Patent Publication Number: US-2010118081-A1

Title: Dead Volume Removal From An Extrusion Printhead

Description:
FIELD OF THE INVENTION 
     The present invention is related to fluid conduit devices, and more particularly to extrusion printheads for micro-extrusion systems. 
     BACKGROUND 
     In order to meet the demand for low cost large-area semiconductors, micro-extrusion methods have been developed that include extruding a paste including a dopant bearing material (dopant ink) along with a sacrificial material (non-doping ink) onto the surface of a semiconductor substrate, and then heating the semiconductor substrate such that a dopant (e.g., phosphorous or boron) disposed in the dopant ink diffuses into the substrate to form the desired doped region or regions. The co-extrusion process utilizes a co-extrusion printhead having to inlet ports for receiving the paste (i.e., dopant ink and non-doping ink), multiple outlet orifices (nozzle openings) that are arranged to co-extrude the paste in the desired manner, and a combination of plenums and flow channels defined in the extrusion printhead that channel the paste between the inlet ports and the nozzle openings. 
     In comparison to screen printing techniques, the co-extrusion of dopant and sacrificial materials on the substrate provides superior control of the feature resolution of the doped regions, and facilitates deposition without contacting the substrate, thereby avoiding wafer breakage. Such fabrication techniques are disclosed, for example, in U.S. Patent Application No. 20080138456, which is incorporated herein by reference in its entirety. 
     A problem with the co-extrusion process described above, and in general with fluid conduit devices (e.g., valves) used for similar or related purposes, is that paste-like materials can stagnate in corners and pockets (dead volumes) of the fluid conduit device (e.g., the printhead described above), making this stagnant material difficult to clean out. More importantly, if the stagnant material sits long enough, it can agglomerate into clog forming material. That is, if the co-extrusion printhead is used and then stored, the stagnant material can dry out, harden, and then form a sizable chunk of clog forming material trapped inside the printhead, thereby increasing manufacturing costs by requiring replacement of clogged printheads and discarding of flawed workpieces. Clogging is one of the most significant risks of extrusion printing technology relative to alternate methods such as screen printing. 
     What is needed is a method for modifying a micro-extrusion printhead (or other similar fluid conduit device) that avoids the clogging problem associated with conventional printheads. What is also needed is a printhead (or other similar fluid conduit device) modified by the novel method. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a novel method that, prior to use in “normal” operation, purposefully traps a hardenable material in the dead volume spaces of flow channels defined through a micro-extrusion printhead (or other fluid conduit device) while leaving the main channel regions open to conduct extrusion material, thereby avoiding the clogging problem associated with conventional printheads by reducing or eliminating regions where the extrusion materials can stagnate and dry out to form clogs. 
     In accordance with an embodiment of the present invention, a method for producing a fluid conduit device (e.g., a micro-extrusion printhead assembly) begins by fabricating a body defining an inlet port, an outlet orifice, and a flow channel communicating between the inlet port and the outlet orifice through the body. The body of the fluid conduit device is fabricated using one or more solid (first) materials, such as a metal or hard plastic, that remain in a solid form during subsequent processing and extrusion process. The present invention is particularly directed to fluid conduit devices in which the body is constructed such that the flow channel includes one or more direction changes (corners) that form dead volume spaces (i.e., regions of the flow channel in which extrusion material remains relatively stagnant during subsequent extrusion processes). In order to prevent stagnant extrusion material from forming clogs, the manufacturing process further includes minimizing/eliminating the dead volume spaces in the flow channel by filling the dead volume spaces with a hardenable (second) material that can be inserted into the flow channel in a liquid form, and then cured or otherwise hardened into a solid form. In accordance with an embodiment of the present invention, during a first phase of dead volume minimization, the hardenable material is introduced as a solid forming fluid (e.g., a liquid, paste, or gel) that fills the entire flow channel space (i.e., the main flow channel regions and the dead volume spaces) as it flows through the micro-extrusion printhead. During a second phase, while the hardenable material is still in the fluid form, a second (non-hardening) fluid is introduced into the printhead in a way that displaces the hardenable material from the main flow channels, but does not displace the hardenable material remaining in the dead volume spaces. In one embodiment the solid forming fluid and the displacement fluid are immiscible. The hardenable material disposed in the dead volume spaces is then solidified and the second fluid is removed from the main flow channels. The resulting micro-extrusion printhead includes the original solid materials (e.g., metal) that define the flow channels, and the solid hardenable material disposed in the dead volumes spaces of the flow channels in a way that does not impede the subsequent passage of a extrusion material (e.g., paste) through the main flow channel regions during “normal” extrusion printing. 
     According to an aspect of the present invention, the hardenable material is hardened (solidified) either while the second (displacing) fluid is still in the flow channel, or after the second fluid is removed from the flow channel. In accordance with one specific embodiment, the hardening process involves a chemical reaction between the hardenable (first) fluid and the displacing (second) fluid, such as the hardening of a two component epoxy. In accordance with another specific embodiment, the hardening process involves utilizing a thermoset material and elevating the temperature of the printhead. In yet another specific embodiment the hardening process includes, after the second phase is completed, subjecting the printhead to ionizing radiation that penetrates the printhead and activates a solidifying reaction in the hardenable material. 
     In a preferred embodiment, the hardenable material undergoes limited volume change during the hardening process. This goal can be achieved, for example, by using a hardenable material with a significant volume fraction (&gt;20%) of solid particles. 
