Furnace Muffle

Abstract

A furnace muffle operative to maintain a clean atmosphere over a wide temperature range. The muffle is formed of fused silica with a flat hearth adapted to directly support a moving conveyor. The muffle includes a downwardly extending V-shaped portion formed along the length of the hearth arranged to hold one or more temperature sensing thermocouples with a minimum of lead-length and to collect contaminants in an area removed from workload being processed.

Description

FIELD OF THE INVENTION

This invention relates to furnaces and more particularly to an improved muffle for moving conveyor muffle furnaces especially adapted to provide efficient heat transfer and to maintain a clean operating atmosphere over a wide range of operating temperatures.

BACKGROUND OF THE INVENTION

Muffle furnaces are widely employed in continuous heat-processing systems, such as in the fabrication of semiconductor and thick film products, in which workpieces are heated while being transported on a conveyor through the muffle. The muffle is generally formed in the shape of a long tube of ceramic or metal and is heated by heater elements arranged in the insulative firebrick to provide the requisite heating. Ceramic muffles are usually fabricated of a high-alumina refractory material having low temperature conductivity, low emmissivity and high mass. Such muffles exhibit considerable thermal inertia, with the result that temperature changes within the furnace cannot be accomplished as rapidly as desirable in many instances. For example, a workpiece traveling through the muffle at a predetermined rate may miss the effect of a temperature change due to the slow reaction time of the ceramic material to the change in temperature. Rapid and precise temperature control is, therefore, not easily accomplished by reason of the slow thermal characteristics of the muffle. A further problem is experienced with such ceramic muffles by reason of contaminants emitted from the material especially at extreme elevated temperatures. The contaminants may take the form of gases and fine dust particles which can markedly reduce the yield of precision products such as thick film circuits being processed in the furnace.

Muffles formed of metal such as Inconel have been employed in an attempt to reduce the effects of thermal inertia; however, the thermal characteristics of the metal are not always suitable for many purposes, and in addition, the metal employed can form contaminants at elevated temperatures. For example, in oxidizing atmospheres above 600.degree. C., metal oxides form which can cause contamination of workpieces being processed. The problem of oxide formation is especially critical at temperatures above 600.degree. C. and for thermal processing at temperatures above this level furnace muffles of conventional construction have generally been found unsatisfactory. The relatively poor thermal characteristics of ceramic and metal muffles detract from the overall thermal efficiency of the furnace in which such muffles are employed, since energy from the heating elements is transmitted by radiation to the outside of the muffle and is transmitted by conduction through the muffle and thence by radiation from the inner muffle surface to the workpieces being processed. Considerable energy is therefore wasted in heating the muffle itself before energy is transmitted to the workpieces.

SUMMARY OF THE INVENTION

Briefly the present invention provides a unique muffle for a conveyor furnace which is effectively transparent to thermal radiation over a wide range of operating temperatures and which maintains a clean operating atmosphere over this temperature range, including high elevated temperatures where muffles of conventional construction can become a major source of contamination. As a particular feature of the invention, the novel muffle construction permits the effective placement of thermocouples in positions to efficiently sense the temperature of workpieces passing therethrough with a minimum of lead length. In one embodiment of the invention the novel muffle construction permits thermocouple placement in a manner such that accurate temperature sensing is accomplished through thermally transparent windows in the muffle wall, with the result that a gas atmosphere can be maintained within a closed muffle structure. The novel muffle is formed of fused silica (often referred to as fused quartz) and is shaped as an elongated tube of generally rectangular cross section and reasonably uniform wall thickness. The lower interior surface of the muffle (hearth) is substantially flat and is adapted to support a moving conveyor belt thereon without need for ancillary mechanical support frames for transport of workpieces through the furnace, and includes an axially and outwardly extending V-shaped portion along the lower muffle surface. This outwardly extending portion is especially adapted to support a number of thermocouples in positions to accurately sense operating temperature, and is so constructed as to collect dust, oxides and other contaminants in an area removed from the work in process and in a position to permit easy removal and cleaning.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1, labeled PRIOR ART, is a cutaway pictorial view of a furnace muffle of conventional design;

FIG. 2 is a cutaway pictorial view of a mold useful in forming a muffle according to the invention;

FIG. 3 is a cutaway pictorial view of a furnace with a fused silica muffle constructed according to the invention;

FIG. 4 is a cutaway pictorial view of an alternative embodiment of a novel fused silica furnace muffle according to the invention; and

