Oxide removal process for high purity silicon carbide powder

Abstract

The present invention refers to the field of silicon carbide production and purification. In particular, the present invention refers to a purification process for oxide removal for high purity silicon carbide powder, i.e., the silicon carbide purification from residual oxide layer up to an oxygen concentration at least below 0.02% wt, preferably below 0.01% wt.

Description

[0001] OXIDE REMOVAL PROCESS FOR HIGH PURITY SILICON CARBIDE POWDER

[0002] TECHNICAL FIELD

[0003] The present invention refers to the field of silicon carbide production and purification.

[0004] In particular, the present invention refers to a purification process for oxide removal for high purity silicon carbide powder, i.e., the silicon carbide purification from residual oxide layer up to an oxygen concentration at least below 0.02% wt, preferably below 0.01% wt.

[0005] In the present invention the inventors particularly intended to focus the attention on SiC powders for crystal growing within the semiconductor industry field. In this specific area of application several material specifications must be considered. Purity grade is usually expressed with a number before the letter N where the number indicates the “number of nines” purity. For example, a material with a purity level of 99.9% is labeled as 3N while a material with a purity level of 99.99999 is labeled as 7N. In the silicon carbide field, the purity indicated with this method consider all the elements apart from excess of silicon and carbon (commonly indicated as free silicon and free carbon), oxygen and nitrogen. To have a high purity material, depending on the synthesis method, the producer can make use of high purity reagents, has to be aware of possible contaminations when handling the material at various stages and can include one or more purification step.

[0006] STATE OF THE ART

[0007] Silicon carbide (SiC), also known as carborundum, is a semiconductor with chemical and physical properties suitable for hard coatings applications or for use as an abrasive in machining processes such as grinding, honing, water-jet cutting, or sandblasting. Currently, silicon carbide is also widely used in the semiconductor industry because of its electrical characteristics which, differently to silicon, make it suitable for applications in high power, high frequency, and high temperature devices.

[0008] Silicon carbide occurs in nature as moissanite, an extremely rare mineral which is found in only minute quantities. Because natural moissanite is extremely scarce, most silicon carbide is synthetic. Nowadays, some processes to manufacture silicon carbide are known, which differ one from the other in terms of the methodology used to carry out the reactions necessary for the formation of the compound, and which also differ in terms of purity status / form in which the synthesized material present itself.

[0009] The simplest process to manufacture silicon carbide is the so-called Acheson method, which is based on heating a mixture of silicon dioxide (S iOi) and carbon (C) in a furnace in the presence of a graphite rod. The final compound, silicon carbide, obtained presents impurities of Nitrogen and Aluminum, which disadvantageously affect the electrical conductivity of the compound. This process is also widely used in the production of low purity SIC for applications in the abrasive field.

[0010] Silicon carbide can also be produced by Chemical Vapor Deposition (CVD), a process that allows to obtain a deposit on a solid support, starting from a precursor that is introduced in gaseous form and decomposes on the surface of the substrate. It is a technologically more complex process than the Acheson and which is used to obtain very high levels of purity.

[0011] A most direct method to synthesize SiC consists in reacting its elemental components carbon and silicon. The method uses ultrapure Si and Carbon reactants in powder form stoichiometri- cally mixed for the synthesis of SiC at a temperature of about 2000°C under vacuum and / or an inert atmosphere, for example, argon.

[0012] US 5,899,752 describes a method of in-situ cleaning a native oxide layer from the surface of a silicon wafer positioned in a vacuum chamber substantially free from oxidizing species by passing at least one non-oxidizing gas over the native oxide layer at a temperature between about 650 C. to about 1025 C, until such native oxide layer is removed.

[0013] US 2012 / 0174944 Al (national phase of WO 2011 / 158558 Al) describes a cleaning method of a SiC semiconductor including the step of forming an oxide film on said SiC semiconductor and the step of removing the oxide film, in which an oxygen plasma is used for forming the oxide film and hydrogen fluoride is used for removing the oxide film.

