WO2005031044A1

WO2005031044A1 - Susceptor for induction-heated epitaxial reactors

Info

Publication number: WO2005031044A1
Application number: PCT/EP2004/010655
Authority: WO
Inventors: Giuseppe Tarenzi; Danilo Crippa; Franco Preti
Original assignee: Lpe Spa
Priority date: 2003-09-25
Filing date: 2004-09-22
Publication date: 2005-04-07
Also published as: ITMI20031841A1

Abstract

The susceptor (1) for induction-heated epitaxial reactors comprises: a) a hollow body (2) in the shape of a truncated pyramid or a prism, provided with openings in its lower and upper faces, suitable for supporting substrates on its lateral faces, and made of an electrically-conductive material, b) a lower closing element (3), and C) an upper closing element (4); characterised in that the lower element (3) and/or the upper element (4) is made of electrically-insulating material.

Description

Susceptor for induction-heated epitaxial reactors DESCRIPTION

The present invention relates to a susceptor for induction-heated epitaxial reactors in accordance with the preamble to claim 1.

The present invention also relates to a closing element for use in such a susceptor and an epitaxial reactor that uses such a susceptor. Epitaxial reactors with susceptors in the shape of a truncated pyramid or a prism, known as barrel reactors, with induction heating, have been known and used for decades. In these reactors the susceptor is heated using an inductor that generates a radio-frequency electromagnetic field (typically from 4KHz to 40KHz). A reactor of this type is well described and illustrated in, for example, international patent application WOOO/58533 filed by the same applicant, which is to be regarded as incorporated herein by reference.

These susceptors are generally composed of a hollow body in the shape of a truncated pyramid and two elements, a lower element and an upper element. The lateral faces of the body are provided with pockets for holding substrates to be processed. Epitaxial reactors are often used to grow layers of silicon, but are also used for growing other semiconductor materials such as, for example, GaAs and GaN. In induction-heated reactors, the body is generally made in one piece and is completely hollow with a single cavity that has the same shape as the body itself and is open to the outside of the body at its upper face and lower face. The lower and upper openings generally have the same shape as the section of the body, which is to say a polygon. The body is, in practice, therefore formed of a series of walls that have a substantially uniform thickness (except for the presence of the pockets). The lower and upper elements serve to close the respective openings. These three parts are traditionally made of graphite covered by a protective layer of silicon carbide. All the parts of the susceptor can in this way be heated by induction since graphite is an electrically-conductive material. The substrates held in the pockets are in their turn heated owing to the heat generated by the susceptor (to be precise there is, as is known, a contribution due to conduction and a contribution due to radiation).

The heating obtained in this manner is fairly uniform and the epitaxial layer grown on the substrates has a fairly uniform resistivity, sufficient to comply with the requirements of the microelectronics industry for many applications. A known and conventional solution for improving the temperature uniformity of the susceptor involves acting on the inductor coils, for example, to concentrate the coils where it is necessary to better heat the susceptor. Different types of reactor (single-wafer reactors) are generally used to obtain epitaxial layers with an extremely uniform resistivity. The market nevertheless demands increasingly uniform epitaxial layers. An object of the present invention is to improve the uniformity of resistivity in the epitaxial layers grown in induction-heated barrel reactors. This object is achieved thanks to the susceptor having the features recited in independent claim No. 1.

The basic idea of the present invention is to make the closing elements of electrically-insulating material instead of electrically-conductive material. Advantageous features of the susceptor in accordance with the present invention are set out in the dependent claims.

For very many years, people only considered the fact that a closing element of electrically-conductive material couples with the electromagnetic field generated by the inductor, thereby heating itself such that it should heat the extreme parts of the susceptor body by conduction. It should be borne in mind that the extreme zones of the reaction chamber always tend to be a little cooler, being close to the gas inlets and outlets and close to the extremes of the inductor. The present invention considers another fact in addition to this, namely that graphite closing elements draw part of the electromagnetic energy emitted by the inductor away from the susceptor body. Now, independently of the scientific explanation, it may be observed that using closing elements made of electrically-insulating material and quartz in particular improves the uniformity of resistivity, in many cases by more than five times. In accordance with another aspect, the present invention also relates to a closing element for the susceptors of epitaxial reactors having the features set forth in independent claim No. 10.

In accordance with yet another aspect, the present invention also regards an induction-heated epitaxial reactor having the features set forth in independent claim No. 19.

The present invention will emerge more clearly from the following description, which should be considered in conjunction with the accompanying drawings, wherein: Fig. 1 is a partly sectioned exploded side view of one embodiment of the susceptor in accordance with the present invention,

Fig. 2 shows an embodiment of the lower closing element in accordance with the present invention (A: cross-sectional view, B: view from above), and

Fig. 3 shows an embodiment of the upper closing element in accordance with the present invention (A: cross-sectional view, B: view from below).

