Patent Application
20060193366
1. Field of the Invention
Embodiments of the invention relate to the field of furnaces, and more specifically, to heating element structure in furnaces.
2. Description of Related Art
Furnaces typically use resistance wires as heating elements. Many applications using furnaces require the heaters to be responsive to temperature changes and maintain a uniform temperature over some time period. A resistance wire typically goes through many thermal cycles during its life. Resistance wires expand, grow, or elongate due to exposure to high temperatures over time. When these wires are held firmly by ceramic separators at some fixed points for mechanical stability, they may expand or elongate beyond these points, leading to premature failure or break.
Existing techniques to provide reliable wire heating elements have a number of drawbacks. One technique uses ceramic separators to restrain the wire heating elements and provide space in the separators for wire elongation. This technique requires using several separators that are embedded in the insulator layer, leading to assembly difficulty and increased cost. In addition, it limits the elongation within the separators. Another technique uses a number of anchors to secure the wires. The anchors are fit to anchor recesses having radial and retaining grooves. This technique requires using specially designed anchors and anchor recesses. It also confines the elongation to within the grooves.
Embodiments of invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
An embodiment of the present invention is a heating element structure. A first tray has an inner boundary and an outer boundary. The inner and outer boundaries define a space. A first heating element fit to the first tray and surrounding the inner boundary generates heat when power is applied. The heating element expands in the space within a temperature range.
In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown to avoid obscuring the understanding of this description.
One embodiment of the invention may be described as a process which is usually depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a program, a procedure, a method of manufacturing or fabrication, etc.
An embodiment of the invention is a heating element structure used in a furnace. The furnace may be positioned horizontally or vertically. The furnace includes a heating core. The heating core has a number of heating element structures stacked on each other. Each of the heating element structures includes a tray and a heating element placed around the tray. The tray may have a number of embodiments. In one embodiment, the tray includes a ring and a disk. The ring has a number of slots or holes to provide an efficient heat transfer when the heating element generates heat. The disk is attached to the ring and provides at least one of a horizontal space and a vertical space to allow expansion or elongation of the heating element when the heating element is exposed to high temperatures over time. The heating element structure provides an efficient heat generation and mechanical stability. By providing the space to allow growth of the heating element, the life of the heating element may be prolonged, avoiding premature failure. In addition, the heating element structures are simple to construct, allowing easy construction of the heating core and reducing assembly costs. The tray assembly also provides highly reliable mechanical support to the heating elements in the heating core.
The shield, or shell, 110 provides a housing or enclosure to house or enclose the heating core 130. It may be made of stainless steel. It may include a top ring 112 to shield the top of the heating core 130 and a side shield 114. Typically the shield 110 has a shape of a circular, oval, or elliptic cylinder. The shield 110 may have structures, parts, or elements to provide mechanical and electrical support for power bars and thermocouples.
The insulation layer 120 provides insulation for the heating core 130. The insulation layer 120 includes a top insulation layer 122 and a side insulation layer 124. The insulation layer 120 may be made of any material that is highly resistant to heat, has a low temperature expansion coefficient, has a low heat transfer coefficient, and maintains its properties over time. An example of such material is a mixture of aluminum oxide (Al2O3) and silicon dioxide or silica (SiO2). As is known by one skilled in the art, any other insulating materials having the above desirable characteristics may be used.
The heating core 130 provides heat generation to an object 135 placed inside the core. The object 135 may be any object, structure, element, or component that needs to be heated at some pre-defined temperature range. In one embodiment, the object 135 is a semiconductor wafer. The temperature range may be any suitable range as required, from 25° C. to 1700° C. For example, for semiconductor wafer applications, the temperature range may be between 500° C. to 1200° C. The heating core 130 has power bars to connect to the power source 160. The heating core 130 may provide heat to a number of zones inside the heating core 130. The heating zones may have different temperature ranges according to the requirements and specifications of the furnace. The power bars are allocated to correspond to the heating zones.
The cap 140 seals the heating core 130 at the top and provides an tight mechanical fit to the top ring 112 to reduce or minimize heat loss. The bottom ring 150 provides mechanical support for the heating core 130.
