US3383497A

US3383497A - Electric resistance heaters

Info

Publication number: US3383497A
Application number: US641389A
Authority: US
Inventors: Darrel M Harris
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1964-12-02
Filing date: 1967-02-03
Publication date: 1968-05-14
Anticipated expiration: 1985-05-14

Description

May 14, 1968 D. M. HARRIS 3,383,497

ELECTRIC RESISTANCE HEATERS Original Filed Dec. 2, 1964 4 Sheets-Sheet 1 1O 3 IO 4 /A INVENTORI DARREL M. HARRIS ATTORNEY May 14, 1968 D. M. HARRIS 3,383,497

ELECTRIC RESISTANCE HEATERS Original Filed Dec. 2, 1964 4 Sheets-Sheet 2 I TEMPERATURE GHACTERISTIGS OF UNCOMPENSATED HEATER TEMPERATURE V5 BRIDGE LENGTH TEMPERATURE IIIO " I I I I I I I I j 2 4 6 8 IO I2 I4 l6 I8 20 22 24 INCHES ALONG BRIDGE LENGTH INVENTOR F 5 DARREL M. HARRIS ATTO R NEY May 14, 1968 D. M. HARRIS 3,383,497

ELECTRIC RESISTANCE HEATERS Original Filed Dec. 2, 1964 4 Sheets-Sheet 3 I190 a BBVOLTS 465 AMPS n70 (b) 30 VOLTS 455 AMPS use A TEMPERATURE (c) 27 VOLTS 44o AMPS IHO- INCHES ALONG BRIDGE LENGTH FIG.6

INVENTOR DARREL M. HARRIS BYW% ATTORNEY May 14, 1968 D. M. HARRIS 3,383,497

ELECTRIC RESISTANCE HEATERS Original Filed Dec. 2, 1964 4 Sheets-Sheet 4 TEMPERATURE CHARACTERISTICS OF COMPENSATED BRIDGE TEMRERATURE V5. BRIDGE LENGTH LEFT SIDE OF BRIDGE TEMPERATURE TEMPERATURE CHARACTERISTICS OF COMPENSATED BRIDGE TEMPERATURE VS. BRIDGE LENGTH TEMPERATURE I RIGHT SIDE OF BRIDGE 40 l l I l I l I I 2 4 6 8 IO l2 I4 I6 I8 20 22 INCHES ALONG BRIDGE LENGTH INVENTOR DARREL M. HARRIS ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE A graphite heating element for epitaxial deposition furnaces which comprises a pair of legs connected by a bight portion, each of the legs is tapered from a point centrally of its end 50 that it is thicker in the center. The taper is such that the cross sectional area provides uniform electric resistance and uniform heat distribution throughout. The bight portion is substantially thicker than each of the legs and includes a larger central aperture at the point of connection with each of the legs. A second modification is also provided and attains uniform electric resistance characteristics and uniform heat distribution by selectively drilling apertures transversely through the legs so that the apertures are concentrated near each of the ends of the legs.

Thisapplication is a division of copending application, Ser. No. 415,363, filed Dec. 2, 1964, now Patent No. 3,351,742, dated Nov. 7, 1967.

This invention relates in general to certain new and useful improvements in heating devices and more particularly to an improved method for maintaining temperature uniformity across the length of electric resistance heaters.

In recent years, semiconductor devices such as silicon controlled rectifiers have found widespread use in the electronics industry. These solid state rectifiers, such as the silicon controlled rectifiers are often formed by depositing an epitaxial silicon film on Wafers formed of generally high purity silicpn. The silicon wafers are normally placed upon a graphite heating element which forms part of an epitaxial silicon furnace and are heated to a temperature where free silicon is deposited on the wafer and becomes bonded to the surface of the wafer. These wafers are then further processed by conventional methods and used in the manufacture of solid state devices such as silicon controlled rectifiers.

