Patent Abstract:
A continuous inkjet printer in which a continuous ink stream is deflected at the printhead nozzle bore without the need for charged deflection plates or tunnels. The printhead includes a primary ink delivery channel which delivers a primary flow of pressurized ink through an ink staging chamber to the nozzle bore to create an undeflected ink stream from the printhead. A secondary ink delivery channel adjacent to the primary channel is controlled by a thermally actuated valve to selectively create a lateral flow of pressurized ink into the primary flow thereby causing the emitted ink stream to deflect in a direction opposite to the direction from which the secondary ink stream impinges the primary ink stream in the ink staging chamber. A method of fabricating the printhead includes layering of the thermally actuated valve over the secondary ink delivery channel formed in a silicon substrate and creating the ink staging chamber over the delivery channels with sacrificial material which is later removed through the nozzle bore etched into the chamber wall formed over the sacrificial material.

Full Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous ink jet printheads which integrate multiple nozzles on a single substrate and in which print nonprint operation is effected by controlled deflection of the ink as it leaves the printhead nozzle.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical printing presses, even though this conventional method requires very expensive setup and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems, for example, being able to produce high quality color images at a high-speed and low cost, using standard paper.  
           [0003]    Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop on demand ink jet. Continuous ink jet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell.  
           [0004]    U.S. Pat. No. 3,373,437, which issued to Sweet et al. in 1967, discloses an array of continuous ink jet nozzles wherein ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet, and is used by several manufacturers, including Elmjet and Scitex.  
           [0005]    U.S. Pat. No. 3,416,153, which issued to Hertz et al. in 1966, discloses a method of achieving variable optical density of printed spots in continuous ink jet printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris.  
           [0006]    U.S. Pat. No. 3,878,519, which issued to Eaton in 1974, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.  
           [0007]    U.S. Pat. No. 4,346,387, which issued to Hertz in 1982 discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a drop formation point located within the electric field having an electric potential gradient. Drop formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging rings, deflection plates are used to deflect the drops.  
           [0008]    Conventional continuous ink jet utilizes electrostatic charging rings that are placed close to the point where the drops are formed in a stream. In this manner individual drops may be charged. The charged drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a “catcher”) may be used to intercept the charged drops, while the uncharged drops are free to strike the recording medium. In the current invention, the electrostatic tunnels and charging plates are unnecessary.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide a high-speed continuous ink jet apparatus and method whereby drop formation and deflection may occur at high repetition.  
           [0010]    It is another object of the present invention to provide a method of producing continuous the jet printing apparatus utilizing the advantages of selecting processing technology offering low cost, high volume methods of manufacture.  
           [0011]    It is yet another object of the present invention to provide an apparatus and method for continuous ink jet printing that does not require electrostatic charging tunnels or deflection plates.  
           [0012]    In accordance with an aspect of the invention, apparatus is provided for controlling ink in a continuous ink jet printer in which a continuous stream of ink is emitted from a nozzle wherein the apparatus comprises a reservoir of pressurized ink, an ink staging chamber having a nozzle bore to establish a continuous flow of ink in a stream, ink delivery means intermediate said reservoir and said staging chamber for communicating ink between said reservoir and said staging chamber, said channel means comprising a primary ink delivery channel and an adjacent secondary ink delivery channel; and a thermally actuated valve positioned, when closed, to block ink flow through said secondary channel and, when opened, to permit ink flow through said secondary channel, whereby opening and closing of said valve results in deflection of said ink stream between a print direction and a non-print direction.  
           [0013]    In accordance with another aspect of the invention, there is provided a method of fabricating a continuous inkjet printhead having a series of inkjet devices each of which includes primary and secondary ink delivery channels, an ink staging chamber having a chamber wall with a nozzle bore aligned with said primary ink delivery channel and a thermally actuated valve positioned over said secondary delivery channel to control, by opening and closing of said valve, deflection of an ink stream emitted from said nozzle bore between print and non-print directions. The fabrication method comprises providing a silicon substrate having a front side and a back side; forming a series of first and second adjacent wells in the substrate corresponding to said primary and secondary ink delivery channels; and depositing a patterned thermally actuated valve device over each of said second wells. The method also includes depositing and patterning sacrificial material over said wells to form a volume corresponding to said ink staging chamber; depositing a chamber wall material over said sacrificial material to define an ink staging chamber wall; etching a nozzle bore in the chamber wall aligned with said first well; and removing said sacrificial material through said nozzle bore thereby forming said ink staging chamber with said valve device released within the chamber. The method further includes etching a channel through the back side of said substrate to said wells to form said primary and secondary ink delivery channels to said ink staging chamber.  
