US 3281517 A
Description (OCR text may contain errors)
5 ma m ET TMN m v 1 N 0 o 7 l NEM W 5 .Mw [UHD A y e *E 1A h \u\ l 8 DP. 2 S 3 MR e M5 V 2 mm 27 Fl L Wm ,3 E .V15 Y R m 'i B E w M H M m. 1 v ,2 F
3 6 9 6 l 9, 1 l y V n. m u M m..
INVENTORS FERDNRND j. HEMMER @Am/:ES RPDr/iom BY 9r- ATTORNEY 2 Sheets-Sheet f F. J. HEMMER ETAL,
VACUUM FURNAGB Exe. 5
Filed Nov. 19,
United States Patent O Mice 3,281,517 VACUUM FURNACE Ferdinand J. Hemmer, Alexandria, and James R. Piedmont, Falls Church, Va., assignors to Melpar, Inc., Falls Church, Va., a corporation of Delaware Filed Nov. 19, 1963, Ser. No. 324,614 9 Claims. (Cl. 13-31) The present invention relates generally to evaporation sources and more particularly to a vacuum furnace having; -a cylindrical, double walled resistance heater; a nozzle for the evaporant; and an electrode clamp for the heater which maintains electrical contact with the heater over extreme temperature ranges.
Since graphite is a brittle material, the heater configuration and dimensions of a graphite heater must be such that the heater can withstand the forces applied during fabrication and handling, as well as the forces produced by thermal expansion of mating parts during operation. These requirements imply a relatively large ratio between heater cross-sectional area and length and ytherefore a low electrical resistance. Consequently, large electrical currents must be supplied in order to generate the heat necessary to produce high temperatures. In addition, the resistance of the electrodes and leads employed to conduct current between the iheater `and the power supply and the contact resistance between the electrodes and the heater must be unusually low, in lorder to achieve efficient operation, and to prevent melting of the electrodes and conductors, since heat generated in any portion of the heating circuit `is proportional to the resistance of that portion 4of the circuit.
To avoid these difficulties, it has generally been the practice to design the heater with circulating water through the electrodes to prevent melting, resulting in very low eliiciency, since much of the heat is wasted.
A serious problem associated with vacuum evaporation sources, in general, is flow of vapor to portions of the vacuum chamber other than to the substrate surface upon which condensation of vapor is desired. This results in great ineiciency and contamination of the vacuum chamber and equipment contained therein. Another serious pr-oblem is variation in thickness of the lm deposited on the substrate after condensation and soliditication of vapor. The irst problem is solved, according to the invention, by depositing the evaporant via a nozzle. Known methods of solving the second problem, non-uniformity of lm thickness, have been costly and cumbersome. Techniques used heretofore have been t-o move the substrate periodically past a stationary source or to place the substrate above the center of a ring of sources. The use of a nozzle also helps solve the second problem.
According to the present invention, many difficulties heretofore associated vwith graphite resistance heaters are overcome by employing a graphite heater having a double walled cylindrical configuration. The charge being evaporated is placed in the cup formed by the inner wall, which extends in a stem on which one contact is secured. The other contact is secured to the outer walls remote from the heater. This construction enables large length to exist between the metal contacts and the graphite heater, whereby the cross section to length ratio of the heater is maintained at a low value, while the contacts are located remotely fr-om the high temperature heater. Also, both contacts are considerably removed from the vapor stream emanating from the charge being heated.
A further problem encountered in resistance vapor furnaces is poor contact between the copper electrodes and the graphite heater, due to differences of expansion over the extreme temperature variations encountered. This is because the materials involved have Widely separated coethcients of thermal expansion. According to the present 3,281,517 Patented Oct. 25, 1966 invention intimate contact is maintained between the electrode and heater by strapping them together and by selecting the proper dimensions with relation to the thermal expansion coetiicients of the heater, electrode, and strap materials. The strap is secured to the copper electrode and extends about the circumference of the cylindrical heater. High contact pressure is maintained between the heater and the electrode because that portion of the electrode located between the heater yand the points at which the strap is secured to the electrode expands suficienly to compensate for expansion of the strap. As an additional feature, uniformity of contact pressure can be further enhanced by securing leaf springs to the exterior surfaces lof the strap in the region near where the strap is secured to the electrode but is free of contact. Since the springs are necessary in any event to take up slack in the strap during assembly, their use to maintain'constant tension as temperature of operation varies is an added dividend.
A further `defect in prior heaters is their inefliciency in delivering evaporant to the substrate surface being` coated. One of the reasons for this is that the vapor tends to travel in all directions. Also there is a great deal of heat loss from the crucibles of the prior art devices so that much of the electrical energy delivered to the heater is not effectively converted to energy for evaporating the charge.
