WO2011110369A1

WO2011110369A1 - Apparatus for thermally treating semiconductor substrates

Info

Publication number: WO2011110369A1
Application number: PCT/EP2011/001256
Authority: WO
Inventors: Uwe Keim; Peter VÖLK
Original assignee: Centrotherm Thermal Solutions Gmbh & Co. Kg
Priority date: 2010-03-12
Filing date: 2011-03-14
Publication date: 2011-09-15
Also published as: DE102010011156A1; DE102010011156B4

Abstract

An apparatus for thermally treating substrates, in particular semiconductor substrates is described. The apparatus comprises a processing tube for receiving a plurality of semiconductor substrates and a resistance heating element radially encompassing the processing tube. The resistance heating element is made of a material which is free of metal. By providing a material which is free of metal for the resistance heating element, the danger of diffusion of metals into the processing space within the processing tube may be obviated.

Description

Apparatus for thermally treating semiconductor substrates

The present invention relates to an apparatus for thermally treating substrates, in particular semiconductor substrates, said apparatus having a processing tube which is substantially transparent to heat radiation, for receiving a plurality of semiconductor substrates, and a resistance heating element radially encompassing the processing tube.

In the semiconductor fabrication field, different apparatuses for thermally treating semiconductor substrates are known, which on the one hand, thermally treat single wafers or on the other hand, batches of semiconductor substrates. In single wafer treatment apparatuses, typically irradiation sources, such as halogen lamps or arc lamps, are used for thermally treating the semiconductor substrates. When thermally treating a batch of semiconductor wafers, typically resistance heating elements made of metal are used.

In order to protect the batch of semiconductor wafers or other materials, which are to be thermally treated, such as crystals, ceramics, sintered materials, glass, and so on, from the metallic resistance heating element, these materials are typically received in a processing tube made of, for example, quartz glass, silicon carbide or borosilicate glass 3.3, such as Duran ® of the Duran Group GmbH. Furthermore, in the processing tube, a desired process gas atmosphere may be adjusted in a controlled manner. It has been shown, however, that metallic resistance heating elements release metallic contaminations into the environment. These contaminants may over time, in particular at high temperatures, diffuse through the processing tube, and may lead to undesired contaminations of the semiconductor substrate. It is therefore known to use double walled processing tubes having an interior tube and an outer tube, which form a clearance therebetween, which may be flushed by a gas. Such a separation of the processing tubes reduces the danger that metals diffuse through the tube processing tubes into the interior processing space, but lead to an expensive configuration for the processing tube. Furthermore, by providing two processing tubes, the metallic resistance heating element is arranged further away from the semiconductor substrate to be processed thereby achieving a less efficient heating thereof. The present invention is directed towards an apparatus of the type mentioned above, which overcomes at least one of the previously mentioned disadvantages. In order to achieve the above problem, an apparatus in accordance with claim 1 is provided. Further embodiments of the invention are shown in the dependent claims.

In particular, an apparatus for thermally treating substrates is provided, said apparatus having a processing tube for receiving a plurality of semiconductor substrates and a resistance heating element radially encompassing the processing tube, wherein the resistance heating element consists of a material which is free of metal. By providing a material which is free of metal for the resistance heating element, the danger of diffusion of metals into the processing space within the processing tube is prohibited. The processing tube may now be optionally designed with a single wall, since the danger of metal diffusion no longer exists, thereby enabling the resistance heating element to be placed closer to the semiconductor substrate.

In accordance with a preferred embodiment, the resistance heating element comprises at least one of the following materials: graphite, carbon fiber reinforced carbon (CFC) and SiC. These materials may be manufactured at high purity in any desired form and they are further highly heat resistant. They each have a small coefficient of thermal expansion compared to a metallic element such that they have high shape stability and may go through many thermal cycles. As a negative temperature coefficient thermistor, they allow high heating rates already at low temperatures and even at low voltages. Furthermore, these materials comprise a lower density compared to metals, such that they comprise a lower thermal mass at comparable structures, whereby less energy is required for heating and cooling the same. Therefore, faster heating and cooling rates may be achieved. In particular for PV systems (PECVD/POCI3), this may lead to a reduction of process time.

