Abstract:
In an embodiment an apparatus for coating a substrate comprises: an array of plasma arcs; a first plurality of reagent manifolds located upstream of the array of plasma arcs and a second plurality of reagent manifolds located downstream of the array of plasma arcs, each manifold having at least one orifice through which a reagent is ejected into a plasma jet issuing from a respective plasma arc; and a controller which modulates the flow of reagent to each manifold according to the contours of the substrate and to the substrate position relative to the plasma arcs and the manifolds.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional of U.S. patent application Ser. No. 12/406,166, filed Mar. 18, 2009, now U.S. Pat. No. 8,049,144, which is a Divisional of U.S. patent application Ser. No. 10/910,143, filed Aug. 3, 2004, now U.S. Pat. No. 7,521,653, the contents of each of which are incorporated herein by referenced thereto. 
    
    
     BACKGROUND 
     The present invention generally relates to a system for coating substrates. More specifically, the invention relates to a plasma arc coating system. 
     Existing multi-plasma arc coaters incorporate resistive heaters to pre-heat a substrate before it enters into a coating chamber or station. These heaters extend over a large distance in the direction of substrate motion. Further, these heaters have poor spatial resolution in the direction transverse to substrate motion, and have a slow thermal response time relative to the substrate residence time at the heater station. 
     Many coating stations continuously supply reagent during the coating process, such as coating stations that employ ring manifolds for individual plasma arcs or racetrack manifolds for arc arrays. These manifolds, however, do not allow the upstream and downstream injection orifices to be fed and switched on and off independently of one another. 
     Moreover, in certain coating stations, arcs and manifolds are placed on opposite sides of the coating station to coat both sides of the substrate, which requires balancing opposing jets to minimize or prevent overspray. However, in a two-sided coating station, balancing opposing jets is difficult to achieve and generally can not consistently be maintained during production runs. Therefore, the opposing jets mix when the jets are not fully intercepted by a substrate, resulting in condensing precursors originating from arcs on opposite sides of the substrates. 
     SUMMARY 
     In an embodiment an apparatus for coating a substrate comprises: an array of plasma arcs; a first plurality of reagent manifolds located upstream of the array of plasma arcs and a second plurality of reagent manifolds located downstream of the array of plasma arcs, each manifold having at least one orifice through which a reagent is ejected into a plasma jet issuing from a respective plasma arc; and a controller which modulates the flow of reagent to each manifold according to the contours of the substrate and to the substrate position relative to the plasma arcs and the manifolds. 
     In an embodiment a method of coating a substrate with a reagent comprises: ejecting a reagent from a first plurality of reagent manifolds located upstream of an array of plasma arcs; ejecting the reagent from a second plurality of reagent manifolds located downstream of the array of plasma arcs, each of the manifolds of the first and second plurality of reagent manifolds having at least one orifice through which a reagent is ejected into a plasma jet issuing from a respective plasma arc; and modulating the flow of reagent to each manifold with a controller according to the contours of the substrate and to the substrate position relative to the plasma arcs and manifolds 
     Further features and advantages will become readily apparent from the following description, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a system for coating a substrate in accordance with an embodiment of the invention; 
         FIG. 2  depicts the substrate with a filler that advances through the system of  FIG. 1 ; 
         FIG. 3A  depicts a coating station of the system of  FIG. 1  in accordance with an embodiment of the invention; 
         FIG. 3B  is a view of the coating station of  FIG. 3A  taken along the line  3 B- 3 B; 
         FIG. 4  depicts a reagent manifold segment with a valve system for the coating station of  FIGS. 3A and 3B  in accordance with an embodiment of the invention; 
         FIG. 5A  depicts a fine heater of the system of  FIG. 1  in accordance with an embodiment of the invention; 
         FIG. 5B  is a view of the heater of  FIG. 5A  taken along the line  5 B- 5 B; 
         FIG. 6A  depicts a coarse heater of the system of  FIG. 1  in accordance with an embodiment of the invention; and 
         FIG. 6B  is a view of the coarse heater of  FIG. 6A  taken along the line  6 B- 6 B. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a system  10  for plasma coating substrates is shown in accordance with an embodiment of the present invention. As its primary components, the system  10  includes a coating station  12 , a coating station  14 , a fine heater  16 , a fine make-up heater  18  located on the entrance side of the coating station  14 , and a coarse heater  20  located on the entrance side of the fine heater  16 . 
