Patent Abstract:
An apparatus and method for controlling temperature profiles in ejection mechanisms is provided. A heater includes a first resistor segment having an electrical resistivity, a second resistor segment; and a coupling segment positioned between the first resistor segment and the second resistor segment. The coupling segment has an electrical resistivity, wherein the ratio of the resistivity of the coupling segment to the resistivity of the first resistor segment is substantially zero. Alternatively, the first resistor segment has an electrical conductivity and the coupling segment has an electrical conductivity, wherein the electrical conductivity of the coupling segment is greater than the electrical conductivity of the first resistor segment.

Full Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to the field of digitally controlled printing devices, and more specifically to an apparatus and method for controlling temperature profiles in ejection mechanisms of these devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     The state of the art of inkjet printing technologies is relatively well developed. A wide variety of inkjet printing devices are available for commercial purchase from consumer desktop printers that produce general documents to commercial wide format printers that produce huge photographic quality posters. Hewlett-Packard Company of Palo Alto Calif., for example, has been particularly active in the development of thermal inkjet printing devices.  
         [0003]     A thermal inkjet printer typically comprises a transitionally reciprocating printhead that is fed by a source of ink to produce an image-wise pattern upon some type of receiver. Such printheads are comprised of an array of nozzles through which droplets of ink are ejected by the rapid heating of a volume of ink that resides in a chamber behind a given nozzle. This heating is accomplished through the use of a heater resistor that is positioned within the print head in the vicinity of the nozzle. The heater resistor is driven by an electrical pulse that creates a precise vapor bubble that expands with time to eject a droplet of ink from the nozzle. After the drop is ejected and the electrical pulse is terminated, the ink chamber refills and is ready to further eject additional droplets when the heater resistor is again energized.  
         [0004]     The quality of an ejected droplet from a thermal inkjet printhead is dependent upon the precision of the vapor bubble that is produced by the heater resistor, and is therefore also dependent upon how evenly the heater resistor produces heat. Since it is also desirable to shape heater resistors to better control the quality of the ejected droplet, physical characteristics such as current crowding become an issue. Since electrical current will always follow the shortest path, current will crowd and produce more heat in the shorter path when there is both a shorter and a longer path for the current to flow within a particular structure.  
         [0005]     U.S. Pat. No. 6,367,147 issued to Giere et al. teaches that multiple heater resistors that are disposed at various angles to one another require coupling devices to connect the resistors and thus turn the current from one heater resistor to another. Since these coupling devices incorporate both short and long paths in which current will flow, the coupling devices must incorporate a compensation resistor to correct the flow of current in a manner that will force the current to flow evenly within the coupling device. In Giere et al., a segmented heater resistor includes a first heater resistor segment and a second heater resistor segment. The coupling device provides serial coupling from the first resistor segment to the second resistor segment with the compensation resistance reducing current crowding within the coupling device.  
         [0006]     Heater resistors that are connected in various series and parallel combinations are also subject to the current crowding effect, unless they provide equal paths for the flow of current. In the case where the current paths are not equal, some form of coupler must afford any change of angle of one resistor to another. In this case, the coupler will exhibit uneven heating through the current crowding effect, and compensation resistance within the coupler must be employed. The use of compensation resistors is complicated, costly and expensive. Additionally they produce a voltage drop within the coupler, causing drive voltage inefficiencies. The present invention is directed to overcoming one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0007]     According to one aspect of the present invention, a heater includes a first resistor segment having an electrical resistivity and a second resistor segment. A coupling segment is positioned between the first resistor segment and the second resistor segment. The coupling segment has an electrical resistivity, wherein the ratio of the resistivity of the coupling segment to the resistivity of the first resistor segment is substantially zero.  
         [0008]     According to another aspect of the present invention, a printhead includes a nozzle and a drop forming mechanism positioned about the nozzle. The drop forming mechanism includes a first resistor segment having an electrical resistivity, a second resistor segment, and a coupling segment positioned between the first resistor segment and the second resistor segment. The coupling segment has an electrical resistivity, wherein the ratio of the resistivity of the coupling segment to the resistivity of the first resistor segment is substantially zero.  
