WO2015044748A1

WO2015044748A1 - Coated susceptor and anti-bowing method

Info

Publication number: WO2015044748A1
Application number: PCT/IB2014/001909
Authority: WO
Inventors: Francesco COREA; Danilo Crippa; Laura GOBBO; Marco Mauceri; Vincenzo Ogliari; Franco Preti; Marco Puglisi; Carmelo VECCHIO
Original assignee: Lpe S.P.A.
Priority date: 2013-09-27
Filing date: 2014-09-24
Publication date: 2015-04-02
Also published as: ITCO20130040A1; CN105637118A

Abstract

The present invention relates to a susceptor for a reactor for epitaxial growth, consisting of a disc-shaped body (20, 30) made of graphite having a first face and a second face; the first face comprises at least one zone, in particular a circular-shaped recess (21) or relief (31) adapted to receive a substrate to be subjected to epitaxial growth; the first face exposes a first upper surface (22, 32) corresponding to such a zone (21, 31) and a second upper surface (23, 33) which surrounds such a zone (21, 31); the second face exposes a lower surface (24, 34); the second upper surface (22, 33) and the lower surface (24, 34) are coated with a layer of silicon carbide; so, the outward curvature of the susceptor is limited during the life thereof, i.e. after having been used for many processes of epitaxial growth of silicon carbide.

Description

TITLE

COATED SUSCEPTOR AND ANTI-BOWING METHOD

DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a susceptor coated with silicon carbide and having at least one zone for receiving substrates to be subjected to "epitaxial growth" of silicon carbide, as well as to a method for limiting the outward curvature of a substrate supporting element during the life thereof, i.e. also after having been used for many processes of epitaxial growth of silicon carbide.

PRIOR ART

Epitaxial growth and the reactors for obtaining it have been known for many decades; they are based on the technique known as "CVD" (Chemical Vapor Deposition).

A technical field in which they are used is that of the production of electronic components; the processes and the reactors for this application are particular because a very high quality of the deposited layers is required and the quality requirements are continuously on the rise. One type of epitaxial reactor uses a "susceptor" which is inserted in a reaction chamber and which supports one or more substrates to be subjected to epitaxial growth (see reference numerals 10 and 1000 in Fig. 1.1 A); as known, the substrates may be perfectly circular or often with a "flat" (see substrate 1000 in Fig. L IB).

The present invention indeed relates to such a susceptor, in particular for high-temperature (1550-1750°C) epitaxial growth of silicon carbide.

Typically, reactors with hot wall reaction chamber are used for the high-temperature epitaxial growth of silicon carbide; the heating of the chamber and susceptor is generally obtained by means of electromagnetic induction or resistance.

Most of the prior art (Fig. 1) relates to the epitaxial growth of silicon, with process temperatures up to 1250°C, and to the corresponding reactors; indeed, only rather recently have electronic components of silicon carbide become slightly more widely used.

At the very beginning, i.e. many decades ago, susceptors were entirely made of graphite.

However, in those days, it was found that the graphite contaminated the substrates because the impurities present in the graphite of the susceptor partially migrated into the

superimposed substrates during the epitaxial growth processes.

So, it was thought to coat the susceptor, which was entirely made of graphite, by depositing a thin layer of silicon carbide (in Fig. 1A, the dashed line indicates the boundary between the outer silicon carbide and the inner graphite); the coating concerned the entire surface of the susceptor. Such a solution proved to be fully satisfactory for many decades, i.e. substantially until the current days.

SUMMARY

Recently, the Applicant realized that the layer of silicon carbide of the susceptor in contact with the silicon carbide substrates subjected to epitaxial growth cause some problems in the case of high-temperature epitaxial growth of silicon carbide, also in light of the increasingly stringent requirements in terms of grown substrate quality and production process quality and speed; a first problem relates to the fact that the substrates tend to stick to the susceptor; a second problem relates to the fact that the silicon carbide of the layer tends to migrate towards the superimposed substrates (it is assumed that first sublimation and then solidification occur).

Recently, the Applicant thus thought of entirely coating the susceptor, which is entirely made of graphite, with a thin layer of tantalum carbide instead of silicon carbide.

The use of such a material substantially solved both problems mentioned above; however, a new problem has arisen; during the processes of epitaxial growth of silicon carbide, silicon carbide deposits not only on the substrates, but also on the face of the susceptor exposed to the precursor gases, i.e. on the surface of the susceptor adjacent to the substrates (see Fig. 17A); the gradual accumulation (process after process) of silicon carbide on a face of the susceptor causes a gradual outward curvature of the susceptor (i.e. such that the central zone rises with respect to the peripheral zone - see Fig. 17B); in other words, after a given number of epitaxial growth processes, the susceptor is slightly bulged, i.e. convex (see Fig. 17B).

Such a change of shape of the susceptor creates problems to both the operation of the reactor and the quality and uniformity of the epitaxial growth processes.

The Applicant thus set the aim to solve the above-described problems.

Such an objective is substantially achieved by virtue of a susceptor having the technical features set forth in the appended claims, which form an integral part of the present disclosure.

