Thermal shield, substrate support, and processing chamber

Abstract

This describes a processing chamber, a substrate support, and a heat shield. The heat shield includes a dish-shaped body having a thickness, outer diameters with first and second edges at opposite ends of the diameter of the dish-shaped body, a front surface defining the thickness, and a rear surface. The front surface has a first longitudinal region including the first edge and a second longitudinal region including the second edge. One or more of the first or second longitudinal regions are coated with an emissive material (i.e., emissivity) to reduce side-to-side temperature variations. In some embodiments, a processing chamber having the heat shield described herein consumes less power compared to a comparative processing chamber without a heat shield.

Description

Technical Field

[0001] Embodiments of the present invention generally relate to thermal shields and, more particularly, to substrate supports and processing chambers having thermal shields for reducing side-to-side temperature variations. Prior Art

[0002] Conventional processing chambers include a substrate support having a support surface and a rear surface defining the thickness of the support body, and a heater within the thickness of the support body. These processing chambers often experience side-to-side temperature variations in the support surface during heating. These conventional processing chambers typically have an asymmetric liner positioned between the substrate support surface and the heater. In these cases, the slit valve side of the liner typically has a higher temperature than the pumping port side. Therefore, reducing side-to-side temperature variations in conventional processing chambers is challenging. Reducing side-to-side temperature variations can advantageously reduce the temperature and power consumption required to heat the support surface.

[0003] Therefore, there is a need in the art for a processing chamber that reduces side-to-side variations in temperature in a substrate supporting surface and consumes less power. Summary of the Invention

[0004] One or more embodiments of the present invention relate to a heat shield. In some embodiments, the heat shield comprises a disk-shaped body having a thickness, a central axis, and an outer diameter, the outer diameter having a first edge and a second edge at opposite ends of the diameter of the disk-shaped body. A front surface and a rear surface define the thickness of the disk-shaped body. The front surface of the disk-shaped body comprises a first longitudinal region and a second longitudinal region, the first longitudinal region comprising the first edge and the second longitudinal region comprising the second edge. The first longitudinal region has one or more of a low emissivity or a medium emissivity, and the second longitudinal region has one or more of a medium emissivity or a high emissivity, wherein the high emissivity is greater than the low emissivity by more than or equal to 0.2.

[0005] Additional embodiments of the present invention relate to substrate supports. In some embodiments, the substrate support comprises a support body having a support surface and a rear surface defining a thickness of the support body, a heater within the thickness of the support body, and a heat shield comprising a dish-shaped body having a thickness, a central axis, and an outer diameter, the outer diameter having a first edge and a second edge at opposite ends of the diameter of the dish-shaped body. The front surface and the rear surface define the thickness of the dish-shaped body. The front surface of the dish-shaped body comprises a first longitudinal region and a second longitudinal region, the first longitudinal region comprising the first edge and the second longitudinal region comprising the second edge. The first longitudinal region has one or more of a low emissivity or a medium emissivity, and the second longitudinal region has one or more of a medium emissivity or a high emissivity, wherein the high emissivity is greater than the low emissivity by more than or equal to 0.2.

[0006] Further embodiments of the present invention relate to a processing chamber. In some embodiments, the processing chamber includes a chamber body having a top wall, a bottom wall, and sidewalls, and containing an interior volume. In some embodiments, the processing chamber includes a substrate support within the interior volume, the substrate support including a support rod having a support body at one end of the support rod. The support body has a support surface and a rear surface defining a thickness of the support body, and a heater within the thickness of the support body. In some embodiments, the processing chamber includes a heat shield, the heat shield including a dish-shaped body having a thickness, a central axis, and an outer diameter, the outer diameter having a first edge and a second edge at opposite ends of the diameter of the dish-shaped body. The front surface and the rear surface define the thickness of the dish-shaped body. The front surface of the dish-shaped body includes a first longitudinal region and a second longitudinal region, the first longitudinal region including a first edge and the second longitudinal region including a second edge. The first longitudinal region has one or more of low emissivity or medium emissivity, and the second longitudinal region has one or more of medium emissivity or high emissivity, with the high emissivity being greater than or equal to 0.2 of the low emissivity. Simple diagram description

[0007] A more detailed description of the invention, briefly summarized above, may be obtained by reference to the embodiments, some of which are illustrated in the accompanying drawings, so that the above-mentioned features of the invention may be understood in detail. It should be noted, however, that the accompanying drawings illustrate only typical embodiments of the invention and are therefore not to be considered limiting of the scope of the invention, as the invention may admit to other equally effective embodiments.