         [0009]     According to another aspect of the present invention, a heater includes a first resistor segment having an electrical conductivity and a second resistor segment. A coupling segment is positioned between the first resistor segment and the second resistor segment. The coupling segment has an electrical conductivity, wherein the electrical conductivity of the coupling segment is greater than the electrical conductivity of the first resistor segment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:  
         [0011]      FIG. 1  is two-dimensional view of a multiple element inkjet heater assembly disposed about an ejection orifice;  
         [0012]      FIG. 2  is a temperature contour plot of a two-leg portion of a multiple element inkjet heater assembly showing the thermal effects produced by current crowding;  
         [0013]      FIG. 3  is a two-dimensional view of one multiple element inkjet heater assembly made in accordance with the present invention;  
         [0014]      FIG. 4  is a two-dimensional view of another multiple element inkjet heater assembly made in accordance with the present invention; and  
         [0015]      FIG. 5  is a temperature contour plot of a two-leg portion of a multiple element inkjet heater assembly with metal interconnects showing the lack of the thermal effects produced by current crowding.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements.  
         [0017]     Referring now to  FIG. 1 , drawn is a two-dimensional view of the substrate of an orifice plate  10  upon which is disposed an inkjet heater assembly  20  that is arranged about an ejection nozzle  30 . Electrical input conductor  40  and electrical output conductor  50  supply current to the inkjet heater assembly  20 . The geometrical construction of the inkjet heater assembly  20  by nature allows a shorter current path around the inside path  60  versus the outside path  70  of the inkjet heater assembly  20 . This physical fact produces a heating profile within inkjet heater assembly  20  shown in  FIG. 2 .  
         [0018]     Referring now to  FIG. 2 , shown again is a two-dimensional view of the substrate of an orifice plate  10 , upon which is disposed an inkjet heater assembly  20  that is arranged about an ejection nozzle  30 . It is instructive to note that both inkjet heater assembly  20  and ejection nozzle  30  are shown in partial views.  FIG. 2  is a thermal profile of the heating that occurs when current flows through inkjet heater assembly  20 . The inside path  60  and inside corner  80  versus the outside path  70  and outside corner  90  of the inkjet heater assembly  20  shows significantly more heating within the inside corner  80  versus the outside corner  90  because of the current crowding effect. That is to say that the shorter current path along inside path  60  and inside corner  80  and the longer current path along outside path  70  and outside corner  90  produces the temperature gradient shown as the highest temperature residing at point  100  and lowest temperature residing at point  110 .  
         [0019]     Higher resistance heater resistors are generally desirable for thermal inkjet applications, to minimize the voltage drops of the electrical feed lines that supply current to the inkjet heater assemblies. However, the use of higher resistances in the inkjet heater resistors that minimize these drops also tend to produce more undesirable heat in the areas that experience current crowding.  
         [0020]     Referring now to  FIG. 3 , drawn is a two-dimensional view of the substrate of an orifice plate  10  upon which is disposed an inkjet heater assembly  20  that is arranged about an ejection nozzle  30 . Electrical input conductor  40  and electrical output conductor  50  supply current to the inkjet heater assembly  20 . The geometrical construction of the inkjet heater assembly  20  by nature allows a shorter current path around the inside path  60  versus the outside path  70  of the inkjet heater assembly  20 . Coupling segments  120  connect individual straight heater resistor elements  130  together to form the inkjet heater assembly  20 . Coupling segments  120  are effectively shaped to transfer current from a first resistor segment to a second resistor segment and can take a variety of shapes or geometries, including triangles, squares, rectangles, etc.  
         [0021]     Referring now to  FIG. 4 , detailed is a two dimensional view of the substrate of an orifice plate  10 , upon which is disposed an inkjet heater assembly  20  that is arranged about an ejection nozzle  30 . This is a slightly different alternative to that described in  FIG. 3 , in that it shows a configuration where the configuration of coupling segments  120 , in addition to being constructed of a straight portion as shown in  FIG. 3 , comprises some radius of curvature  125 .  
         [0022]     Referring back to  FIGS. 3 and 4 , the conductivity of these coupling segments  120  is in the order of 100 times greater than the conductivity of the materials used to produce the individual heater resistor elements  130 . Coupling segments  120  can be constructed of copper, aluminum, alloys of copper and aluminum, or in fact any highly conductive metal that is compatible with the process used to manufacture the orifice plate  10 . It is also instructive to note at this point that while a nozzle plate is discussed, the present invention also includes a printhead or a cartridge where the nozzle is formed in a body, where portions of the body forms an inkjet chamber. Alternatively, the ratio of resistivity of the coupling segment  120  to the resistivity of the resistor element  130  can be selected such that a low ratio result is produced, for example, a resistivity ratio of 1 to 100.  
