Patent Application
20100193505
The present disclosure relates to heating elements. More particularly, the present disclosure relates to strip heating elements for furnaces, e.g., semiconductor processing furnaces, that have a circuitous path including straight and radiused segments that advantageously accommodates thermal expansion and contraction.
In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Conventional heating elements are generally formed of wire or sheet metal of various designs and geometries. However, wire patterned elements are generally limited in operating temperature by virtue of being embedded or semi-embedded in a surrounding medium, such as insulation. Further, wire patterned elements are typically not precision formed, are highly labor intensive and have a medium ratio of surface to mass resulting in fast heating and cooling. For sheet metal heating elements, those formed as primarily square patterns suffer from non-uniformity, while those with continuously curving patterns produce high stresses, both effects being more pronounced when the heating element expands at operating temperatures.
A substantially uniformly radiating and substantially stress free heating element, even at operating temperatures, would be advantageous. Such a heating element can be included in a furnace to improve processing of items. For example such a heating element can be included in a semiconductor processing furnace for the processing of semiconductor wafers.
An exemplary heating element comprises a continuous planar strip, wherein a path of the continuous strip from a first end to a second end is circuitous and includes a plurality of repeating cycles, each repeating cycle including a plurality of first straight segments, a plurality of second straight segments and a plurality of radiused segments, wherein a length of the first straight segment is greater than a length of the second straight segment, and wherein an angular sum of a single cycle of the circuitous path is greater than 360 degrees.
An exemplary embodiment of a heating assembly comprises the heating element mounted in spaced relation to the insulating substrate by a plurality of mounting members.
An exemplary method of manufacturing a heating assembly comprises forming a heating element body from a resistance alloy, the heating element body including a continuous planar strip with an emitting surface and a plurality of mounting members, bending the plurality of mounting members out of plane relative to the continuous strip, and inserting the plurality of mounting members into a substrate until an integrated spacer on the mounting members contacts the substrate, wherein a path of the continuous strip from a first end to a second end is circuitous and includes a plurality of repeating cycles, each repeating cycle including a plurality of non-parallel first straight segments, a plurality of second straight segments and a plurality of radiused segments, wherein a length of the first straight segment is greater than a length of the second straight segment, and wherein an angular sum of a single cycle of the circuitous path is greater than 360 degrees.
Another exemplary method of manufacturing a heating assembly comprises forming a heating element body from a resistance alloy, the heating element body including a continuous planar strip with an emitting surface, and inserting a plurality of mounting members through an opening integrally formed on the continuous strip and into a substrate until a spacer associated with the mounting members contacts the substrate, wherein a path of the continuous strip from a first end to a second end is circuitous and includes a plurality of repeating cycles, each repeating cycle including a plurality of non-parallel first straight segments, a plurality of second straight segments and a plurality of radiused segments, wherein a length of the first straight segment is greater than a length of the second straight segment, and wherein an angular sum of a single cycle of the circuitous path is greater than 360 degrees.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The following detailed description can be read in connection with the accompanying drawings in which like numerals designate like elements and in which:
An exemplary embodiment of a heating element 10 comprises a continuous planar strip 12 and a plurality of mounting members 14. A path of the continuous strip 12 from a first end 16 to a second end 18 is circuitous and includes a plurality of repeating cycles 20. Each repeating cycle 20 includes a plurality of non-parallel first straight segments 22, a plurality of second straight segments 24 and a plurality of radiused segments 26. An angular sum of a single cycle of the circuitous path is greater than 360 degrees.
The heating element 10 has an emitting surface 30 that generally extends in and generally is contained in a first plane. Within this first plane, the plurality of first straight segments 22 are oriented generally laterally to an axis 32 oriented from the first end 34 of the heating element 10 to a second end 36 of the heating element 10, e.g., within ±15 degrees of perpendicular to the axis 32. The plurality of second straight segments 24 are oriented generally longitudinally to the axis 32, e.g., within ±15 degrees of parallel to the axis 32. In an exemplary embodiment, any two consecutive first straight segments are generally (within ±15 degrees, alternatively within ±5 degrees) non-parallel and any two consecutive second straight segments are generally (within ±15 degrees, alternatively within ±5 degrees) parallel. Alternatively, any two consecutive first straight segments 22 are strictly non-parallel and/or any two consecutive any two consecutive second straight segments 24 are strictly parallel. The axis 32 is conventionally oriented in the X-axis direction.