     In accordance with a specific embodiment, the present invention is utilized to modify a micro-extrusion printhead assembly utilized in a micro-extrusion system that forms parallel extruded lines of functional material on a substrate surface. According to an aspect of the present invention, the micro-extrusion printhead includes a layered nozzle structure sandwiched between a first (back) plate structure and a second (front) plate structure. The layered nozzle structure is made up of stacked metal (or other rigid material) plates including a top nozzle plate, an optional bottom nozzle plate, and a nozzle outlet plate sandwiched between the top and bottom nozzle plates (or between the top nozzle plate and the second plate structure). The various plates and structures of the printhead are etched or otherwise formed to define openings that, when the plates are operably assembled to form the layered nozzle structure, combine to define flow channels extending between inlet ports formed on back plate structure and outlet orifices (nozzles) formed by layered nozzle structure. In a specific embodiment, each nozzle is formed by an elongated nozzle channel that is etched or otherwise formed in the nozzle outlet plate, and portions of the top and bottom nozzle plates that serve as upper and lower walls of the extrusion nozzle. According to the present invention, dead volume spaces (e.g., corner regions inside the flow channel and disposed at interfaces between adjacent plates) are filled with the solid forming fluid, and then processed as described above to form hardenable material that prevents the stagnation of. 
     According to another embodiment of the present invention, the associated micro-extrusion system includes a co-extrusion printhead assembly that is constructed to co-extrude two different materials in order to form closely spaced high-aspect ratio gridline structures on a substrate surface or narrow printed lines of dopant bearing paste, wherein the co-extrusion printhead assembly is modified to include the clog-preventing structures in the manner described above. Similar to the single material extrusion embodiments described above, the co-extrusion printhead assembly includes upper an lower plate structures that serve to guide the two extrusion materials via separate conduits from corresponding inlet ports to a layered nozzle structure, and a layered nozzle structure that is formed in accordance with the various specific embodiments described above to bias the extruded bead toward the target substrate. However, in the co-extrusion embodiment, the extruded bead includes a sacrificial material and a gridline (functional) material arranged such that the gridline material forms a high-aspect ratio gridline structure that is supported between two sacrificial material portions (the sacrificial portions are subsequently removed). The formation of such co-extruded bead structures requires the compression of the gridline material between the two sacrificial material portions, which is achieved by utilizing a three-part nozzle channel including a central channel and two side channels that converge with the central channel at a merge point located adjacent to the nozzle orifice (opening). The gridline material is transferred through the central nozzle channel by way of a first flow channel, and the sacrificial material is transferred through the two side nozzle channels by way of second and third flow channels such that the gridline material is compressed between the two sacrificial material portions at the merge point, and is forced through the nozzle orifice (opening) to form a high-aspect ratio gridline structure (bead) that is supported between the two sacrificial material portions. As with the single material extrusion printhead, the co-extrusion printhead is fabricated to include clog preventing portions located in dead volumes of the first flow channel feeding the central nozzle channel, and the second and third flow channels feeding the side nozzle channels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where: 
         FIG. 1  is a side view showing a portion of a micro-extrusion system including a micro-extrusion printhead assembly formed in accordance with an embodiment of the present invention; 
         FIG. 2  is a side view showing the micro-extrusion system of  FIG. 1  in additional detail; 
         FIG. 3  is an exploded cross-sectional side view showing a generalized micro-extrusion printhead assembly utilized in the system of  FIG. 1 ; 
         FIGS. 4(A) and 4(B)  are cross-sectional side views showing a portion of a micro-extrusion printhead assembly during normal operation; 
         FIGS. 5(A) and 5(B)  are cross-sectional side views depicting the formation of a clog by stagnant extrusion material in the printhead assembly portion of  FIG. 4(B) ; 
         FIGS. 6(A) ,  6 (B) and  6 (C) are simplified cross-sectional side view showing portions of a micro-extrusion printhead assembly during the formation of clog-preventing structures according to an embodiment of the present invention; 
         FIG. 7  is a simplified cross-sectional side view showing the printhead assembly portion of  FIG. 6(C)  during a subsequent extrusion process; 
         FIG. 8  is a front view showing a micro-extrusion system including a generalized co-extrusion printhead assembly according to another embodiment of the present invention; 
         FIG. 9  is an exploded perspective view showing the co-extrusion printhead assembly of  FIG. 8  in additional detail; 
         FIG. 10  is an exploded partial perspective view showing a portion of the printhead assembly of  FIG. 9  in additional detail; 
         FIG. 11  is a simplified exploded partial perspective view showing a portion of a generalized layered nozzle structure utilized in the co-extrusion printhead assembly of  FIG. 8 ; and 
         FIG. 12  is a cross-sectional side view showing an exemplary co-extruded gridline structure generated on a substrate surface by the co-extrusion printhead assembly of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is described below with specific references to an improvement in micro-extrusion systems, but is applicable to any similar fluid conduit device. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “upper”, “top”, “lower”, “bottom”, “front”, “rear”, and “lateral” are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. In addition, the phrases “integrally connected” and “integrally molded” is used herein to describe the connective relationship between two portions of a single molded or machined structure, and are distinguished from the terms “connected” or “coupled” (without the modifier “integrally”), which indicates two separate structures that are joined by way of, for example, adhesive, fastener, clip, or movable joint. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. 
       FIG. 1  is a simplified side view showing a portion of a micro-extrusion system  50  for forming parallel extruded material lines  55  on upper surface  52  of a substrate  51 . Micro-extrusion system  50  includes an extrusion printhead assembly (fluid conduit device)  100  that is operably coupled to a material feed system  60  by way of at least one feedpipe  68  and an associated fastener  69 . 