         [0023]     Referring now to  FIG. 5 , shown again is a two-dimensional view of the substrate of an orifice plate  10 , upon which is disposed an inkjet heater assembly  20  that is arranged about an ejection nozzle  30 . It is instructive to note that both inkjet heater assembly  20  and ejection nozzle  30  are shown in partial views.  FIG. 5  is a thermal profile of the heating that occurs when current flows through inkjet heater assembly  20 . The inside path  60  and inside corner  80  versus the outside path  70  and outside corner  90  of the inkjet heater assembly  20  show essentially zero heating within the inside corner  80  an the outside corner  90  of the coupling segment  120 . The use of pure metals such as copper, aluminum, alloys of copper and aluminum, or in fact any highly conductive metal that is compatible with the process used to manufacture the orifice plate  10 , reduces heating in coupling segment  120  while the crowding of current still exists. That is to say that the shorter current path along inside path  70  and inside corner  80  and the longer current path along outside path  70  and outside corner  90  would still produce a temperature gradient within coupling segment  120  due to the properties of Ohm&#39;s law. Power dissipated within a structure is related to I 2 (R) where I=Current and R=Resistance. By making the coupling segment  120  of a metal essentially prevents heat from being generated within the coupling segment  120  by minimizing the resistance multiplier in the above equation.  
         [0024]     In  FIG. 5 , the heat in upper resistor arm  140  and lower resistor arm  150 , shown in hatch is a high temperature. The heat in coupling segment  160 , shown as black is a low temperature. Heat gradients  170  are showing that the heat transitions from high temperatures in the resistor arms  140  and  150  to a low temperature in coupling assembly  160 .  
         [0025]     The resistor element(s)  130  and/or the coupling segments  120  can also be constructed from polysilicon that have high and low resistivity regions. Through doping (or the addition of impurities) the resistivity of polysilicon can be varied from about 800 micro-ohms per centimeter to 80,000 micro-ohms per centimeter. This is enough, for example, to obtain a 100 to 1 ratio in resistivity. This is accomplished by doping the polysilicon lightly in a first region thus creating a region of high resistivity, and doping the polysilicon heavily in a second region thus creating a region of low resistivity. Dopants that are suitable for such purposes are elements such as Phosphorus, Boron or Arsenic. By doping the coupling segment  120  heavily and then doping the upper resistor arm  140  and lower resistor arm  150  less heavily, favorable heating profiles such as discussed above are also achieved.  
         [0026]     Referring back to  FIGS. 3-5 , the ratio of resistivity of the coupling segment  120  to the resistivity of the resistor element  130  is substantially zero. In this sense, current crowding still exists but the resistivity of coupling segment  120 , as compared to the resistivity of the resistor element  130 , is so low that little or no heat is generated within coupling segment  120 . Although one example embodiment discloses a resistivity ratio of at least 1 to 100, other resistivity ratios will work depending on the specific application contemplated. Example resistivity ratios include ratios greater than 1 to 100.  
         [0027]     The same is true for the conductivity ratio of the coupling segment  120  as compared to the conductivity of the materials used to produce the individual heater resistor elements  130 . In this sense, current crowding still exists but the conductivity of coupling segment  120 , as compared to the conductivity of the resistor element  130 , is so high that little or no heat is generated within coupling segment  120 . Although one example embodiment discloses that the conductivity of the coupling segment  120  is in the order of at least 100 times greater than the conductivity of the materials used to produce the individual heater resistor element  130 , other conductivity ratios will work depending on the specific application contemplated. Example conductivity ratios include ratios greater than 100×.  
         [0028]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.  
       PARTS LIST  
       [0000]    
       
           10  orifice plate  
           20  inkjet heater assembly  
           30  ejection nozzle  
           40  electrical input conductor  
           50  electrical output conductor  
           60  inside path  
           70  outside path  
           80  inside corner  
           90  outside corner  
           100  highest temperature  
           110  lowest temperature  
           120  coupling segment  
           125  radius of curvature  
           130  resistor element  
           14  upper resistor arm  
           150  lower resistor arm  
           160  coupling segment  
           170  heat gradient

Technology Classification (CPC): 1