[0008] FIG1 illustrates a schematic cross-sectional view of a processing chamber according to one or more embodiments of the present invention;

[0009] Figure 2 shows an enlarged view of region II of Figure 1;

[0010] FIG3 illustrates a schematic top view of a heat shield according to one or more embodiments of the present invention;

[0011] FIG4 shows a schematic top view of the heat shield of FIG3; and

[0012] FIG. 5 illustrates a diagrammatic isometric view of a heat shield according to one or more embodiments of the present invention. Implementation Method

[0013] Before describing several exemplary embodiments of the present invention, it should be understood that the present invention is not limited to the details of the architecture or processing steps described in the following description. The present invention is capable of other embodiments and of being practiced or carried out in various ways.

[0014] As used in this specification and the accompanying claims, the term "substrate" refers to a surface or portion of a surface on which processing is performed. Unless the context clearly indicates otherwise, one skilled in the art will also understand that reference to a substrate may also refer to only a portion of the substrate. Furthermore, reference to deposition on a substrate may refer to both a bare substrate and a substrate having one or more films or features deposited or formed thereon.

[0015] As used herein, "substrate" refers to any substrate or material surface formed on a substrate upon which film processing is performed during the manufacturing process. For example, substrate surfaces upon which processing may be performed include materials such as silicon, silicon oxide, strained silicon, silicon-on-insulator (SOI), carbon-doped silicon oxide, amorphous silicon, doped silicon, germanium, gallium arsenide, glass, sapphire, and any other material, such as metals, metal nitrides, metal alloys, and other conductive materials, depending on the application. Substrates include, without limitation, semiconductor wafers. Substrates may be exposed to pretreatment processes to polish, etch, reduce, oxidize, hydroxylate, anneal, UV cure, electron beam cure, and / or bake the substrate surface. In addition to film processing directly on the substrate surface itself, any of the disclosed film processing steps may also be performed on underlying layers formed on the substrate, as described in greater detail below, and the term "substrate surface" is intended to include such underlying layers as the context dictates. Thus, for example, where a film / layer or portion of a film / layer has been deposited onto a substrate surface, the exposed surface of the newly deposited film / layer becomes the substrate surface.

[0016] As used in this specification and the accompanying claims, the terms "substrate support" and "substrate support pedestal" are used interchangeably.

[0017] As used in this specification and the appended claims, the use of relative terms such as "above" and "below" should not be construed to limit the scope of the invention to physical orientations in space. Thus, the use of relative terms should not be limited to directions indicated by gravity.

[0018] Embodiments of the present invention advantageously provide thermal shields for substrate supports and for processing chambers to reduce side-to-side variations in the temperature of a substrate support surface. Embodiments of the present invention advantageously provide processing chambers that consume less power than comparable processing chambers that do not include the thermal shields described herein.

[0019] The present invention provides methods for use in a single wafer processing chamber 100. FIG. 1 shows a schematic cross-sectional view of the processing chamber 100 according to one or more embodiments of the present invention. FIG. 2 shows an enlarged view of area II of the processing chamber 100 of FIG. The enlarged view of FIG. 2 shows a substrate support 200 and a thermal shield 300.

[0020] The processing chamber 100 has a chamber body 110 with a top wall 106 (i.e., a processing chamber lid), at least one side wall 104, and a bottom 102. The chamber body 110 and the top wall 106 define an interior volume 109. The processing chamber 100 has a gas injector 130 (also known as a gas distribution plate or showerhead). The type of gas injector 130 used will depend, for example, on the type of process being performed and the type of showerhead or gas injector. For example, a processing chamber 100 configured to operate as an atomic layer deposition apparatus may have a showerhead or vortex-type gas injector. A processing chamber 100 configured to operate as a plasma station may have one or more electrode and / or ground plate configurations to generate a plasma while allowing the plasma gas to flow toward a substrate.

