Silicon carbide substrate and method of growing SiC single crystal boules

Abstract

The present invention relates to a silicon carbide (SiC) substrate with improved mechanical and electrical characteristics. Furthermore, the invention relates to a method for producing a bulk SiC crystal in a physical vapor transport growth system. The silicon carbide substrate comprises an inner region (102) which constitutes at least 30% of a total surface area of said substrate (100), a ring shaped peripheral region (104) radially surrounding the inner region (102), wherein a mean concentration of a dopant in the inner region (102) differs by at least 1-10.sup.18 cm-.sup.3 from the mean concentration of this dopant in the peripheral region (104).

Claims

1. Silicon carbide substrate, comprising: a doped inner region (102) which constitutes 45%±15% of a total surface area of said substrate (100), a doped ring shaped peripheral region (104) radially surrounding the inner region (102), wherein a mean concentration of a dopant in the inner region (102) differs by at least 1.10.sup.18 cm.sup.−3 from the mean concentration of this dopant in the peripheral region (104), and wherein a bow of the substrate (100) is less than 25 μm and/or a warp of the substrate (100) is less than 40 μm.

2. Silicon carbide substrate according to claim 1, wherein said dopant comprises nitrogen, and wherein the nitrogen dopant concentration is lower in the peripheral region (104) than in the inner region (102).

3. Silicon carbide substrate according to claim 1, wherein the mean concentration of said dopant in the inner region (102) differs by at least 5.10.sup.18 cm.sup.−3 from the mean concentration of this dopant in the peripheral region (104).

4. Silicon carbide substrate according to claim 1, wherein the substrate (100) has an electrical resistivity in a range from 12 mΩ cm to 26 mΩ cm.

Description

(1) The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the present invention. These drawings together with the description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the embodiments may form—individually or in different combinations—solutions according to the present invention. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein:

(2) FIG. 1 shows a schematic top view of a silicon carbide substrate according to a first advantageous embodiment of the present invention;

(3) FIG. 2 shows a schematic top view of a silicon carbide substrate according to a second advantageous embodiment of the present invention;

(4) FIG. 3 shows a schematic cross sectional view of a PVT growth apparatus according to a first embodiment of the present invention;

(5) FIG. 4 shows a schematic cross sectional view of a PVT growth apparatus according to a further embodiment of the present invention;

(6) FIG. 5 shows a schematic cross sectional view of a PVT growth apparatus according to a further embodiment of the present invention;

(7) FIG. 6 shows a schematic cross sectional view of a PVT growth apparatus according to a further embodiment of the present invention;

(8) FIG. 7 shows a schematic cross sectional view of a PVT growth apparatus according to a further embodiment of the present invention;

(9) FIG. 8 shows a schematic cross sectional view of a PVT growth apparatus according to a further embodiment of the present invention;

(10) FIG. 9 shows a schematic cross sectional view of a PVT growth apparatus according to a further embodiment of the present invention.

(11) The present invention will now be described in more detail with reference to the Figures. Turning first to FIG. 1, a schematic representation of a SiC substrate 100 (also sometimes called “wafer” in the following) according to the present invention is shown. According to the present invention, the SiC substrate 100 comprises in a radial direction (as indicated by the arrow r) a first region 102 and a second region 104. The first region 102 is surrounded by the second ring shaped region 104. As indicated by the letter “A”, the first region 102 has a first mean concentration of a dopant such as nitrogen, whereas the second region 104 has a second mean concentration (“B”) which is different from the first mean concentration. According to an exemplary embodiment of the present invention, the mean concentration of the dopant differs between the first region 102 and the second region 104 by at least 1.Math.10.sup.18 cm.sup.−3, preferably by 5.Math.10.sup.18 cm.sup.−3. In particular, the mean concentration of nitrogen in the first region 102 may be higher than the nitrogen concentration in the second peripheral region 104.

