SYSTEM FOR GROWING SILICON CARBIDE CRYSTALS

Description

DESCRIPTION of the industrial invention in the name of LPE SpA with headquarters in Bollate (MI). The present invention relates to a system for growing silicon carbide crystals according to the preamble of claim 1. Various proposals have been put forward in the past for growing silicon carbide crystals of a quality suitable for use in the microelectronics industry at very high temperatures (over 1800°C). An initial and fundamental proposal was put forward by Nisshin Steel in 1992; this is illustrated in European patent EP554047. Nisshin Steel's idea involves mixing reaction gases containing silicon and carbon, feeding the gas mixture into a high-temperature reaction chamber, and depositing the mixed silicon and carbon onto a substrate, growing a crystal. Nisshin Steel's embodiment involves an intermediate-temperature pre-chamber in which solid silicon carbide particles form. This idea was taken up again in 1995 by OKMETIC; OKMETIC's solution is described in international patent application WO97 / 01658. A second and fundamental proposal was put forward by Jury Makarov in t 1999; this is illustrated in international patent application WOOO / 43577. Makarov's idea involves separately feeding reaction gases containing silicon and carbon into a high-temperature reaction chamber, and then bringing them into contact near a substrate in such a way that the silicon and carbon deposit. cci a . A PATENTS 13JUNE^3' directly on the substrate while growing a crystal; Makarov's invention aimed to avoid silicon carbide deposits along the walls of the chamber and therefore envisaged forming the silicon carbide only in the vicinity of the substrate, i.e. the growing crystal. Studying the solution proposed by Makarov, we realized that it is critical from both the chemical kinetics and fluid dynamics points of view. The purpose of this invention is to provide a third and fundamental proposal, different and better than the previous ones. This object is achieved by the system for growing silicon carbide crystals having the characteristics set forth in independent claim 1. The idea underlying the present invention is to introduce carbon-containing gases and silicon-containing gases into a reaction chamber through separate inlet means and to bring these gases into contact in a central region of the chamber distant from the growth substrate. In this way, the concentration profile and the velocity profile are substantially constant in the radial direction (of course, there are some inevitable edge effects); thus, a constant growth rate, a uniform crystalline structure and a uniform chemical composition are obtained over the entire cross-section of the substrate. Advantageous aspects of the present invention are set forth in the dependent claims. The present invention will become clearer from the following description, taken in conjunction with the attached drawings, in which: Fig.l is a schematic sectional view that helps to understand the description of the teachings of the present invention, Fig.2 shows, in simplified sectional view, a first example of embodiment of the present invention, and Fig.3 shows, in simplified sectional view, a second example of embodiment of the present invention, The system for growing silicon carbide crystals on substrates according to the present invention comprises a chamber extending in one direction; typically this direction is vertical; the chamber provides: separate inlet means for carbon-containing gases and silicon-containing gases, support means for substrates placed in a first extreme zone of the chamber, unloading exit means placed in the vicinity of the support means, heating means capable of heating the chamber to a temperature above 1800°C; the inlet means for silicon-containing gases are positioned, shaped and sized so that the silicon-containing gases enter a second extreme zone of the chamber; The inlet means for carbon-containing gases are positioned, shaped, and sized so that the carbon and silicon come into substantial contact in a central area of ​​the chamber, distant from both the first and second extreme zones. In Fig.1, the reference 1 indicates the chamber, the reference 10 indicates the volume enclosed by the chamber, the reference 2 indicates the inlet means for silicon-containing gases, the reference 3 indicates the inlet means for carbon-containing gases, the reference 4 indicates the support means for substrates (a substrate is shown applied to the means 4 and indicated with a black stripe), the reference 5 indicates the exhaust outlet means, the reference 21 indicates an evaporation cell for the means 2 (which will be mentioned and illustrated below), the references 22A and 22B indicate two possible embodiments of central cores for the means 2 (which will be mentioned and illustrated below), the reference ZI indicates an indicative height of the first extreme zone of the chamber, the reference Z2 indicates an indicative height of the second extreme zone of the chamber, the reference ZC indicates an indicative height of the central zone