Patent Publication Number: US-7909434-B2

Title: Printhead and method of printing

Description:
BACKGROUND OF THE INVENTION 
     Inkjet printing technology is used in many commercial products such as computer printers, graphics plotters, copiers, and facsimile machines. One type of inkjet printing, known as “drop on demand,” employs one or more inkjet pens that eject drops of ink onto a print medium, such as a sheet of paper, to produce dots on the print medium. Printing fluids other than ink, such as preconditioners and fixers, can also be utilized. The pen or pens are typically mounted to a movable carriage that scans or traverses back-and-forth across the print medium. The print medium is advanced between scans in a direction perpendicular to the scanning direction. As the pens are moved repeatedly across the print medium, they are activated under command of a controller to eject drops of printing fluid at appropriate times. The ejection of the drops is controlled so as to form a desired image on the print medium. 
     An inkjet pen generally includes at least one fluid ejection device, commonly referred to as a printhead, from which the drops of printing fluid are ejected. One common printhead architecture includes a substrate having at least one fluid feed hole and a plurality of drop generators arranged around the feed hole. Each drop generator includes a firing chamber in fluid communication with the fluid feed hole and a nozzle in fluid communication with the firing chamber. A fluid ejector, such as a resistor or piezoelectric actuator, is disposed in each firing chamber. Activating the fluid ejector causes a drop of printing fluid to be ejected through the corresponding nozzle. Printing fluid is delivered to the firing chamber from the fluid feed hole to refill the chamber after each ejection. Generally, only one subset of drop generators is fired at a time to reduce peak current draw. A subset of nozzles that fires simultaneously is referred to as an “address,” and a set of adjacent nozzles containing one instance of each address is called a “primitive.” 
     To provide high image quality, each nozzle of the printhead should be able to accurately and repeatedly deposit the desired amount of printing fluid in the proper pixel location on the print medium. However, printhead aberrations can cause misplaced drops that vary from the desired location on the print medium, resulting in what is known as dot placement error. Such dot placement error can have a component in the direction that the carriage is scanned, which component is known as scan axis directionality (“SAD”) error. Dot placement error can also have a component in the direction that the print medium is scanned, which component is known as paper axis directionality (“PAD”) error. 
     Printheads are typically constructed so that the nozzles are arranged in two or more columns, each lying perpendicular to the scan axis. In some designs, the nozzles of each column are located at the same axial location relative to the scan axis (i.e., in a straight line perpendicular to the scan axis). Such a configuration is often referred to as an “inline” architecture. With inline designs, the time that elapses between firing can result in SAD error. Other printhead designs strive to reduce SAD error by employing staggered nozzle columns in which various nozzles in a column are located at slightly different locations relative to the scan axis. A staggered nozzle layout is often, but not always, accomplished by providing the drop generators with different shelf lengths. As used herein, the term “shelf length” refers to the distance, for a given drop generator, from the center of the nozzle to the edge of the fluid feed hole adjacent to that drop generator. Staggered printhead designs reduce SAD error by matching the distances between nozzles to the distances traveled by the carriage in the time between firings. 
     However, material deformations can occur during the fabrication of printheads with staggered designs that create systematic concentricity variations from nozzle to nozzle. These concentricity variations can cause PAD error, which is generally considered to be more problematic than SAD error because it is difficult to compensate for and leads to banding defects. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of an inkjet pen. 
         FIG. 2  is a perspective view of one embodiment of a printhead. 
         FIG. 3  is a partial cross-sectional view taken along line  3 - 3  of  FIG. 2 . 
         FIG. 4  is a partial cross-sectional view taken along line  4 - 4  of  FIG. 3 . 
         FIG. 5  is a partial cross-sectional view of a printhead showing an alternative inline architecture. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  shows an illustrative inkjet pen  10  having a printhead  12 . The pen  10  includes a body  14  that generally contains a printing fluid supply. As used herein, the term “printing fluid” refers to any fluid used in a printing process, including but not limited to inks, preconditioners, fixers, etc. The printing fluid supply can comprise a fluid reservoir wholly contained within the pen body  14  or, alternatively, can comprise a chamber inside the pen body  14  that is fluidly coupled to one or more off-axis fluid reservoirs (not shown). The printhead  12  is mounted on an outer surface of the pen body  14  in fluid communication with the printing fluid supply. The printhead  12  ejects drops of printing fluid through a plurality of nozzles  16  formed therein. Although only a relatively small number of nozzles  16  is shown in  FIG. 1 , the printhead  12  may have two or more columns with more than one hundred nozzles per column, as is common in the printhead art. The columns are generally perpendicular to the scan axis of the inkjet pen  10 . The scan axis, represented by arrow A in  FIG. 1 , is the axis that the pen  10  is traversed along when in use. Appropriate electrical connectors (such as a “flex circuit”)  18  are provided for transmitting signals to and from the printhead  12 . 