(12) According to the present invention, the circular inner region 102 takes up at least 45%±15% of the total wafer surface. Accordingly, the ring shaped outer region 104 of course also takes up at least 45%±15% of the total wafer surface. As a result, it can be avoided that a tensile ring-shaped stress is generated in the peripheral region 104. Rather, in the peripheral region 104, a compressive stress prevails which results in a tensile, radially acting stress in the first region 102. This tensile stress in the first region 102 counteracts a preset deflection due to the growth of convex crystals (or at least reduces this effect).

(13) By introducing impurity atoms with different mean concentrations in the different first and second regions, mechanical stress can selectively be generated inside the silicon carbide substrate in a way that the thermal tension which is generated during the growth procedure is compensated. As a result, the SiC substrate 100 according to the present invention is low strain or ideally strainless after removal of all near surface disturbing layers by applying the final polishing steps. Consequently, the SiC substrate 100 does not have stress induced geometric errors and therefore has low bow and warp values. This advantageous geometry allows for an excellent thermal coupling of such substrates in an epitaxial reactor, the thermal coupling being decisive for homogeneous high-quality epitaxial layer growth. Furthermore, high-quality electronic components can be fabricated on a substrate according to the present invention. Moreover, also the material loss can be reduced during machining because due to the compensation of stress no distortion is occurring and therefore the material removal is decreased significantly. As already mentioned above, known thick substrates exhibit even with the thermal or mechanical stress low bow and warp values. However, the SiC substrates according to the present invention may have much lower thicknesses and still exhibit excellent geometric characteristics. For instance, ratios of the thickness and the diameter in a range between 0.002 and 0.004 can be achieved.

(14) The SiC substrate 100 according to the present invention may be distinguished by the following features when providing a nitrogen dopant concentration which is about 5.Math.10.sup.18 cm.sup.−3 higher in the first (inner) region 102 than in the second (outer) region 104: The dimensions may be chosen so that the diameter is 100 mm or even 150 mm or even 200 mm with a wafer thickness of less than 1000 μm and more than 200 μm, for example 350 μm±25 μm. The overall dislocation density as indicated by the etch pit density (EPD) may amount to 50 000 cm.sup.−3, preferably stay below 10 000 cm.sup.−2. The electrical resistivity may be in a range between 12 mΩcm and 26 mΩcm, preferably between 18 mΩcm and 22 mΩcm.

(15) It could be shown that these SiC substrates have a bow of less than 25 μm, even below 15 μm, and a warp of less than 40 μm, even below 30 μm.

(16) Depending on the intended subsequent epitaxial layers and on the required optical and semiconductor material properties, the SiC substrate may have one of the more than 200 possible SiC polytypes that have been found up to date. As this known in the art, the most common polytypes include 3C, 2H, 4H, 6H, 8H, 9R, 10H, 14H, 15R, 19R, 20H, 21H, and 24R, where (C), (H) and (R) are the three basic cubic, hexagonal and rhombohedral crystallographic categories. In the cubic zinc-blende structure, labelled as 3C—SiC or β-SiC, Si and C occupy ordered sites in a diamond framework. In hexagonal polytypes nH—SiC and rhombohedral polytypes nR—SiC, generally referred to as α-SiC, nSi—C bilayers consisting of C and Si layers stack in the primitive unit cell. Preferably, the substrate according to the present invention is of the 4H polytype. Moreover, the orientation of the substrate is between 0° and 8°, preferably 4° off-axis. Polytype and orientation is usually controlled by the orientation of the seed crystal or by tilting during the crystal preparation.

(17) FIG. 2 shows a further aspect of the substrate according to the present invention. When fabricating the SiC substrate in a PVT system, it can be expected that there will be no sharp delimiting line between the regions 102 and 104. Rather, there is a transitional region 106 having a gradient of the mean dopant concentration that lies between the values A and B of regions 102 and 104, respectively. The dopant concentration in the transitional region 106 is marked with the letter “C”. It has to be understood, that in the transitional region 106 the dopant concentration is variable along the radial direction r to lead from the higher value A in the central region 102 to the lower value B in the peripheral region 104.