           [0014]    These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    In the drawings:  
         [0016]    [0016]FIG. 1 shows a simplified block schematic diagram of one exemplary printing apparatus according to the present invention.  
         [0017]    [0017]FIG. 2 shows in schematic form a cross-section of a segment of a continuous ink jet printhead illustrating principles of the present invention.  
         [0018]    FIGS.  3 - 17  show in schematic form the steps employed in a method of producing a continuous ink jet printhead in accordance with a feature of the invention.  
     
    
     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.  
         [0020]    Referring to FIG. 1, a continuous ink jet printer system includes an image source  10  such as a scanner or computer which provides raster image data, outline image data in the form of a page description language, or other forms of digital image data. This image data is converted to half-toned bitmap image data by an image processing unit  12  which also stores the image data in memory. A plurality of valve control circuits  14  read data from the image memory and apply time-varying electrical pulses to a set of electrically controlled micro-valves that are part of a printhead  16 . These pulses are applied at an appropriate time, and to the appropriate nozzle in the printhead, so that drops formed from a continuous ink jet stream will form spots on a recording medium  18  in the appropriate position designated by the data in the image memory.  
         [0021]    Recording medium  18  is moved relative to printhead  16  by a recording medium transport system  20 , and which is electronically controlled by a recording medium transport control system  22 , which in turn is controlled by a micro-controller  24 . The recording medium transport system shown in FIG. 1 is a schematic only, and many different mechanical configurations are possible. For example, a transfer roller could be used as recording medium transport system  20  to facilitate transfer of the ink drops to recording medium  18 . Such transfer roller technology is well known in the art. In the case of page width printheads, it is most convenient to move recording medium  18  past a stationary printhead. However, in the case of scanning print systems, it is usually most convenient to move the printhead along one axis (the sub-scanning direction) and the recording medium along the orthogonal axis (the main scanning direction) in a relative raster motion.  
         [0022]    Micro-controller  24  may also control an ink pressure regulator  26  and valve control circuits  14 . Ink is contained in an ink reservoir  28  under pressure. In the non-printing state, continuous ink jet drop streams are unable to reach recording medium  18  due to an ink gutter  17  that blocks the stream and which may allow a portion of the ink to be recycled by an ink recycling unit  19 . The ink recycling unit reconditions the ink and feeds it back to reservoir  28 . Such ink recycling units are well known in the art. The ink pressure suitable for optimal operation will depend on a number of factors, including geometry and thermal properties of the nozzles and thermal properties of the ink. A constant ink pressure can be achieved by applying pressure to ink reservoir  28  under the control of ink pressure regulator  26 .  
         [0023]    The ink is distributed to the back surface of printhead  16  by an ink channel device  30 . The ink preferably flows through slots and/or holes etched through a silicon substrate of printhead  16  to its front surface, where a plurality of nozzles and heaters are situated. With printhead  16  fabricated from a silicon substrate, it is possible to integrate valve control circuits  14  with the printhead.  
