Patent Publication Number: US-8534818-B2

Title: Printhead including particulate tolerant filter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned, U.S. patent application Ser. No. 12/767,824, entitled “PRINTHEAD INCLUDING FILTER ASSOCIATED WITH EACH NOZZLE”, Ser. No. 12/767,826, entitled “CONTINUOUS PRINTHEAD INCLUDING POLYMERIC FILTER”, Ser. No. 12/767,828, entitled “METHOD OF MANUFACTURING PRINTHEAD INCLUDING POLYMERIC FILTER”, Ser. No. 12/767,827, entitled “PRINTHEAD INCLUDING POLYMERIC FILTER”, all filed concurrently herewith. 
     FIELD OF THE INVENTION 
     This invention relates generally to the field of digitally controlled printing systems and, in particular, to the filtering of liquids that are subsequently emitted by a printhead of the printing system. 
     BACKGROUND OF THE INVENTION 
     The use of inkjet printers for printing information on recording media is well established. Printers employed for this purpose can include continuous printing systems which emit a continuous stream of drops from which specific drops are selected for printing in accordance with print data. Other printers can include drop-on-demand printing systems that selectively form and emit printing drops only when specifically required by print data information. 
     Continuous printer systems typically include a printhead that incorporates a liquid supply system and a nozzle plate having a plurality of nozzles fed by the liquid supply system. The liquid supply system provides the liquid to the nozzles with a pressure sufficient to jet an individual stream of the liquid from each of the nozzles. The fluid pressures from the liquid supply required to form the liquid jets in a continuous inkjet are typically much greater than the fluid pressures from the liquid supply employed in drop-on-demand printer systems. 
     Different methods known in the art have been used to produce various components within a printer system. Some techniques that have been employed to form micro-electro-mechanical systems (MEMS) have also been employed to form various printhead components. MEMS processes typically include modified semiconductor device fabrication technologies. Various MEMS processes typically combine photo-imaging techniques with etching techniques to form various features in a substrate. The photo-imaging techniques are employed to define regions of a substrate that are to be preferentially etched from other regions of the substrate that should not be etched. MEMS processes can be applied to single layer substrates or to substrates made up of multiple layers of materials having different material properties. MEMS processes have been employed to produce nozzle plates along with other printhead structures such as ink feed channels, ink reservoirs, electrical conductors, electrodes and various insulator and dielectric components. 
     Particulate contamination in a printing system can adversely affect quality and performance, especially in printing systems that include printheads with small diameter nozzles. Particulates present in the liquid can either cause a complete blockage or partial blockage in one or more nozzles. Some blockages reduce or even prevent liquid from being emitted from printhead nozzles while other blockages can cause a stream of liquid jetted from printhead nozzles to be randomly directed away from its desired trajectory. Regardless of the type of blockage, nozzle blockage is deleterious to high quality printing and can adversely affect printhead reliability. This becomes even more important when using a page wide printing system that accomplishes printing in a single pass. During a single pass printing operation, usually all of the printing nozzles of a printhead are operational in order to achieve a desired image quality and ink coverage on the receiving media. As the printing system has only one opportunity to print a given section of media, image artifacts can result when one or more nozzles are blocked or otherwise not working properly. 
     Conventional printheads have included one or more filters positioned at various locations in the fluid path to reduce problems associated with particulate contamination. Even so, there is an ongoing need to reduce particulate contamination in printheads and printing systems and an ongoing need for printhead filters that provide adequate filtration with acceptable levels of pressure loss across the filter. There is also an ongoing need for effective and practical methods for forming printhead filters using MEMS fabrication techniques. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a printhead includes a nozzle plate, a filter, and a plurality of walls. Portions of the nozzle plate define a plurality of nozzles. The filter, for example, a filter membrane, includes a plurality of pores grouped in a plurality of pore clusters. Each of the plurality of walls extends from the nozzle plate to the filter membrane to define a plurality of liquid chambers positioned between the nozzle plate and the filter membrane. Each liquid chamber of the plurality of liquid chambers is in fluid communication with a respective one of the plurality of nozzles. Each liquid chamber of the plurality of liquid chambers is in fluid communication with the plurality of pores of a respective one of the plurality of pore clusters. The respective one of the plurality of pore clusters includes two pore sub-clusters spaced apart from each other by a non-porous portion of the filter membrane. 