(18) Importantly, it has to be noted that FIGS. 1 and 2 are schematic representations and are not to scale regarding the dimensions of regions 102, 104, and 106, and regarding the area ratios of these regions with respect to each other.

(19) FIGS. 3 to 9 illustrate different embodiments of PVT processes for growing single crystal boules which can be sliced into substrates as described with reference to FIGS. 1 and 2.

(20) Physical vapor transport (PVT), also known as seeded sublimation growth, has been the most popular and successful method to grow large sized SiC single crystals. U.S. Pat. No. 8,747,982 B2 describes an advantageous fabrication method that can be used and modified to fabricate SiC substrates according to the present invention.

(21) A first example of generating a dopant profile in a radial direction during growth of an SiC single crystal 108 will be explained with reference to FIG. 3. In the following, the growing SiC single crystal 108 will also be referred to as “boule”.

(22) FIG. 3 shows a schematic view of a PVT growth cell 110, wherein PVT growth of a SiC single crystal 108 is carried out in a graphite crucible 112 sealed with a graphite lid and loaded with a sublimation source 114 disposed in a source material compartment 116 at the bottom of the crucible 112. A single crystal seed (not visible in the Figures) is arranged at the crucible top. A thermal insulation material surrounds the crucible 112 and is only open in the region of a heat dissipation channel which generates the temperature gradient which is necessary for re-condensation (not shown in the Figures).

(23) The sublimation source 114 is usually a polycrystalline SiC grain or powder synthesized in a separate process. The loaded crucible 112 is placed inside a growth chamber where it is surrounded by the thermal insulation (not shown in the Figures). Inductive or resistive heating (not shown in the Figures) is used to bring the crucible 112 to a suitable temperature, generally between 2000° C. and 2400° C. for the PVT growth of a SiC single crystal on the SiC single crystal seed. The growth chamber may for instance be made of fused silica, and an RF coil is positioned with respect to the crucible 112 such that during growth of the single crystal the temperature of the sublimation source 114 is maintained higher than the temperature of the seed crystal (typically with a difference of 10 to 200K).

(24) Upon reaching a suitably high temperature, the sublimation source 114 vaporizes and fills a growth compartment 118 of the crucible 112 with a vapor of silicon, Si.sub.2C and SiC.sub.2 molecules. The temperature difference between the sublimation source 114 and the seed crystal forces the vapor to migrate and to condense on the seed crystal, thereby forming a growing single crystal boule 108. In order to control the growth rate, PVT growth is typically carried out in the presence of a small pressure of inert gas, usually between 0.1 mbar and 100 mbar.

(25) In addition to known arrangements as the ones shown in U.S. Pat. No. 8,747,982 B2, the present invention provides a gas inlet 120 which is arranged centrally within the growth compartment 118 in order to provide a directed gas stream 122 that is directed towards a central region of the growing crystal 108. Thereby, a dopant concentration difference occurs between those regions of the growth compartment 118 which are close to an inner wall 124 of the crucible 112 and the central region. By providing a higher dopant concentration close to the center of the growing crystal 108, a higher dopant concentration is built into the growing crystal lattice in the center compared to the concentration that is built in in the peripheral regions. Depending on the particular parameters of the gas stream 122, a concentration profile as shown in FIG. 1 or 2 can be realized for the finally processed SiC substrates that are produced from the crystal 108.

(26) In the shown embodiment, the gas stream 122 contains nitrogen as a dopant. However, of course, also other suitable gases, e.g. ammonia, can be introduced into the growth compartment 118 via the inlet 120.