     The materials are supplied in a paste through pushing and/or drawing techniques (e.g., hot and cold) in which the materials are pushed (e.g., squeezed, etc.) and/or drawn (e.g., via a vacuum, etc.) through flow channels  115  and  125 - x  formed in extrusion printhead assembly  100 , and out one or more outlet orifices (exit ports, or nozzle outlets)  169  that are respectively defined in a lower portion of printhead assembly  100 . Micro-extrusion system  50  also includes a X-Y-Z-axis positioning mechanism  70  including a mounting plate  76  for rigidly supporting and positioning printhead assembly  100  relative to substrate  51 , and a base  80  including a platform  82  for supporting substrate  51  in a stationary position as printhead assembly  100  is moved in a predetermined (e.g., Y-axis) direction over substrate  51 . In alternative embodiment (not shown), printhead assembly  100  is stationary and base  80  includes an X-Y axis positioning mechanism for moving substrate  51  under printhead assembly  100 . 
       FIG. 2  shows material feed system  60 , X-Y-Z-axis positioning mechanism  70  and base  80  of micro-extrusion system  50  in additional detail. The assembly shown in  FIG. 2  represents an experimental arrangement utilized to produce solar cells on a small scale, and those skilled in the art will recognize that other arrangements would typically be used to produce solar cells on a larger scale. Referring to the upper right portion of  FIG. 2 , material feed system  60  includes a housing  62  that supports a pneumatic cylinder  64 , which is operably coupled to a cartridge  66  such that material is forced from cartridge  66  through feedpipe  68  into printhead assembly  100 . Referring to the left side of  FIG. 2 , X-Y-Z-axis positioning mechanism  70  includes a Z-axis stage  72  that is movable in the Z-axis (vertical) direction relative to target substrate  51  by way of a housing/actuator  74  using known techniques. Mounting plate  76  is rigidly connected to a lower end of Z-axis stage  72  and supports printhead assembly  100 , and a mounting frame  78  is rigidly connected to and extends upward from Z-axis stage  72  and supports pneumatic cylinder  64  and cartridge  66 . Referring to the lower portion of  FIG. 2 , base  80  includes supporting platform  82 , which supports target substrate  51  as an X-Y mechanism moves printhead assembly  100  in the X-axis and Y-axis directions (as well as a couple of rotational axes) over the upper surface of substrate  51  utilizing known techniques. 
     As shown in  FIG. 1  and in exploded form in  FIG. 3 , layered micro-extrusion printhead assembly  100  includes a first (back) plate structure  110 , a second (front) plate structure  130 , and a layered nozzle structure  150  connected therebetween. As described in additional detail below, back plate structure  110  and front plate structure  130  are solid (e.g., metal) structures, and at least one of back plate structure  110  and front plate structure  130  defines one or more hollow regions that form one or more flow channels to conduct the extrusion material from an inlet port  116  to layered nozzle structure  150 . Back plate structure  110  and front plate structure  130  also serve to rigidly support layered nozzle structure  150  such that extrusion nozzles  163  defined in layered nozzle structure  150  are pointed toward substrate  51  at a predetermined tilted angle θ 1  (e.g., 45° ), whereby extruded material traveling down each extrusion nozzle  163  toward its corresponding nozzle orifice  169  is directed toward target substrate  51 . 
     Each of back plate structure  110  and front plate structure  130  includes one or more integrally molded or machined metal (or other rigid material) parts. In the disclosed embodiment, back plate structure  110  includes an angled back plate  111  and a back plenum  120 , and front plate structure  130  includes a single-piece metal plate. Angled back plate  111  includes a front surface  112 , a side surface  113 , and a back surface  114 , with front surface  112  and back surface  114  forming a predetermined angle θ 2  (e.g., 45°; shown in  FIG. 1 ). Angled back plate  111  also defines a (first) flow channel portion  115  that extends from a threaded countersunk inlet port  116  defined in side wall  113  to a (first) elbow (i.e., a bend)  118  located adjacent to a bore outlet defined in back surface  114 . Back plenum  120  defines a (second) flow channel portion  125 - 1  that is aligned to communicate with the bore outlet located adjacent to elbow  118 , a (third) flow channel portion  125 - 2  that communicates with flow channel portion  125 - 1  by way of a neck region  127 , and a (fourth) flow channel region  125 - 3  that communicates with flow channel  125 - 2  by way of a (second) elbow  128 , and an outlet that feeds into extrusion nozzles  163  in the manner described below. As illustrated, flow channel portions  115 ,  125 - 1 ,  125 - 2  and  125 - 3  cooperates to feed extrusion material from inlet port  116  to layered nozzle structure  150  during an extrusion process. 
     Referring in to the lower portion of  FIG. 1 , front plate structure  130  includes a front surface  132  and a beveled lower surface  134  that form predetermined angle θ 2  (shown in  FIG. 1 ). 
