Patent Publication Number: US-2004040833-A1

Title: Apparatus and method for plasma treating an article

Description:
BACKGROUND OF INVENTION  
       [0001] The invention relates to an apparatus for generating a substantially uniform plasma. More particularly, the invention relates to an apparatus for generating a substantially uniform plasma for treating an article. Even more particularly, the invention relates to an apparatus that is capable of generating a controllable, adjustable plasma for treating an article.  
       [0002] Plasma sources are used to provide a variety of surface treatments for a number of articles. Examples of such surface treatments include deposition of various coatings, plasma etching, and plasma activation of the surface. The characteristics of the plasma treatment process are strongly affected by the operating parameters of the plasma source.  
       [0003] Individual plasma sources, such as expanding thermal plasma (also referred to hereinafter as “ETP”) sources, can be used to treat surface areas having a diameter in the range of about 10-15 cm. During plasma treatment, conditions within the plasma source may drift; e.g., cathode-to-anode distance may change over time due to erosion of the cathode, or cathode voltage or operating pressure may change. To counteract such drift, particularly changes in cathode-to-anode distance, disruption of the process and disassembly of the plasma source are usually required.  
       [0004] An array of multiple plasma sources may be used to coat larger substrate areas. Ideally, the individual plasmas generated by each of the plasma sources in the array should have the same characteristics. In practice, however, source-to-source variation in plasma characteristics and, consequently, in the resulting plasma treatment, is commonly observed. A significant amount of the variability is related to the previously described variations in the individual plasma sources.  
       [0005] Drift within a single plasma source cannot be counteracted in real time; such corrections require disruption of the process and disassembly of the plasma source. Where multiple plasma sources are used, minimization of source-to-source variation in the generated plasmas is often desirable. Therefore, what is needed is an apparatus that is capable of generating a stable, controllable plasma. What is also needed is an apparatus for plasma treating an article using a stable, controllable, adjustable plasma. What is further needed is a plasma source that generates a plasma and is adjustable so as to alter the properties of the generated plasma.  
       SUMMARY OF INVENTION  
       [0006] The present invention meets these and other needs by providing an apparatus that generates at least one plasma that is stable and adjustable in real time. In one embodiment, the apparatus includes multiple plasma sources that can either be “tuned” in real time to generate plasmas that are similar to each other or, conversely, “detuned” to generate dissimilar plasmas. The apparatus may be used to provide plasma treatment—such as, but not limited to, coating, etching, heating, lighting or illumination, and activation—for an article. The invention also provides a plasma source in which operating parameters are adjustable in real time. Methods of providing such plasmas and treating an article using such plasmas are also disclosed.  
       [0007] Accordingly, one aspect of the invention is to provide an apparatus for generating a substantially controllable plasma. The apparatus comprises: at least one plasma source, the plasma source comprising a plasma chamber in which the substantially controllable plasma is generated, at least one cathode and an anode disposed in the plasma chamber, the at least one cathode and the anode being separated by a gap, the gap being adjustable, a power source coupled to the anode and the at least one cathode for providing a voltage across the anode and the at least one cathode, a plasma gas inlet for introducing a gas for generating the plasma (hereinafter referred to as a “plasma gas”) from a plasma gas source into the plasma chamber at a plasma gas flow rate, and a sensor for monitoring conditions within the plasma chamber; and a second chamber in fluid communication with the plasma chamber through an exit port, wherein the second chamber is maintained at a second pressure that is less than a first pressure in the plasma chamber, and wherein the substantially controllable plasma flows from the plasma chamber into the second chamber through the exit port.  
       [0008] A second aspect of the invention is to provide a plasma source for generating a substantially controllable plasma. The plasma source comprises: a plasma chamber in which the substantially controllable plasma is generated; an anode disposed at a first end of the plasma chamber, the first end having an exit port through which the substantially controllable plasma exits the plasma chamber; at least one adjustable cathode disposed in the plasma chamber, wherein the at least one adjustable cathode is movable to establish a gap between the anode and the at least one adjustable cathode; a power source coupled to the anode and the at least one adjustable cathode for providing a voltage across the anode and the at least one adjustable cathode; a plasma gas inlet for introducing a plasma gas from a plasma gas source into the plasma chamber at a plasma gas flow rate; and at least one sensor for detecting and monitoring conditions within the plasma chamber.  