     It should be noted that in some applications the inkjet pen has a page wide array in which the printhead is as wide as the print medium and is consequently not scanned across the page. Only the print medium page is advanced relative to the printhead. The present invention is equally applicable to these types of pens and printheads. In this case, the “scan axis” refers to the direction perpendicular to the page axis; i.e., the direction that the page is moved. 
     Referring to  FIGS. 2 and 3 , the printhead  12  includes a substrate  20  having at least one fluid feed hole  22  formed therein with a plurality of drop generators  24  arranged around the fluid feed hole  22 . The fluid feed hole  22  is an elongated slot extending generally perpendicular to the scan axis A and in fluid communication with the printing fluid supply. Each drop generator  24  includes one of the nozzles  16 , a firing chamber  26 , a feed channel  28  establishing fluid communication between the fluid feed hole  22  and the firing chamber  26 , and a fluid ejector  30  disposed in the firing chamber  26 . The nozzles  16  are thus arranged in two columns, one on each side of the fluid feed hole  22 , lying substantially perpendicular to the scan axis A of the inkjet pen  10 . The fluid ejectors  30  can be any device, such as a resistor or piezoelectric actuator, capable of being operated to cause drops of fluid to be ejected through the corresponding nozzle  16 . 
     In the illustrated embodiment, an oxide layer  32  is formed on a front surface of the substrate  20 , and a thin film stack  34  is applied on top of the oxide layer  32 . As is known in the art, the thin film stack  34  generally includes an oxide layer, a metal layer defining the fluid ejectors  30  and conductive traces, and a passivation layer. A fluidic layer assembly  36  comprising a primer layer  38 , a chamber layer  40  and an orifice layer  42  is formed on top of the thin film stack  34 . The fluidic layer assembly  36  defines the firing chambers  26 , the feed channels  28  and the nozzles  16 . Although  FIGS. 2 and 3  illustrate one possible printhead configuration, namely, two rows of drop generators about a common feed hole, it should be noted that other configurations may also be used in the practice of the present invention. 
     Turning now to  FIG. 4 , it is seen that the printhead  12  has a “dual inline” architecture rather than a traditional inline design having no stagger or a staggered design having multiple nozzle locations with a unique nozzle location for each address. With the dual inline architecture, all of the nozzles  16  of each column are located at one of two different axial positions relative to the scan axis A of the inkjet pen  10  (nozzle locations shown in dotted lines in  FIG. 4 ). That is, although the nozzles  16  of each column are distributed along the length of the column, nozzles are located at just two different points along the scan axis A. This dual inline architecture can be accomplished in one embodiment by providing two different shelf lengths for the drop generators  24 . The shelf length (i.e., the distance between the center of the nozzle  16  and the edge of the fluid feed hole  22  for a given drop generator) determines the location of the nozzle  16  relative to the scan axis A. In the illustrated embodiment, the printhead  12  has only two discrete shelf lengths for all of the drop generators  24 , with adjacent drop generators  24  alternating between the two shelf lengths. This means that the drop generators  24  include a first set of drop generators  24   a , each having a first shelf length L 1 , and a second set of drop generators  24   b , each having a second shelf length L 2 , so that all drop generators  24  have either the first shelf length L 1  or the second shelf length L 2 . 
     In the illustrated embodiment, the first shelf length L 1  is greater than the second shelf length L 2 , and the difference between these two shelf lengths is set to substantially minimize or reduce dot placement error. In one possible embodiment, a preferred shelf length differential (L 1 -L 2 ) is in the range of about 0.25 to 2.0 times the dot width column of the printhead  12 , and more preferably is about one-half of the dot column width. The “dot column width” of a printhead is the spacing between the centroids of two dots printed by the same nozzle and is dependent on the resolution of the printhead. The resolution, typically measured in dots per inch (dpi), is the number of dots that can be printed per unit length and is a function of how frequently the printhead can fire per unit length of carriage motion. For example, a printhead having a resolution of 1200 dpi can print 1200 dots in a one inch line along the print medium, meaning that the dots are spaced apart by 1/1200 of an inch. Accordingly, the dot column width of the printhead would be 1/1200 of an inch. In this example, the preferred shelf length differential would be 1/2400 of an inch, which is one-half of the dot column width. 
