Abstract:
A system and method for modularly interfacing wet components of a coating apparatus to the remainder of the coating apparatus comprising a station whereupon a substrate is located. A fluid cart houses the wet components of the apparatus which comprise a dispensing head in fluid connection to the fluid cart. The fluid cart can be connected to a station and a variety of different types of coating operations subsequently performed. The fluid can be readily disconnected from the station after the coating operation is complete, permitting a different fluid cart to be attached to the same station. The first fluid cart can then receive required servicing without idling the station to which it had been attached. Various embodiments of the means for coating the substrate and powering and controlling equipment of the coating apparatus are disclosed.

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is being concurrently filed with commonly assigned U.S. patent application, Ser. No. 09/227,692, filed Jan. 8, 1999, now abandoned, entitled “INTELLIGENT CONTROL FOR EXTRUSION HEAD DISPENSEMENT”; U.S. patent application, Ser. No. 09/227,362, filed Jan. 8, 1999, now U.S. Pat. No. 6,092,937, entitled “LINEAR DEVELOPER”; U.S. patent application, Ser. No. 09/227,607 filed Feb. 16, 2000, entitled “MOVING HEAD, COATING APPARATUS AND METHOD”; U.S. patent application, Ser. No. 09/227,381, filed Jan. 8, 1999, entitled “SYSTEM AND METHOD FOR CLEANING AND PRIMING AND AN EXTRUSION HEAD”; U.S. patent application, Ser. No. 09/227,459, filed Jan. 8, 1999, entitled “SYSTEM AND METHOD FOR ADJUSTING A WORKING DISTANCE TO CORRESPOND WITH THE WORK SURFACE”; the disclosures of which are incorporated herein by reference. Reference is also made to the following co-pending and commonly assigned U.S. patent application entitled “LINEAR EXTRUSION COATING SYSTEM AND METHOD, Ser. No. 09/148,463, filed Sep. 4, 1998; and U.S. patent application entitled “SYSTEM AND METHOD FOR PROVIDING COATING OF SUBSTRATES, Ser. No. 09/201,543, filed Nov. 30, 1998; the disclosures of which are incorporated herein by reference. The present application is a continuation of co-pending, commonly assigned Provisional U.S. Patent Application, Ser. No. 60/070,985 now expired, entitled “INTELLIGENT CONTROL SYSTEM FOR EXTRUSION HEAD DISPENSEMENT”, filed Jan. 9, 1998 the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to the precision coating of surfaces and more particularly to extrusion coating of substrates wherein fluid storage, delivery, and dispensing means are contained within a module which can be removably attached to a station containing the remainder of a complete coating apparatus. 
     BACKGROUND 
     It is often necessary or desired to provide a coating of a particular substrate. For example, in the video electronics industry it is often desired to coat panels which will serve as flat panel displays (FPD) to be incorporated into television sets, computer monitors and the like. It is important in such applications to ensure the accuracy and consistency of coating thicknesses across the panel. 
     In the prior art, the fluid delivery means, including fluid supply, pumps, and dispenser or fluid extrusion head assembly, as well as the chuck, substrate and means for distributing a coating of the fluid on the substrate were all part of a single integrated coating apparatus assembly. As such, when it was necessary to change coating fluids, or perform other operations on the fluid delivery means, the entire coating apparatus would be idled. Fluid changeover operations include time consuming tasks such as cleaning all tubing, pumping mechanisms, and essentially all surfaces where residue of the previous coating material could be present. This thoroughness is often necessary because of potentially dangerous chemical reactions between two different coating materials to be used in succession, and because of the danger of cross-contamination between coating materials used in different processes. The idle time for the coating apparatus is expensive and wasteful given that mechanisms unrelated to the fluid delivery system are idled by the operations necessary for fluid changeover. Accordingly, a need exists in the art for a system and method wherein a chuck assembly adapted to position and hold substrates to be coated as well as other components and materials used in the coating process but not part of the fluid delivery system are not left idle during fluid delivery system cleaning operations. 
     Additionally, in order to avoid dripping or smearing coating material which has gathered around the extrusion head after a coating operation, it is often necessary to clean the extrusion head before a new coating operation begins. In the prior art, cleaning of extrusion mechanisms is usually accomplished manually, potentially leading to inconsistent results and disruption and delay of the coating operations. Therefore, it is a problem in the art that manual cleaning operations are inconsistent and unreliable. 
     In order to ensure that coating material is applied consistently and evenly right from the start of the coating operation, it is desirable to ensure that a bead is fully and properly formed at the extrusion head prior to starting the coating process. A problem in the prior art exists with respect to properly priming fluid extrusion heads so as to ensure that a proper bead is formed prior to extruding fluid over the substrate, that a consistent rate of coating fluid flow is thereafter achieved, and that the extrusion head can be quickly moved from the priming mechanism to the substrate. 
     Generally, in prior art coating systems, there is a single pump mechanism located remotely from the extrusion head with appropriate fluid conducting means leading from the pump the head. The use of a single pump, while perhaps economical, makes it difficult to precisely control fluid flow at the extrusion head. Specifically, it may be difficult to start and stop at precisely defined moments and to establish the precise fluid flow rate desired. 
     In prior art systems, variation in the height of the extrusion head with respect to the substrate can cause breaking of the coating bead and variation in coating thickness. The causes of such height variation include part dimension variation, part placement error, and gradual drift in machine dimensions over time. Accordingly, there is a need in the art for a system and method for ensuring constant extrusion head height over the substrate being coated. 
     Accordingly, there is a need in the art for a method and system for coating in which the idle time for apparatus not part of the fluid delivery process to be minimized while fluid is being changed or recharged. 
     A still further need in the art exists for a cleaning station whose functions are automatically accessible to a fluid dispenser in between coating operations. 
     A still further need in the art exists for a priming station which can be accessed automatically by a fluid dispenser in between coating operations. 
