US 3291715 A
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INVENTOR. GERALD S. ANDERSON ATTORNEY United States Patent St. Paul, Minn, assignor, by mesne Beverly Hills,
This invention relates to a method and apparatus for generating a localized plasma and more particularly to a method and apparatus for producing a localized plasma for sputtering a target material.
Conventional sputtering techniques involve development of a plasma or a discharge in which a target material is situated. This plasma is produced by application of energy which generates a great deal of heat. Very often a substrate material which is to be treated in the plasma is a material which is very sensitive to high temperatures or due to the physical shape or size of the material the substrate may be damaged or otherwise altered by the high temperatures generated in the plasma. In many of these situations, such as sputtering, a plasma for sputtering the target may be the best technique of working with a physically small target and substrate to effectuate coating of the substrate. Consequently, it is desirable to limit the generation of heat in the plasma and still accomplish the sputtering of the target material and treatment of the substrate.
It is therefore an object of the present invention to provide a new and improved method and apparatus for generating a localized plasma.
It is a further object of the present invention to provide a new and improved method and apparatus for producing a localized plasma for sputtering a target material.
It is another object of the present invention to provide a new and improved method and apparatus of generating a localized plasma which minimizes the development of high temperatures in the region surrounding a substrate material to be treated.
It is yet another object of the present invention to provide a new and improved method and apparatus of using A.C. energy and particularly radio frequency (R.F.) energy for generating a localize-d plasma.
These and other objects of the invention will become more apparent when the specification is considered in view of the drawings in which:
FIGURE 1 is a front view in cross section of an apparatus for generating a localized plasma and FIGURE 2 is a fractional view of an alternate embodiment of a portion of the apparatus shown in FIGURE 1.
Refer first to FIGURE 1 of the drawings. In order to establish a zone in which a plasma may be developed, certain environmental conditions must be established. First of all the zone must be evacuated and charged with a gas which is ionizable. Such a zone is established by utilizing an envelope or bell jar 11. This bell jar 11 may be composed of glass or some other material which will withstand a vacuum. The bell jar 11 is mounted on a base 12 and a vacuum seal is provided between the bell jar 11 and the base 12 by simply utilizing conventional gasket type sealed or similar devices.
The envelope or bell jar 11 after it has been evacuated is then charged with an ionizable gas. Such a gas might be any one of the noble gases or mercury. The gas must be such that when it is subjected to electrical energy such as an electrical field which might be generated by an AC. current, it will ionize and produce a high concentration of positive ions and electrons.
Normally a plasma is developed by application of energy to the zone 13 of the bell jar 11. When ionization occurs, the plasma is developed throughout zone 13 enclosed by the bell jar 11. Such a plasma generates a great deal of heat due to the fact that a relatively large mass of gas is ionized and is in a high state of excitement within a relatively large volume. Normally, the temperatures which accompany the ionization in such a zone are sufficiently high to destroy or severely damage many small parts which may be treated in the bell jar 11.
In order to reduce the volume of gas which is energized and thus reduce the developed high temperatures, a second envelope or tube 14 is utilized. This tube 14 extends into the envelope or bell jar 11 through the walls of the jar. This tube 14 encloses a second zone wherein ionization is accomplished. An extension 16 of the tube 14 extends beyond the zone 13 and thus is not within the bell jar 11. This extension 16 is a closed volume. Portion 17 of the tube 14 extends into the zone 13 and contains an opening 18 which communicates with zone 13. Thus it is noted that the zone 19, established by the tube 14, and the zone 13 communicate through the opening 18 in the tube 14. This means that the type of gas and the pressure of that gas within zone 19 is the same as that which exists in the zone 13.
Next a target 21 is placed in the opening 18 between the ends 22 and 23 of the tube 14. If imaginary lines are drawn extending the sides of the tube 14 at the ends so that the ends close, the target material 21 would be situated within these lines. Thus, although the target material is in the zone 13, it is also confined within a zone which is a part of zone 19.
An electrode 24 is situated within the zone 13 and is energized by a source of DC. energy 26. The electrode 24 is connected to the positive side of the DC source 26 and the negative side of the source 26 is connected to the target material 21.
Energy may be applied to the system by any number of methods. For example, A.C. particularly radio frequency (R.F. energy) may be applied to the extension 16 of tube 14. This RF. energy is applied by utilizing an RF. loop 27. The R.F. loop is situated close to the extension 16 so that the electrical field developed by the radio frequency energy in the loop 27 ionizes the mercury or other gas which is in the zone 19. An R.F. source 28 may be connected to the RF. loop 27 by suitable switches. It is to be noted that RF. energy is preferred, however A.C. energy of other frequencies than those lying in the radio frequency band may be utilized with success.