In order to reduce the thermal mass of the resistance heating element, it is pre- ferably formed as a foil shaped heating element. A foil shaped heating element is a laminar element having a small thickness in comparison to its width and length. In the following, a laminar element having a low thickness is to be understood to be a laminar element having a thickness of not more than 10% of its width and length. Preferably, the thickness of the laminar element is smaller than 3% of its width and length. The foil heating element may for example, be a CFC formed component (plate, tube etc.) having a small wall thickness or a shaped graphite foil. For a maximized and homogeneous heat introduction into the processing tube, the resistance heating element preferably encompasses the processing tube in substance completely over the longitudinal extension of the resistance heating element. In substance, completely is considered to encompass an area of larger than 80% and preferably larger than 90% of radial coverage.

In order to protect the environment with respect to the heating power of the resistance heating element and to protect the resistance heating element from the environment, a housing may be provided which encloses the resistance heating element and isolates the same with respect to the environment. Preferably, the housing encloses the resistance heating element in an airtight manner. Means may be provided for generating a vacuum or a protective gas atmosphere within the housing. The materials mentioned above with respect to the resistance heating element have a thermal stability in an oxygen free atmosphere in a temperature region well above 2000° Celsius which is more than sufficient for most thermal treatment steps for semiconductor substrates. In an oxygen atmosphere, the thermal stability, however, is substantially reduced. Therefore, the resistance heating element should be kept in an atmosphere which is free of oxygen or in a vacuum when it is used for thermal treatments which require temperatures which are above the thermal stability of the resistance heating element in an oxygen atmosphere. The processing tube may for example be made of glass, in particular quartz glass or borosilicate glass 3.3, such as Duran ® or SiC or graphite, which may each comprise high purity and the required thermal stability.

Preferably, the resistance heating element is made of a material which shows the characteristics of a negative temperature coefficient thermistor in a first temperature range and which shows the characteristics of a positive coefficient resistor above said first temperature region. This enables an efficient and fast heating of the resistance heating element at low temperatures and low voltages. Changing the characteristics prohibits the resistance heating element from being primarily heated along a small current pass and thus facilitates a better temperature distribution within the resistance heating element. Preferably, the first temperature region is below or equal to a temperature region, in which the resistance heating element is typically used. In one embodiment of the invention, the processing tube is substantially transparent to the heat radiation radiated by the resistance heating element.

The invention will be described in more detail herein below with reference to the drawings. In the drawings: Fig. 1 shows a perspective sectional view of an apparatus for thermally treating semiconductor substrates;

Fig. 2 shows a perspective view of a resistance heating element, which may be used in the apparatus according to Fig. 1 ;

Figs. 3A-C show different embodiments of resistance heating elements which may be used in the apparatus for thermally treating semiconductor substrates in accordance with Fig. 1 .

In the following description, terms such as above, below, right, and left refer to the representation in the drawings and they should not be seen in a limiting sense, even though they may represent a preferred orientation. Fig. 1 shows a perspective sectional view of a heating apparatus 1 for thermally treating substrates 2. The heating apparatus 1 has a processing tube 4, a resistance heating element 6 and a housing 8. The processing tube 4 is for example, as known, a glass tube which may for example be made of quartz glass or borosilicate glass 3.3, such as for example Duran ®. The processing tube may for example also be made of SiC or graphite. The processing tube 4 has a round cross section and is closed at one end 10, while the other end may be open in order to allow introduction or removal of, for example, a batch of semiconductor substrates 2, which are typically provided in a holding device, into the processing tube 4 or therefrom. In the following, the description refers to the treatment of semiconductor substrates even though other substrates may be treated in the heating apparatus 1 . The open end of the processing tube may be closed or sealed in an appropriate manner during a thermal treatment process. Furthermore, input and/or exhaust ducts for process gases may be provided, in order to set a desired process gas atmosphere in the interior of the processing tube 4. The processing tube 4 is radially encompassed by the resistance heating element 6 adjacent to its closed end portion 10. The resistance heating element 6 extends over a longitudinal section of the processing tube 4, in which the batch of semiconductor substrates is received during the thermal treatment thereof. The resistance heating element 6, which can be seen in a perspective view in Fig. 2, is made of an appropriate material, which is free of metal, such as for example graphite, carbon fiber reinforced carbon (CFC), SiC or another appropriate material which is free of metal. It is important, that the chosen material has sufficient thermal and mechanical stability for the thermal treatment of semiconductor substrates, i.e. that it has sufficient mechanic stability at the required process temperature. Even though the resistance heating element is shown as a single element, it may be formed in multiple parts. Furthermore, several of the shown resistance heating elements 6 may be provided adjacent to each other in an axial direction. In doing so, several heating zones may be provided over the length of the processing tube.

As is shown in Fig. 2, the resistance heating element 6 has a tubular shape, in which cut-outs 12 are provided. At each of the opposite end regions of the resistance heating element 6, a contacting element 14 is provided, in order to enable electrical contact to the resistance heating element 6.

