SE546244C2 - Growth module for simultaneous radial growth of multiple silicon carbide seeds - Google Patents

Abstract

A reactor growth module (1) adapted for a physical vapor transport growth, comprising a crucible (2) extending longitudinally in a vertical direction (V), a seed holder arrangement (7) provided on an inside of said crucible (2), a source material (5) arranged to surround said seed holder arrangement (7) and extending longitudinally along an inner wall of said crucible (2), wherein said seed holder arrangement is adapted to receive and hold at least three seed crystal substrates (6, 6’, 6”), characterized in that the seed holder arrangement (7) is provided such during a physical vapor transport process, said seed substrates (6, 6’, 6”) grow in a radial direction (R) which is substantially perpendicular to said vertical direction (V).

Description

GROWTH MODULE FOR SIMULTANEOUS RADIAL GROWTH OF MULTIPLE SILICON CARBIDE SEEDS Technical field The present invention relates to a growth module and an arrangement for simultaneous and radial growth of multiple silicon carbide crystals in a furnace or hotzone.

Background Silicon Carbide (SiC) is a wide-bandgap semiconductor which possesses desirable electrical, mechanical, thermal, and chemical properties. ln addition to high radiation resistance, high thermal conductivity and high maximum current density, the semiconductor devices made of SiC can be based on material properties able to withstand 10 times higher electric fields compared to silicon-based devices. ln recent years SiC has emerged as a robust semiconductor material for making electrical inverters and converters for electric vehicles and other applications that require high- voltage electronics. ln contrast to silicon, SiC crystals cannot be grown using conventional congruent stoichiometry melt growth techniques at industrially feasible pressures and temperatures. According to theoretical calculations of SiC phase diagram a stochiometric melt can be obtained only at temperature of about 3227°C and pressure around 109 Pa.

However, SiC can be grown from a vapor phase at industrially acceptable temperatures and pressures. The Physical Vapor Transport (PVT), also called a seeded sublimation method, is currently the most common technique for bulk SiC crystal growth from a vapor phase. The growth is typically performed in a graphite container which is surrounded by a thermal insulation at temperatures between 1800 and 2400°C, creating what is conventionally called a "hot-zone", and at pressures varying from 10 to 104 Pa (0.1 to 100 mBar).

The hot-zone is designed in a way that the temperature gradient inside the crucible is created between the SiC source powder heated to a higher temperature and the SiC seed situated at the slightly cooler area. The growth of SiC crystal is obtained upon the sublimation of the SiC source powder and condensation of SiC vapor species (SizC, SiCz, Si) on the SiC seed.

The PVT growth of SiC seeds is essentially not very efficient process. There is a lot of power needed for hot-zone heat-up to the working temperature by the conventional means of RF heating and at that time only one crystal is produced at the top of the mentioned hot-zone, where the seed is generally placed. Needed hot- zone temperature observation point and cooling, which also greatly influences the necessary temperature gradient, is usually achieved by opening the hole in thermal insulation of the hot-zone. This step further decreases the electrical efficiency of the sublimation growth design. The PVT reactors themselves usually also have quite a large footprint and need a lot of facilities connected for each.

There is hence a need for a new and efficiency improved PVT reactor, hot-zone and growth concept for production of silicon carbide ingots.

Summafl lt is with the above considerations in mind and others, that the embodiments described in this disclosure have been made.

This disclosure recognizes the fact, that there is a need for an improved crystal growth arrangement of Silicon Carbide (SiC) growth systems to increase their growth speed and efficiency. lt is an object of this disclosure to provide an arrangement and method that solves the aforementioned problems.

The invention is defined by the appended independent claims. Embodiments are set forth in the appended dependent claims and in the following description.

According to a first aspect there is provided a reactor growth module adapted for a physical vapor transport growth, comprising a crucible extending longitudinally in a vertical direction, a seed holder arrangement provided on an inside of said crucible, a source material arranged to surround said seed holder arrangement and extending longitudinally along an inner wall of said crucible, wherein said seed holder arrangement is adapted to receive and hold at least three seed crystal substrates, characterized in that the seed holder arrangement is provided such during a physical vapor transport process, said seed substrates grow in a radial direction which is substantially perpendicular to said vertical direction.

This configuration of the reactor growth module allows for an enhancement of the efficiency for PVT growth of silicon carbide ingots. The multiple and vertically oriented (or slightly tilted) seeds are thus arranged within or inside one hot-zone in the radially symmetrical assembly surrounded by a growth cavity and source powder.

