other words, even if the seed (seed crystal substrate) is flat, step is present on the flat growth surface. For this reason, it is relatively easy to achieve the growth without polytype modification through a mechanism called lateral growth or step growth.
In contrast, in the growth on a just plane, that is, a basal plane, the random two-dimensional nucleation and spiral growth intrinsic to SiC occur at many locations within the plane. Since the layered structure of the polytype is not exposed, the stacking order of the polytype is not preserved, so that small regions functioning like nuclei of seed crystals of different polytypes are developed, enlarged, and spread.
Nevertheless, when a seed (seed crystal substrate) with an off angle is used, the grown ingot does not have point symmetry, so that the percentage of the straight body (length of the straight body/full length) is low (i.e., the length of the straight body is short). This results in a problem of low wafer yield.
Meanwhile, Patent Document 3 discloses that an end face side of a grown silicon carbide single crystal is sliced to a predetermined thickness and used as an end face of a seed.
However, the studies by the present inventors have revealed that even when the seed crystal substrate as mentioned above is used, if the diameter of the seed crystal substrate is smaller than the inner diameter of the accommodation container, the effect of suppressing the formation of different polytypes is small.
The present invention has been made in view of the above-described problems. An object of the present invention is to provide a method for manufacturing a silicon carbide single crystal so that a silicon carbide single crystal can be manufactured with high straight-body percentage and little formation of different polytypes even in growth with no off-angle control, that is, the growth is directed onto a basal plane which is hardly inclined from a C-axis <0001>.
To achieve the object, the present invention provides a method for manufacturing a silicon carbide single crystal by sublimating a silicon carbide raw material in a growth container to grow a silicon carbide single crystal on a seed crystal substrate, wherein a substrate used as the seed crystal substrate comprises:
a diameter of the seed crystal substrate is 80% or more of an inner diameter of the growth container.
Utilizing such a grown ingot end face as the seed (seed crystal substrate) and setting the diameter of the seed crystal substrate to 80% or more of the inner diameter of the growth container make it possible to manufacture a silicon carbide single crystal with high straight-body percentage while suppressing formation of different polytypes.
As described above, according to the present invention, an end face of a grown ingot is utilized as the seed crystal substrate, and the diameter of the seed crystal substrate is set to 80% or more of the inner diameter of the growth container. Thus, it is possible to manufacture a silicon carbide single crystal with high straight-body percentage while suppressing formation of different polytypes.
Hereinafter, the present invention will be described in detail with reference to the drawings as an example of embodiments. However, the present invention is not limited thereto.
Herein below, a method for manufacturing a silicon carbide single crystal according to the present invention will be described with reference to
An apparatus 1 for growing a silicon carbide single crystal shown in
The growth container 4 has a growth chamber for disposing the seed substrate 2, and a sublimation chamber for disposing the silicon carbide raw material 3, and is formed of, for example, heat-resistant graphite. In addition, during crystal growth, a crystal is grown in an inert gas atmosphere under reduced pressure by setting the growth container 4 in an external container 9 made of SUS or quartz, and supplying an inert gas such as Ar while vacuum exhausting.
As the heater 8, a heater for performing RH (resistance heating) or RF (radio-frequency) heating can be used. In addition, by using a pyrometer as the temperature-measurement sensor 7, the temperature measurement can be performed with precision through the hole 6 for temperature measurement (upper-portion temperature measurement hole) in the heat-insulating material 5 from outside the growth container 4 without contact. In the present invention, a substrate used as the seed substrate 2 to be disposed inside the growth container 4 has a {0001} plane with an off angle of 1° or less as a surface to be placed on the growth container 4, and a convex-shaped end face of a grown ingot (top crystal) as a single crystal growth surface. Note that it is sufficient for the placement surface to have the {0001} plane with an off angle of 1° or less, and the lower limit of the off angle can be 0°. Further, the seed (seed crystal substrate) 2 has a diameter which is 80% or more, more preferably 90% or more, of the growth container 4. Note that the upper limit of the diameter of the seed 2 can be as large as 100% of the inner diameter of the growth container 4. Moreover, the thickness at the thickest portion can be 5 mm or more, preferably 1 cm or more.