In the recent years, it has become a common practice to employ resistance heating elements formed of graphite in these epitaxial silicon furnaces. These heating elements are generally U-shaped in horizontal cross section and consist of a pair of legs which are connected by a bight portion. The legs are generally provided with terminal connectors at their free ends or the ends remote from the bight portion for ultimate connection to a suitable source of electrical cur ent. A suitable amount of electrical current is then passed through the heating element to heat the element to the desired reaction temperature. However, by reason of the fact that graphite is suitable for use as an electrical heating element, it inherently includes a certain amount of internal resistance which interferes with the passage of electrical current. Therefore, as the electrical current traverses the length of each of the legs in the heating element, a slight voltage drop is developed across the entire length of the heating element and the temperature at the ends of the legs which are connected to the bight portion suffer a temperature drop. The terminal connectors are normally secured in clamps which are water cooled. Consequently, there is a rather large heat loss near the terminal ends of the legs which are secured 3,383,497 Patented May 14, 1968 to the clamps. As a result thereof, the ends of the legs are cooler than the center portion thereof with the result of a non-linear temperature distribution across the legs forming part of the graphite heater. Moreover, there are local hot spots or heat concentrations along portions of the legs forming part of the heater.

As a result of this occurrence, graphite heatingelements have been seriously limited in their effective length, and consequently, the resultant capacity of epitaxial silicon furnaces and similar types of furnaces have been greatly hampered.

It is therefore, the primary object of the present inventionto provide an electric resistance heating element which has compensated resistance characteristics for maintaining uniform temperature distribution throughout its efiective length.

It is another object of the present invention to provide an electric resistance heating element of the type stated which is highly efficient and relatively inexpensive tosired resistance characteristics throughout its effective,

length for attaining uniformity of temperature.

With the above and other objects in view, my invention resides in the novel features of form, construction, arrangement, and combination of parts presently described and pointed out.

In the accompanying drawings:

FIGURE 1 is a perspective view of a heater constructed in accordance with and embodying the present invention;

FIGURE 2 is a vertical sectional view taken along lines 22 of the heater of FIGURE 1;

FIGURE 3 is a perspective view of a modified form of heater constructed in accordance with and embodying the present invention;

FIGURE 4 is a vertical sectional view taken along line 44 of FIGURE 3;

FIGURE 5 is a graph of the temperature characteristic of an uncompensated bridge showing temperature versus bridge length;

FIGURE 6 is a graph of the temperature characteristic of a bridge compensated by selectively drilling apertures in accordance with the method of the present invention, showing temperature versus bridge length for various power settings;

FIGURE 7 is a graph of the temperature characteristic of a modified form of compensated bridge constructed in accordance with the present invention and showing temperature versus bridge length for one leg of the bridge; and

FIGURE 8 is a graph of the temperature characteristic of the bridge of FIGURE 7 showing temperature versus bridge length for the other leg of the bridge.

Generally speaking, the present invention is concerned with graphite heating elements, and a method of altering the cross sectional area along selected portions of the length of graphite heaters. The manufacture of epitaxial silicon wafers has always been a costly process inasmuch as it is necessary to maintain a high degree of purity, and uniform crystal structure. However, it has been extremely ditficult to maintain uniform crystal structure and moreover, to maintain uniformity of thickness of an epitaxial layer deposited on a silicon wafer. Various modifications of epitaxial silicon furnaces were performed and alternate methods were used in order to attempt to obtain epitaxial layers on silicon wafers. However, the prior art was completely devoid of an explanation for non-uniform thicknesses of epitaxial silicon layers. It has been found in connection with the present invention, that film quality can be maintained with a high degree of control by maintaining a substantially uniform temperature throughout the entire length of the graphite heater. Mooreover, umformity of temperature in the epitaxial desposition process produces films with substantially uniform resistivities. It has also been found that uniformity of temperature aids in control of film thickness.

In accordance with the present invention, it has been found that by removing selected portions of the graphite heater at selected areas along its length by drilling small apertures, it is possible to reduce the cross sectional area of such portions and thereby alter the resistance characteristics of these particular areas. In this manner, it is possible to maintain a substantially non-uniform electrical resistance characteristic thruoghout the entire length of the heater and this, in turn, provides substantially uniform temperature distribution throughout the effective length of the heater.

The present invention also provides a modified form of graphite heater with substantially widened legs. Moreover, the legs are of the modified form of heater. The legs do not have a uniform cross sectional thickness but have a smaller cross sectional thickness at their transverse ends, that is the free ends which are adapted for ultimate attachment to a furnace. In this manner, it is possible to maintain a substantially non-uniform electrical resistance characteritic by adjusting the thickness of the bridge legs so that it is possible to maintain a substantially uniform temperature distribution for the effective length of each of the legs.