     A still further need in the art exists for more precisely controllable flow of coating material at the extrusion head. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved by a system and method in which the wet components, including the fluid supply, pumping means, fluid dispensing head and utility station operations are located on a carrier, or other device, hereinafter referred to as a mobile device or cart, although it should be appreciated that the present invention may be embodied in any number of devices not completely consistent with the chosen nomenclature, which is removably attachable to the remainder of the coating apparatus. The mobile device is preferably in the form of a cart or fluid station which contains all or substantially all of the components in the coating apparatus which come into contact with coating fluid. The portion of the coating apparatus not part of this mobile fluid module, or fluid cart would preferably contain a chuck and shuttle mechanism, or other transport means. The non-fluid portion of the coating apparatus will be referred to as a base station or work station. 
     The fluid cart and base station preferably both contain means for being secured together in preparation for a coating operation employing a chosen cart. Means for accomplishing this attachment include but are not limited to clips, clamps, rollers on beams which are forced against a rigid surface, and grippers which may be actuated by electrical, pneumatic, and hydraulic means. Preferably, in the context of a plurality of carts with different coating materials and different types of dispensers on board, and possibly, a plurality of base stations, any cart can be mechanically attached, and appropriately interfaced to any station among the plurality of carts and stations, wherein the interface may serve to transfer information, power, and facility connections such as exhaust or drain connections, by a variety of means including electric, pneumatic, hydraulic, or wireless. 
     Preferably, both the fluid cart and base station both comprise means for communicating with the other. The need for coordinating fluid flow rates with relative velocity of the dispenser with respect to a substrate among other parameters create a desire for such communication. The communication link between the cart and base station may be made by wire or cable or may be wireless, and is preferably under computer control. If the communication is accomplished via a hard wire connection, this connection will preferably be automatically made when the cart and base station are mechanically joined. Alternatively, the wired connection may be made manually either before or after a rigid mechanical attachment between the cart and base station is accomplished. Upon removing a cart from a base station, all connections made when first joining the cart and base station are disconnected. 
     In a preferred embodiment, the dispenser, coating head, or extrusion head is associated with a cart as is other equipment with comes into direct contact with the coating fluid. This approach obviates the need to clean the dispenser in between coating operations involving different fluids, and minimizes the amount of mechanical connection and disconnection necessary when changing carts. A preferred embodiment of the cart includes a cowl assembly substantially sealing any exposed wet components, such as the aforementioned head, in order to allow their continued contact with the coating fluid without fear of contamination or evaporation when not in use. Accordingly, various carts may be configured for particular coating operations which are repeated throughout a day or week without requiring the cart to be fully checked when idle, but expected to be subsequently put back into service. 
     In order to properly service fluid carts when they are not attached to base stations and engaged in coating activity, docking stations are deployed which provide connections for operating equipment on the cart, such connections including but not limited to electrical, pneumatic, hydraulic, and wireless. In a preferred embodiment, servicing the carts requires that the various pumps, switches, and other devices be powered and properly controlled. The docking station also provides appropriate connections and control as well as appropriate containers and supplies for removing unwanted fluid from the cart, supplying fresh fluid to the cart, and for appropriately cleaning equipment on the cart. 
     Preferably, fluid supplied to the cart from the docking station includes fresh coating material, and fresh solvent of one or more types used in the head cleaning and priming operations on the fluid cart. Fluid removed from the cart to the docking station includes coating material which is not needed in a subsequent operation, or which has degraded over time to an unacceptable level of quality, or otherwise become contaminated, and used solvent material from the cleaning and priming assembly. The used solvent from the cleaning and priming operations may optionally be subsequently transferred to recycle and recovery station. 
     The following discussion acknowledges various possible relative motion configurations which may be used by a coating apparatus adapted according to the present invention. For example, a primarily moving head configuration is one in which the majority of the relative motion of a substrate with respect to a dispenser or dispensing head is due to movement of the dispenser over a mostly stationary substrate. Conversely, a primarily moving substrate, or moving chuck configuration is one in which the majority of the coating motion is due to motion of the chuck and substrate (it is assumed here that the substrate does not move with respect to the chuck) under a mostly stationary head, such as by the substrate being moved with respect to the dispenser during deposit of the fluid or by allowing a pool of fluid to be deposited and then spinning the chuck and substrate to distribute the fluid. Motion of the dispenser with respect to substrate could be initiated by means on either the cart or the base station, or both. 
     In a preferred embodiment of the invention, the base station is able to perform the same fluid removal and replenishment operations with respect to the fluid cart as described above in connection with the docking station. 
     Preferably, upon attachment of a preferred embodiment cart to a base station, a preferred embodiment dispenser would then be appropriately mechanically attached to a mount on the base station, whether a primarily moving head or primarily moving substrate configuration is in place. In the case of the moving head configuration, the mount for the dispenser would be attached to a shuttle mechanism or transport system which would move the dispenser over the substrate. In the case of a moving substrate configuration, the dispenser would be mounted on a point accurately fixed with respect to the chuck and substrate moving underneath the mounting point. Of course, the cart may include a shuttle or gantry assembly allowing the dispenser to be mounted thereon and positioned properly with respect to the base station without actually mounting the dispenser to the base station, if desired. 
     Even after being mechanically positioned on the base station, the dispenser would remain in communication with the cart, as the cart would continue to supply fluid to the dispenser, and in a preferred embodiment, to supply power and control information to a pump integrally mounted on the dispenser. Therefore, the cart will at least have a fluid connection to the dispenser. Additional possible connections between the dispenser and fluid cart include but are not limited to electrical control cabling, wireless broadcast, pneumatic lines, hydraulic lines (other than for coating fluid). 
     Mechanically mounting the dispenser on a portion of the base station permits the benefits the positioning accuracy of various parts of the base station with respect to each other to govern the relative locating accuracy between the dispenser and the substrate surface, and diminishes the level of precision required in the positioning of the cart with respect to the base station. The cart should nevertheless preferably be rigidly enough attached to the station so that no disruption in the various fluid and other connections occurs. However, mechanically mounting the dispenser on the base station means that imperfect mating of a cart and a base station when they are first attached, or minor relative movement of two with respect to each other during coating should not affect the mechanical precision of the coating operation. Moving the dispenser from the cart to the base station mount can be accomplished either manually, or automatically, preferably under computer control. 