[0014] Barbouche M. el al., Impact of rapid thermal annealing on impurities removal efficiency from silicon carbide for optoelectronic applications, The International Journal of Advanced Manufacturing Technology (2019), https: / / doi.or / 10.1007 / s00170-019-Q4556-7, describe how to enhance silicon carbide SiC purity for silicon passivation form 3N to 5N.

[0015] Advantages of the known processes are mainly found in the homogeneity and purity degrees of the compound obtained; on the other hand, these processes are expensive and involve the use of complex equipment for accurate process control, with particular regard to temperature control. Moreover, the maximum removal level of the oxidized layer formed on the SiC grains during the preparation process is still not completely satisfactory for particular applications.

[0016] The need therefore remains for an efficient purification process that allows the convenient production of a silicon carbide characterized by a very high purity degree, in particular as far as regards the residual presence of oxides impurities. Preferably, said final oxygen concentration should be at least below 0.02% wt, or possibly even below 0.01% wt.

[0017] SUMMARY OF THE INVENTION

[0018] It is an object of the present invention to overcome the drawbacks of the prior art. Particularly, it is an object of the present invention to provide a process for the purification of silicon carbide which allows to obtain the removal of oxide impurities up to a high purity degree.

[0019] The present inventors have now found that removing the residual oxide impurities at a suitable high temperature under an inert atmosphere gives the desired response to above mentioned technical problem of obtaining a final SiC powder with the desired high degree of purity.

[0020] Moreover, the present inventors have also found that the high temperature SiC powder obtained can be advantageously cooled by using a properly selected gas flux, e.g., a H2 or He flow, in the reaction chamber.

[0021] Accordingly, it is a first aspect of the present invention a process for cleaning the powder grains of SiC from the possible oxide layer on them (oxide layer removal), by submitting them to a proper heating step under an inert atmosphere, as disclosed in the appended independent claim.

[0022] It is another aspect of the present invention a step of speeding up cooling of the obtained powder SiC grains by flowing them with a proper inert gas flux, as disclosed in the appended dependent claim.

[0023] It is a further object of the present invention to provide a process for silicon carbide purification, which allows to obtain a high purity material, namely a material having a purity degree of at least 6N, i.e. higher than 99,9999% wt, as disclosed in the appended dependent claim.

[0024] Other aspects of the present invention are disclosed in the appended dependent claims, which form an integral part of the present description.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention will be described below with reference to some exemplary and non-limiting embodiments shown, for example in the annexed drawings.

[0027] Figure 1: discloses SiC crystals (encircled areas) with unreacted C after the synthesis of SiC from a mixture of Silicon (Si) powder and Graphite (C) powder.

[0028] Figure 2: discloses residual oxides, SiOx (encircled area), on a SiC crystal after the removal of graphite excess.

[0029] Figure 3: discloses clean SiC crystals after the final cleaning step (from oxide).

[0030] Figure 4: discloses SiC crystals with a N content of 150 ppm (left) and 0.5 ppm (right).

[0031] DETAILED DESCRIPTION OF THE INVENTION

[0032] While the preparation of SiC can be implemented in several alternative ways (SiC can also be produced with other techniques of the art, such as, for example, CVD or Acheson process), the following is just a preferred way of preparation of the present invention, disclosed by way of a preferred, but non-limiting example.

[0033] In this preferred case, Silicon Carbide is produced by mixing high purity (6N grade) Silicon and Graphite powders with a selected granulometry and in a defined molar ratio. The said mixture is placed in a graphite crucible having an elongated cylindrical shape. The crucible is positioned in an airtight container provided inside with a hot zone. Advantageously, the process comprises the step of removing air from the airtight container so as to obtain an inert atmosphere inside the airtight container. Moreover, the process also comprises the steps of heating the crucible by means of a heating device (e.g., an induction furnace) until reaching a final temperature (TF) of at least 2200°C; maintaining the crucible at said final temperature (TF) for a second time interval At2 until complete Silicon reaction; and reducing or deactivating said heating device after said second time interval At2 (cooling down step). Particularly, at least during the steps of heating the crucible and maintaining the crucible at the final temperature, an internal pressure of the airtight container is controlled to be substantially maintained at a predetermined pressure value by means of addition / removal of an inert gas (e.g., Argon, (Ar)).

