Precise thermal processing apparatus

Disclosed is apparatus for subjecting a workpiece to a controlled temperature comprising a light-weight muffle of high diffusivity material having an interior high emissivity surface, an air cooled envelope surrounding the muffle having an infrared reflective interior surface, and a housing which maintains a vacuum in the space outside of the muffle. Heaters and temperature sensors are spaced about the muffle. The heaters are controlled by an electronic feedback control system which actuates the heaters in response to signals received from the temperature sensors. The apparatus is ideally suited for use in the controlled diffusion processing techniques employed in the fabrication of electronic devices because it applies on the order of 90% of the generated heat to the workpiece, can be rapidly heated up and cooled, and can maintain a selected temperature within narrow limits.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for processing materials at elevated 
temperatures characterized by efficient use of energy and precise 
temperature control. 
In many industrial processes involving the heat treatment of metals, the 
material must be heated to a selected high temperature and maintained at 
that temperature for a prescribed time. Sensitive thermochemical 
processing techniques such as those involving controlled diffusion in 
semi-conductor materials necessitate very precise temperature control. For 
example, the processing of electronic materials for gas discharge displays 
typically requires a heat treatment step wherein the temperature of the 
material must be kept within about .+-.1.degree. C. Failure to attain this 
precision results in a loss of quality control in the diffusion process 
and in insufficient reliability to afford the yields required in 
successful commercial production. The more precisely the temperature of a 
processing furnace can be maintained, the more reliable the thermal 
processing and the higher the yield of products. Thus, although state of 
the art furnaces afford control in the range of 5.degree. C. to 
1.0.degree. C., further improvements enabling temperature control on the 
order of 10.sup.-1 .degree. C. to 10.sup.-3 .degree.C., would be of 
significant commercial value. 
Currently available diffusion furnaces rely for precise operation on the 
presence of a large quantity of stored heat which acts as a "thermal 
buffer". These furnaces comprise an array of massive ceramic insulating 
blocks which surround a hollow muffle, typically made of stainless steel, 
which in turn encloses the material to be processed. The blocks are heated 
by thermostatically controlled resistance heaters (or similar devices). 
They tend to establish thermal equilibrium with the muffle by transferring 
heat thereto via radiation, convection, and conduction. The interior 
surface of the muffle then radiates to the material being processed. 
The main advantage of the "thermal mass" type diffusion furnace described 
above is that it affords an inexpensive means of controlling temperature, 
generally within limits of .+-.1.degree. C. Such precision is attainable 
because the large amount of heat stored in the ceramic mass acts as a 
thermal buffer to moderate effects tending to alter the thermal 
environment of the muffle itself. However, this approach to furnace 
construction can rarely provide levels of control more refined than about 
.+-.1.degree. C. This is because the heat is stored as "sensible heat". 
When called upon to buffer the effects of a thermal disturbance, the hot 
ceramic mass either absorbs or gives up some of the stored heat. This of 
course necessarily results in a change of temperature of the ceramic 
itself, and thus of the muffle and workpiece, but because of the quantity 
of heat present in the ceramic mass, a large external loss or gain results 
in only a small change in the temperature of the muffle or the workpiece. 
Thus, the thermal storage approach reduces the effect of external 
disturbances of the thermal environment, but cannot eliminate these 
effects. 
From the foregoing it is obvious that the larger the stored thermal mass, 
the less a given heat gain or loss will influence the temperature of the 
muffle. However, capital costs and requirements of efficient energy use 
effectively prohibit very large furnaces from being economical, because 
once a thermal mass furnace has attained a given steady-state operating 
condition, it must be run substantially continuously, irrespective of the 
frequency of its use. It can be shut down for maintenance only rarely. The 
reason for this is that the time necessary to obtain steady-state 
operating conditions is generally too long to allow the unit to be cycled. 
Furthermore, cycling creates the risk of losing the desired thermal 
condition, and with it part or all of a day's production. In practice, 
these furnaces accordingly remain energized twenty-four hours a day, seven 
days a week. 
