Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2000-02-02
2001-05-01
Huff, Mark F. (Department: 1734)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298060, C204S298180, C204S298260, C204S298140, C204S298170, C204S192120
Reexamination Certificate
active
06224725
ABSTRACT:
DESCRIPTION
The present invention relates to apparatus for magnetron sputtering, more particularly to apparatus for unbalanced magnetron sputtering, and most particularly to apparatus for enhancing the plasma density in a magnetron sputtering apparatus by independent control of the number and energy of plasma-forming electrons.
The term “unbalanced magnetron sputtering” (UMS) refers in the art to magnetron sputtering wherein the flux densities in north and south poles of a magnetron are unequal, resulting in some lines of magnetic force closing at infinity. These lines are referred to herein as “unclosed” or “open” lines. Because ionizing electrons follow a spiral path around magnetic lines of force, unclosed lines can provide a convenient pathway for guiding electrons away from the region of plasma immediately adjacent to the sputtering target and toward the substrate, thereby increasing the plasma volume and density in the vicinity of the growing depositional film. This can be beneficial in applications wherein a large or irregular object is the substrate to be coated with hard and corrosion-resistant materials, and particularly where the substrate is negatively biased to accelerate ions toward the growing film. Such bias can improve the quality of sputtered thin-films by improving density, composition, and microstructure. Therefore, the highest possible ionization density is desirable.
The principles of UMS are disclosed in a pair of publications by Window and Savvides in the Journal of Vacuum Science and Technology, A4(2), March/April 1986, and A4(3), May/June 1986. Structure and operation of a single unbalanced magnetron is described.
A “coating range” within which a substrate may be coated may be established by the magnetic coupling of two or more opposed UMS devices, as disclosed in U.S. Pat. No. 5,196,105 issued Mar. 23, 1993 to Feuerstein et al. Magnetic field lines which do not close within one UMS device may close with corresponding lines of another UMS device to form a magnetic boundary to electrons and ions within the coating range between the devices. The coating range can be bounded further by an independent magnetic field provided by an additional magnet or magnets disposed outboard of the sputtering target or targets of the UMS devices such that poles of opposite polarity are opposite one another on both sides of the coating range. This arrangement provides a “cage” of magnetic flux lines for retaining electrons and plasma in the coating range between the UMS devices. The coating range can be further broadened to accommodate large or irregular substrates by addition of another pair of UMS devices disposed orthogonally to the first pair. Other similar configurations are possible, as disclosed in U.S. Pat. No. 5,556,519 issued Sept. 17, 1996 to Teer.
A limitation of pure magnetron sputtering is that deposition of sputtered material on a substrate is substantially ballistic between the target and the substrate and thus is limited in deposition energy to the kinetic energy of the target atoms being sputtered. It is known that improved coatings can be achieved through negative electrical biasing of the substrate to make it more attractive to bombardment by ions. For example, the coating of opposite or multiple surfaces of a substrate using parallel opposed magnetron devices on opposite sides of a substrate can be improved by providing controlled-voltage anodes outboard of the targets for accelerating low-energy electrons which escape the plasma, as disclosed in U.S. Pat. No. 4,871,434 issued Oct. 3, 1989 to Mûnz et al., which energizes electrons to form further ions by collision with working gas atoms. The resulting increased ion density at the substrate permits reduction of the substrate bias voltage without reduction in the bias current. In this arrangement, however, there is no lateral constraint of the plasma other than the physical boundaries of the coating chamber.
A further problem, common to all magnetron sputtering schemes whether balanced or unbalanced, is that a field potential of typically 500 V or greater is necessary across the electric sheath overlying the target surface to provide sufficient acceleration of argon ions to cause sputter displacement of target atoms. The sheath also accelerates secondary electrons emitted from the target into the plasma. The collision of these accelerated electrons with argon atoms sustains the plasma sputtering reaction. However, electrons of 500 eV energy are relatively inefficient at ionization of argon atoms. The optimum energy for such electrons is known to be in the range of 50 eV to 100 eV. Therefore, in the known art, the energy needs for argon ion acceleration run counter to the energy needs for high-efficiency plasma formation.
Further, in the known art, the plasma is formed in the magnetron magnetic field in the vicinity of the target and extends to the deposition substrate only by diffusion.
Thus, there is a need for magnetron sputtering means whereby electrons may be generated, magnetically guided, and accelerated into the working gas at energies optimal for the ionization thereof, independent of the voltage across the electric sheath, and there is a further need for means whereby electrons may be accelerated toward the substrate to form a plasma in the vicinity thereof to intensify ion-bombardment of the substrate coincident with sputter deposition of target material.
It is a principal object of the invention to provide an improved apparatus for unbalanced magnetron sputtering wherein electrons for ionizing the working gas are provided independently of and in addition to electrons emitted from the target surface.
It is a further object of the invention to provide an improved apparatus for unbalanced magnetron sputtering wherein auxiliary electrons are provided at an energy level substantially optimal for ionization of the working gas which thereby increase the plasma density.
It is a still further object of the invention to provide an improved apparatus for unbalanced magnetron sputtering wherein both the number and the energy of auxiliary electrons may be independently controlled to control the density of the plasma.
It is a still further object of the invention to provide an improved apparatus for unbalanced magnetron sputtering wherein an insulating substrate is biased inherently and without resort to conventional radio frequency power supplies and controls.
Briefly described, an improved unbalanced magnetron sputtering apparatus in accordance with the invention has a conventional target which may be circular and planar. Adjacent the non-sputtered side of the target is a conventional arrangement of magnets wherein a central portion of the target is backed by a first magnetic pole, for example, south, and the peripheral portion of the target is backed by a second magnetic pole, for example, north. The respective poles are sized in cross-section or strength such that the flux passing through the central pole differs from (is greater or less than) the flux passing through the peripheral pole. The polepieces are connected across their distal extremities, thus creating conventionally a magnetic field in space above the surface of the target, which field at its extreme is parallel to that surface. Energizable primary cathodes are provided conventionally adjacent to the magnets to generate an electric sheath along the target surface. Because the polepieces are appropriately sized or of appropriate field strength, additional field lines extend from one polepiece which cannot close in the other polepiece but rather extend into space in a range of directions and generally toward a substrate to be coated. In a presently preferred embodiment, the central polepiece has fewer lines of magnetic flux passing through it.
Outboard of the target and electrically isolated therefrom is an independently-controllable secondary electrode, preferably a cathode, formed of a non-ferromagnetic material and having a surface facing in the same general direction as the sputterable surface of the target. The peripheral magnet polepi
Chacko-Davis Daborah
Harris Beach LLP
Huff Mark F.
Isoflux, Inc.
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