of the chamber. Furthermore, in Fig.l, the indicative distribution of the gases entering the chamber from means 2 and 3 is indicated with dotted lines, and the axis of symmetry of the chamber is indicated with a dash and dot line (the chamber of the system according to the present invention is not necessarily symmetrical with respect to an axis). Through the system specified above, the concentration profile and the velocity profile are substantially constant in the radial direction at least in the first extreme region of the chamber (of course there are some inevitable edge effects); thus, a growth rate is obtained -the* constant, a uniform crystalline structure and a uniform chemical composition across the entire cross-section of the substrate placed on the support media. Furthermore, since the inlet zone of the silicon-containing gases is far from the mixing zone (central zone ZC) with the carbon-containing gases, and since the chamber is at a very high temperature, any particles Liquid silicon particles formed at or before entering the chamber evaporate, and thus there is no risk of solid silicon carbide particles forming due to contact between the carbon and the liquid particles. These solid silicon carbide particles (especially if large) are difficult to break down by sublimation and are extremely dangerous because they can irreparably damage the growing crystal if they strike its surface. Finally, since the inlet zone of the silicon-containing gases is distant from the mixing zone (central zone ZC) with the carbon-containing gases, it is possible to ensure that the concentration profile and the velocity profile of the silicon-containing gases, at their meeting, are substantially constant in the radial direction (of course there are some inevitable edge effects). According to the present invention, three zones are identified in the chamber: a first extreme zone (ZI), a central zone (ZC), and a second extreme zone (Z2). In all the examples illustrated in the figures (particularly in Fig. 1), the chamber has a substantially cylindrical shape and extends mainly in a substantially vertical direction (the most advantageous choice); the first extreme zone ZI corresponds to the upper zone of the cylinder and the second extreme zone Z2 corresponds to the lower zone of the cylinder. If low gas flows are used in a system according to the present invention (as is preferable), the vertical orientation of the chamber will cause any liquid silicon particles (especially large ones) to tend to remain at the bottom until they evaporate. For example, if the internal diameter of the chamber is 150mm, the The second extreme zone can extend from the base to a height of approximately 50 mm, the central zone can extend from a height of approximately 100 mm to a height of approximately 150 mm, and the first extreme zone can extend from a height of approximately 200 mm to a height of approximately 250 mm. By carefully selecting the various gas outlets and the gas flow velocities and flow rates, the lengths of the various zones and the distances between them can be significantly reduced to less than half. Naturally, since the silicon-containing and carbon-containing compounds enter the chamber in gaseous form, and since lateral diffusion is very high due to the high temperature, it is not possible to precisely define the contact zone and the degree of mixing. The exhaust outlet media can be used to discharge everything: reaction products, unreacted and / or unsettled compounds and elements, carrier gases, attack gases, and, possibly (!), solid particles that detach from the chamber walls and / or from the growing crystal. The temperature of about 1800°C corresponds approximately to the temperature limit of normal CVD processes for silicon carbide growth; furthermore, this temperature of about 1800°C constitutes a frontier temperature: typically below 1800°C there is nucleation of 3C-type SiC and typically above 1800°C there is nucleation of 6H or 4H-type SiC; finally, this temperature of about 1800°C ensures that the silicon is in the gaseous phase in the pressure range (0.1-1.0 atmospheres) and dilutions (1%-20%) of interest. If the means of entry for carbon-containing gases are positioned, shaped and sized in such a way that the carbon and silicon come into substantial contact in an area distant from the chamber walls (as is, in part, the case in Fig.l), the silicon carbide deposits along the internal walls of the chamber are much more limited. The chamber of the system according to the present invention can advantageously provide inlet means for anti-nucleation gas; these can be positioned, shaped and sized in many different ways and also combined with each other; hydrochloric acid [HCl] can advantageously be used as an anti-nucleation gas; this compound reacts with silicon in the gaseous phase preventing nucleation phenomena; hydrochloric acid can advantageously be used in combination with hydrogen. The chamber of the system according to the present invention can advantageously provide inlet means for etching gases; these can be positioned, shaped