US 2664852 A
Description (OCR text may contain errors)
Jan. 5, 1954 E. E. CHADSEY, JR 2,664,852
VAPOR COATING APPARATUS Filed April 27, 1950 IN V EN TOR.
ATTORNEY Patented Jan. 5, 1954 VAPOR COATING APPARATUS Earl E. Chadsey, Jr., Newton, Mass., assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Application April 27, 1950, Serial N0. 158,423
This invention relates to coating and more particularly to vacuum-deposition coating of the type wherein a metal, such as aluminum, is vaporized in a vacuum and the vapors thereof are condensed on a substrate which is moved past the source of the metal vapors.
In vacuum-deposition coating, it is desired to have a relatively large source of the coating metal, such as aluminum, so that large areas of substrate may be coated without shutting down the operation of the coating device. In the prior art this problem has been approached in several ways. In one prior art method, a large source of the metal, such as aluminum, is provided and vaporization takes place from the whole source. In another method, a relatively small source of aluminum is provided and aluminum is fed thereto in solid form.
In the first prior art method the radiant heat transferred to the substrate is very excessive unless the aluminum is heated to a high temperature. However, heating to this high temperature has the disadvantage that the rate of vapor emission at these high temperatures is so great that the substrate must be moved at an impracticably high speed past the source. This first method also has the disadvantage that aluminum at high temperatures is very reactive with practically all materials, and the containers that hold such high temperature aluminum require special surface coatings to prevent reaction between the aluminum and the container. In the second method, Where solid aluminum is fed to the vapor source, this feeding has numerous disadvantages. In the first place, the solid aluminum may cause a vapor shadow which interferes with the uniformity of the coating operation. In the second place, this feeding involves a relatively complex control of the amount of feed. In the third place, this feeding of cold metal is apt to cause considerable changes in the temperature of the aluminum being evaporated, thereby changing the rate of vapor emission.
Accordingly, it is a principal object of the present invention to provide an improved process for vapor-deposition coating with metals, such as aluminum, wherein the above-mentioned disadvantages are overcome.
Another object of the invention is to provide an improved process for vapor-deposition coating of the above type wherein a large supply of mol ten metal is maintained within the vacuum coating chamber, but only a small portion of the surface of the molten metal is used for vaporizing the metal.
Another object of the invention is to provide an improved vacuum-deposition coating apparatus which provides for accurate coating control by providing a steady vapor stream, which permits a high temperature of evaporation of the metal, and which can operate over a long period of time.
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others, and the apparatus possessing the construction, combination of elements and arrangement of parts which are exem plified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.
For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:
Fig. 1 is a diagrammatic, schematic, partially sectional view of one embodiment of the invention; and
Fig. 2 is a diagrammatic, schematic, partially sectional view of an alternative embodiment of the invention.
In the practice of the present invention there is provided a usual vacuum coating chamber which can be evacuated to pressures in the micron range. Within this chamber, or adjacent thereto, there is provided a large body of the metal to be coated by vacuum-deposition procedures. In a preferred form of the invention this metal comprises aluminum, and for simplicity of description aluminum will be referred to hereafter.
This large body of aluminum is preferably positioned in a container which can be heated to a point somewhat above the melting point of aluminum so as to maintain the aluminum therein in a molten condition. Associated with the container for holding this relatively large body of molten aluminum, there is provided an evaporating zone in which the aluminum may be heated to a relatively high temperature, on the order of 1300 C. or above. At this temperature the heat transmitted by radiation from a unit area of the evaporating metal surface to a unit area of the substrate is less than the heat trans: mitted to a unit area of the substrate by the vapors condensing thereon. This evaporating rating area so that the vapor emission rate from the total area will not be too high, it being apparent that extremely high total vapor emission rates require unduly high substrate speeds. In practicing the process of the invention, the aluminum in the large source is maintained at a relatively low temperature, just above its melting point for example, and is fed from the large source to the evaporating zone to maintain a predetermined amount of aluminum in the evaporating zone. Thus, most of the aluminum in the coating apparatus is maintained at a sufficiently low temperature so that it does not have an appreciable vapor pressure (1. e. less than about .001 mm. Hg abs.) and is not particularly reactive with the container materials which confine the molten aluminum. However, thealuin'inum which is being evaporated is raised to a high temperature, on the order of 1300" C. and above, so that there is minimum radiation per gram. of aluminum evaporated, the evaporating aluminum having a vapor pressure .on the order of 0.3 mm. Hg abs. and above.