[0034] Relevant Parameters:

[0035] Silicon (Si) powder and Graphite (C) powder; wherein

[0036] - both powders are of 6N purity grade minimum; and wherein

[0037] - Si grains average diameter is from 0,1 mm to 5 mm; preferably from 1 mm to 4 mm or from 1 mm to 3 mm; most preferably <2 mm;

[0038] - C grains average diameter is from 0,1 mm to 5 mm; preferably from 1 mm to 4 mm or from 1 mm to 3 mm; most preferably <2 mm; and wherein

[0039] - the molar ratio (MR) Si:C is between 1.1: 1 (excess of Si) and 1: 1.2 (excess of C).

[0040] Final reaction

[0041] TF=1800-2400 °C

[0042] Reaction Time:

[0043] At2= 1-5 h

[0044] Purification phase (1): graphite excess removal At the end of the reaction the possible excess of graphite present in the resulting SiC powder is removed by heating the SiC powder at a temperature from 600°C to 1150°C; preferably from 650°C to 1100°C or from 750°C to 950°C; most preferably at 850°C under air flux (i.e., under oxidizing atmosphere).

[0045] The powder enriched in graphite can be obtained in the above reaction or can also be produced with other technique such as CVD or the Acheson process.

[0046] The thermal reaction of graphite with Oxygen (O2) produces CO and CO2 that are removed in the gas phase.

[0047] There are three possible reasons for the presence of C in the resulting SiC:

[0048] - C not reacted with Si due to a molar ratio of the reagents with an excess of C;

[0049] - Unreacted C due to non-uniform reaction in the crucible (e.g., Si migration during the heating process);

[0050] - Graphitization of the resulting SiC due to overheating in some regions of the crucible that lead to the decomposition of the SiC to Si and C.

[0051] Purification phase (2): oxide layer removal / cleaning

[0052] During the previous purification phase (1) from the excess of graphite, part of the SiC can be oxidized by the hot air flux, thus forming a SiOx layer on the powder grains.

[0053] In order to remove this layer of oxide impurities, the SiC powder is heated at a temperature from 1800°C to 2300°C, preferably at about 1900°C (± 25°) in an inert atmosphere free from N2 (preferably, Ar) after vacuum washing the reaction chamber, for avoiding doping by N2.

[0054] As a matter of fact, during this heat treatment the residual oxide SiOx evaporates (SiO has m.p.=1702°C and b.p.=188O°C) from the SiC powder and at least 6N pure grade SiC is obtained; wherein the powder average grain size is below 5mm, as measured according to any of the known methods for measuring particle size, e.g., using dynamic image analysis (DIA).

[0055] While the synthesis of Silicon Carbide can be performed through the process above exemplified or also adopting one of the processes known in the art, the subsequent purification phases (1) and / or (2) are necessary if there is the need to remove possible residual impurities (i.e., graphite excess (purification (1)) and / or the oxidized layer (SiOx) on the SiC grains (purification (2)).

[0056] In particular, looking at the analysis of C and O at the various phases of preparation reported in the following Table:

[0057] It clearly results that purification phase (1) lowers the free C% amount, while purification phase (2) minimizes the residual 0% amount.

[0058] As already stated above, In the present invention the inventors intended to focus the attention on SiC powders for crystal growing within the semiconductor industry field. In this specific area of application several material specifications must be considered.

[0059] As already explained, purity grade is usually expressed with a number before the letter N where the number indicates the “number of nines” purity. For example, a material with a purity level of 99.9% is labeled as 3N while a material with a purity level of 99.99999 is labeled as 7N and so on. In the silicon carbide field, the purity indicated with this method considers all the elements apart from excess of silicon and carbon (commonly indicated as free silicon and free carbon), oxygen and nitrogen. To obtain a high purity material, the producer can make use of high purity reagents, has to be aware of possible contaminations when handling the material at the various stages and can include one or more purification step. Crystal growing for semiconductor industry need purity grade of at least 5N, more preferably 6N.

