RELATED APPLICATION

[1] The present application claims priority to U.S. Provisional Application Nos.

61/940,753, filed February 17, 2014 and U.S. Provisional Application No. 61/986,461, filed April 30, 2014, both entitled SYSTEMS AND METHODS FOR GENERATING METAL OXIDE COATINGS, the contents of both of which are hereby incorporated by reference. The present application also incorporates by reference in its entirety the published U.S. patent application no. 13/211,884 entitled "Deposition systems with a rotating drum," filed on August 17, 2011.

FIELD OF THE INVENTION

[2] The invention relates generally to a thin film deposition system. More specifically, the invention relates to a deposition system with separate metallization and reaction zones for deposition of reactive films.

BACKGROUND

[3] DC Magnetron Sputtering is a thin film deposition technique. For example, sputtering can occur in an environment containing Argon gas (Ar). A negative DC potential is applied to a conductive metal "target." A plasma discharge is established to ionize the gas thereby creating Ar+ ions. The positively charged Ar+ ions accelerate towards the negatively charged target and causes ejection of the target through sputtering, which in turn creates a metal film on an opposing placed substrate.

[4] Introduction of reactive gases such as O ₂ or N ₂ can cause the film to take on properties of the compounds created by the reaction of these gases with the deposited metal film. Further ionization and acceleration of these reactive gases can enhance the reactivity between the gas and the film in addition to improving the density of the film as well as influence other film properties such the film stress, hardness, index and absorption. Conventional deposition systems are complex and suffer from certain shortcomings including reduced wafer throughput and material contamination issues, which limits film quality and requires extended preventative maintenance cleaning of the deposition equipment.

[5] Further, in conventional DC Reactive Magnetron Sputtering (DCRMS) systems for oxide coating, the contamination of the sputtering target with even small amounts of 0 ₂ (e.g., part per million (ppm)) will cause target arcing. Also, if enough reactive gas, such as 0 ₂ , encounters the target it will cause the target surface to convert to a dielectric/ insulator in which case the DC power supply may shut down, thereby interrupting the process. Because of these shortcomings, various systems have been developed to monitor the optical plasma properties and provide feedback to the 0 ₂ Mass flow controller ( Gencoa, Speedflo® or Reactive Sputtering Inc., IRESS) to prevent target poisoning. Other more mechanical means include shielding of the magnetron with high -pressure gas curtains. Sometimes this shielding is done in concert with long sputter throw distance and high pumping speed to prevent arcing. (Scobey, Reactive magnetron sputtering apparatus and method, CA 2254354 Al).

[6] Hence, there is thus a need for improved deposition systems and methods.

BRIEF SUMMARY

[7] In one aspect, a deposition system is disclosed, which comprises a

metallization zone having a sputtering source for generating metal atoms, and a reaction zone having an ion source for generating reactive ions. The system comprises a rotatable mount on which at least one substrate can be mounted, where the rotatable mount is configured for alternatingly moving said at least one substrate between the metallization zone and the reaction zone. Further, the system comprises a movable aperture positioned relative to said sputtering source such that at least a portion of the metal ions pass through the aperture. In many embodiments, the movable aperture is disposed between the sputtering source and the rotatable mount.

[8] In some embodiments, the deposition system can further include an enclosure

(herein also referred to as a vestibule) in which the sputtering source is at least partially enclosed. The enclosure can include an opening that provides an egress port for metal ions generated by the sputtering source. The movable apertures can be moved (e.g., along a linear dimension) relative to the opening of the enclosure such that the aperture would receive at least a portion of the metal ions exiting the enclosure through said opening. In some embodiments, the aperture can be in substantial register with the opening so as to maximize the number of metal ions exiting the enclosure that would pass through the aperture. In some embodiments, the aperture can be moved relative to the sputtering source to adjust the pressure within an enclosure in which the sputtering source is disposed. Further, in some embodiments, the aperture itself can form the opening of the enclosure (vestibule) in which the sputtering source is enclosed.

[9] The aperture can be formed in a aperture plate that is configured to move linearly relative to the sputtering source. A variety of mechanisms can be employed to effect the movement of the aperture plate. By way of example, a plurality of linear motion feedthroughs can be employed to move the aperture plate, as discussed in more detail below.

[10] The sputtering source can generate a flux of metal ions that exhibit an angular flux distribution relative to a central axis. By way of example, the angular flux distribution can be cosinosoidal, as discussed in more detail below. The aperture can be utilized to narrow the angular distribution of the flux. As discussed below, such narrowing of the angular distribution of the metal atoms can be achieved not only with movable aperture but also with a fixed aperture that is positioned relative to the sputtering source to obtain a desired angular distribution of the flux. In some embodiments in which a movable aperture is employed, the aperture can be configured to move along said central axis to narrow the angular distribution of the flux of metal atoms (to collimate the flux). In some embodiments, the aperture is configured to provide a collimated beam of atoms characterized by a maximum divergence angle that is equal to or less than about 30 degrees, e.g., in a range of about 10 to about 30 degrees.

[11] A variety of sputtering and ion sources can be employed. In some

embodiments, the sputtering source is a magnetron and the ion source is configured to generate O2 ions.

[12] In another aspect, a coated substrate is disclosed that includes an underlying substrate (e.g., a silicon substrate), and a metal oxide film (e.g., an aluminum oxide film) that is disposed over a surface of the substrate, wherein the metal oxide film is substantially free of noble gas impurities (e.g., argon, neon, or xenon). By way of example, the concentration of a noble gas impurity in the metal oxide film (coating) can be less than 1 percent, or preferably less than 0.5%, or more preferably in a range of 1-10 parts per million. Further, in some

embodiments, the thickness of the metal oxide film can be in a range of about 0.1 microns to about 50 microns.

[13] Further understanding of various aspects of the invention can be obtained with reference to the following detailed description in conjunction with the associated drawings, which are described briefly below. BRIEF DESCRIPTION OF THE DRAWINGS

[14] The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[15] FIG. 1 is schematic top view of an embodiment of a deposition system according to the invention.

[16] FIG. 2 is partial cross-sectional view of the deposition system shown in

FIG. 1.

[17] FIG. 3 is perspective view of an embodiment of a drum usable with the deposition system in FIG. 1

[18] FIG. 4 is a schematic view of an embodiment of a metallization zone usable with the deposition system in FIG. 1

[19] FIG. 5 is a schematic view of an embodiment of a sputtering mask positioned relative to two substrates.