With the apparatus thus situated and charged with gas, application of energy to the tube 14 at the extension 16 generates a plasma within the zone 19 which is confined to the zone 19. It is confined to the zone 19 even in the area of the target 21 where zone 19 communicates with zone 13. It has been found through experiments that the plasma which is developed in the zone 19 does not invade zone 13 which surrounds the portion 17 of the tube to any substantial degree. Thus the amount of heat which is generated may be maintained at a minimum simply because the volume of gas which is ionized and thus generating heat energy is reduced to a minimum by the tube 14. Additionally, part of the plasma, the plasma in extension 16 extends beyond the envelope .or chamber 11 and thus does not contribute heat energy to the zone 13. A substrate 34 which is to be coated with material which is sputtered off the target 21, is situated in zone 13 where it is maintained relatively cool throughout the sputtering operation. The substrate 34 may be relatively temperature sensitive but since heat development is reduced to a minimum, it may be coated by sputtering techniques. The sputtered material dislodged from target 21 coats the substrate 34 as it diffuses throughout the zone 13. Some advantage may be gained by applying a voltage to the substrate 34 through a terminal 36, however this is not essential to achieve coating of the substrate 34. Thus it is noted that the substrate 34 is situated in a position near the target 21 and yet it is in a relatively cool zone outside the plasma.
The plasma apparently is confined to the small area around the target 21 because the envelope 14 provides a boundary for confining the direction of the electron flow in the plasma. In other Words, the electrons which occur at the opening 13 are travelling in a direction normal to the plane of the end 23 of the tube and thus produce a plasma only within the imaginary boundaries of the tube 14 which surround the target material 21. Some development of plasma beyond the end of the tube 14 does occur but it has been found that the amount of ionization or development of plasma is relatively insignificant. In other words, a plasma is not developed in the zone 13 except to the extent that there is a small leakage at the opening 18.
The plasma can be further confined to the immediate vicinity of the target material 21 by application of an electromagnetic field in the proper direction illustrated by arroW 29. A Winding 31 is placed about the tube 14 and energized by a DC. source 32, thus providing a magnetic field in the direction shown which tends to focus electrons in the vicinity of the target material 21 so that the electrons do not escape from the area and develop a plasma in zone 13. This further reduces the amount of heat generation since it additionally reduces the amount of leakage plasma which is developed about the opening in tube 14.
Another method of reducing the amount of leakage plasma which is present in the zone 13 is illustrated in FIGURE 2 of the drawings. In this case, an aperture 33 is provided at only one place in the tube 14, thus providing a minimum area where leakage may occur.. The target 21 is still confined within the plasma developed in the tube 14 and is effectively treated with a development of a minimum temperature in the vicinity of the target material.
It is to be understood that the above described arrangements of method and apparatus are simply illustrative of an application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.
Now, therefore, I claim:
1. A device for sputtering material from a target electrode to coat a substrate, which comprises:
a first chamber for enclosing a first zone containing an ionizable gas and said substrate;
a tubular plasma chamber fabricated in the form of a closed loop for enclosing a second zone, said loop passing through the wall of said first chamber and having at least one portion extending within said first chamber, said portion of said loop Within said first chamber being provided with at least one opening, said loop receiving said gas through said opening;
means for ionizing said gas within said plasma chamber to produce a plasma therein; holder means for mounting said target within said plasma chamber adjacent to said opening; and means for applying an electric potential to said target to bombard said target with ions from said plasma and sputter said material through said opening onto said substrate. 2. A device for producing a localized plasma to permit coating of a heat sensitive substrate, which comprises:
first chamber means for enclosing a first volume of an ionizable gas and said substrate; means mounted outside said first means for ionizing a volume of ionizable gas other than said first volume; second chamber means for enclosing a second volume of said ionizable gas, said second means having a tubular, looped configuration including oppositely disposed sections received within said first means and provided with opposed openings located remotely from said substrate, said second chamber means having a portion extending outside said first means and adjacent to said ionizing means so that said second volume of gas is ionized to form a plasma, said portion extending through said first chamber means into communication with said opposed sections for enclosing said second volume apart from said first volume, said opposed sections being efiective to maintain substantially all of said plasma within said second means and between said opposed openings in said second means; holder means for mounting a target between said opposed openings for exposure to said plasma, said holder means also locating said target to permit deposition of atom sputtered from said target onto said substrate; and means for applying an electrical potential to said target to urge ions of said plasma from said opposed sections into collision with said target to effect ion bombardment of said target so that atoms are sputtered from said target for deposition on said substrate.
References Cited by the Examiner UNITED STATES PATENTS 4/1941 Burkhardt et al. 204-192 3/1949 Johnson 204-298 1/1962 Klein 313-63 2/1962 Wehner 204-192 3,120,621 2/1964 Gunther et al. 310-4 3,143,477 8/1964 Dolique 176-3 3,160,566 12/1964 Dandl et al. 176-7 FOREIGN PATENTS 809,323 7/1961 WestGermany.
JOHN H. MACK, Primary Examiner.
R. K. MIHALEK, Assistant Examiner.
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