In an exemplary embodiment, a single cycle 20 of the circuitous path includes two first straight segments 22, two second straight segments 24 and four radiused segments 26. The single cycle 20 includes two lobes 38. Each lobe 38 includes two radiused segments 26 and one second straight segment 24. The one second straight segment 24 separates the two radiused segments 26.
The radiused segment can take any suitable form.
The radiused segments 26 in both the pseudo-sinusoidal and the pseudo-square form of the heating element 10 have both an interior radius r1 and an exterior radius R2. Each radiused segment 26 has an associated angle α that represents the angular change in direction of the circuitous path over the length Lr of the radiused segment 26. With regard to individual radiused segments 24, exemplary embodiments of the radiused segments have an angle α that is between 90 degrees and 135 degrees, i.e., 90°<α<135°, alternatively between 90 degrees and 100 degrees, i.e., 90°<α<100°,
In exemplary embodiments, an angular sum β of the angles α of one lobe 38 is greater than 180 degrees, preferably greater than 180 degrees to about 200 degrees, more preferably about 185 to about 190 degrees. For example, the angular sump of a lobe 38 can be expressed as:
β=Σαn
where n=number of radiused segments in the lobe. As each cycle includes two lobes, an angular sum of the angles α associated with a single cycle of the circuitous path is greater than 360 degrees, preferably greater than 360 degrees to about 400 degrees, more preferably about 370 to about 380 degrees.
The angular sum β greater than 180 degrees results in the two first straight segments 22 adjacent the lobe 38 being non-parallel. This non-parallel relationship can be seen in both
The circuitous path of the heating element 10 can be idealized as a line 50 located at a centerline of the planar heating element 10.
It can be understood by one skilled in the arts that the uniformity of power dissipation of an emitter is higher for a homogenous conductor of uniform cross-section and surface area. It is therefore desirable to maximize the ratio of the length of straight segments to the length of curved segments. It has been determined empirically that the following relationship yields a result with high uniformity, high fill-factor (ratio of substrate surface power to emitter surface power) and minimizes stress in the emitter. Furthermore, this relationship accommodates and controls expansion during transient conditions and over the useful life of the heating element.
In exemplary embodiments, the lengths of the first straight segments 22, the second straight segments 24 and radiused segments 26 in a single cycle are such that they satisfy the following relationship:
where L1.A is the length L1 of a first first straight segment 22, L1.B is the length L1 of a second first straight segment 22, L2,A is the length L2 of a first second straight segment 24, L2,B is the length L2 of a second second straight segment 24, Lr,a is the length Lr of a first radiused segment 26 and Lr,b is the length Lr of a second radiused segment 26. Alternatively, the relationship above is greater than 2.2, further from greater than 2.2. to less than 10.0 or less than 5.0. This relationship represents the ratio of the length of straight segments to the length of radiused segments. For a uniform width of the emitter surface, this is also the ratio of surface areas of straight segments to radiused segments. An example of a suitable width is 8 mm. The length is measured at the center of the emitter path, i.e., along line 50.
The heating element 10 includes a plurality of mounting members 14. The mounting members 14 extend from the periphery 60 of the continuous strip 12 at a plurality of locations along the circuitous path. The mounting members can be located at any suitable position. In one embodiment and as shown in
As shown in
In a first embodiment, the mounting members 14 include a base end 62 and a distal end 64 and have an integrated spacer 66 at the base end 62. A length the integrated spacer 66 extends from the base end 62 defines a stand-off distance for the continuous strip 12 when the heating element 10 is mounted to a substrate 102. A washer 68 or other plane surface can be optionally included to prevent the integrated spacer 66 from embedding into the insulation 102.
As shown in
As used herein, the second plane is different from the first plane in which the emitting surface 30 generally extends and generally is contained. As an example, the first plane is oriented substantially consistent with an XY-plane and the second plane is substantially consistent with a YZ-plane or a XZ-plane of a right-handed, three-dimensional Cartesian coordinate system.
In a further exemplary embodiment, a combination of the integrated mounting members, such as mounting members 14 shown in
The heating element 10 comprises a power terminal 110 at the first end 34 or the second end 36 of the heating element 10. In alternative embodiments, the power terminals are located at locations other than the first end or second end. The power terminal connects to an electrical circuit of, for example, a semiconductor processing furnace.
The heating element 10 can be formed from any suitable material. For example, in an exemplary embodiment, the heating element 10 is formed from a resistance alloy, preferably an iron chromium aluminum alloy. Other suitable resistance alloys include nickel chromium or ceramic alloys, such as molybdenum disilicide or silicon carbide. The resistance alloy can be formed into a heating element body by, for example, cutting the heating element body from a sheet of material, casting a heating element body, machining a heating element body, extruding, pressing, punching or canning a heating element body, or combinations of such methods.