     As mentioned above, the inventive mechanism is not restricted to an embodiment in which the dispenser is mounted to a portion of the base station in preparation for the coating operation. In an alternative embodiment, the dispenser could remain on or mountably attached to the fluid cart during coating. This embodiment may place an increased burden on the accuracy of positioning of the cart or at least the mechanism used to mount the dispenser with respect to the base station while the two are attached where relative positions of the dispenser and substrate are critical. 
     Using the present invention, each cart may be associated with a particular fluid or with a particular size or type of coating head. When a cart becomes unusable such as because the fluid supply has been exhausted, the fluid supply becomes unusable due to degradation over time, or because the current manufacturing process requires using a different coating fluid, the attached cart can be readily and rapidly disconnected from the base station. A new cart can then be immediately attached to the main station, and the dispensing head on the new cart attached to an appropriate mounting position on the base station. 
     Coating operations can thus quickly resume independently of the time consuming task of cleaning and readying for operation the fluid system on the old cart. The old cart can be cleaned and prepared for renewed operation at a docking station in parallel with the resumption of coating operations at the very same base station. The idle time experienced in the systems of the prior art is thereby minimized since coating operations need cease only during the disconnection and connection of carts and relocation of dispensing heads. 
     In a preferred embodiment of the present invention, utilities for servicing the fluid dispensing head may be located on each fluid cart within a range of travel of the fluid dispensing head as carried by a shuttle mechanism or other transport system of the preferred embodiment base station. Locating such utilities on the cart permits the features of the utilities, including but not limited to cleaning and priming of the dispenser, to be optimized for the fluid and dispensing head resident on that cart. For example, a cart carrying a particular fluid and a particular dispenser would contain scrubbers and solvents particularly suited for both the fluid and dispenser associated with that cart. Otherwise stated, the various components present on, or associated with a particular cart can be coordinated with each other for optimum system performance. 
     A shuttle on the base station can be automatically programmed to stop at utility stations on the fluid cart at appropriately selected times, such as between coating operations. A set of utilities could include a scrubbing station at which bulk coating material would be removed from the dispensing head through a combination of physical scrubbing with brushes in combination with use of a solvent. Such a scrubbing station is particularly useful if the most recently used coating material is highly viscous. 
     Another operation among these utilities preferably consists of a rinsing station at which a powerful solvent removes any material remaining from the most recent coating operation, even if the dispensing has been cleaned at the scrubbing station. The solvent used at the rinsing station is preferably selected so as to remove any solvent remaining from the scrubbing operation and any residual coating material, and so as to evaporate rapidly after completion of the rinsing operation thereby obviating the need for any further cleaning action. 
     Yet another operation among these utilities could consist of a priming station at which the dispensing head could be placed so as to ensure that a full and consistent bead of coating fluid is made ready at the dispensing head in preparation for the next coating operation, as well as to extract any coating fluid from the dispenser which may have been contaminated with or diluted by a cleaning solvent or other matter. A preferred embodiment for such a priming station consists of rotating cylinder upon which coating fluid is placed in the smallest quantity necessary to establish a consistent bead or to remove contaminated coating fluid from the dispenser. In this embodiment, holding the dispensing head stationary in proximity to the rotating cylinder effectively simulates moving the dispensing head over a certain length of surface material, without requiring the space such a length of surface material would occupy. 
     In a preferred embodiment of the present invention, at least one primary pump located remotely from the dispensing head, preferably on the fluid cart, would pressurize the fluid connections leading up to a dispensing head assembly, and a second smaller pump, preferably integrated into the dispensing head assembly, would accurately control the flow of fluid to be dispensed or extruded onto the substrate surface. Implementation of such an integrated pump on head arrangement would require communication and control connections to be made between the cart and the dispensing head. These connections, as well as the main coating material fluid connection are preferably coiled and located in such a manner as to not have their function disrupted in any way by motion of the dispenser on a shuttle mechanism or other transport means. A preferred mechanism for avoiding interference with the various cables, tubes, hoses and the like, necessary for connections to the dispenser and shuttle mechanisms and other devices, is the deployment of troughs preferably along the sides of the base station in which these connecting means may be placed. 
     In a preferred embodiment of the present invention, the means for moving the head and substrate to be coated with respect to each other is achieved by moving the head over a mostly stationary substrate. Using this embodiment, the footprint of the apparatus in the horizontal plane is much reduced with respect to a configuration in which the substrate travels a distance equal to its own length underneath a fluid dispenser. Using the moving head embodiment, the length of the system need only exceed the length of the substrate by the amount necessary for the fluid dispensing mechanism to move clear of the substrate, for purposes of substrate placement and removal, and possibly for the placement of utilities to service the fluid dispenser in between coating operations. 
     The moving head embodiment is adaptable to large substrate sizes as the nature of the chuck assembly design would change little with increasing substrate size. A single coating apparatus can be used with substrates of different sizes by employing a head of appropriate length and ensuring the shuttle mechanism has sufficient travel to cover the lengths of the various substrates to be coated. Where a larger substrate cannot be accommodated by a particular coating apparatus, the principal changes required for such apparatus to accommodate a larger substrate would be to appropriately increase either the width and/or travel of the shuttle mechanism and the length (span) of the fluid dispenser, and to adjust the size the chuck. Increasing the size of the substrate does not significantly increase the stresses on the moving means in a moving head embodiment. Whereas, in a moving substrate environment, the weight to be carried by the moving means, and the stresses thereon increase considerably with increasing substrate size. 