The power source 160 provides power to the heating core to generate heat when power is applied. The power source 160 is connected to the heating core 130 via the power bars. The power source 160 may have a power controller 165 that controls the amount of current and/or voltage to the heating core 130. By receiving different amounts of current or voltage via the individual power bars, the heating core 130 is able to generate different heat profiles in the corresponding heating zones.
The heating element structures 2101 to 210N are aligned and stacked on one another. In one embodiment, the furnace 100 is a vertical furnace. The heating element structures 2101 to 210N, therefore, are stacked in a vertical direction. For a vertical arrangement, consideration is taken to ensure that the heating element structures at the bottom of the heating core 130 (e.g., the heating element structures 210N and 210N-1) are designed to sustain the weight of all the heating element structures above them. The number N of the heating element structures vary according to the applications or the number of zones divided in the heating area.
In general, the heating element structures 2101 to 210N have similar shape and construction style. Therefore, the construction and assembly of the heating core 130 are greatly simplified. The heating element structures 210k (k=1, . . . , N) includes a tray 220k and a heating element 230k that fits to the tray 220k. The heating element 230k generates heat when power is applied. Due to constant heating, the heating element 230k may expand or elongate over time. The construction of the tray 220k allows a space 242k in the vertical dimension and a space 250k in the horizontal dimension to accommodate the expansion or elongation of the heating element 230k. This prevents the deformation of or structural and/or mechanical damage to the heating element 230k that may reduce the life of the heating element 230k.
The tray 220 typically has a circular, oval, or elliptical shape that fits the inside of the housing provided by the shield 110 and the insulation layer 120 shown in
The heating element 230 is fit to the tray 220 and surrounds the inner boundary 312 to generate heat when power is applied. The heating element 230 may expand or elongate in at least one of the spaces 240 and 242 within a temperature range. In essence, the spaces 240 and 242 allow expansion in both horizontal and vertical directions, or in three-dimensional space. The heating element 230 may be a wire having a wire shape. The wire shape may be one of a helical shape, a solid shape, and a flat shape. The helical shape provides a large surface area for high wattage without high watt density. For heating applications, the wire 230 may expand or elongate over time. It may not return to its original length after being exposed to high temperatures causing a continuous growth with temperature cycling. The spaces 240 and 242 provide sufficient room for the wire 230 to grow or expand over time. This prevents premature failure of the wire 230. The heating element 230 may be made of any suitable material for heat generation. Commonly used material may be Kanthal, Nikrothal, Super-Kanthal, Molybdenum Discilicide, etc. It may have a coefficient of thermal expansion of approximately 15E-6[K−1] at 1000° C.
The ring 410 defines the inner boundary 312 and has a ring height 414. It has a plurality of slots, or holes, 412 spaced around the inner boundary 312. Typically, the slots 412 are spaced at equal distances to provide uniform heat transfer. The heating element 230 is placed around, or surrounds, the plurality of slots 412. For clarity, the heating element 230 is only shown partially in
The disk 420 is attached to the ring 410 near or at the inner boundary 312. It may be attached to close to the inner or outer diameters 416 or 418 of the ring 410. It has or defines the outer boundary 316. The surface of the disk 420 essentially defines the space 330 between the inner boundary 312 and the outer boundary 316. The disk has a recess 422 so that a post or a bar may be inserted to align the stacked trays. In addition, the recess 422 may define the end of the heating element 230 where a connecting bar or a power bar may be connected. The disk 420 may have two recesses 422 on two sides of its diameter to provide two alignment points when the trays are stacked on one another. The disk may also be made of aluminum oxide or silica.
The ring 410 and the disk 420 may be attached together through any attachment mechanism such as welding or gluing. Alternatively, both may be integral to form a single-piece as the tray 220.
There may be several embodiments to form the ring 410. The purpose is to create a number of guides or posts around the inner boundary 312 and a number of slots or holes to provide efficient heat transfer from the heating element 230 to the core when power is applied.
The heating element 320k in the heating element structure 210k is connected to the heating element 320k+1 in the heating element structure 210k+1 by a connecting bar 510k. The connecting bar 510k is attached, or welded, to the end of the heating element 320k at the recess 422k and to the end of the heating element 320k+1 at the recess 422k+1. Similarly, a power bar 520j is attached, or welded, to the end of the heating element 320j at the recess 422j and to the end of the heating element 320j+1 at the recess 422j+1. The power bar 520j serves dual purposes. One is to connect the two heating elements together like the connecting bar 510k. One is to provide a terminal to connect to the power source 160 (
The heating elements 3201 to 320N are, therefore, connected in a zigzag pattern from the top tray to the bottom tray, forming an electrically continuous wire. Power bars 520j's are provided at selected heating zones to provide proper electrical power for different amounts of heat to be generated.