According to the first aspect, the seed holder arrangement may further comprise a central hollow portion extending through an upper portion and lower portion of said crucible.

The inner part of the assembly encompassed by radially placed SiC seeds is thus designed in a way to provide sufficient cooling and promote the mass transfer towards the seeds from the hotter outer regions containing SiC powder source material. The estimated efficiency increase in this kind of reactor is approximately three times in case where six 150 mm seeds are arranged in a hexagonal pattern forming the inner assembly for growth.

The reactor growth module may further comprise at least one inductive heater or heater arrangement, which is arranged to surround said crucible.

The inductive heater thus provides heating of said crucible and its inner parts, i.e. the hot-zone.

The least one inductive heater and said crucible may be moveable in relation to one another.

According to the first aspect the source material may be comprised in a porous graphite tube, or in a graphite tube with drilled openings, or is sintered to shape.

According to one alternative the module may be rotatable to 90° from a vertical symmetry axis to the horizontal symmetry axis. This means that the vertical axis becomes a horizontal axis. However, the seeds still grow a radial direction perpendicular to this axis, i.e. radially as seen in a direction from the center to the inner walls of the crucible.

The seed holder assembly may be rotatable inside said crucible and around a vertical axis.

According to an alternative growth module, the crucible is rotatable around a vertical axis or tilted vertical axis.

This means that either the holder body or the entire "hot-zone assembly", i.e. the crucible with its inner components and insulation, can be rotated to equalize the growth rates of the seed crystals, which may be especially advantageous if the reactor assembly is rotated to have a horizontal symmetry axis.

According to a second aspect there is provided a growth module chamber or system comprising at least two rector growth modules according to the first aspect, and a joint vacuum and gas system.

According to a third aspect there is provided a physical vapor transport method, comprising of a reactor growth module including a crucible extending longitudinally in a vertical direction, wherein the method comprises providing a seed holder arrangement on an inside of said crucible, providing a source material arranged to surround said seed holder arrangement and extending longitudinally along an inner wall of said crucible, providing said seed holder arrangement with seed crystal substrates, providing at least one inductive heater surrounding said crucible, placing said seed holder arrangement and seed crystal substrates inside the crucible comprising the source material, and increasing a temperature inside said crucible by said inductive heater until a growth temperature is reached; and keeping said growth temperature and; increasing a thickness of said seed crystal in a direction which is substantially perpendicular to said vertical direction. According to a fourth aspect there is provided the use of the reactor growth module according to the first aspect, or the chamber according to the second aspect in a physical vapor transport process.

Brief description of drawinqs Embodiments of the present solution will now be described, by way of example, with reference to the accompanying schematic drawings.

Fig. 1 shows a schematic cross-sectional side view of a PVT reactor comprising the innovative radial growth configuration.

Fig. 2 shows a schematic cross-sectional top view of a PVT reactor comprising the innovative radial growth configuration.

Fig. 3 shows a schematic side view of an example growth chamber with two vertically stacked radial growth modules.

Description of Embodiments For the conventional crystal growth of Silicon Carbide (SiC) material a process called the Physical Vapor Transport (PVT) is used. This method is also referred to as the modified Lely method, or the "seeded sublimation growth". lt uses a high-quality seed crystal "wafer" (thin crystalline disk of SiC material also called a substrate or seed) to begin the growth process of the SiC ingot. Usually, the process is carried out at temperatures between 1800 and 2400°C in an inductively heated closed graphite crucible surrounded by thermal insulation. This standard PVT growth setup is carried out in an argon environment and due to the used isostatic graphite innate porosity or intentional openings in the crucible can be described as a quasi-closed system. Also, a small flow of nitrogen gas is often introduced into the process Chamber to obtain the SiC with n-type semiconducting properties.

The main objective of increased growth efficiency in this inventive idea is achieved by the change of crystal growth orientation and corresponding hot-zone (growth environment) geometry, allowing the placement of multiple seeds inside it for simultaneous growth.

This favorable geometry can be roughly described as the seeds being vertically and preferably symmetrically placed inside a growth crucible or module at the outer part or perimeter of a cooled central tube (the seed holder or a central part of the seed holder) in a radially symmetric pattern, while being surrounded by a free space for their growth expansion. The seeds are encased or enclosed by an outer shell, placed on the inside of the crucible, of heated source powder from which the sublimated growth species are supplied. This outer source powder can be for example hold in place by porous graphite, by graphite tube with slots or holes in it, or even sintered together to hold its shape. The encapsulating crucible acts as a heater material for the hot-zone and can be made from graphite, refractory coated graphite, or solid refractory compound altogether. The heating is provided by induction coil/coils, which are also presumed to be radially symmetric but can be split in several sections or completely separated from each other for temperature optimization.

