Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2000-09-28
2002-04-16
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298170, C204S298190
Reexamination Certificate
active
06372098
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to the field of magnetron sputtering. More specifically, the present invention is directed to methods and systems for providing magnetic fields within a magnetron sputtering device to achieve high target utilization.
BACKGROUND OF THE INVENTION
Magnetron sputtering is a technique for coating objects that generates a stream of coating material by sputtering a target through the use of a plasma discharge. Sputtering is a process in which material is dislodged from the surface of a material by collision with high-energy particles. In magnetron sputtering devices, high-energy particles in the form of plasma ions are directed towards the target under the action of an imposed magnetic field. Sputtering is controllable through the proper application of plasma parameters, such as pressure, power, and gas, and a magnetic field, which may also be controllable. In vacuum, the sputtered materials travel from the magnetron toward one or more workpieces and adhere to the workpiece surface. Through the judicious choice of plasma gases, magnetic design and physical layout, a wide variety of materials, including metals, semiconductors and refractory materials, can be sputtered to desired specifications. Magnetron sputtering has thus found acceptance in a variety of applications including semiconductor processing, optical coatings, food packaging, magnetic recording, and protective wear coatings.
Commonly used magnetron sputtering devices include a power supply for depositing energy into a gas to strike and maintain a plasma, magnetic elements for controlling the motion of ions, targets for generating coating material through sputtering by the plasma, and provisions for mounting or holding one or more workpieces for coating. Sputtering is accomplished with a wide variety of devices having differing electrical, magnetic, and mechanical configurations. The configurations include: various types of electrodes, one of which may be the target; sources of DC or AC electromagnetic fields or radio frequency energy to produce the plasma; and permanent magnets, electromagnets or some combination thereof to direct the ions. In addition, the vacuum chamber is connected to a vacuum pump and a gas supply for controlling the environment within the chamber. Target materials used with DC or mid-frequency AC sputtering are chosen from conductive elements or alloys which form conductive materials, such as metals, metal oxides and ceramics, and typically include, but are not limited to, silver, tin, zinc, titanium, chromium, or indium. Non-conductive materials may be sputtered using RF sputtering methods.
In practice, a plasma is struck within the vacuum chamber, and magnetic fields are used to accelerate ions in a plasma onto a target, thus enhancing sputtering from the target. In addition to sputtering the target, ion bombardment heats the target and other components. The performance of electrodes, magnetic elements and targets may be improved when the various components are cooled. This cooling helps to control temperature dependent material properties that might alter or degrade the magnetic field and also increases the stability and lifetime of components. When properly maintained, the electrodes and magnetic elements generally have long lifetimes, on the order of a decade or more. The targets must be replaced when new materials are to be sputtered, or when sputtering has reduced the thickness of the target to depletion or unacceptable levels. Thus the magnetic elements have relatively low maintenance requirements as compared to the target, which must be replaced at regular intervals during normal use.
The location and strength of magnetic fields, especially adjacent to the target surfaces, have great practical importance in magnetron sputtering devices. It is well known in the art that the interaction between the change in shape of the sputtered surface and the magnetic field over the target surface results in an acceleration of sputtering at locations where sputtering has begun. Thus it is common for targets to erode rapidly at certain locations, leaving other locations relatively uneroded. The faster the sputtering, the quicker the thickness of the target is eroded. As a result, a target with increased thickness is sometimes used to prolong the target lifetime. The increased thickness may increase the amount of material available for sputtering, but can adversely affect the total percentage of target material consumed during sputtering. The fraction of target material consumed during sputtering before the target must be replaced is sometimes called the “target utilization.” Utilization is greatly affected by the maximum rate of sputtering which may be concentrated in a focused region of the target surface. Even if the average sputtering rate over the surface is small, the peak sputtering rate at a particular target location can limit the total amount of time before the target must be replaced. Thus, sputtering uniformly across the entire target surface over the target lifetime can maximize utilization.
One example of a prior art magnetron is shown in FIG.
5
. The side view of
FIG. 5
is a representative cross section of a target
503
, including an initial front surface
507
and a back surface
505
, and a horseshoe magnet
501
as used in a planar magnetron sputtering device. The magnetic poles (N and S) and magnetic field lines (dotted lines) at an initial front surface
507
are also shown. Also shown is a sputtered front surface
507
′. During use, the target surface undergoes a loss or erosion of material due to sputtering which modifies the surface shape from initial front surface
507
to sputtered front surface
507
′. The change in shape of front surface
507
can affect the strength of the magnetic field at the target surface, especially for a non-magnetic target material, resulting in a change in the location of further sputtering. With sputtering faster at the center than at the edge, prior art target utilization tends to be low. Utilization in many prior art magnetron sputtering devices is in the range of 17-25%
Magnets used to control sputtering and increase target utilization and lifetime are generally designed through an iterative process to select the proper size, shape, strength, and location of the magnets. Cooling requirements for the target or magnet may put further restraints on the size, material, and shape of the magnets. Obtaining an optimal design usually involves the modeling and prediction of the optimum design followed by the deposition of a number of workpieces under a variety of conditions to optimize the magnetic field. Existing magnets impose some restrictions that make design optimization difficult. For example, Bernick (U.S. Pat. No. 5,736,019, issued Apr. 7, 1998) discloses a magnet design that provides improved target utilization in some cases. While the Bernick design is an improvement over some prior art systems it incorporates tapered magnets which are expensive, difficult to cool, difficult to manufacture, and provides limited means for tuning. This complexity adds to the cost of optimizing and of manufacturing the final product. Further by way of example, Manley (U.S. Pat. No. 5,262,028) addresses the need to provide improved magnetic fields by including magnets of differing magnetic orientation, with some poles oriented parallel and other poles oriented perpendicular to the target. While this combination of magnets does allow for some modification of the magnetic field at the portion of the target being sputtered, it requires a large number of magnets, and either a further increase in the magnetic field or a decrease in the desired thickness of the target.
There is a need in the art of magnetron sputtering devices for method and an apparatus that provides higher target utilization than that associated with prior art devices. In addition, there is a need for a magnet assembly having a small number of magnets, sized and shaped for easy manufacture and assembly. There is also a need for magnets th
Ceelen Hans Peter Theodorus
Newcomb Richard L.
Neida Philip H. Von
Nguyen Nam
Pace Salvatore P.
The BOC Group Inc.
Ver Steeg Steven H.
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