       [0009] A third aspect of the invention is to provide an apparatus for generating a substantially controllable expanding thermal plasma. The apparatus comprises: at least one expanding thermal plasma source, the at least one expanding thermal plasma source comprising: a plasma chamber in which the substantially controllable plasma is generated; an anode; at least one adjustable cathode disposed in the plasma chamber, wherein the at least one adjustable cathode is movable to establish a gap between the anode and the at least one adjustable cathode; a power source coupled to the anode and the at least one adjustable cathode for providing a voltage across the anode and the at least one adjustable cathode; a plasma gas inlet for introducing a plasma gas from a plasma gas source into the plasma chamber at a plasma gas flow rate; and at least one sensor for detecting and monitoring conditions within the plasma chamber; and a second chamber in fluid communication with the plasma chamber through an exit port, wherein the second chamber is maintained at a second pressure that is less than a first pressure in the plasma chamber, and wherein the substantially controllable plasma flows from the plasma chamber into the second chamber through the exit port.  
       [0010] A fourth aspect of the invention is to provide a method for generating a substantially controllable plasma. The method comprises the steps of: providing at least one plasma source, the at least one plasma source comprising: a plasma chamber; an anode; at least one adjustable cathode disposed in the plasma chamber; a power source coupled to the anode and the at least one adjustable cathode; a plasma gas inlet; and at least one sensor; providing a plasma gas through the plasma gas inlet to the plasma chamber in each of the at least one plasma source; generating a plasma in the plasma chamber; monitoring at least one parameter within the plasma chamber; and controlling the plasma, wherein the plasma is controlled by adjusting conditions within the plasma chamber, based upon the monitoring of the at least one parameter.  
       [0011] A fifth aspect of the invention is to provide a method for treating an article using a substantially controllable expanding thermal plasma. The method comprises the steps of: providing at least one expanding thermal plasma source, wherein the at least one expanding thermal plasma source comprises: a plasma chamber; an anode; at least one adjustable cathode disposed in the plasma chamber; a power source coupled to the anode and the at least one adjustable cathode; a plasma gas inlet; and at least one sensor; providing a plasma gas through the plasma gas inlet to the plasma chamber in each of the at least one plasma source; generating a plasma in the plasma chamber; monitoring at least one parameter within the plasma chamber; and controlling the plasma, wherein the plasma is controlled by adjusting conditions within the plasma chamber, based upon the monitoring of the at least one parameter; forming an expanding thermal plasma by expanding the plasma through an exit port into a second chamber in fluid communication with the plasma chamber, wherein the second chamber contains the article and is maintained at a second pressure that is less than a first pressure in said plasma chamber; and impinging the expanding thermal plasma on a surface of the article, thereby treating the article.  
       [0012] These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0013]FIG. 1 is a schematic representation of an apparatus for generating a substantially controllable plasma;  
     [0014]FIG. 2 is a plot of plasma chamber pressure as a function of cathode length, measured at a constant flow rate of argon gas into the plasma chamber;  
     [0015]FIG. 3 is a plot of cathode voltage as a function of cathode length, measured at a constant flow rate of argon gas into the plasma chamber;  
     [0016]FIG. 4 is a plot of cathode voltage and plasma chamber pressure as a function of time;  
     [0017]FIG. 5 is a plot of deposition profiles of silicon carbide films deposited using multiple plasma sources in both the tuned and detuned states; and  
     [0018]FIG. 6 is a plot of individual Taber delta haze values for an abrasive resistant silicone coating as a function of substrate location and position with respect to the individual ETP sources. 
    
    
     DETAILED DESCRIPTION  
     [0019] In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top”, “bottom”, “outward”, “inward”, and the like are words of convenience and are not to be construed as limiting terms.  