The idea underlying the present invention is that of coating beforehand a susceptor made of graphite only partially with silicon carbide; in order to receive the substrates to be subjected to epitaxial growth at least one specific zone is included, which can be either lowered with respect to the part of susceptor which surrounds it, or raised, or possibly even at the same level.

It is thus the object of the present invention also a method which avoids the curvature of a substrate supporting element (in particular, a susceptor) during the life thereof, i.e. also after having been used for many processes of epitaxial growth of silicon carbide.

LIST OF FIGURES The present invention will become more apparent from the following detailed description to be considered in conjunction with the accompanying drawings, in which:

Fig. 1 shows a simplified section view and a partial top view of a disc-shaped body of a susceptor according to the prior art with a substrate inserted in a recess thereof,

Fig. 2 shows a diagrammatic (section) view of a disc-shaped body of a susceptor according to a first embodiment of the present invention - this figure is evidently not in scale,

Fig. 3 shows a diagrammatic (section) view of a disc-shaped body of a susceptor according to a second embodiment of the present invention - this figure is evidently not in scale,

Fig. 4 shows a simplified section view of a first example of a substrate supporting element to be used in combination with a susceptor body,

Fig. 5 shows a simplified section view of a second example of a substrate supporting element to be used in combination with a susceptor body,

Fig. 6 shows a simplified section view of a third example of a substrate supporting element to be used in combination with a susceptor body,

Fig. 7 shows a simplified section view of a fourth example of a substrate supporting element to be used in combination with a susceptor body,

Fig. 8 shows a simplified section view of a fifth example of a substrate supporting element to be used in combination with a susceptor body,

Fig. 9 shows a simplified section view and a partial top view of an example of a coupling of a supporting element and a frame to be used in combination with a susceptor body (the susceptor body is partially shown, in a simplified manner),

Fig. 10 shows a simplified, partial section view of a first combination of a susceptor body and a substrate supporting element,

Fig. 1 1 shows a simplified, partial section view of a second combination of a susceptor body and a substrate supporting element,

Fig. 12 shows a simplified, partial section view of a third combination of a susceptor body and a substrate supporting element,

Fig. 13 shows a simplified, partial section view of a fourth combination of a susceptor body and a substrate supporting element,

Fig. 14 shows a simplified, partial section view of a fifth combination of a susceptor body and a substrate supporting element,

Fig. 15 shows a simplified, partial section view of a sixth combination of a susceptor body and a substrate supporting element, Fig. 16 shows a simplified section view and a top view of a seventh combination of a susceptor body and a substrate supporting element,

Fig. 17 shows two simplified section views (not in scale) of a flat susceptor plane (A) before being used in an epitaxial reactor (with the subsequent depositions of silicon carbide being diagrammatically illustrated) and of a curved, i.e. bulged, susceptor (B) after a series of processes of epitaxial growth of silicon carbide, and

Fig. 18 shows a simplified section view (not in scale) of a slightly counter-curved susceptor before being used an epitaxial reactor.

Such a description and such drawings are provided by way of mere example and therefore are non-limiting.

It is worth noting that such a description considers various innovative concepts (and ways to implement them) which are independent from one another, but which can be advantageously combined with one another.

As easily apparent, the present invention, the main advantageous aspects of which are defined in the appended claims, can be implemented in various manners.

DETAILED DESCRIPTION

Fig. 2 and Fig. 3 are diagrammatic; in particular, the dimensions of the susceptor bodies are distorted to highlight their details and they have a single zone adapted to receive substrates for simplicity; in actual fact, the susceptor bodies may have one or more zones adapted to receive substrates, typically identical to one another.

It is useful to explain that, in many figures, parts which are adjacent are shown as being slightly spaced apart only to allow to better view their shapes.

The susceptors according to the present invention, in particular those in Fig. 2 and Fig. 3, are produced complete with the corresponding various coating layers before being used in the processes of "epitaxial growth" of silicon carbide, i.e. in processes of depositing layers of silicon carbide on substrates to be treated.

Fig. 2 shows a susceptor for a reactor for epitaxial growth consisting of a disc-shaped body 20 entirely made of graphite having a first face and a second face. The body is typically placed within a reaction chamber so as to be horizontal, and thus the first face corresponds to the upper face and the second face corresponds to the lower face. The first face comprises a circular-shaped recess 21 adapted to receive a substrate to be subjected to epitaxial growth. As will be more apparent below, the recess may receive the substrates either directly or indirectly, for example, by means of a supporting element; naturally, the size of the recess in the first case is smaller than the size of the recess in the second case. The first face thus exposes a lowered upper surface 22, corresponding to the bottom of recess 21 , and a raised upper surface 23, which surrounds recess 21 , while the second face exposes a lower surface 24.

The raised upper surface 23 is coated with an exposed layer 27 of silicon carbide.

At least part of the lower surface 24 is coated with an exposed layer 28 of silicon carbide; in Fig. 2, layer 28 entirely coats surface 24 for simplicity of production.

Layers 27 and 28 are made before using the susceptor in processes of "epitaxial growth" of silicon carbide, i.e. processes of depositing silicon carbide on substrates to be treated.

