US3776535A - Method for evaporating aluminum - Google Patents

Abstract

Method for evaporating aluminum utilizing a carbon liner having a recess therein by depositing the aluminum in the recess and then heating the aluminum in the recess in a location generally at the center of the recess. The outer margin of the liner is cooled to a greater extent than the inner portion of the liner so that the outer margin of the liner is cooler whereby the outer edge of the aluminum in the recess will be cooled to such an extent that the outer margin is not molten so that the aluminum will not creep over the edge of the liner.

Description

United States Paent [1 1 Sp 1 Dec. 4, 1973 [54] METHOD FOR EVAPORATING ALUMINUM 3,437,734 4/1969 Roman et al. 13/31 3 29 24 7 6 [75] Inventor: Vernon C. Spellman, Santa Rosa, 5 12 1:3

3,029,777 4/1962 Cerych et al 266/34 R [73] Assignee: Optical Coating Laboratory, Inc.,

Santa Ro c lifl Primary Examiner-Gerald A. Dost Art P lD.Fl h tal. 22 Filed: Mar. 13, 1972 omey e r e pp 233,936 57 ABSTRACT Related [15. Application Data Method for evaporating aluminum utilizing a carbon [62] Division of Ser. No. 29,410, April 17, 1970, Pat. No. liner having a recess therein y depositing the alumi- 3,684,263 num in the recess and then heating the aluminum in the recess in a location generally at the center of the [52] U5. C1. 266/34 R, 118/49 reeess- The outer m g of h liner i ool d o a [51] Int. Cl. .L C23c 17/00 greater extent than h nn r po io of the liner so [58] Field of Search 13/31; 266/34 R, that the outer m gin f h liner is ool r whereby 266/39, 43; 263/48; 118/49, 49,1 495 the outer edge of the aluminum in the recess will be cooled to such an extent that the outer margin is not [56] Referen Cit d molten so that the aluminum will not creep over the Smith l3/3l edge of the liner.

2 Claims, 2 Drawing Figures METHOD FOR EVAPORATING ALUMINUM CROSS REFERENCE TO RELATED APPLICATION This application is a divisional of application Ser. No. 29,410, filed Apr. 17, 1970 now US Pat. No. 3,684,268.

BACKGROUND OF THE INVENTION In the evaporation of certain materials in vacuum chambers certain problems are encountered. For example, in the evaporation of aluminum, difficulty has been encountered in that the aluminum, when melted, wets the surface of the material which is utilized for the crucible or the liner for the crucible so that upon cooling the aluminum contracts and cracks the liner which have been utilized. There is, therefore, a need for a new and improved source which can be utilized for evaporating materials.

SUMMARY OF THE INVENTION AND OBJECTS The source for use in evaporating materials consists of a crucible which is formed of a good heat conducting material. The crucible is provided with an upwardly facing recess which is formed by outwardly and upwardly tapered side walls. A liner is seated in said recess. The liner is formed of a material different from the crucible. The liner also has an upwardly facing recess formed therein. The liner is also provided with an outer side wall which is tapered upwardly and outwardly so that its taper is substantially identical to the taper of the side walls forming the recess in the crucible, the size of the liner being such so that a substantial portion of the tapered outer surface of the liner is in contact with the tapered side wall of the crucible. It is also preferable that the liner have a size so that its bottom wall is spaced about the top surface of the bottom wall for the recess in the crucible.

In general, it is an object of the present invention to provide a source for use in evaporating materials in a vacuum in which the liner will not break or crack during heating and cooling of the source.

Another object of the invention is to provide a source of the above character which is particularly suitable for evaporating metals and particularly aluminum.

Another object of the invention is to provide a source of the above character in which the metal will not tend to creep over the edge of the liner.

Another object of the invention is to provide a source of the above character in which there is a hot molten pool in the center during operation and in which the edges of the pool are relatively cool.