Next, the inventive method for manufacturing a silicon carbide single crystal will be described using
First, as described in
Next, as described in
Next, as described in
Next, as described in
Next, as described in
Finally, as described in
A SiC single crystal is manufactured in a manner as described above. In the seed crystal substrate, an end face of a grown ingot is used as the growth surface of the single crystal, and the diameter of the seed crystal substrate is 80% or more of the inner diameter of the growth container. These make it possible to manufacture a silicon carbide single crystal with a high straight-body percentage while suppressing formation of different polytypes.
Hereinafter, the present invention will be more specifically described by showing Examples and Comparative Examples. However, the present invention is not limited thereto.
A SiC single crystal with a diameter of 100 mm was grown according to the flow described in
After the SiC single crystal growth, the straight-body percentage (straight body/full length) was measured and found to be 90%. Moreover, a wafer cut out with a multi-wire saw was ground, mirror-polished, and CMP polished. Then, the surface of the wafer was observed. As shown in
A SiC single crystal was grown under the same conditions as in Example 1, except for using a seed crystal substrate with a diameter of 84 mm (the diameter of the seed crystal substrate/the inner diameter of the growth container was 80%).
After the SiC single crystal growth, the straight-body percentage (straight body/full length) was measured and found to be 85%. After the SiC single crystal growth, a wafer cut out with a multi-wire saw was ground, mirror-polished, and CMP polished. Then, the surface of the wafer was observed, and no formation of different polytypes was found. Further, crystals were prepared, resulting in five batches in total. Table 2 shows the result. As shown in Table 2, no different polytypes were formed in all the five batches (i.e., the incidence of different polytypes was 0%).
A SiC single crystal was grown under the same conditions as in Example 1, except for using a seed (seed crystal substrate) with the main surface being the {0001} plane and a thickness of 1 mm (uniform).
Crystals were prepared, resulting in ten batches in total. Table 1 shows the result. As shown in Table 1, different polytypes were formed and all the ten batches were disordered.
A SiC single crystal was grown under the same conditions as in Example 1, except for using a seed (seed crystal substrate) with the main surface being tilted by 4° from the {0001} plane in the <11-20> direction and a thickness of 1 mm (uniform).
After the SiC single crystal growth, the straight-body percentage (straight body/full length) was measured and found to be 40%. Crystals were prepared, resulting in five batches in total. Table 1 shows the result. As shown in Table 1, no different polytypes were formed in all the five batches.
A SiC single crystal was grown under the same conditions as in Example 1, except for using a seed crystal substrate with a diameter of 79 mm (the diameter of the seed crystal substrate/the inner diameter of the growth container was 75%).
After the SiC single crystal growth, a wafer cut out with a multi-wire saw was ground, mirror-polished, and CMP polished. Then, the surface of the wafer was observed. Further, crystals were prepared, resulting in ten batches in total. Table 2 shows the result. As shown in Table 2, the formation of different polytypes was observed in two batches out of the ten batches (i.e., the incidence of different polytypes was 20%).
A SiC single crystal was grown under the same conditions as in Example 1, except for using a seed crystal substrate with a diameter of 52 mm (the diameter of the seed crystal substrate/the inner diameter of the growth container was 50%).
After the SiC single crystal growth, a wafer cut out with a multi-wire saw was ground, mirror-polished, and CMP polished. Then, the surface of the wafer was observed. Further, crystals were prepared, resulting in ten batches in total. Table 2 shows the result. As shown in Table 2, the formation of different polytypes was observed in three batches out of the ten batches (i.e., the incidence of different polytypes was 30%).
A SiC single crystal was grown under the same conditions as in Example 1, except for using a seed crystal substrate with a diameter of 21 mm (the diameter of the seed crystal substrate/the inner diameter of the growth container was 20%).
After the SiC single crystal growth, a wafer cut out with a multi-wire saw was ground, mirror-polished, and CMP polished. Then, the surface of the wafer was observed. Further, crystals were prepared, resulting in ten batches in total. Table 2 shows the result. As shown in Table 2, the formation of different polytypes was observed in five batches out of the ten batches (i.e., the incidence of different polytypes was 50%).
Table 2 reveals that when the diameter of the seed crystal substrate is as large as 80% or more of the inner diameter of the growth container, the formation of different polytypes is sufficiently suppressed.
It should be noted that the present invention is not limited to the above-described embodiments. The embodiments are just examples, and any examples that have substantially the same feature and demonstrate the same functions and effects as those in the technical concept disclosed in claims of the present invention are included in the technical scope of the present invention.
Number | Date | Country | Kind |
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2018-048559 | Mar 2018 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2019/005730 | 2/18/2019 | WO | 00 |