     According to another aspect of the invention, the printhead can include a liquid source that is in liquid communication with each nozzle of the plurality of nozzles through each liquid chamber and the respective one of the plurality of pore clusters associated with each liquid chamber. The liquid source is configured to provide liquid under pressure sufficient to eject a jet of liquid through each nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  shows a simplified schematic block diagram of an example embodiment of a printing system made in accordance with the present invention; 
         FIG. 2  is a schematic view of an example embodiment of a continuous printhead made in accordance with the present invention; 
         FIG. 3  is a schematic view of an example embodiment of a continuous printhead made in accordance with the present invention; 
         FIG. 4A  is a cross-sectional side view of a jetting module including an example embodiment of the invention; 
         FIG. 4B  is a cross-sectional plan view of a jetting module including another example embodiment of the invention; 
         FIG. 5A  shows sectional plan and side views of a nozzle, a liquid chamber and a portion of a filter membrane including an example embodiment of a pore cluster configuration according to the present invention; 
         FIG. 5B  shows sectional plan and side views of a nozzle, a liquid chamber and a portion of a filter membrane including another example embodiment of a pore cluster configuration according to the present invention; 
         FIG. 6  shows flow conditions of a liquid as it flows through a filter membrane having the pore configuration of  FIG. 5B ; 
         FIG. 7  is a flow chart representing a method for manufacturing an integrated filter membrane/nozzle plate unit in accordance with an example embodiment of the invention; 
         FIGS. 8A through 8F  show processing stages in the formation of an integrated filter membrane/nozzle plate unit according to the method described in  FIG. 7  with  FIG. 8F  also showing a cross-sectional side view of a jetting module including another example embodiment of the present invention; 
         FIG. 9A  is a cross-sectional side view of a jetting module including another example embodiment of the present invention; and 
         FIG. 9B  is a cross-sectional side view of a jetting module including another example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     The example embodiments of the present invention are illustrated schematically and not to scale for the sake of clarity. One of the ordinary skills in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention. 
     As described herein, the example embodiments of the present invention provide a printhead or printhead components typically used in inkjet printing systems. However, many other applications are emerging which use inkjet printheads to emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. As such, as described herein, the terms “liquid” and “ink” refer to any material that can be ejected by the printhead or printhead components described below. 
     Referring to  FIGS. 1-3 , example embodiments of a printing system and a continuous printhead are shown that include the present invention described below. It is contemplated that the present invention also finds application in other types of printheads or jetting modules including, for example, drop on demand printheads and other types of continuous printheads. 
     Referring to  FIG. 1 , a continuous inkjet printing system  20  includes an image source  22  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  24  which also stores the image data in memory. A plurality of drop forming mechanism control circuits  26  read data from the image memory and apply time-varying electrical pulses to a drop forming mechanism(s)  28  that are associated with one or more nozzles of a printhead  30 . These pulses are applied at an appropriate time, and to the appropriate nozzle, so that drops formed from a continuous inkjet stream will form spots on a recording medium  32  in the appropriate position designated by the data in the image memory. 
     Recording medium  32  is moved relative to printhead  30  by a recording medium transfer system  34 , which is electronically controlled by a recording medium transfer control system  36 , and which in turn is controlled by a micro-controller  38 . The recording medium transfer system  34  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 transfer system  34  to facilitate transfer of the ink drops to recording medium  32 . 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  32  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 an orthogonal axis (the main scanning direction) in a relative raster motion. 
     Ink is contained in an ink reservoir  40  under pressure. Unlike drop-on-demand printheads, a continuous flow of liquid  52  is provided through printhead  30 , the continuous flow of liquid  52  having pressure sufficient to form the continuous jets of liquid  52  from which continuous inkjet drop streams are formed. In the non-printing state, the continuous inkjet drop streams are unable to reach recording medium  32  due to an ink catcher  42  that blocks the stream and which may allow a portion of the ink to be recycled by an ink recycling unit  44 . The ink recycling unit reconditions the ink and feeds it back to reservoir  40 . 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  40  under the control of ink pressure regulator  46 . Alternatively, the ink reservoir can be left unpressurized, or even under a reduced pressure (vacuum), and a pump is employed to deliver ink from the ink reservoir under pressure to the printhead  30 . In such an embodiment, the ink pressure regulator  46  can include an ink pump control system. As shown in  FIG. 1 , catcher  42  is a type of catcher commonly referred to as a “knife edge” catcher. 