     Layered nozzle structure  150  is disposed between back plate structure  110  and front plate structure  130 , and includes two or more stacked plates (e.g., a metal such as aluminum, steel or plastic) that combine to form one or more extrusion nozzles  163 . In the simplified embodiment shown in  FIG. 3 , layered nozzle structure  150  includes a top nozzle plate  153 , a bottom nozzle plate  156 , and a nozzle outlet plate  160  sandwiched between top nozzle plate  153  and bottom nozzle plate  156 . Top nozzle plate  153  defines an inlet port (through hole)  155 , and has a (first) front edge  158 - 1 . Bottom nozzle plate  156  is a substantially solid (i.e., continuous) plate having a (third) front edge  158 - 2 . Nozzle outlet plate  160  includes a (second) front edge  168  and defines an elongated nozzle channel  162  extending in a predetermined first flow direction F 1  from a closed end  165  to an nozzle orifice  169  defined through front edge  168 . When operably assembled (e.g., as shown in  FIG. 1 ), nozzle outlet plate  160  is sandwiched between top nozzle plate  153  and bottom nozzle plate  156  such that elongated nozzle channel  162 , a front portion  154  of top nozzle plate  153 , and a front portion  157  of bottom nozzle plate  156  combine to define elongated extrusion nozzle  163  that extends from closed end  165  to nozzle orifice  169 . In addition, top nozzle plate  153  is mounted on nozzle outlet plate  160  such that inlet port  155  is aligned with closed end  165  of elongated channel  162 , whereby extrusion material forced through inlet port  155  flows in direction F 1  along extrusion nozzle  163 , and exits from layered nozzle structure  150  by way of nozzle orifice  169  to form bead  55  on substrate  51 . 
     Referring again to  FIG. 1 , when operably assembled and mounted onto micro-extrusion system  50 , angled back plate  111  of printhead assembly  100  is rigidly connected to mounting plate  76  by way of one or more fasteners (e.g., machine screws)  142  such that beveled surface  134  of front plate structure  130  is positioned close to parallel to upper surface  52  of target substrate  51 . One or more second fasteners  144  are utilized to connect front plate structure  130  to back plate structure  110  with layered nozzle structure  150  pressed between the back surface of front plate structure  130  and the back surface of back plenum  120 . In addition, material feed system  60  is operably coupled to flow channel  115  by way of feedpipe  68  and fastener  69  using known techniques, and extrusion material forced into flow channel  115  is redirected by elbow  118  to layered nozzle structure  150  by way of flow channel portion  125 - 1 , neck  127 , flow channel portion  125 - 2 , elbow  128 , and flow channel  125 - 3 . The extrusion material exiting flow channel portion  125 - 3  enters the closed end of nozzle  163  by way of a third elbow formed at inlet  155  by closed end  165  (both shown in  FIG. 3 ), and flows in direction F 1  down nozzle  163  toward outlet  169 . Referring to  FIG. 3 , the extrusion material flowing in the nozzle (i.e., traveling in direction F 1  along channel  162 ) flows in (or parallel to) a lateral extrusion plane E defined by the nozzle outlet plate  160 , and is directed through the outlet orifice (printhead nozzle)  169 . Referring back to  FIG. 1 , because the extruded material is guided along the extrusion plane E at the tilted angle θ 2  as it exits nozzle orifice  169 , layered micro-extrusion printhead  100  reliably directs the extruded material toward substrate  51  in a manner that facilitates high volume solar cell production. 
     According to the present invention, as shown in  FIG. 1 , clog-preventing structures  170  are formed in dead volume spaces communicating with flow channels  115  and  125 - 1  to  125 - 3  of printhead assembly  100  in order to prevent extrusion material from stagnating in these regions and forming clogs. In particular, the body formed by front plate structure  110 , back plate structure  130  and layered nozzle structure  150  is composed substantially of one or more solid (first) materials (e.g., stainless steel), and clog-preventing structures  170  composed of a hardenable (second) material are permanently attached to the inside wall of the body in elbow regions  118  and  128 , and in neck region  127  at locations (i.e., dead volume spaces) where flow of extrusion material could otherwise stagnate and harden. The hardenable (second material) may be for example wax, thermoplastic, thermoset plastic, epoxy of various types including two-component acrylic or urethane epoxy. An example epoxy that may be used is Loctite Hysol™ E-60HP epoxy adhesive which has a 60 minute work life and high peel and high shear strength. By forming clog-preventing structures  170  in these dead volume spaces using the novel method described below, printhead assembly  100  reliably extrudes material during the production of solar cells without interruption due to clogs, thereby reducing manufacturing costs. 
     For optimal flow properties during the displacement of the hardenable material by the non-hardenable material, the latter should have similar Theological properties, notably the viscosity at the shear levels induced during displacement. For example, a suitable displacement fluid can be prepared by dissolving a viscosifier such as cellulose ether, for example Methocel K100M available from the Dow Chemical Corporation, in distilled water. 
     According to a preferred embodiment at least one of the nozzle structure materials, the output geometry, and the internal conduit geometry of printhead assembly  100  are modified to cause the extrusion material (bead) traveling through extrusion nozzle  163  (i.e., in or parallel to the lateral extrusion plane E) to be reliably directed (angled) toward the target substrate as it leaves the printhead nozzle orifice. Printhead assembly  100  that include the desired modifications are described in additional detail in co-owned and co-pending U.S. patent application Ser. No. ______ entitled “DIRECTIONAL EXTRUDED BEAD CONTROL”, filed with the present application, which is incorporated herein by reference in its entirety. In an alternative embodiment, a structure is provided to direct airflow against the extruded bead to achieve the desired bias against the substrate, as set forth in co-owned and co-pending U.S. patent application Ser. No. ______ entitled “MICRO-EXTRUSION SYSTEM WITH BEAD DEFLECTING MECHANISM”, filed with the present application, which is incorporated herein by reference in its entirety. 