[0021] In FIG. 1 , processing chamber 100 has a substrate support 200 within interior volume 109. Referring to FIG. 1 and FIG. 2 , substrate support 200 includes support rods 205, a support body 210 having a support surface 220 and a back surface 230 defining a thickness T SB of support body 210, and a heater (not shown) within thickness T SB of support body 210. The heater can be any suitable heater known to those skilled in the art, including, but not limited to, a resistive heater. In some embodiments, support body 210 comprises an electrostatic chuck and includes one or more electrodes for chuck- ing a wafer to support surface 220. In some embodiments, thickness T SB of support body 210 ranges from 0.25 inches to 1 inch.

[0022] In some embodiments, the substrate support 200 includes a support bracket 275 to support the heat shield 300. The support bracket 275 can be any suitable support mechanism known to those of ordinary skill in the art. In some embodiments, the support bracket 275 includes a separate body design that allows the support bracket 275 to be positioned around an existing support rod 205.

[0023] Without wishing to be bound by any particular theory of operation, power consumption is the amount of power required to heat the substrate support surface 220. The inventors have discovered that a processing chamber 100 having a thermal shield 300 as described herein consumes less power, e.g., power consumption is reduced by at least 10%, including at least 15%, or at least 20%, when compared to a processing chamber that does not include a thermal shield.

[0024] In the embodiment depicted in FIG. 1 , the processing chamber 100 has a slit valve 120 on the left and a pumping port 140 on the right. Those skilled in the art will recognize that the specific arrangement of the slit valve 120 and the pumping port 140 is merely exemplary and should not be construed as limiting the scope of the present invention. As used herein, the term "slit valve" refers to an area that can be opened or closed, configured to allow for the insertion or removal of a substrate.

[0025] The processing chamber 100 typically includes a vacuum source (not shown) connected to the exhaust port 105 for maintaining a reduced pressure state within the interior volume 109. The space between the support surface 220 of the support body 210 and the gas distribution plate 130 is referred to as the processing region 125. In some embodiments, a pumping port 140 is positioned adjacent to the processing region 125 to exhaust gas from the processing region 125. In some embodiments, the pumping port 140 is connected to a vacuum source (not shown) for exhaust purposes. The vacuum source connected to the pumping port 140 and the vacuum source connected to the exhaust port 105 can be the same source or different sources.

[0026] One or more embodiments of a heat shield 300 are depicted in Figures 2-5. The heat shield 300 includes a dish-shaped body 310 having a thickness T TS , a central axis 315 , and an outer diameter 320 . In some embodiments, the thickness T TS of the dish-shaped body 310 of the heat shield 300 ranges from 0.05 inches to 0.5 inches. When the heat shield 300 has a dish shape, one side of the dish is proximate to the slit valve 120 and the other side is proximate to the pumping port 140 . In embodiments where the slit valve 120 and the pumping port 140 are on opposite sides of the processing chamber, the heat shield 300 has a first edge 355 and a second edge 365 at opposite ends of the diameter D of the dish-shaped body 310 . In embodiments where the slit valve 120 and the pumping port 140 are spaced differently at angles other than 180° relative to the central axis 315 , the first edge 355 and the second edge 365 are located near the circumference of the disc-shaped body of each of the slit valve 120 and the pumping port 140 .

[0027] The dish-shaped body 310 may comprise any suitable material known to those skilled in the art. In some embodiments, the dish-shaped body 310 comprises aluminum. In some embodiments, the dish-shaped body 310 comprises stainless steel, a ceramic material, or a quartz material.

[0028] The dish-shaped body 310 has a front surface 330 and a rear surface 340 that define a thickness T TS of the dish-shaped body 310. In some embodiments, as shown in Figures 4 and 5 , the front surface 330 includes a first longitudinal region 350 and a second longitudinal region 360. The first longitudinal region 350 includes a first edge 355, and the second longitudinal region 360 includes a second edge 365. In one or more embodiments, the support body 210 includes a first edge 255 (i.e., a first side) and a second edge 265 (i.e., a second side). In some embodiments, the first edge 255 and the second edge 265 of the support body 210 are aligned with the first edge 355 and the second edge 365 of the dish-shaped body 310 of the heat shield 300.