(27) FIG. 4 shows a second advantageous embodiment of a growth cell 110 according to the present invention. Again, the crystal 108 grows from a seed crystal arranged in a growth compartment 118. A sublimation powder source 114 is contained in a source material compartment 116. The bulk density of the source material should be in the range of 1.0 g.Math.cm.sup.−3 to 2.6 g.Math.cm.sup.−3, preferably in the range of 1.4 g.Math.cm.sup.−3 to 1.8 g.Math.cm.sup.−3. The grain size (D50) of the source material should be in the range of 100 μm to 1000 μm, preferably in the range of 300 μm to 500μm. In contrast to known arrangements, however, the powder source 114 is not homogeneously distributed across the whole diameter of the crucible 112. According to the present invention, the center of the source material compartment 116 comprises an enriched source material 126 that is additionally enriched with a dopant. The concentration of the doping element in the enriched SiC powder is at least 1.Math.10.sup.20 cm.sup.−3, preferably 5.Math.10.sup.20 cm.sup.−3. The concentration of the doping element in the lower doped outer source material is lower than 5.Math.10.sup.17 cm.sup.−3, preferably lower than 1.Math.10.sup.17 cm.sup.−3. During the heating process, a stream 128 of the dopant is generated when vaporizing the enriched source material 126. Additionally, also a gradual dilution of the dopant can be provided at the interface between the undoped source material and the enriched source material 126.

(28) The advantage of this embodiment compared to the embodiment shown in FIG. 3 can be seen in the fact that firstly no gaseous dopant sources have to be handled and that, secondly, during the growth process no continuous access to the crucible 112 is required.

(29) FIGS. 3 and 4 relate to the idea to actively enhance the concentration of a dopant, such as nitrogen, in the center of the growth compartment 118. FIGS. 5 and 6, on the other hand, describe embodiments where the concentration of the dopant is actively decreased in the peripheral regions of the growth compartment 118.

(30) As shown in FIG. 5, the growth cell 110 comprises a growth compartment 118 and a source material compartment 116. The source material compartment 116 is filled with a sublimation source material 114 as this is known for conventional growth cells. According to the present invention, the inner wall 124 of the crucible 112 is at least partly covered with a solid getter material 130. The getter material 130 selectively attracts and binds dopant atoms that are present in the growth compartment 118, thereby reducing their concentration in a region close to the wall 124 of the crucible 112. Consequently, the dopant concentration in the central region of the growth compartment 118 is higher than the concentration in the peripheral regions and the growing crystal 108 comprises a central region with a higher dopant concentration compared to the peripheral regions. Due to the reduced availability of the dopant in the peripheral region of the growth compartment 118, a lower dopant concentration is incorporated into the growing crystal 108 at the outer region compared to the central region.

(31) For instance, when using nitrogen as a dopant, the solid getter material 130 may comprise a metal such as tantalum, tungsten, niobium, molybdenum or hafnium or alloys thereof. These elements bind nitrogen irreversibly by forming nitride bonds. Other suitable getter materials may of course also be used. The design of the getter is such that it is in cylindrical shape with a diameter slightly larger than the seed diameter, with a radial gap of 2 mm, preferably with a gap of 1 mm, to be close enough for effective gettering, with a thickness in the range of 0.5 mm to 3 mm, and a minimum length larger than the crystal length maintaining absorption of nitrogen at the crystal rim during the complete crystal growth process.

(32) FIG. 6 shows a further advantageous example of a growth cell 110 according to the present invention. This arrangement is similar to the one shown in FIG. 5 in that a getter material is provided in the periphery delimiting the growth compartment 118. Instead of a solid getter layer, however, granular or powdery getter particles 132 are provided according to this embodiment. In order to keep these getter particles 132 in place, a porous cover wall 134 is provided at the inside of the growth compartment 118. The porous cover wall 134 may for instance be formed from graphite that allows a diffusion of the molecules and atoms from the growth compartment to the getter material and vice versa. Therefore, the cover wall may have a bulk density of 1.0 g.Math.cm.sup.−3to 2.0 g.Math.cm.sup.−3, preferably of 1.2 g.Math.cm.sup.−3.