[20] FIG. 6A is a perspective view of an embodiment of a deposition system.

[21] FIG. 6B is a perspective view of an embodiment of a reaction zone usable with the deposition system in FIG. 6A.

[22] FIG. 7A is a perspective view of an embodiment of a deposition system.

[23] FIG. 7B is a perspective view of an embodiment of a reaction zone usable with the deposition system in FIG. 7A.

[24] FIG. 8 A is a schematic view of an embodiment of a metallization zone usable with the deposition system in FIG. 7A. [25] FIG. 8B is a schematic view of an embodiment of the metallization zone.

[26] FIG. 8C is a schematic view of an embodiment of the metallization zone with an aperture formed by an aperture shield.

[27] FIG. 9 schematically depict a partial view of an embodiment according to the present teachings that includes a movable aperture disposed in front of a sputtering source.

[28] FIG. 10 schematically depicts collimation of a sputter metal atoms by using an aperture according to the present teachings.

[29] FIG. 11 A schematically shows the pinch-off effect when a non-collimated metal flux is employed to fill a via, and

[30] FIG. 1 IB schematically shows the filling of a via by using a collimated flux of metal atoms in accordance with an embodiment of the present teachings.

DETAILED DESCRIPTION

[31] Embodiments of deposition systems described herein provide for deposition of metallic films and their subsequent reaction with improved throughput and quality over conventional systems. For example, and without limitation, an aluminum film is deposited on a silicon wafer and then subsequently reacted with oxygen to form Α1 ₂ 0 ₃ as a dielectric in a semiconductor component. Other example films are SiO ₂ , TiN and TiC. In addition, an aluminum film deposited on a glass substrate is subsequently reacted with oxygen to form an alumina film, providing a scratch-resistant layer on the glass for consumer electronics applications. Flexible polymeric (plastic) substrates may also be used. In other examples, multiple metallic film layers are deposited, each with a different index of refraction to produce a high quality optical coating. In another example, multiple reactive gases are alternately used to form SiOxNx materials. In another example, multiple metallic targets are used with an inert gas in the reaction zone to form precise multilayer films for X-ray mirrors. Many combinations of metal films and reactive gases are usable with the embodiments described herein in an efficient manner with less cross contamination of materials and improved consistency of deposition.

[32] The term "about" as used herein denotes a variation of at most 5%, or preferably at most 1%, of a numerical value.

[33] FIG. 1 shows an embodiment 10 of a deposition system in accordance with the invention. The embodiment 10 includes an outer enclosure 12 with a separate metallization zone 14 and a reaction zone 16. A drum 18 has a plurality of substrates 20 affixed to the outer surface of the drum 18 so that rotation of the drum 18 causes at least one of the substrates 20 to alternately pass through the metallization zone 14 and the reaction zone 16. In other embodiments the drum 18 is replaced with a different shape other than circular. For example, a hexagonal drum is used to accommodate larger substrates. It is envisioned that other drum shapes are used in other embodiments to accommodate substrates of different size, thickness and shapes.

[34] The metallization zone 14 includes a magnetron 22 fed by an inert gas source

24. The magnetron 22 is enclosed in a vestibule 26 and has an aperture 27. The aperture allows pressure introduced into the vestibule to build up to a pressure higher than in the reaction zone. The inert gas source is preferably Argon but can also be other inert gas such as Xenon for example. A pair of outer baffles 28 are arranged to assist in the maintaining of the pressure developed from the gas source 24 feeding an inert gas into the metallization zone 14 and exiting through a pump (not visible in FIG. 1). The baffles 28 extend close to the drum 18 while maintaining separation from the substrates 20 as they rotate through the metallization zone 20. The distance of the aperture from the drum surface 18 also determines the pressure buildup in the vestibule 26.

[35] The reaction zone 16 includes an ion source 30 fed by a gas source 32. The gas is preferably a reactive gas including but not limited to Oxygen or Nitrogen but may also be an inert gas such as Argon. In one embodiment, a hollow cathode electron source 42 is used to inject electrons into the reaction zone 16 to maintain plasma neutrality or a zero net charge of positively and negatively charged ions in the reaction zone 16. A pair of inner baffles 38 and outer baffles 40 assist with maintaining the pressure in the reaction zone 16 caused by the inflow of gas from the gas source 32 and exiting through a pair of differential pumps 36. The reaction zone 16 is further enclosed by an enclosure 34 to assist with maintaining the reaction zone pressure. In addition, the metal film being deposited on the substrate 20 is itself an additional selective pump of the reactive gas in the reaction zone. Both the differential pumps 36 and the film itself act to reduce any residual reactive gas to levels that cannot appreciably penetrate into the higher pressure metallization zone 14. A final pumping step is the subsequent selective pumping of any reactive gases that enter the metallization zone 14 by the sputtered metal atoms and are getter-reacted on the inner walls of 26. The low mean free path of any 0 ₂ molecules, for example within the higher Argon background pressure metallization zone 14 causes any 0 ₂ gas to be reacted and gettered before contact with the target.

[36] In one embodiment, the system 10 is pumped to a base pressure P and the metallization zone 14 is filled with Argon gas, with the metallization zone 14 maintained at a pressure Pa, 1x10 - ^" 3 to 1x10 - ^" 2 Torr for example. A reactive gas such as 0 ₂ is flowed into the reaction zone 16 through source 32 and maintained at a pressure Pr, where Pr is substantially less than Pa, lxlO ^"4 to 5xl0 ^"4 for example. The Argon gas is positively charged in the metallization zone 14 with the magnetron 22 by igniting a plasma discharge with a direct current (DC), pulsed or radio frequency (RF) power supply. The positively charged Argon ions strike a metal target mounted on the magnetron 22 resulting in sputtering of a metal film on the substrate 20 as it passes through the metallization zone 14. The substrate 20 then passes through the reaction zone 16 upon rotation of the drum 18. The reactive gas is ionized into a plasma with the ion source 30. In one embodiment the ion source is an electron cyclotron resonance (ECR) plasma source. The substrate 20 with the deposited metal film is then reacted and densified in the reaction zone 16. For example, an aluminum film would be converted to A1 ₂ 0 ₃ if 0 ₂ were the reactive gas in the reaction zone 16 and aluminum were the target used in the metallization zone 14. The ion source 30 should be understood to include any plasma or ion source suitable to activate the reactive gas or species (e.g. 0 ₂ ) and accelerate into the substrate 20 to cause a substantially complete reaction between the reactive gas and the deposited metal to form a stoichometric film or alloy and to densify the film. FIG. 2 further illustrates the system described in FIG. 1. The pump, a cryogenic pump for example, 52 is used to discharge the Argon used in the metallization zone 14 and in part the reactive gas used in the reaction zone 16. The reaction gas is discharged through a combination of pump 52 and the differential pumps 36 controlled in part by the spacing of the baffles 38 and 40 relative to the rotating drum 18.