     The ink is distributed to printhead  30  through an ink channel  47 . The ink preferably flows through slots or holes etched through a silicon substrate of printhead  30  to its front surface, where a plurality of nozzles and drop forming mechanisms, for example, heaters, are situated. When printhead  30  is fabricated from silicon, drop forming mechanism control circuits  26  can be integrated with the printhead. Printhead  30  also includes a deflection mechanism which is described in more detail below with reference to  FIGS. 2 and 3 . 
     Referring to  FIG. 2 , a schematic view of continuous liquid printhead  30  is shown. A jetting module  48  of printhead  30  includes an array or a plurality of nozzles  50  formed in a nozzle plate  49 . In  FIG. 2 , nozzle plate  49  is affixed to jetting module  48 . However, as shown in  FIG. 3 , nozzle plate  49  can be integrally formed with jetting module  48 . 
     Liquid  52 , for example, ink, is emitted under pressure through each nozzle  50  of the array to form streams, also commonly referred to as jets, of liquid  52 . In  FIG. 2 , the array or plurality of nozzles extends into and out of the figure. 
     Jetting module  48  is operable to form liquid drops having a first size or volume and liquid drops having a second size or volume through each nozzle. To accomplish this, jetting module  48  includes a drop stimulation or drop forming device  28 , for example, a heater or a piezoelectric actuator, that, when selectively activated, perturbs each stream or jet of liquid  52 , for example, ink, to induce portions of each stream to break-off from the stream and coalesce to form drops  54 ,  56 . 
     In  FIG. 2 , drop forming device  28  is a heater  51 , for example, an asymmetric heater or a ring heater (either segmented or not segmented), located in a nozzle plate  49  on one or both sides of nozzle  50 . This type of drop formation is known with certain aspects having been described in, for example, one or more of U.S. Pat. No. 6,457,807 B1, issued to Hawkins et al., on Oct. 1, 2002; U.S. Pat. No. 6,491,362 B1, issued to Jeanmaire, on Dec. 10, 2002; U.S. Pat. No. 6,505,921 B2, issued to Chwalek et al., on Jan. 14, 2003; U.S. Pat. No. 6,554,410 B2, issued to Jeanmaire et al., on Apr. 29, 2003; U.S. Pat. No. 6,575,566 B1, issued to Jeanmaire et al., on Jun. 10, 2003; U.S. Pat. No. 6,588,888 B2, issued to Jeanmaire et al., on Jul. 8, 2003; U.S. Pat. No. 6,793,328 B2, issued to Jeanmaire, on Sep. 21, 2004; U.S. Pat. No. 6,827,429 B2, issued to Jeanmaire et al., on Dec. 7, 2004; and U.S. Pat. No. 6,851,796 B2, issued to Jeanmaire et al., on Feb. 8, 2005. 
     Typically, one drop forming device  28  is associated with each nozzle  50  of the nozzle array. However, a drop forming device  28  can be associated with groups of nozzles  50  or all of nozzles  50  of the nozzle array. 
     When printhead  30  is in operation, drops  54 ,  56  are typically created in a plurality of sizes or volumes, for example, in the form of large drops  56  having a first size or volume, and small drops  54  having a second size or volume. The ratio of the mass of the large drops  56  to the mass of the small drops  54  is typically approximately an integer between 2 and 10. A drop stream  58  including drops  54 ,  56  follows a drop path or trajectory  57 . 
     Printhead  30  also includes a gas flow deflection mechanism  60  that directs a flow of gas  62 , for example, air, past a portion of the drop trajectory  57 . This portion of the drop trajectory is called the deflection zone  64 . As the flow of gas  62  interacts with drops  54 ,  56  in deflection zone  64 , it alters the drop trajectories. As the drop trajectories pass out of the deflection zone  64  they are traveling at an angle, called a deflection angle, relative to the un-deflected drop trajectory  57 . 
     Small drops  54  are more affected by the flow of gas than are large drops  56  so that the small drop trajectory  66  diverges from the large drop trajectory  68 . That is, the deflection angle for small drops  54  is larger than for large drops  56 . The flow of gas  62  provides sufficient drop deflection and therefore sufficient divergence of the small and large drop trajectories so that catcher  42  (shown in  FIGS. 1 and 3 ) can be positioned to intercept one of the small drop trajectory  66  and the large drop trajectory  68  so that drops following the trajectory are collected by catcher  42  while drops following the other trajectory bypass the catcher and impinge a recording medium  32  (shown in  FIGS. 1 and 3 ). 