     [0020] Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing an embodiment of the invention and are not intended to limit the invention thereto.  
     [0021] Turning now to FIG. 1, an apparatus  100  for producing a substantially controllable plasma of the present invention, comprising a first plasma source  102 , a second plasma source  202 , and a second chamber  140 , is shown. The present invention is not limited to the embodiment represented in FIG. 1; apparatus  100  may comprise a single plasma source or more than two plasma sources as well. It is understood that, while various features of first plasma source  102  are described in detail and are referred to throughout the following description of the invention, the following description is applicable to second plasma source  202  as well.  
     [0022] First plasma source  102  comprises a plasma chamber  104 , a cathode  106 , and an anode  108 . Cathode  106  is disposed within, and extends into, plasma chamber  104 . While a single cathode  106  is shown in FIG. 1, it is understood that plasma source  102  may include multiple cathodes  106 . Anode  108  is located at one end of plasma chamber  102 . An exit port  118  provides fluid communication between plasma chamber  104  and second chamber  140 . The substantially controllable plasma generated within plasma chamber  104  exits plasma chamber  104  through exit port  118  and enters second chamber  140 . In one embodiment, exit port  118  may comprise an orifice formed in anode  108 . In another embodiment, exit port may comprise at least one “floating” (i.e., electrically insulated from both cathode  106  and anode  108 ) cascade plate  122  separating anode  108  from the rest of plasma chamber  102 . Alternatively, exit port  118  may be located in a floating wall in one of plasma chamber  102  and second chamber  140 .  
     [0023] A gas for generating the plasma (hereinafter referred to as a “plasma gas”) is injected into plasma chamber  104  through at least one plasma gas inlet  114 . the plasma gas may comprise at least one inert or non-reactive gas, such as, but not limited to, a noble gas (i.e., He, Ne, Ar, Xe, Kr). Alternatively, in embodiments where the plasma is used to etch the surface, the plasma gas may comprise a reactive gas, such as, but not limited to, hydrogen, nitrogen, oxygen, fluorine, or chlorine. Flow of the plasma gas may be controlled by a flow controller, such as a mass flow controller, located between a plasma gas source (not shown) and the at least one plasma gas inlet  114 . A first plasma is generated within plasma chamber  104  by injecting the plasma gas into plasma chamber  104  through the at least one plasma gas inlet  114  and striking an arc between cathode  106  and anode  108 . The voltage needed to strike an arc between cathode  106  and anode  108  is provided by power source  112 . In one embodiment, power source  112  is an adjustable DC power source that provides up to about 100 amps of current at a voltage of up to about 50 volts. Second chamber  140  is maintained at a second chamber pressure by a vacuum system (not shown), which is substantially less than a first plasma chamber pressure. In one embodiment, second chamber  140  is maintained at a pressure of less than about 1 torr (about 133 Pa) and, preferably, at a pressure of less than about 100 millitorr (about 0.133 Pa), while plasma chamber  104  is maintained at a pressure of at least about 0.1 atmosphere (about 1.01×10 4  Pa). As a result of the difference between the first plasma chamber pressure and the second chamber pressure, the first plasma passes through exit port  118  and expands into second chamber  140 .  
     [0024] Second chamber  140  is adapted to contain an article  160  that is to be treated with the plasmas produced by apparatus  100 . In one embodiment, such plasma treatment of article  160  comprises injecting at least one reactive gas into the plasma produced by apparatus  100  and depositing at least one coating on a surface of article  160 . The surface of article  160  upon which the at least one plasma impinges may be either planar or non-planar. Apparatus  100  is capable of providing other plasma treatments in which at least one plasma impinges upon a surface of article  160 , such as, but not limited to, plasma etching at least one surface of article  160 , heating article  160 , lighting or illuminating article  160 , or functionalizing (i.e., producing reactive chemical species) a surface of article  160 . The characteristics of the plasma treatment process are strongly affected by the operating parameters of the plasma source. Among such operating parameters are the operating pressure within the plasma source, plasma resistance, the potential across the cathode and anode, the plasma current, and the cathode-to-anode distance.  