(33) According to the present invention, the getter particles 132 may comprise one or more nitrogen binding metals such as tantalum, tungsten, niobium, molybdenum or hafnium as alloy or mixture, in a granulated or powdered form. Nitrogen which is present as a dopant in the growth compartment 118 diffuses through the porous graphite wall 134 and irreversibly forms nitride bonds with the getter particles 132. Thereby the lateral nitrogen distribution close to the growth front of the crystal 108 is influenced in a way that less nitrogen is available for being built into the crystal at the margin than within the center of the crystal 108. To balance the getter functionality over the whole time of the crystal growth process, the composition of grain and powder sizes has to be adjusted in the range of 0.01 mm to 1 mm, preferably in the range of 0.05 mm to 0.5 mm, in order to offer an optimized free surface of the getter.

(34) A concentration profile according to the present invention as shown in FIGS. 1 and 2 can be achieved thereby.

(35) FIG. 7 shows a further advantageous example of a growth cell 110 according to the present invention. According to this particular embodiment, the source material compartment 116 is completely filled with a dopant enriched powder source material 126. For controlling a stream 128 of dopant, e. g. a stream of nitrogen, which is directed towards the center of the crystal 108, a filter element 136 is arranged at the interface between the source material compartment 116 and the growth compartment 118. The filter element 136 is arranged to cover a peripheral region of this interface and to leave open a central region in order to allow dopant, such as nitrogen, to pass towards the crystal 108 when the powder source material 126 is evaporated.

(36) As shown in FIG. 7, the dopant filter 136 may for instance have a ring-shaped form in order to provide a more or less ring-shaped area of the growth compartment 118 which has a reduced dopant concentration compared to the central area as indicated by the dopant stream 128. The filter may comprise a metal such as tantalum, tungsten, niobium, molybdenum, or hafnium as alloy or mixture, in either a granulated or powdered form embodied in a graphite capsule that allows a diffusion of the molecules and atoms from the powder compartment to the growth compartment via the filter. The filter has a bulk density of 1.0 g.Math.cm.sup.−3 to 2.0 g.Math.cm.sup.−3, preferably of 1.2 g.Math.cm.sup.−3. To balance the getter functionality over the whole time of the crystal growth process, the composition of grain and powder sizes has to be adjusted in the range of 0.01 mm to 1 mm, preferably in the range of 0.05 mm to 0.5 mm, in order to offer an optimized free surface of the getter. The height of the filter capsule has to be adjusted such that the getter capability is maintained throughout the crystal growth process, having a thickness in the range of 1 mm to 20 mm, preferably in the range of 5 mm to 10 mm.

(37) Of course, the ideas according to the embodiments shown in FIGS. 3 to 7 may also be combined with each other in any arbitrary way that might be necessary for producing a desired dopant concentration profile in the silicon carbide substrates.

(38) As already mentioned above, the production times for growing SiC single crystals may be significantly reduced by simultaneously growing two crystals instead of one. In order to achieve such a simultaneous growth, the principles of European patent EP 2 664 695 B1 may be adapted to the ideas according to the present invention.

(39) As shown in FIG. 8, a growth cell 210 according to the present invention may comprise two growth compartments 118, 119. The growth compartments 118, 119 are arranged symmetrically with respect to a source material compartment 116 that comprises a sublimation source 114. In each of the growth compartments 118, 119 one crystal 108, 109 is growing from a seed crystal (not shown in the Figures). Similar to the arrangement shown in FIG. 3, a gas inlet 120 is provided for introducing nitrogen gas towards each of the growing crystals 108, 109. Of course, any other gaseous dopant, e.g. ammonia may also be covered by this embodiment.

(40) In each of the growth compartments 118, 119 a dopant inlet 121, 123 is arranged in a way that the respective central region of the growth compartments 118, 119 are supplied with a higher concentration of dopant than the peripheral regions.