Another object of the invention is to provide a source of the above character in which there is not an undue accummulation of slag during operation of the crucible.

Another object of the invention is to provide a source of the above character which is relatively efficient.

Additional objects and features of the invention will appear from the following description in which the preferred embodiments are set forth in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a top plan view of a source for evaporating materials incorporating the present invention.

FIG. 2 is a cross-sectional view taken along the line 22 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawings, the source for the evaporation of materials consists of a crucible 11 which is formed of a suitable good heat conducting material such as copper. The crucible 11 has a circular geometry. However, it should be appreciated that if desired other geometries can be utilized; for example, a rectangular geometry. The crucible is provided with a relatively large upwardly facing recess 12. The recess is formed by an upwardly and outwardly tapered side wall 13 and a bottom circular planar bottom wall 14. A downwardly facing recess 16 is also provided within the crucible 11 and is adapted to receive a cooling fluid such as water which is utilized for cooling the crucible. The crucible is provided with a plurality of threaded holes 17 which are adapted to receive screws so as to make a watertight connection with the base or other support member (not shown) which has provisions for supplying the cooling fluid to the recess 16.

A liner 21 is seated within the recess 12 in the crucible. The liner 2-1 is formed of a material which is different from the material of the crucible 11. It is possible that the liner 21 can be formed of different materials and still be useful in the present invention. However, for use in evaporating certain materials such as aluminum, it has been found to be preferable to utilize a liner which is made of a high purity carbon. The liner is provided with an upwardly facing concave recess 22 which preferably is shaped so that it forms part of a sphere, or in other words it has a spherical surface. The size of the recess is such that a small annular planar generally horizontal surface adjoins the side wall forming the recess 22.

The liner 21 is provided with an outer side wall 24 which is also tapered upwardly and outwardly with a taper which is substantially identical to the taper provided for the side wall 13 of the recess 12 in the crucible 11. By way of example, the liner may have a thickness of approximately five-eighths of an inch at its thickest dimension and may be provided with a taper of approximately 5. However, if desired, a taper ranging from 2 to can possibly be used.

The liner 21 has a size with respect to the recess 12 and the crucible so that it will seat within the recess but in such a manner that the bottom surface 26 of the liner is spaced above the bottom wall 14 of the recess 12 to provide a space 27 between the bottom surface 26 and the bottom wall 14 for a purpose hereinafter described. In order to increase the extent which this space surrounds the liner 21, an annular step or recess 28 has been provided in the side wall 24 of the liner in such a manner so that it extends upwardly from the bottom surface 26 to approximately one-half of the side wall. This recess 28 provides additional air space which is in communication with the space 27.

From the construction noted even with the step or recess 28, there is a substantial area of direct contact between the upper portion of the side wall 24 and the side wall 13 forming the recess in the crucible 11.

Operation and use of the source may now be briefly described as follows. Let it be assumed that the source has been provided with a suitable continuously flowing cooling fluid and that it is to be utilized in conjunction with appropriate means for supplying heat to the material which is to be evaporated in the crucible. By way of example, one or more electron guns of a conventional type can be utilized for directing electron beams into the material in the liner to evaporate the same. The crucible can be operated either in a batch mode or a continuous mode. In the batch mode, the crucible is preloaded by placing a certain amount of the material to be evaporated within the recess 22 of the liner. If the crucible is to be utilized in the continous mode, wire of the desired material can be fed continously into the recess of cavity 22. One or more electron guns are then placed in operation to form a puddle of the material.