Thereby, the progressive outward curvature of susceptor 30 can be considerably limited; indeed, it has been empirically verified that the deformations due to the layers of silicon carbide on the upper and lower surfaces tend to compensate each other; this also applies even if, when using the susceptor for processes of epitaxial growth of silicon carbide, further silicon carbide is deposited on the upper surface - naturally, a progressive deformation of the susceptor cannot be entirely avoided also because of such a further deposition.

There are two alternatives with regards to the problem of material migration from the bottom of recess 21.

According to first alternative (shown in Fig. 2), the entire lowered upper surface 22 is coated with an exposed layer of graphite; such a layer typically corresponds to the graphite of the disc-shaped body. Indeed, the evolution of materials has led to obtain graphite of excellent quality, and thus the possible minor contamination due to the graphite, in particular of the contained impurities, is not harmful.

According to the second alternative, (not shown in Fig. 2) the entire lowered upper surface is coated with an exposed layer of tantalum carbide. Indeed, the sublimation of tantalum carbide is negligible at the epitaxial growth temperatures of silicon carbide (1550-1750°C). Furthermore, it was thought to make susceptors entirely of sintered silicon carbide because these could have avoided substantial problems of migration of material towards the superimposed substrates or substantial problems of deformation during use. However, such a solution was rather costly, taking into account the fact that the susceptor must be replaced after a given period of use.

By way of example, the size of the disc-shaped body of the susceptor may be: diameter 200- 400 mm, thickness 5- 10 mm, diameter of the recess 100-200 mm, depth of the recess 1-4 mm, number of recesses from 1 to 10; by way of example, the size of the substrates of silicon carbide (homoepitaxial growth) may be: thickness of 250-500 μ and diameter of 100-200 mm; in general, the processes of epitaxial growth of silicon carbide for electronic

applications include depositions of 2-20 μ, which however may be extended to 100 μ and more. In the example shown in Fig. 2, the layer 27 of silicon carbide on the upper surface 23 reaches the inner side 25 of recess 21 ; in the example shown in Fig. 2, the layer 27 of silicon carbide on the upper surface 23 reaches the outer side 26 of the disc-shaped body 20;

typically and as shown in Fig. 2, both such features are achieved.

Typically, the lower surface 24 is flat, the raised upper surface 23 is flat, and the lowered surface 22 is either perfectly flat or slightly concave.

The lower surface 24 may be coated with a layer of silicon carbide either entirely or only in an annular zone or only in a central zone; indeed, compensation for deformation is the most important aspect.

Instead, with regards to the upper surface 23 of the susceptor, a complete coating is by far preferable; indeed, when the susceptor is used for processes of epitaxial growth of silicon carbide, silicon carbide is thus deposited again and only on the silicon carbide underneath. Thus, it can be expected that the thickness of the silicon carbide on the susceptor is uniform (i.e. independent from the horizontal position) and remains uniform during the entire life of the susceptor. Furthermore, it may be expected that the silicon carbide has uniform physical features on top (regardless of the horizontal position and of the vertical position).

The thickness of the layer of silicon carbide on the lower surface 24 may be, for example, in the range from 10 μ to 100μ for the entire life of the susceptor.

The thickness of the layer of silicon carbide on the upper surface 23 may be, for example, in the range from 10 μ to 100 μ for the entire life of the susceptor; at the end of the life of the susceptor (i.e. before possible maintenance thereof), such a thickness may reach and even exceed 1000 μ.

In general, the thickness of the layer of silicon carbide on the upper surface 23 may be either equal to or different from the thickness of the layer of silicon carbide on the lower surface 24. However, experiments were carried out and better results were obtained when the thickness of the layer of silicon carbide on the upper surface 23 was greater than the thickness of the layer of silicon carbide on the lower surface 24, probably because the area of the upper layer 27 is smaller than the area of the lower layer 28 due to the presence of recess 21. According to one of these experiments, a susceptor entirely made of graphite was treated directly in the reaction chamber of the epitaxial reactor; firstly, it was degassed for several minutes at a temperature of about 1650°C under a hydrogen flow, then a "sacrificial substrate" was placed in the recess, then about 20 μ were deposited on the upper surface 23 (and on the "sacrificial substrate"), then the "sacrificial substrate" was removed from the recess, then the susceptor was turned and about 10 μ were deposited on the lower surface 24, then the susceptor was turned, the "sacrificial substrate" was placed in the recess and about 20 μ were deposited on the upper surface 23 (and on the "sacrificial substrate"), finally the "sacrificial substrate" was removed from the recess; such a susceptor provided excellent results from all points of view during its entire life. In all cases, it is worth noting that the properties of the layers of silicon carbide produced during the initial treatment of the susceptor were influenced by the production method of the layers themselves.

Variants of the solution diagrammatically shown in Fig. 2 are possible.

The inner side 25 of recess 21 may be coated with an exposed layer of silicon carbide; by first approximation, this coating does not contribute to limiting the curvature of the susceptor, but can facilitate the production of the susceptor.

The inner side 26 of the disc-shaped recess 20 may be coated with an exposed layer of silicon carbide; by first approximation, this coating does not contribute to limiting the curvature of the susceptor, but can facilitate the production of the susceptor.

The lowered upper surface 22 (smooth in Fig. 2), may be at least in part rough or rugged or knurled.