Fig. 1 illustrates schematically a PVT reactor growth module 1 for growing a silicon carbide ingot according to the inventive idea. Not shown in Fig. 1 is a quartz chamber wall, which usually separates a heating coil from the hot-zone of the crucible and provides a closed space for, e.g. pressure and dopant control. Such a quartz chamber wall is illustrated in Fig. 3 which is described below.

The reactor growth module 1 comprises a crucible 2, which is heated through inductive heaters 3 (3, 3', etc.) arranged to surround the crucible The growth module 1 is further provided with insulating material 4, such as a rigid graphite felt, or soft graphite felt. This insulating material 4 is arranged to enclose the growth module crucible 2, but also provides an opening for temperature monitoring and cooling to achieve desired temperature gradients inside the crucible. lnsulating material 4 may also be present on the inside of the crucible 2, if necessary for the temperature field optimization. The growth module without the coil and vacuum chamber enclosure (not shown) is called a "hot-zone".

The crucible 2 has an upper portion 8 and a lower portion 9 as seen in a vertical direction V to which will be referred to in the following disclosure.

The source material 5 usually comprises a form of polycrystalline SiC powder containing grains with various desirable size distribution.

Inside the crucible 2, the source powder material 5 is arranged as a substantially tube-shaped source extending in a vertical direction V and is hence situated at or juxtaposed to the inner walls of the crucible 2 to form a growth cavity, or distance C, between the seeds 6 (6, 6', 6", etc.) or a smaller than C distance between an outer growth surface of the seeds 6 and an inner surface of the SiC powder source 5 if sintered, or inner surface of a porous holding tube if not. The distance C can be varied with crucible diameter, seed size and number, and crucible and/or powder enclosure shape.

The distance C will thus decrease with the crystals growing on the seeds in a direction R which is substantially perpendicular to the vertical direction V.

The source material 5 may be, as mentioned, held in place by e.g., porous graphite tube, graphite tube with drilled openings, or sintered to shape.

By substantially tube-shaped means, that the source material is shaped to surround the seed holder arrangement 7 which is arranged at the center of the crucible 2. The source material 5 may, for instance, have a hexagonal shape (not shown) or any other suitable form to correspond to the seed holder arrangement. As an example the cross-section of the substantially tube-shaped source material 5 may thus be hexagonal to correspond to a holder carrying six seeds. lt may also comprise voids, vertical and horizontal separations, and other favorable features for the advantageous behavior during growth.

At least one of the inductive heaters 3 or inductive coils surrounds the loaded crucible 2 with its surrounding felt or insulating material The inductive heaters or coils 3 are preferably moveable in a vertical direction V, i.e., up and down along the hot-zone symmetry axis (vertical axis). As an alternative the heater 3 can be fixed and the hot-zone moveable in relation to the coils. By moving either the heater 3 or the hot-zone the temperature of the hot-zone and other temperature fields within the crucible may be controlled during growth.

The source material 5 sublimes appreciably at temperatures above 1800°C, forming various elemental and molecular species. At high temperatures and in an inert gas atmosphere SiC sublimes to form; Si, SizC, and SiCz in concentrations generally governed by the stoichiometry and temperature of the source and surrounding environment. The vapors are transported towards the seed by the radia| and in smaller part axial thermal gradients of the setup and condense to form SiC crystals on the inside of the growth cavity on cooler seeds 6 and possibly on their holder The seeds 6 (or the crystal seed wafers, seed material, or substrates) must be kept in place during the PVT process, and conventionally a holder body 7 adapted to hold and retain the seed is used for this purpose.

The seeds 6, 6', 6", etc. (depending on the number of seeds that can be and are fitted inside) for growing SiC ingots are placed vertically on the inside of the crucible 2 and held in place by a seed holder assembly 7. This means that the seeds 6 are held in a vertical position such that they grow, during operation of the PVT process, in a direction which is perpendicular or at least substantially perpendicular to the vertical direction V, i.e. in a radia| direction R as seen from a center of the crucible to the inner walls of the crucible. The seeds can also be slightly tilted if favorable for growth uniformity, e.g., because of convection effects in a gas phase. This means that the distances shown in Fig. 1 as C, C', and C" would follow the rule C'C>C".