Embodiments of the disclosed heating element and heating assembly provide several advantages, either singly or in combination. For example, the pseudo-sinusoidal and pseudo-square patterns comprise a substantial portion of straight segments with substantially uniform width yielding highly uniform surface temperatures.
In the pseudo-sinusoidal pattern, the difference between the highest and the lowest temperature (ΔT) along the emitting surface is about 8° C. (
Heating elements disclosed herein can be incorporated into a furnace, such as a furnace for processing semiconductors. In such an application, multiple heating elements are positioned in an array or zone and are controlled for heating by a temperature control circuit.
The radii of the radiused segments are sized to maximize temperature uniformity and minimize stress. The inside radii have a particular low stress when compared to the conventional designs consisting primarily of curved sections. Further, the uniformity of surface temperatures is much improved relative to conventional designs consisting of square patterns with little or no radii at the corners.
The heating elements disclosed herein have a high surface loading factor, also known as a fill factor. Here, the pseudo-square and pseudo-sinusoidal heating elements have more emitter surface area than conventional designs consisting primarily of curved sections. This is, at least in part, because the angular sum of a single cycle is greater than 360 degrees. This puts the first straight segments in non-parallel relationship with a resulting longer length than parallel segments, and therefore, more emitting surface. Further, the distance D2 is minimized while the distance D1 is varied to accommodate the length (L2) of the first straight segment 22. This contributes to a high fill factor while increasing temperature uniformity and lowering stress in the lobes 38. An example of a typical surface loading of total active area is approximately 145% of the emitter loading.
The heating elements disclosed herein contribute to controlling thermal expansion effects. Materials forming the heating element expand upon heating proportional to the coefficient of thermal expansion of the material. This expansion can cause the heating element to flex and bend, resulting in the emitter surface having a variable position relative to a piece to be heated (and, therefore, making the temperature profile more non-uniform). In extreme situations, the flexing and bending can result in short circuiting. The heating elements disclosed herein control and minimize the effects the thermal expansion. For example, the non-parallel orientation of the first straight segments direct a portion of the thermal expansion in the lateral direction into the longitudinal direction, and therefore maintains the orientation relative to the piece to be heated and a more uniform temperature profile. In another example, the edges of the pattern can be curved or bent along the lateral axis in order to direct the thermal expansion toward the insulating substrate to permit placement of the heating element closer to adjacent objects, such as additional circuits, with reduced instance of short circuiting.
The use of mounting means with a stand-off distance can also contribute to improved performance. Alternating support locations along the length of the circuitous path allows thermal expansion of the heating element to be directed into a twisting or torsional movement of the heating element between the supports and not just in planar movement.
The heating elements disclosed herein are free-radiating. That is, the mounting members provide a stand-off distance for the heating element relative to the insulating substrate. The architecture allows for heat to emit from all sides evenly and without the use of extra electrical energy to compensate for, for example, heating a substrate in surface contact to the heating element. Thus, the free-radiating heating element lowers the operating temperature of the emitter. Further, such a heating element will have a longer life at the same substrate power density as conventional heating elements or, alternatively, can operate at higher power densities over comparably life times.
The disclosed embodiments result in a high performance heating element combining low mass and high surface area. The disclosed patterns enable a high degree of automation in the fabrication and assembly process and precise geometries yielding uniform heating and consistent performance.
Several variations of the heating element can be made. For example, the heating element can have different thicknesses, varying, for example, from about 0.5 mm to about 10 mm. Also for example, the heating element can have different widths, based on the width of the straight segments, varying, for example, from about 5 mm or longer. These variations in width and thickness can be suitably incorporated as long as the basic features of the geometry are maintained, i.e., the non-parallel first straight segments and a substantially straight overall pattern.
While the drawing figures disclose embodiments that are substantially planar in configuration, it will be appreciated by those skilled in that art that the disclosed geometries can be applied to assemblies that have curved surfaces such as cylinders or semi-cylinders. The variation that incorporates the separate mounting means specifically lends itself to those configurations by allowing the emitter to be appropriately formed to conform to the curved surface, and then fixed in place by the separate mounting means. Curved surface configurations may also be constructed by arranging the emitter segments so that they run along the axial length of the curved surface, or by approximating the desired curved geometry with a series of planar panels.
Although described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 61/202,206, filed Feb. 5, 2009, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61202206 | Feb 2009 | US |