     Referring also to  FIG. 2 , various configurations of the system  10  involve impinging an activated reagent, or, in some implementations, activated reagents, on one or both sides of a substrate  22  as it advances through the coating stations  12 ,  14 . The substrate  22  can be a rear window, a roof panel or other component of a vehicle. The substrate  22  can be made of polycarbonate. The heaters  16  and  20  ensure that the substrate  22  is at the appropriate temperature before entering the coating station  12  and the heater  18  re-heats the substrate  22  to make up for any heat loss during transit before entering the subsequent coating station  14 . 
     Each substrate  22  is attached to a conveyor system by a tab  24 . In certain implementations, a filler  26  is also attached to the conveyor system by a set of tabs  28 . Alternatively, the substrate  22  can be mounted to a respective filter  26 . 
     The filler  26  is closely spaced such that it is a virtual extension of the edge of the substrate  22 . The filler  26  is a disposable or re-useable part made from, for example, polycarbonate or aluminum that forms a border around the substrate  22 . In such a configuration, the filler  26  acts a collector of extraneous heat and coating material. As such, use of the filler promotes uniform temperature and coating thickness over the substrate. In particular embodiments, the temperature over the substrate  22  is about 65° C.±10° C. at the entrance of the coating station  12 , and the coating thickness is between about 2 μm to 3 μm after the coating station  14 , as the conveyor system moves the substrates (and optionally the fillers) through the system  10  at a speed of about 2.5 cm/s. Continuous flow of the coating reagents into the manifolds results in wastage of material between the substrates. Moreover, that continuous flow creates extra cleaning and maintenance burdens. 
     Various embodiments of the system  10  are able to reduce the tendency for relatively thick plasma coating at the edges of the substrate, where greater coating thicknesses tend to occur relative to the center of the substrate  22 . Since there is a preferred range for the coating thickness (that is, not too thin, which compromises abrasion-resistance, and not too thick, which compromises water immersion performance), it is desirable to minimize thickness variability across the substrate so as to make the coating process more robust. The relatively thick coating at the edges is a probable contributing factor to less robust water immersion performance at the edges of the substrates. 
     Further, the system  10  minimizes extraneous coating material on the vacuum chamber walls of the coating stations  12 ,  14  (and on the fillers  26  when they are employed), when there are gaps between successive substrates, which occurs when the adjacent trailing and leading edges of successive substrates do not nest because of their different contours, or because the substrate transport system provides for independent motion or articulation of substrates to accommodate complex shapes or to accommodate a combination of a stationary heating step with a scanning coating step. 
     Referring now to  FIGS. 3A and 3B , each coating station  12 ,  14  includes a vacuum chamber with a plurality, for example, of six plasma arcs  30  on each side of the vacuum chamber. Each station  12 ,  14  further includes an upstream reagent manifold segment  32  and a downstream reagent manifold segment  34  associated with, for example, a pair of arcs. As such, each side of the coating station chamber is provided with three upstream reagent manifold segments  32  and three downstream reagent manifold segments  34 . Various embodiments of the coating stations  12 ,  14  involve injecting coating reagents through the manifold segments  32 ,  34  independently of one another. A set of oxygen manifold segments  35  are also associated with respective manifold segments  32 ,  34 . A simple control mechanism can be employed to switch the oxygen manifold segments  35  on and off individually, or a combination of two or more oxygen manifold segments  35  can be operated together. 
     As shown in  FIG. 3B , the leading edge  37   a  of an advancing substrate  22  first passes the upstream manifold segments  32 , then the array of arcs  30 , and finally the downstream manifold segments  34 , and the trailing edge  37   b  is the last portion of the substrate  22  to pass by the downstream manifold segments  34 . The arcs  30  are typically fed with an inert gas, such as argon, which is heated to the point of partial ionization and issues into the vacuum chamber as jets (from the respective arcs) directed towards the substrate to be coated. Coating reagents are introduced in vapor form between the arcs and the substrate at a controlled rate by injection orifices distributed on the manifold segments  32  and  34  adjacent to the respective arcs. A controller  40  ( FIG. 4 ) directs the operation of the upstream and downstream manifold segments  32 ,  34 , such that the twelve manifold segments can operate independently of one another according to the profiles of the leading and trailing edges of the substrate  22 . 
