US2527747A - Apparatus for coating articles by thermal evaporation - Google Patents

Description

Oct. 31, 1950 M. N. LEWIS ETAL APPARATUS FOR comm; ARTICLES BY 'IHERIAL EVAPORATION Filed Jan.

1 n M; /M

Patented Oct. 31, 1950 APPARATUS FOR COATING ARTICLES BY THERMAL EVAPORATION Margaret N. Lewis and Park H. Miller, Jr., Philadelphia, Pa., assignors to the United States of America as represented by the Secretary oi War Application January 3, 1946, Serial No. 638,875

1 Claim. 1

This invention relates in general to methods and apparatus for coating, and more particularly to methods and apparatus for coating b thermal vaporization.

Heretofore, the thermal vaporization in a vacuum of metals such as germanium and silicon to produce a thin coating on a metal backing member has been done by heating a small quantity of such metal in a crucible by means of an ordinary resistance heating element. Such a method resulted in impurities in the film deposited on the backing member. Such impurities, it was found, emanated from the crucible material itself. For example, when a. quantity of silicon or germanium was placed in a tantalum crucible and directly heated by passing an electric current through the crucible, it was found that the tantalum reacted with the silicon at the temperatures used. To prevent this reaction, beryllium oxide was tried as a crucible and heated by a tungsten wire coil. However, traces of beryllium were also found in spectographic pictures of the evaporated films.

In order to obviate the difliculties mentioned above, it was found that the metal to be vaporized in vacuo could be brought up to the required temperature by means of direct electron bombardment so that the evaporated films were free of an discernible impurities due to reaction of the coating metal with the crucible.

It is an object of our invention to provide an apparatus and method for coating by thermal vaporization in which the contamination of the evaporated film due to reaction between the coating metal and the crucible is avoided.

It is another object of our invention to provide an apparatus and method for coating by thermal vaporization in which the coating material is brought to its vaporization temperature while maintaining the crucible at a much lower temperature.

It is another object of our invention to provide a. method of applying a coating in which the coating metal is vaporized in a vacuum by means of direct electron bombardment.

It is another object of our invention to provide an apparatus for coating in which the coating metal is vaporized in a vacuum by means 01' direct electron bombardment.

The above and other objects and advantages will become readily apparent from the following specification when read in conjunction with the accompanying drawing in which the single figure is a semi-diagrammatic view of one embodiment of our invention.

Referring to the drawing, it is seen that the (Cl. ill-12.2)

reference numeral l0 refers to a rigid base member of electrical insulating material. A metal support or pedestal ll carries a crucible l2 also made of metal, preferably tantalum. A quantity of the coating metal l3 which is to be vaporized is placed in the crucible II.

A cathode H which is to be a source of electrons for bombarding the coating metal I3 is connected by means of the conductors l5 and I6 to a source or electrical energy IT. A focussing element l8, preferably of cylindrical configuration is so situated as to focus the electrons emitted from the cathode H. In order to attract the electrons emitted from the cathode H to the metal I 3 to be vaporized, a, source of electrical energy i9 is provided, the positive side of which is connected through conductor 20 to the metal table or support II. The negative side of the source of electrical energy 19 is connected to the negative side of the cathode potential supply I".

The focussing element It is connected to the negative side of a source of electrical energy H by means of a conductor 22. The positive side of the source of electrical energy 2| is connected to the positive source of the source of electrical energy I 9.

The article to be coated indicated by the reference numeral 23 is supported upon a backing member 24 by any suitable means. The entire apparatus is enclosed by a transparent bell jar 25, the interior of which may be evacuated by connecting a vacuum pump to the hose connection 28.

The degree of vacuum required for vaporization by electron bombardment must be such as to permit the establishment of a stead flow of electrons from the cathode to the coating metal H but need not be any higher than 10- mm. of mercury. The high voltage supply I9 should be such .as to supply a direct current voltage of between 3,500 to 5,000 volts at 10 to 15 milliamperes. The negative potential at which the focussing element i8 is to ,be placed should be sufficient to cause a narrow beam formation of electrons emitted from the cathode I4 so that as far as possible substantially all of the electrons emitted from the cathode will impinge upon the supply of coating metal l3.

Our invention avoids the possibility of contamination of evaporated films due to reaction between the coating metal and the crucible. By bringing the coating metal to the evaporating temperature and maintaining the crucible at temperatures below the vaporization temperature 3 of the material forming the crucible, reaction between the coating metal and the crucible does not occur, and contamination of the evaporated film is obviated.

While we have disclosed one embodiment oi our invention it is obvious that many changes may be made without departing from the scope of the invention as set forth in the appended claims.

Having thus disclosed our invention what is claimed is:

A device for coating an article by thermal evaporation comprising an evacuated chamber, supporting means within said chamber for said article, an electrically conducting base member in the chamber, a container in the chamber for a coating metal to be vaporized, said container being supported by said base member, electron emitting means including a cathode positioned intermediate said article supporting means and said electrically conducting member and in a straight line therewith, a first source of direct current potential, means connecting the positive terminal of said source of potential to said electrically conducting base member and the negative terminal to said cathode: means for focusing the electrons from said cathode on said material to be vaporized, said means for focusing comprising a metal cylinder having its axis on said straight line and positioned so as to surround the cathode, a second source of direct current potential and means connecting said second source between said focusing means and said electrically conducting bas member.

MARGARET N, LEWIS.

PARK H. MHLER, J R.

REFERENCES CITED The following references are of record in the tile oi this patent:

UNITED STATES PATENTS Number Name Date 1,326,794 Larsen Dec. 30, 1919 1,584,728 Case May 18, 1926 2,103,623 Kott Dec. 28, 1937 2,157,478 Burkhardt, et a1 May 9, 1939 2,239,842 Burkhardt, et a1. Apr. 22, 1941 2,435,997 Bennett Feb. 17, 1948

Images