     [0025] In one embodiment, the plasmas generated by at least one of first plasma source  102  and second plasma source  202  are expanding thermal plasmas (also referred to hereinafter as “ETP”). In an ETP, a plasma is generated by ionizing the plasma source gas in the arc generated between at least one cathode  106  and anode  108  to produce a positive ion and an electron. For example, when an argon plasma is generated, argon is ionized, forming argon ions (Ar + ) and electrons (e − ). The plasma is then expanded into a high volume at low pressure, thereby cooling the electrons and positive ions. In the present invention, the plasma is generated in plasma chamber  104  and expanded into second chamber  140  through exit port  118 . As previously described, second chamber  140  is maintained at a substantially lower pressure than the plasma chamber  104 . In an ETP, the positive ion and electron temperatures are approximately equal and in the range of about 0.1 eV (about 1000 K). In other types of plasmas, the electrons have a sufficiently high temperature to substantially affect the chemistry of the plasma. In such plasmas, the positive ions typically have a temperature of about 0.1 eV, and the electrons have a temperature of about 1 eV, or 10,000 K, or higher. Consequently, the electrons in the ETP are too cold and thus have insufficient energy to cause direct dissociation of any gases that may be introduced into the ETP. Such gases may instead undergo charge exchange and dissociative recombination reactions with the electrons within the ETP.  
     [0026] The characteristics of the plasma generated by plasma source  102  depend in part upon gap  110 , which is defined herein as the spacing between cathode  106  and anode  108 . FIGS. 2 and 3 are plots of plasma chamber pressure and cathode voltage as a function of cathode length, respectively. In this particular embodiment of the invention, gap  110  decreases as the cathode length increases. In each plot, the cathode-to-anode distance was systematically varied and the data was collected using a constant flow rate of argon gas into the plasma chamber. As seen in FIG. 2, the pressure of the plasma decreases with decreasing cathode-to-anode distance. Similarly, the voltage needed to sustain the plasma decreases with decreasing cathode-to-anode distance, as illustrated in FIG. 3.  
     [0027] Changes—or “drift”—in cathode-to-anode may occur during operation of a plasma source. Drift may be caused by erosion of either the cathode or anode, deposition of material on either the cathode or anode, mechanical settling or seating of plasma source components, and thermal expansion of plasma source components. Variation of cathode voltage and plasma pressure as functions of time (expressed in FIG. 4 as the number of experiments run under the same conditions) are shown in FIG. 4. It is understood that some factors—such as deposition of material on either the cathode or anode—may cause drift in a direction opposite that shown in FIG. 4. As drift occurs with passing time, both the cathode voltage and plasma pressure that are needed to sustain the plasma shift (i.e., either increase or decrease) as well. These trends are consistent with those shown in FIGS. 2 and 3. Typically, cathode-to-anode distance cannot be adjusted in real time; complete disassembly of the plasma source is usually required to make the adjustment.  
     [0028] The present invention provides a plasma source  102  in which gap  110  (i.e., cathode-to-anode distance) is adjustable in real time to a desired distance in response to selected conditions within plasma chamber  104 , such as, but not limited to, plasma pressure, cathode voltage, plasma current, and plasma gas flow rate. At least one sensor  116  monitors and detects any change in such conditions within plasma chamber  104 . The sensor(s) selected for use in plasma source  102  depends upon the property to be monitored. Non-limiting examples of the at least one sensor  116  that may be used monitor conditions within plasma chamber  104  include: a pressure sensor, such as a transducer, in fluid communication with plasma chamber  104 ; a voltmeter (or any similar voltage measurement device) for measuring and detecting cathode voltage; and an ammeter for measuring and detecting plasma current. Any change detected by the at least one sensor  116  is fed to a controller, which then adjusts gap  110  by changing the position of one of cathode  106  and anode  108  to maintain the selected conditions within desired ranges.  