         [0024]    Turning to FIG. 2, a segment of printhead  16  is shown schematically in cross-section. In the illustration the printhead includes an ink staging chamber  40  having a nozzle bore  42  from which ink under pressure is emitted in a stream directed toward the recording medium  18 . The pressurized ink from reservoir  28  is communicated via the channel device  30  to the staging chamber  40  by ink delivery channel means  30  which, for each ink jet nozzle comprises a primary ink delivery channel  44  and an adjacent secondary ink delivery channel  46 . In the embodiment illustrated, a thermally actuated valve  50 , shown in solid line, is positioned within the staging chamber  40  over the secondary channel  46  thereby blocking the flow of ink through the secondary channel  46 . With the flow of ink through channel  46  blocked, the pressurized ink flowing through the primary channel  44  is emitted through nozzle bore  42  without deflection as stream  52  shown in solid line. The nozzle bore  42  is preferably axially aligned with the primary ink delivery channel  44  and the secondary ink delivery channel is axially offset from the primary channel in a direction opposite to the desired deflection direction of ink stream as represented by dotted outline  52   a.  When valve  50  is thermally actuated by signals from valve control circuits  14  to raise up as shown by dotted lines  50   a , pressurized ink flows through secondary channel  46  creating a lateral flow through the staging chamber  40  that combines with the ink flowing axially through the primary channel  44  to the nozzle bore  42 . The result of this lateral flow it to cause the deflection of the stream  52  as shown in dotted line  52   a.  Thus, opening and closing of the valve results in deflection of the ink stream between a print direction and a non-print direction depending on the position of the gutter  17   
         [0025]    A method by which the printhead of FIG. 2 may be fabricated in accordance with a feature of the invention will now be described with reference to FIGS. 3 through 16. To begin the process, as shown in FIG. 3, an oxide layer  80 , preferably in the thickness range of from 0.1 to 1.0 micron, is formed on a silicon substrate  82 . This oxide layer is patterned and etched to form an array of rectangular shaped openings  84  as seen in the plan view of FIG. 4. The openings may be staggered as shown in order to allow for access to electrical contact terminals from opposite sides of the substrate. It will be appreciated that these figures are schematic in nature to illustrate the steps of the fabrication process and are not drawn to scale. A resist layer  86  is next applied to the substrate  82  as shown in FIG. 5 by a known spin coating technique and is lithographically patterned. This pattern is etched into the silicon substrate  82  to form substrate wells  90  and  92  in the substrate  82  preferably in the depth range of from 1 to 100 microns as shown in FIG. 6. These wells will ultimately become the primary and secondary ink delivery channels  44  and  46 , respectively. In the preferred embodiment illustrated in FIG. 6, well  90  is formed as a cylindrical hole while well  92  is formed as a rectangular slot, although it will be appreciated that other configurations may be employed.  
         [0026]    In FIG. 7, the resist layer  86  is stripped and a conformal second oxide layer  94  is grown on the substrate  82 . Since the 2 nd  oxide layer is thermally grown the growth takes place at the substrate  82 , 1 st  oxide layer  80  interface. So realistically this is where the 2 nd  oxide layer is formed, under the 1 st  oxide layer with thickness in the range of from 0.1 to 1 micron. In FIG. 8, a first sacrificial layer  100  is deposited. The deposited thickness is enough to completely fill substrate wells  90  and  92  as well as the rectangular-shaped openings of modified oxide layer  80 . In the preferred embodiment this layer is polysilicon. Alternatively, polyimide may be used. The first sacrificial layer  100  is then made planar to oxide layer  80  in FIG. 9 by chemical mechanical polishing. The chemical mechanical polishing process is designed to etch the first sacrificial layer  100  and stop on the modified oxide layer  80  creating a planarized first sacrificial layer  100   a.    
         [0027]    In FIG. 10, a third oxide layer  102  is then deposited preferably in the thickness range of from 0.1 to 1 micron. This is followed by deposition and patterning of a lower valve actuator layer  104  as shown in FIGS. 10 and 11. The criteria for the lower thermal actuator layer  104  are i) high coefficient of thermal expansion; ii) resistivity between 3-1000 μΩ-cm; iii) high modulus of elasticity; iv) low mass density; and v) low specific heat. Metals such as aluminum, copper, nickel, titanium, and tantalum, as well as alloys of these metals meet these requirements. In the preferred embodiment, the metal is an aluminum alloy. In FIG. 12, an upper actuator layer  106  is then deposited and then removed in the areas above the planarized first sacrificial layer  100   a  except for the material deposited on the lower actuator layer  104  and a small protective region  106   a  adjacent the lower actuator layer  104 . The third oxide layer  102  not protected by the upper actuator layer  106  is also removed during this step. The criteria for the upper actuator layer  106  are i) low coefficient of thermal expansion; and ii) the layer should be electrically insulating. Dielectric materials such as oxides and silicon nitride meet these requirements. In the preferred embodiment, the dielectric material is an oxide. The protective region  106   a,  along with the third oxide layer  102 , completely encloses the lower actuator layer  90 , protecting it from the ink.  