     A shuttle mechanism which carries the fluid dispenser preferably rides on an air bearing or alternative support and guidance mechanisms such as rolling contact with a rail system, or low friction contact surface, located underneath the chuck assembly, the shuttle mechanism thereby forming a single continuous rigid loop structure. The rigidity of this design optimizes the precision with which the coating apparatus can operate. This configuration also limits the width of the apparatus by obviating the need for a support surface beyond the width of the chuck assembly, thereby further reducing the footprint of the coating apparatus. The shuttle mechanism, with its air bearing below the chuck a carriage to carry the fluid dispenser above the chuck and substrate, and structural links connecting the two, effectively envelops the chuck thereby restricting the permitted thickness of the chuck assembly and equipment contained therein. 
     Although the modular fluid cart concept is preferably employed in conjunction with coating apparatus employing a moving head apparatus, the concept is not limited to this configuration. The modular fluid cart could be attached to a base station wherein the substrate to be coated moves underneath a mostly stationary dispensing head, or extrusion head. More generally, the base station to which the fluid cart is attached could employ any combination of dispenser and substrate movement in order to accomplish an appropriate coating motion. 
     Accordingly, it is a technical advantage of the present invention that the idle time for apparatus not part of the fluid delivery process is minimized while coating fluid is changed or recharged. 
     It is a further advantage of a preferred embodiment of the present invention that the various devices and utilities located on a particular fluid cart may be selected for optimal interaction with each other. 
     It is a still further advantage of the present invention that flow of coating material at the dispensing head is more precisely controllable than in prior art systems. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 depicts an perspective view of a coating apparatus with a fluid cart according the preferred embodiment of the present invention attached to a base station, with a dispenser suitably mounted on the base station; 
     FIG. 2 depicts a top view of fluid cart and base station of the preferred embodiment; 
     FIG. 3 depicts a perspective view of a preferred embodiment of the base station of the present invention; 
     FIG. 4 depicts a view of the front of a fluid cart according to a preferred embodiment of the present invention; 
     FIG. 5 depicts a view of the rear of a fluid cart according to a preferred embodiment of the present invention; 
     FIG. 6 depicts a plumbing diagram illustrating the flow of fluid through the extrusion mechanism; 
     FIG. 7 depicts a partial cross-sectional view of the pump on head apparatus that is integrally connected to the extrusion head; 
     FIG. 8 depicts an isometric view of a docking station and carts attachable thereto according to a preferred embodiment of the present invention; and 
     FIGS. 9A-9C show a substrate coating station module of the present invention having an extrude and spin sub-module deployed therein. 
    
    
     DETAILED DESCRIPTION 
     The present invention is described in the context of depositing a coating on a surface of a variety of devices including but not limited to flat panel displays and integrated circuit substrates. This is a known manufacturing technique which is described for illustrative purposes only. The process liquid may be a photoresist, developer, etchant, chemical stripper, solder mask, or any other liquid chemical, or other sufficiently available substance to allow its use in coating of substrates used in the manufacture of devices such as integrated circuits, flat panel displays and the like, as well as other sophisticated devices such as multi-chip modules (MCM&#39;s) and high density interconnect (HDI) chips used in mainframe computers, telecommunication switching systems, military electronics and other high-end devices. The present invention is not limited to any particular coating liquid, substrate or end product, and the principles of the invention should be broadly construed to any coating application regardless of the particular liquid, substrate or end product. 
     FIG. 1 depicts an isometric view of a coating apparatus  100  according to a preferred embodiment of the present invention, comprising a preferred embodiment fluid cart  200  joined with a preferred embodiment base station or work station  300 . 
     The fluid cart  200  is a preferred embodiment of a preferably mobile, modular, removably attachable fluid delivery system comprising all or substantially all of the components of the coating apparatus which contact coating fluid in the course of operation of said coating apparatus  100 . The modular removably attachable fluid delivery system is preferably mobile with said mobility being enabled by rolling contact or low friction contact with a floor, or rolling or sliding contact with a rail system. Alternatively, the fluid cart could be transported without contacting a floor or other surface, by mechanized means, under either manual or automatic control. 
     On the preferred embodiment base station  300 , a motion interface which is preferably a bearing mechanism which could be an air slide made of granite or other hard rigid material, but alternatively could comprise a rail system with frictional or rolling contact, or electromagnetic suspension, forms a foundation along which the shuttle mechanism or transport system travels for cleaning and coating operations. The preferred embodiment fluid cart  200  is at one end of the air slide, and an electrical control unit  104 , preferably part of the base station, at the other end of the air slide  103 , though it may be remotely located. The fluid cart  200  may include a utility station  102 . The utility station, if included, preferably includes facilities for scrubbing, rinsing, and/or priming the dispenser or dispensing head  101 . 
     A chuck  105  is suspended above the air slide  103  to allow a bearing for a shuttle mechanism or other transport system to pass beneath, and the dispensing head  101  above, the chuck. The chuck  105  supports the substrate  107 . The bearing  106  of shuttle mechanism  108  rides along air slide  103  underneath the chuck  105 , while the dispenser, or dispensing head  101  moves above the chuck  105  supporting substrate  107 . The dispensing head is preferably a linear extrusion head attached to fluid manifold preferably containing a bead forming orifice substantially as described in U.S. Pat. No. 4,696,885, titled “METHOD OF FORMING A LARGE SURFACE AREA INTEGRATED CIRCUIT”, the disclosure of which is incorporated herein by reference. It will be appreciated that the present invention may be practiced employing a work station which comprises apparatus for spin coating fluid onto a substrate, as shown in FIGS. 9A-9C. 
     The travel of the shuttle mechanism  108  preferably will be at least long enough to permit the dispensing head  101  to completely coat the largest substrate to be placed on the apparatus  100 , although this range may be reduced by providing for some movement of the substrate during coating, and to clear the substrate by a sufficient distance to permit the substrate to be removed by external personnel or machinery. 