[0060] Another important parameter for the SiC powder is the Nitrogen (N) content. Electrical properties change with the N doping of the SiC wafer, so it is important to be able to control the amount of N also in the raw materials SiC powder for crystal growing. N doping varies from crystal saturation (that corresponds to about 4xl09cm’3for 4H-SiC) down to a very low level such as N <0.1ppm to produce what is known as N-free semi-insulating material, useful for electronics, automotive and telecommunication industry. To control the amount of N in the SiC powder it is possible to play with high vacuum, controlled amount release of N gas, and / or gas flux of N or N-inert gas mixtures such as N-Ar mixture during the phase of the synthesis of SiC and / or purification phase (2) (i.e., cleaning of the oxide layer) of the production process. In purification phase (1) (calcination) the temperature is too low for incorporating N in the SiC crystal lattice. Color of semi-insulating SiC is white (transparent grains) as in Figure 4 right, while the presence of N in the crystal is visible from the green color of the SiC grains (shown in dark-gray in Figure 4, on the left) that gets darker and darker as N increases.

[0061] Free carbon value indicates the amount of C in the resulting powder that is not creating SiC crystal grains. Unfortunately, as clearly results from the above disclosure, what is obtained from every phase of preparation and / or purification of SiC is a high-temperature powder that can be handled only after a proper cooling step that lowers the temperature to an acceptable level. However, the cooling steps which must necessarily follow the preparation phase (i.e, the synthesis of SiC) and the purification phase (2) (i.e., the oxide layer removal), are indeed very time consuming. Thermalization of the crucible can take several hours. To speed up said cooling steps it has been found very useful and simple to use an inert gas flux in the reaction chamber. However, the choice of the gas, or the mixture of gases, is crucial, and has to take into account possible chemical reactions between the gas and the powder, the crucible, and all the components of the furnace chamber.

[0062] The cooling phases are necessary after the preparation phase of SiC above described as well as after the purification phase (2) (i.e., the oxide layer removal).

[0063] Among the possible cooling gases “compatible” with the single phases of the process, the inventors unexpectedly found that, gases such as H2 and He are very efficient for reducing the duration of the cooling steps as much as possible, thus greatly reducing the overall times of the process and also the cost thereof.

[0064] Thus, cooling with a flow of H2 and / or He resulted extremely beneficial; indeed, the cooling phase duration was at least lower than 60 % and provided a reduction of at least about 35 % of the overall time of the process.

[0065] However, when H2 is used as the cooling gas, it to be taken into account the fact that H2 can react with Carbon (graphite) at temperatures higher than 1300°C forming CH4 and C2H2. Thus, the temperature can preferably be lowered from 1800°C-2400 °C (end of the reaction of preparation of SiC), or from 1800°C-2300°C (end of purification phase (2)), to 1300°C with a flux of Argon and then cooling is continued with the flux of H2 until the desired temperature is reached.

[0066] In turn, when He is the cooling gas used, due to the fact that it is a totally inert gas in the reaction or purification environment, the cooling can be performed by directly using the flow of He from the beginning.

[0067] As already mentioned, the previously described purification phase (2) of the SiC powder grains from a layer of oxide, and the corresponding next cooling phase, can be used also for conveniently purifying SiC obtained through other synthetic ways, like, for example, the Acheson or the CVD process. Moreover, it can also be used in different situations, for example, for in-situ cleaning away a native oxide layer formed on the surface of a silicon wafer during its production as described in US 5,899,752. So, in a particularly preferred embodiment, the main aspect of the present invention is a method for realizing the purification of SiC powder grains from impurities formed by an oxide layer comprising:

[0068] - a purification phase by a thermal treatment at a temperature from 1800°C to 2300°C, preferably at about 1900°C (± 25°), under an inert atmosphere free from N2 (preferably, under Ar) after vacuum washing the reaction chamber; said thermal treatment being followed by:

[0069] - a cooling phase performed by providing a cooling gas flow into the reaction chamber, said cooling gas being selected from H2 or He.