The susceptor for induction-heated epitaxial reactors in accordance with the present invention comprises: a) a hollow body in the shape of a truncated pyramid or a prism, which is provided with openings in its lower and upper faces, is suitable for supporting substrates on its lateral faces, and is made of an electrically-conductive material, b) a lower closing element, and c) an upper closing element.

The lower and/or upper element is made of electrically-insulating material.

The idea of the present invention, as can be seen, is very simple but produces significant and unexpected results (improvements of up to more than five times in the uniformity of resistivity of the layers grown).

Moreover, it has the advantage of being extremely easy to apply even to epitaxial reactors that have been installed and in operation for years, with the same results; all that is needed is to replace one or both of the closing elements.

In Fig. 1 reference 1 generally indicates the susceptor as a whole, the reference 2 indicates the body which is hollow (as illustrated by the cross-sectional part on the left-hand side of the figure), the reference 3 indicates the lower closing element and the reference 4 indicates the upper closing element 4.

The susceptor body in Fig. 1 has the shape of a truncated pyramid with a section in the shape of a heptagon (seven-sided polygon). In the lower zone, the edges of the pyramid are heavily chamfered (two of these chamfered edges are visible in Fig. 1, where they basically take the form of triangles). Laterally, there are seven lateral faces, each of which features two substantially circular pockets for two substrates (the substrates are generally circular but feature a "flat"). The number of pockets is not in itself relevant to the present invention. The upper zone of the susceptor body in Fig. 1 does not feature any pockets and is used, in conjunction with the wall of the bell jar surrounding the susceptor, to create a zone for heating the reaction gases that are injected into the bell jar from above. The body of the susceptor in Fig. 1 has a single internal cavity with a shape that corresponds exactly to the external shape of the body. The two shapes could be different, according to alternative embodiments of the present invention.

The lower and upper openings of the susceptor body in Fig. 1 have a shape that is exactly the same as the lower and upper faces of the pyramid. The two shapes could be different in alternative embodiments of the present invention. If a closing element is made of insulating material, the currents induced by the inductor of the epitaxial reactor's induction heating system are substantially zero. It can therefore be stated that such a closing element does not substantially absorb any of the electromagnetic energy emitted by the inductor. All the electromagnetic energy is therefore substantially available for heating the susceptor body and the closing elements remain relatively cool. Above all, the extreme part of the susceptor body next to the closing element considered seems to benefit from this. More uniform heating results in the susceptor body having a more uniform temperature (any temperature inhomogeneities generally extend in a vertical direction in barrel reactors). A more uniform susceptor body temperature results in the substrates being grown having a more uniform temperature, which in turn causes the layers grown to have more uniform resistivity .

In the experiments conducted, the best results were obtained using electrically- insulating material both to make the lower closing element and to make the upper closing element, although a considerable improvement in uniformity of resistivity was nevertheless obtained even with a single electrical insulator element. In embodiments of the present invention where only one of the two closing elements is made of electrically-insulating material, the other closing element is made of electrically-conductive material. In this case, the conductive closing element could be made as a separate part from the susceptor body, but could be made, equivalently, as one piece with the susceptor body. The one-piece embodiment, using silicon carbide coated graphite, for example, solves a number of mechanical coupling problems but is more costly since it requires a more complicated production process .

Many of the considerations expressed regarding the closing elements in the description hereinafter apply both to the lower closing elements and to the upper closing elements. When this is not the case, it is stated explicitly. If the closing element in accordance with the present invention is made of a single material, this material must not only be an electrically-insulating material but also be a refractory material (which is to say comfortably able to withstand the high temperatures that are generated in the reaction chamber of an epitaxial reactor, which are generally in excess of 1,000 degrees centigrade) and inert (which is to say comfortably able to withstand the environment in the reaction chamber of an epitaxial reactor both during the growth processes and during the cleaning processes, such that it does not release substances able to spoil the growth processes). This material can advantageously be a thermal insulator. Indeed, in this way, the closing element draws less heat from the susceptor body, particularly in the zones adjoining the mechanical contact between them.

Alternatively, the closing element in accordance with the present invention could, for example, be made of two materials. In this case, the closing element could comprise a core (the major part of the element) made of a first material and a surface zone (in the form of a layer or plate) made of a second material. In accordance with the present invention, the first material, for the core, is an electrically-insulating and refractory material, while the material for the surface zone is inert and refractory.

From the experiments conducted, the most suitable material for making closing elements in accordance with the present invention is quartz. Indeed, this is an excellent electrical insulator and a good thermal insulator, as well as being refractory and inert. Moreover, the cost of this material is reasonable.