Recess 21 may house a substrate supporting element (see Fig. 10, Fig. 1 1 , Fig. 12, Fig. 13,

Fig. 14) instead of only one substrate; such an element may be stably laid at the bottom 22 of recess 21.

The recess of the disc-shaped body of the susceptor may have a radial, annular widening on top and/or at the bottom (see Fig. 13 and Fig. 14). Such a widening, in particular the upper widening, may be used to house a corresponding widening of the supporting element (see Fig. 13 and Fig. 14); as will be explained below, such a widening, in particular the lower widening, may have other purposes.

Recess 21 may house the combination of a substrate supporting element (see reference numeral 91 in Fig. 9) and a frame for the supporting element (see reference numeral 97 in Fig. 9) instead of either only a substrate or only a substrate and a supporting element; such a frame may be inserted in recess 21 and then stably laid at the bottom 22 of recess 21.

Unlike Fig. 2, the recess of the susceptor may possibly comprise, at least, one through hole, i.e. which extends from one face to the other of the disc-shaped body of the susceptor; this possibility will be better understood when Fig. 16 will be described in detail.

Fig. 3 shows a susceptor for a reactor for epitaxial growth which is very different from that in Fig. 2. With regards to the coating of the graphite body, the susceptor in Fig. 3 is similar (but not identical) to that in Fig. 2, and thus considerations similar to those already made for the susceptor in Fig. 2 apply.

It consists of a solid disc-shaped body 30 entirely of graphite having a first face and a second face. The first face comprises a circular-shaped recess 31 adapted to receive a substrate to be subjected to epitaxial growth.

The reference numerals in Fig. 3 have the following meaning: reference numeral 32 corresponds to a raised upper surface of the top of relief 31 , reference numeral 33 corresponds to a lowered upper surface which surrounds relief 31 , reference numeral 34 corresponds to a lower surface, reference numeral 35 corresponds to an outer side of relief 3 1, reference numeral 36 corresponds to an outer side of the disc-shaped body 30, reference numeral 37 corresponds to a layer of silicon carbide on the upper surface 33, reference numeral 38 corresponds to a layer of silicon carbide on the lower surface 34. The analogy or duality between reference numerals 20, 21 , 22, 23, 24, 25, 26, 27, 28 and reference numerals 30, 31 , 32, 33, 34, 35, 36, 37, 38 is apparent; however, it is worth noting that while the side 25 is inner and thus potentially not directly exposed to the precursor gases, the side 35 is outer and thus typically exposed to the precursor gases (unless particular measures are taken, for example protection elements outside the susceptor).

The layers 37 and 38 are made before using the susceptor in processes of "epitaxial growth" of silicon carbide, i.e. processes of depositing silicon carbide on substrates to be treated.

Thereby, the progressive outward curvature of the susceptor 30 can be considerably limited.

The inner side 35 of relief 31 may be coated with an exposed layer of silicon carbide; by first approximation, this coating does not contribute to limiting the curvature of the susceptor, but can facilitate the production of the susceptor.

The inner side 36 of the disc-shaped recess 30 may be coated with an exposed layer of silicon carbide; by first approximation, this coating does not contribute to limiting the curvature of the susceptor, but can facilitate the production of the susceptor.

Relief 31 , as recess 21 , may also receive the substrates either directly or indirectly, for example by means of a supporting element (see Fig. 15) or the combination of a substrate supporting element and a frame for the supporting element; such a frame may be mounted on the relief 31 and then stably laid on the top 32 of relief 3 1 (see Fig. 15 for similitude).

Relief 31 may have a height, by way of example, in the range from 1 mm to 6 mm.

It is worth noting that the surface 32 of the top of relief 31 may be either perfectly flat or slightly concave, and smooth or rough or rugged or knurled.

The surface 32 may be shaped further; for example, there could be at least one recess and/or at least one relief to allow a stable mechanical coupling between susceptor body and substrate supporting element (see Fig. 15) or susceptor body and frame.

It is worth noting that the layers of silicon carbide, produced on the body of the susceptor

(corresponding to elements 27 and 28 in Fig. 2 and corresponding to elements 37 and 38 in Fig. 3) before using it in an epitaxial reactor, can cause a deformation of the body itself, in particular a slight counter-curvature (see Fig. 18 for example).

Reference was previously made to substrates to be subjected to epitaxial growth. Such supports are thought and designed to be used in combination with a susceptor body; such a combination forms a susceptor which is more complex than the usual susceptors according to the prior art (see Fig. 1). The disc-shaped bodies in Fig. 2 and Fig. 3 are certainly suited to the purpose, even if the supporting elements which will be described in detail thereinafter may be used with susceptor bodies either equal to these or different therefrom.

The susceptor body to be combined with one or more supports substantially consists of a typically solid disc-shaped body having a first face and a second face; the first face comprises at least one zone adapted to receive a substrate, i.e. a supporting element for a substrate, indirectly.

The supporting element is placed in this zone; for example, in case of a recess, the supporting element is typically inserted in the recess and then laid at the bottom thereof (see Fig. 10, Fig. 1 1 , Fig. 12, Fig. 13, Fig. 14); in the case of a relief, the supporting element is typically mounted on the relief and laid on the top (see Fig. 15).