[0070] In a further preferred embodiment, said purification method of the present invention further comprises:

[0071] 1) - a first synthetic step of preparing SiC, in which Silicon (Si) powder and Graphite (C) powder, wherein both powders are of 6N purity grade minimum, and wherein

[0072] - Si grains average diameter is from 0,1 mm to 5 mm; preferably from 1 mm to 4 mm or from 1 mm to 3 mm; most preferably <2 mm; and wherein

[0073] - C grains average diameter is from 0,1 mm to 5 mm; preferably from 1 mm to 4 mm or from 1 mm to 3 mm; most preferably <2 mm; and wherein

[0074] - the molar ratio (MR) Si:C is between 1.1: 1 (excess of Si) and 1: 1.2 (excess of C), are mixed and reacted at a temperature of 1800°C-2400 °C for a reaction time of 1-5 h, under an inert atmosphere free from N2, preferably, Argon (Ar); and / or

[0075] 2) - a second purification step of removing possible graphite excess (calcination phase)_pre- sent in the resulting SiC powder from step 1 above performed by heating said SiC powder at a temperature from 600°C to 1150°C, preferably from 650°C to 1100°C or from 750°C to 950°C, most preferably at 850°C under air flow (i.e., under oxidizing atmosphere), wherein the thermal reaction of graphite with air Oxygen (O2) produces CO and CO2 that are removed with the gas phase; and wherein both steps 1) and 2) precede the purification phase from oxide layer described above.

Claims

C l a i m s1. A purification method for realizing the oxide removal from SiC powder grains comprising the following phases:- a purification phase performed by a thermal treatment at a temperature from 1800°C to 2300°C under an inert atmosphere free from N2 after vacuum washing the reaction chamber; said thermal treatment being followed by:- a cooling phase performed by providing a cooling gas flow into the reaction chamber.

2. The method according to claim 1, in which said thermal treatment is performed at a temperature of about 1900°C (± 25°C).

3. The method according to claim 1 or 2, in which said inert atmosphere free from N2 is Ar.

4. The method according to anyone of the preceding claims, in which in the cooling phase said cooling gas is selected from H2 or He.

5. The method according to claim 4, in which, when the selected cooling gas is H2, the temperature reached during the said purification phase is pre-cooled to 1300°C with Ar before providing the flow of H2.

6. The purification method according to anyone of claims from 1 to 4, which further comprises the following steps:1) - a first synthetic step of preparing SiC, in which Silicon (Si) powder and Graphite (C) powder are mixed and reacted at a temperature of 1800°C-2400 °C for a time of 1-5 h, under an inert atmosphere free from N2; and / or2) - a second purification step of removing possible graphite excess_present in the SiC powder resulting from step 1) above, performed by heating said SiC powder at a temperature from 600°C to 1150°C under an air flow which oxidizes graphite excess with air O2 producing CO and CO2 that are removed with the gas phase; wherein both steps 1) and 2) precede the purification phase from oxide layer of claim 1.

7. The method according to claim 5, in which in step 1) Si powder and C powder are both of 6N purity grade minimum.

8. The method according to claim 5, in which in step 1) Si powder grains average diameter is from 0,1 mm to 5 mm.

9. The method according to claim 5, in which in step 1) C powder grains average diameter is from 0,1 mm to 5 mm.

10. The method according to claim 5, in which in step 1) the molar ratio Si:C is between 1.1: 1 and 1: 1.2.

11. The method according to claim 5, in which in step 1) the inert atmosphere free from N2 is Ar.

12. The method according to claim 5, in which in step 2) SiC powder is heated from 650°C to 1100°C or from 750°C to 950°C. most preferably at 850°C.

13. The method according to claim 5, in which in step 2) SiC powder is heated at 850°C.

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