In the present invention, the effective length of the legs of the heater refers to that portion of a length of a leg which presents a usable surface for accommodation of silicon wafers.

Referring now in more detail and by reference characters to the drawings which illustrate practical embodiments of the present invention, A designates a graphite heater or so-called bridge generally comprising a pair of horizontal legs 1, 2 which are connected by a bight portion 3. The legs 1, 2 are provided with top and bottom faces 4, 5 respectively, interior faces 6 and exterior faces 7.

Each of the legs 1, 2 is drilled near the free end thereof to provide small apertures 8 for selectively reducing the cross sectional area of each of the legs 1, 2. The apertures 8 extend the width of each ofthe legs 1, 2, that is from the exterior face 7 to the interior face 6. Moreover, the apertures 8 are sufficiently small, in the range of approximately inch diameter in order to eliminate any effect on the structural characteristics of the heater A. It has been found that by selectively drilling the apertures 8 at a point midway between the upper and lower faces of each of the legs 1, 2 there is practically no reduction in internal strength. In many cases, it has been found that superior results are obtained when the apertures 8 are separated by increasingly smaller distances closer to the free end of the legs 1, 2. Thus, the spacing between each of the apertures 8 is increased as the distance of the aperture from the free end of the legs 1, 2 increases in the form of a geometric proportional increase.

It is not necessarily preferable to space the apertures 8 in the form of a geometric progression. In some cases, depending upon the particular graphite heater, it has been found that suitable results are obtained when the apertures 8 are located in groups of variable spacings or of constant spacings. For example, in the graphite heater illustrated in FIGURE 1, it was found that a group of apertures spaced from each other by a distance X, followed by a second group of apertures spaced from each other by a distance of 2X, followed by a third group of apertures spaced from each other by a distance of 3X was found V to produce very suitableresults. The

method of selectively placing the apertures is hereinafter described in detail.

It is also possible, but not absolutely necessary to provide a similar set of apertures 9 near the ends of the legs 1, 2 which are formed with the bight 3, substantially as shown in FIGURE 1. Again, each of the apertures 9 is placed midway between the top and bottom faces of the legs 1, 2 and is located so that it can present a substantially uniform temperature distribution across the length of the legs 1, 2. It has been found that by selectively drilling the apertures in this manner, the strength of the entire heater A is maintained, and yet the cross sectional area is adjusted so that a substantially uniform temperature distribution is maintained throughout its entire length.

It is also known that the current passing through the legs 1, 2 and through the bight portion 3 has a tendency ,to travel through the shortest current path in materials such as the graphite, from which the heating element A is formed. For this reason, the conventional heaters usually experience large current densities around the inner point of connection between the bight 3 and the legs 1, 2. This type of condition creates a substantially higher temperature along the inner margin of the bight portion 3 and a substantially cooler temperature at the outer corners of the common connection of the legs 1, 2 and the bight 3. In order to eliminate this condition, the heater A is formed with a pair of recesses 10 which extend toward the exterior wall of the bight 3, in the manner as shown in FIGURE 1. The recesses 10 are somewhat circular and extend for the full vertical length of the bight 3, that is, they extend from the top face thereof to the bottom face thereof. It has been found that by producing this type of recess, the current traveling through the legs 1, 2 is forced to move around the recess 10 and thereby create a higher temperature in a region which extends centrally along the bight 3. Moreover, the area between each of the recesses 10 forms a heat sink for heat dissipation in the region of high current density. In this manner, it is possible to provide a more substantially uniform temperature distribution throughout the length of the bight 3. Furthermore, this type of arrangement has eliminated the conditions of'the prior art where the ends of the legs .1, 2 were substantially cold with respect to the inner margin thereof. In effect, the recesses 10 thereby form a heat sink 11 which is located between each of the legs 1, 2.