To control the temperature of the workpieces contained within a thermal 
mass furnace, it is necessary and only necessary to regulate the 
temperature of the muffle. Thus, the temperature distribution on the 
muffle completely determines all thermal aspects of the processing steps. 
At the temperature required in the processes discussed above (typically 
500.degree. C. or more), by far the most important mechanism of heat 
transfer is radiation. At 500.degree. C., the radiant heat flux from a 
stainless steel surface having an emissivity of 0.9 will be approximately 
6,000 BTU/hr. ft.sup.2. If this material is radiating to a background of 
high emissivity (e.g., greater than 0.7) at, for example, 35.degree. C., 
the return of radiation from the background to the surface will be 
negligible.Since natural convection can remove only about 1,000 BTU/hr. 
ft.sup.2 from steel, at least about 88% of the heat loss from steel occurs 
by radiation at temperatures of 500.degree. C. or above. Accordingly, in 
the context of the diffusion furnace, both natural convection and 
conduction can be neglected as an insignificant method of heat transfer 
between the muffle and workpiece when compared with the rapid and 
efficient radiative transfer. 
Thus, since the most important aspect of thermal interaction between the 
muffle and the material being processed is radiation, control of this 
mechanism of heat exchange is most important. Similarly, the most 
important mechanism of heat loss from the muffle is radiation, and again, 
radiation is the most important mechanism to control. (Convection within 
the muffle must, of course, be controlled to prevent local variations of 
temperature. This is done by conventional means.) 
SUMMARY OF THE INVENTION 
The instant invention provides a novel type of apparatus for maintaining a 
workpiece at a selected precise temperature. The apparatus comprises a 
light weight muffle of material having a high thermal diffusivity and an 
interior surface characterized by high emissivity. An envelope having an 
interior surface that is reflective to the infrared completely encircles 
the muffle. A housing surrounds the envelope and contains a vacuum in the 
space between the muffle and housing. Heater means and temperature sensing 
means are placed in thermal communication with the muffle; electronic 
means is provided for controlling the heat output of the heaters in 
response to signals received from the temperature sensors. 
With the furnace construction of the invention, the muffle's convective or 
conductive heat losses (which can represent as much as 12% of its total 
heat loss) are eliminated; its radiant environment is controlled with the 
reflective envelope. The electronic controls, light-weight muffle of high 
thermal diffusivity and infrared reflective envelope are used in 
combination to enable one to maintain a selected muffle temperature with 
high precision. In general, the muffle temperature may be controlled at 
least as precisely as in the thermal storage type furnaces described 
above, and certain embodiments of the apparatus of the invention are able 
to maintain a temperature with a precision equal to or greater than 
.+-.10.sup.-2 .degree. C. 
In preferred embodiments, the muffle is made of stainless steel and is less 
than about 0.1 inch thick. Optimally, the muffle should be just strong 
enough to withstand the vacuum applied to its exterior and the 
temperatures to which it is subjected. A conduit communicates between the 
interior of the muffle and the exterior of the housing for providing 
atmospheres of controlled composition and pressure to the workpiece 
holding space within the muffle. The heater means preferably comprises one 
or more electrical resistance heaters fixed to or made an integral part of 
the exterior surface of the muffle. The preferred temperature sensing 
means comprise a plurality of thermocouples spaced about the muffle. 
Since no reflective surface has a reflectivity of 100 percent, a certain 
fraction (typically less than 2%) of the IR radiation impinging on the 
envelope will be absorbed as heat. Thus, the envelope must be cooled. This 
can be done by providing a thermal conductive link between the envelope 
and the housing, for example, by using the housing as the substrate for 
the infrared reflective surface. Alternatively, the envelope may be cooled 
by providing it with an emissive outside surface and providing an emissive 
interior surface on the housing, thereby providing radiative coupling 
between the envelope and housing. The preferred cooling means comprises 
cooling coils mounted in thermal communication with the body of the 
envelope. 
