Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2000-04-07
2002-08-20
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298210, C204S298260, C204S192120
Reexamination Certificate
active
06436252
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of magnetron sputtering, more particularly to methods and apparatus for producing coatings on the inside surfaces of cylindrical, curved or irregularly shaped objects, such as the internal surfaces of pipes.
BACKGROUND OF THE INVENTION
Sputter deposition is a process in which the surface of a workpiece is coated with a film/coating of material that is sputtered, i.e., physically eroded by ion bombardment of a target, which is formed of, or coated with, a consumable material (often termed a target material) to be sputtered. Sputtering is conventionally implemented by creating a glow discharge plasma over the surface of the target material in a low pressure gas atmosphere, termed the sputtering gas. Gas ions from the plasma are accelerated by an electric field to bombard and eject atoms from the surface of the target material. These atoms travel through the gas environment until they impact the surface of the workpiece or substrate to be coated where they are deposited, creating a coating layer. For reactive sputtering, the sputtering gas includes a small proportion of a reactive gas, such as oxygen, nitrogen etc., which reacts with the atoms of the target material to form an oxide, nitride etc. coating. DC and pulse (example AC) sputtering techniques are well known, over a wide range of frequencies, including RF sputtering.
In a typical sputtering operation, a target of, or coated with, the consumable material to be sputtered is placed within the low pressure gas plasma and is connected as the cathode. Ions from the sputtering gas, usually a chemically inert noble gas such as argon, bombard the surface of the target and knock off atoms from the target material. The workpiece to be coated is typically placed proximate to the cathode, i.e., within sputtering proximity, such that it is in the path of the sputtered target atoms, such that a film of the target/consumable material is deposited on the surface of the workpiece. The sputtered atoms leave the target surface with relatively high energies and velocities, so that when the atoms bombard the workpiece surface, they intermix into the atomic lattice of the workpiece surface, creating a strong bond.
While the overall yield of the sputtering process, that is the number of atoms sputtered per incident ion, depends on the energy of the incident ions, the overall sputtering rate not only depends on the energies of the incident ions, but also on the number of ions impacting the surface. Both the ion energy and the number of ions are dependent both on the level of ionization in the gas plasma (glow discharge) and on the location of this plasma with respect to the target surface. Therefore, it is desirable that the ions in the plasma be produced adjacent to the target surface, so that their energies are not dissipated by collisions with intervening gas atoms. While the number of gas atoms available for ionization (residual gas pressure) needs to be high, the requirement for minimum interference with sputtered particles acts in opposite direction. Consequentially, there is a need for high ionization efficiency and relatively low gas atom concentration.
One known way to improve the efficiency of glow discharge sputtering is to use magnetic fields to confine electrons to the glow region in the vicinity of the cathode/target surface. This process is termed magnetron sputtering. The addition of such magnetic fields increases the rate of ionization. In magnetron sputtering devices, electrons emitted from the target surface accelerate to a drift velocity that is orthogonal to both the directions of the electric field and the magnetic field as measured near the surface of the target. In most magnetron sputtering devices, the paths traveled by these electrons form a closed loop. Furthermore, such devices are typically designed so that the magnetic field lines form arches along which the electrons drift. As the electrons are emitted from the target surface, they move in proximity to the target surface, “frozen by the magnetic field”, thereby increasing the average ionization probability, but typically cause uneven erosion of the target material as a result of the non-uniform ionization probability along the target surface.
Magnetron sputtering increases the efficiency of the plasma generation because all of the electrons caught in the magnetic field have an increased effective path length in the proximity of the target, that is, each electron emitted from the target surface has a much longer distance of travel while in proximity to the target. The result is that the electrons have collisions with a much higher number of gas atoms, while still near the target. Accordingly, the resulting higher intensity plasma has more ions available to bombard the surface, resulting in a higher sputtered flux.
Magnetron sputtering processes are classified as planar or cylindrical. The planar (circular, rectangular and triangular shaped) magnetron sputtering devices generally suffer from non-uniform erosion, with the area of maximum erosion in the shape of a racetrack centered around the magnet position, rendering the target unusable after use, even while relatively large amounts of useful target material still remain. Also, planar magnetrons often employ large magnet structures, making them generally useless for creating films inside structures with hollow workpieces having annular cavities, such as narrow diameter pipes etc.
Several different types of cylindrical magnetron sputtering devices have been developed, as disclosed and summarized by Thornton et al., “Cylindrical Magnetron Sputtering”, 1978 Academic Press, Inc., pp. 75-113. Cylindrical magnetron sputtering devices are used to coat cylindrical workpieces, such as the inside surfaces of pipes. Basically, the target material in a cylindrical magnetron sputtering device is in the form of an elongated tube. In U.S. Pat. No. 4,031,424, issued to Penfold, solenoid coils are disposed around the outside of the magnetron chamber to provide the confining magnetic fields, and to create magnetic fields having flux lines parallel to the axis of the elongated cathode target. Such cylindrical magnetrons tend to be somewhat more even in their erosion patterns, however, they suffer from undesirable end effects, and the solenoid coils are complex and bulky. In a cylindrical magnetron, the electron drift in direction perpendicular to both the electrical and magnetic fields causes the electrons to orbit around a central target post. Unfortunately, the electrons tend to leak out or escape their orbits near each end of the central post, resulting in lower ionization intensities, and thus lower sputtering rates at each end of a cylindrical target. Furthermore the Penfold magnetron sputtering device uses an elongated target that may not be bent or shaped to follow the contours of irregularly shaped objects. If the targets in such magnetrons were to be bent or shaped to the contours of irregularly shaped objects, the magnetic field strength over the target surface would not be uniform, resulting in marked non-uniformity in the plasma sheet, and thus in a non-uniformly sputtered flux along and around the surface of the target.
U.S. Pat. No. 4,376,025 issued to Zega attempts to solve some of the problems associated with the Penfold cylindrical magnetron sputtering apparatus by reorienting the magnetic flux lines circularly around the axis of the elongated rod-like target material, as opposed to the axial orientation used by Penfold. Zega discloses a cylindrical magnetron device utilizing a tubular current-carrying electrode disposed within a tubular target cathode.
Instead of using a separate solenoid coil to generate the plasma confining magnetic field, Zega uses a high current carrying hollow electrical conductor disposed within a tubular target to generate a circumferential magnetic field that surrounds the tubular target. The disadvantage of this approach is that, while very efficient with small diameter targets, it becomes less efficient as the target diameter increa
Gorodetsky Alexander S.
Tzatzov Konstantin K.
Cantelmo Gregg
Greenlee, Winner & Sullivan
Nguyen Nam
Surface Engineered Products Corp.
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