Abstract:
A heater is provided. The heater includes a first material having a circular form and a first sheet resistivity. The first material has a first radius of curvature. The heater also includes a second material having a circular form and a second sheet resistivity. The second material is positioned adjacent to the first material and has a second radius of curvature. The first radius of curvature is greater than the second radius of curvature and the first sheet resistivity is less than the second sheet resistivity.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to the field of liquid droplet ejection, for example, inkjet printing, and more specifically to an apparatus for controlling temperature profiles in liquid droplet ejection mechanisms.  
       BACKGROUND OF THE INVENTION  
       [0002]     The state of the art of inkjet printing, as one type of liquid droplet ejection, is relatively well developed. A wide variety of inkjet printing apparatus are available for commercial purchase from consumer desktop printers that produce general documents to commercial wide format printers that produce huge photographic quality posters.  
         [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 driven by an electrical pulse that creates a precise vapor bubble that expands with time to eject a droplet of ink from the nozzle. Upon the drop being ejected and the electrical pulse 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 printer is dependent upon the precision of the vapor bubble that is produced by the heater resistor, and is therefore dependent upon how uniformly the heater resistor produces heat. Since it is desirable to shape heater resistors to better control the quality and trajectory of the ejected droplet, these shapes can also create design issues of their own. Heater resistors of various shapes are known. More specifically, heaters in the form of rings are known. U.S. Pat. No. 6,588,888 by Jeanmaire et al. teaches that heaters that are disposed within droplet forming mechanisms can be formed in a ring shape or a partial ring shape.  
         [0005]     Inkjet heater resistors by their nature must reside in compact areas, such as within a small printhead. When these resistors are placed within miniature enclosures and are constructed of various curved shapes, current flows through the shortest path that is available. That is to say that if there is a source of current that flows through a conductor, and that conductor provides both a short and a long path to the flow of current, the current will bias itself to take the shorter path. This is defined as current crowding, since more current will flow within the shorter portion of the conductor than the longer portion of the conductor. This being understood, the two paths of current within a conductor will also produce a non-uniform heating profile due to the non-uniform current flow. This is known and addressed in U.S. Pat. No. 6,367,147 by Giere et al., wherein the inventors use current balancing resistors to minimize such effects.  
         [0006]     The ability of a material to resist the flow of electricity is a property called resistivity. Resistivity is a function of the material used to make a resistor and does not depend on the geometry of the resistor. Resistivity is related to resistance by: 
 
 R=pL/A  
 
 Where R is the resistance (Ohms); p is the resistivity in (Ohms-cm); L is the length of the resistor; and A is the cross sectional area of the resistor. In thin film applications, a property known as sheet resistance (Rsheet) is commonly used in the analysis and design of heater resistors. Sheet resistance is the resistivity of a material divided by the thickness of the heater resistor constructed from that material, the resistance of the heater resistor determined by the equation: 
 
 R=R sheet( L/W ) 
 
 where L is the length of the heater resistor and W is the width of the heater resistor. 
 
         [0007]     The construction of heater resistors using the CMOS process is desirable and lends particular efficiencies to ink jet printer manufacturing. Moreover, the selective doping of the base polysilicon with elements such as Arsenic, Boron and Phosphorus produce variable sheet resistivities. These resistivities can vary from a minimum of 1 milliohm-cm to 100 ohm-cm. This ability to selectively dope the base sheet resistances allows the construction of heater resistors in the same polysilicon as other necessary structures. Additionally, by adding electronic drivers and the like to the base structure reduces costs and improves process efficiencies by a reducing production steps and the eliminating the need for other materials.  
         [0008]     Inkjet heater resistors constructed of a circular shape are subject to the current crowding effect. Additionally, the doping of polysilicon to create heater resistors is both cost-effective and desirable in the full utilization of the CMOS process to produce inkjet printheads. The present invention is directed towards overcoming one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0009]     According to one feature of the present invention, a heater includes a first material having a circular form and a first sheet resistivity. The first material has a first radius of curvature. The heater has a second material having a circular form and a second sheet resistivity. The second material is positioned adjacent to the first material and has a second radius of curvature. The first radius of curvature is greater than the second radius of curvature and the first sheet resistivity is less than the second sheet resistivity. 