(41) In an analogous way as explained with reference to FIGS. 5 and 6, the growth cell 210 may also make use of the idea of reducing the dopant concentration in the peripheral regions by providing a granulated or powdered getter material, e.g. metals such as tantalum, tungsten, niobium, molybdenum or hafnium as alloy or mixture. FIG. 9 shows an example where getter particles 132, 133 are arranged behind porous cover walls 134, 135 that allow a diffusion of the molecules and atoms from the growth compartment to the getter material and vice versa. Therefore, the cover wall shows a bulk density of 1.0 g.Math.cm.sup.−3 to 2.0 g.Math.cm.sup.−3, preferably of 1.2 g.Math.cm.sup.−3. To balance the getter functionality over the whole time of the crystal growth process the composition of grain and powder sizes has to be adjusted in the range of 0.01 mm to 1 mm, preferably in the range of 0.05 mm to 0.5 mm, in order to offer an optimized free surface of the getter.

(42) By binding a certain amount of the dopant atoms from the peripheral regions of the growth compartments 118, 119, the getter particles 132, 133 generate a concentration gradient of the dopant inside the growth compartments 118, 119. In particular, the dopant concentration, e.g. nitrogen, in the center of the growth compartments 118, 119 is higher than in the peripheral region in order to lead to a higher dopant concentration being built into the growing crystals 108, 109 in their central regions.

(43) Instead of providing getter particles 134, 133, also a solid getter material as shown in FIG. 5 may be provided. As explained for the previous embodiments, the chosen getter material depends on the dopant of which the concentration profile has to be shaped. For nitrogen for instance tantalum, tungsten, hafnium, niobium, molybdenum, or hafnium as alloy or a mixture thereof can be used. The design of the solid getter is such that it is in cylindrical shape with a diameter slightly larger than the seed diameter, with a radial gap of 2 mm, preferably with a gap of 1 mm, in order to be close enough for effective gettering, with a thickness in the range of 0.5 mm to 3 mm and a minimum length larger than the crystal length maintaining absorption of nitrogen at the crystal rim during the complete crystal growth process.

(44) According to the present invention, the mean concentration of a dopant in the inner region 102 differs by an absolute value of at least 1.Math.10.sup.18 cm.sup.−3 from the mean concentration of this dopant in the peripheral region 104. When assuming for instance that the mean concentration in the inner region 102 is in a range between 3.Math.10.sup.18 cm.sup.−3 and 3.Math.10.sup.19 cm.sup.−3 and that the mean concentration of the dopant in the peripheral region 104 is lower than in the inner region 102, this would mean that the absolute difference of at least 1.Math.10.sup.18 cm.sup.−3 would correspond to relative differences between 3% and 50% in relation to the mean concentration of the dopant in the peripheral region 104. This relationship can be derived from the following calculation.

(45) For a mean concentration of the dopant in the inner region of 3.Math.10.sup.19 cm.sup.−3, the mean concentration in the peripheral region is calculated as 3.Math.10.sup.19 cm.sup.−3−1.Math.10.sup.18 cm.sup.−3=2.9.Math.10.sup.19 cm.sup.−3, so that the value of 1.Math.10.sup.18 cm.sup.−3 corresponds to 3%.

(46) On the other hand, for a mean concentration of the dopant in the inner region of 3.Math.10.sup.18 cm.sup.−3, the mean concentration in the peripheral region is calculated as 3.Math.10.sup.18 cm.sup.−3−1.Math.10.sup.18 cm.sup.−3=2.Math.10.sup.18 cm.sup.−3, so that the absolute value of 1.Math.10.sup.18 cm.sup.−3 corresponds to 50%.

REFERENCE NUMERALS

(47) TABLE-US-00001 Reference Numeral Description 100 SiC substrate 102 Inner region 104 Outer region 106 Transitional region 108, 109 SiC single crystal boule 110, 210 Growth cell 112 Crucible 114 Sublimation source material 116 Source material compartment 118, 119 Growth compartment 120, 121, 123 Gas inlet 122 Gas stream 124 Inner wall of crucible 126 Dopant enriched source material 128 Stream of dopant 130 Solid getter material 132, 133 Getter particles 134, 135 Porous cover wall 136 Dopant filter