Patent Application
20070080056
The invention relates to a system and apparatus for depositing films on surfaces, and more particularly to a method and apparatus for reducing or eliminating non-uniformities and variations in deposited film thickness during magnetron sputtering operations.
A variety of methods exist to apply coatings, such as thin films, to substrates, such as glass. Generally, sputtering is a technique for forming a thin film on a substrate. Sputtering techniques include diode DC sputtering, triode sputtering, and magnetron sputtering.
Magnetron sputtering has become a widely used sputtering technique. Films formed by sputtering can be important for numerous devices, such as semiconductors and window glass. Typical films created by these processes include metallic materials such as silver, aluminum, gold, and tungsten, or dielectric materials such as zinc oxide, tin oxide, titanium oxide, silicon oxide, silicon nitride, and titanium nitride. Magnetron sputtering involves providing a target, including or formed of a metal or dielectric material, and exposing this target to a plasma in a deposition chamber. Ions formed in the plasma may be accelerated toward the target due to the presence of an electric field. Momentum from this ion bombardment is transferred to atoms on the target's surface, thereby causing atoms of the target to gain enough energy to leave the surface of the target. Some of the atoms that have been rejected from the surface of the target in this manner are deposited on a substrate, thereby providing thin, uniform coating layers on substrates.
The gas used to form the plasma may be an inert, non-reacting gas, such as argon. Alternately, or additionally, reactive gases, such as nitrogen or oxygen, may be used to form the plasma. Reactive gases may combine with sputtered atoms during the formation of the sputtered coating. Deposition of reacted compounds, such as zinc oxide, tin oxide, etc., may be achieved in this manner.
To improve the efficiency of the sputtering process (i.e., to improve sputtering rate), the number of available ions may be increased by increasing the density of the plasma. To obtain a high density plasma, an electric field and a magnetic field may be used together to produce a resultant force on electrons that tends to keep the electrons in a region near the surface of the target (i.e., the“plasma-containing region,” or“confinement region”). The resultant force on electrons in such a region is governed by the vector cross product of the electric and magnetic fields (the“E×B” drift path). For example, a magnetic field may be formed such that the magnetic lines of flux are in a direction that is generally parallel to the surface of the target. An electric field may be provided (e.g., by applying a voltage to the target) to accelerate electrons in a direction perpendicular to the surface of the target. The resultant force on the ions is defined by the“E×B” drift path and is in a direction perpendicular to both the electric and magnetic fields, governed by the“right hand rule.” This force on the electrons results in“electron drift paths,” which may be used to keep the electrons near the surface of the target, where they may collide with other neutral atoms or molecules (from the plasma or sputtered atoms from the target), thereby causing further ionizations and increasing the sputtering rate.
The magnetic fields used in sputtering magnetrons are typically provided by placing one or more magnets behind the target to help shape the plasma and focus the plasma in an area adjacent the surface of the target (i.e., the“confinement region”). The magnetic field lines may, for example, emanate from a magnet (or magnets) placed behind the target, penetrating through and forming arcs over the target surface such that the magnetic field lines are substantially parallel to the target surface. The plasma may be concentrated near the surface of the target by wrapping and joining the magnetic field lines upon themselves to form a closed-loop“racetrack” pattern. This can be done, for example, with appropriately sized and shaped magnetic elements. A“planar magnetron” configuration is shown in FIGS. 1 (a) and (b) illustrating racetrack-shaped plasma-containing electron drift paths.
Planar magnetrons tend to develop racetrack-shaped grooves eroded into the targets, caused by continued sputtering in a racetrack pattern that is largely static relative to the target. Erosion is strongest near the center of the path formed by the magnets (due to the increased confinement of plasma in this area), which tends to create a“V-shaped” racetrack groove in the target surface. As the groove deepens, uniformity of the film being deposited tends to get worse and sputtering rates tend to decline. The utilization of target material is typically quite low for planar magnetrons as a result, with target utilization in some cases falling in the range of about 15% to 30% of total target volume.