FIG. 5 is a fragmentary side view, partly in section, of a lower portion of the muffle shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

A moving conveyor furnace of conventional construction is illustrated in FIG. 1 (designated PRIOR ART) and includes an elongated arched roof fused silica muffle 10 supported within a suitably insulated structure, built up, for example, of firebrick 12 and having a plurality of customary electrical heating elements 14 disposed around the exterior of muffle 10. The electrical heating elements 14 are used to elevate the work passing through the furnace to the selected operating temperature, and in a long furnace may include several heating zones (not shown) which are controlled by thermocouple sensors to be described below. The use of a fused silica muffle 10 permits operation at extremely high temperatures as required in certain processes. As previously mentioned, one of the principal advantages of a fused silica muffle is that it is inert and clean and thus assists in achieving contamination-free processes.

As illustrated in FIG. 1, the base of the muffle 10 (hearth) includes a thicker central portion 16, that is, thicker than the ends of the muffle base and thicker than the sidewalls and arched roof. The reasons for this prior art construction will be discussed below.

To complete the furnace structure shown in FIG. 1, an endless conveyor belt 18 formed of woven stainless steel wire, chain-link or other suitable material, passes through the length of muffle 10 supported upon a generally V-shaped rectangular channel 20 which extends throughout the muffle 10. Due to the nature of the muffle cross section which includes thicker center portion 16, channel 20 is supported upon an appropriate plurality of brackets 22 and in this manner the conveyor 18 is maintained in relatively flat configuration so that workpieces being treated in the furnace will travel through the furnace in a plane as the conveyor advances from one end of the furnace to the other.

As previously mentioned, temperature control is provided for the furnace and for this purpose one or more thermocouple sensors 26 are disposed beneath the channel 20 centrally of the muffle. Each thermocouple 26 is placed beneath an opening 24 in the channel 20 and as illustrated is connected to axial lead wires which run the length of the muffle in auxiliary channels 28. Thus, if a large number of thermocouples are used along the length of the furnace, each of the channels 28 will contain a bundle of leads which at the ends of the furnace are connected to the heater power controls.

The prior art furnace construction just described in connection with FIG. 1 presents certain disadvantages which detract from the ultimate performance in the processing of precision products such as thick film circuits, integrated circuits and the like, despite the advantages otherwise achieved from the use of fused silica for the muffle 10. The principal limitation is the channel structure 20 which is not only costly, but since it is made of metal the conveyor belt and thermocouple lead wire support structure contributes substantially to contamination especially at elevated temperatures and thus tends to nullify the advantage achieved from the use of fused silica in the muffle itself. Also as is evident from FIG. 1, the thermocouple sensors 26 which are beneath the conveyor 18 and sense the temperature through openings 24 are in a position to be contaminated by the oxides, dust and other materials which tend to drop through opening 24 during operation of the furnace. Contamination which accumulates within the muffle is at all times in close proximity to the workpieces transported by conveyor belt 18 and oftentimes these products are extremely sensitive to contamination and are critically affected thereby. Contamination which falls on the thermocouple 26 can result in erroneous temperature sensing and power system heat control.

For an understanding of the limitations of these prior art fused silica muffle constructions, it is appropriate at this point to describe briefly the general manner in which these muffles are fabricated. A fused silica muffle is formed within a horizontal mold which has an internal cavity of the length and external configuration of the muffle desired. Thus, muffle 10 of FIG. 1 is formed within a mold (usually separate sections of cast iron) where the cavity is of the arched roof exterior configuration illustrated. A graphite painted high temperature heater rod, typically a Globar rod, is inserted into a "barrel" of sand whose axial length is equal to that of the mold cavity. By heating this rod slowly, a tube of liquid sand (or fused silica) of somewhat plastic consistency is formed over the surface of the rod while the remainder of the sand is in granular form. The heater rod and the hot fused silica surrounding it is then withdrawn from the barrel and the heater rod is in turn withdrawn to provide a hollow tube of soft fused silica. One end of this tube is pinched off and a blow pipe is inserted in the other. The tube is then placed into the cast iron mold and blown to extend and conform with the interior of the mold. Upon cooling, a fused silica muffle is obtained. This may be cut to any desired length for furnace use.

Examining the muffle fabrication process more closely, it may be seen that when the tube of soft fused silica is placed on the bottom of the mold, the lowermost outer surface of the tube will be flattened somewhat and then as the air pressure is increased to extend the tube to its proper shape the fused silica flows away from the central base portion leaving the latter thicker than the remaining walls. Contact between the fused silica tube and the mold causes chilling of the lower portion of the tube and corresponding increase in the viscosity of this portion. As a result, as the tube is blown, the lower viscosity material more readily flows toward the walls of the mold leaving a thicker central base portion. It is in this fabrication process that the muffle 10 shown in FIG. 1 with the bulge in the center of the base is obtained.

In accordance with the principles of the present invention, a fused silica muffle for furnace use is formed in a manner which substantially eliminates the problems discussed above in connection with prior designs and which provide significantly superior performance. In this regard, reference is made to FIG. 2 which shows in diagrammatic form the cast iron mold 30 which may be used in the fabrication of one embodiment of the novel fused silica muffle. As shown, the mold 30 is formed of a pair of mating separable sections defining an internal cavity 31 which is much like the configuration previously discussed in connection with FIG. 1 except that a V-shaped outwardly extending section 32 is provided in the base wall of the cavity. The base of the cavity is otherwise planar.

As shown in FIG. 2, an elongated tube 34 of molten silica is disposed within the mold cavity in contact with the shoulders of the V-shaped portion 32. Tube 34 is in a generally plastic state and air or other suitable gas of predetermined pressure is introduced to cause expansion into contact with the walls of cavity 31. The tube 34 is thus blown into a muffle configuration as indicated in the dotted outline 36 conforming to the mold cavity. During expansion of the tube 34 into engagement with the cavity interior surface, the fused silica flows outwardly into engagement with the cavity wall in such a manner that the wall thickness is generally uniform as shown with the exception of the region in the outermost vertex of the V-shaped section. The contact area between tube 34 and the mold cavity is substantially reduced compared with the prior art method described above, with the result that less chilling occurs and the viscosity of tube 34 is essentially uniform. Any wall thickening which may be produced occurs in the vertex of the V-shaped portion and does not interfere with the flatness of the muffle floor.

The novel fused silica muffle formed in the manner shown in FIG. 2 is embodied into a furnace as illustrated in FIG. 3 which in many respects is structurally the same as the prior type furnace shown in FIG. 1. Thus the muffle 38 is encased within a structure of insulating firebrick 40 and is surrounded by a plurality of customary heating elements 42. The furnace may have axial separate temperature zones each of which is individually controlled, but this has not been illustrated since the separation or division into zones does not form part of the present invention. The muffle base includes flat interior surfaces 44 and 46 adapted to support a moving conveyor 48 formed as mentioned earlier of woven stainless steel wire, chain-link or the like, but as will be observed the conveyor rides smoothly upon the generally flat interior surfaces and does not require auxiliary metal support. In the illustrated embodiment, the outwardly extended section 50 formed axially along the length of the muffle hearth is of generally V-shaped cross section this configuration being derived from the mold shown in FIG. 2. The V-shaped portion of the muffle shown in FIG. 3 is of such a width that it does not interfere with the flatness of the conveyor 48 which travels through the length of the muffle. In the drawing FIG. 3, the width of the V-shaped cross section has been exaggerated somewhat relative to the full width of the muffle base for purpose of clarity.

As illustrated in FIG. 3, the novel fused silica muffle is provided with an opening 51 through which the fused quartz tube 52 having an interior closed end is fitted. Tube 52 contains the thermocouple 53 whose leads 55 are suitably spaced by insulator 57. The leads 55 which extend beneath the muffle may be taken out through the side of the furnace through an appropriate insulating tube (not shown). It will be observed that the thermocouple 53 in its quartz tube 52 is appropriately positioned beneath the conveyor, and hence in a position to sense the temperature of the workpieces traveling through the furnace in a manner which is less susceptible to drafts through the muffle. The thermocouples may be conveniently removed by withdrawal from the openings 51 for servicing and most significantly, contamination such as that diagrammatically illustrated at 59 falling from the conveyor will drop into the vertex of the V and will not interfere with operation of the thermocouple. The number of thermocouples and hence the number of openings 51 which are provided in the wall of the V-shaped section will depend upon the needs of the particular process. These thermocouples may be used to maintain a uniform temperature throughout the length of the muffle or if the heaters are divided into zones, thermocouples may be used to establish different temperatures axially along the furnace.