[37] In one example, the system 10 is operated by rotating the drum 18 at a fixed speed. A reactive gas is set to a pressure Pr and the ion source is ignited. The energy and flux of the ion source is adjusted for the required reaction. Argon gas is flowed into the metallization zone 14 and the flow of Argon is adjusted to a pressure Pa. The magnetron 22 is then ignited to form a plasma and the power to the magnetron 22 is adjusted to a power P such that 5-7

Angstroms (A) of metal film are deposited on a substrate 20 for each revolution of the drum 18. The substrate 20 is then passed through the reaction zone to react the deposited film and then the drum subsequently passes the substrate back to the metallization zone 14 to deposit another metal film. With this method, the film growth is rapid, stable and predictable due to the linear relationship of magnetron power to metal film deposition rate. As the target in the magnetron 22 erodes predictably and the deposition rate drops, the operator of the system 10 increases the power to the magnetron 22 to maintain a substantially constant deposition rate. In one example, this adjustment of magnetron power is controlled automatically by an algorithm based on the time the system 10 has been in operation.

[38] For example, if 5A (e.g. 5xl0 ^~10 m) are deposited for each revolution of the drum 18, at a drum revolution of lOOrpm, one will deposit 500A of reacted film in one minute and 5000A in ten minutes. The rate of deposition is further limited by the reaction time of the reactive gas, which is further limited by the maximum ion flux or beam current that can be delivered by the ion source 30.

[39] A further example of settings used in the system 10 uses a cathode measuring three inches by six inches (the cathode being part of the magnetron 22 described below) and 900 watts of DC power applied to the magnetron 22, which produces a deposition rate of 32A of aluminum for each second that the substrate is in the metallization zone. This is referred to as the "static" deposition rate. For a drum 18 rotating at 60rpm, a "dynamic" deposition rate of 1.15 A/rev. is produced for a typical metallization zone and substrate. If the magnetron 22 power is increased to 1500 watts the static deposition rate would be approximately 1.66 greater than the rate using 900 watts resulting in a 53A/sec static deposition rate. In one example of system 10 a dynamic rate of 5A/rev, of aluminum is deposited using 3.6kW of magnetron power and a drum 18 rotation rate of 60rpm. A reactive flux of oxygen 0 ₂ ⁺ must be maintained at a sufficient magnitude to convert the deposited aluminum film to a stoichiometric A1 ₂ 0 ₃ film for pass of the substrate 20 through the reaction zone 16. According to the molecular formula for A1 ₂ 0 ₃ 50% more 0 ₂ ions are required than aluminum to completely react the film. For example, a metal deposition rate of 5A/rev. is equivalent to a metal flux of 3x10 15 atoms/cm 2 /sec. at a drum 18 revolution rate of 60rpm. An 0 ₂ ionized flux of similar magnitude with an average energy of approximately 30-60 eV/atom is used to react the film to produce A1 ₂ 0 ₃ . It is important not to exceed an upper limit of the eV/atom energy by too much, because re-sputtering of the deposited film will interfere with the deposition process. In addition, beyond this energetic threshold the energetic ions can re-sputter portions of the film causing non- stoichiometric films or implant into the film thereby causing localized stress defects. This upper limit is determined by the specific film being deposited.

[40] Because the reactive time in the reaction zone 16 is limited by the ion current or flux, it is important that the ion source 30 be as close to the substrate 20 as possible. With a linear ion source is it possible to get much closer to the substrate 20 than to arrange a series of circular sources along the height of the drum 18. If a circular ion source is used, the arrangement of the sources must be raised above the substrate so that the ion source height and separation from the substrate 20 gives a flux distribution, (which is the sum of the individual sources) being uniform across the substrate. For example, for a drum 18 that is approximately three feet in height, the ion sources should be 5 - 8 inches above the substrate. This close proximity is not possible with multiple circular sources due to beam superposition issues. In contrast, the magnetron 22 does not have to be in such close proximity to the substrate 20 because the magnetron 22 can support large power densities. [41] The close proximity of the ion source 30 to the substrate 20 and the baffles 38 and 40 to the drum 18 can limit the substrate 20 thickness and curvature. In this case, the drum 18 can have a retro-machined or recessed surface within which the substrate 20 is held. FIG. 3 shows a drum 18 with a retro-machined surface for each substrate 20. In one example, the top of the substrate 20 is "proud" (e.g. flush) to the surface of the drum 18. Further variations to the drum shape are considered within the scope of the invention, including replacing the substrate with another object to receive a metal film.

[42] Uniformity and stress of film deposition is further enhanced by adjusting the pressure Pa in the metallization zone 14 and the target to substrate 20 distance or the pressure- distance product (PxTsd). Adjusting Tsd is facilitated by mounting the magnetron 22 within a flanged housing with adjustment rods 72 as shown in FIG. 4. Adjusting the pusher rods 72 moves the magnetron 22 closer to the substrate 20 mounted on the drum 18. In a preferred embodiment, the magnetron 22 includes a mask 82 between a target 78 and the substrate 20. An anode 76 is disposed between the target 78 and a cathode 74 and is designed to have a small annular portion between the target 78 and the substrate 20. In one example, the anode is biased to a ground potential and the cathode 74, which electrically communicates with the target 78 is biased to a negative potential less than ground. The positively charged inert molecules (e.g. Ar+) will accelerate towards the negatively charged target 78 and impact the target resulting in sputtering.

[43] To achieve optimum uniformity of deposited material on the substrate 20 using a linear magnetron, the magnetron length should be substantially equal to the drum 18 height plus twice the target width. For example if the drum 18 is one meter in height and the magnetron is 10 cm. wide, the magnetron length should be approximately 1.2 meters. In one example, gas lines to and from the gas source 24 and supplying the magnetron 22 are chosen to substantially eliminate a pressure gradient along the magnetron length so enhance film linear uniformity.