     When catcher  42  is positioned to intercept large drop trajectory  68 , small drops  54  are deflected sufficiently to avoid contact with catcher  42  and strike the print recording medium  32 . As the small drops are printed, this is called small drop print mode. When catcher  42  is positioned to intercept small drop trajectory  66 , large drops  56  are the drops that print. This is referred to as large drop print mode. 
     Referring to  FIG. 3 , jetting module  48  includes an array or a plurality of nozzles  50 . Liquid, for example, ink, supplied through channel  47  (shown in  FIG. 2 ), is emitted under pressure through each nozzle  50  of the array to form streams or jets of liquid  52 . In  FIG. 3 , the array or plurality of nozzles  50  extends into and out of the figure. 
     Drop stimulation or drop forming device  28  (shown in  FIGS. 1 and 2 ) associated with jetting module  48  is selectively actuated to perturb the stream or jet of liquid  52  to induce portions of the stream to break off from the stream to form drops. In this way, drops are selectively created in the form of large drops and small drops that travel toward a recording medium  32 . 
     Positive pressure gas flow structure  61  of gas flow deflection mechanism  60  is located on a first side of drop trajectory  57 . Positive pressure gas flow structure  61  includes first gas flow duct  72  that includes a lower wall  74  and an upper wall  76 . Gas flow duct  72  directs gas flow  62  supplied from a positive pressure source  92  at downward angle θ of approximately a 45° relative to the stream of liquid  52  toward drop deflection zone  64  (also shown in  FIG. 2 ). Optional seal(s)  84  provides an air seal between jetting module  48  and upper wall  76  of gas flow duct  72 . 
     Upper wall  76  of gas flow duct  72  does not need to extend to drop deflection zone  64  (as shown in  FIG. 2 ). In  FIG. 3 , upper wall  76  ends at a wall  96  of jetting module  48 . Wall  96  of jetting module  48  serves as a portion of upper wall  76  ending at drop deflection zone  64 . 
     Negative pressure gas flow structure  63  of gas flow deflection mechanism  60  is located on a second side of drop trajectory  57 . Negative pressure gas flow structure includes a second gas flow duct  78  located between catcher  42  and an upper wall  82  that exhausts gas flow from deflection zone  64 . Second duct  78  is connected to a negative pressure source  94  that is used to help remove gas flowing through second duct  78 . Optional seal(s)  84  provides an air seal between jetting module  48  and upper wall  82 . 
     As shown in  FIG. 3 , gas flow deflection mechanism  60  includes positive pressure source  92  and negative pressure source  94 . However, depending on the specific application contemplated, gas flow deflection mechanism  60  can include only one of positive pressure source  92  and negative pressure source  94 . 
     Gas supplied by first gas flow duct  72  is directed into the drop deflection zone  64 , where it causes large drops  56  to follow large drop trajectory  68  and small drops  54  to follow small drop trajectory  66 . As shown in  FIG. 3 , small drop trajectory  66  is intercepted by a front face  90  of catcher  42 . Small drops  54  contact face  90  and flow down face  90  and into a liquid return duct  86  located or formed between catcher  42  and a plate  88 . Collected liquid is either recycled and returned to ink reservoir  40  (shown in  FIG. 1 ) for reuse or discarded. Large drops  56  bypass catcher  42  and travel on to recording medium  32 . Alternatively, catcher  42  can be positioned to intercept large drop trajectory  68 . Large drops  56  contact catcher  42  and flow into a liquid return duct located or formed in catcher  42 . Collected liquid is either recycled for reuse or discarded. Small drops  54  bypass catcher  42  and travel on to recording medium  32 . 
     Alternatively, deflection can be accomplished by applying heat asymmetrically to stream of liquid  52  using an asymmetric heater  51 . When used in this capacity, asymmetric heater  51  typically operates as the drop forming mechanism in addition to the deflection mechanism. This type of drop formation and deflection is known having been described in, for example, U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000. It is understood that these deflections are purposely created and are different than undesired deflections created by particulate contamination of a printhead filter. 
     Alternatively, deflection can be accomplished by applying heat asymmetrically to filament of liquid  52  using an asymmetric heater  51 . When used in this capacity, asymmetric heater  51  typically operates as the drop forming mechanism in addition to the deflection mechanism. This type of drop formation and deflection is known having been described in, for example, U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000. 