A cylindrical magnetron target assembly is shown in a partial cut-away perspective view in
Although rotation of the cylindrical magnetron targets improves target utilization, it has been difficult to simultaneously optimize target utilization and sputtering uniformity. If, on the one hand, the confinement field at the turnarounds is substantially the same strength as along the straight portions, then excess target erosion occurs at the turnarounds. This is because a point of the target that rotates tangentially through the arc of the turnaround spends more time in the high-density portion of the plasma than do points that rotate perpendicularly through the straight portions of the racetrack. A common method to improve target utilization is to weaken the magnetic confinement at the turnaround in order to compensate for the excess time a point spends in the plasma at said turnaround. However, the weakened magnetic confinement leads to electron losses and changes in the drift velocity which, in turn, result in spatial variations of plasma density and hence non-uniform sputtering rate. These effects are not immediately self-correcting and extend well beyond the immediate vicinity of the turnaround. So even though the turnaround may be away from the substrate, non-uniform deposition on the substrate may result from issues at each turnaround.
The result of uneven erosion patterns is that by the time the target must be replaced due to nearly complete erosion near the target ends, the central portion of the target still retains a substantial amount of sputterable target material. Better target utilization is desirable to minimize waste of target material. Moreover, replacing used targets is a time-consuming and expensive operation, typically requiring the sputtering line to be shut down for a significant period of time.
In certain embodiments of the invention, a cathode target assembly for use in sputtering target material onto a substrate includes a generally cylindrical target and a magnetic array, the magnetic array adapted to provide a plasma confinement region adjacent an outer surface of the target comprising a plurality of electron drift paths along the length of the target. In certain embodiments, the magnetic array, or portions thereof, may be further adapted to oscillate generally axially with respect to the cylindrical target in order to further promote efficient target utilization along the length of the target.
In certain other embodiments of the invention, a magnetic array for generating a plasma-confinement region during sputtering operations may include a plurality of magnetic elements adapted to be disposed within a cylindrical target to provide a plurality of magnetic flux loops along a length of the cylindrical target.
In an alternate embodiment of the invention, a magnetic array for generating a plasma-confinement region during sputtering operations may include a magnetic element adapted to be disposed within a cylindrical target to provide a generally serpentine-shaped magnetic flux loop along a substantial length of the cylindrical target.
In certain embodiments of the invention, a method of sputtering material from a cylindrical cathode target may include: providing a deposition chamber having a cathode target assembly and a conveyor for moving a substrate through the deposition chamber in proximity to the cylindrical cathode target of the cathode target assembly; forming a plurality of plasma-containing confinement regions along a length of the cylindrical cathode target; rotating the cylindrical cathode target relative to a magnetic array disposed within the cylindrical cathode target; and moving the substrate through the deposition chamber in proximity to the cylindrical cathode target, wherein the magnetic array comprises a plurality of magnetic elements adapted to provide a plurality of magnetic flux loops along a length of the cylindrical cathode target.
FIGS. 1 (a) and (b) are schematic representations of the formation of electron drift path “racetracks” used in a planar magnetron sputtering process.
The following detailed description should be read with reference to the drawings, in which like elements in different drawings are numbered identically. The drawings depict selected embodiments and are not intended to limit the scope of the invention. It will be understood that embodiments shown in the drawings and described below are merely for illustrative purposes, and are not intended to limit the scope of the invention as defined in the claims.
Cylindrical target 30 preferably is rotatable about its longitudinal axis, typically by means of a motor (e.g., an electric motor) or other such motive device. In some embodiments, the motive device/rotating means comprises a drive end block 40 containing a motor suitable for rotating cylindrical target 30. Cathode target assembly 10 may be provided with a support end block 50, which may be suitable for supporting the cylindrical target 30 opposite the drive end block 40. In some embodiments, support end block 50 houses a cooling fluid inlet 60 and a cooling fluid outlet 70. Cooling fluid inlet 60 and cooling fluid outlet 70 may provide cooling water to cylindrical target 30 to cool it during the sputtering process. Alternatively, cathode target assembly 10 may be cantilevered, and may not include a support end block 50.
The cathode target assembly 10 includes a magnetic array assembly 80 carried within the cylindrical target 30 for generation of a plasma confinement field adjacent a surface of the target 30. The magnetic array assembly 80 may be disposed within cylindrical target 30 (e.g., within an interior recess bounded by the cylindrical target). A framework 82 (or similar support means) may be provided for supporting the magnetic array assembly 80, optionally independently of rotation of the target 30 (e.g., in a static, or substantially static, state). In the embodiment shown in
Framework 82 and key 84 may additionally be adapted to allow movement of the magnetic array assembly 80, or portions thereof, in a direction generally along the longitudinal axis of the target 30, according to certain embodiments of the invention.
A variety of substrates are suitable for use in the present invention. In most cases, the substrate is a sheet of transparent material (i.e., a transparent sheet). However, the substrate is not required to be transparent. For example, opaque substrates may be useful in some cases.
However, it is anticipated that for most applications, the substrate will comprise a transparent or translucent material, such as glass or clear plastic. In many cases, the substrate will be a glass sheet. A variety of known glass types can be used, and soda-lime glass is expected to be preferred.
Substrates of various size can be used in the present invention. For example, the invention can be used to process large-area substrates. Certain embodiments of the invention can process a substrate having a width of at least about .5 meter, preferably at least about 1 meter, perhaps more preferably at least about 1.5 meters (e.g., between about 2 meters and about 4 meters), and in some cases at least about 3 meters.
Substrates of various thickness can also be used with certain embodiments of the invention.
Commonly, substrates with a thickness of about 1-5 mm are used. Some embodiments involve a substrate with a thickness of between about 2.3 mm and about 4.8 mm, and perhaps more preferably between about 2.5 mm and about 4.8 mm. In some cases, a sheet of glass (e.g., soda-lime glass) with a thickness of about 3 mm is used.
The invention is particularly advantageous in processing large area substrates, such as glass sheets for architectural and automotive glass applications. Substrates of this nature commonly have a width of at least about .5 meter, more commonly at least about one meter, and typically greater than about 1.5 meters (e.g., between about 2 meters and about 4 meters). Accordingly, the target is preferably adapted to sputter target material substantially across the entire width of such a substrate (i.e., a substrate having a width in one or more of the above ranges). With large area substrates in particular (especially those formed of glass), it is desirable to convey the substrates through a deposition chamber in a horizontal orientation, rather than in a vertical orientation.
Certain embodiments of the invention provide a magnetic array assembly 80 within (e.g., mounted within an axially-extending central cavity defined by) a cylindrical target 30 for generating a plurality (at least two, in some cases at least three, optionally at least five, or even at least seven) of electron drift paths arranged to simultaneously improve film deposition uniformity and target utilization. By providing a number of smaller electron drift paths along the length of the target (optionally having their respective centers maintained at longitudinally spaced-apart locations), rather than one long“racetrack”-shaped electron drift path, the magnetic field distribution, and hence, the overall plasma confinement field, is substantially similar in strength and structure along the length of the target, regardless of non-uniformities that may be present in the individual electron drift paths, when considering the averages over short distances. The plasma confinement fields present near the ends of the target are not substantially different from those along the central length of the target. Although there may be a periodic variation in sputter rate along the target length, the multiple distributions of the flux of sputtered material results in more uniform erosion of target material. Further improvements in uniformity of erosion, and hence, in target utilization, may be achieved by the additional use of magnet-bar oscillators, which periodically shift the magnets a small distance with respect to the target. If the periodic variation of the confinement results in periodic variation in film deposition, film deposition can be further improved by using a plurality of sputtering targets, wherein the magnetic arrays per the invention, are in slightly different positions in each target so that the average film, deposited by the plurality of targets, is even more uniform.
The use of multiple electron drift paths along the length of a cylindrical cathode target may be particularly useful when reactive gases (e.g., comprising oxygen and/or nitrogen) are used in the sputtering process. During sputtering operations in which reactive gases are used, the problem of non-uniform deposition rates near the ends of a target may be compounded, particularly when operating in the transition between metal-mode and full poisoned mode. The regions of more intense plasma (usually at the ends of the target) sputter faster than low-intensity regions, thus more quickly consuming the reactive gas making the high intensity regions more metallic. The more metallic regions, in turn, have an even higher sputter rate. The interaction is very non-linear and results not only in variations of deposition rate, but also causes variations in film stoichiometry.