Figures from Fig. 4 to Fig. 9 also show a substrate 2000 for a better understanding thereof; the substrate 2000 is made of silicon carbide because this is the most typical application of the present invention, although the present invention is not limited to silicon carbide substrates; the substrate 2000 has a flat because this is typical, even if the present invention is not limited a substrates with a flat.

The supporting element comprises at least one circular disc.

In the case of the example in Fig. 4, the element consists of a flat circular disc 41 ; the surface of the disc 41 on which the substrate 2000 rests may be either perfectly flat or slightly concave; the diameter of the disc is slightly larger than the diameter of the substrate, for example greater than 1 -3 mm; by way of example, the thickness of the disc 41 may be in the range from 1 mm to 3 mm.

The supporting element may comprise a circular disc with an annular edge which is raised with respect to the disc; thereby, a recess in which substrates may be housed is defined. In case of the example in Fig. 5, the element consists of a flat circular disc 5 1 with a raised edge 52; the thickness of the raised edge is uniform and approximately equal to the thickness of the disc, i.e., by way of example, 1 -3 mm. Again by way of example, the total thickness of the element may be in the range from 2 mm to 4 mm and the depth of the recess may be in the range from 250 to 1000 μ.

In case of the example in Fig. 6, the element consists of a flat circular disc 61 with a raised edge 63; the thickness of the raised edge is uniform and high, i.e., by way of example, 3-10 mm. Again by way of example, the total thickness of the element may be in the range from 2 mm to 4 mm and the depth of the recess may be in the range from 250 to 1000 μ.

The raised edge has a lower portion and an upper portion, which are different to each other; the two portions are adjacent to each other and the lower portion is adjacent to the circular disc.

In the case of the example in Fig. 7, there is a flat circular disc 71 and the upper portion 75 of the edge, raised with respect to disc, projects radially outwards with respect to the lower portion 74; furthermore, both the portion 74 and the portion 75 are annular. By way of example, the total thickness of the element may be in the range from 2 mm to 6 mm, the protrusion of the upper portion may be in the range from 3 mm to 10 mm, the thickness of the upper portion may be in the range from 1 mm to 3 mm, and the depth of the recess may be in the range from 250 to 1000 μ.

The supporting element may comprise a circular disc and a ring; the ring is joined to the circular disc at the upper surface thereof; the ring is adapted to surround substrates; thereby, a recess in which substrates may be housed is defined.

In the case of the example in Fig. 8, the element consists of a circular flat disc 81 and a circular ring 86; the ring 86 is joined to the circular disc 81 at the upper surface thereof and is distant from the edge of the disc 81. By way of example, the thickness of the ring is in the range from 1 mm to 3 mm, the distance between edge of the disc and ring may be in the range from 3 mm to 10 mm, the total thickness of the element may be in the range from 2 mm to 4 mm, and the depth of the recess may be in the range from 250 to 1000 μ.

The following considerations apply in general for all supporting elements.

The diameter of the recess defined within the supporting element is typically slightly greater than the diameter of the substrate, for example greater than 1-3 mm; the depth of the recess defined within the supporting element is typically slightly greater than the thickness of the substrate and is, for example, between 250 and 1000 μ.

The raised edge (for example the edge 52, the edge 63, the edge 74 and 75, and similarly the ring 86) may be either circular or shaped, for example shaped so as to be complementary to the outer shape of the substrate (see Fig. 9B for example).

The supporting element exposes a resting surface for the substrates, for example made of graphite or tantalum carbide. Such a surface may be smooth, or alternatively, at least in part rough or rugged or knurled. Such a surface may be flat or alternatively slightly concave. The supporting element be either entirely made of graphite, or entirely made of tantalum carbide, or made of graphite entirely coated with tantalum carbide; it is worth noting that tantalum carbide is an expensive material, much more expensive than graphite. The solutions in which the entire outer surface of the supporting element is made graphite and/or tantalum carbide are particularly suited to the cases in which the element is not directly exposed to precursor gases and thus silicon carbide is not deposited thereon during the epitaxial growth processes; the example in Fig. 4 typically falls into one of these cases; the example in Fig. 5 could fall into one of these cases if the thin upper thickness of the raised edge 52 is ignored.

Alternatively, the supporting element may be made of graphite coated at least in part with silicon carbide; the possible coating does not concern the surface on which the substrate, which is preferably made of graphite or tantalum carbide, rests.

Such solutions are particularly suited to the cases in which the element is directly exposed to the precursor gases and thus silicon carbide is deposited thereon, during the epitaxial growth processes. The examples in Fig. 6, Fig. 7 and Fig. 8 typically fall into these cases; figures from Fig. 1 1 to Fig. 14 show an upper surface of the support which is directly exposed to the precursor gases; Fig. 15 shows an upper surface and an outer side surface of the support which are directly exposed to the precursor gases (less particular measures).