Silicon carbide is an alternative material, but does, however, need to be used in its polycrystalline or amorphous form so that it largely behaves as an electrical insulator. This material has the disadvantage of currently being quite costly.

In this case, it is best to take into account that this material is a good thermal conductor, such that the mechanical coupling between the closing element and the susceptor body needs to be designed in such a way as to limit the flow of heat from the susceptor body to the closing element. It is worth noting at this point that the materials used for the various parts of the susceptor need to be chosen with great care. It is necessary to take into account that an epitaxial reactor operates in a temperature range of between ambient temperature and a temperature in the order of one thousand degrees centigrade (where processes for the epitaxial growth of silicon are concerned), such that the various properties of the materials (like their electrical conductivity and thermal conductivity, for example) need to be evaluated for this entire temperature range. In the light of the conditions that occur in the reaction chamber of an epitaxial reactor, the best way of fitting a closing element on the susceptor body is to slot it into the respective opening in the body. The slot fitting could use a protuberance on the closing element which slots and locks into the respective opening in the body. If this protuberance is made of a material with a low thermal dilation coefficient (such as quartz, for example, like the rest of the closing element), this protuberance could be continuous (without interruptions) and there would be no problems of the covering element and body being permanently locked together. This is the case in the embodiments in all the figures. It is easy to make this protuberance in the shape of a circle. This is the case in all the embodiments in all the figures. This shape has the advantage that if it is fitted inside a polygonal opening, the contact between the circle and polygon is limited to a few points, such that good thermal contact is not produced between the body of the susceptor (hot) and the closing element (cold).

The closing element in accordance with the present invention can basically comprise a polygonal plate that matches the respective face of the susceptor body.

This is the case for both the closing elements in Fig. 1 and the lower closing element in Fig. 2. In the embodiments shown in these figures, the plate reaches the edge of the susceptor body.

If the diameter of the polygon is 300mm, the thickness of the plate can, for example, be 6mm or from 3mm to 9mm.

Such a closing element is very easy to make and works well. The large hole in the centre of the element is used to fit the susceptor on the reactor's rotation system, but is not relevant for the purposes of the present invention.

Alternatively, the closing element in accordance with the present invention can basically comprise a prism with a polygonal section matching the respective face of the susceptor body. This is largely the case for the upper closing element in Fig. 3.

If the diameter of the polygon is 300mm, the height of the former can, for example, be 19mm or from 15mm to 30mm.

Such a closing element has considerable mass and therefore tends to have a fairly low temperature during the epitaxial growth processes and, in all cases, a lower temperature than the plate-like closing element.

In this case, above all, due to the considerable mass of the closing element, it is necessary to design the rotation system for the susceptor in the epitaxial reactor appropriately. In order to overcome the problem of the considerable mass of the prism-shaped closing element, a recess can be incorporated in the face thereof that faces the body when the element is fitted on the susceptor body. In order to obtain a sufficient reduction in mass, it is preferable for the recess to extend over most of the face and, naturally, have a not insignificant depth. This is the case of the upper closing element in Fig. 3. In the embodiment shown in Fig. 3, there is a circular protuberance and the recess has a cylindrical shape and a diameter that is slightly smaller than that of the protuberance.

The small hole in the centre of the element is used for fitting other devices, but this is not relevant for the purposes of the present invention. A phenomenon worthy of consideration, since it is relevant to the present invention, concerns the "incrustations" that form on the susceptor during the epitaxial growth processes. These "incrustations" differ depending on the conditions, in particular the temperature, in which they are formed. During an epitaxial growth process, for silicon for example, the silicon does not grow only on the substrates located in the susceptor pockets, but also grows on the parts of the susceptor that are exposed to the reaction gases. The material that grows on the susceptor forms "incrustations". This growth largely occurs on the hot parts. The growth naturally depends on the availability of growth material and therefore on the flow configuration of the gases in the reaction chamber. The "incrustations" can change the geometry of the reaction chamber (when they reach a given thickness) and therefore prevent a uniform and well-controlled growth process on the substrates. Small solid particles can become detached from the "incrustations" as a result of the mechanical action of the gas flow, and these particles can damage the surfaces of the layers growing on the substrates. It is therefore necessary to periodically carry out cleaning processes on the reaction chamber and respective susceptor without disassembling the reactor ("in situ"). Typically, the susceptor is heated and so, consequently, is the chamber, into which HC1 that dissolves the incrustations is subsequently fed. The frequency of the cleaning processes can vary considerably. In some extreme cases, a cleaning process is carried out for each growth process, although it is more common to carry out one cleaning process for around every ten growth processes. The use of closing elements in accordance with the present invention, which only heat up slightly during the growth processes, results in a much more limited formation of "incrustations" on said elements. It should also be stated that the "incrustations" which form on the closing elements in accordance with the present invention are more difficult to remove, partly because said elements are more difficult to heat. In the experiments conducted, it was noted that by using closing elements in accordance with the present invention, the "incrustations" on the flat surface are reduced, but that a "collar" is formed on the border between the closing element and the body of the susceptor, and that this "collar" is very often difficult to remove with the cleaning processes and very dangerous for the substrates being grown because it is exposed very directly to the flow of gases. To overcome this problem, the upper covering element can be made to project laterally with respect to the susceptor body, when it is fitted on the latter. "Eaves" are thus formed. The "collar" forms under the "eaves" and the "eaves" protect the "collar" from the mechanical action due to the flow of gases. Moreover, the "eaves" keep the flow of reaction gases away from the border and therefore reduce the concentration of silicon available for the formation of "incrustations" on the border. The projection of the element is advantageously located around the entire perimeter of the element.