     The substrate  107  is preferably raised from the chuck  105  prior to removal of the substrate  107  from the coating apparatus  100  preferably using substrate lift pins  109  located underneath the substrate surface, or alternatively by reversing the vacuum in the chuck, gliding or rotating devices under the substrate to raise the substrate, or by lifting a portion of the substrate which protrudes beyond the surface of the chuck  105 . The travel of the shuttle mechanism  108  will preferably also be long enough so that in addition to clearing the substrate  107 , the shuttle mechanism will be able to gain access to utility station  102 . 
     In order to minimize the system footprint and improve coating performance, particularly on the leading edge (edge nearest the utility station) of the substrate  107 , the substrate  107  will be located as close as possible to the utility station  102 . Preferably, the shuttle mechanism  108  carries the dispensing head  101  to the utility station  102  for head cleaning and for priming of the bead either before or during the loading of the substrate  107 . The preferred embodiment shuttle mechanism  108  then carries the dispensing head  101  to the near edge of the substrate  107  (the side closest to the utility station  102 ) so that coating of the substrate  107  may begin. The shuttle mechanism  108  then carries the dispensing head  101  across the substrate at a carefully monitored and preferably predetermined rate, preferably under computer control, while the dispensing head  101  dispenses coating material at a controlled rate over the substrate  107 . 
     The shuttle velocity and coating fluid dispensing or extrusion rate, while preferably predetermined, may also be selected just prior to coating or even during coating if the coating operation would benefit from doing so. Such in-process change in linear transport velocity (coating velocity) and fluid extrusion rate would preferably be controlled and coordinated by control means so as to ensure a fluid coating possessing the desired characteristics. Reasons for changing the selection of coating velocity and fluid extrusion rate could result from changes in ambient air conditions, and the parameters of the fluid including but not limited to viscosity, temperature, pressure, and the proportion of solid matter dissolved in the fluid. 
     In a preferred embodiment, the rate at which the shuttle mechanism  108  moves is coordinated with the rate of fluid flow to the dispenser  101  from the fluid cart  200 . Enabling such coordination requires that the base station  300  and the fluid cart  200  have communication means between them, which preferably include but are not limited to electrical wiring and wireless communication. 
     Once the shuttle mechanism  108  has traveled to a point where the dispensing head  101  has coated the entire substrate  107 , fluid flow to the dispensing head  101  is preferably discontinued. In a preferred embodiment, the dispenser  101  comprises an integrally mounted pump-on-head  110  which can accurately control the flow of fluid to the dispenser, such fluid having been initially pressurized and delivered to the dispenser by pumping means located on the fluid cart  200 . The pump-on-head assembly  110  permits initiation and discontinuation of fluid flow to occur more accurately and rapidly than when using only remote pumping means, such as that located on the fluid cart  200 . Alternatively, the fluid will not have been initially pressurized by remote pumping means, and the pump-on-head assembly will perform all required fluid pressurization and flow control by itself. 
     Preferably, the substrate  107  is then removed prior to moving the shuttle mechanism  108  back to the utility station  102  to avoid any accidental dripping of coating material onto the substrate  107 . The shuttle mechanism  108  is then preferably moved to the utility station  102 , and another substrate  107  subsequently loaded onto the chuck  105 . Employing a preferred sequence of operations, the dispensing head  101  is never above a substrate  106  except when performing a controlled coating operation. Of course, in alternative embodiments, such as when a coating fluid is of sufficient viscosity so as not to present a drip hazard, the sequence of head movements may be different than that outlined above. For example, after servicing, the head may be moved over a substrate loaded onto chuck  105  to begin coating from the far edge toward the near edge. Of course, substrate  107  may alternatively be loaded after the head has moved to the far end, if desired. Similarly, the head may be returned to a home position, passing over a freshly coated substrate, prior to removal of the substrate from the chuck, if desired. 
     Alternately, the shuttle  108  could remain in position near the electrical unit  104  end of the base station after the coating operation and permit external machinery to remove the substrate  107  from the coating apparatus  100 . Such an approach could be used for example, if the nature of coating material was such that the dispenser  101  did not require cleaning and priming prior to the next coating operation, or if the coating material is sufficiently viscous that no danger exists of dripping material onto the substrate during shuttle  108  travel back toward the fluid cart  200 . Moreover, the return travel of the head may be utilized in coating a second substrate loaded after removal of the first and while the shuttle remains in the position near electrical unit  104 , thus improving throughput of the coating apparatus. It is noted that the preferred embodiment pump on head  110  permits negative pressure to be applied to the dispenser thereby aiding in the prevention of any unwanted extrusion. 
     In a preferred embodiment, a height sensing and adjustment mechanism is implemented on the shuttle mechanism  108  to fine tune the gap between the dispensing head  101  and the substrate  107  in real time during the coating operation. Sensing means is appropriately zeroed while the head  101  is at the correct height, and a correction signal is subsequently generated whenever the height deviates above or below the preset level. The height sensing means can consist of a rod with a roller base which rolls along the substrate, or a surface parallel to the substrate. Such an arrangement would provide direct linear position feedback reporting the height of the dispenser or dispensing head above the substrate. An alternative means for height measurement would be to measure dispenser height over the substrate based upon the position of the height adjustment motor on the shuttle mechanism  108 . Using motor position information for height control constitutes indirect position feedback. Alternative technologies for conducting height sensing include optical, sonic, ultrasonic, hall effect, mechanical contact using friction or rolling contact, air back pressure, and electromagnetic. These methods also constitute direct position feedback. 
     A control system, preferably comprising computer hardware and software, converts this signal into information suitable to drive a motor or other driving means to restore the dispensing head to the proper height. This process of height self-correction begins at the start of the coating process and continues throughout the coating process. Control of the automatic height correction process can be handled either by main host software or delegated to a control sub-system which performs the height control function without burdening the main host software. 
     FIG. 2 depicts a top view of fluid cart and base station of the preferred embodiment. The fluid cart  200  and base station  300  are shown separated but ready for attachment. 