In one preferred form of the invention, the molten aluminum is fed by means of induced .currents existing in the aluminum as a result .of the heating thereof. In this form of the invention, the aluminum is heated in the evaporation zone by a high frequency induction coil, and the physical flow currents produced in the body of the aluminum by induced eddy currents are utilized for forcing the aluminum from the low-temperature pool thereof to the high-temperature evaporation zone.
In another embodiment of the invention, the
aluminum is fed from the low-temperature sup- 1 ply thereof to the evaporation zone by means of a difference in pressure between the pressure existing over the surface of the pool of lowtemperature aluminum and the aluminum in the evaporation zone. at a very low pressure, on the order of 1 mm. Hg abs. or less, a very small pressure over the surface of the low-temperature pool of metal is suflicient to lift the aluminum to a very considerable height.
Instill other forms of the invention the feed of the molten aluminum may be achieved by gravity flow, or actual pumping thereof, by means of the application .of a mechanical force, such as a centrifugal force, .to the molten low-temperatune aluminum.
In the above embodiments of the invention the evaporation zone preferably comprises a second container, and the low-temperature molten .aluminum is preferably fed thereto at a point below the evaporation surface to prevent lowering of the temperature of this evaporation surface as .a result of the feed of the cooler aluminum thereto. This low-temperature molten aluminum is preferably fed to the bottom of this second container and rises in the second container, it being heated as it rises to the high temperature desired for the rapid high-temperature evaporation at the evaporation surface.
This second container, which comprises the evaporation Zone, preferably confines the hightemperature aluminum therein so that the evaporation surface of the high-temperature aluminum has a smaller area than the surface area of the molten aluminum in the first container.
Referring now to Fig. 1, there is shown one preferred embodiment of the invention wherein the feeding of the molten aluminum to the evaporation zone is achieved by physical flow currents in the aluminum produced by induced electrical Since the evaporation zone is therebetween the substrate 20.
' tures on the order of 1300 C. and above.
4 l eddy currents. The apparatus of Fig. 1 comprises a vacuum-tight housing l0 which defines therewithin a vacuum chamber l2, this vacuum chamber being arranged to be evacuated to very low pressures, on the order of one micron, by means of a vacuum pumping system schematically indicated at [4. Within the vacuum chamber there is provided a means for supporting the substrate to be coated, this means comprising a first spool 16 and a second spool l8 carrying For holding the relatively large supply of metal, there is included a low-temperature zone generally indicated at 22,
while for evaporating the metal there is provided a high-temperature zone indicated generally at The low-temperature zone comprises a con tainer 24 which may be formed of carbon or the like, this container holding a large body of molten aluminum 26.
For feeding the molten aluminum 26 from the low-temperature zone in the bottom of the carbon container 24., there is provided a guide means shown as a hollow, cylindrical tube .28, which extends from the low-temperature zone 22 to the high-temperature zone 21. Cylindrical tube 28 thus serves as a second and smaller container which holds the high-temperature aluminum. The large container 24 is preferably partially closed by a cover plate 30, of a refractory material, which prevents the passage of radiation and low-temperature vapors from the large body of aluminum to the substrate 20. A spacing 32 between the cover plate 30 and the feeding tube 28 permits the retru'n of excess aluminum from the high-temperature zone back to the lowtemperature zone. For heating the aluminum in both the low and high-temperature zones, there is provided an induction coil 34. This coil .is preferably arranged so that a majority of the heating current is induced in the upper portion of the feeding tube 28 and the aluminum contained therein. However, .a sufficient amount of the current is induced in the low-temperature body of aluminum to maintain the aluminum molten therein. This induction .coil .34, due to the creation of eddy currents in the tube 28 and in the molten aluminum, also acts to cause :a physical flow of the aluminum upwardly from the low-temperature, large body thereof into the upper end of the tube .28 and thus .into the hightemperature evaporation zone. The tube 28 and the aluminum therein become highly heated due to these induced eddy currents. A suitable power source 36 is indicated for supplying an alternating current to the induction coil 34.