[44] In a preferred embodiment, a mask 82 as shown in FIG. 5 is used to enhance deposition uniformity. The mask 82 is interposed between the magnetron 22 and the substrate 20 and is shaped to account for local film variations. For a flat substrate 20 the mask 82 produces a linear wave front of sputtered material to the substrate 20. Various alterations to the mask are used to shape the wave front to match the surface of non-planar substrates, for example a convexed or concaved substrate. FIG. 5 further illustrates the positioning of the mask 82 relative to a substrate 20a that has just exited the metallization zone 14 and a substrate 20b that is within the metallization zone 14. The deposited metal film uniformity primarily determines the reacted film uniformity. This is because the ion source 30 is typically operated in the saturation mode. In other words, slightly more current is delivered to the ion source 30 than is needed to cause saturation of the film stoichiometry. For example, if the voltage of the ionized reactive gas is kept sufficiently low (e.g. less than 100-125eV) the deposited reacted film's molecular bonds will not be broken and stoichiometry is maintained in the reacted film. Once stoichiometry is reached, the film is stable. In an embodiment where the system 10 is configured to make alloys, the ion source 30 is operated at a setting between zero energetics and less than saturation to obtain a film of any composition.

[45] In one embodiment, the rotating drum 18 is cooled with a liquid, for example water and has an RF, DC or pulsed electrical bias applied to enhance activation or assist through re-sputtering in the planarization of a substrate with photolithographic features. The linear magnetron 22 in the present invention easily lends itself to a hidden anode 76 design as shown in FIG. 4. In prior solutions using reactive sputtering, the anode is also the substrate and becomes coated with an insulator. The electrons in the plasma can therefore no longer return to ground, and an accumulation of charges causes the plasma to spread out to the substrate seeking a return path to ground. The magnetron plasma now is in contact with the substrate, which results in undesirable substrate heating. A hidden anode 76 as shown in FIG. 4 overcomes this problem because the anode will not be coated with an insulator and this in turn keeps the plasma confined to the target 78 thereby reducing the substrate heating.

[46] In prior solutions, the magnetron 22 is purposely operated in an unbalanced mode. In this mode the plasma is made to extend to the substrate 20 thereby adding an ion assist to the substrate from the Argon ions in the magnetron plasma. This plasma extension provides for film and gas reactivity however it is combined with the metallization zone. In this unbalanced configuration, a high power and high film deposition rate will put severe constraints on the substrate temperature and corresponding cooling requirements. Unlike the prior solutions, the present invention separates the metallization zone 14 and the reaction zone 16 in an effective manner.

[47] In a preferred embodiment, the substrates 20 are mounted vertically as shown in FIG. 3 and a linear magnetron 22 is now used for film deposition. Every point on a vertical line of the substrate spend the same time under the deposition flux while in the metallization zone 14 thus substantially eliminating the center to edge film thickness variation or gradient along the substrate 20. Prior solutions that are encumbered by this film gradient are required to use a delta- shaped magnetron to produce uniform films at added expense and system complexity. Even with the delta-shaped magnetron, a mask is still required to enhance uniformity. In the present invention, no such gradient exists, thereby allowing use of a standard linear magnetron with standard targets.

[48] Due to the complete separation between the metallization zone 14 and the reaction zone 16, the system 10 operates in an open-loop with little dependency between the zones and without the risk of reactive sputter hysteresis which "poisons" or oxidized the metal target 78 and consequently shuts down the sputtering process. Furthermore separation of the zones avoids the requirement to use pulsed sputtering or other devices to reduce arc events due to target poisoning although in one example pulse sputtering is used to enhance ionization at the target for effecting film properties. Separation of the metallization zone 14 and the reaction zone 16 and consequently the reduction in target poisoning, requires a pressure differential between the zones of approximately one order of magnitude and a physical separation greater than the mean free path of the reactive gas. In one example, the metallization zone 14 has a pressure Pr of lxlO ^"3 torr while the reaction zone 16 has a pressure Pr of lxlO ^"4 torr. This pressure differential and mean free path difference provides a diffusive gas barrier to further enhance the isolation of the two zones. It is envisioned that a greater distance between the two zones will also permit a reduction in the pressure differential and that various combinations of pressure differential and physical separation are within the scope of the invention. The present invention overcomes limitations of previous solutions that required complicated feedback loops involving optical plasma spectroscopy or mass flow controller feedback loops to control gas flow and the magnetron power supply.

[49] The present invention also overcomes limitations in previous solutions that require the cathode of the magnetron to be encased in a differentially pumped enclosure rather than just enclosing the reaction zone 16 in a differentially pumped enclosure. Advantageously, the present approach facilitates a system with multiple metallization zones because only the common reaction zone 16 is differentially pumped as shown in FIGs. 6A and 6B. In FIG. 6A a system 90 has an enclosure 12 with an opening 92 for the ion source 30. The reaction zone 16 is encased in an enclosure 34 and is differentially pumped with pumps 36. In FIG. 6B, the enclosure 34 includes a pair of inner baffles 38 and a pair of outer baffles 40. The differential pumps 36 are positioned outside of the inner baffles 38 and inside of the outer baffles 40. The reaction zone 16 is within the inner baffles 38.

[50] The pumps 36 and the baffles 38 and 40 are chosen so that the pressure outside of the reaction zone 16, (between the reaction zone 16 and the metallization zone 14) is reduced to a pressure substantially less than the reaction zone 16, lxlO ^"5 torr for example. In this case if the pressure outside of the reaction zone is Pr and the conductance of the baffles is Cb (measured in torr-litres/sec) and the reduced pressure outside the enclosure 34 is Po then the speed of the pumps 36 is calculated by the equation Spump = (Pr-Po)/Cb. By enclosing the reaction zone 16 and differentially pumping this zone separately from the rest of the system, pump sizes and system size are reduced, allowing the target and ion source to be more loosely coupled.

[51] In one of the preferred embodiments the reaction zone 16 includes an additional pump 112 positioned between the inner baffles 38 as shown in FIGs. 7A and 7B. With the addition of pump 112, the baffles 38 and 40 can be moved closer to the drum 18 for improved isolation of the reactive gases from the rest of the system. In another embodiment, the pair of inner baffles 38 is moved closer to the drum 18 and the pair of outer baffles 40 are moved further away from the drum 18 such that the differential pumps 36 providing differential reduction of the reactive gas can now be used to also pump the inert gas from the metallization zone thereby eliminating the need for pump 52 shown in FIG. 2. The speed of the pump 112 is limited by the conductance of the aperture created by the outer baffles 40 and the speed of the pump 112.