     In the preferred embodiment of the present invention, the fluid cart  200  comprises fluid storage and pumping means  201 . The pumping means may comprise one pump, or a plurality of pumps, connected to a fluid reservoir. Fluid pressure supplied by pumping means on the cart is sufficient to provide controlled fluid flow to the dispensing head  101 , even when the dispensing head is located at the far end of the base station  300  from the pumping means on the fluid cart. In the preferred embodiment, pumping means on the cart may be supplemented by a pump on head  110  which is integrally mounted on the dispenser or dispensing head  101 . 
     In the preferred embodiment, the dispenser head mounts  202  on the fluid cart  200  support the dispenser  101  while it is located on the fluid cart  200 . Before beginning coating with a recently attached cart, the dispenser in communication with said cart is preferably moved either manually or automatically from the fluid cart mounts  202  to the base station mounts  301  located on the shuttle  108  or other transport system. In the case of the fluid cart mounts  202 , and the base station mounts  301 , mounting may occur by a variety of means including but not limited to clamps, clips, nut and bolt attachment, ball joint, spring loaded locking mechanism, or electrically, pneumatically or hydraulically powered gripping means. 
     In the preferred embodiment, the rigid attachment of the dispenser  101  to the shuttle  108  ensures that the accuracy of placement of the dispenser  101  on the station benefits from the tolerances present in the machining of the base station  300  and the precision with which the shuttle  108  is located with respect to the stationary parts of the base station  300 . After attachment to the base station mounts  301 , the accuracy of dispenser location with respect to the base station  300  or a substrate located on the base station  300  is independent of the precision present in the attachment of the fluid cart  200  to the base station  300 . 
     In a preferred embodiment, a utility station  102  is present on the fluid cart. The utility station  102  preferably comprises means for scrubbing, rinsing and priming the dispensing head  101 . The utility station  102  is preferably located at such a point on the fluid cart  200  that the shuttle  108  can carry the dispenser to the portions of the utility station  102  under automatic control. 
     In the preferred embodiment, connections between the fluid cart  200  and the dispenser  101  are maintained while the dispenser is mounted on the shuttle  108  and traveling over the base station  300 . These connections should be set up so as to permit the dispenser to travel without any disturbance in the function of the various devices serviced by the connections and so as not to physically interfere with the operation of the shuttle  108  or any other component of the cart  200  or station  300 . A plurality of connections between the cart  200  and the dispenser  101  are preferably used including but not limited to fluid cable  206 , and control cable  207 . The fluid cable  206  carries fluid from the fluid storage and pumping means  201  on the cart to the dispenser  101 , as well as return lines for fluid from the pump on head  110  back to the fluid reservoir. 
     In a preferred embodiment, the control cable  207  exercises control over equipment located on the dispenser  101  and receives information from sensory devices located on the dispenser (not shown). This control over dispenser equipment may be accomplished by electrical, hydraulic or pneumatic means. The equipment on the dispenser to be controlled may include but is not limited to valves, motors, and an integrally mounted pump on head  110 . Sensory devices located on the dispenser  101  may include but are not limited to devices for measuring fluid flow velocity, fluid pressure, air or fluid temperature, coating thickness, or still other characteristics of fluid within the dispenser, as well as non-fluid characteristics such as dispenser height over a substrate to be coated, ambient air characteristics, mechanical vibration, and direct measurement of dispenser velocity. 
     In a preferred embodiment of the present invention, the cables leading from the fluid cart  200  to the dispenser or extrusion head  101  are physically arranged so that they do not interfere with any function of the cart  200  or base station  300 . In the preferred embodiment, space is provided for slack in cables  206  and  207  in troughs along the sides of the base station. Alternatively, the cables could be suspended above the workspace. 
     The cart  200  and base station  300  are preferably mechanically attached to permit proper operation of the coating apparatus as a whole. Accordingly, the preferred embodiment fluid cart  200  has attachment means  203  which mate with base station  300  attachment means  303 . In a preferred embodiment, means for interfacing control information are incorporated within attachment means  203  and  303 . Such means include but are not limited to electrical contacts, fiber optic contacts, and pneumatic or hydraulic connections. Means for accomplishing the mechanical attachment include but are not limited to clamps, rollers on beams which are forced against a rigid surface, clamps, and grippers which may be actuated by electrical, pneumatic, magnetic, electromagnetic, and hydraulic means. 
     Control interface cable  205  on the cart  200  is shown leading from attachment means  203  back to a junction box  208 . In a preferred embodiment, a junction box  208  establishes all needed electrical contact between the control interface cable  205  leading to electrical interface at the attachment means  203  and the various electrical control equipment, utilities or power supplies located on the fluid cart. Any pneumatic and hydraulic connections made between the cart  200  and the base station  300 , may also be integrated into the attachment means  203  and  303 . 
     Alternatively, the electrical and other utility connections between the cart  200  and base station  300  could be made separately from the mechanical attachment means  203  and  303 . In this alternative embodiment, cables leading to modular connectors for any or all of electrical, pneumatic or hydraulic connections from both the cart  200  and the base station  300  could be joined, either manually or automatically, once the cart  200  and base station  300  are in sufficient proximity to permit such connections. Preferably, the connection of electrical and other utility cables between the cart  200  and base station  300  are made first, and the cart  200  is then fully docked, or mechanically attached to the base station. Alternatively, the connection of modular utility cables between the cart  200  and base station  300  occurs after the cart and station have been attached mechanically. The utility and mechanical connections could also be made simultaneously. 
     In a preferred embodiment, control means  204  are mounted on the cart. This control means may comprise a connection to a host computer, a dedicated processor, personal computer, programmable logic controller, or other control device. Preferably, control means  204  on the cart  200  controls the primary pump located on the fluid cart, a pump on head, or integrally mounted head pump  110  if present on the dispenser. The control means can also control equipment within the utility station  102  if disposed on the fluid cart  200 , such as rotation of scrubbers and a priming roller, and the flow of fluid to a scrubbing and rinsing stations. In a preferred embodiment, the cart control means  204  receives information from sensory devices such as a height sensor. Additional possible sensory information which can be received by the control means  204  includes fluid characteristics such as pressure, temperature, and fluid flow velocity and dispenser mechanical characteristics such as position and velocity with respect to the substrate  107 . 
     In a preferred embodiment, control means  302  attached to or in communication with the base station  300  controls the position and velocity of the shuttle mechanism  108  or other transport system, in at least two linear dimensions, such as the coating direction and vertical motion. The base station control means  302  would preferably also control the general sequence of events of the coating apparatus  100 . More specifically, in a preferred embodiment, the base station control means  302  would typically instruct the shuttle  108  to move above various components of the utility station  102 , and when finished, to proceed to a starting point for the coating operation. When all is ready for the coating operation, the base station control means  302  would preferably move the shuttle at a controlled velocity over the substrate to be coated. When the coating operation is complete, the control means  302  can move the shuttle  108  to move clear of the substrate, in either direction, to permit external equipment to pick up the coated substrate. 
     As will be discussed in more detail later, in an alternative preferred embodiment, the functions of the cart control means  204  and base station control means  302  may be performed, or at least coordinated, by a single computer. A single computer could perform the coordination and still employ other processors to which the host computer delegates certain tasks such as, for example, height sensing and adjustment, or fluid flow control. 
     In a preferred embodiment, coordination between cart control means  204  and base station control means  302  is employed for a number of the functions of the coating apparatus  100 . For example, while the coating process is under way, there is a need to coordinate the velocity of the shuttle  108  carrying the dispenser  101  with the rate of deposition or extrusion of the coating fluid. Further, where height adjustment is employed, the height sensor preferably feeds height measurement information back through the control cable  207  to control means  204  which routes this information to base station control means  302  via control interface cable  205  and interface within the attachment means  203 . The recited list of examples is illustrative and not comprehensive. 
     In order to properly ascertain various process parameters, the cart  200  and base station  300  will preferably identify the other component of its “identity”. In this context, the identity of a cart  200  refers to the type of fluid and type of dispensing head it is carrying. Communication of such information as quantity of fluid stored on the cart, presence or absence of an integrated head pump  110 , and presence or absence of a height sensor on the dispenser could also be communicated. 
     In the reverse direction, a base station  300  would preferably communicate the length of travel of the shuttle. Other information communicated by the base station  300  would preferably describe the nature of the substrate to be coated, and the type and amount of coating material required for that substrate. The information exchanged between the two components of a full coating apparatus  100  would preferably ensure, before actual coating begins, that the two components are properly matched for the coating operation to be undertaken. 
     In an alternative preferred embodiment, a single centralized control system, preferably in the form of a personal computer, in communication with the various devices on both the cart  200  and the base station  300  would control the individual devices and perform any required coordination between two or more functions which must function in concert with one another for the coating operation to be successful. With this embodiment, the coordination between shuttle velocity and fluid flow rate would preferably be performed by having the computer or other centralized control means simultaneously communicate with the shuttle mechanism  108 , fluid delivery means  201  and  110 , and sensors indicating the velocity of the shuttle and the fluid, perform any required calculations, and control both the pumps and the shuttle so as to achieve a coating satisfying all process parameters, including but not limited to thickness, uniformity, and material purity. 
     In order to have access to all equipment to be controlled, and all sensory information within the coating apparatus  100 , the centralized computer would preferably be connected to the base station on a quasi-permanent basis, thereby having ongoing uninterrupted access to the equipment and sensors on the base station  300 . Specifically, contact between the computer and the base station would be independent of the connection status between the cart and station. With this arrangement, the computer would acquire the ability to communicate with, and control equipment on the fluid cart  200  only once a control cable connection or other interfacing mechanism is implemented between the fluid cart  200  and the base station  300 . 
     Having discussed embodiments in which control means are located on both the cart  200  and the base station  300 , and an embodiment in which a single centralized computer performs all the required monitoring and control, it is noted that the inventive mechanism is not limited to any particular number of control systems or to any set of possible distributions of computing activity between such control systems. The invention may be practiced with any number of control systems and with a wide range of possible distributions of control and sensory measurement receiving between them. A few examples have been discussed above purely for purposes of illustration and are not meant to limit the invention to the control configurations discussed therein. 
     The interchangeable fluid cart invention has been discussed in the context of a base station in which the relative motion between the dispenser  101  and the substrate  107  results from a shuttle mechanism  108  carrying a dispenser  101  over a mostly or completely stationary substrate. The invention is however, not limited to this “moving head” configuration. The invention may be practiced with a base station in which a chuck supporting a substrate moves with respect to a mostly or completely stationary dispenser. 
     FIG. 3 depicts a perspective view of a preferred embodiment of the base station of the present invention. The base station embodiment of FIG. 3 shows roller locking mechanism  306  for mechanical attachment to a fluid cart (not shown). 
     FIG. 4 depicts a view of the front of a fluid cart according to a preferred embodiment of the present invention. Preferred embodiment dispenser head mounts  202  are shown with no head present. A preferred embodiment utility station  102  is also shown. 
     FIG. 5 depicts a view of the rear of a fluid cart according to a preferred embodiment of the present invention. 
     FIG. 6 illustrates the elements and interconnections of the fluid supply bay  610  which is remotely located from the dispensing head but still within the cart, as well as the extrusion head module  620  which is integrally mounted to the dispensing head which may be an extrusion head. FIG. 6 illustrates the micro-dispenser or “pump-on-head” assembly  621 , wherein a pump is directly integrated with the extrusion head for the purposes described herein. Various forms of dispensers may be used in conjunction with pump on head concept, of which the extrusion head  800  is but one example. 
     Process fluid for deposition on a substrate comes from fluid supply bay  610 . The fluid supply bay  610  consists of a processed fluid reservoir  611 , feed pump  612 , and drain bottle  613 . Process fluid to be deposited by the extrusion head  800  is fed from the process fluid reservoir  611  to the feed pump  612  and is then filtered within a filter housing  614 . A feed pump useful in the present invention is illustrated by the pump shown in U.S. Pat. No. 5,167,837 to Snodgrass et al, entitled “FILTERING AND DISPENSING SYSTEM WITH INDEPENDENTLY ACTIVATED PUMPS IN SERIES”, which is hereby incorporated by reference, although other devices may be used as well. 