[0029] The use of "first" and "second" in describing edges and sides of a component is used to indicate that the component, part, or area is distinct and should not be taken as limiting the scope of the present invention to any particular physical orientation. For example, in some embodiments, the first edge 355 is aligned with the slit valve 120, and in some embodiments, the second edge 365 is aligned with the pumping port 140. In some embodiments, the first edge 355 is aligned with the pumping port 140, and in some embodiments, the second edge 365 is aligned with the slit valve 120.

[0030] 4-5 , in some embodiments, the first longitudinal zone 350 extends a distance along the diameter D from the first edge 355 toward the central axis 315. In some embodiments, the second longitudinal zone 360 ​​extends a distance along the diameter D from the second edge 365 toward the central axis 315. In some embodiments, the first longitudinal zone 350 extends to the second longitudinal zone 360, forming a boundary line BL, which extends perpendicular to the diameter D. In some embodiments, the boundary line BL passes through the central axis 315 such that 50% of the surface area of ​​the front surface 330 includes the first longitudinal zone 350 and 50% of the surface area of ​​the front surface 330 includes the second longitudinal zone 360. In some embodiments, the boundary line BL intersects the diameter D at a point within 30% to 70% of the distance from the first edge 355 to the second edge 365.

[0031] The inventors have unexpectedly discovered that coating one or more of the first longitudinal zone 350 or the second longitudinal zone 360 ​​with materials having different emissivities reduces side-to-side temperature variations. Without wishing to be bound by any particular theory of operation, it is believed that emissivity can be controlled by modifying conditions on the surface, such as by applying a process or coating to the surface. In some embodiments, the first longitudinal zone 350 has one or more of low emissivity or medium emissivity. In some embodiments, the second longitudinal zone 360 ​​has one or more of medium emissivity or high emissivity.

[0032] Without wishing to be bound by any particular theory of operation, emissivity is the amount of thermal energy radiated from the surface of a material. In some embodiments, the emissivity of a material ranges from 0 to 1. In some embodiments, the emissivity material (i.e., emissivity) can be any suitable material known to those of ordinary skill in the art. In some embodiments, low emissivity ranges from 0 to 0.35, including less than or equal to 0.2. In some embodiments, low emissivity is approximately 0.15. In some embodiments, low emissivity includes a clean aluminum surface. In some embodiments, medium emissivity ranges from 0.36 to 0.65. In some embodiments, medium emissivity is approximately 0.6. In some embodiments, medium emissivity includes anodized aluminum surfaces. In some embodiments, high emissivity ranges from 0.66 to 1. In some embodiments, high emissivity is approximately 0.8. In some embodiments, high emissivity includes a black coating. In some embodiments, high emissivity is greater than low emissivity by at least 0.2.

[0033] The inventors have discovered that when the heat shield 300 is positioned a distance from a surface (e.g., a surface of a substrate support), each of the first longitudinal zone 350 and the second longitudinal zone 360 ​​having emissivity reduces side-to-side variations in the temperature of the surface (e.g., a surface of a substrate support).

[0034] Referring again to FIG. 2 , in some embodiments, the heat shield 300 is positioned a distance from the support body 210 such that a gap G exists between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300. The gap G between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300 can be adjusted by any suitable means known to those skilled in the art. In other words, the heat shield 300 can be positioned such that the front surface 330 of the heat shield 300 is closer to or further away from the rear surface 230 of the support body 210, thereby reducing or increasing the size of the gap G. Without wishing to be bound by any particular theory of operation, it is believed that as the gap G between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300 increases, the emissivity effect is reduced and the reduction in side-to-side temperature variation is reduced. In some embodiments, the gap G between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300 is less than or equal to 10 mm. In some embodiments, the gap G between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300 is 6 mm.

[0035] The inventors have discovered that each of the first longitudinal region 350 and the second longitudinal region 360 having emissivity reduces side-to-side variations in the temperature of the support surface 220 of the substrate support 200. In some embodiments, the temperature of the support surface 220 is measured along one or more of the first edge 255 or the second edge 265 of the support body 210. The temperature may be measured by any method or using any instrument known to those of ordinary skill in the art.

[0036] In some embodiments, the first longitudinal region 350 and the second longitudinal region 360 are configured to reduce side-to-side variations in the temperature of the support surface 220 .