and sized in many different ways and even combined with each other; hydrochloric acid [HCl] can advantageously be used as an etching gas; this compound attacks solid deposits and solid particles of silicon and silicon carbide (particularly if polycrystalline); hydrochloric acid can advantageously be used in combination with hydrogen. Attack gas inlet means may be positioned, shaped, and sized to cause gas inlet in the first extreme region of the chamber (as in the examples in Fig. 2 and Fig. 3), i.e., in the vicinity of the support means and the exhaust outlet means. Such means may serve to prevent the exhaust outlet means from becoming blocked. due to material deposits. In the examples in Fig. 2 and Fig. 3, these means comprise a hollow sleeve (which also acts as the chamber wall in the upper area of ​​the chamber), communicating with a suitable duct, which has a plurality of holes facing towards the inside of the chamber. Anti-nucleation gas inlet means can be positioned, shaped, and sized to cause gas inlet in the second extreme zone of the chamber (as in the example in Fig. 2), i.e., in the vicinity of the silicon-containing gas inlet means. Such means can serve to reduce the presence of liquid silicon particles in the chamber, particularly in the second zone of the chamber. In the example in Fig. 2, such means comprise a plurality of nozzles arranged in a ring and oriented at an angle of approximately 45° toward the center of the chamber. Anti-nucleation gas inlet devices can be positioned, shaped, and sized to induce gas ingress into the central region of the chamber. These devices can serve to reduce the presence of liquid silicon particles in the chamber, particularly in the central region. Etch gas inlet means can be positioned, shaped, and sized to create a gas flow essentially only along the chamber walls. Such means can serve to remove and / or prevent silicon carbide deposits along the chamber walls; however, when designing such an etch gas flow along the walls, its effect on the chamber walls must be taken into account, as they must be adequately protected. The attack gas inlet means may be designed to admit into the chamber an attack gas, typically hydrochloric acid, combined with a carrier gas, typically hydrogen (alternatively, argon or helium, or a mixture of two or more of these gases); the attack gas to carrier gas ratios could be, for example, 1000 slm for hydrogen and 1-2000 slm for hydrochloric acid. The support means of the system according to the present invention can also advantageously include inlet means for etching gas (as in the examples of Fig. 2 and Fig. 3); these can be positioned, shaped, and sized to cause gas ingress around the substrates. Such means can serve to remove silicon carbide deposits (especially polycrystalline silicon carbide) at the periphery of the support means and to limit lateral crystal growth. In this case, the support means can consist, for example, of a thick disk equipped with an internal cavity and mounted on a tube communicating with the cavity (as in the examples of Fig. 2 and Fig. 3); the tube is thermally and chemically insulated; the etching gas is injected into the tube, flows into the cavity, and exits through a plurality of holes drilled on the periphery of the disk. The system according to the present invention can advantageously include means for rotating the support means during the growth process (as in the examples of Fig. 2 and Fig. 3). In this way, a better uniformity of the growth conditions at the crystal surface is achieved. The system according to the present invention can advantageously include means for retracting the support means during the growth process (as in the examples in Fig. 2 and Fig. 3). In this way, during growth, the crystal surface is essentially always in the same position in the chamber, regardless of the length of the grown crystal, and therefore it is easier to control the growth conditions at the crystal surface. The means of moving the support means can be advantageously protected from both the heat and the chemical environment of the reaction chamber (as in the examples in Fig.2 and Fig.3). In all the examples illustrated in the figures, the support means are designed to support only one substrate, which is the simplest situation. In accordance with the present invention, the silicon-containing gas inlet means can be positioned, shaped and sized in many different ways. The simplest way to implement such devices is with a duct that opens into the second zone of the chamber; if the chamber is vertical and cylindrical, the duct will typically be vertical and central. This duct communicates with the system chamber, and therefore the temperature at the end of the duct will be quite high, although lower than that of the chamber. The outlet of the duct into the chamber can be advantageously made with a fluid-dynamic distributor designed to make the velocity profiles uniform and avoid lateral vortices. To limit the entry of liquid silicon particles into the chamber, this duct can advantageously be provided in correspondence with one of its sections terminal, a silicon evaporation cell; such a cell is schematically shown in Fig. 1 and designated with the reference 21; the most typical and simple way to evaporate liquid silicon particles is by heating; in fact, in Fig. 1, a graphite sleeve covered with a suitable material suitable for being heated by induction and radiation is schematically shown. To heat silicon-containing gases, this duct can advantageously feature a central core at one of its terminal sections; the central core can be heated by radiation from the duct walls; the core can be shaped and sized in various ways; particular shapes and / or sizes can be aimed at maximizing heat exchange between the duct walls and the core and between the core and the gas. To better distribute the gases contained within the chamber, this duct can advantageously feature a central core at one of its terminal sections; the core can be shaped and sized in various ways; particular shapes and / or dimensions can be aimed at avoiding vortices and controlling any condensation phenomena along the walls. The central core can therefore serve, if shaped and sized appropriately, both to heat and to distribute the gas. In Fig.1, only two examples of such cores are shown for information purposes (to be precise, this figure represents them in section and not yet mounted in the terminal section of the duct); the first core, indicated with the reference 22A, has a cylindrical shape with two semi-spherical ends and is suitable for being inserted completely into the terminal section of the duct; the second core, indicated with the reference 22B, has an inverted conical shape with a spherical cap at the base and is designed to be placed above the exit of the duct in such a way that the top of the cone is inserted into the duct but without occluding it. To limit the entry of liquid silicon particles into the chamber, the inlet means for silicon-containing gases can advantageously comprise a beaker-shaped element with an opening facing the conduit (as in the example in Fig. 3). In this way, the beaker is heated by radiation from the chamber walls, and the gas flowing into the beaker is rapidly heated to a high temperature by the walls of the beaker. Rapid heating is highly advantageous because it reduces the time during which the silicon is below the silicon dew point and thus the growth time of the silicon particles (and thus their size). Furthermore, any particles (particularly liquid silicon particles) tend to be retained in the beaker until they evaporate. Better results are obtained if the conduit extends inside the beaker (as in the example in Fig.3); in this way, in fact, the sudden variations foreseen in the path that goes from the duct to the chamber tend to eliminate the liquid silicon particles by impact. Even if Fig.3 shows a cylindrical glass, this glass can be appropriately shaped and sized both with regards to the external surface and the internal surface; particular shapes and / or sizing can be aimed at avoiding vortices, maximising heat exchanges between the walls of the chamber and the glass and between glass and gas, to check for any condensation phenomena along the walls. In accordance with the present invention, the inlet means for carbon-containing gases can be positioned, shaped and sized in many different ways. The inlet means for carbon-containing gases may comprise a plurality of nozzles arranged in a ring that open into the second zone of the chamber (as in the example in Fig. 2, where the nozzles are substantially facing upward); in the case of a vertical cylindrical chamber, the ring and the chamber are typically coaxial, and the ring is typically positioned at the base of the cylinder (as in the example in Fig. 2) or on the lower part of the cylindrical wall. The nozzles must be shaped and sized such that the jet of carbon-containing gas substantially contacts the silicon in a central zone of the chamber; the shape of a nozzle determines the direction and shape of the gas jet. The inlet means for carbon-containing gases may comprise a plurality of ducts arranged in a crown which open into the central area of ​​the chamber (as in the example in Fig.3); in the case of a vertical cylindrical chamber, the crown and the chamber are typically coaxial and the ducts are •r typically all equal and parallel; for a good result, the average diameter of the crown can be chosen to be approximately equal to 2 / 3 of the internal diameter of the chamber. In the example in Fig. 3, these ducts are in communication with a hollow disk adjacent to the base of the chamber; a series of small ducts open into the cavity of the disk; the small ducts branch out like branches from a large coaxial duct. The inlet means for carbon-containing gases may include a crown-shaped conduit opening into the central region of the chamber; in the case of a vertical cylindrical chamber, the crown and the chamber are typically coaxial; to allow for good distribution of the silicon-containing gases (which enter the second region of the chamber), the mean diameter of the crown should be only slightly smaller than the internal diameter of the chamber; in this case, an additional crown-shaped lead-in gas conduit may be provided, positioned around the crown-shaped carbon-containing gas conduit and close to the chamber walls, to keep the chamber walls free of silicon carbide deposits. The inlet means for carbon-containing gases should be designed to achieve good mixing with the silicon-containing gases and a broad, uniform distribution of the gases within the chamber, and to avoid vortices; possible backdiffusion of the carbon-containing gases toward the silicon-containing gas inlet should also be taken into account. For both silicon-containing gas inlet media and carbon-containing gas inlet media, the goal is to bring carbon and silicon to the substrate and not •1* on the walls of the room. The inlet means for precursor gases (containing silicon or carbon) are typically adapted to admit into the chamber a precursor gas associated with and then diluted in a carrier gas which may be hydrogen or argon or helium or a mixture of two or more of these gases; the precursor gas to carrier gas ratios could be, for example, 1Oslm for carrier gas and 1-2slm for precursor gas. The most typical silicon-bearing precursor gas is silane [SiH4]; it may be advantageous to mix silane [SiH4] with hydrochloric acid [HC1] in order to prevent (or at least limit) the formation of silicon droplets anywhere in the ducts; alternatively, compounds containing both silicon and chlorine such as dichlorosilane [DCS], trichlorosilane [TCS] and silicon tetrachloride [SiC14] can be used. The precursor gases carrying carbon can be propane [C3H8], ethylene [C2H4], acetylene [C2H2]; of these, the most stable compound at high temperatures is acetylene, the easiest to handle is propane, and the compromise is ethylene. Since very high temperatures must be maintained in the chamber, it is advantageous for the heating means to be of the induction type and to be capable of heating the walls of the chamber; in none of the figures have the heating means been highlighted. It is preferable to maintain a predetermined temperature profile; in particular, it is advantageous for the temperature of the central zone of the chamber to be very high (2200°C-2600°C) while the temperature of the first zone (and therefore of the substrate and the growing crystal) to be a little lower (1800°C-2200°C) to favor the condensation of the silicon carbide; the temperature of the first zone (the inlet zone for the silicon-containing gases) must be very high (2200°C-2600°C) but can also be slightly lower (2000°C-2400°C) than the temperature of the central zone. In a first case, the heating means can therefore be suitable determine the following temperatures in the chamber: in the first zone, a temperature in the range of 1800-2200 degrees, preferably around 2000 degrees, in the central area, a temperature in the range 2200- 2600 degrees, preferably around 2400 degrees, in the second zone, a temperature in the range of 2000-2400 degrees, preferably around 2200 degrees. In a second case, the heating means can then be used to determine the following temperatures in the chamber: in the first zone, a temperature in the range of 1800-2200 degrees, preferably around 2000 degrees, in the central area, a temperature in the range 2200- 2600 degrees, preferably around 2400 degrees, in the second zone, a temperature in the range of 2200-2600 degrees, preferably around 2400 degrees. It is advantageous to provide that the support means include temperature control means. The support means of the system according to the present invention are typically made of graphite covered with a layer of SiC or Tac; these therefore also act as a heating element both by induction and by radiation. To control t The temperature of the support media can be advantageously controlled by a gas flow, for example hydrogen; a hydrogen flow of 25 slm absorbs a power of approximately 1 kW to heat to 2000°C from room temperature. In this case, the support media can consist, for example, of a thick disk with an internal cavity and mounted on a tube communicating with the cavity; the tube is thermally and chemically insulated; the refrigerant gas is injected into the tube, flows into the cavity, and exits through a plurality of holes drilled on the periphery of the disk. In the examples in Fig. 2 and Fig. 3, the gas flow inside the support means can be advantageously used both for attack and for temperature control. Many of the components of the system according to the present invention can be made of graphite; typically these parts will need to be covered with a protective layer, for example, of SiC and TaC (which is more resistant). In Fig.2 and Fig.3 the same references used in Fig.1 have been used to identify elements with identical or similar functions. Although the drawings illustrate only two specific embodiments of the present invention, it is clear from the above description that the present invention is capable of being realized in a great many ways given by the combination of the many variations provided for its component means.

Claims

CLAIMS 1. A system for growing silicon carbide crystals on substrates comprising a chamber extending in one direction, wherein the chamber provides: separate inlet means for carbon-containing gases and silicon-containing gases, support means for substrates placed in a first extreme zone of the chamber, unloading exit means placed in the vicinity of the support means, heating means capable of heating the chamber to a temperature above approximately 1800°C; wherein the inlet means for silicon-containing gases are positioned, shaped, and sized such that the silicon-containing gases enter a second extreme region of the chamber; characterized in that the inlet means for carbon-containing gases are positioned, shaped, and sized so that the carbon and silicon come into substantial contact in a central area of ​​the chamber, distant from both the first and second extreme zones.

2. System according to claim 1, wherein the inlet means for carbon-containing gases are positioned, shaped and sized such that the carbon and silicon come into substantial contact in an area also distant from the walls of the chamber.

3. System according to claim 1 or 2, wherein the chamber provides inlet means for attack gas positioned, shaped and sized in such a way as to cause gas to enter the first extreme zone of the chamber.

4. System according to claim 1 or 2 or 3, wherein the chamber includes inlet means for anti-nucleation gas positioned, shaped and sized so as to cause gas inlet into the second extreme region of the chamber.

5. System according to one of the preceding claims, wherein the chamber includes inlet means for anti-nucleation gas positioned, shaped and sized in such a way as to cause gas inlet in the central area of ​​the chamber.

6. System according to one of the preceding claims, wherein the chamber includes the inlet means for attack gas positioned, shaped and sized in such a way as to create a gas flow substantially only along the walls of the chamber.

7. System according to one of the preceding claims, wherein the supporting means provide inlet means for attack gas positioned, shaped and sized in such a way as to cause gas ingress around the substrates.

8. System according to one of the preceding claims, comprising means adapted to rotate the support means during the growth process. •t 9. System according to one of the preceding claims, comprising means adapted to retract the support means during the growth process.

10. System according to one of the preceding claims, wherein the inlet means for silicon-containing gas comprises a conduit opening into the second zone of the chamber.

11. System according to claim 10, wherein said duct includes, at one of its terminal sections, a silicon evaporation cell.

12. System according to claim 10 or 11, wherein said duct includes, at one of its terminal sections, a central core suitable for heating the gases containing silicon and / or distributing them in the chamber.

13. System according to claim 10 or 11 or 12, wherein said inlet means for silicon-containing gas comprises a cup-shaped element having an opening facing the conduit.

14. System according to claim 13, wherein the duct extends inside the glass.

15. System according to any one of claims 1 to 14, wherein the inlet means for carbon-containing gases comprise a plurality of nozzles arranged in a ring which open into the second zone of the chamber.

16. System according to one of claims 1 to 14, wherein the inlet means for carbon-containing gases comprise a plurality of ducts arranged in a ring which open into the central area of ​​the chamber.

17. System according to one of claims 1 to 14, wherein the inlet means for carbon-containing gases comprise a crown-shaped duct opening into the central area of ​​the chamber.

18. System according to one of the preceding claims, wherein the heating means are of the induction type and are suitable for heating the walls of the chamber.

19. System according to one of claims 1 to 18, wherein the means of heating are designed to determine the following temperatures in the room: in the first zone, a temperature in the range 1800- 2200 degrees, preferably around 2000 degrees, in the central area, a temperature in the range 2200- 2600 degrees, preferably around 2400 degrees, in the second zone, a temperature in the range 2000- 2400 degrees, preferably around 2200 degrees.

20. System according to one of claims 1 to 18, wherein the means of heating are designed to determine the following temperatures in the room: in the first zone, a temperature in the range 1800- 2200 degrees, preferably around 2000 degrees, in the central area, a temperature in the range 2200- 2600 degrees, preferably around 2400 degrees, in the second zone, a temperature in the range 2200- 2600 degrees, preferably around 2400 degrees.

21. System according to one of the preceding claims, wherein the means Supporting means include temperature control. The Mandatory Eng. Alberto De Ros (Registration No. 562BM) Della Dragotti & Associates Srl ADR / ac