     
    
     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 a two dimensional view of an inkjet orifice surrounded by a ring heater;  
         [0012]      FIG. 2  is a detail of a non-uniform temperature profile produced by an uncorrected ring heater;  
         [0013]      FIG. 3  is a detail of a corrected temperature profile produced by a corrected ring heater;  
         [0014]      FIG. 4  is a detail of a two dimensional view of an inkjet orifice surrounded by a ring heater and accompanied by its cross-sectional view of it&#39;s construction;  
         [0015]      FIG. 5  is a detail of a two dimensional view of an inkjet orifice surrounded by a ring heater and accompanied by its cross-sectional view of it&#39;s construction;  
         [0016]      FIG. 6  is a detail of a two dimensional view of an inkjet orifice surrounded by a ring heater and accompanied by its cross-sectional view of it&#39;s construction;  
         [0017]      FIG. 7  is a detail of a two dimensional view of an inkjet orifice surrounded by a ring heater and accompanied by its cross-sectional view of it&#39;s construction; and  
         [0018]      FIG. 8  is a detail of a corrected temperature profile produced by a corrected ring heater using selective doping. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     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 elements common to the figures.  
         [0020]     Referring to  FIG. 1 , drawn is a two dimensional view of the substrate of an orifice plate  10  upon which is disposed an inkjet heater  20  which is arranged about an ejection nozzle  30 . An electrical input conductor  40  and an electrical output conductor  50  supply electrical current to the inkjet heater  20 . The circular or ring-like construction of the inkjet heater  20  by its physical nature allows a shorter current path around the inside path  60  versus the outside path  80  of the inkjet heater  20 . Also shown for means of clarification are an inside portion  70  of the inkjet heater  20  and an outside portion  90  of the inkjet heater  20 . Disposed between the outside portion  90  of the inkjet heater  20  and the ejection nozzle  30  is an unused portion of the base substrate  100  from which the orifice plate  10  is constructed.  
         [0021]     Referring now to  FIG. 2 , shown is the detail of a non-uniform temperature profile  110  that will occur in an uncorrected inkjet heater  20 . The application of a specific electrical current across the electrical input conductor  40  and the electrical output conductor  50  (from  FIG. 1 ) results in non-uniform heating of the inkjet heater  20 . It should be noted that only ½ of the inkjet heater  20  is detailed for purposes of clarity. It is apparent that, for a given voltage drop, the thermal gradient induced into an uncorrected inkjet heater  20  ranges from 287 degrees Centigrade in the outside path  80  of the inkjet heater  20  to 418 degrees Centigrade in the inside path  60  of the inkjet heater  20 . Thusly, the variation in temperature across the inkjet heater  20  totals 131 degrees Centigrade and cause problems in thermal bubble formation.  
         [0022]     Referring now to  FIG. 3 , shown is the detail of a uniform temperature profile  120  that will occur in a corrected inkjet heater  20  when applying one of a variety of possible correction methods of the present invention. Again it should be noted that only ½ of the inkjet heater  20  is detailed for purposes of clarity. It is apparent from the uniform temperature profile  120  that the temperature gradient in a corrected inkjet heater  20  ranges from 484 degrees Centigrade in the outside path  80  of the inkjet heater  20  to 500 degrees Centigrade in the inside path  60  of the inkjet heater  20 . It should also be noted that the same specific voltage drop is applied as in the prior example. Thus the variation in temperature across the inkjet heater  20  is reduced to total only 16 degrees Centigrade and will substantially eliminate undesired effects in thermal bubble formation.  
         [0023]     Referring now to  FIG. 4 , a drawing is shown that details a two dimensional view of a orifice plate  10  that comprises an inkjet heater  20  that is arranged about an ejection nozzle  30 . An electrical input conductor  40  and an electrical output conductor  50  supply electrical current to the inkjet heater  20 . The ringed construction of the inkjet heater  20  by nature of physics allows a shorter current path around the inside path  60  versus the outside path  80  of a current flowing through inkjet heater  20 . Additionally  FIG. 4  details the construction of the orifice plate  10  in cross-sectional view built upon a base substrate  100 . Establishing a flow of current through input conductor  40  and output conductor  50  that flows through the inkjet heater  20  creates the non-uniform heating profile previously discussed in  FIG. 2 . This non-uniform heating is corrected by using a method as shown in the profile drawing of  FIG. 4 . In this implementation, the outside portion  90  of the inkjet heater  20  is thicker than the inside portion  70  of the inkjet heater  20 , and their relative widths are equal. This situation establishes a condition wherein the outside portion  90  of the inkjet heater  20  has a larger cross-sectional area than the inside portion  70  of the inkjet heater  20 . A larger cross-sectional area exhibits lower resistance to current flow than a smaller cross sectional area. Thus, the resistance change brought about by a corresponding change in cross-sectional area will normalize the current flow to be uniformly distributed through the inkjet heater  20 . Current that flows by virtue of current crowding through the path of lowest resistance will be denied that ability by making all the current paths through the heater resistor  20  equal to each other. This fact enables an equal flow of current through the heater resistor  20 , and whose temperature profile embodies the uniform temperature profile  120  discussed in  FIG. 3 .  