The cut-outs 12 are provided in a longitudinal direction of the resistance heating element 6 between the contact elements 14 and are arranged such that a nonlinear current path is provided between the contact points of the contact elements 14. The cut-outs 12 should be arranged such that during operation, current flows through large areas of the resistance heating element, in order to achieve a homogeneous temperature distribution therein.

The inner diameter of the resistance heating element is dimensioned such that the process tube may be received therein, as shown on Fig. 1 . The resistance heating element 6 should be dimensioned such that only a small clearance is formed between the processing tube 4 and the resistance heating element 6. This clear- ance should prohibit mechanical contact between the processing tube 4 and the resistance heating element 6 and at the same time, should facilitate good heat transfer between the two by conduction of heat and/or convection.

The thickness of the resistance heating element 6 is thin compared to its width and length, such that the resistance heating element 6 as a whole may be called a foil heater. The thickness is chosen such that a sufficient mechanical stability of the resistance heating element 6 is achieved.

As shown in Fig. 1 , the resistance heating element 6 covers the processing tube 4, in substance, completely in a radial direction over the length of the resistance heating element 6. Radial coverage is only not provided in the area of the cut-outs 12. This enables a good and homogeneous input of heat into the interior of the processing tube 4. It is, however, as mentioned above, also possible to provide several heating elements 6 which may generate different temperature regions within the processing tube 4 or which may provide an intermediate main heating with stronger heating at the respective ends, in order to compensate for any losses at the ends.

The housing 8 is also provided at the end region 10 of the processing tube 4. The housing 8 completely encloses the resistance heating element 6 and a portion of the processing tube 4. The housing 8 has an end wall 16 adjacent the closed end 10 of the processing tube 4, as well as an opposite end wall 18. The end wall 18 has a centered opening for passing the processing tube 4 therethrough, as shown in Fig. 1 . The opening in the end wall 18 is dimensioned such that the processing tube 4 is received therein in a close fitting manner. A seal or sealing means may be provided between the processing tube 4 and the end wall 18, in order to close the interior space of the housing 8 in an airtight manner. Input and exhaust ducts (not shown) may be provided to the interior space of the housing 8 via a vacuum and/or a predetermined gas atmosphere. In particular, a protective gas atmosphere may be set in the interior space. The contact elements 4 are led through a radial wall portion of the housing 8, again in a gas tight manner, in order to not impair the gas atmosphere within the housing 8.

The housing 8 is made of an appropriate material, preferably metal free, which may on the one hand, provide thermal insulation of the resistance heating element and on the other hand, is capable of containing a desired gas atmosphere in its interior. The interior space may be lined with an appropriate insulating material.

Figs. 3A to 3C each show a perspective view of alternative configurations of the resistance heating element 6. In Figs. 3A to 3C, the same reference signs are used as previously, when the same or equivalent elements are described. The resistance heating element 6 of Fig. 3A has a tube shape, in which a single cut-out 12 is provided. The cut-out 12 extends helically between the end portions of the resistance heating element 6. At each end portion, a contact element 14 is provided. By means of the helical cut-out, a helical current path is formed between the contact elements 14.

The resistance heating element 6, in accordance with Fig. 3B, also has, in sub- stance, a tube shape in which larger areas are cut-out. The tube shape is formed by longitudinally extending segments 20, which are adjacent in a circumferential direction of the tube form. Adjacent segments are connected at one of their ends via a segment 22 extending in the circumferential direction. The radially extending segments 22 are alternatingly provided at opposite end portions of the resistance heating element 6 in order to form a serpentine configuration of the segments 20, 22. Again, contact elements 14 are provided. In this embodiment, the contact elements 14 are provided at the same end of the resistance heating element. The resistance heating element 6 is cut open at 24 to ensure that current flowing between the contact elements 14 follows the serpentine path.

Fig. 3C shows a further embodiment of the resistance heating element 6 which again, has a tube shape in which cut-outs 12 are provided. At the end portions, contact elements 14 are provided. The cut-outs 12 are arranged such that between the contact elements 14, two substantially parallel and nonlinear current paths are formed .