Taking the thicknesses of the epitaxial layers usually grown into account, together with the growth conditions, a projection of no more than 5mm should be sufficient for all cases of practical interest. The "collar" that can form at the location of the lower closing element is less troublesome. In this case, in order to avoid disturbing the laminar flow of the gases along the lateral faces of the susceptor, the lower element should be laterally recessed with respect to the susceptor body, when it is fitted on the latter. The present invention brings innovations to the closing elements of the susceptors for induction-heated "barrel" reactors.

There are therefore two wholly-parallel aspects to the present invention: the susceptor as a whole and the closing element (which can be used above and below, excepting for differences that are not relevant for the purposes of the present invention). The accompanying claims reflect these parallel aspects. The susceptor in accordance with the present invention can comprise a single closing element made of electrically-insulating material or two closing elements made of electrically-insulating material. In the second case, the closing elements can be different from one another, including in terms of the advantageous features of the present invention.

For example, a susceptor in accordance with the present invention could comprise a lower closing element as shown in Fig. 2 and an upper closing element as shown in Fig. 3. This specific combination is particularly advantageous. The susceptor in accordance with the present invention can be advantageously used in induction- heated epitaxial reactors.

It is worth noting at this point that epitaxial reactors can also comprise more than one susceptor.

Claims

CLAIMS 1. Susceptor for induction-heated epitaxial reactors comprising: a) a hollow body in the shape of a truncated pyramid or a prism, which is provided with openings in its lower and upper faces, is suitable for supporting substrates on its lateral faces, and is made of an electrically-conductive material, b) a lower closing element, and c) an upper closing element; characterised in that the lower element and/or upper element is made of electrically-insulating material.

2. Susceptor according to claim 1, wherein the lower element and/or upper element is made of electrically-insulating, refractory, inert and preferably thermally-insulating material.

3. Susceptor according to claim 2, wherein the lower element and/or upper element is made of quartz.

4. Susceptor according to any one of the preceding claims, wherein the lower and/or upper element is fitted in the respective opening of the hollow body, using a protuberance that is preferably continuous and in the shape of a circle.

5. Susceptor according to any one of the preceding claims, wherein the lower and/or upper element basically comprises a polygonal plate that matches the respective face of the body.

6. Susceptor according to any one of the preceding claims, wherein the lower and/or upper element basically comprises a prism with a polygonal section matching the respective face of the body.

7. Susceptor according to claim 6, wherein the lower element and/or upper element has a recess in the face that faces the body, with said recess preferably extending over most of the face.

8. Susceptor according to any one of the preceding claims, wherein the upper element projects laterally with respect to the body.

9. Susceptor according to claim 8, wherein the projection of the element is located around the entire perimeter of the element and is no greater than 5mm.

10. Closing element for the susceptor of epitaxial reactors, characterised in that it is made of electrically-insulating material.

11. Closing element according to claim 10, characterised in that it is made of electrically-insulating, refractory, inert and preferably thermally-insulating material.

12. Closing element according to claim 11, characterised in that it is made of quartz.

13. Closing element according to any one of claims 10 to 12, characterised in that it is provided with a protuberance that is preferably continuous and in the shape of a circle for slotting into a typically polygonal opening in a susceptor.

14. Closing element according to any one of claims 10 to 13, characterised in that it basically comprises a polygonal plate.

15. Closing element according to any one of claims 10 to 13, characterised in that it basically comprises a prism with a polygonal section.

16. Closing element according to claim 15, characterised in that it has a recess in the face that faces the body when the element is fitted on the susceptor body, with said recess preferably extending over most of the face.

17. Closing element according to any one of claims 10 to 16, characterised in that it is able to project laterally with respect to the susceptor body when it is fitted on the susceptor body.

18. Closing element according to claim 17, wherein the projection is located around the entire perimeter of the element and is no greater than 5mm.

19. Induction-heated epitaxial reactor, characterised in that it comprises at least one susceptor according to any one of claims 1 to 9.