     The controller  40  modulates the reagent flows to the respective individual manifold segments  32 ,  34  according to the position of the substrate edge relative to a particular manifold segment. Specifically, the controller  40  directs the operation of a three-way valve  42  associated with each manifold segment. The valve  42  either directs vaporized reagent from a reservoir  44  that is shared by multiple manifold segments to the respective manifold segments via conduits  45  and  47  or diverts the reagent away from the associated manifold segments, providing for rapid modulation of reagent flow to the manifold segments  32 ,  34  and associated jets while the reagent flow from the reservoir  44  continues uninterrupted. The diverted flow can be directed, via a conduit  49 , into a relatively cool condensation vessel  46 , also shared by multiple manifold segments, that condenses the reagent vapor, which can then be recycled into the primary liquid reagent reservoir  44  via an optional conduit  51 . In sum, the valve  42  modulates the reagent flow to the respective individual manifold segments  32  or  34  by directing the continuous reagent flow from the reservoir  44  either to the manifold segment or to the condensation vessel  46  for recycling. The close proximity of the valve to the wall of the vacuum chamber (indicated by the dashed line) is intended to minimize the volume of residual vaporized reagent between the valve  42  and the manifold segment immediately after the vapor flow has been switched to the condensation vessel  46 . The residual vapor can continue to flow into the manifold  32  or  34  after the valve  42  is switched to divert the flow away from the manifold, which may be undesirable since the residual vapor flow tends to limit the effective speed of response of the flow modulation means. Although the reservoir  44  and the condensation vessel  46  are described above as being shared by multiple manifold segments, the reservoir  44  and the condensation vessel  46  can be associated with a single manifold segment. 
     Thus, the system  10  reduces or minimizes excess coating at the edges of substrates by modulating reagent flow to the upstream and downstream manifolds, according to specific protocols, as a leading or trailing substrate edge crosses in front of a respective array of arcs  30 . Reagent flows to a manifold only when there is a substrate present, reducing the flux of coating precursors during edge coating and limiting the opportunity for coating precursors originating on one side of the substrate to reach the opposite side. The optimum timing of the individual switching events, expressed in terms of a local edge position relative to the individual manifolds, can be determined empirically. The desired timing can then be easily implemented, since it is only a matter of equipment design to provide for tracking of the position of the substrate with sufficient accuracy and resolution. 
     Accordingly, the system  10  minimizes extraneous coating on the vacuum chamber walls of the coating stations  12 ,  14 , as well as on the fillers if they are employed, because coating reagent is diverted to the condensation vessel  46  when there are gaps between successive substrates, minimizing the frequency of chamber wall cleaning (and cleaning of fillers if employed) and maximizing utilization of reagent. As mentioned previously, the system  10  improves the uniformity of plasma coating thickness, making the plasma coating process more robust with respect to upper and lower specification limits for coating thicknesses. 
     As shown in the illustrated embodiment, each coating station includes six upstream and six downstream manifold segments. However, depending on the application, each station may include greater or fewer segments. The manifolds are segmented to accommodate a variety of contours of substrate edges. With the arc array oriented transverse to the direction of substrate motion, different segments of a contoured substrate edge generally pass the arc array at different times. Segmenting the manifolds provides for local flow switching according to a desired protocol. 
     Note that non-uniform substrate temperature is promoted by preferential heating of substrate edges, by variations in substrate thickness, and by substrate shape complexity. There is, however, a preferred range for substrate temperature for the coating process, since a temperature that is too low tends to compromise water immersion performance, and a temperature that is too high risks substrate distortion and thermal damage. Thus, it is desirable to maximize temperature uniformity across the substrate so as to make the coating process more robust. Accordingly, the system  10  provides for improved consistency of plasma coating performance across a substrate by improving substrate temperature uniformity during the coating process. 
     Referring back to  FIG. 1 , as well as to  FIGS. 5A and 5B , each of the fine heater  16  and fine make-up heater  18  includes a programmable two-dimensional array of heat sources  50  positioned on each of the opposing walls of the heaters. The heat sources  50  are under the direction of a controller  140  and are programmable with a fast response time relative to the substrate residence time in the heater and produce a small spot size (i.e., the heated area on the substrate at a working distance of up to several inches) relative to characteristic dimensions of the substrate. The fast response times of the heat sources allow controlled heating of the substrate to produce a desired substrate temperature profile for the coating process and also minimization of waste heat. Arranged as an array, the heat sources  50  are capable of projecting a contiguous heated area onto a passing or stationary substrate. 
     The optional fine make-up heater  18  compensates for cooling of the substrate between the coating stations  12  and  14  and provides for controlled substrate temperature profile for the coating station  14 , which is analogous to the role of the fine heater  16  with respect to the first coating station  12 . 