It is accordingly an object of the invention to provide a furnace for 
maintaining a workpiece at a selected precise temperature which is energy 
efficient and capable of more precise control than is presently possible 
in the known thermal storage type furnace. Other objects are to provide a 
furnace having a muffle isolated such that heat loss by convection or 
conduction is minimized or eliminated and to provide a furnace 
characterized by low thermal inertia and essentially zero heat storage. 
Another related object is to provide a furnace which may be turned off 
when not in use for a short period, and later turned back on and brought 
to operating conditions in a short time. 
These and other objects and features of the invention will be apparent from 
the following description of some preferred embodiments and from the 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The apparatus of the invention is capable of maintaining a workpiece at a 
given precise temperature and of heating a workpiece at a controlled rate 
because of the combination of three fundamental construction features. 
Specifically, the construction of the muffle, the structural environment 
of the muffle within the furnace, and the unique temperature sensing and 
heating means are used in combination to attain the advantages of the 
invention as set forth in detail below. 
The muffle is constructed of a material having a high thermal diffusivity, 
and is characterized by a high emissivity interior surface. The approach 
involves fabricating the muffle of a thin, light material so that 
essentially zero heat is stored, and the muffle is capable of rapidly 
eliminating heat gradients formed in its structure by thermal diffusion 
and infrared radiation transfer. The presently preferred material for the 
muffle is stainless steel less than about 100 mils thick. To optimize the 
precision of the temperature maintainable in the furnace, the muffle 
should be made as thin and as light-weight as possible, bearing in mind 
that for most furnace applications it will be called upon to withstand 
pressures upwards of 14 psi and temperatures upwards of 500.degree. C. In 
apparatus designed for heating workpieces under a vacuum, the muffle may 
be made of extremely thin material since the pressure on its opposite 
sides will be the same. It is believed that best results will be obtained 
when the muffle is made of an inert metal on the order of 0.01 inch thick 
and having a diffusivity of 0.27 ft.sup.2 /hr. or more. 
The structural environment of the muffle is designed to minimize heat 
transfer from the muffle to anything other than its interior and the 
workpiece it contains. Convective heat losses are eliminated by evacuating 
the space exterior to the muffle. Conductive heat losses are minimized by 
employing low thermal conductivity material in the necessary support 
structure which holds the muffle in position and by other design features. 
Radiative heat loss, which is the mechanism by which essentially all of 
the heat will be lost from the muffle under the conditions described 
above, is minimized by enclosing the muffle in an envelope having a highly 
reflective interior surface. It is currently within the skill of the art 
to produce infrared radiation reflective coatings on any one of a large 
variety of substrate materials that will exhibit a reflectivity greater 
than 97%. Coatings are currently available, e.g., gold films protected 
with an infrared transparent metal oxide coating such as titanium oxide, 
which are capable of remaining at 98% reflectivity for extended periods 
(even in air at 500.degree. C.). These can also withstand repeated 
cleanings. It is especially advantageous that the reflective coating used 
on the interior surface of the envelope in the apparatus of the instant 
invention is located in a vacuum and thus resists degradation. 
In combination with these features, one or more heaters, preferably 
electric heaters such as resistance heaters of infrared lamps (but in any 
case devices which are capable of precise thermal control) are provided in 
contact with, or slightly spaced apart from, the exterior surface of the 
muffle. The thermal output of the heater or heaters is controlled by an 
electronic feedback system which samples the temperature of the muffle 
through temperature sensing devices such as thermocouples, and employs the 
incoming signals to control heater output. 
The principal limitations on the precision with which the temperature of 
the muffle may be regulated are: 
1. The precision with which the temperature of the surface of the muffle 
can be measured; 
2. The speed with which electronic controls can react to a change in the 
sensed temperature; 
3. The speed with which power input can be applied to the heating elements; 
and 
4. The rate at which the temperature of the muffle can respond to changes 
in power input. 