It is possible to provide a modified form of graphite heater B substantially as shown in FIGURES 3 and 4, generally comprising a pair of

horizontal legs

12, 13 which are connected by a bight portion 14. The legs 12, =13 have top faces 15, bottom faces 15, interior faces 16 and exterior faces 17. Moreover, the

legs

12, 13 are tapered from each of their transverse ends that is their free ends, which are adapted for ultimate attachment to a suitable furnace, such as an epitaxial silicon furnace. They are tapered in such a manner so that they have a slightly smaller cross sectional thickness at each of the ends than in the center portion thereof, when referring to the vertical dimension of the

legs

12, 13 reference being made to FIGURE 5. Thus, it can be seen that the thickness of the legs increases as the distance from the free ends thereof increases. The angle of taper of each of the legs '12, 13 is so adjusted so that a cross sectional area of the

legs

12, 13 is maintained in order to provide a substantially constant uniform temperature distribution across the lengths of the

legs

12, 13.

Each of the

legs

12, 13 is connected by the bight 14 which is slightly thicker in the vertical dimension, reference being made to FIGURE 3, than each of the

legs

12, 13. For heaters having overall lengths .of approximately 22 inches, the legs have an overall thickness of approximately 0.260 inch at the center portion and a thickness of approximately 0.215 inch at each of the ends. The bight 14 has an overall thickness of approximately 0.500 inch and has an overall length of approximately 1% inches so that it is substantially thicker than the overall thickness of the legs '12, 13. Moreover, by reference to FIGURE 4, it can be seen that the distance separating each of the

legs

12, 13 has been substantially reduced so that it is only approximately of an inch spacing. By reducing the spacing between each of the

legs

12, 13, it has been found that it is possible to eliminate much of the radiation from the internal walls of the

legs

12, 13 and thereby maintain a more uniform temperature distribution across the length .of each of the

legs

12, 13. The graphite heater B is also provided with a circular recess 19 at the point of connection with each of the two

legs

12, 13. The circular recess 19, which has a diameter of approximately /2 inch, serves substantially the same purpose as each of the recesses in the graphite heater A. In addition, the thickness of the bight 14 provides a larger current path so as to reduce the current density and thereby aids in maintaining uniform temperature across the bight 14.

At each of the free ends of the

legs

12, 13 that is the ends where they are normally attached to a clamp forming part of a furnace, the legs integrally merge into enlarged

terminal portions

20, 21 respectively. The enlarged portions have a length of approximately 1 inches and have an overall thickness of approximately A1, inch. Moreover, the terminal ends 20, 21 merge with the legs .12, '13 at an angle of taper of approximately 30". It has been found that by selectively adjusting the angle of taper in order to adjust the thickness of the

legs

12, 13, it is possible to provide a substantially uniform temperature distribution across the entire length of the

legs

12, 13.

It should be understood that the present invention is not limited to heaters with only two legs and heaters with the particular shape and dimensions illustrated. For example, it is possible to provide heaters in accordance with the present invention which have three or more legs connected by a common bight portion. In this event, each of the legs of the heaters would be compensated in cross sectional area in order to maintain uniform temperature distribution across each of the legs.

In order to selectively alter the efifective cross sectional area of the graphite heaters at selected portions of their length, the graphite heaters are normally mounted in a suitable epitaxial silicon furnace in the same manner as when used in a commercial operation. The free ends of the heaters are secured in clamps which may or may not be water cooled, depending upon the particular furnace. It is, of course, understood that the heater is normally cleaned for use in the same manner as it would be if it were to be used in a commercial operation.

Thereafter, it is necessary to measure the temperature produced at various selected portions along the length and width of the heaters. This can be accomplished by attaching thermocouples to the heater and connecting the leads thereof to a suitable temperaure readout device. However in actual practice, it has been found to be convenient to use an optical pyrometer. The graphite heater is then enclosed in a bell jar and current is passed through the heater in order to raise the temperature to a point within a range of normal operating temperatures. After the temperatures along selected portions of the length of the heater are recorded, the heater is cooled and the bell jar or so called hat is removed. At this point, the desired cross sectional area of the legs of the heater can be determined by simple electrical relationships. After the desired cross sectional area at selected portions along the length have been determined for the entire length thereof, the desired cross sectional area can be attained by removing the required amount of the cross sectional area. As pointed out above, this is accomplished by drilling small apertures. The apertures are sufficiently small so that they do not interfere with the internal strength of the heater, but yet, are sufiicient in number so that they sufiiciently alter the cross sectional area of the legs to accomplish the intended purpose.