Such a partial coating of silicon carbide is useful for limiting the outward gradual curvature of the supporting element caused by the deposition of silicon carbide. With this regard, the considerations made in connection to the disc-shaped susceptor body and to the layers of silicon carbide apply. Thus, it is advantageous for the entire exposed upper surface (i.e. which does not support a substrate) of the supporting element to be coated with a layer of silicon carbide and for at least part of the lower surface of the supporting element to be coated with a layer of silicon carbide; for example, the lower surface of the circular disc may be coated either entirely, or only in an annular zone, or only in a central zone. In the case of raised edge (see Fig. 7), a layer of silicon carbide could be located, alternatively or additionally, on the back of the disc, on the outer lower side of the edge and/or on the back of the protrusion.

Fig. 9 shows the coupling of a supporting element 91 and a frame 97 which surrounds it; in Fig. 9A, such a coupling is shown during a step of inserting in a recess 99 of the body 90 of the susceptor and, subsequently, will stably lay at the bottom of recess 99; at the end of the operation, the supporting element 91 will also be stably laid at the bottom of recess 99;

alternatively, the supporting element 91 may be slightly spaced apart (for example 0.5 mm) from the bottom of recess 99. The element 91 in Fig. 9 resembles the element 41 in Fig. 4 but there is a groove to couple with the frame 97 on the lower face. The element + frame coupling in Fig. 9 resembles the supporting element in Fig. 6 as a whole.

In general, the frame comprises a hole and the supporting element is inserted, typically stably, in the hole; the frame (together with the element) is placed at a specific zone, for example a recess (as shown in Fig. 9 - see Fig. 9A) or a relief of the susceptor body;

typically, it is stably laid (as shown in Fig. 9 - see Fig. 9A); the hole may be through (as in Fig. 9 - see Fig. 9A) or blind; the hole may be either circular or shaped (as shown in Fig. 9B - see Fig. 9B), for example shaped so as to be complementary with the outer shape of the substrate.

The frame may be advantageously made of graphite (either totally or partially) coated with silicon carbide or entirely of silicon carbide. In case of partial coating, it is advantageous to provide a layer of silicon carbide which entirely coats the upper surface of the frame and layer of silicon carbide which entirely coats the lower surface of the frame for the reasons explained above; the thickness of the two layers may be advantageously the same.

Advantageously, the supporting element be entirely made of graphite or entirely made of tantalum carbide or made of graphite entirely coated with tantalum carbide. Differentiating the material between frame and supporting element is evidently very advantageous.

In general, the supporting element and/or the frame may advantageously comprise an inner side with a surface which is partially cylindrical and partially flat (Fig. 9B - this could also applied to the figures from Fig. 4 to Fig. 8).

The figures (Fig. 10, Fig. 1 1 , Fig. 12, Fig. 13, Fig. 14, Fig. 15) show a series of examples of combinations of a susceptor body and a substrate supporting element; other combinations are possible. In figures Fig. 10, Fig. 1 1 , Fig. 12, Fig. 13, Fig. 14, the element is stably placed at a recess of the body; the depth of the recess of the body corresponds to the total thickness of the supporting element. In Fig. 15, the element is stably placed at a slight relief of the body.

Advantageously, all the bodies have an outer bevel on the upper face of the body at the edge.

In the example in Fig. 10, the supporting element corresponds to the supporting element in Fig. 5; the body of the susceptor has a bevel on the upper face of the body at the recess.

In the example in Fig. 1 1 , the supporting element corresponds to the supporting element in

Fig. 6.

In the example in Fig. 12, the supporting element corresponds to the supporting element in Fig. 8.

In the example in Fig. 13, the supporting element corresponds to the supporting element in Fig. 7; the recess has a radial, annular widening which starts at a given distance from the bottom of the recess; the shape of the recess is complementary to the outer shape of the supporting element.

In the example in Fig. 14, the supporting element corresponds to the supporting element in Fig. 7; the recess has a radial, annular widening which starts at the bottom of the recess; therefore, there is a gap between the inner side of the recess and outer side of the supporting element; in this example, such a gap has a triangular section but other shapes are

alternatively possible.

In the example in Fig. 15, the supporting element corresponds to the supporting element in Fig. 7; in this example, the diameter of the top of the relief corresponds to the diameter of the disc of the supporting element, even if this is not indispensable. In Fig. 15, the surface of the relief is shaped to obtain stable mechanical couplings; in particular, it has an annular groove in which a ring is inserted which projects from the lower surface of the supporting element so that the body with relief and the supporting element are well coupled to one another. Fig. 15 also shows the lower face of the body of the susceptor; in general, this comprises a seat adapted to receive a pin for guiding the rotation of the susceptor.

In the example in Fig. 16, the recess of the susceptor comprises a through hole, i.e. which extends from one face to the other of the disc-shaped body of the susceptor, and the supporting element is similar to the supporting element in Fig. 4; such a example compresses a plurality of recesses, in particular four recesses, and a corresponding plurality of supporting elements. In this example, both the shape of the recess and the shape of the supporting element corresponds to the shape of the substrate which is, in particular, circular with a flat; the diameter of the recess is slightly greater than the diameter of the substrate. In Fig. 16, the supporting element differs from that in Fig. 4 in that it has a groove on the lower face for coupling with the susceptor body, in particular with the bottom of the recess which simply consists of a rectangular section ring. Fig. 16 also shows the lower face of the susceptor body; in general, this comprises a seat adapted to receive a pin for guiding the rotation of the susceptor.