By placed vertically thus means that the seeds are placed to face the source material 5 which is arranged on the inside of the walls of the crucible 2, but not at its top 8 or bottom This arrangement is thus significantly different to a conventional PVT reactor where the seed is arranged at the top of the crucible, and hence grows in a vertical and downwards direction, and where the source material is conventionally arranged at a lower section 9 (bottom 2/3) of a crucible.

The seed holder assembly 7 arranged at a center part of the crucible may be provided with a hollow portion 10, extending from the upper part 8 of the crucible 2 to the lower part 9. The hollow portion may also be interrupted with solid graphite or refractory material parts to fasten the seed holder arrangement 7 to the crucible 2. The hollow portion 10 is important for heat exchange, sufficient cooling of the seeds and introduction of temperature gradient from source powder towards the seeds to condensate the SiC vapors on them and form the SiC ingot.

As seen in Fig. 2 illustrating a top view of the growth module reactor 1, the seed holder 7 may hold six seeds (6, 6', etc.) in a hexagonal configuration in this example.

The seed holder may however be adapted to hold at least three seeds 6, 6', 6" and up to any number in an appropriate configuration, depending on the size of the reactor, hot-zone, growth needs, etc. This means that the holder arrangement may, as seen in a top or bottom view, have a triangular, square, pentagonal, hexagonal, heptagonal, octagonal, etc. pattern or cross-section. The number of seeds is preferably such that they can be placed symmetrically around the hollow portion The seed crystal 6 is nowadays most commonly 150 mm in diameter, but may alternatively be 100 mm, 200 mm or any other size depending on the reactor and growth needs (such as expansion, area selection, available sizes of the seed crystals, etc.).

The design and construction of the holder arrangement 7 and the surrounding crucible 2, source material 5 and insulation material 4 may vary depending on the number of seeds in the holder, the type of reactor used, and on the size and dimensions of the seed crystals.

The seed crystals or substrates must however have an adequate size or number for a given reactor size to allow for an acceptable temperature difference between the edges or outer surface thereof compared to their axial line. lf the seed crystal is too big or equivalently their number is too low, the seed crystals would experience largely different temperatures at their edges compared to their axial center line due to the different distance from the source powder or its holding tube. These distances are shown in Fig. 2 as C and C*, respectively. This means that the growth cavity, or the distance between the growing ingot and the source material must be kept as such that a desirable temperature gradient is maintained in the reactor 1 between the source material 5 and the seeds 6. At this high temperature, low pressure and in the inert gas moderating (mean-free path controlling) atmosphere, the SiC source material 5 sublimes and condense on the cooler seed crystals ln most cases, the temperature in a source powder is around 2300°C at the hottest parts while the seed temperature is usually kept between 2000 and 2200°C when growing common 4H-SiC polytype. The crystal front surface temperature will also vary depending on its thickness changes while growing.

Estimated efficiency increase in this kind of reactor is approximately three times where six 150 mm seeds are placed in hexagonal pattern forming the inner tube-like assembly for growth. This increase is not higher mainly due to a larger hot-zone dimensions needed to encompass all parts necessary and consequently the required bigger coil/coils generally consuming about two times more power for heating to the same temperature according to our numerical simulations. ln case of a smaller seeds with the same reactor size as for conventional growth, the efficiency would be better. The seeds however can't be too big (or equivalently their number too low, i.e., the square or triangular pattern) for a given reactor size, as they would experience largely different temperatures at their edges compared to their axial center line, as mentioned above, due to C and C* distance difference. This configuration might however be preferred in some scenarios where the higher gradients are needed.

As illustrated in Fig. 3 a chamber 20 may comprise several reactor growth modules 1, 1', etc., having their separate (or alternatively partially shared) heating coils (i.e., coil 3' on Fig. 3 which could comprise of one coil, or two separate coils) on the outside of the quartz enclosure 21. Here the growth modules 1, 1' are arranged in a vertically stacked configuration (distance VD apart or stacked directly one on top of another) such that the overall vacuum and gas inlet system is shared or joint between those stacked modules in one growth chamber 20 or growth system 20. Fig. 3 contains the previously mentioned example of quartz enclosure tube 21, which has been omitted from other drawings for clarity. As illustrated in Fig. 3 each growth module 1, 1' is provided with a holder arrangement 7, 7' holding SiC seed substrates 6, 6' respectively. Each growth module 1, 1' further is provided with a substantially tube-like source material 5, 5' and hollow portion 10, 10'. ln the growth chamber or system 20 each module 1, 1' may be operated independently of the other modules.