         [0024]     Referring now to  FIG. 5 , an additional drawing is shown that details a two dimensional view of a orifice plate  10  that comprises an inkjet heater  20  that is arranged about an ejection nozzle  30 . An electrical input conductor  40  and an electrical output conductor  50  supply electrical current to the inkjet heater  20 . The ringed construction of the inkjet heater  20  by nature of physics allows a shorter current path around the inside path  60  versus the outside path  80  of a current flowing through inkjet heater  20 . Additionally  FIG. 5  details the construction of the orifice plate  10  in cross-sectional view built upon a base substrate  100 . Establishing a flow of current through input conductor  40  and output conductor  50  that flows through the inkjet heater  20  creates the non-uniform heating profile previously discussed in  FIG. 2 . This non-uniform heating is corrected by using a method as shown in the profile drawing of  FIG. 5 . In this implementation, the outside portion  90  of the inkjet heater  20  is wider and has a higher doping than the inside portion  70 . The outside portion  90  of the inkjet heater  20  has a larger cross-sectional area than the inside portion  70  of the inkjet heater  20 . This condition creates a proper normalization. Current that wants to flow by virtue of current crowding through the path of lowest resistance will be denied that ability by making all the current paths through the heater resistor  20  equal to each other. This fact enables an equal flow of current through the heater resistor  20 , and whose temperature profile embodies the uniform temperature profile  120  discussed in  FIG. 3 .  
         [0025]     Referring now to  FIG. 6 , a drawing is shown that details a two dimensional view of a orifice plate  10  that comprises an inkjet heater  20  that is arranged about an ejection nozzle  30 . An electrical input conductor  40  and an electrical output conductor  50  supply electrical current to the inkjet heater  20 . The ringed construction of the inkjet heater  20  by nature of physics allows a shorter current path around the inside path  60  versus the outside path  80  of a current flowing through inkjet heater  20 . Additionally  FIG. 6  details the construction of the orifice plate  10  in cross-sectional view built upon a base substrate  100 . Establishing a flow of current through input conductor  40  and output conductor  50  that flows through the inkjet heater  20  creates the non-uniform heating profile previously discussed in  FIG. 2 . This non-uniform heating is corrected by using a method as shown in the profile drawing of  FIG. 6 . In this implementation, the outside portion  90  of the inkjet heater  20  is thicker than the inside portion  70  of the inkjet heater  20 , and their relative widths are unequal, inside portion  70  being thinner than outside portion  90 . This situation establishes a condition wherein the outside portion  90  of the inkjet heater  20  has a larger cross-sectional area than the inside portion  70  of the inkjet heater  20 . This condition over-compensates the equalization of the resistance of inkjet heater  20 , and causes excessive current to flow in the outside portion  90 . Selectively doping the inside portion  70  slightly heavier than outside portion  90  will cause a change in the sheet resistivity, making the inside portion  70  more conductive than the outside portion  90  and will normalize the current flow to be uniformly distributed through the inkjet heater  20 . Current that wants to flow by virtue of current crowding through the path of lowest resistance will be denied that ability by making all the current paths through the heater resistor  20  equal to each other. This fact enables an equal flow of current through the heater resistor  20 , and whose temperature profile embodies the uniform temperature profile  120  discussed in  FIG. 3 .  