     [0029] In one embodiment, plasma source  102  includes at least one adjustable cathode  106 . Gap  110  may be set to a predetermined distance by moving cathode  106 . As seen if FIGS. 2 and 3, respectively, changes in plasma chamber pressure or cathode voltage, as detected and monitored by the at least one sensor  116 , are indicative of changes in gap  110 . Cathode drift may, for example, be indicated by shifts or changes in cathode voltage and plasma chamber pressure. Plasma chamber pressure data obtained during statistical process control as feedback, for example, may be used to control gap  110 . Compensation of cathode drift may be achieved during operation of plasma source  102  by movement of adjustable cathode  106  in response to input by the at least one sensor  116  to maintain gap  110  at the selected distance. Variation due to cathode drift of the plasma generated by plasma source  104  is thus eliminated or significantly reduced by such adjustment of cathode  106 . Such movement of the adjustable cathode  106  may be performed in real time either manually or by a controller.  
     [0030] In some instances, it may be desirable to vary the properties of the plasma generated by plasma source  102  over time. Non-limiting examples of situations in which plasma properties may be altered during operation include deposition of multiple layers on a single substrate or performing multiple plasma treatments of a single article. The ability to adjust gap  110  in real time permits the properties of the plasma generated by plasma source  102  to be modified in a controllable fashion without disassembly of plasma source  102 .  
     [0031] Movement of adjustable cathode  106  may be accomplished by a pressure means coupled to the movable cathode  106 . In one embodiment, pressure means include a pressure plate coupled to a rear portion of adjustable cathode  106  by either set screws or a screw drive. As conditions dictate, gap  110  may be increased or decreased by either applying or releasing pressure to the pressure plate, or gap  110  may be maintained at a constant value as adjustable cathode  106  erodes during operation of plasma source  102  by applying pressure plate as needed. In another embodiment, pressure means may comprise a pneumatic drive coupled to adjustable cathode  106 . The pneumatic drive may increase, decrease, or maintain gap  110  at a selected value—as conditions dictate—by moving adjustable cathode  106  accordingly.  
     [0032] In another embodiment, plasma source  102  further includes a screw or worm drive for moving adjustable cathode  106 , thereby adjusting gap  110 . In yet another embodiment, adjustable cathode  106  comprises a wire, and movement of adjustable cathode  106  to either increase, decrease, or maintain gap  110  at a selected value is achieved by coupling a wire feed to adjustable cathode  106 .  
     [0033] Adjustable cathode  106 , in one embodiment, is movable in a direction that is normal to cascade plate  122 . Here the longitudinal axis of adjustable cathode  106  is concentric with exit port  118 . Alternatively, adjustable cathode  106  is movable in a direction parallel to cascade plate  122 .  
     [0034] In one embodiment, cathode  106  is movable and anode  108  is fixed during operation of plasma source  102 . In other embodiments, however, gap  110  may be adjusted by providing first plasma source  102  with a movable anode  108 . Movement of anode  108  may be accomplished by mechanisms similar to those previously described for providing movement of adjustable cathode  106 . First plasma source  102  may also include both a movable cathode  106  and a movable anode  108 .  
     [0035] In those embodiments in which apparatus  100  includes more than one plasma source, second plasma source  202  includes features corresponding to those of first plasma source  102 , which are described herein. For example, plasma source  202  includes cathode  206 , anode  208 , gap  210 , at least one plasma gas inlet  214 , at least one sensor  216 , exit port  218 , and cascade plate  222 . The voltage needed to strike an arc between cathode  206  and anode  208  is provided by either power source  112  or a separate power source.  
     [0036] In some instances, it is desirable to treat article  160  uniformly with a plasma. The characteristics (e.g., coating thickness, degree of etching or activation) of a region treated by a single plasma source, such as an ETP source, generally exhibit a profile having a Gaussian distribution about the axis of the plasma source. When multiple plasma sources are used to treat article  160 , uniformity may be promoted by positioning the individual plasma sources such that the resulting Gaussian distributions overlap. The profile, as well as the width and height of the distributions, are dependent in part upon the characteristics of the plasmas that are used to treat the substrate. The characteristics of each of the plasmas are in turn dependent upon the conditions—such as cathode voltage, plasma gas pressure and cathode-to-anode distance (gap  110 )—used to generate the plasmas within the individual plasma sources.  