     The heat sources  50  provide spatially-resolved heating for a moving or stationary substrate, which can compensate for preferential heating of substrate edges, and for substrate-specific features and shape complexity that tend to promote non-uniform substrate temperatures. Referring in particular to  FIG. 5B , as each area element of the substrate  22  is heated by a heat source  50 , the instantaneous intensity of that source is adjusted in a programmed fashion according to the desired local temperature of that area element. As such, the heat sources turn on only when the substrate is present, as indicated by the activated heat sources  60 . 
     Note that since substrate heating by the plasma arcs  30  during the coating process may also be non-uniform, it may be desirable to compensate for this by programming the substrate temperature profile after the heaters  16  and/or  18  to be non-uniform in a complementary way. For example, in regions of the substrate that are preferentially heated by the arcs  30  (e.g. a convex surface with a relatively small working distance to the arc array), it may be desirable to leave that region relatively cool during exposure in the fine heater  16 , as well as in the fine make-up heater  18 . 
     The fast response time of the programmable array of heat sources  50  facilitates controlling the temperature profile on individual substrates to allow for different treatment of successive substrates, according to their particular features. A further benefit of the fast response time is a more efficient use of heater power and less extraneous heating of the vacuum chamber, and filler if used, since individual heat sources  50  are active only when a substrate is present to absorb the projected heat. The significance of this benefit is increased by shifting more of the heating burden away from the coarse heater  20  and onto the programmable heat sources in the heaters  16  and  18 . The heaters  16  and  18  promote temperature uniformity in the substrates during plasma coating, which, in turn, promotes a more robust coating process. Moreover, each side of the substrate to be coated can be exposed to a respective programmable array of heat sources before the coating process; that is, the operation of the array of heat sources  50  on one side of the substrate may be different than that of the opposite array of heat sources in each of the heaters  16  and  18 . 
     Since the operation (both the intensity and the on and off time cycles) of the array of heat sources  50  is fully programmable, the system  10  can accommodate different substrates, for example, substrates with different shapes, sizes, thicknesses, and made from different materials with different thermal responses. This feature of the heat sources  50  is unlike conventional heat sources in that the substrate heating process can be rapidly modulated to minimize wasted power and extraneous heating of the vacuum chamber, and of the filler if used, and can be substrate specific to promote a narrow substrate temperature range during coating. 
     Provision for the coarse heater  20  is optional. For the coarse heater  20 , fast response time and small spot size are not required. Thus, as shown in  FIGS. 6A and 6B , the course heater includes, for example, a set of longitudinal heat sources  61  and associated reflectors  63  that raise the substrate temperature part way to a desired temperature, without targeting a particular temperature profile. A purpose of the coarse heater  20  is to reduce the burden on, and in some cases the length in the direction of substrate motion of, the programmable array of heat sources  50  in the fine heater  16 . Accordingly, the coarse heater  20  is located upstream of the fine heater  16  so that the substrate first passes the coarse heater before advancing through the fine heater. 
     The system  10  may also provide for in-situ measurement of the local substrate temperature, for example, through the use of a series of infrared temperature sensors  62  (FIG. I), following substrate exposure to the programmable fine heater  16 , and optionally following the coating station  12 , the fine make-up heater  18 , and the second coating station  14 . These temperature measurements help characterize the substrate temperature profile before (and optionally after) the plasma coating process. This profile can be used to adjust the program of the programmable array of heat sources  50  to establish the program appropriate for a specific substrate prior to actual production. Alternatively, the profile can be monitored during production to allow real-time adjustment of the program to maintain a desired temperature profile. 
     The system  10  is also able to identify the type and shape of substrate to be heated and then recall from a library of information or a database the appropriate program for the array of heat sources  50 , a feature which may be of interest if different substrates are to be coated in a common production run. 
     In certain implementations, the energy output of each heat source  50  is between about 200 W to 400 W. The heat sources  50  can be spaced apart (center to center) between about 3 inches to 6 inches. Any suitable lamp that provides the necessary energy and spot size may be used. For example, the heat sources  50  may be quartz lamps or halogen lamps. 
     In particular implementations, the substrates are heated in the heaters  16 ,  18  while stationary. Specifically, the conveyor system moves a substrate into the heaters  16 ,  18 , and the substrate then remains stationary as selected heat sources  50  in the presence of the substrate turn on. This implementation allows for a simplified control system, a shorter array of heat sources in the direction of movement of the substrates, and hence a shorter overall footprint for the system  10 . 
     The above and other implementations are within the scope of the following claims.