       FIGS. 4(A) to 5(B)  describe the clogging problem addressed by the present invention, and  FIGS. 6(A) to 6(C)  describe the novel method for preventing the clogging problem with reference to a micro-extrusion printhead assembly similar to that utilized in the described above with reference to  FIGS. 1-3 . Each of these figures illustrates a portion of a printhead assembly  100 A including an interface between a top plate structure  110 A and a layered nozzle structure  150 A that are essentially identical to those described above. For example,  FIG. 4(A)  shows a partial cross-sectional view of printhead assembly portion  100 A prior to the formation of clog-preventing structures according to the present invention, where printhead assembly portion  100 A includes a layered nozzle structure  150 A sandwiched between a back plenum  120  and a front plate structure  130 A, which are similar to those described above with reference to  FIG. 3 . Back plenum  120  defines a flow channel portion  125 - 3  that communicates with an elbow (closed end)  165 A of a nozzle  163 A by way of a neck region  155 A defined at the interface between back plenum  120  and top nozzle plate  153 A. In combination with side walls formed by nozzle outlet plate  160 A (not shown), portion  154 A of top nozzle plate  153 A and portion  157 A of bottom nozzle plate  156 A form the upper and lower walls, respectively, of nozzle  163 A, whereby extrusion material entering nozzle  163 A from conduit  125  generally flows along the dashed line F 1  to outlet orifice  169 A. Note that front edge  168 A of nozzle outlet plate  160 A, front edge  158 - 1 A of top nozzle plate  153 A, and front edge  158 - 2 A of bottom nozzle plate  156 A are coplanar with front edge  128  of back plenum  120 , forming a front edge of printhead assembly  100 A. In one exemplary embodiment, a thickness of top nozzle plate  153 A is 300 microns, bottom nozzle plate  156 A is 25 microns thick, nozzle outlet plate  160 A is 50 microns thick, a width of nozzle orifice  169 A is 200 microns, and a length of nozzle  163 A is 2000 microns. 
       FIG. 4(B)  is a simplified cross-sectional side view showing the portion of  FIG. 4(A)  during operation of printhead assembly  100 A. Referring to  FIG. 4(B) , extrusion material  180  inserted into printhead assembly  100 A by way of an inlet port (e.g., see inlet port  116  of  FIG. 1 ) is forced from flow channel portion  125 - 3  through neck region  155 A to elbow  165 A of nozzle  163 A, where it turns and passes between upper/lower wall portions  154 A and  157 A to outlet orifice  168 A, at which point the extrusion material is extruded onto upper surface  52  of substrate  51  in the form of a bead  55 . In particular, extrusion material passing through top plate structure  110 A is eventually conducted to flow channel portion  125 - 3 , at which point the extrusion material encounters resistance due to the narrowing of the flow channel at neck region  155 A. Those skilled in the art will recognize that portions of extrusion material located adjacent the center of the flow channel (e.g., adjacent to dashed line arrow F 1 ) tends to be pushed by fluid pressure through neck  155 A and into nozzle  163 A, while portions of extrusion material located near the wall surface outside neck region  155 A (e.g., annular corner region  129  at the interface between back plenum  120  and top nozzle plate  153 A, and angled corner region  166  formed at the inter face of nozzle outlet plate  160 A and bottom nozzle plate  156 A) tends to stagnate. As a result, as shown in  FIG. 5(A) , stagnant extrusion material portions  180 - 1  and  180 - 2  remain in corner regions  129  and  166  after the extrusion process is completed, and often remains even after a rinsing fluid is passed through flow channel portion  125 - 3  and nozzle  163 A (i.e., through neck  155 A and elbow  165 A). During periods between uses, extrusion material portions  180 - 1  and  180 - 2  dry out and form solid pieces. As illustrated in  FIG. 5(B) , extrusion material portions  180 - 1  and  180 - 2  may become displaced and/or combine to lodge in neck  155 A or elbow  165 A, forming a clog (blockage) that reduces or prevents the flow of extrusion material  180  between flow channel portion  125 - 3  and nozzle  163 A during a subsequent extrusion operation, leading to the problems described above. 
       FIGS. 6(A) to 6(C)  are cross-sectional side views showing a method for modifying printhead assembly  100 A according to an embodiment of the present invention. First, as indicated in  FIG. 6(A) , a solid forming (first) fluid  170 A is caused to flow through the flow channel of printhead assembly  100 A (e.g., such that fluid  170 A fills flow channel portion  125 - 3  and nozzle  163 A, including all dead volume spaces located adjacent to neck  155 A and elbow  165 A). As described above, this process involves feeding the solid forming fluid into an inlet port (e.g., inlet port  116 ; see  FIG. 1 ) of printhead assembly  100 A. In one embodiment, solid forming fluid  170 A includes a hardenable material (i.e., fluid  170 A forms a solid structure when subject to a curing or other hardening process). Next, as indicated in  FIG. 6(B) , a second (displacing) fluid  175  is caused to pass between the inlet port and the exit port (i.e., through flow channel portion  125 - 3  and nozzle  163 A) such that the solid forming fluid is displaced from the flow channel by the displacing liquid except for portions  170 A- 1  and  170 A- 2  of the solid forming fluid remaining in corner regions  129  and  165 A of the flow channel. In accordance with a preferred embodiment, displacing fluid  175  and hardenable fluid  170 A are immiscible. Finally, as indicated in  FIG. 6(C) , the remaining portions of the solid forming fluid in the dead volume regions are solidified utilizing a selected hardening process (indicated by wavy arrow H) to form clog preventing structures  170 - 1  and  170 - 2  that are permanently attached to the inside walls of the flow channel. In one embodiment the solidifying process is performed while the displacing fluid is still inside the flow channel for example by using a two component epoxy that cures in the presence of the displacing fluid with a cure time longer than the time needed to displace the epoxy. In another example the solid forming material is a thermoplastic that is displaced at a temperature above its melt point and then cooled. Exemplary thermoplastics include acrylic, polyacetal, polyethylene terephthalate, polycarbonate, polyetheretherketone, polypropylene, polystyrene and polyvinyl chloride. In another embodiment, the solidifying process is performed after the displacing fluid is drained from the flow channel. In one embodiment, the solid forming fluid includes a thermoset material (e.g., epoxy or polyimide), and the solidifying process is performed by elevating the temperature of the printhead assembly. In another alternative embodiment, the solidifying process is performed by subjecting the printhead to ionizing radiation that activates a solidifying reaction in the hardenable material (e.g., diglycidyl ether of bisphenol F epoxy resin) disposed in the solid forming fluid. 