     [0037] In one embodiment, conditions within first plasma source  104 —and, consequently, the first plasma produced by first plasma source  102 —are adjustable with respect to conditions within plasma chamber  204  and second plasma, and vice versa. For example, first plasma source  102  and second plasma source  202  may be “tuned” to eliminate or minimize variation between the first plasma and the second plasma by setting at least one of plasma pressures, cathode voltages, and gaps  110 ,  210  of first plasma source  102  and second plasma source  202  to be equal to each other. Using any of the means that have been described above for adjusting movable cathodes  106 ,  206 , gap  110  and gap  210  may then be maintained at the same value during operation by moving adjustable cathodes  106 ,  206  (or, in some embodiments, adjustable anodes  108 ,  208 ) in response to input from sensors  116 ,  216 . Such tuning of first plasma source  102  and second plasma source  202  may be advantageous, for example, for depositing a coating having a substantially uniform profile of at least one selected property over a large surface area of a planar substrate.  
     [0038] Conversely, first plasma source  102  and second plasma source  202  may be “detuned” by moving adjustable cathodes  106 ,  206  to provide gaps  110 ,  210  of unequal size, thereby producing a first plasma and second plasma that are dissimilar with respect to each other. Detuning may be desirable, for example, for plasma treating a non-planar substrate. In such instances, the working distance (i.e., the distance between a plasma source and the substrate surface) for first plasma source  102  may differ from the working distance for second plasma source  202 . At the point at which they impinge on the surface of article  160 , the properties of the first plasma (generated by first plasma source  102 ) would consequently differ from those of the second plasma (generated by second plasma source  202 ). Differences in working distances for the individual plasma sources may be compensated for by moving adjustable cathodes  106 ,  206  (or, in some embodiments, adjustable anodes  108 ,  208 ) to provide gaps  110 ,  210  of unequal size to produce first and second plasmas that have essentially the same properties at their respective points of impingement upon the surface of article  160 .  
     [0039] The characteristics of the plasma generated by first plasma source  102  also depends upon the pressure of the plasma gas within plasma chamber  104  and the voltage (or potential) of cathode  106 . Thus, the characteristics of the plasma may also be controlled by adjusting at least one of the pressure of the plasma gas within plasma chamber  104  and the voltage of cathode  106 . Plasma gas pressure may be monitored by the at least one sensor  116  and adjusted accordingly. One means of adjusting the plasma gas pressure is by controlling the flow of plasma gas into plasma chamber  104  through plasma gas inlet  114 . Means for controlling the flow of plasma gas into plasma chamber  104  include, but are not limited to, needle valves and mass flow controllers. The cathode voltage (or potential) may be similarly monitored by the at least one sensor  116 , and adjusted by adjusting power supply  112  accordingly. It is understood that, in those embodiments having a second plasma source  202 , the characteristics of the plasma generated by second plasma source  202  may be similarly controlled by adjusting at least one of the plasma gas pressure within plasma chamber  204  and the voltage of cathode  206  in response to input provided by the at least one sensor  216 .  
     [0040] In one embodiment, the present invention permits at least one of the pressure of the plasma gas within plasma chamber  104  and the cathode voltage of cathode  106  to be adjustable with respect to the plasma gas pressure within plasma chamber  204  and the cathode voltage of cathode  206 , respectively. Thus, conditions within plasma chamber  104  and thus the first plasma produced by first plasma source  102 —are adjustable with respect to conditions within plasma chamber  204  and the second plasma produced by second plasma source  202 , and vice versa. For example, the first plasma generated by first plasma source may be either “tuned” to eliminate or minimize variation between the first plasma and the second plasma or “detuned” to be dissimilar with respect to each other.  