[52] With the improved isolation of the reactive gases as shown in FIGs. 7 A and

7B, the linear magnetron 22 can be positioned further from the substrate 20 as shown in FIG. 8A. Advantageously, this further reduces particles on the substrate 20. Because the inert gas is introduced within the magnetron 22 locally, then the open area through which the inert gas is pumped is effectively a sputter aperture with conductance Ca.

[53] FIG. 8B shows an embodiment of the metallization zone 14 where the plasma from the magnetron 22 is substantially confined by a pair of inner baffles 142. FIG. 8C shows one of the preferred embodiments of the metallization zone 14 where the plasma is confined by an aperture 27 formed by an aperture shield 26. If the pressure in this sputter vestibule 26 is fixed at pressure Pa and assuming the main pump 52 speed is much higher than the speed of the differential pumps 36 and the main pump 52 has a speed P _spe ed, then the size of the aperture can then be calculated to maintain P _a . The distance of the aperture plate from the substrate holder (e.g. drum) 18 also contributes to the pressure build up in the vestibule 26.

[54] The vertical orientation of the deposition in the metallization zone 14 advantageously reduces the possibility of particles landing on the substrate 20. Unlike prior solutions that used downward facing deposition the requirement for shutting down the system for maintenance is reduced and the use of full-face erosion targets to minimize target re-deposition is eliminated. Delta magnetrons as used in the prior solutions have a long meandering non- sputtered zone, which becomes saturated with re-deposited film. This film will over time "spall" off due to stress, which greatly contributes to undesirable particles reaching the substrate 20. By using a linear magnetron 22 whose re-deposition area is substantially less than a delta magnetron, particles are reduced in thick coatings.

[55] A further advantage of the system 10 with monolayer deposition, and high speed passing through the zone with inert gas and subsequent conversion to a dense

stoichometric reacted layer is the significant reduction of the inert gas in the substrate 20. By reducing the inert gas (e.g. Argon) in the substrate 20, defect induced stress variation is greatly reduced. The reacted film stress is then controlled by film densification with a process known as atomic penning due to the ion source. The stress control is achieved by controlling the atom/ion ratio (e.g. metal atom flux to ion flux ratio) and momentum exchange of the ion at the surface. By controlling magnetron 22 power, anode 76 current and anode 76 voltage the film stress can be varied or controlled or alternatively, magnetron 22 power, anode 76 current and anode 76 voltage can be held constant and the atom/ion ratio and metal rate per revolution of the drum 18 is controllable by varying the drum 18 rotation speed.

[56] In one embodiment a second magnetron is added to the system 10 to form high and low index metal oxide layers on a substrate 20 to produce high quality optical coatings. For example, a silicon target in one magnetron and a tantalum target in a second magnetron can provide any number of Si0 ₂ /Ta ₂ 0 ₅ optical coatings including anti-reflection, band bass and blocking coatings. The formation of the optimal layers is easily monitored by a gated optical monitor. Other embodiments use more than two magnetrons to provide even more complex structures. In another embodiment, two magnetrons are used each with its own metal target and the reactive gas in the reaction zone 16 is replaced with Argon to rapidly form precise metal multilayer films from low Z materials for X-ray mirrors. [57] In another embodiment for multilayer coatings, an additional (e.g. second or more) magnetron is mounted within a single attached vestibule 26. The additional magnetron is used and indexed with a motor controller. In this way a layer A is deposited with magnetron A then halted and magnetron B is then rotated in position in front of the aperture, power is applied to magnetron B and a film B is deposited. This embodiment of a multiple target sputtering system greatly reduces machine foot print and cost of ownership.

[58] In another embodiment, the gas source 32 is pulsed or alternated between different gases to form different combinations of films. For example, a Silicon target is used, with 0 ₂ and N ₂ reactive gases used sequentially to produce SiN and Si0 ₂ layers. Alternatively, an 0 ₂ /N ₂ mixture is used to form SiON _x materials.

[59] The aperture 27 disposed in front of the magnetron sputtering source, as shown schematically in FIG. 1, can serve a number of different functions. For example, the aperture can allow the buildup of a pressure of a sputtering gas, such as argon, in the magnetron vestibule (in some embodiments, the sputtering gas can be fed directly in to the magnetron vestibule). Such a pressure gradient can facilitate the shielding (isolation) of the sputtering zone from the reactive zone (e.g., the oxidation zone discussed above), which can in turn diminish, and preferably eliminate, the contamination of the sputtering target by the reactive gases, such as oxygen.

[60] In some embodiments, pumping is not directly applied to the magnetron vestibule. In such embodiments, the spacing of a rotatable mount, e.g., the drum discussed above, and the aperture as well as the aperture size in combination with the pumping speed of the chamber pump (e.g., the pump 52 shown in FIG. 2) can determine the pumping speed on the sputtering gas, e.g., argon, in the magnetron vestibule and hence the degree of pressure buildup within the magnetron vestibule.

[61] By way of example, in some embodiments in which the sputtering gas is argon and the reactive gas is oxygen, the aperture can allow the buildup of a high argon pressure at the sputtering target (cathode). For example, if the sputtering is carried out at 10 mtorr and the oxygen (0 ₂ ) pressure in the ion source is 2x10 ^" torr, a pressure gradient of greater than 10 can develop between the sputtering source and the ion source (i.e., between the sputtering zone and the reactive zone), thus inhibiting the reactive ions, e.g., oxygen ions, from infiltrating the sputtering zone.

[62] Moreover, the spatial separation of the sputtering zone from the reactive zone can further shield the two zones from one another and hence inhibit cross contamination between them. For example, in some embodiments of the present teachings, the ion source for generating reactive ions, e.g., 0 ₂ ions, is disposed at a location opposed to the sputtering source such that the ion source is angularly separated by 180° around the perimeter of the rotatable mount relative to the sputtering source. In some such embodiments, the ion source can include a vestibule into which a reactive gas, such as 0 ₂ , can be fed.