Upon START, the process 600 forms a first tray having an inner boundary and an outer boundary (Block 610). The inner and outer boundaries define a space. Next, the process 600 fits or places a first heating element to the first tray to surround the inner boundary (Block 620). The first heating element and the height of the first tray defines a vertical space above the first heating element. The first heating element generates heat when power is applied. It may expand, elongate, or deform in the space within a temperature range during its life.
Then, the process 600 attaches a connecting bar or a power bar to a first end of the first heating element at the post to connect the first heating element to a second heating element in a second tray (Block 630). The power bar is connected to a power source. The second tray is stacked above or below the first tray. The process 600 is then terminated.
Upon START, the process 610 forms a ring defining the inner boundary (Block 710). The ring has a plurality of slots spaced around the inner boundary. The ring has a ring height. The ring height is fit to the size or diameter of the heating elements in addition to a vertical space, if necessary. The plurality of slots provides heat transfer to the core when the heating element generates heat.
Next, the process 610 attaches a disk to the ring at the inner boundary (Block 720). The disk defines the outer boundary. It has a recess to fit a post. The process 610 is then terminated. The disk and the ring may be attached, glued, or welded together from two separate pieces. Alternatively, they may be constructed integrally to form a single piece.
The process 610 may be modified to form the tray in accordance to
The top insulation ring 810, the side insulation ring 820, and the outer shell 830 are similar to the corresponding parts of the system 100 shown in
The heating core 840 has specific grooves machined or formed into the outer diameter of the core cylinder in a spiral or parallel shape. The core 840 may be in the shape of a cylinder, polygon or rectangle, or any suitable shape. The depth of the groove is determined by the shape and size of the heating element and the space to allow for expansion.
The formed channel 910 is formed into the outer diameter of the core cylinder in a spiral or parallel shape. It may be implemented by a specific machining procedure. The formed channel 910 has slots (or holes) 920 that are made at equally distant spaces in the center of the groove. The purpose for the slots 920 is to allow for quick heat transfer from the heating element to the process area, at the center of the heater. The equally distant sections between the slots 920 provide for two functions; first, to support and preserve the original shape of the cylinder; and second, to serve as a barrier to the heating element should it try to move out of its location.
The formed channel 910 has groves 915 that are designed specifically for the shape of heating element 1010. The groove 915 provides a space 1020 to allow for the expansion or growth of the heating element 1010. The channel 910 also supports the heating element 1010 against gravitational forces. The heating structure is therefore mechanically stable.
The heating element 1010 is essentially similar to the heating element 230 shown in
Within the channel, the space 1020 is provided to allow for the expansion, elongation, or movement of the heating element 1010 at high temperatures. The size of the space 1020 may be similar to the size of the space 240 shown in
Placement of parallel channels or one channel in shape of a spiral allows for the uniform and continuous placement of the heating element 1010 throughout its length. This maintains a uniform temperature throughout the heater and more specifically, the area located in the center called “flat zone”. The flat zone is where the semiconductor product will be processed, and the uniformity of the temperature impacts the semiconductor product performance. This area is typically controlled to a tolerance of +/−0.1-0.25 degrees C., depending on the application. The ability to control the resistance wire spacing accurately in the fashion explained above contributes greatly to achieving the required temperature uniformity.
The manufacturing process continues by placing supporting insulation rings at the two ends of the heater element, adjacent to the core of the heater. Stainless steel rings are then welded at the two ends, finalizing the structure of the heating element. Following this step, the necessary parts are added to the outer stainless steel shell for the support of terminal bars, providing the connection to power lines, and the support of thermocouples. The final step is to place the warning labels on the heater.
As in the embodiment shown in
While the invention has been described in terms of several embodiments, those of ordinary skill in the art will recognize that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.
This patent application claims the benefits of U.S. Provisional Application, titled “Heating Element With Efficient Heat Generation And Mechanical Stability”, Ser. No. 60/646,383, filing date Jan. 24, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60646383 | Jan 2005 | US |