     A dual inline architecture can also be implemented without two different shelf lengths. For example,  FIG. 5  shows an alternative embodiment of a printhead  112  having a dual inline architecture. That is, all of the nozzles  116  of each column are located at one of two different axial positions relative to the scan axis A of the inkjet pen. The distance along the scan axis A between the first and second axial positions of the nozzles  116  is set to substantially minimize or reduce dot placement error. For example, this distance can be in the range of about 0.25 to 2.0 times the dot width column of the printhead  112 , and more preferably about one-half of the dot column width. In this embodiment, cutouts  144  are formed in the edges of the fluid feed hole  122  adjacent to the first group drop generators  124   a . The depth of the cutouts  144  in the direction of the scan axis A is equal to the distance along the scan axis A between the first and second axial positions of the nozzles  116 . In this way, the nozzles  116  of each column are located at one of two different axial positions, but each nozzle has a shelf length associated with it that is substantially equal to the shelf lengths of the other nozzles  116 . The drop generators  124  of both groups thus have substantially equal fluidic shelf lengths L. Other implementations may be employed to create equal fluidic shelf lengths for a dual inline architecture. 
     Referring again to  FIGS. 2-4 , to eject a droplet from one of the nozzles  16 , printing fluid is introduced into the associated firing chamber  26  from the fluid feed hole  22  via the associated feed channel  28 . The associated fluid ejector  30  is activated or fired to force a droplet through the nozzle  16 . For example, if the fluid ejectors  30  are resistors, the associated resistor is activated with a pulse of electrical current, which causes the resistor to produce heat that heats the printing fluid in the firing chamber  26 . This forms a vapor bubble in the firing chamber  26  and forces a droplet of printing fluid through the nozzle  16 . The firing chamber  26  is refilled after each droplet ejection with printing fluid from the fluid feed hole  22  via the feed channel  28 . While the drop generators  24  can be configured to eject droplets of either uniform or different drop weights, the first group drop generators  24   a  and the second group drop generators  24   b  do not necessarily produce droplets of different drop weights. In fact, the first group drop generators  24   a  and the second group drop generators  24   b  can produce droplets of equal or substantially equal drop weights. The multiple drop generators  24  are typically fired in a predetermined firing order. Generally, the firing order for the dual inline architecture will be such that all of the drop generators of one nozzle location are fired before any of the drop generators of the other nozzle location are fired. Furthermore, it is preferred, although not required, that each primitive has an even number of addresses. 
     As mentioned above, the drop generators  24  in each column alternate between first group drop generators  24   a  and second group drop generators  24   b . Alternating adjacent drop generators  24  between the two shelf lengths means that, for any given drop generator  24 , its two adjoining drop generators are positioned the same along the scan axis A with respect to that drop generator. In others words, a drop generator&#39;s positioning and spacing along the scan axis A relative to the drop generator immediately adjacent to it on one side is the same as the drop generator&#39;s positioning and spacing along the scan axis A relative to the drop generator immediately adjacent to it on the other side. Consequently, the relative positioning of the two adjoining nozzles is the same for any given nozzle  16 . The dual inline architecture thus eliminates asymmetry or systematic concentricity variations from nozzle to nozzle. 
     Because the nozzles  16  of each column are located at two discrete locations relative to the scan axis of the inkjet pen  10 , the dual inline architecture reduces SAD error by 50% as compared to conventional inline architectures. While this reduced SAD error may not be as good as that obtained with a conventional staggered design, it is acceptable for many applications. Furthermore, the dual inline architecture provides substantially smaller PAD error than conventional staggered designs because there are little or no nozzle-to-nozzle concentricity variations. Other advantages of the dual inline architecture include the need to tune only two shelf lengths and the reduced need for stagger compensation because there are only two configurations that need to be matched and optimized for drop velocity, drop weight, R-life, aerosol, etc. Faster refill speeds are enabled because trajectory errors associated with puddling are reduced. Furthermore, there are no incremental costs or processing involved with the dual inline architecture. 
     While specific embodiments of the present invention have been described, it should be noted that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.