In a preferred form of the invention, the container 24 comprises carbon or a carbon graphite mixture which is relatively .inert to molten aluminum at temperatures below about 1000 C. The feeding tube 28 may be formed of carbon, but preferably comprises carbon or the like having a surface stratum of zirconium or a similar group IVa or group Va metal carbide, which is relatively inert .to molten aluminum at tempera- If such a zirconium carbide surface stratum is employed, the wetting action of the aluminum thereon assists in the lifting action of the induced eddy currents in the aluminum, this wetting action being described more fully in the copending application of Godley filed on even date therewith. In one preferred embodiment of the Fig.
1 form of the invention, the carbon container 24 and the feeding tube 28 have a wall thickness of A inch. When an alternating current of 10,000 C. P. S. is. used, the induced current has a skin depth of about .7 inch so that most of the induced current flows within the aluminum.
In the operation of the device of Fig. 1, a substrate is positioned in thevacuum chamber I2, and the chamber is evacuated to a low pressure, on the order of one micron Hg abs, by means of the vacuum pumping system I4. The aluminum 26, in solid form, is then heated by supplying current to the induction coil 34. As the aluminum becomes molten, it becomes more concentrated in the center of the container 24 by the action of the eddy currents flowing within the aluminum, and the central portion of the aluminum rises up within the feeding tube 28 as shown, the top portion of this lifted column of aluminum extending up into the high-temperature evaporation zone 21. As a result of these eddy currents, the aluminum in the evaporation zone is heated to relatixely high temperatures, preferably above 1300 C. to cause rapid evaporation of the aluminum. This high-temperature evaporation gives a low amount of radiant heat per gram of aluminum coated on the substrate. The relatively low temperature of the aluminum in the bottom of the container 24, in the lowtemperature zone 22, prevents attack of the molten aluminum by the carbon crucible 24. This arrangement has the additional advantage that any aluminum carbide scum which forms into the tube 28 enters the tube at the bottom thereof, any aluminum carbide contained in this latter portion of the aluminum being violently agitated and remaining intermixed so as not to form a scum. The aluminum in the high-temperature zone 21 is also being violently agitated with the resultant prevention of scum formation, as described more fully in the copending application of Chadsey et al., Serial No. 134,988, filed December 24, 1949, now Patent No. 2,643,201.
The embodiment of the invention described in Fig. 1 thus provides a coating device wherein the major portion of the aluminum is confined at relatively low temperatures, and the aluminum is fed from the large, low-temperature zone to a small, high-temperature evaporation zone where a rapid evaporation of the high-temperature aluminum is achieved.
Referring now to Fig. 2', where like numbers refer to like elements in Fig. 1, there is shown an alternative embodiment of the invention r wherein the aluminum is fed from the low-temperature, large body thereof to the high-temperature evaporation zone by gas pressure on the surface of the low-temperature body of aluminum. In this embodiment of the invention the low-temperature zone 46 comprises a gas-tight container 42 in which a carbon container 44 is positioned for confining low-temperature molten aluminum. The means for feeding molten aluminum from the low-temperature zone to the high-temperature evaporation zone 45 comprises a tube 40 which extends from below the surface of the aluminum in the low-temperature zone to the high-temperature zone. For heating the aluminum in the high-temperature zone I confined thereby, is preferably shielded by a re- 'in the low-temperature zone 40 by means of an induction coil 56. Induction coils 4B and 56 are preferably suitably energized from power sources which are not shown.
In one preferred embodiment of the Fig. 2 form of the invention, the feeding tube 46 is connected in a gas-tight manner with the gas-tight housing 42 to permit the creation within the housing 42 of a gas pressure greater than that existing in the coating chamber. For providing this gas pressure there is included a pipe 58 leading from a suitable source 60 of gas. This source 60' may comprise a source of an inert gas, such as argon or the like, or may be a bleed valve to the atmosphere. It is preferred that an inert gas be employed so as to prevent reaction between the aluminum in the low-temperature zone and the gas.