In the case of the heater B, the variance in temperature for the new heater can be determined in the manner described above in connection with the heater A. Thereafter, it is possible to shave the upper and lower surfaces of the heater B in order to achieve the proper amount of taper so that a proper cross sectional area is maintained at selected portions along the length of the heater.

The invention is further illustrated by but not limited to the following examples:

EXAMPLE 1 This example illustrates the non-linear temperature distribution across the length of the heater when the heater is not compensated by alteration of cross sectional area at selected portions of its length for maintaining uniform temperature distribution. A graphite heater having' an oveall length of approximately 24 inches and an overall width of approximately 3%; inches was mounted in an epitaxial silicon furnace. Each of the arms had a width of approximately 1% inches in the transverse dimention. Moreover, each of the legs had an overall thickness of approximately A inch and each of the legs was separated by approximately 11 inches. The bell jar temperature was maintained at approximately 730 C. throughout the entire experiment. The jar was maintained upder the pressure of 40 microns, and the power supplied to the heater was 28 volts at 470 amperes.

An optical pyrometer was used to measure the temperature of each leg at each of the twenty-three wafer positions on each leg. Hydrogen gas was then passed into the bell jar at a rate of 14 liters per minute at room temperature and the following temperatures of each of the wafer positions was measured and are recorded below.

Table l LEFT LEG Wafer Distance Position From Temperature,

N0. Attached C.

Terminal (inches) RI GHT LE G The wafers nearest the free terminals of the heater are wafers 1 and 46. The temperature of the bridge was plotted as a function of the bridge length as illustrated 7 in FIGURE and it shows the non-uniform temperature distribution across each of the'legs. Moreover, FIGURE 5 has two curves inasmuch as each of the legs of the graphite heater was not of the same temperature across its length. Y EXAMPLE 2 This example also illustrates the non-linear temperature distribution across the length of the heater, similar to the heater in example 1 when'the heater is not compensated by alteration of the cross sectional area at selected portions of its length. Hence, the heater in, this example was also not capable of maintaining a relatively. constant uniform temperature distribution across its entire length. The graphite heater used in this example has approximatelythe same dimentions as the graphite heater employed in Example 1. However, the bell jar temperature was maintained at 730 C. at the start ofthe experiment and at approximately 655 C. at the end of he experiment. The bell jar was maintained underja. pressure of approximately 40 microns and this power supplied to the heater was 26 volts at 435 amperes.

An optical pyrometer was used to measure the temperature of twenty-two wafer positions on one leg of the heater and tweny-on wafer positions on the opposite leg of the heater. The bell jar was maintained under a vacuum of 40 microns and the following temperatures of each of the wafer positions were measured and are recorded below.

Table II LEFT LEG Dlstance From No. Attached Terminal (inches) Tcrnpegature,

RIGHT LEG EXAMPLE 3 a bell jar. The heater had an overall width of approximately 3%; inches and each leg had an overall width in the transverse dimension of approximately 1% inches. The two legs had reliefs which were formed by slots having a thickness of A; inch and extended inwardly from the interior surface of the bight portion for a distance of approximately inch. The slots terminated in slightly enlarged apertures having a radius of approximately A; inch. Eachofvthe legs is provided with 17 apertures extending for the entire width of the leg and having a diametral cross section of approximately 0.067 inch. The 17 apertures started at a point 3 inches from the free terminal margins of each of the legs and extended to a point 5 inches from the free terminal margins of the legs or for an overall distance of 2 inches. Accordingly, the 17 holes were spaced by distances of /3 inch. The legs were then provided with 8 apertures extending to a length of 7 inches from the free terminal margins and accordingly, each of the 8 apertures was separated by distances of inch from each other. Thereafter, each of the legs was provided with two apertures separated by /2 inch and two additional apertures separated by 1 inch thereby providing a total of 29 apertures which extended from a length 3 inches inwardly from the terminal margins of the legs to a point l0 inches inwardly from the free terminal margins of the legs.

The heater was maintained under a vacuum of 20 microns at room temperature at the outset and then hydrogen was passedinto the hat at a rate of 15 liters per minute at atmospheric pressure. Current was supplied to the heater until the hat temperature was raised to 740 to 750 C. throughout the experiment. The power supplied to the heater was at 30 volts with 455 amperes.