         [0026]     Referring now to  FIG. 7 , a drawing is shown that details a two dimensional view of a orifice plate  10  that comprises an inkjet heater  20  that is arranged about an ejection nozzle  30 . An electrical input conductor  40  and an electrical output conductor  50  supply electrical current to the inkjet heater  20 . The ringed construction of the inkjet heater  20  by nature of physics allows a shorter current path around the inside path  60  versus the outside path  80  of a current flowing through inkjet heater  20 . Additionally  FIG. 7  details the construction of the orifice plate  10  in cross-sectional view built upon a base substrate  100 . Establishing a flow of current through input conductor  40  and output conductor  50  that flows through the inkjet heater  20  creates the non-uniform heating profile previously discussed in  FIG. 2 . This non-uniform heating is corrected by using a method as shown in the profile drawing of  FIG. 7 . In this implementation, the outside portion  90  of the inkjet heater  20  is sloped  130  in relation to the inside portion  70  of the inkjet heater  20 , and their relative widths in relation to one another are equal. It should be understood that in keeping with the prior descriptions they can also be unequal, and that the sloped  130  condition can also be an arcuate  140  condition or exhibit some uniform or non-uniform radius of curvature. This configuration establishes a situation wherein the outside portion  90  of the inkjet heater  20  has a larger cross-sectional area than the inside portion  70  of the inkjet heater  20 . A larger cross-sectional area exhibits lower resistance to current flow than a smaller cross sectional area. Thus, the resistance change brought about by a corresponding change in cross-sectional area will normalize the current flow to be uniformly distributed through the inkjet heater  20 . Current that wants to flow by virtue of current crowding through the path of lowest resistance will be denied that ability by making all the current paths through the heater resistor  20  equal to each other. This fact enables an equal flow of current through the heater resistor  20 , and whose temperature profile embodies the uniform temperature profile  120  discussed in  FIG. 3 .  
         [0027]     Referring now to  FIG. 8 , a drawing is shown that details a two dimensional view of a orifice plate  10  that comprises an inkjet heater  20  that is arranged about an ejection nozzle  30 . An electrical input conductor  40  and an electrical output conductor  50  supply electrical current to the inkjet heater  20 . The ringed construction of the inkjet heater  20  by nature of physics allows a shorter current path around the inside path  60  versus the outside path  80  of a current flowing through inkjet heater  20 . Establishing a flow of current through input conductor  40  and output conductor  50  that flows through the inkjet heater  20  creates the non-uniform heating profile previously discussed in  FIG. 2 . This non-uniform heating is corrected by using a method as shown in  FIG. 8 . By more heavily doping the outside portion  90  of the inkjet heater  20  than the inside portion  70  of the inkjet heater  20 , a normalization of sheet resistance can also be accomplished. It should be noted that this is detailed in  FIG. 8 , by showing a greater density of dots (doping) within outside portion  90  than the density of dots (doping) within inside portion  70  of inkjet heater  20 . This situation establishes a condition wherein the outside portion  90  of the inkjet heater  20  has a lower resistance than the inside portion  70  of the inkjet heater  20 . Thus, the resistance change brought about by a corresponding change in area doping will normalize the current flow to be uniformly distributed through the inkjet heater  20 . Current that wants to flow by virtue of current crowding through the path of lowest resistance will be denied that ability by making all the current paths through the heater resistor  20  equal to each other. This fact enables an equal flow of current through the heater resistor  20 , and whose temperature profile embodies the uniform temperature profile  120  discussed in  FIG. 3 . It should be noted here that people skilled in the art will realize that an inkjet heater  20  can be divided into a plurality of correction regions and, for purposes of clarity, the previous discussions have been limited to two regions. Doping of the heater can be varied across an inkjet heater  20  in a multiplicity of rings that can vary in thickness and in width due to individual engineering needs. Additionally, for the corrected results shown in  FIG. 3 , the resistivity across the inkjet heater  20  was varied as the square of its radius, when using silicon as a base material. It should be understood by those skilled in the art that the optimum resistivity variation across the inkjet heater  20  will vary as the base material varies, (for example silicon vs. glass) based upon the thermal environment.  
         [0028]     Although the present invention has been described with reference to inkjet printheads, it is recognized that printheads of this type are being used to eject liquids other than inkjet inks. As such, the present invention finds application as a liquid droplet ejector for use in areas other than and/or in addition to its inkjet printhead application.  
         [0029]     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  
           30  ejection nozzle  
           40  electrical input conductor  
           50  electrical output conductor  
           60  inside path  
           70  inside portion  
           80  outside path  
           90  outside path  
           100  base substrate  
           110  non-uniform temperature profile  
           120  uniform temperature profile  
           130  sloped  
           140  arcuate