Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1999-03-18
2001-05-22
McDonald, Rodney (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192120, C204S298160, C335S306000, C335S209000
Reexamination Certificate
active
06235164
ABSTRACT:
FIELD OF INVENTION
The invention relates to sputtering apparatus for depositing thin films of magnetically orientable material on substrates and to vacuum processing apparatus for magnetically orienting the material on the substrates.
BACKGROUND
Conventional sputtering devices include a vacuum chamber enclosing a target electrode of selected material and a substrate onto which the material is to be deposited. Air within the chamber is evacuated to a low pressure and is partially replaced by an ionizable gas, such as argon. A power supply applies a negative potential to the target electrode. Gas ions strike the target, causing an emission of atoms from the target into a plasma from which the target material is deposited on the substrate.
In certain applications, such as the manufacture of magnetic recording heads, thin films of magnetic material need to be applied with a predetermined magnetic orientation. The thin films are deposited by sputtering apparatus in a predetermined orientation by exposing the substrate to a uniform magnetic field (i.e., equal magnitude field lines extending in a common direction). Ordinarily, permanent magnets placed in the vicinity of the substrate are used to generate the magnetic field.
For example, most commercial implementations locate a pair of permanent magnets on opposite sides of the substrate to generate the required field across the substrate. However, only a small portion of the magnetic field between the electromagnets exhibits the necessary uniformity, and this limits the area of the substrate over which the target material can be deposited with the required magnetic orientation.
U.S. Pat. No. 5,026,470 to Bonyhard et al. discloses an alternatively designed sputtering apparatus having a polygon-shaped electromagnet located beneath the substrate. A spiral coil having a plurality of sides forming a polygon is embedded in a pallet assembly for producing a plurality of uniform magnetic fields that extend perpendicular to each of the sides. A plurality of substrates is positioned on the pallet assembly with respective edges aligned with one of the sides. However, this apparatus is very large, difficult to manufacture, and inefficient to operate with smaller lot sizes.
After coating and subsequent processing, the substrates are cut into smaller units that are used for such purposes as heads for disk drives. An industry trend now requires larger substrates (e.g., 15.25 centimeters squared) to be coated with more accurately aligned magnetic domains (e.g., within one degree alignment). The increased accuracy provides improved yield from the substrate, and the increased area of the substrate permits more heads to be manufactured simultaneously.
The larger substrates are difficult to coat with the present designs of sputtering apparatus. For example, the permanent magnets located at either side of the substrate must be spaced at large distances to produce the required uniformity. However, the added spacing requires impractically large magnets be used to produce the required field strength. Similarly, interferences between magnetic fields on each side of the pallet assembly of Bonyhard et al. can significantly limit the size of substrates that can be coated with accurately aligned magnetic material.
Although a uniform magnetic field is required in the vicinity of the substrate for magnetically orienting particles of the target material as they are deposited on the substrate, the same magnetic field in the vicinity of the target can cause uneven erosion of the target and variations in the thickness of deposited target material across the surface of the substrate. The magnetic field reacts with an electric field in the vicinity of the target causing emitted electrons to drift across the target and to increase local ionization and accompanying bombardment of one end of the target.
The sputtering device of Bonyhard et al. provides for rotating the pallet assembly to provide a more uniform coating of the target material on the substrates. However, this adds to the size and complexity of the device and requires the use of uniformity shields which cut off part of the sputtered flux in order to produce a uniform film deposition.
SUMMARY OF INVENTION
The invention improves sputtering of thin magnetically oriented films by enabling large substrates to be coated with accurately aligned magnetic domains. A highly uniform magnetic field is produced in the vicinity of a substrate with a practically sized and powered electromagnet. The invention also provides for depositing magnetic material with a uniform thickness across substrate surfaces without adding significant size or complexity to the new apparatus and without using shields to cut off part of the deposited flux.
The improved sputtering apparatus includes the usual features of a vacuum chamber enclosing a target electrode and a substrate holder made from a nonmagnetic material. A substrate is mounted on the holder with a prepared surface facing away from the holder. The electromagnet is located just outside of the vacuum space adjacent to the holder and includes a plate-shaped core and a series of parallel windings that are distributed between two ends of the core.
The electromagnet produces a uniform magnetic field within a defined area of a plane located at a predetermined distance from the electromagnet corresponding to the location of the prepared surface of the substrate. The magnetic field is defined by a locus of field strength vectors that vary in absolute magnitude but have substantially uniform components of magnitude (e.g., within a tolerance of five percent) within the defined area of the plane. The uniform components of magnitude are also aligned in a uniform direction within the same area of the plane to produce a so-called “easy” axis of magnetization on the substrate surface that varies from uniformity by as little as plus or minus one degree.
This high degree of accuracy is obtained over a large area of the substrate surface (e.g., over 15 centimeters squared) by specially arranging the windings of the electromagnet on the plate shaped core. For example, the windings can be divided into groups for carrying different amounts of current. More current is carried by the windings closest to the two ends of the core to compensate for changes in the strength of the magnetic field close to the ends. The current within each group of windings is adjusted to produce the uniform components of field strength in the uniform direction along the easy axis of magnetization across the entire surface of the substrate. In addition, the current can be adjusted to compensate for the buildup of magnetic material on the substrate or elsewhere in the vacuum chamber, which can increasingly shunt the desired magnetic field.
Similar results can be obtained by varying the density of the windings between the two ends of the core. For example, the windings can be arranged with increasing numbers of winding layers approaching the two ends of the core to appropriately vary the resulting magnetic field. Improved directional uniformity is obtained by specially shaping a core of the magnet to further compensate for variations in the magnetic field at both ends of the magnet.
However, we have also discovered that magnetically permeable field shapers can be positioned next to the substrate to greatly increase the area of the substrate surface that can be magnetically oriented with a given size electromagnet. The field shapers are made of a paramagnetic material that is specially shaped and oriented to align a much larger portion of the magnetic field to a uniform direction.
The invention also provides for minimizing the harmful effects of the magnetic field on the target to provide a more uniform coating thickness across the substrate. For example, the plate-shaped electromagnet can be powered by an alternating current that periodically reverses the direction of the magnetic field. The alternating current has a cycle rate less than 10 hertz to prevent the formation of significant eddy currents in the electromagnet core.
R
Ballentine Paul H.
Gerrish Kevin S.
Heimanson Dorian
Stephens, II Alan T.
CVC Products Inc.
Eugene Stephens & Associates
McDonald Rodney
Ryan Thomas B.
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