       FIG. 7  shows a printhead assembly portion  100 B after the solidifying process of the present invention is completed to form a printhead assembly portion  100 B, and an extrusion material  180  is passed through printhead assembly portion  100 B in the manner described above to form beads  55  on upper surface  52  of a target substrate  51 . Top plate structure  110 A (including plenum  120 ), bottom plate structure  130 A, and layered nozzle structure  150 A of printhead assembly portion  100 B form a body composed substantially of a first material (e.g., stainless steel) that defines a hollow flow channel (i.e., including flow channel portion  125 - 3  and nozzle  163 A) that communicates between an inlet (not shown) and outlet orifices (e.g., nozzle outlet orifice  169 ). According to the present invention, printhead assembly portion  100 B includes a first clog-preventing structure  170 - 1  composed of a hardenable material (e.g., epoxy) that is permanently attached to the printhead material in neck region  129 , and a second clog-preventing structure  170 - 2  composed of the hardenable material that is permanently attached to the printhead material in elbow region  165 A. With clog-preventing structure  170 - 1  and  170 - 2  thus disposed, substantially all of extrusion material  180  is easily removed from the flow channel at the end of the extrusion process, thereby preventing the clogging problem described above. 
       FIGS. 8-12  illustrate a co-extrusion system  50 E according to another embodiment of the present invention. Co-extrusion system  50 E includes a Z-axis positioning mechanism and X-Y axis positioning mechanism that are constructed and function in a manner similar to that described above with reference to  FIGS. 1 and 2 . As set forth in the following paragraphs, co-extrusion system  50 E differs from the above-described embodiments in that it includes material feed system  60 E having means for supplying two extrusion materials to a printhead assembly  100 E, and printhead assembly  100 E includes means for co-extruding the two extrusion materials in a manner that generates parallel high-aspect ratio gridline structures (described below with reference to  FIG. 12 ). As set forth in the description below, co-extrusion printheads are multi-layered devices typically including eleven or more layers of individually machined material that is stack bonded to form the completed printhead assembly. Although the channels within one layer can often be designed to have a swept structure to minimize dead volume, other dead volume is unavoidable, particularly at the coupling between channels in adjacent layers. As described in additional detail below, the present invention involves disposing clog-preventing structures similar to those described above into these dead volume spaces in order to facilitate reliable co-extrusion processing. 
     Referring to  FIG. 8 , material feed system  60 E represents exemplary experimental arrangement utilized to produce solar cells on a small scale, and those skilled in the art will recognize that other arrangements would typically be used to produce solar cells on a larger scale. Referring to the upper portion of  FIG. 8 , material feed system  60 E includes a pair of housings  62 - 1  and  62 - 2  that respectively support pneumatic cylinders  64 - 1  and  64 - 2 , which is operably coupled to cartridges  66 - 1  and  66 - 2  such that material forced from these cartridges respectively passes through feedpipes  68 - 1  and  68 - 2  into printhead assembly  100 E. As indicated in the lower portion of  FIG. 8 , the Z-axis positioning mechanism (partially shown) includes a Z-axis stage  72 E that is movable in the Z-axis (vertical) direction by way of a housing/actuator  74 E (partially shown) using known techniques. Mounting plate  76 E is rigidly connected to a lower end of Z-axis stage  72 E and supports printhead assembly  100 E, and a mounting frame (not shown) is rigidly connected to and extends upward from Z-axis stage  72 E and supports pneumatic cylinders  64 - 1  and  64 - 2  and cartridges  66 - 1  and  66 - 2 . 
       FIG. 9  is an exploded perspective view showing micro-extrusion printhead  100 E in additional detail. Micro-extrusion printhead  100 E includes a first (back) plate structure  110 E, a second (front) plate structure  130 E, and a layered nozzle structure  150 E connected therebetween. 
     Back plate structure  110 E and front plate structure  130 E serve to guide the extrusion material from corresponding inlet ports  116 - 1  and  116 - 2  to layered nozzle structure  150 E, and to rigidly support layered nozzle structure  150 E such that extrusion nozzles  162 E defined in layered nozzle structure  150 E are pointed toward substrate  51  at a predetermined tilted angle (e.g., 45°), whereby extruded material traveling down each extrusion nozzle  162 E toward its corresponding nozzle orifice  169 E is directed toward target substrate  51 . 