[63] The enclosure around the magnetron and the aperture provide another advantage in that if any reactive gas, such as 0 ₂ , finds its way into the magnetron enclosure, it can be quickly gettered by the forward sputtered flux to the surrounding walls of the enclosure and/or the aperture plate. In other words, the infiltrating reactive gas can be effectively pumped away.

[64] In some embodiments, as shown schematically in FIG. 9, a system 900 for generating an oxide coating according to the present teachings includes a movable aperture 901a that is disposed in front of a sputtering source 902, e.g., a magnetron (in this figure, the reaction zone is not depicted). In this embodiment, the aperture 901a is in the form a rectangular opening formed within an aperture plate 901. The aperture 901a can function as the output opening of an enclosure (vestibule) 904 around the magnetron. In some embodiments, the enclosure itself can include an opening through which the metal atoms can exit the enclosure, and the aperture 901a can be placed in front of this opening, and preferably in substantial register therewith. The movable aperture 901a is configured to move linearly back and forth along an axial direction, which can correspond to a central axis of a flux of metal atoms generated by the sputtering source and propagating toward the aperture. Similar to the previous embodiments, the system 900 includes a rotatable mount 902' on which a plurality of substrates 903 can be mounted. In this embodiment, the substrates are mounted on the rotatable mount such that when each substrate is aligned with the aperture 901a, the axial axis is perpendicular to the substrate surface.

[65] A variety of mechanisms can be used to move the aperture plate relative to the sputtering source and the rotatable mount. By way of example, in this embodiment, the sputtering source (e.g., a magnetron) is enclosed within an enclosure 904. A set of linear motion feedthroughs 905 are coupled to the aperture plate 901. Each of the linear motion feedthroughs 905 includes an external rotating knob 905a, which is is external to the enclosure 904 (and in this embodiment external to the vacuum chamber), that can be rotated, e.g., manually or by a motor. A coupler 905b couples rotational motion of the knob to an internal rod 905c, which is located within the enclosure and is coupled at its distal end to the aperture plate, and transforms the rotary motion of the external section of the knob to a linear motion of the internal rod 905c, which in turn causes linear movement of the aperture plate and hence the aperture. A variety of commercially available linear motion feedthroughs, such as those marketed by MDC corporation of Haywood, California, U.S.A. can be employed. In this embodiment, four linear motion feedthroughs are employed, though only two are shown in FIG. 9.

[66] The movable aperture allows adjusting the pressure within the sputtering source vestibule (chamber) 904, e.g., which can form an enclosure around the magnetron. For example, for a given flow rate of a sputtering gas, such as argon, into the sputtering source vestibule, the closer the aperture is to the sputtering source the higher will be the pressure of gas within the chamber. The variation of the pressure within the sputtering source vestibule can be used to modulate the stress in a metal film sputtered onto a substrate, e.g., from a compressive to a tensile stress. For example, in some embodiments, at a very low pressure (e.g., a pressure in a range of 0.75 to 2 mtorr) the stress in the sputtered metal film can be compressive and at a high pressure (e.g., a pressure greater than 30 mtorr) the stress can be tensile.

[67] In conventional ion assisted sputtering systems for generating coatings, the sputtering and reactive gases are in communication with one another in a single chamber. As a result, it may not be possible to vary the operational pressures of the sputtering source and the ion source over their entire respective operational ranges. For example, an ion source (e.g., an 0 ₂ ion source) may not perform well at over the entire range in which the pressure of a sputtering gas in a magnetron can be varied. For example, in many cases the maximum operational pressure of an ion source can be about 1 mtorr, which is less than the maximum operational pressure of a magnetron, e.g., 30 to 50 mtorr. In the systems according to the present teachings, due to the isolation of the sputtering source from the reactive ion source, the pressure of the sputtering source (e.g., a magnetron) and the pressure of the ion source can be varied

independently. For example, the pressure of the sputtering source can be varied over its entire operational range without adversely affecting the operation of the ion source. By way of example, in some embodiments of the present teachings in which a movable aperture is disposed in front of a sputter magnetron (See, e.g., FIG. 9), the pressure of the magnetron (e.g., the pressure within the magnetron vestibule) can be varied over its entire operational range at a constant flow rate of a sputtering gas (e.g., argon) by moving the aperture plate (and hence the aperture) closer or farther from the rotatable mount, which decreases or increases the pumping speed applied to the magnetron vestibule with a concomitant increase or decrease in the sputtering gas (e.g., argon) pressure. This change in the pressure of the magnetron vestibule has no detrimental effect on the operation of the ion source due to its isolation from the magnetron.

[68] In some embodiments, the independent control of the sputtering gas pressure and the reactive gas pressure allows independently adjusting the stress in the sputtered metal film generated in the sputtering zone and the stress in the oxide film generated via oxidation of the metal film in the reaction zone. For example, in some embodiments, the metal film can be deposited onto a substrate in a tensile, compressive or neutral stress state. The pressure of the reactive ion source (e.g., an 0 ₂ ion source) can then be controlled to generate the metal oxide in a desired stress state. In other words, the stresses of the metal and oxide films can be adjusted separately to have unprecedented control in adjusting the stress in the resultant metal oxide film (e.g., an aluminum oxide film). For example, if the metal film is sputtered onto a substrate in a compressive state, the pressure of the reactive ion source can be adjusted to generate a metal oxide film at a net zero stress. Typically, as the ion energy increases the compressive stress in the generated metal oxide film increases. By way of example, if the metal film is generated during the sputtering phase in a tensile stress state, the energy of the reactive ions (e.g., 0 ₂ ions) during the reaction phase can be increased to counter the tensile stress so as to generate a metal oxide film that exhibits a slight compressive stress or a neutral stress. In some embodiments, the energy of the reactive ions (e.g., 0 ₂ ions) can be in a range of about 65 to 150 eV.

[69] In some embodiments, a system according to the present teachings facilitates adjusting the pressure of the sputtering gas (e.g., argon) based on the desired stress in the deposited metal film. In some implementations, the pressure of the sputtering gas (e.g., argon) can be pulsed in the sputtering vestibule through the process cycle (or at least a portion thereof) to average out the metal film stress. For example, the argon pressure within the magnetron vestibule can be varied during different cycles of sputtering as a substrate mounted on a rotatable mount passes multiple times through the sputtering zone so as to obtain a desired average metal stress. In some embodiments, the sputtering pressure can be adjusted so as to impart a tensile stress to the metal film. In other words, the metal film can be biased with a tensile stress. As the ion source can be operated independently of the sputtering source as discussed above, the ion source can be configured, including the pressure of the reactive gas and the energy of the reactive ions, in view of the biased tensile stress of the metal film to tune the stress in the resultant metal oxide film to a desired value. For example, the ion source can be configured to bias the metal film toward a less tensile state, which in combination with the tensile bias provided by the sputtering source, can result in a metal oxide film having a neutral or a slightly compressive stress state. Such tuning of the stress of the resultant metal oxide film can be advantageous in a variety of applications, including deposition of metal oxide films (e.g., aluminum oxide) on flexible substrates, such as plastic substrates.