The present invention obviates these difficulties by employing a converging-diverging nozzle at the crucible mouth. The nozzle, in addition to providing a sharply defined vapor path, increases the evaporant temperature because it reects considerable radiant heat back to the charge. Also, the nozzle reduces the probability of evaporant blowout due to rapid expansion of adsorbed gas during the initial heating operation.
By utilizing a converging-diverging nozzle, uniformity of evaporant deposited on the `substrate is maintained to a great degree. This is because there are collisions of evaporant molecules at the nozzlethroat, which tend to cause an even distribution thereof at any cross section of the vapor stream. Also, the diverging mouth of the nozzle restricts the stream boundary so that the molecules of the evaporant do not scatter widely.
A further objection of many prior art furnaces is that a chemical reaction occurs between the heater and the evaporant if contact as well as radiant heat is employed. Such a reaction is promoted by the extreme temperatures at whi-ch the heater and evaporant are maintained. It is detrimental to the operation of the system because the evaporant is contaminated with molecules from the heater, while the heater erodes. lA feature of the present invention is that chemical reactions between the evaporant and heater are eliminated by utilizing `a crncible liner fabricated from boron nitride, an extremely inert material.
It is accordingly an object of the present invention to provide a new and improved furnace of the type utilized to deposit an evaporant on a substrate.
A further object of the present invention is to provide a resistance furnace employed for vaporizing materials wherein problems associated with melting of copper electrodes are obviated without sacrificing power etciency.
Another object of the present invention is to provide a new and `improved vacuum resistance furnace wherein chemical reaction between the heater and evaporant is prevented.
An additional object of the present invention is to provide Ia vacuum furnace of great efliciency in transferring heat energy to an evaporant.
Yet another object of the present invention is to provide a vacuum furnace for `depositing an evaporant on a substrate with uniform thickness.
Still a further object of the present invention is to provide a new and improved clamp for materials having widely differing coeflicients of thermal expansion and which are subjected to large temperature variations.
An additional object of the invention is to provide a new and improved means for establishing electrical contact between a resistance heater and an electrode, capable of delivering large currents to the heater in such a manner that .intimate contact between the heater and electrode is maintained over all temperatures encountered.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:
FIGURE 1 is an elevation cross sectional view of a preferred embodiment of the present invention taken along the lines 1-1 of FIGURE 2;
FIGURE 2 is a top Iview of the heater assembly of FIGURE l;
FIGURES 3 and 4 illustrate further embodiments of the nozzle;
FIGURE 5 is an illustration of an alternative form for A locating the evaporant charge in the crucible; and
suspended from the heater walls.
Reference is now made to FIGURES 1 and 2 of the drawings wherein heater assembly 11 and substrate 12 which is to be coated are enclosed within jar 13, maintained under a vacuum of approximately *5 mm. of Hg in a conventional manner. Assembly 11 includes a double walled resistance heater 14, fabricated of graphite and having its spaced outer and inner walls 15 and 16 concentric with each other. Copper electrodes 17 and 18, for supplying large currents (on the order of 400 amperes) fromcopper power leads 25 and 26 to series connected heater segments l5 and 16, are connected to the outer and inner walls 15 and 16, respectively. The connection to inner wall 16 is made to stem 20, to which electrode 18 is joined, at a point lower than the lowest point of outer wall 15 and the outer wall connection is established at a point slightly below the base of crucible 10. Leads 25 and 26 are maintained in contact with bores in electrodes 17 and 18, which bores are remotely located from heater 14, by stainless steel nut and bolt assemblies 27 at the end of each electrode. To generate maximum heat and produce maximum temperature near source material 19 (between 600 C. and 2500 C.), inner wall 16 is machined relatively thin where it forms crucible 10. Crucible 10 carries cup like liner 21 in which material 19 is located.
Liner 21 is manufactured from an electrically insulating refractory material, preferably boron nitride, to prevent the ow of current through charge 19. Liner 21 also physically and chemically isolates charge 19 from heat source 14, hence the deleterious effects of contamination and source erosion involved in chemical and electrical reactions between the heater and the evaporant materials are prevented. By utilizing la replaceable liner that is easily'inserted into and withdrawn from the cavity between inner wall 16, the same heater may be utilized when different exaporant materials are utilized. Gas removal during evacuation and outgassing from the interior of walls 15 and 16 is attained with radially vextending openings 22 in the walls.