     Referring to the upper portion of  FIG. 9 , back plate structure  110 E includes a molded or machined metal (e.g., aluminum) angled back plate  111 E, a back plenum  120 E, and a back gasket  121  disposed therebetween. Angled back plate  111 E includes a front surface  112 E, a side surface  113 E, and a back surface  114 E, with front surface  112 E and back surface  114 E forming predetermined angle θ 2  (e.g., 45°). Angled back plate  111 E also defines a pair of bores (not shown) that respectively extend from threaded countersunk bore inlets  116 - 1  and  116 - 2  defined in side wall  113 E to corresponding bore outlets defined in back surface  114 E. Back plenum  120 E includes parallel front surface  122 E and back surface  124 E, and defines a pair of conduits (not shown) extending from corresponding inlets  126 - 1  and  126 - 2  defined through front surface  122  to corresponding outlets (not shown) defined in back surface  124 E. Similar to the description provided above, the bores/conduits defined through back plate structure  110 E feed extrusion material to layered nozzle structure  150 E. 
     Referring to the lower portion of  FIG. 9 , front plate structure  130 E includes a molded or machined metal (e.g., aluminum) front plate  131 E, a front plenum  140 E, and a front gasket  141  disposed therebetween. Front plate  131 E includes a front surface  132 E, a side surface  133 E, and a beveled back surface  134 E, with front surface  132 E and back surface  134 E forming the predetermined angle described above. Front plate  131 E defines several holes for attaching to other sections of printhead assembly  100 E, but does not channel extrusion material. Front plenum  140 E includes parallel front surface  142 E and back surface  144 E, and defines a conduit (not shown) extending from corresponding inlet  148  to a corresponding outlet  149 , both being defined through front surface  142 E. As described below, the conduit defined in front plenum  140 E serves to feed one of the extrusion materials to layered nozzle structure  150 E. 
     Similar to the single material embodiment, described above, layered nozzle structure  150 E includes a top nozzle plate  153 E, a bottom nozzle plate  156 E, and a nozzle outlet plate  160 E sandwiched between top nozzle plate  153 E and bottom nozzle plate  156 E. As described in additional detail below, top nozzle plate  153 E defines a row of substantially circular inlet ports (through holes)  155 - 1 E and a corresponding series of elongated inlet ports  155 - 2 E that are aligned adjacent to a (first) front edge  158 - 1 E. Bottom nozzle plate  156 E is a substantially solid (i.e., continuous) plate having a (third) front edge  158 - 2 E, and defines several through holes  159 - 6 E, whose purpose is described below. Nozzle outlet plate  160 E includes a (second) front edge  168 E, and defines a row of three-part nozzle channels  162 E that are described in additional detail below, and several through holes  159 - 7 E that are aligned with through holes  159 - 6 E. When operably assembled, nozzle outlet plate  160 E is sandwiched between top nozzle plate  153 E and bottom nozzle plate  156 E to form a series of nozzles in which each three-part nozzle channel  162 E is enclosed by corresponding portions of top nozzle plate  153 E and bottom nozzle plate  156 E in the manner described above, with each part of three-part nozzle channel  162 E aligned to receive material from two inlet ports  155 - 1 E and one elongated inlet port  155 - 2 E. As described in additional detail below, this arrangement produces parallel high-aspect ratio gridline structures (beads) in which a gridline material is pressed between two sacrificial material sections. 
     In addition to top nozzle plate  153 E, bottom nozzle plate  156 E and nozzle outlet plate  160 E, layered nozzle structure  150 E also includes a first feed layer plate  151  and a second feed layer plate  152  that are stacked over top nozzle plate  153 E and served to facilitate the transfer of the two extrusion materials to nozzle outlet plate  160 E in the desired manner described below. First feed layer plate  151  is a substantially solid (i.e., continuous) plate having a (fourth) front edge  158 - 4 E, and defines several Y-shaped through holes  155 - 3 E located adjacent to front edge  158 - 4 E, and several feed holes  159 - 1 E whose purposes are described below. Second feed layer plate  152  is disposed immediately below first feel layer plate  151 , includes a (fifth) front edge  158 - 5 E, and defines several substantially circular through holes  155 - 4 E located adjacent to front edge  158 - 5 E, and several feed holes  159 - 2 E whose purposes are described below. 
     As indicated by the dashed arrows in  FIG. 9  and described in additional detail in  FIGS. 10 and 11 , two extrusion materials are fed by way of two separate paths in a substantially Z-axis direction through the various layers of layered nozzle structure  150 E to nozzle outlet plate  160 E. The two flow paths are described in detail in the following paragraphs. 
     Referring to the upper portion of  FIG. 9 , gridline material  56  injected through inlet port  116 - 1  is fed downward through opening  121 - 1  in back gasket  121  and into opening  126 - 1  defined in back plenum  120 E. The gridline material then exits back plenum  120 E and passes through aligned openings  159 - 1 E to  159 - 5 E respectively formed in first feed layer plate  151 , second feed layer plate  152 , top nozzle plate  153 E, nozzle outlet plate  160 E, and bottom nozzle plate  156 E before entering opening  149 - 1  of front plenum  140 E. As indicated in  FIG. 9  and in additional detail in  FIG. 10 , the gridline material is then redirected by front plenum  140 E and moves upward from opening  149 - 2  through opening  159 - 6 E formed in bottom nozzle plate  156 E and opening  159 - 7 E formed in nozzle outlet plate  160 E. As indicated in the upper portion of  FIG. 10  and in  FIG. 11 , the gridline material then enters the rearward end of elongated openings  159 - 7 E, and is redirected in a substantially horizontal direction along arrow F 1 A to the front end of elongated opening  159 - 7 E. The gridline material is then forced downward into a central channel  167  of three-part nozzle channel  162 E. As described in additional detail below, the gridline material then flows along central channel  167 E in the direction of arrow F 1 , and is compressed between corresponding sacrificial material portions before exiting from orifice  169 E. 
     Referring again to the upper portion of  FIG. 9 , sacrificial material  57  injected through inlet port  116 - 2  is fed downward through opening  121 - 2  in back gasket  121  and into opening  126 - 2  defined in back plenum  120 E. The sacrificial material is dispersed by plenum  120 E and is passed into the rearward end of Y-shaped elongated channels  155 - 3 E, which are formed in first feed layer plate  151 . As indicated by dashed arrows in  FIGS. 9 and 11 , the sacrificial material flows along each Y-shaped elongated channel  155 - 3 E to a split front end region, where the sacrificial material is distributed through corresponding openings  155 - 4 E disposed in second feed layer plate  152  and openings  155 - 1 E disposed in top nozzle plate  153 E, and then into opposing side channel  165 E of three-part nozzle channel  162 E. As described in additional detail below, the sacrificial material then flows along side channels  165 E, and presses against the corresponding gridline material before exiting from orifice  169 E. 
     Referring to  FIG. 11 , nozzle output plate  160 E includes a plate that is micro-machined (e.g., using deep reactive ion etching) to include arrowhead-shaped three-part nozzle channel  162 E including a central channel  167 E and opposing (first and second) side channels  165 E. Central channel  167 E is separated from each side channel  165 E by an associated tapered finger of plate material. Central channel  167 E has a closed end that is aligned to receive gridline material from the front end of elongated opening  159 - 7 E of top nozzle plate  153 E, and an open end that communicates with a merge point  166 E. Similarly, side channels  165 E have associated closed ends that are aligned to receive sacrificial material from corresponding openings  155 - 1 E of top nozzle plate  153 E, and open ends that communicate with a merge point  166 E. Side channels  165 E are angled toward central channel  167 E such that sacrificial material is fed against opposing sides of the gridline material flowing in central channel  167 E. 
     As shown in  FIG. 12 , the gridline material and sacrificial material co-extruded through each nozzle outlet orifice  169 E (see  FIG. 11 ) of co-extrusion printhead assembly  100 E during the extrusion process forms an elongated extruded structure  55 E on surface  52  of substrate  51  such that the gridline material of each structure  55 E forms a high-aspect ratio gridline structure  56 , and such that the sacrificial material of each structure  55 E forms associated first and second sacrificial material portions  57 - 1  and  57 - 2  respectively disposed on opposing sides of the associated high-aspect ratio gridline  56 . The shape of extruded structures  55 E (i.e., the aspect ratio of gridline material  56  and the shape of sacrificial portions  57 - 1  and  57 - 2 ) are controllable through at least one of the shapes of the one or more outlet orifices and internal geometry of printhead assembly  100 E, characteristics of the materials (e.g., viscosity, etc.), and the extrusion technique (e.g., flow rate, pressure, temperature, etc.). As set forth in the specific embodiment described below, the structure within the printhead assembly and the shape of the nozzle outlet orifices may be modified to further enhance the extrusion process. Suitable gridline materials  56  include, but are not limited to, silver, copper, nickel, tin, aluminum, steel, alumina, silicates, glasses, carbon black, polymers and waxes, and suitable sacrificial materials  112  include plastic, ceramic, oil, cellulose, latex, polymethylmethacrylate etc., combinations thereof, and/or variations thereof, including combining the above with other substances to obtain a desired density, viscosity, texture, color, etc. To limit the tendency for the materials to intermix after extrusion, extruded beads leaving co-extrusion printhead  100 E can be quenched on substrate  51  by cooling the substrate using known techniques. Alternately, the gridline (ink) material used may be a hot-melt material, which solidifies at ambient temperatures, in which case co-extrusion printhead  100 E is heated, leaving the extruded structures to solidify once they are dispensed onto substrate  51 . In another technique, the materials can be cured by thermal, optical and/or other means upon exit from co-extrusion printhead  100 E. For example, a curing component can be provided to thermally and/or optically cure the materials. If one or both materials include an ultraviolet curing agent, the material can be bound up into solid form in order to enable further processing without mixing. 
     Techniques for fabricating the various printheads described above are described, for example, in co-owned and co-pending U.S. patent application Ser. No. 11/555,512, entitled “EXTRUSION HEAD WITH PLANARIZED EDGE SURFACE”, which is incorporated herein by reference in its entirety. Alternatively, the laminated metal layer arrangements described herein, the extrusion printheads of the present invention can be manufactured by electroplating metal up through features in a patterned resist structure, by brazing together layers of etched plate metal, by generating structures out of photo-definable polymer such as SU8, or by machining or molding. 
     As set forth above, co-extrusion printhead  100 E includes eleven layers of individually machined material that is stack bonded to form the completed printhead assembly, and dead volume spaces are unavoidable, particularly at the coupling between channels in adjacent layers. Accordingly, as shown in simplified form in  FIG. 11 , according to another embodiment of the present invention clog-preventing structures  170  are formed at each coupling between channels in adjacent layers. 
     Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, although described with specific reference to micro-extrusion printheads, the present invention may be utilized in other fluid conduit devices as well, such as valves.