[70] In some embodiments, the apertures utilized in the system according to the teachings of the invention, either fixed or movable apertures, can be employed to collimate the sputtered atoms. By way of example, referring again to FIG. 9, the aperture 901a can be moved relative to the magnetron such that the maximum divergence of atoms passing through the aperture to reach a substrate is less a desired threshold. The angular distribution of the flux of the emitted sputtered atoms can be characterized by a cosinosoidal function. For example, the emitted flux can be characterized by Φ= Cos ⁿ 0, where Φ denotes the flux of sputtered atoms at an angle Θ relative to a central axis (e.g., the axial direction depicted in FIG. 9) of the flux, which can be normal to the substrate onto which the atoms are directed. For a large value of n (e.g., n greater than about 3), the flux is more forward focused, and for a small value of n (e.g., n equal to less than about 3), the flux is spread or "flattened" out.

[71] With reference to FIG. 10, in absence of the aperture 901a, certain atoms that are emitted at large angles relative to a normal to the substrate (e.g., at angles and θ ₂ ) would strike the substrate at high grazing angles of incidence. Atoms striking the substrate surface at high grazing angles tend to adhere poorly to the substrate surface. With the aperture plate 901 in place, these off-normal atoms strike the apertures plate and hence are prevented from reaching the substrate. More specifically, in this example, the atoms emitted at angles greater than θ ₃ strike the inner surface of the aperture plate and hence do not reach the substrate. In other words, in this example, the aperture lowers the spread of the angular distribution of the atoms by ensuring that the maximum divergence of the emitted atoms that pass through the aperture is θ ₃ . By moving the aperture along a direction normal to the substrate, the maximum angular distribution of the emitted atoms can be adjusted. As the spread of the angular distribution of the emitted atoms narrows, the number of atoms leaving the aperture decreases, thereby lowering the deposition rate of atoms on the substrate. This can impose a practical limit on the minimum spread of the angular distribution of the emitted atoms. In some embodiments, the aperture plate is positioned such that the maximum divergence angle of the emitted atoms as they exit the aperture, namely, θ ₃ , is in a range of about 10 to about 30 degrees. In some embodiments, rather than a single aperture, a plurality of apertures (e.g., 2 to 5 apertures) can be placed in series and spaced relative to one another to adjust the angular spread of the emitted atoms. In some embodiments, the use of multiple apertures allows further reducing the angular spread of the emitted atoms beyond a level that is practical by using a single aperture.

[72] The aperture plate can be designed to obtain a desired collimation of the emitted atoms, that is, a desired angular spread of the atoms characterized by a maximum divergence angle, by using a variety of different methods. For example, in some embodiments, a pinhole method can be employed to measure the angular distribution of cathode flux for a particular magnetron. The measured angular distribution together with a given separation of the cathode from the aperture can be employed to design a plate of areal dimension. Further detail regarding a pinhole method can be found in J. A, Thornton and D.W. Hoffman JVST 18,2, March 1981, which is herein incorporated by reference in its entirety. In some cases, the dimension obtained via measurement can be further optimized by theoretical modeling that takes into account the conductance of the aperture, gas phase scattering within the vestibule, and the distance of the aperture from the rotatable mount, among others.

[73] The use of an aperture according to the present teachings to collimate atoms emitted from a sputtering target can provide a number benefits. In some applications, such collimation can provide improved result in planarization of microelectronic structures. For example, the use of emitted atoms that are not collimated according to the present teachings to planarize or fill up a via or trench can lead to the deposition of atoms on the top of the via around the edge of the opening, thereby leading to pinch off of the top of the via (tear drop effect) that would prevent the filling of the bottom of the via, as shown schematically in FIG. 11 A. In contrast, the use of an aperture in front of the sputter target, e.g., a via or trench, can result in a more uniform deposition of the sputtered atoms. In particular, the addition of an aperture in front of the sputtering substrate can greatly reduce the arrival of high-angle of incident atoms at the substrate. In some embodiments, the use of a plurality of apertures placed in series and spaced by a predetermined distance (or distances) relative to one another can remove almost entirely the high angle of incident atoms from the sputtered flux. In other words, using collimation, the metal atoms propagating at high angles of incident relative to a direction normal to the substrate can be eliminated. The arrival of the metal atoms within an angular distribution characterized by smaller incident angles relative to the normal can allow complete filing of a via or a trench as shown schematically in FIG. 1 IB. The filled via or trench can be subsequently planarized, e.g., by chemical mechanical planarization.

[74] As discussed above, the isolation of the metal sputtering zone and the reaction zone according to the present teachings provides a number of advantages. As discussed in more detail below, another advantage of such isolation of the two zones is that it can greatly reduce, and in some cases eliminate, the possibility of sputtering gas (i.e., the gas in the sputtering vestibule) implanting in the resultant metal oxide coating (such a sputtering gas is typically argon, though it is possible to use other noble gases, such as neon or xenon). This in turn lowers, and in some cases eliminates, the contamination of the resultant metal oxide film (coating) by the sputtering gas, e.g., argon. It is known that the inclusion of impurities in metal films sputtered on substrates can lead to a component of compressive stress in such films, especially at higher cathode voltages. In many cases, the sputtering impurity is argon atoms. In particular, at low sputtering pressures (e.g., pressures in a range of about 0.5 to about 5 mtorr), neutral argon atoms can reflect off the cathode at the full potential of the cathode and can be implanted in the substrate to cause compressive stress. At higher sputtering pressures, there is some probability that the sputtering gas (e.g., argon) can be implanted beneath actively growing layers. By using the teachings of the invention, and in particular by isolating the sputtering source from the ion source, this component of the compressive stress in the resultant metal oxide film can be reduced, and preferably eliminated. Further, the ion source can be utilized to control the stress in the metal oxide film in repeatable and controllable manner, e.g., in a manner discussed above.