Using supporting elements or element + frame couplings extends the life of the susceptor body; indeed, maintenance (for example for removing the deposited silicon carbide) and/or the replacements concentrates thereon. For maintenance purposes, the element + frame coupling is advantageous because such a supporting element which is always protected practically does not require maintenance.

Using supporting elements or element + frame couplings provides flexibility; indeed, the shape of the recess or relief of the body of the susceptor is substantially independent from the shape and/or size of the substrate. Incidentally, different supporting elements may be associated to the same recess or relief of the body of the susceptor, in particular with recesses of different shape and/or size (for example complementary to the other shape of different substrates). As mentioned, the laying surface may be advantageously at least in part either rough or rugged or knurled. This processing tends to avoid sticking with the superimposed body and/or slipping of the superimposed body.

According to how the present invention is implemented, such a consideration may apply either to the surface of the susceptor on which the substrates lay, or to the surface of the supporting element on which the substrates lay, or to the surface of the susceptor on which the supporting elements lay.

Thus, it is apparent that there is a method for limiting the outward curvature (i.e. "bulging") of a substrate supporting element following a plurality of processes of depositing layers of silicon carbide on substrates to be treated.

Such a method is applied to susceptors, to supporting elements (in strict sense) and to the element + frame couplings.

Such a method considers an exposed upper surface and an exposed lower surface and includes coating both the exposed upper surface and the exposed lower surface with a layer of silicon carbide; such a coating must be produced in advance, i.e. before the processes of depositing layers of silicon carbide on substrates to be treated. In order to avoid curvature, such a coating may be limited to the exposed upper surface and to the exposed lower surface In case of a susceptor consisting of a disc-shaped body (see, for example, the susceptors in Fig. 2 and Fig. 3), such a coating typically concerns a surface of the upper face (for example, elements 22 and 33) and a surface of the lower face (for example, elements 24 and 34).

By virtue of this method, the shape of the body of the component (and of possible pockets) only slightly changes during its operative life with respect to its ideal shape; the body could be either flat at the beginning and slightly curved (i.e. bulged or convex) at the end or slightly counter-curved (i.e. concave) at the beginning and slightly curved (i.e. bulged and convex) at the end.

It is worth noting that in a susceptor, for example similar to that in Fig. 2 or Fig. 3, or a supporting element, for example similar to one in Fig. 6 or Fig. 7, may be made by means of a mechanical processing which creates a slight initial counter-curvature, for example similar to that shown in Fig. 18. Thereby, the gradual accumulation (process after process) of silicon carbide on the upper face causes a progressive curvature, and thus a progressive flattening of the susceptor or supporting element; a further of silicon carbide could cause a slight bulging of the susceptor or supporting element.

Therefore, such a preventive mechanical counter-curvature and the preventive chemical coating described above could also be performed on the same susceptor or supporting element for the same purpose. The susceptors of the reactors for epitaxial growth are simultaneously used to support and heat the substrates which are subject to epitaxial growth.

In case of high-temperature epitaxial growth of silicon carbide, the susceptor is placed within a reaction chamber of the hot wall type; typically, the heating is of the induction type and allows to heat the walls of the chamber and the susceptor simultaneously.

Typically, there is a rather small gap between the lower wall and the upper wall of the chamber; it may be, for example, a parallelepiped of a few centimeters in height; the discshaped body of the susceptor is normally inserted in a recess of the lower wall of the chamber in which it may rotate about its axis.

The rotation is obtained, in general, by means of a specific gas flow (i.e. there is no shaft which transmits a rotary motion to the susceptor); for this reason, the horizontal position of - the susceptor in the chamber is known with an accuracy of a few millimeters and the horizontal position of the substrate in the recess is known with an accuracy of a few tenths of a millimeter; furthermore, in general, it is not possible to know either the angular position of the susceptor or the angular position of the substrates. Because of these position

uncertainties, it is not easy to handle the substrates above all when they are in the chamber. With regards to the handling of the substrates, the present invention advantageously includes the possibility of handling the supporting elements and/or the element + frame couplings. This means that it is no longer necessary to load and unload an entire susceptor, which is cumbersome and heavy, with the substrates.

This means that it is possible to unload the substrates (together with the supporting elements or the element + frame couplings), without damaging them, at rather high temperatures thus reducing the downtime of the epitaxial reactor. The unloading temperature may be, for example, in excess of 500°C, and may even reach 800-1000°C; it is preferable to use tools made of quartz or silicon carbide for handling objects this hot.

Fundamentally, there are three ways for automatically unloading a supporting element or an element + frame coupling while the susceptor body is inside the reaction chamber of the epitaxial reactor:

A) by means of mechanical traction and action from the top, for example on a corner, B) by means of pneumatic suction and action from the top, for example on a flat surface, C) by means of mechanical thrust and action from the bottom;

naturally, loading is carried out in the opposite sense.

For example, method A is suitable for the solution shown in Fig. 10; the inner bevel may be used for the mechanical action on the corner of the supporting element of an appropriate tool. For example, method B leans itself to the solutions shown in Fig. 1 1 , Fig. 12 and Fig. 13; the wide upper surface of the edge of the supporting element may be used for the pneumatic action of an appropriate tool on the supporting element.