In the following, operation of the heating apparatus 1 will be explained in more detail. Firstly, a batch of semiconductor substrates 2 is loaded into the processing tube 4 and is brought into the vicinity of the resistance heating element 6. The processing tube 4 is then closed and a desired process gas atmosphere is set therein. Thereafter, current is applied to the resistance heating element 6 via the contact elements 14 and the resistance heating element 6 is thereby heated to a desired process temperature. Alternatively, it is also possible that the resistance heating element 6 is already previously heated to a desired standby temperature. This may be the process temperature or a temperature below said process temperature. The temperature of the resistance heating element 6 may be sensed by a pyrometer (not shown). The materials mentioned above, in particular, graphite and CFC are so called negative temperature coefficient thermistors, whose electrical resistance initially lowers with an increase in temperature. Therefore, it is possible, even at low voltages, to provide high heating rates. At an increased temperature, the characteristic of a negative temperature coefficient thermistor typically changes to the characteristic of a positive temperature coefficient thermistor. By appropriate choice of material, high surface loads to the heating material are possible, and these materials have a lower thermal mass compared to a metallic heating element, which facilitates heating thereof. An oxygen free gas atmosphere is set in the housing 8 either via a vacuum and/or the introduction an appropriate gas, in particular, a protective gas, such that the desired materials for the resistance heating element 6 provide sufficient thermal stability.

After the resistance heating element 6 is heated to the predetermined process temperature, it is held at this temperature for a predetermined time and is subsequently cooled down in order to finish treatment of the semiconductor wafers 2. Alternatively, it is also possible that the resistance heating element 6 is brought to the standby temperature or is held at the standby temperature, respectively. Subsequently, the semiconductor wafers are unloaded from the processing tube 4.

The present invention was described herein above with respect to preferred embodiments of the invention, without being limited to these specific embodiments. In particular, the structure and shape of the individual elements may differ from the structures and shapes as shown.

Claims

Claims:

1. An apparatus (1 ) for thermally treating substrates, said apparatus having a processing tube (4) for receiving a plurality of substrates and a resistance heating element (6) radially encompassing the processing tube (4), said resistance heating element (6) is made of a material which is free of metal and has a tube shape having at least one cut-out (12), wherein the at least one cutout (12) is arranged such that it blocks a linear current pass between contact points for electrically contacting the resistance heating element (6) and thereby forms at least one nonlinear current path between said contact points.

2. The apparatus (1 ) of claim 1 , characterized in that the resistance heating

element comprises at least one of the following materials: graphite, carbon fiber reinforced carbon (CFC) or SiC.

3. The apparatus (1 ) of claim 1 or 2, characterized in that the resistance heating element (6) is a foil heating element.

4. The apparatus (1 ) according to any one of the preceding claims, characterized in that the resistance heating element (6), in substance, completely

encompasses the processing tube (4) in a radial direction over the longitudinal extension of the resistance heating element (6).

5. The apparatus (1 ) according to any one of the preceding claims, characterized by a housing (8), which encloses the resistance heating element (6) and isolates the same from the surrounding environment.

6. The apparatus (1) according to any one of the preceding claims, characterized in that the housing (8) encloses the resistance heating element (6) in a gas tight manner.

7. The apparatus (1) according to claim 5 or 6, characterized by means for

generating a vacuum or a protective gas atmosphere in said housing (8).

8. The apparatus (1 ) according to any one of the preceding claims, characterized in that said resistance heating element (6) is made of a material that shows the characteristics of a negative temperature coefficient thermistor up to a first temperature region and shows the characteristics of a positive temperature coefficient thermistor at a temperature above said first temperature region, wherein the first temperature region is lower or equal to the temperature region at which the resistance heating element is typically operated.

9. The apparatus (1 ) according to any one of the preceding claims, characterized in that a single cut-out (12) is provided, which helically extends in the longitudinal direction of the tube shaped resistance heating element (6), wherein the contact points for electrically contacting the resistance heating element (6) are provided at opposite ends of the tube shape of the resistance heating element (6) in a longitudinal direction thereof.

10. The apparatus (1 ) according to any one claims 1 to 8, characterized by a

plurality of cut-outs (12), which extend in longitudinal direction of the tube shape resistance heating element, and which are arranged such that strip shaped longitudinally extending segments (20) are formed, which are adjacent in a circumferential direction of the tube shape, wherein adjacent segments are connected at one of their ends via segments (22) extending in said

circumferential direction, and wherein said circumferentially extending segments (22) are alternatingly arranged at opposite end portions of the resistance heating element (6) to provide a serpentine configuration of the segments (20, 22).

1 1. The apparatus (1 ) according to any one of claims 1 to 8, characterized in that a plurality of cut-outs (12) is provided which extend transverse to the longitudinal extension of the tube shape resistance heating element (6) and which are arranged such that between contact points at opposite ends of the resistance heating element (6) two separate nonlinear current paths are formed.