Because source 19 in crucible 10 must be subjected to such extreme temperatures and copper has a relatively low melting point, 1083 C., it is necessary that there exist a substantial temperature gradient between electrodes 17 and 18 and the crucible. Such a gradient is established by utilizing the double walled construction whereby the power clamps 17 and 18 are shielded from the high temperatures generated in the thin inner wall of the heater. This arrangement enables clamps 17 and 18 to have a large cross sectional area contacting walls 15 and 20 while the graphite heater has maximum length to cross sectional area ratio for a given overall size. In consequence, the contact resistance is quite low compared to heater resistance andthe ratio of heat generated in the heater to heat generated at the contacts is very large. By placing clamps 17 and 18 below crucible 10 heating of clamps 17 and 18 by radiation from crucible 10 is minimized.
To reduce heat loss by radiation, reflectors 24, made of a material capable of withstanding extreme temperatures, e.g. tantalum, are disposed concentrically about the outer wall of heater 14 and at the top and bottom thereof. For evaporations requiringin excess of 2000 C., reflectors 24 should be placed between the crucible 10 and the electrodes 17 and 18. This position of reflectors 24 shields electrodes 17 and 18 from the intense radiation of crucible 10 and provides more efficient operation.
Maximum heating efiiciency is achieved through contact between the charge and the heater. This is ac' complished by removing liner 19 when there is no possibility of chemical reaction between the evaporant and heater 14. To promote maximum deposition efiiciency on substrate 12 of evaporant 19, converging-diverging nozzle 23 is positioned in liner 21 and is supported at the top of heater 14.
The converging-diverging configuration of nozzle 23 results in a highly efficient deposition process because th'e vapor stream, the boundaries of which are designated by reference numeral 25, is confined to a relatively narrow path having symmetrical cross sections of substantially constant -radial density. The stream density is quite uniform at any given elevation becaunse a random distribution of vapor molecules occurs as a result of molecular collisions at the nozzle throat. Hence, deposition of the evaporant on substrate 12 is substantially uniform over the area of interest.
The evaporant drifts from the area proximate to the bottom of crucible 10 to the throat of nozzle 23 at its point of smallest diameter and then travels through the diverging portion with increased speed. The small outlet area increases the efficiency of the heater since it reduces, to a large extent, the escape of radiant heat from the crucible interior, i.e., the converging walls of nozzle 23 reflect heat radiation back towards charge 19 to promote additional heating of the latter.
The nozzle tends to confine the vapor stream within'a desired conical shaped path 25 because of the following factors:
(a) -If ow involves relatively high vapor pressure and flow rate the nozzle provides a smooth transition from the high pressure at the source material 19 to the low pressure at the crucible 10 outlet and thus reduces scattering of molecules that would otherwise occur after an a-brupt increase in cross-sectional area of the liow channel with its attendant rapid drop in pressure.
(b) If flow is molecular (relatively low vapor pressure and flow rate) with few collisions between vapor molecules upstream from the nozzle throat, the diverging portion of the nozzle tends to direct the vapor molecules into paths parallel to the axis of the nozzle. This occurs because the angle of incidence at which a vapor molecule strikes the sides of the exhaust cone increases with each successive collision with the cone surface.
(c) Since vapor molecule have high velocities and momentum when emitted from a nozzle, there is less deflection, and therefore less scattering, when vapor molecules collide with residual gas molecules.
The nozzle also improves operation, particularly for evaporant materials having poor thermal conductivity but high thermal emissivity, by causing the combined con# figuration of the crucible and nozzle to approach that of 'a black lbody, within which temperature is uniform throughout. This improves efciency by raising the tem- -perature at thecharge surface, where vapor can most easily escape, relative t-o the temperature below the charge surface. The resultant reduction in pressure gradient through lthe charge also minimizes the blowing out of solid charge particles which might otherwise damage the film deposited an substrate 12. Blow out of particles is also reduced by the small size of the exhaust channel of the nozzle.
The problem of evaporant coating the Walls of nozzle 23 is eliminated by maintaining the molecules in the nozzle in a gaseous state. This is accomplished by maintaining the nozzle at high temperature through heat generated in lthe thin heater walls 16 in the area from the crucible base up nozzle 23 past its throat.
Nozzle 23 has a tendency to reduce the vapor fiow rate because of its small diameter throat. This tendency, however, is more than overcome by the added heating efficiency introduced by the nozzle. The converging diverging nozzle 23 results in a considerably greater efiiciency than is achieved with a sharp edged orifice having the same fiow area as the nozzle throat at its smallest diameter.
To maintain the pressure between clamps 17 and 18 with heater 14 at a sufficiently high level to enable an extremely low impedance connection to exist therebetween for all temperatures that the system must withstand (25 C. to 3000" C.), the special strap construction 31, best illustrated in FIGURE 2 is provided. Since similar strap constructions are provided for electrodes 17 and `1S, a description of the strap f-or electrode 17 suffices for both.