     The filtered process fluid is then pumped by the feed pump  612  to the pump-on-head assembly  621  of the extrusion head module  620  so that the fluid may be deposited on a substrate. Excess process fluid received by the feed pump  612  is returned to the reservoir  611 , with a small quantity of air and process fluid moving through vent  615 . 
     FIG. 7 depicts a partial cross-sectional view of the preferred embodiment pump on head apparatus that is integrally connected to the extrusion head. Fluid flow from the feed pump  612  (FIG. 6) passes through a three way recirculation valve  624  that routes the fluid flow to either the process fluid reservoir  611  (FIG. 6) in the fluid supply bay  610  (FIG. 6) through output  701  or to the micro-dispenser  621  through conduit  626 . The process fluid is driven through the micro-dispenser  621  by a pump drive means  702 . The pump drive means  702  comprises a drive motor (not shown) coupled through a transmission assembly  703  to a positively driven rod and seal arrangement  704 . The rod and seal arrangement  704  is hydraulically coupled to an internal drive diaphragm  622  (FIG. 6) within the micro-dispenser  621 . The drive motor actuates the drive rod  704  in precise and measurable movements to displace a desired amount of hydraulic fluid. The displaced hydraulic fluid drives the diaphragm  622  (FIG. 6) to displace an amount of process fluid through the micro dispenser  621  to extrusion head  800  or back to the fluid reservoir  611 . 
     Other pumping means could include centrifugal, reciprocating, peristaltic, pressure vessel with precisely regulated pressure and/or flow controls, piston, diaphragm (single, dual, continuous or single shot, and pneumatic or hydraulically activated), gravity feed, and progressive cavity. 
     The direction of process fluid flow depends on whether or not the extrusion head  800  is in an active or inactive mode and the settings of an isolation valve  625  and vent valve  623 . When the head is inactive, the isolation valve  625  closes and the vent valve  623  opens to direct flow of the process fluid back to the process fluid reservoir  611  of the fluid supply bay  610 . During active operation, the vent valve  623  closes and the isolation valve  625  opens to direct flow of process fluid out of the micro-dispenser  621  through outlet port  629 . 
     Referring back to FIG. 6, the neural network system, or other control system  601  preferably controls the steady-state fluid flow by monitoring the flow rate at points  626  and  630 . Point  626  will measure the flow rate into the pump-on-head assembly  621 . To ensure that the system has steady-state flow during the active and inactive periods, the neural network system  601  can control the openings of the recirculation valve  624 , the vent valve  623  and/or isolation valve  625  to further control fluid flow. The neural network system can also control the pumping rate in a very precise manner to effect the desired flow rate changes. It is noted here that control schemes other than a neural network can be used. 
     The micro-dispenser or pump-on-head assembly  621  may also be configured to function as a vacuum pump to withdraw process fluid from the extrusion head and cease providing the process fluid. Otherwise stated, the pump-on-head assembly can supply negative pressure to the extrusion head. This enables an extrusion to be stopped at a more precise point on the substrate than would otherwise be possible and permits fluid flow to be stopped more instantaneously than otherwise possible. In prior art embodiments, the process fluid continued to flow until the extrusion head was emptied or until capillary action halted fluid flow from the extrusion head manifold. An extrusion head vent valve  628  may also be used to vent extraneous process fluid and/or release excess pressure from the extrusion head and limit excess flow. The vented process fluid returns to the process fluid reservoir  611  within the fluid supply bay  610  through a conduit  629 . The extrusion vent valve  628  may also be controlled by the neural network to correct fluid flow anomalies that reach the extrusion head pump-on-head assembly. 
     FIG. 8 depicts an isometric view  800  of a docking station  801  and fluid carts  802  attachable thereto according to a preferred embodiment of the present invention. FIG. 8 shows the docking station  801 , with one cart  802  attached thereto, one other cart  802  not attached to the docking station, and one empty cart connection station  804  within the docking station  801 . On each cart  802  are connection, or “utility connections” for power, information and facilities (including pneumatic, electrical, possibly wireless communication connections)  803 . 
     When a fluid cart  802  is finished with a coating operation it is brought to the docking station  801  and attached to a cart connection station  804 . The fluid cart  802  is both mechanically put in place, and utility connections  803  connected appropriately to the docking station  801 . The utility connections permit the equipment on the fluid cart  802  to be properly powered and controlled while the cart  802  is serviced at the docking station  801 . Effectively, the docking station is able to provide all the power and control to the fluid cart  802  normally provided by a base station of the coating apparatus with some extra services added. 
     Services available to the fluid cart  802  while stationed at the docking station  801  include cleaning of the cart  802  in general, cleaning of the surfaces which contact specialized fluids such as coating material and solvents in particular, replenishment of coating material and solvents, and removal of used solvent and coating material which is either unsuitable for a succeeding coating operation using that cart  802 , or which has degraded over time or because of contamination. Accordingly, the utility connections  803  comprise means for connecting pneumatic, electrical, and hydraulic lines between the cart  802  and the docking station  801 . Wireless communication can also be implemented if desired. In particular, the utility connections  803  will comprise the ability to transmit specialized fluids such as solvents for use in the cleaning and priming assembly, and coating material, both fresh and used, between the cart  802  and the docking station. Used fluids, both solvent and coating material, upon removal from the cart, may optionally be treated, either at the docking station or at another location so as to recycle the used fluids and recover fluid reusable at a future time. 
     In an alternative preferred embodiment of the coating apparatus, the base station will comprise all the services and connection provisions described above in connection with the docking station for servicing the fluid cart, including removal of used fluids and replenishment of solvents and coating material. It is noted that use of the docking station will generally be more efficient as it permits a base station to continue coating operations with another cart attached. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.