The Fig. 2 embodiment of the invention is preferably formed of the same materials as those employed in the Fig. l embodiment. In this case the container 44 preferably comprises carbon, as does the connecting tube 46. The'tube 46 also preferably includes a surface stratum of a carbide of the group IVa and group Va metals. The gas-tight housing 42 preferably comprises a metal such as steel. The gas supply tube 58 may also be formed of metal.
In the operation of the Fig. 2 device, the coating chamber I2 is evacuated. The low-temperature zone 40 may also be evacuated at this time through a suitable connection (not shown). Heat is then supplied to the low-temperature zone 40 by means of induction coil 56, and gas is provided through the tube 58 to provide a pressure differential between the interior of the gas-tight housing 42 and the evacuated chamber l2 to force the molten aluminum up to the hightemperature evaporation zone 45. Since the vacuum chamber [2 is at a very low pressure, less than 1 mm. Hg abs, the absolute pressure over the surface of the molten aluminum does not have to be very high. When the aluminum reaches the hightemperature evaporation zone, it is rapidly evaporated by heat supplied by the induction coil 48, this heat preferably raising the temperature of the aluminum in the evaporation zone to temperatures on the order of 1300 C. or above. The level of the aluminum in evaporation zone 45 may be readily controlled by controlling the pressure of the gas existing above the surface of the molten aluminum in the gas-tight housing 42. As will be apparent, this pressure is essentially constant, it being necessary to increase the pressure slightly as the level of aluminum in the gas-tight housing falls due to evaporation of the aluminum from the high-temperature zone. Since the aluminum in the low-temperature zone 40 is relatively inert, it is a relatively simple matter to measure the level of the aluminum therein by mechanical or electrical means, this indication of level being utilized to control the degree of gas pressure existing within the gas-tight housing 42.
As mentioned previously, the present invention has been described in connection with preferred embodiments thereof, but other forms may be equally utilized. For example, other mechanical forces may be employed for feeding the aluminum from the low-temperature zone to the hightemperatin'e evaporation zone. For example, centrifugal force may be employed. Equally, gravity may be utilized for causing the fiow of molten aluminum from a relatively low-temperature, large body thereof to a high-temperature evaporation zone. While induction heating has been shown for simplicity of illustration, other forms, such as radiant heating, are equally practicable.
Since certain changes may be made in the above process and apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
Apparatus for coating a heat-sensitive substratewith aluminum by vacuum-evaporation of said aluminum and condensation of said aluminum upon said substrate, said apparatus comprising means defining a vacuum-tight chamber, a container mounted in said vacuum chamber for confining molten aluminum, a heater surrounding said container for melting aluminum in said container and maintaining said aluminum at a. temperature not much in excess of the melting point thereof, aheat-resistant carbon tube mounted vertically in said chamber for confining high-temperature molten aluminum within said 8 vacuum chamber so that vapors from said hightemperature aluminum maybe condensed on-said substrate, said tube having an interior surface stratum comprising a carbide of the class consisting of the carbides of the metals titanium. zirconium, hafnium, vanadium, columbium and tantalum, a second heater for heating molten aluminum at the top of said tube to a temperature on the order of 1300 C. and above, said tube extending below the surface of the molten aluminum in said container, pneumatic means for forcing aluminum up said tube from said container to the top of said tube, a radiation shield between the upper portion of the outer surface of said tube and said substrate for preventing radiation from said tube to said substrate, and means for moving said substrate past the top of said tube.
EARL E. CHADSEY, JR.
References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,143,723 Walker et al Jan. 10, 1939 2,153,786 Alexander et al Apr. 11, 1939 2,363,781 Ferguson Nov. 28, 1944 2,382,432 McMa-nus et a1 Aug. 14, 1945 2,384,578 Turner Sept. 11, 1945 2,450,853 Colbert et a1 Oct. 5. 1948 2,508,500 De Lange et a1 May 23, 1950 2,584,660 Bancroft Feb. 5, 1952