The end of electrode 17 nearest heater 14 is machined to have the same curvature as the outer heater Wall under ambient conditions. A thin strip of flexible refractory material, tantalum having a thickness of 0.003 inch in one preferred emb-odiment, surrounds the entire periphery of louter cylindrical wall 15 of heater 14 except in the area proximate copper electrode 17. Strap 32 is secured in place on the electrode by stainless steel nut and bolt set 33 to maintain intimate contact between electrode 17 and heater wall 15. Cantilever leaf springs 34 are secured to either side of electrode 17 by nut and bolt set 33. Springs 34, preferably made of molybdenum, extend along the exterior surface of strap 32 to a point near which heater 14 an-d electrode 17 make c-ontact and are used during assembly to take up the slack on strap 32. They also help to maintain constant contact pressure between heater 14 and electrode 17 over the -operating temperature range.
In operation, intimate contact between copper electrode 17 and the wall of graphite heater 14 is maintained over all operating temperatures despite the considerable difference in their coefiicients of thermal expansion.
This occurs -because the materials and dimensions of heater 14, electrode 17, and strap 32 have been selected so that electrode 17 pushes heater 14 against strap 32 with a slightly greater pressure as temperature rises, i.e., the expansion of copper electrode 17 between the points where strap 32 is secured to electr-ode 17 and the point where electrode 17 contacts heater 14 (distance A in FIGURE 2), is high enough to compensate for the amount by which the `thermal expansion of strap 32 exceeds the expansion lof the circumference of heater 14.
The deposition of evaporant on substrate 12 can be adjusted by utilizing the modified configurations of FIG- URES 3 or 4. In FIGURE 3, a -pin 41 is inserted in the crucible base at its center and extends vertically to form a tip at the center of the throat for nozzle 23. Thereby, an annular nozzle is provided t-o improve the uniformity of deposition thickness because the tendency of evaporant to accumulate at the center of the path is obviated. In FIGURE 4, the converging diverging throat of nozzle 23 is misaligned relative to the axis of crucible 10. Hence, the vapor stream is directed to areas on substrate 12 remote from the point aligned with the crucible axis.
'Ilo increase the rate of vapor evaporation, hence the time required for complete deposition, the modification of FIGURE 5` may be employed. In this embodiment, the evaporant is coated on wire mesh 43 that is vertically -mounted on the crucible floor. Because the evaporant has a great deal of open or exposed surface area, it is ra-pidly heated so that 4the rate of flow from the wire to substrate 12 is increased over the solid configuration of FIGURE l.
According to the embodiment of FIGURE 6, crucible liner 2.1 is modified so that it is supported only -at the shoulder of heater 14 between its inner and outer walls 15 and 16. Since liner 21 does not contact heater wall 16 at the highest temperature area thereof, chemical reactions which cause evaporant contamination and heater erosion are minimized. Also, the selection of materials for liner 21 is increased to include low as well as high resistivity substances by utilization of this technique. This is because the possibility of short circuiting around the heater is obviated by the single connection at an equipotential point, since voltage is constant at any given elevation around the circumference. While nozzle 23 has been excluded from FIGURE 6, it is to be understood that it may be included if desired or necessary.
Reference is now made to FIGURE 7 of the drawings wherein the crucible of FIGURE 1 is modified so that bottom stem 20 is excluded. In this configuration, which is more easily manufactured than that of FIGURE l, the narrow inner wall 15 of heater 14 is extended to its contact location with electrode 18.
This configuration permits the loading of a larger charge 19 in the crucible liner 21 and location of the charge near the level of maximum heater temperature which occurs approximately midway between the ends of the inner portion of heater 14 where the cross-sectional area is made small to produce greater resistance and therefore greater heat generation. The heater vent hole 22 is located at the bottom o f the heater for convenience of fabrication in this configuration.
While we have described 4and illustrated several specific embodiments of our invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and sc-ope of the invention as defined in the appended claims.