     [0041] Tuning of the first plasma and the second plasma may be achieved by adjusting at least one of plasma pressures and cathode voltages of first plasma source  102  and second plasma source  202  to be equal to each other. Conversely, the first and second plasma may be detuned by adjusting at least one of plasma pressures and cathode voltages of first plasma source  102  and second plasma source  202  to be dissimilar with respect to each other. Plasma pressures within plasma chambers  104 ,  204  may be monitored by the at least one sensors  116 ,  216 , respectively. Using the means for controlling the flow of plasma gas into each of plasma chambers  104 ,  204  through plasma gas inlets  114 ,  214  described above, the plasma pressures within each of plasma chambers  104 ,  204  may be adjusted in response to the input provided by the at least one sensors  116 ,  216 . Similarly, cathode voltages of cathodes  106 ,  206  may be monitored by the at least one sensors  116 ,  216 , respectively, and adjusted with respect to each other by adjusting power supply  112  accordingly.  
     [0042] An example of such tuning and detuning of the plasmas produced by multiple plasma sources is shown in FIG. 4. The profiles of a-Si x C y :H films deposited on a substrate by injecting vinyltrimethylsilane (VTMS) into plasmas generated by multiple ETP sources are plotted as a function of lateral position on the substrate. The film profiles correspond to the properties—such as, but not limited to, temperature, density, cross-sectional area, and reactant concentration—of the plasmas that are used to deposit the films. The squares in FIG. 4 represent the film profile that is obtained when the two sources that are detuned; i.e., operated at different plasma pressures and cathode voltages. The dissimilar plasmas produce a profile in which the thickness of the deposited exhibits a significant variation. The diamonds in FIG. 4 represent the film profile that is obtained when the pressures and voltages of the two sources have been tuned to be equal. The resulting profile exhibits less variation than that obtained using detuned plasma sources.  
     [0043] Another aspect of the invention is to provide an article having at least one coating disposed on a surface of the article, wherein the at least one coating is deposited by the method described herein using apparatus  100 . The at least one coating is substantially uniform and has a selected property that exhibits a variation of less than about 10% across the surface of the article. The selected property of the at least one coating may be one of coating thickness, abrasion resistance, ultraviolet radiation absorbance, infrared radiation reflectivity, modulus, hardness, oxygen permeability, water permeability, adhesion, surface energy, thermal conductivity, and electrical conductivity. The at least one coating may comprise an abrasion-resistant coating, an ultraviolet filtering coating, an infrared reflective coating, an oxygen- or moisture-barrier coating, an anti-reflective coating, a conductive coating, interlayers, an adhesion layer and combinations thereof. The coatings and deposition methods are described in U.S. Pat. No. 6,420,032, entitled “Adhesion Layer for Metal Oxide UV Filters” by Charles Dominic Iacovangelo et al.; U.S. Pat. No. 6,426,125, entitled “Multilayer Article and Method of Making by Arc Plasma Deposition” by Barry Lee-Mean Yang et al.; U.S. Pat. No. 6,261,694, entitled “Infrared Reflecting Coatings” by Charles Dominic lacovangelo; and U.S. Pat. No. 6,376,064, entitled “Layered Article with Improved Microcrack Resistance and Method of Making” by Steven Marc Gasworth et al., all of which are incorporated herein by reference in their entirety.  
     [0044] The advantages and salient features of the present invention are illustrated by the following example:  
     EXAMPLE 1  
     [0045] An abrasive resistant silicone (SiO x C y ) coating was deposited on a polycarbonate LEXAN® substrate using an array of expanding thermal plasma (ETP) sources. Each of the ETP sources included an adjustable cathode of the present invention. The ETP plasma sources were tuned by equalizing the pressures within the respective plasma chambers. The coating was formed by injecting oxygen (O 2 ) and octamethyltetracyclosiloxane (D4) into each of the ETPs. The resulting coating had a thickness of about 2 microns. The abrasion resistance of the coating was determined by a 1000 cycle Taber abrasion test. A plot of the individual Taber Delta haze values as a function of substrate location and position with respect to the individual ETP sources is shown in FIG. 6. The coating exhibited a uniform 2% increase in haze with a standard deviation of 0.6% across the large area substrate.  
     [0046] While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention.