Patent Publication Number: US-6702419-B2

Title: System and method for delivering droplets

Description:
FIELD OF THE INVENTION 
     The invention relates generally to a system and method for delivering droplets on a substrate. More particularly, the invention relates to aligning nozzles in ink-jet print heads with respect to target points on a substrate. 
     BACKGROUND OF THE INVENTION 
     One conventional type of printer forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land on the medium. Such a printer can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate control hardware and software. A conventional print cartridge for an inkjet type printer comprises an ink containment device and a fingernail-sized apparatus, commonly known as a print head, which heats and expels ink droplets in a controlled fashion. Other conventional inkjet type printers use piezo elements that can vary the ink chamber volume through use of the piezo-electric effect to expel ink droplets in a controlled fashion, e.g., the ink ejection device of Asai as described by U.S. Pat. No. 5,764,247. Piezoelectric nozzles are not typically implemented in cartridges, but instead are provided as nozzle arrays. 
     In some applications, more than one inkjet print cartridge will be designed into a printer. Usually this multiple print cartridge assembly is created to accommodate multiple colors of ink. Properly controlling the arrangement of various droplets of ink of different colors will result in a wide spectrum of perceivable colors. The clarity and quality of the resultant image is affected by the accuracy of the placement of the ink droplets on the medium. Printers that use multiple print cartridges, or alternatively an array of nozzles, to cooperatively form a single image usually require mechanical or electronic adjustment so that ink droplets printed by one nozzle align at precise locations on the receiving medium relative to those printed by another nozzle in the printer. 
     Cartridge-to-cartridge alignment has been eliminated in some printers with the use of a single multi-color ink cartridge having a print head employing three sets of orifices arranged in a group and receiving one color of ink for each group on the print head. Such a single multi-color print cartridge is inherently self-aligning due to the precise positioning of one set of orifices relative to another on the single orifice plate on the multi-color print cartridge. Even for this cartridge, however, unless other compensation is made, the orifice plate of the print head should be oriented precisely perpendicular to the direction of travel for accurately printed results. 
     Mechanical alignment of print cartridges is simple but expensive, requiring precision features created in the orifice plate of the print head, precision alignment of the cartridges during manufacture to alignment structures or secondary milling of alignment structures or adjustment within the printers cartridge carriage. In each of these foregoing implementations, there are stringent requirements on the printer and the cartridge carriage for either precision during manufacture and long-term stability, or complex adjustability and human intervention. Electronic alignment typically requires printing ink droplet dots on a separate region of the medium, scanning the medium with a detector for these dots, and then establishing time delays within the printer to compensate for the measured offsets. Again, printer complexity or human intervention and judgment are required to optimize this form of alignment. 
     In U.S. Pat. No. 5,448,269, Beauchamp et al. use a test pattern for multiple ink-jet cartridge alignment for bi-directional printing. 
     In U.S. Pat. No. 5,451,990, Sorenson et al. use specified test patterns as a reference for aligning multiple ink-jet cartridges. 
     In U.S. Pat. No. 5,600,350, Cobbs et al. teach multiple ink-jet print cartridge alignment by scanning a reference pattern and sampling the same with reference to a position encoder. 
     The method of aligning a print head provided by U.S. Pat. No. 6,193,350 by Hadley requires the creation and use of a delay time to solve the problems associated with dynamic alignment of the print head to multiple sequential targets. 
     Another prior art alignment technique includes mounting the print head in a fixed mechanical position. The print head is mounted to a mechanical frame which itself is mounted to the printing platform. The print head position including the position of the nozzles is known to the limit of the mechanical tolerances. The tolerances of the mechanical mounting can easily exceed the maximum tolerances required for precise ink jet printing for organic light emitting devices (typically 1-10 micrometers). The position of the nozzles within the mechanical frame of the print head can vary due to manufacturing tolerances of the print head. 
     The alignment system of U.S. Pat. No. 6,234,602 requires the printing of a test pattern according to a first data set, acquiring a second data set representative of the test pattern, fitting a first waveform representative of the first data set to the second data set to determine an initial fit offset value, partitioning the second data set into third data sets, fitting a measuring construct to each of the third data sets, and calculating an actual print head alignment offset value for each of the third data sets using the initial date offset with comparison data representative of comparing the measuring construct and the second data set. 
     Ink-jet printing has become popular as a deposition technique for the polymer, or small molecule, solutions used in organic electronics, such as organic light-emitting devices (OLEDs). One key challenge for this technique is the delivery of the solution droplets to target points of the substrate with a precision of approximately 1 to 10 micrometers. To meet this challenge, nozzles of the multi-nozzle print heads are aligned with great precision to the absolute position and the rotational angle of the substrate. 
     For some print head arrangements, as shown in FIG. 1, the array print head  10  is rotated with respect to the print direction (v)  12 , in order to achieve a print pitch (p v )  14  different from the “physical” pitch (p)  16  of the print head. Such arrangements enable a print head  10  having nozzles  18  disposed at predetermined spaced intervals to deliver droplets for a variety of target  20  separations. Unfortunately, the rotation of the print head  10  with respect to the print direction  12  renders the problem of angle alignment of the print head to the substrate targets  20  even more difficult. 
     But, the advantages created by this angle alignment frequently justify the effort. One advantage is that print heads  10  rotated relative to the print direction  12  enable easy adjustment of nozzle  18  spacing to compensate for thermal expansion of the substrate. Note also that a wide variety of target  20  separations can be accomplished by using different patterns of nozzles  18  disposed on a print head  10 ; e.g., every other nozzle, or every third nozzle. 
     As shown in FIG. 2, for many prior art systems the substrate  25  is located on a sample chuck and kept in position by vacuum suction or by using a mechanical clamp. For droplet deposition, either the substrate  25  is positioned on a substrate stage  27  and moved underneath the print head  10  while the print head position remains fixed, or the print head is mounted on a stage and is positioned over the substrate while the substrate is kept at a fixed position. The problem of adjusting the angle of the print head  10  and the substrate  25  rotation is described in greater detail below with reference to FIG.  4 . 
     An automatic inkjet nozzle inspection system from ImageXpert Inc. in Nashua, N.H. acquires an image of the nozzles, but does not determine the orientation of the nozzles or nozzle plate. The ImageXpert allows for determination of the nozzle plate rotation relative to the screen of their system, but not relative to the substrate (medium). 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention is implemented in embodiments that are based on a system for delivering droplets on a substrate. The system comprises nozzles adapted to deliver droplets to target points on the substrate, a first camera, and means for determining an angular rotation of the substrate in a coordinate system. In some embodiments the means for determining the angular rotation includes a second camera. The substrate has a top surface and alignment marks. The nozzles have droplet delivery openings oriented towards the substrate for delivery of the droplets. The first camera is oriented substantially towards the nozzle openings, and the second camera is oriented substantially towards the substrate top surface. 
     A second aspect of the invention is implemented in embodiments that are based on a method for delivering droplets on a substrate. The method comprises providing nozzles and obtaining an angular displacement value in a coordinate system. The nozzles include a first nozzle and a second nozzle. The nozzles are disposed in proximity of the substrate. The angular displacement corresponds to at least two alignment marks where the alignment marks are disposed on the substrate. The method further comprises sequentially placing the first nozzle and the second nozzle in a position in a viewing area of a first camera, registering sets of coordinates in the coordinate system, and determining an angular rotation of the first nozzle and the second nozzle in the coordinate system. The first nozzle and the second nozzle each have at least one opening for the delivery of droplets. The first camera is oriented substantially towards the nozzle openings. 
     A third aspect of the invention is implemented in embodiments that are based on a method for assessing the suitability of a print head having at least two nozzles for manufacturing. The print head has at least a first nozzle and a second nozzle. The first nozzle has at least one droplet delivery opening. The method comprises obtaining an angular displacement value in a coordinate system, sequentially placing the first nozzle and the second nozzle within a viewing area of a first camera, registering sets of coordinate data in the coordinate system, and comparing the sets of coordinate data and the angular rotation data to predetermined suitability values. The alignment marks disposed on a substrate. The first camera faces substantially towards the nozzle openings. Each set of coordinate data is associated with one of the nozzles. The comparing step is performed within a system for delivering droplets. 
     These, and other, aspects of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A clear conception of the advantages and features constituting the invention, and of the components and operation of model systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments. The embodiments are illustrated in the drawings accompanying and forming a part of this specification, wherein like reference characters (if they occur in more than one view) designate the same parts. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. 
     FIG. 1 illustrates a print head disposed with a rotational displacement relative to a droplet target array. 
     FIG. 2 illustrates a prior art method of aligning nozzles in a print head. 
     FIG. 3 illustrates the placement of the downward looking camera over an alignment mark to obtain stage coordinates x 1 , y 1 , according to a process step of one embodiment of the invention. 
     FIG. 4 illustrates a print head with a substrate that is also rotationally displaced relative to a coordinate system. 
     FIG. 5 illustrates the placement of the downward looking camera over an alignment mark to obtain stage coordinates x 2 , y 2 , according to a process step of one embodiment of the invention. 
     FIG. 6 illustrates the placement of the upward looking camera over a first nozzle to obtain stage coordinates x 3 , y 3 , according to a process step of one embodiment of the invention. 
     FIG. 7 illustrates the placement of the upward looking camera over a last nozzle to obtain stage coordinates x 4 , y 4 , according to a process step of one embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention and the various features and advantageous details thereof are explained more fully with reference to the embodiments that are illustrated in the accompanying drawings and detailed in the following description of preferred embodiments. Descriptions of well-known components and processing techniques are omitted so as not to unnecessarily obscure the invention in detail. 
     System for Delivering Droplets 
     One embodiment of the invention, as shown in FIG. 3 provides a system for delivering droplets on a substrate  25 . The system for delivering droplets includes nozzles  18  (see FIG.  1 ), a first camera  30 , and means for determining an angular rotation of the substrate  25  in a coordinate system. Commercially available multi-nozzle print heads, such as provided by Ink Jet Technology, Inc. or Spectra Inc. can be used to implement this embodiment. In some embodiments, the angular rotation of the substrate can be determined “manually,” i.e., by observation of fiducial marks using a microscope and a crosshair. 
     The nozzles  18  are adapted to deliver droplets to target points  20  (see FIG. 1) on the substrate  25 . In some embodiments, the means for determining an angular rotation of the substrate  25  in a coordinate system comprise a second camera  35 . 
     The substrate  25  has a top surface and alignment marks  40 . The nozzles  18  have droplet delivery openings oriented towards the substrate  25  for delivery of the droplets on the substrate. The first camera  30  is oriented substantially towards the nozzle  18  openings. The second camera  35  is oriented substantially towards the substrate  25  top surface. 
     Some prior art printing tools, as shown in FIG. 2, also use alignment marks  40  on the substrate  25 . In the prior art, the alignment marks  40  are first used to adjust the substrate  25  relative to the axes of a coordinate system  45  to set a first (substrate) angle (α)  50  (see FIG. 4) by using a rotary stage (not shown). After setting the first angle  50  to zero, a second (nozzle array—γ) angle  55  is adjusted. In some embodiments, the nozzle array angle  55  is defined by the rotational separation of the major axis  60  of the print head  10  from the x-axis of the coordinate system  45 . To adjust the second angle  55 , a print head viewing camera  65  views a print head  10  surface to detect alignment points  70 . The alignment points  70  are disposed on the print head  10 . The nozzle array angle  55 , is then calculated based on the view angle  75 . 
     Some embodiments use two separate cameras to align a nozzle array. As shown in FIG. 3, a downward looking camera  30  can be disposed in a fixed position relative to the print head  10 . Camera  30  can then be used to align the substrate  25  through determination of the substrate angle  50  enclosed by the substrate and the x-axis of the fixed coordinate system  45 . Camera  30  is in a downward looking orientation in the embodiment depicted by FIG.  4 . But, the downward looking orientation is not a critical limitation for camera  30 . Instead, for some embodiments, camera  30  can be positioned to view the alignment marks  40  disposed on the substrate  25 . In a preferred embodiment, downward looking camera  30  is disposed having a focal plane within a rotational displacement relative to the substrate  25  plane of approximately 5 to 175 degrees. 
     In some embodiments as shown in FIG. 3, an additional upward looking camera  35  is mounted in a fixed position relative to the substrate  25 . Camera  35  can be used to determine the coordinates of the nozzles  18 , instead of the coordinates of the frame of the print head  10  as in earlier systems. Camera  35  is in an upward looking position in the embodiment depicted by FIG.  4 . But, this orientation is not a critical limitation. Instead for some embodiments of the invention, camera  35  can be oriented to view the nozzle  18  openings, or nozzle alignment features corresponding to the nozzles. In a preferred embodiment, upward looking camera  35  is disposed having a focal plane within a rotation displacement relative to a plane formed by selected nozzles  18  of approximately 5 to 175 degrees. Commercially available CCD cameras having suitable optics, such as those used in the ImageXpert system, can be used to implement these embodiments. In one embodiment, the entire ImageXpert system can be used instead of the upward looking camera  35  thereby providing the optical system plus image recognition software for automatic detection of the nozzles. 
     The combination of the two alignment procedures described below, in the Method for delivering droplets section, allows for a precise determination of the angular relation between the substrate  25  and the nozzle  18  array disposed on the print head  10 , i.e., the nozzle array/substrate angle (β)  80  that is shown in FIG.  4 . 
     Some embodiments include a substrate chuck mounted on a substrate stage  27 , and a print head  10  mounted at a fixed position. However, the procedure is easily transferable to a setup with a fixed-position substrate  25  and a nozzle  18  array mounted to a nozzle array stage (not shown). Alternatively, the substrate  25  can be fixed in the x-direction and movable in the y-direction, while the nozzle  18  array can be fixed in the y-direction and movable in the x-direction; or vice-versa for the substrate and the nozzle array. 
     In some embodiments the droplets can be delivered on the substrate to form at least one of organic light emitting devices, organic thin film transistors, organic solar cells, electronic devices comprising both organic and inorganic materials, and hybrid TFT, and hybrid diode devices. The organic light emitting devices can comprise an array at least of one of a high-resolution passive matrix and a high-resolution active matrix display. 
     In some embodiments the droplets can comprise at least one of a liquid solution of one or more polymers, a dispersion of one or more polymers, a liquid solution of one or more oligomers, a dispersion of one or more oligomers, a liquid solution of one or more small molecules, and a dispersion of one or more small molecules. 
     In some embodiments the droplets can be adapted to act as at least one of print emissive materials, charge transporting materials, charge-injecting materials, charge-conducting materials, and light-emissive materials. The light-emissive materials can comprise electro-luminescent polymers based on the poly(phenylene-vinylene) (PPV) backbone; polymers based on the fluorene backbone; poly(phenylene-vinylene) copolymers, fluorene copolymers, and copolymers comprising combinations of poly(phenylene-vinylene) polymers and fluorene polymers. The charge-conducting polymers can comprise at least one of doped polyaniline solutions, doped polyethylene-dioxy-thiophene solutions, and polyethylene-dioxy-thiophene solutions doped with polystyrene sulfonic acid. 
     For some embodiments as depicted in FIG. 3, alignment marks  40  are arranged on the substrate  25  so that the substrate angle  50  can be determined by detecting the position of at least two of the alignment marks. In a preferred embodiment, the alignment mark  40  spacing is large enough to provide delivery of droplet dots within approximately 0.1 to 30 micrometers of their desired position. (See the Example provided below after the Method for delivering droplets section.) 
     Method for Delivering Droplets 
     Another embodiment provides a method for delivering droplets on a substrate. The method comprises providing nozzles, obtaining an angular displacement value in a coordinate system, sequentially placing the nozzles in a position in a viewing area of a first camera, registering sets of coordinates in the coordinate system, and determining an angular rotation of the nozzles. In some embodiments the method also includes delivering droplets on the substrate from at least one of the nozzles. 
     The nozzles include a first nozzle and a second nozzle, and are disposed in proximity of the substrate. The angular displacement corresponds to at least two alignment marks. The alignment marks are disposed on the substrate. The first nozzle and the second nozzle each have at least one opening for the delivery of droplets. The first camera is oriented substantially towards the nozzle openings. Each set of coordinates is associated with one of the nozzles. The angular rotation values (gamma) of the first nozzle and the second nozzle are provided in the coordinate system. 
     According to some embodiments, a first step of an alignment procedure includes moving the substrate stage  27  until a first alignment mark  41  is centered in the downward looking camera  30 . Upon centering camera  30  on alignment mark  41 , the stage coordinates x 1  and y 1  are registered. In a following step, the substrate stage  27  is moved until a second alignment mark  42  is located underneath the same camera (FIG.  3 ). The coordinates are registered again (x 2  and y 2 ), and the substrate angle  28  (α) (see FIG. 4) of the substrate in the coordinate system  45  can be determined by a simple trigonometric calculation, i.e., tan α=(y 1 −y 2 )/(x 1 −x 2 ). Further alignment marks can be used in order to enhance the precision of the angle measurement. This technique of determining the substrate position/rotation has been used in prior art printing systems. 
     As shown in FIG. 6, some embodiments include a second upward looking camera  35 . The substrate stage  27  can be moved until the first nozzle  18   a  of the print head  10  array is located above the center of the upward looking camera  35 . When this occurs, the first nozzle coordinates  90  (x 3  and y 3 ) are registered. Accordingly, the coordinates  95  (x 4  and y 4 ) of the last nozzle  18   b  of the array are determined, as described in FIG.  7 . The nozzle array angle  55  (γ) of the nozzle  18  array relative to the x-axis of the coordinate system  45  is now easily determined by a trigonometric calculation, i.e., tan γ=+/−(y 3 −y 4 )/(x 3 −x 4 ). 
     Instead of the first nozzle  18   a  and the last nozzle  18   b,  other nozzles of the array can be used. Increasing the number of nozzle coordinates increases the accuracy of the calculation of angle  55 . The accuracy of this measurement can also be increased be using small characteristic nozzle alignment features  85  as target points for coordinate determination, such as a certain corner of the opening for the rectangular nozzle orifices, as shown in FIG. 7, and by using image recognition applications and associated hardware elements. Image recognition applications that can be used with this embodiment include XCaliper, Optimas. The image recognition systems can either detect dark or light areas (with adjustable threshold), distinguishing the nozzle orifice from the background of the nozzle plate, and determine the center point of the nozzle opening. In other embodiments, the software can be “taught” to search for a certain geometric shape, such as for a rectangular or circular nozzle orifice in the image provided by the camera. 
     As indicated in FIG. 4, the precise nozzle array/substrate angle  80  (β) of the nozzle  18  array with respect to the substrate  25  can now be determined by adding the measured nozzle array angle  55  (γ) and the measured substrate angle  50  (α). No mismatches due to improper machining of the substrate  25  mount or the print head  10  mechanics degrade the precision of this alignment procedure. 
     EXAMPLE 
     In one example of the invention, the goal is to print lines with a spacing of 353.55 micrometers onto a substrate. Using a print head with a physical nozzle pitch of 500 micrometers, we can use an angle of 45 degrees between the substrate and the nozzle array. If we allow for a misplacement of droplets between neighboring print lines (353.55 micrometers+/−misplacement) of 10 micrometers, we can allow for a maximum angular error of 10 micrometers/(500 micrometers*cos 45)=0.02828 radians=1.6 degrees. This maximum angular error includes the uncertainty of the determination of the nozzle array angle and of the substrate angle; both of which add quadratically to the overall error. 
     Print Head Suitability Assessment Method 
     Another embodiment provides a method to assess the suitability of a print head for manufacturing comprising obtaining an angular displacement value corresponding to at least two alignment marks, sequentially placing a first nozzle and a second nozzle within a viewing area of a first camera, registering sets of coordinate data associated with the nozzles, determining angular rotation data corresponding to the first nozzle and the second nozzle, and comparing the sets of coordinate data and the angular rotation data to predetermined suitability values. 
     At least one nozzle has at least one droplet delivery opening. The angular displacement value corresponds to at least two alignment marks. The alignment marks are disposed on a substrate. The first camera faces substantially towards the nozzle openings. Each set of coordinate data is associated with one of the nozzles. 
     In some embodiments of the suitability assessment method, on-line statistics of the manufacturing precision of the nozzle plate are obtained. Such statistics enable the nozzle array user to eliminate faulty and off-specification print heads  10 . This is a very useful feature in inkjet machines used for high-volume manufacturing of displays. The nozzle plate statistics can be supported by a computer, using automatic pattern recognition software. The statistics can be generated by storing a nozzle position for each nozzle, calculating a separation between neighboring nozzles, i.e., (d i,i+1 =Sqrt((x i+1 −x i ) 2 +(y i+1 −y i ) 2 )) or similar statistical algorithms, determining an average and a standard deviation of these nozzle separations, and then comparing the statistics with manufacturer&#39;s specifications. 
     Various additions, modifications and rearrangements of the features of the invention may be made without deviating from the spirit and scope of the underlying inventive concept. It is intended that the scope of the invention as defined by the appended claims and their equivalents cover all such additions, modifications, and rearrangements. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means-for.” Expedient embodiments of the invention are differentiated by the appended claims.