Regarding the first limitation, it will be appreciated that devices 
presently exist which are capable of measuring temperature with almost 
arbitrary precision, e.g., commercially available calibrated thermocouples 
or thermopiles are capable of measuring temperature with a precision of 
10.sup.-1 .degree. C. to 10.sup.-3 .degree. C. The limiting factors on 
points two and three are the speed with which changes in muffle 
temperature can be sensed and the speed of the thermal response of the 
heaters to a given power input. Again, both of these are so rapid that 
they need not be considered further. Regarding factor four, the speed of 
thermal response of the muffle depends entirely upon its mass, the spacing 
of the heaters, and the characteristic thermal diffusivity of the material 
from which it is made. Accordingly, it is preferred that a plurality of 
heaters spaced about the muffle be employed, and that the mass of the 
muffle be as small as possible. 
For transmission of the thermal signal over distance d into a material 
having thermal diffusivity a, the time t required is: t=d.sup.2 /a. Since, 
for 430 stainless steel, a.apprxeq.0.27 ft.sup.2 /hr. it may be seen that 
for a muffle having a wall thickness of 10 mils, thermal diffusion across 
the wall will occur in about 0.1 second. Given this response, it should be 
possible to control the temperature of the interior surface of the muffle 
with the same precision as that of the outside surface, i.e., to within 
about .+-.0.1.degree. C. to .+-.0.01.degree. C., provided one uses 
suitably sensitive temperature sensors and electronic controls, which are 
currently available. 
The only significant mechanism for heat loss from the outside of the muffle 
is through thermal radiation. However, infrared radiation emitted from the 
exterior surface of the muffle or the electric heaters is reflected by the 
infrared reflective coating on the interior of the envelope surrounding 
the muffle. Such radiation may return directly to the muffle or it may 
undergo several reflections before returing. While proper construction of 
the interior surface of the envelope can minimize the number of 
reflections which occur (see copending U.S. application Ser. No. 663,370, 
filed Mar. 3, 1976 entitled "Heat Recuperator", the disclosure of which is 
incorporated herein by reference), as a "worst case" example, it will be 
assumed that thermal radiation from the muffle or heaters must undergo an 
average of 5 reflections before returning to the muffle. In this 
situation, if the reflectivity of the interior surface of the envelope is 
98% (easily obtained by current coating techniques), then the strength of 
the reflected beam returning to the muffle would be (0.98).sup.5 or 90.4% 
of the strength of the emitted beam. Thus, on the average, the effective 
emissivity of the envelope would be 1.0-0.904=0.096; the effective 
emissivity of the muffle would be at least about 0.90. On the basis of 
these assumptions, it may readily be calculated that the infrared 
reflective interior surface of the envelope will reduce the radiant heat 
loss from the muffle to a maximum of about 9% of the black body radiant 
flux emitted at the temperatures of interest. 
The small amount of heat absorbed by the envelope must be dissipated; 
otherwise the muffle and reflective envelope will eventually reach thermal 
equilibrium, resulting in possible thermal degradation of the envelope 
structure. 
Referring to the drawing, FIGS. 1 and 2 schematically illustrate a furnace 
embodying the invention wherein access to a cylindrical workpiece holding 
space 10 is provided by a series of doors arranged radially to the central 
axis of the holding space. The apparatus comprises a housing 12 having a 
door 14 which, when closed, forms a seal with the housing in engagement 
with elastomeric bead 16. An elongated, hexagonal cross-section envelope 
18 having a hinged door 20 is mounted on supports 22 within housing 12. An 
enclosed, cylindrical muffle 24 of sheet stainless steel is mounted on 
supports 26 coaxially with envelope 18 and has a sealable access door 38. 
A means for producing a vacuum such as pump 28 communicates via pipe 30 
with the interior of the housing and serves to produce subatmosphereic 
pressures in the space bounded by the exterior of muffle 24 and the 
interior of housing 12. A conduit 32 communicates between the workpiece 
holding space 10 on the interior of the muffle 24 and the exterior of 
housing 12. 
As shown in FIG. 2, the envelope 18 comprises a corrugated structure having 
a plurality of serpentine conduits 34 through which cooling fluid is 
forced by a pump of the like (not shown). The interior surface 36 of 
envelope 18 is provided with an infrared reflective coating such as vapor 
deposited gold which, preferably, is protected by an infrared transparent 
film, e.g., a film of an oxide of zirconium, titanium, or aluminum. To 
promote effective heat exchange between fluid passing through conduits 34 
and envelope 18, the substrate for the reflective coating should 
preferably comprise a material having a high thermal conductance such as 
copper. 