For example, method C leans itself to the solutions shown in Fig. 14 and Fig. 15; an appropriate tool may operate from the bottom underneath the protrusion of the edge of the supporting element. For such a purpose, the tool may comprise, for example, two long fingers which by translating insinuate under the protrusion and are then lifted pushing the supporting element upwards.

As mentioned, similar considerations to those for handling the supporting elements apply to the handling of the frames. For example, the frame in Fig. 9 does not leave a gap between frame and recess, but an alternative solution could include it by virtue of a different shape of the frame and/or of the recess. Again, for example, the frame in Fig. 9 or a variant thereof, could lay on a relief like that shown in Fig. 15.

Claims

1. Susceptor for a reactor for epitaxial growth, consisting of a disc-shaped body made of graphite having a first face and a second face,

wherein said first face comprises at least one circular-shaped zone adapted to receive a substrate to be subjected to epitaxial growth, whereby said first face exposes a first upper surface corresponding to said at least one zone and a second upper surface which surrounds said at least one zone,

wherein said second face exposes a lower surface,

characterized in that said second upper surface and said lower surface are coated beforehand with a layer of silicon carbide.

2. Susceptor according to claim 1 , characterized in that only said second upper surface and said lower surface are coated with a layer of silicon carbide.

3. Susceptor according to claim 1 or 2, consisting of a disc-shaped body made of graphite having a first face and a second face,

wherein said first face comprises at least one circular-shaped recess adapted to receive a substrate to be subjected to epitaxial growth, whereby said first face exposes a lowered upper surface corresponding to the bottom of said at least one recess and a raised upper surface which surrounds said at least one recess,

wherein said second face exposes a lower surface,

characterized in that said raised upper surface and said lower surface are coated beforehand with a layer of silicon carbide.

4. Susceptor according to claim 3, wherein said lower surface is coated with a layer of silicon carbide either entirely or only in an annular zone or only in a central zone.

5. Susceptor according to claim 3 or 4, wherein the thickness of the layer of silicon carbide on the upper surface is greater than the thickness of the layer of silicon carbide on the lower surface.

6. Susceptor according to claim 3 or 4 or 5, wherein said lowered upper surface is coated with a layer of graphite.

7. Susceptor according to claim 3 or 4 or 5, wherein said lowered upper surface is coated with a layer of tantalum carbide.

8. Susceptor according to any one of the preceding claims from 1 to 7, wherein said lowered upper surface is either rough or rugged or knurled.

9. Susceptor for a reactor for epitaxial growth according to claim 1 or 2, consisting of a disc-shaped body made of graphite having a first face and a second face,

wherein said first face comprises at least one circular-shaped relief adapted to receive a substrate to be subjected to epitaxial growth, whereby said first face exposes a raised upper surface corresponding to the top of said at least one relief and a lowered upper surface which surrounds said at least one relief,

wherein said second face exposes a lower surface,

10. Susceptor according to claim 9, wherein said lower surface is coated with a layer of silicon carbide either entirely or only in annular zone or only in a central zone.

1 1. Susceptor according to claim 9 or 10, wherein the thickness of the layer of silicon carbide on the upper surface is greater than the thickness of the layer of silicon carbide on the lower surface.

12. Susceptor according to claim 9 or 10 or 1 1 , wherein said raised upper surface is coated with a layer of graphite.

13. Susceptor according to claim 9 or 10 or 1 1 , wherein said raised upper surface is coated with a layer of tantalum carbide.

14. Susceptor according to any one of the preceding claims from 9 to 13, wherein said raised upper surface is either rough or rugged or knurled.

15. Susceptor according to any one of the preceding claims from 1 to 14, comprising at least one supporting element for said substrate, and wherein said at least one supporting element lays on said at least one zone.

16. Susceptor according to any one of the preceding claims from 1 to 14, comprising at least one supporting element for said substrate and at least one frame for said supporting element, wherein said at least one frame comprises a hole, and wherein said at least one frame lays on said at least one zone, in particular is either inserted in said at least one recess or mounted on said at least one relief, and said at least one supporting element is inserted in said hole.

17. Reactor for epitaxial growth comprising at least one susceptor for supporting and heating substrates according to any one of the preceding claims and a hot wall type reaction chamber in which said at least one susceptor is contained, and being of the type for depositions of layers of silicon carbide on said substrates, and preferably with induction heating.

18. Method for limiting the outward curvature of a substrate supporting element for a reactor for epitaxial growth following a plurality of processes of depositing layers of silicon carbide on substrates, wherein said substrate supporting element has an exposed upper surface and an exposed lower surface, characterized in that said exposed upper surface and said exposed lower surface are coated beforehand ? with a layer of silicon carbide.

19. Method according to claim 18, characterized in that only said exposed upper surface and said exposed lower surface are coated beforehand with a layer of silicon carbide.

20. Method according to claim 18 or 19, wherein said substrate supporting is a susceptor consisting of a disc-shaped body made of graphite having a first face and a second face, wherein said first face comprises at least one circular-shaped zone adapted to receive a substrate to be subjected to epitaxial growth, whereby said first face exposes a first upper surface corresponding to said at least one zone and a second upper surface which surrounds said at least one zone,

wherein said second face exposes a lower surface,