The electron beams from the guns are positioned so that the hottest point in the puddle will be near the center of the puddle and away from the outer margins of the liner. The outer margins of the puddle are generally cooler than the center of the puddle for the reason that there is relatively good contact between the upper portion of the liner 21 and the copper crucible 11 which is water cooled. This means that the outer edge of the liner is continously being cooled and in fact is cooled to such an extent that the aluminum will not remain molten at the outer edge of the liner. Since aluminum is not molten at the outer edge of the liner, there is no tendency for the aluminum to move out or creep over the very edge of the liner but rather will be contained in the center region of the liner. The liner 21 which is formed of carbon although it is an excellent conductor also serves as an insulator. Because of this fact, the temperature of the material being melted in the liner 21 increases toward the center. There is also additional insulation provided between the copper crucible and the liner which insulates the bottom portion of the liner from the crucible. This is the space 27 and the annular recess 28. This ensures that the hottest portion of the aluminum puddle is not cooled by the water cooled crucible l l but that only the upper outer margin of the pool is cooled. In addition, this greatly increases the efficiency of the crucible, and makes it possible to maintain the aluminum very hot so that it will evaporate very rapidly. The aluminum has a tendency to travel to the hottest spot and therefore it will have a tendency to travel towards the middle of the liner so that there will be no tendency for the aluminum to creep over the sides of the liner even though the recess 22 may be very full.

As hereinbefore described, the liner is provided with a spherical recess. It has been found that a recess having this shape is particularly advantageous when the source is used for evaporating aluminum. It has been found that the aluminum will wet the surface of the carbon liner and therefore when the aluminum cools it adheres to the carbon liner. However, upon cooling it is found that the carbon readily shears along the interface between the carbon and the cooled aluminum thus preventing the liner from cracking or breaking and thus enabling the source to be retired without the necessity of replacing the liner. The shearing action takes place because the carbon is considerably weaker than aluminum. The carbon particles which have been sheared off of the carbon liner up reheating will float out to the outer edge of the puddle and will form a sort of slag at the extreme edge of the aluminum puddle along with other impurities that are in the parent metal. When sufficient impurities and other slag accumulates, it may be necessary to eventually replace the liner. However, it has been found that a liner of this type can be reheated or refired many times for the evaporation of aluminum without the necessity of replacing the same.

The taper which has been provided on the liner and in the recess ensures that there is continuously good contact between the upper portion of the liner and the crucible regardless of expansion and contraction during all stages of heating the liner and the material contained therein. As pointed out above, this tight interface is necessary to maintain the outer margin of the liner relatively cool so that the aluminum will have no tendency to creep up the sides of the liner.

It has been found that it is possible to utilize the source for other materials as, for example, dielectrics. When such is the case, the liner can be machined so that the recess 22 is much larger and so that the wall thickness of the liner is only approximately .100 inch thick. In this way it is possible to greatly increase the capacity of the liner. This type of crucible also operates very satisfactorily because the liner has a slight amount of insulation heat wise as hereinbefore pointed out even though it is a good conductor electrically. This greatly increases the efficiency of the source since large quantities of the heat will not be lost through the watercooled crucible.

It is apparent from the foregoing that there has been provided a source which is particularly useful in evaporating materials in a vacuum chamber and particularly for use with electron guns. There is no tendency for the material which is being evaporated to creep over the edges of the liner. In addition, the material does not crack or break the liner when it cools. It is particularly satisfactory for evaporating aluminum which is one of the more difficult materials to evaporate.

I claim:

1. In a method for evaporating aluminum in a carbon liner having an upwardly facing recess therein forming a minor portion of the surface of a sphere, depositing aluminum in the recess, heating the aluminum in the recess in a location generally at the center of the recess to form a centrally disposed molten puddle of aluminum in the recess, cooling the outer margin of the liner to a greater extent than the inner portions of the liner so that the outer margin of the liner is cooler whereby the outer edge of the puddle of aluminum within the recess in the liner will be cooled to such an extent that the outer margin of the aluminium within the recess is not molten so that the aluminum will not creep over the edge of the liner.

2. A method as in claim 1 together with the step providing insulation for the bottom of the liner to minimize cooling of the central portion of the lines.

Claims (1)

1. 2. A method as in claim 1 together with the step providing insulation for the bottom of the liner to minimize cooling of the central portion of the lines.

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