1. A vacuum evaporation source for vaporizing a charge of evaporant material, said source comprising:
(a) a resistance heater, including 1) spaced concentric inner and outer walls of high electrical resistance refractory material, said outer wall being at -least co-extensive with said inner wall,
(2) a bridge of said refractory material continu- -ously joining one end of each wall with the corresponding end of the other wall,
(3) an extension of said refractory material joined to and closing the other end of said inner wall and forming therewith a crucible for receiving said charge, said extension and the other end of said outer wall each having a thickness much greater than the thickness of said inner wall so that said inner wall constitutes a region of higher resistance to the lflow of electrical current;
(b) a pair of electrodes for supplying current to said resistance heater;
(c) resilient means for maintaining high contact pressure between a first of said electrodes and said extension and between the second of said electrodes and said outer wall proximate the other end thereof, at the regions of said greater thickness; to maintain said electrodes at a lower temperature than the temperature of said inner wall during passage of current through said electrodes, at positions remote from said charge Iand its vapor stream; and to promote electrical contact between said electrodes and said regionsA of Igreater thickness throughout the passage of current therethrough despite differences in thermal coecient of expansion between the materials of which said resistance heater, said electrodes, and said resilient means are composed.
2. The combination according to claim 1 wherein is further included means for controlling the direction and flow of the vapor stream issuing from said crucible.
3. The combination according to claim 1 wherein is included a crucible liner of electrically insulative and chemically inert material, relative to the materials of which said resistance heater and said charge are composed, for holding said charge.
4. The combination according to claim 3 wherein said resistance heater is comprised of graphite, said liner is comprised of boron nitride, said electrodes are comprised of copper, and said resilient means is comprised of tantalum.
5. The combination according to claim 1, wherein each of said electrodes comprises an electrically conductive power lead, an electrically conductive bus clamped to said power lead and having an end surface conforming in shape to that portion of the resistance heater with which the respective electrode is to Ibe maintained in contact; and wherein said resilient means includes a metallic strap fastened at its ends to said bus and encompassing said resistance heater at said portion thereof to urge said end surface of said bus against said portion, and at least one leaf spring for maintaining said strap in tension during temperature variations accompanying current ow through said electrodes.
6. The combination according to claim 2, wherein said means for controlling direction and ow of ythe vapor stream comprises a converging-diverging nozzle substantially housed within said inner wall and forming an exit from said crucible for material vaporized therein.
7. A vacuum evaporation device comprising a graphite resistance heater having a pair of concentric annular walls separated by an air gap, each wall having a pair of ends, one end of the innerwall of said pair of walls being joined with the adjacent end of the outer wall of said pair of walls, the other end of said inner wall being closed and having a solid substantially cylindrical portion projecting therefrom, said cylindrical portion and the other end of said outer wall each having a thickness substantially greater than vthe thickness of the inner wall; first and second electrodes maintained in contact with said cylindrical portion and said other end of said outer wall, respectively, under substantially uniform contact pressure throughout temperature variations accompanying current tlow therethrough; and a nozzle substantially confined within a portion of the space encompassed by said inner wall adjacent said one end thereof and spaced from said closed end to form therewith a chamber for receiving material to be vaporized, said nozzle having an aperture converging from said chamber to la constricted throat region and diverging beyond said throat region to control the direction and `flow of the vapor stream from said chamber when said material is vaporized,
8. The combination according to claim 7 wherein each of said electrodes is maintained in uniform contact pressure with the respective portion of said resistance heater by a strap looped about said respective portion and fastened to the respective electrode, each said strap having a coeflcient of thermal expansion less than that of the respective electrode, and a separate leaf spring associated with each strap and commonly fastened therewith for maintaining substantially constant tension in said strap during said temperature variations.
9. The combination according to claim 8 wherein each said strap is composed of tantalum, and each of said electrodes is composed of copper.
References Cited by the Examiner UNITED STATES PATENTS 1,015,091 1/1912 Simpson 13-31 1,691,079 11/ 1928 Nightingale 339-230 X 2,288,235 6/ 1942 Foley 338-332 X 2,337,679 12/ 1943 Ostenberg A 13-31 2,440,135 4/ 1948 Alexander. 2,615,060 10/1952 Marinace et al 13-22 X 2,693,521 11/ 1954 Alexander. 2,772,318 11/1956 Holland 13-25 2,793,609 5/ 1957 Shen et al. 118-49 2,798,932 7/1957 Evans 118-49 X 2,996,418 8/1961 Bleil v.- 118-49 X 2,998,376 8/1961 Smith 118-49 X 3,000,998 8/196-1 Wiora 339-230 X 3,075,263 1/1963 Iuckniess et al 13-22 X 3,117,210 1/1964 Herb 13-31 X 3,139,474 6/ 1964 Weech 13-31 FOREIGN PATENTS 140,197 1/ 1935 Austria.
166,663 1/ 1956 Australia.
915,619 7/ 1946 France. 1,022,715 1/ 1958 Germany.
RICHARD M. WOOD, Primary Examiner.
ANTHONY BARTIS, Examiner.
V. Y. MAYEWSKY, Assistant Examiner.