[75] In conventional batch or load locked sputtering processes, either in reactive or non-reactive or ion assisted deposition, the sputtering gas, which is typically argon, is always present in the process gas, and the process gas is in communication with the substrate continuously during the entire film deposition cycle. So, for layers greater than a monolayer ( 1 monolayer is 1 atomic layer of material) there is a chance that the argon gas can become entrained or trapped in subsequent layers of the substrate.

[76] In contrast, in many embodiments of the present teachings, only 1 monolayer of metal can be deposited per revolution of the rotatable mount. The deposited monolayer of metal is fully consumed or oxidized by subsequent exposure to reactive ions, e.g., 0 ₂ generated by an ion source running at 100 % 0 ₂ . Therefore, in many embodiments, due to the thinness of the coating, no argon atoms, or 0 ₂ molecules, are implanted below these very thin metal layers. Consequently, in many embodiments, the substrate is free of any implanted argon atoms. For example, in some embodiments, the concentration of the argon atoms in the resultant metal oxide film can be less than 1 percent, or preferably less 0.5%, or more preferably a 1-10 parts per million.

[77] In many embodiments, after the metal film deposition, during the

bombardment of the substrate surface by reactive ions, e.g., by 0 ₂ ions during an oxidization phase, any free argon atoms left on the film surface can be removed by energetic reactive ions (e.g., 0 ₂ ions) bombarding the metal film surface. Further, as noted above, since the build-up of a metal oxide layer on a substrate surface under processing is achieved by periodic deposition of thin layers of metal followed by the reaction of those layers with reactive ions, e.g., 0 ₂ , in many embodiments, no argon is buried beneath multi-layers of the reacted film. Hence, the resultant oxide film (e.g., aluminum oxide film) is substantially free of sputtering gas atoms (e.g., argon atoms).

[78] By ensuring that substantially no sputtering gas atoms (e.g., argon atoms) are incorporated in the oxide film formed on a substrate surface, a component of stress associated with presence of sputtering gas atoms as impurities in the film can be removed. This can also significantly reduce, and preferably eliminate, the effect of variation of voltage in a sputtering source (e.g., a magnetron), which can occur through the lifetime of the target and also with pressure, on the build-up process of the oxide film. As a result, the stress in the resultant metal oxide film can be determined to a first order by the energy of the reactive ions bombarding the substrate and the reactive ion flux. In some embodiments, the discharge voltage and the discharge current of an ion source can determine the energy of the reactive ions and the ion flux.

[79] In some embodiments, the stress in an ion assisted film process can be determined by the ion to atom ratio. In such embodiments, the stress can be controlled by the ratio of the rotational speed of the rotatable mount, the magnetron power (metal rate/rev) and the ion beam current. More generally, the film quality, such as index of refraction, optical transmission and mechanical stability, of the metal oxide film can be affected by the ion/atom ratio. In many embodiments, the atom/ion ratio is about 1, and preferably greater than 1, e.g., in a range of about 1 -10. [80] As a result, coated substrates are produced comprising an underlying substrate and a metal oxide film disposed on a surface of the substrate. Films may be disposed over multiple surfaces, and these can be the same or different. Various types of substrates that can be used include, for example, crystalline materials such as silicon and sapphire, flexible materials such as polymeric substrates and various plastics, including polycarbonates or polymethyl methacrylates, and glass such as soda-lime, borosilicate, or aluminosilicate glass, including chemically-strengthened alkali aluminosilicate glass (such as the material referred to as Gorilla® glass available from Corning). The thickness of the substrate can vary, depending on the targeted application and on cost, and can be, for example, greater than about 0.1 mm, including from about 0.2 mm to about 5 mm, from about 0.3 mm to about 2 mm, or from about 0.5 mm to about 1.0 mm. The thickness of the metal oxide film can also vary in a range of about 0.1 microns to about 50 microns, including about 0.5 microns to about 20 microns and about 1 micron to about 10 microns. In addition, the metal oxide film is preferably substantially free of noble gas impurities (e.g., argon, neon, or xenon), as described above.

[81] The metal oxide film can impart improved properties to the substrate. Thus, by combining a relative thick substrate material with a deposited thin metal oxide surface film, the resulting coated substrate would have the desirable surface characteristics of the metal oxide film while also taking advantage of desirable bulk properties of the underlying substrate material, These coated substrates differ significantly from composite substrates (comprising multiple discrete layers of material) in that no bonding of layers is used or required, which are often a site of delamination and failure of a composite substrate. As a specific example, an aluminum oxide film would be expected to provide improved scratch resistance, abrasion resistance, and/or hardness to the surface of a glass substrate upon which it is disposed. Additional benefits may also expected for this coated substrate, depending on the thickness and properties of the alumina coating and on the type and thickness of the glass, including, for example, improved of modified optical properties, high flexural and mechanical strength, fracture toughness (i.e., the ability of the coated substrate containing a crack or scratch to resist fracture), modulus, and smudge resistance (improved or modified oleophilicity or hydrophilicy). Other metal oxide films would be expected to provide corresponding improvements or modifications to other or similar substrates, and such combinations would be recognized by one skilled in the art.

[82] The resulting coated substrates can be used in a variety of different end-use application. As a specific example, a coated substrate comprising A1 ₂ 0 ₃ disposed on a glass substrate could be used as a cover plate for various consumer electronic devices. The electronic device can be any known in the art comprising a display or display element, such as mobile or portable electronic devices including, but not limited to, electronic media players for music and/or video, such as an mp3 player, mobile telephones (cell phones), personal data assistants (PDAs), pagers, laptop computers, or electronic notebooks or tablets. The cover plate can be affixed to the display surface of the display element of the device or it can be a separate protective layer that can be placed or positioned over or on top of the display element and later removed if desired. Preferably, the alumina coated surface of the coated substrate in this specific example is a front, exterior-facing surface of the cover plate, in order to take advantage of the properties or the coating, such as scratch resistance, but can also be side-facing surfaces to improve fracture resistance and cracking. The overall thickness of the cover plate can vary depending on a variety of factors, such as the number of alumina layers and the size of the device but, in general, is less than or equal to about 5 mm, such as less than or equal to about 3 mm and less than or equal to about 1 mm. [83] All publications referred to herein are incorporated by reference.

[84] While the invention has been shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims.

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