[0037] In some embodiments, the first longitudinal zone 350 has an emissivity of 0.15 (e.g., low emissivity) at the first edge 355. In some embodiments, the second longitudinal zone 360 ​​has an emissivity of 0.8 (e.g., high emissivity) at the second edge 365. In some embodiments, when the first longitudinal zone 350 has an emissivity of 0.15 (e.g., low emissivity) at the first edge 355 and the second longitudinal zone 360 ​​has an emissivity of 0.8 (e.g., high emissivity) at the second edge 365, and the gap G between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300 is 6 mm, the temperature variation between the first edge 355 and the second edge 365 can range from 3°C to 4°C.

[0038] In some embodiments, the first longitudinal zone 350 has an emissivity of 0.15 (e.g., low emissivity) at the first edge 355. In some embodiments, the second longitudinal zone 360 ​​has an emissivity of 0.6 (e.g., medium emissivity) at the second edge 365. In some embodiments, when the first longitudinal zone 350 has an emissivity of 0.15 (e.g., low emissivity) at the first edge 355 and the second longitudinal zone 360 ​​has an emissivity of 0.6 (e.g., medium emissivity) at the second edge 365, and the gap G between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300 is 6 mm, the temperature variation between the first edge 355 and the second edge 365 can range from 2°C to 3°C.

[0039] In some embodiments, the first longitudinal zone 350 has an emissivity of 0.6 (e.g., medium emissivity) at the first edge 355. In some embodiments, the second longitudinal zone 360 ​​has an emissivity of 0.8 (e.g., high emissivity) at the second edge 365. In some embodiments, when the first longitudinal zone 350 has an emissivity of 0.6 (e.g., medium emissivity) at the first edge 355 and the second longitudinal zone 360 ​​has an emissivity of 0.8 (e.g., high emissivity) at the second edge 365, and the gap G between the rear surface 230 of the support body 210 and the front surface 330 of the heat shield 300 is 6 mm, the temperature variation between the first edge 355 and the second edge 365 can be less than or equal to 2°C, including less than or equal to 1°C.

[0040] Reference throughout this specification to "one embodiment," "some embodiments," "various embodiments," "one or more embodiments," or "an embodiment" means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of phrases such as "in one or more embodiments," "in some embodiments," "in various embodiments," "in one embodiment," or "in an embodiment" in various places throughout this specification are not necessarily referring to the same embodiment of the present invention. Furthermore, in one or more embodiments, the particular features, structures, materials, or characteristics may be combined in any suitable manner.

[0041] Although the present invention has been described herein with reference to specific embodiments, it will be understood that these embodiments are merely illustrative of the principles and applications of the invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the invention without departing from the spirit and scope of the invention. Therefore, it is intended that the invention encompass modifications and variations that come within the scope of the appended claims and their equivalents.

[0042] 100: Processing chamber 102: bottom 104: Sidewall 105: Exhaust port 106: Top wall 109: Internal volume 110: Chamber body 120: Slit valve 125: Processing Area 130:Gas Injector 140: Pumping port 200: substrate support 205:Support rod 210: Support body 220: Support surface 230: rear surface 255: First edge 265: Second Edge 275:Support bracket 300:Heat mask 310: disc-shaped body 315: Central Axis 320:Outer diameter 330: front surface 340: rear surface 350: first longitudinal zone 355: First Edge 360: Second longitudinal zone 365: The Second Edge BL:Borderline D: diameter G: Gap II: Region TSB:Thickness T TS:Thickness

[0043] Domestic storage information (please note the order of storage institution, date, and number) none Overseas deposit information (please note the order of deposit country, institution, date, and number) none

Claims

1. A heat shield comprising: a dish-shaped body having a thickness, a central axis, and an outer diameter having a first edge and a second edge at opposite ends of a diameter of the dish-shaped body, a front surface, and a rear surface, the front surface and the rear surface defining the thickness of the dish-shaped body, the front surface of the dish-shaped body including a first longitudinal region and a second longitudinal region, the first longitudinal region including the first edge and the second longitudinal region including the second edge, the first longitudinal region having one or more of a low emissivity or a medium emissivity, and the second longitudinal region having one or more of a medium emissivity or a high emissivity, the high emissivity being greater than or equal to 0.2 than the low emissivity.

2. The heat shield as described in claim 1, wherein the disc-shaped body comprises aluminum.

3. The heat shield as described in claim 2, wherein the low emissivity comprises a clean aluminum surface and the high emissivity comprises a black coating.

4. The heat shield as claimed in claim 3, wherein the first longitudinal region extends a distance from the first edge along the diameter toward the central axis, and the second longitudinal region extends a distance from the second edge along the diameter toward the central axis.

5. The thermal mask as claimed in claim 4, wherein the first longitudinal region extends to the second longitudinal region to form a boundary line that extends perpendicular to the diameter.

6. The heat shield as claimed in claim 5, wherein the boundary line travels through the central axis such that 50% of the surface area of ​​the front surface includes the first longitudinal region and 50% of the surface area of ​​the front surface includes the second longitudinal region.

7. The thermal mask as claimed in claim 5, wherein the boundary line intersects the diameter at a point within 30% to 70% of a distance from the first edge to the second edge.

8. The thermal shield as described in claim 3, wherein the low emissivity is in the range of 0 to 0.35 and the high emissivity is in the range of 0.66 to 1.

9. The thermal shield as claimed in claim 1, wherein the emissivity comprises an anodized aluminum surface.

10. A thermal shield as described in claim 9, wherein the emissivity is in the range of 0.36 to 0.

65.

11. The heat shield as claimed in claim 1, wherein a dimension of each of the first longitudinal region and the second longitudinal region is reduced by the side-to-side temperature variation of a surface located within 10 mm of the front surface.

12. The thermal mask as claimed in claim 1, wherein the first longitudinal region has an emissivity of 0.15 at the first edge and the second longitudinal region has an emissivity of 0.8 at the second edge.

13. The thermal mask as claimed in claim 1, wherein the first longitudinal region has an emissivity of 0.15 at the first edge and the second longitudinal region has an emissivity of 0.6 at the second edge.

14. The thermal mask as claimed in claim 1, wherein the first longitudinal region has an emissivity of 0.6 at the first edge and the second longitudinal region has an emissivity of 0.8 at the second edge.

15. A substrate support member, comprising: A support body having a support surface and a rear surface defining a thickness of the support body, and a heater within the thickness of the support body; and a heat shield comprising a dish-shaped body having a thickness, a central axis and an outer diameter having a first edge and a second edge at opposite ends of a diameter of the dish-shaped body, a front surface and a rear surface, the front surface and the rear surface defining the thickness of the dish-shaped body, the heat shield being positioned at a distance from the support body such that a gap exists between the rear surface of the support body and the front surface of the heat shield, the front surface of the dish-shaped body including a first longitudinal region and a second longitudinal region, the first longitudinal region including the first edge and the second longitudinal region including the second edge, the first longitudinal region having one or more of a low emissivity or a medium emissivity, and the second longitudinal region having one or more of a medium emissivity or a high emissivity, the high emissivity being greater than or equal to 0.2 than the low emissivity.

16. The substrate support as claimed in claim 15, wherein the low emissivity comprises a clean aluminum surface, the medium emissivity comprises an anodized aluminum surface, and the high emissivity comprises a black coating.

17. The substrate support as described in claim 15, wherein the gap is less than or equal to 10 mm.

18. The substrate support as described in claim 16, wherein the gap is 6 mm.

19. The substrate support as claimed in claim 15, wherein the first longitudinal region and the second longitudinal region are configured to reduce side-to-side temperature variation in the support surface.

20. A processing chamber, comprising: a chamber body having a top wall, a bottom wall, and a side wall and containing an internal volume; a substrate support member within the internal volume, the substrate support member including a support rod having a support body at one end of the support rod, the support body having a support surface and a rear surface defining a thickness of the support body, and a heater within the thickness of the support body; and a heat shield including a dish-shaped body having a thickness, a central axis, and an outer diameter, the outer diameter having a first edge and a second edge at opposite ends of a diameter of the dish-shaped body, a front surface, and a rear surface, the front surface and... The rear surface defines the thickness of the disc-shaped body. The heat shield is positioned along the support rod at a distance from the support body, such that there is a gap between the rear surface of the support body and the front surface of the heat shield. The front surface of the disc-shaped body includes a first longitudinal region and a second longitudinal region. The first longitudinal region includes the first edge and the second longitudinal region includes the second edge. The first longitudinal region has a low emissivity and the second longitudinal region has a high emissivity, which is greater than or equal to 0.2 than the low emissivity.

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