In certain embodiments of the invention, a portion of an electron drift path 510 may overlap a portion of an adjacent electron drift path 510 to define an overlapping region 514. In this context, the term overlap means that portions of two drift paths are located at the same longitudinal locale (i.e., are the same distance from a given end 30E of the target) on the target. Thus, a target rotational path 520 that falls within an overlapping region 514 will cross multiple (i.e., at least two) electron drift paths 510 during rotation of the target 30 relative to the magnetic array assembly 80. The amount of overlap may be defined in a number of ways, as would be appreciated by one of ordinary skill in the art. For example, one might define a“percent overlap” to be the length of an overlapping region 514 divided by the longitudinal electron drift path displacement 516, as shown in
In certain embodiments of the invention, the oblique angle 512 and the amount of overlap may be chosen such that an area of relatively high erosion rate on one drift path 510 falls along the same target rotational path 520 as an area of relatively low erosion rate of a nearby or adjacent drift path 510 to provide more uniform target depletion.
The plurality of electron drift paths 510 in the exemplary embodiment of
The plurality of electron drift paths 510 in the exemplary embodiment of
The magnetic array assembly 80 associated with the embodiment of
Certain embodiments of the invention provide for longitudinal movement (e.g., oscillation) of magnetic array assembly 80 within cylindrical target 30. The magnetic array assembly 80 can thus be movable in a generally axial direction (i.e., longitudinally) by about one-half centimeter or more, and desirably more than about one centimeter, such movement being preferred to substantially even out the target wear pattern 86 of a cathode target assembly 10. In some embodiments of the invention, the entire magnetic array 80 is adapted to move generally axially up to 4 centimeters or more. An example of an apparatus for providing oscillation of magnetic array assembly 80 is described in U.S. patent application Ser. No. 11/171,054 (filed Jun. 30, 2005, titled“Cylindrical Target With Oscillating Magnet For Magnetron Sputtering”), the entire contents of which are hereby incorporated by reference. Oscillators of this nature can be obtained commercially from General Plasma Inc. (Tucson, Arizona, USA). Thus, in certain method embodiments, a magnetic array assembly of the described nature is moved (e.g., in a back-and-forth manner) longitudinally during sputtering.
Preferably, the substrate support 640 is configured for maintaining (e.g., supporting) the substrate 20 in a horizontal configuration while the substrate 20 is being coated (e.g., during conveyance of the substrate 20 through the deposition chamber 600). Thus, the support 640 desirably is adapted to convey a sheet-like substrate 20, and preferably multiple sheet-like substrates that are spaced-apart from one another, through the deposition chamber 600 while maintaining the/each substrate 20 in a horizontal orientation (e.g., wherein a top major surface 614 of the/each substrate 20 is upwardly oriented while a bottom major surface 612 of the/each substrate 20 is downwardly oriented). In the embodiments shown in the present figures, the substrate support 640 comprises a plurality of spaced-apart transport rollers 610. Typically, at least one of the rollers 610 is rotated (e.g., by energizing a motor operably connected to the roller) such that the substrate 20 is conveyed through the deposition chamber 600 along the path of substrate travel 645. When the substrate is conveyed over such rollers, the bottom surface 612 of the substrate 20 is in direct physical (i.e., supportive) contact with the rollers 610. The substrate 20 is typically conveyed through the deposition chamber 600 at a speed of about 100-500 inches per minute. In certain embodiments of the invention, the substrate 20 is a sheet of glass that is on the substrate support 640 during conveyance, and wherein other sheets of glass are also on the substrate support 640, such sheets of glass being spaced-apart from one another on the substrate support 640 and conveyed in such a spaced-apart configuration. While the illustrated substrate support 640 comprises a plurality of spaced-apart rollers 610, it is to be appreciated that other types of substrate supports can be used.
In certain embodiments, such as that illustrated in
In certain embodiments, such as that illustrated in
Thus, embodiments of a METHOD AND APPARATUS FOR CYLINDRICAL MAGNETRON SPUTTERING USING MULTIPLE ELECTRON DRIFT PATHS are disclosed. One skilled in the art will appreciate that the invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the invention is limited only by the claims that follow.