According to another embodiment (not shown in the figures) the complete growth module setup with the heater coil may be 90° rotated, i.e. placed horizontally. Even if the vertical axis V thus in reality becomes a horizontal axis the seeds grow in a radial direction R, perpendicular to this vertical axis V.

Even further it might be needed, in above stated case with horizontal symmetry axis placement, that the holder arrangement or the whole hot-zone is rotatable and kept in continuous rotation during the growth process. This rotation of vertical axis horizontal and the rotation of holder assembly could be preferred in case of vertical space constrains in reactor placement or if advantageous for any other reason, e.g. the crystal quality, temperature field, gas flow, gas species uniformity, gravitational effect on formed graphite particles, etc.

Modifications and other variants of the described embodiments will come to mind to one skilled in the art having benefit of the teachings presented in the foregoing description and associated drawings. Therefore, it is to be understood that the embodiments are not limited to the specific example embodiments described in this disclosure and that modifications and other variants are intended to be included within the scope of this disclosure. Furthermore, although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Therefore, a person skilled in the art would recognize numerous variations to the described embodiments that would still fall within the scope of the appended claims. As used herein, the terms "comprise/comprises" or "include/includes" do not exclude the presence of other elements or steps. Furthermore, although individual features may be included in different claims (or embodiments), these may possibly advantageously be combined, and the inclusion of different claims (or embodiments) does not imply that a combination of features is not feasible and/or advantageous. ln addition, singular references do not exclude a plurality. Finally, reference signs in the claims are provided merely as a clarifying example and should not be construed as limiting the scope of the claims in any way.

Claims (10)

Claims

1. A reactor growth module (1) adapted for a physical vapor transport grovvth cornprising silioort carbicšo growth, comprising a crucible (2) extending Iongitudinally in a vertical direction (V), at least one inductive heater (3) arranoed to surround said crucible tå) a seed holder arrangement (7) provided on an inside of said crucible (2), a source material (5) arranged to surround said seed holder arrangement (7) and extending Iongitudinally along an inner wall of said crucible (2), wherein said seed holder arrangement is adapted to receive and hold at least three seed crystal substrates (6, 6', 6"), characterized in that the seed holder arrangement (7) is provided such during a physical vapor transport process, said seed substrates (6, 6', 6") grow in a radial direction (R) which is substantially perpendicular to said vertical direction (V).

2. The reactor growth module (1) as claimed in claim 1, wherein said seed holder arrangement (7) further comprises a central hollow portion (10) extending through an upper portion (8) and lower portion (9) of said crucible (2).

3. “The reactor growth module (1) as claimedgg io--eEašm--åarrg one of preceding claims, wherein said at least one inductive heater (3) and said crucible (2) are moveable in relation to one another.

4. The reactor growth module (1) as claimed in any one of the preceding claims, wherein said source material (5) is comprised in a porous graphite tube, or in a graphite tube with drilled openings, or is sintered to shape.

5. The reactor growth module (1) as claimed in any one of the preceding claims, wherein said module (1) is rotatable to 90° from a vertical symmetry axis to the horizontal symmetry axis.

6. The reactor growth module (1) as claimed in any one of the preceding claims, wherein said seed holder assembly (7) is rotatable inside said crucible (2) around a vertical axis.

7. The reactor growth module (1) wherein said crucible (2) is rotatable around a vertical axis.

8. A growth module chamber (20) comprising at least two stacked rector growth modules (1, 1') as claimed in any one of claims 1 to åí, and a joint vacuum and gas system.

9. A physical vapor transport method cornpršsino silicort carbide orovvth, comprising of a reactor growth module (1) including a crucible (2) extending longitudinally in a vertical direction (V), the method comprising: providing a seed holder arrangement (7) on an inside of said crucible (2), providing a source material (5) arranged to surround said seed holder arrangement (7) and extending longitudinally along an inner wall of said crucible (2), providing said seed holder arrangement (7) with seed crystal substrates (6, 6', 6"), providing at least one inductive heater (3) surrounding said crucible (2), placing said seed holder arrangement (7) and seed crystal substrates (6, 6', 6") inside the crucible (2) comprising the source material (5), and increasing a temperature inside said crucible (2) by said inductive heater (3) until a growth temperature is reached; and keeping said growth temperature and; increasing a thickness of said seed crystal (6, 6', 6") in a direction (R) which is substantially perpendicular to said vertical direction (V).

10. The use of the reactor growth module (1) in any one of claims 1 to ål, or the chamber (20) as claimed in claim 9, in a physical vapor transport process comprising silicon carbide orovtfth.