         [0028]    In FIG. 13 a,  a second sacrificial layer  110  is deposited and lithographically patterned. The second sacrificial layer encloses the rectangular shaped opening  84  (FIG. 13 b ) including the thermally actuated valve  50  and substrate well  90 ,  92 . In the preferred embodiment, this material is photoimageable polyimide. This material can be spun on and patterned by masked exposure and development. The material is then final cured at 350 C. to provide a layer preferably in the thickness range 2-10 microns. A slight etchback in an oxygen plasma can be performed to adjust the final thickness and descum the surface. After subsequent removal, the volume occupied by this second sacrificial layer will become the in ink staging chamber  40  (FIG. 2).  
         [0029]    In FIG. 14, a thick chamber wall layer  112  is then deposited with a preferred thickness so that all regions between the second sacrificial layer  110  will be filled up and result in a thickness on top of the second sacrificial layer  110  that is greater than 1 micron. In the preferred embodiment this material is an oxide layer. Other materials such as silicon nitride or oxynitrides can be used as well as combinations of this material to form the chamber wall layer  112 . This layer can then be planarized by chemical mechanical polishing with a preferred final thickness of the chamber wall layer  112  above the second sacrificial layer  110  to be greater than 1 micron.  
         [0030]    In FIG. 15, the chamber wall layer  112  is next patterned and etched to form the nozzle bore  42  for the ejection of ink. The etch process also opens up a through-hole  116  in the chamber wall as well as in the upper actuator layer  106  so that electrical contact can be made to the lower actuator layer  104  which in turn activates the thermally actuated valve  50 . In FIG. 16, the back side of the silicon substrate  82 , is then patterned and ink feed channels  30  are etched into the silicon substrate  10  until they meet the liner oxide  94  coating the bottoms of the wells  90  and  92 . The first sacrificial layer  100   a , and second sacrificial layer  110  are then removed through the nozzle bore  42  with plasma etchants which do not attack the chamber wall layer  112 . This step will create the ink staging chamber  40 , clear away the sacrificial layer from wells  90  and  92 , and release the thermal actuator  50  (FIG. 2) comprised of lower actuator layer  104  and upper actuator layer  106 . For polyimide sacrificial layers an oxygen plasma is used. For polysilicon sacrificial layers XeF 2  (Xenon Difluoride) or SF 6  (Sulfur Hexafluoride) is used. Finally the liner oxide  94  coating the bottoms of the wells  90  and  92  is removed by etching from the back of the silicon substrate  10  thereby creating the primary and secondary ink delivery channels  44  and  46  (FIG. 17). Once the thermal valve actuator is released upon removal of the sacrificial layers, the bottom layer  104  of the actuator will be in a state of tensile stress that will cause the actuator to bend towards the opening of the secondary ink delivery channel thereby minimizing any leakage while the actuator is in the off (closed) state. More importantly, some minimal leakage can be tolerated in the off state. Such minimal leakage will cause a slight deflection of the ink stream  52  resulting in an initial deflection bias. However, this will not significantly affect the operation since what is most important is the change in deflection of the ink stream between the closed and open state of the thermal actuator.  
         [0031]    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 spirit and scope of the invention.  
                                         PARTS LIST                                10   image source       12   image processing unit       14   valve control circuits       16   printhead       17   ink gutter       18   recording medium       20   recording medium transport           system       22   transport control system       24   micro-controller       26   ink pressure regulator       28   ink reservoir       30   ink channel device       40   ink staging chamber       42   nozzle bore       44   primary ink delivery channel       46   secondary ink delivery channel       50   thermally actuated valve       52   ink stream       80   first oxide layer       82   silicon substrate       84   openings       86   resist layer       90, 92   substrate wells       94   conformal oxide layer       100   first sacrificial layer       104   lower thermal actuator layer       106   upper actuator layer       110   second sacrificial layer       112   chamber wall layer       116   through hole

Technology Classification (CPC): 1