As shown in FIG. 3 an opening 54 is provided in one of the inclined walls of the V-shaped section 50 and similar openings may be provided along the length of the muffle to assist in the cleanout of contaminants which accumulate through use. These contaminants may be blown out by compressed air piped through one or more openings 54 or conversely by suction through tubes inserted through one or more openings 54.

As is apparent from FIG. 3 the moving conveyor 48 is supported directly on the interior planar surface of the muffle hearth eliminating as mentioned earlier the need for cumbersome and costly metal-supporting structures; but apart from cost, the reduction of the amount of metal in the muffle significantly enhances the cleanliness of furnace operation and sharply lowers the contaminants which otherwise appear in the process.

The novel fused silica muffle described with reference to FIG. 3 is particularly suitable in use with furnace atmospheres of air or other gas where leakage from the muffle interior through the cleanout openings 54 and from thermocouple mounting holes 51 is not particularly critical. In some instances, however, it is necessary to employ a critical controlled gaseous atmosphere within the muffle and for such purposes openings communicating with the muffle exterior such as openings 51 and 54 cannot be employed.

An embodiment of the present invention especially suited to operation with critical gas atmospheres is shown in FIGS. 4 and 5. Here the muffle is generally rectangular but with essentially the same V-shaped outward extension and lower surface illustrated previously in connection with FIG. 3. The relative width of the V-shaped extension has been exaggerated for clarity but in a practical embodiment the width is sufficiently narrow to permit operation of the conveyor belt 48 in a horizontal plane. A transverse channel 60 is provided in the outermost portion of the vertex of V-shaped section 58 to accommodate each thermocouple sensor 70 (the structure of the sensor being essentially the same as illustrated and described with reference in FIG. 3). As will be observed, the channel 60, which may be formed by a high speed grinding wheel, is of rectangular cross section and is arranged to leave a thin-wall window 62 in the vertex; that is to say, the channel 60 does not breach the integrity of the muffle structure. The thin window 62 is sufficiently radiation transparent to permit the thermocouple to sense the temperature of the work passing above on the conveyor belt and thus permits processing control of furnace temperature. The muffle shown in FIG. 4 has a flat roof and except for the ends is completely sealed. Contaminants which fall into the base of the V section may from time to time be blown out by compressed gas introduced at an end of the muffle. The rectangular configuration presents a smaller cross section than the muffle 38 shown in FIG. 3. The advantage here is that due to the smaller cross section, higher gas velocity is attainable with minimum gas consumption.

From the foregoing it may be seen that the novel fused silica muffle structures illustrated and described herein provide an exceptionally clean environment. The temperature control may be efficiently maintained in one or more zones and specialized gaseous atmospheres may be used as required. The endless conveyor belt which transports the work from the furnace may be smoothly operated without auxiliary metal channels thus enhancing performance by reducing cost. Apart from the improved operation due to cleanliness and thermocouple placement, both versions of the novel muffle, illustrated in FIGS. 3 and 4, provide superior strength and stability by virtue of the rib structure resulting from the V-shaped outwardly extending sections.

Structural variations of the novel muffles illustrated herein may be made to adapt these to specialized operating environments.

Claims

What is claimed is:

1. A muffle for a conveyor furnace comprising:

an elongated unitary hollow member formed of fused silica;

a hearth integrally formed as the base wall of said member and having substantially flat coplanar surfaces adapted to directly support a moving conveyor thereon; and

a section centrally disposed along the length of said hearth between said coplanar surfaces and outwardly extending therefrom,

said section being adapted to support one or more thermocouple sensors thereon in efficient thermal sensing position.

2. A furnace muffle according to claim 1 wherein said outwardly extending section is adapted to collect along the length thereof contaminants which may be present within said muffle and includes one or more openings disposed in a wall of said section along the length thereof for removal of said contaminants.

3. A furnace muffle according to claim 1 wherein said outwardly extending section is of generally V-shaped cross section and said unitary hollow member has a substantially uniform wall thickness.

4. A furnace muffle according to claim 3 wherein a selected outermost region of said outwardly extending section includes a radiation transparent window adapted to transmit thermal energy within said member to a thermocouple sensor disposed adjacent said window.

5. A furnace muffle according to claim 3 wherein a selected outermost region of said outwardly extending section has a relatively thin wall operative to provide a radiation transparent window for transmission of thermal energy from within said member to a thermocouple sensor disposed adjacent said thin wall.

6. A furnace muffle according to claim 1 wherein said outwardly extending section includes one or more openings in a wall thereof for support of respective ones of said thermocouple sensors within said section and below said moving conveyor.