An optical pyrometer was used to measure the temperature of 21 silicon wafer positions on each of the legs and the temperatures and the distances from the free terminal margins of the legs are recorded in the following table. (See Table III below.) The temperature of the bridge was measured and tabulated in Table III as a function of the bridge length for three different power settings. These data of temperature versus bridge for each of the power settings were plotted as shown in Figure 6. By reference thereto it can be seen that the three power settings were maintained at (a) 33 volts and 465 amperes, (b) 30 volts and 455 amperes, (c) 27 volts and 440 amperes. It can thus be seen, that for the greater portion of the length of each of the legs of the bridge, a substantially uniform temperature distribution had been maintained.

EXAMPLE 4 This example also illustrates the temperature distribution across the legs of .a graphite heater when the latter has been compensated for cross sectional area in order to maintain uniform temperature distribution, in the same manner as the graphite heater of Example 3. The graphite heater employed in this particular example was substantially similar to the graphite heater employed in Example 3 and Table III LEFT LEG Distance Wafer From Temperature, 0. Position No. trttaehedI ermina a b 0 (Inches) 1, 118 1, 103 1, 082 1, 183 1, 175 1,148 1, 192 1,184 1,158 1, 190 1, 184 1, 153 1,188 1,180 1,153 1,185 1, 175 l, 1, 185 1,175 1, 150 1,185 1,175 1, 150 1,185 1,175 1, 150 1,185 1,175 1,150 1, 185 1, 1, 150 1, 1, 375 l, 350 1, 385 1, 175 l, 150 1, 385 1,175 1, 150 1, 135 1, 175 1.150

Table IllContinued Table I V--Cont1nued LEFT LEG RIGHT LEG Distance From Temperature Temperature D1stance u o a Wafer From Temperature, 0. Water No. glitlalcllggc'figsi) No. 1, C. No. 2, G. PositionNo. Attached Terminal (a) (b) (c) m 1,175 1 166 (Inches) 11 1,173 1,166 1s 1, 135 1, 175 1, 1.50 12 1, 173 1, 160 19 1, 185 1, 175 1,150 12 12g 20 1,185 1, 175 1,150 15 173 1,158 21 16 1' 173 1' 152 22 23 1, 109 1,192 1,102 g 19 1: 107 11153 RIGHT LEG 20 3 1,122

1,125 1,103 1, 032 21 1,1 1,1 2 1,196 1,190 1, 173 22 1,160 1,144 1, 213 1,200 1, 155 0 1,200 1,187 1.155 7 it 123 These temperature d1str1but1ons could also be plotted as g 1:187 11177 1, 150 a function of the length of each of the legs to obtain i. 12; 1g? curves similar to those plotted in FIGURE 7. It could ii 1:187 11177 11150 thus be seen that a substantially uniform temperature 12 distribution was maintained across each of the legs. 1 1', 187 1',177 1', 152 13 1,157 1,177 EXAMPLE 5 i; HZ; 1 5 This example illustrates the temperature distribution 10 1,187 across the legs of the graphite heater B which has been 5 1 i1 i77 1:152 compensated for cross sectional area in order to main- 22 1,185 1,177 1.152 tain uniform temperature distribution. The graphite heater 23 1,186 ,177 1,149

was similarly secured to an epitaxial silicon furnace and enclosed by a bell jar. The apertures formed in the side walls of the legs were substantially the same as those formed in the heater of Example 3.

The heater was maintained under a vacuum of approx1- mately 20 microns at room temperature. The heater was the vacuum purged, purged at room temperature and then hydrogen was passed into the hat at a rate of l5 l1ters per minute. Current was supplied to the heater unt1l the hat temperature was raised to approximately 740 C.

-to 750 C. throughout the experiment. The power supplied to the heater was 40 volts at 455 amperes. After approximately fifteen minutes the temperature of the wafers was measured and is recorded in the table set forth below as Temperature No. 1. Shortly thereafter, trichlorosilane was passed into the hat at a rate of approximately 0.5 gram per minute.

A total of 19 silicon wafers was placed upon each of the legs and the distance from the free terminal margins of the legs are recorded in the table as set forth below. Recorded in the column designated as Temperature No. 2 is the temperature of the bridge after the addition of the trichlorosilane.