Muffle 24 has a sealable access door 38 and an interior workpiece support 
40. Its interior surface 42 is characterized by high emissivity. A 
plurality of resistance type heating elements 44 are wound 
circumferentially about the muffle with an axial separation. Power is 
supplied to the heating elements through leads 46 which deliver current 
under the control of feedback control means 48. Lastly, a plurality of 
thermocouples 50 are spaced about the surface of muffle 24 and deliver a 
signal to control means 48 via leads 52. 
Referring to FIG. 3, a cross-section of a second furnace embodying the 
invention is shown. The embodiment of FIG. 3 is similar to that set forth 
in FIGS. 1 and 2 except for certain changes in the muffle heating means 
and the cooling means servicing the envelope 18. Specifically, in place of 
resistance heaters 44, a plurality of elongated infrared lamp heaters 60 
are axially arranged about the circumference of muffle 24 and out of 
contact with the muffle. Instead of the corrugated structure of the 
envelope, the embodiment of FIG. 3 comprises an envelope which disposes of 
its accumulated heat by conduction to the housing 12 through a plurality 
of heat conducting support struts 62 and by emitting infrared radiation 
from its exterior surface 64 to the interior surface 66 of housing 12. 
High emissivity coating on the exterior of envelope 18 and on the interior 
of housing 12 will facilitate this mechanism of heat loss and form a 
radiative couple between the envelope and housing. 
In operation, with a workpiece on support 40, a vacuum is drawn in the 
space between the exterior of the muffle 24 and the interior of the 
housing 12 by vacuum producing means 28. The muffle is then heated to a 
selected high temperature by energizing the heaters 44 (or 60). During the 
temperature rise, the interior surface 42, and to a lesser extent, the 
exterior surface of muffle 24, emit infrared radiation. Radiation emitted 
by interior muffle surface 42 is absorbed and used to raise the 
temperature of the workpiece and any atmosphere within space 10. As air 
within space 10 expands, it vents itself through conduit 32. The radiation 
emitted from the exterior surface of muffle 24 or outwardly from the 
heaters will impinge upon the interior infrared radiation reflective 
surface 36 of envelope 18. At least 90% of this radiation will be returned 
to the muffle. That portion of the energy absorbed by the envelope will be 
dissipated by cooling fluid passing through openings 34, or, in the case 
of the embodiment of FIG. 3, by radiation emitted from the exterior 
surface 64 of envelope 18 and absorbed by housing 12, and/or by conduction 
through struts 62. 
When a selected workpiece temperature has been reached (as indicated by the 
signals from thermocouples 50), further energizing of the heaters 44 or 60 
is controlled by feedback control means 48 so that the temperature of the 
muffle is maintained within precise limits. Thus, the workpiece and muffle 
exchange infrared radiation and quickly thermally equilibrate. A certain 
fraction of the infrared radiation emitted from the exterior surface of 
the muffle is lost to the body of envelope 18, this is removed at a 
constant rate by the cooling mechanism. 
Shut down of the furnace is easily accomplished by terminating power to the 
heaters and continuing to cool the envelope. 
Thus, it may be seen that the apparatus of the invention is capable of 
employing energy in an efficient manner and is easily started up and shut 
down. Furthermore, because of the interaction of the features as described 
above, an improved degree of thermal precision is possible. 
From the foregoing it will be obvious that various modifications can be 
made in the apparatus without departing from the spirit or scope of the 
invention. Thus, it is contemplated, for example, that conduit 32 may be 
used to evacuate workpiece holding space 10 or to provide a controlled 
atmosphere within the holding space as dictated by the use to which the 
apparatus is to be put, and that various structural changes can be made 
further to improve the operation of furnaces constructed in accordance 
with the foregoing. Accordingly, other embodiments are within the 
following claims.