Table IV LEFT LEO Distance From Temperature Temperature Wafer No. Attached Ter- No. 1, C. No. 2, C.

minal (inches) RIGHT LEG B had an overall length of 24 inches and an overall width of 5% inches. Each leg had a width of approximately 2% inches separated by a slot of Ms inch between each leg. Each of the legs of the graphite heater had an overall thickness in the vertical dimension of approximately inch. At its free end, the graphite heater terminated in an enlarged head portion having a length of 1 inches and an overall thickness of .260 inchfat a point midway between their length. At the bridge end, each of the legs was integrally formed with enlarged head portions having a length of approximately 1% inches and an overall thickness of .500 inch. Moreover, each of the legs at the point of attachment to the enlarged head portions had overall thicknesses of .230 inch. At points of attachment to the head portions at the free end, the legs had overall thicknesses of approximately 0.215 inch. At the head portion, the enlarged bridge was formed with an aperture having a diameter of approximately /2 inch, and is located at the point midway between each of the legs.

The heater was attached to an epitaxial silicon furnace having means for water cooling the heater at the point of attachment and was enclosed within a bell jar. The cooling water inlet temperature was maintained at 27 C. and the outlet water temperature was recorded to be 62 C. At the start of the operation, the pressure within the bell jar was reduced to 200 microns. The bell jar was then purged with gaseous hydrogen for approximately ten minutes. Gaseous hydrogen was thereafter passed into the bell jar at approximately 35 liters per minute (standard pressure and temperature). Current was then supplied to the heater until the temperature of the bell jar was raised to 780 C. The power supplied to the heater was 17.2 volts at 1070 amperes.

Shortly thereafter trichlorosilane gas was passed into the heater at a rate of 1.28 grams per minute. A doping gas, phosphorus trichloride, was added to the bell jar at a rate of approximately 25 milliliters per minute. A total of 35 wafers were placed on each leg of the heater and temperatures are recorded in the table set forth below (Table V). It is to be noted that there were two rows of wafers on each leg. The temperature of the heater was plotted as a function of the heater length as illustrated in FIGURES 7 and 8 and shows the relatively uniform temperature distribution across the length of the legs. The graph of FIGURE 7 shows the temperature of the wafer positions in each row on the left leg, and the graph of FIGURE 8 shows the temperature of the wafer positions in each row on the right leg.

Table V LEFT LEG Outer Row Inner Row Distance From Temper- Distance From Temper- Wafer Attached ature, Wafer Attached ature, No. Terminal C No Terminal 0.

(Inches) (Inches) RIGHT It should be understood that changes and modifications in the form, construction, arrangement and combination of parts presently described and pointed out may be made and substituted for those herein shown without departing from the nature and principle of my invention.

Having thus described my invention, what I desire to claim and secure by Letters Patent is:

1. An electrical heating element formed substantially of carbonaceous resistance material for use in epitaxial deposition furnaces and the like and being adapted to be heated by the passage of an electric current therethrough; said heating element comprising a plurality of longitudinally extending substantially coplanar legs, a bight portion extending across and operatively connecting one of the transverse ends of each of said legs, said legs having free opposed transverse ends adapted for operative attachment to a source of electrical power, each of said legs having a plurality of transversely extending selectively spaced apertures etfective to reduce the cross sectional area of the legs, said apertures being sized to produce differences in the longitudinal cross sectional area and in resistance characteristics of said heating element to create a substantially uniform temperature across the length of each of said legs.

2. The electrical heating element of claim 1 further characterized in that a recess is formed in said bight portion adjacent each of said legs at the point of attachment to said bight portion to maintain uniformity of temperature in said bight portion.

3. The electrical heating element of claim 2 further characterized in that said recesses extend vertically.

4. The electrical heating element of claim 1 further characterized in that said heating element has only two legs.

5. The electrical heating element of claim 1 further characterized in that the spacing between each of said apertures is increased as the distance of the apertures from the transverse ends of the legs increases in the form of a geometric proportional increase.

References Cited FOREIGN PATENTS 12/1921 Germany. 10/1955 Germany.

RICHARD M. WOOD, Primary Examiner.

VOLODMYR Y. MAYEWSKY, Examiner.