     Deflection can also be accomplished using an electrostatic deflection mechanism. Typically, the electrostatic deflection mechanism either incorporates drop charging and drop deflection in a single electrode, like the one described in U.S. Pat. No. 4,636,808, or includes separate drop charging and drop deflection electrodes. 
     As shown in  FIG. 3 , catcher  42  is a type of catcher commonly referred to as a “Coanda” catcher. However, the “knife edge” catcher shown in  FIG. 1  and the “Coanda” catcher shown in  FIG. 3  are interchangeable and work equally well. Alternatively, catcher  42  can be of any suitable design including, but not limited to, a porous face catcher, a delimited edge catcher, or combinations of any of those described above. 
       FIG. 4A  is a cross-sectional side view of a jetting module  48  of printhead  30  including an example embodiment of the invention. Specifically, cross-sectional views of a nozzle plate  49  and a channel  47  are shown. For clarity, various other structures including drop forming device  28 /heater  51  are not shown. In this example embodiment, channel  47  has been formed in a separate component which has been assembled into jetting module  48 . Specifically, channel  47  is formed from a substrate  87 . 
     Nozzle plate  49  is formed from a substrate  85 , various portions of substrate  85  defining a plurality of nozzles  50 . For clarity, only four (4) nozzles  50  are shown. It is understood that other suitable numbers of nozzles  50  can be employed in other example embodiments. 
     Jetting module  48  includes a filter adapted for filtering particulate matter from the continuous flow of liquid  52 . In particular, jetting module  48  includes filter membrane  100 . Filter membrane  100  is adapted for filtering portions of the continuous flow of liquid  52  that is provided by channel  47 . Filter membrane  100  includes a plurality of pores  110  adapted for filtering particulate matter in the continuous flow of liquid  52 . 
     Jetting module  48  includes a plurality of liquid chambers  53 , each of the liquid chambers  53  providing a portion of liquid  52  to a respective one of nozzles  50 . In this example embodiment, filter membrane  100  is separated from nozzles  50  by the plurality of liquid chambers  53 . The liquid chambers  53  provide for fluid communication between nozzles  50  and pores  110 . Each liquid chamber  53  can be positioned for fluid communication with a different one of the plurality of nozzles  50 . 
     In this example embodiment, each liquid chamber  53  is positioned for fluid communication with a single different one of the nozzles  50 . Each liquid chamber  53  is defined by a walled enclosure at leas partially defines by wall(s)  55 . Each wall  55  extends from nozzle plate  49  to filter membrane  100  and helps define liquid chambers  53  that are positioned between nozzle plate  49  and filter membrane  100 . In addition to being in fluid communication with a respective one of the plurality of nozzles  50 , each liquid chamber  53  of the plurality of liquid chambers  53  is in fluid communication with a plurality of pores  110  of a respective one of the plurality of pore clusters  120 , described in more detail below, of filter membrane  100 . 
     Each of the walled enclosures can take various forms including walled enclosures that define circular, rectangular and elliptical spaces. Liquid chambers  53  of the present invention can provide various benefits. For example, liquid chambers  53  can be employed to reduce acoustical crosstalk between nozzles  50 . The walled enclosures employed to define liquid chambers  53  can be used to provide structural support for various printhead components. Added structural support may be required to withstand the rigors of a manufacturing process by way of non-limiting example. 
       FIG. 4B  schematically shows a plan sectional view of jetting module  48  including another example embodiment of the present invention. In this example embodiment, filter membrane  100  includes a planar member positioned to span across or “bridge” the liquid chambers  53  (i.e. liquid chambers  53  and nozzles  50  being shown in broken lines). The plurality of pores  110  adapted for filtering particulate matter from the continuous flow of liquid  52  are shown positioned in the planar member. Each of the pores  110  can include various sectional shapes suitable for filtering the continuous flow of liquid  52 . For example, pores  110  including circular sectional shapes are shown. The size of the pores  110  can vary in accordance with a measured or anticipated size of particulate manner within liquid  52 . Circular shaped pores  110  can include diameters on the order of four (4) microns although other pore shapes, sizes, and pore arrangement patterns are permitted. In some example embodiments, pores  110  are sized such that an area of each pore  110  is less than half of the area of each nozzle  50 . In the illustrated embodiment, each of the plurality of pores  110  has a uniform size when compared to other pores of the plurality of pores  110 . Each pore  110  forms an opening through filter membrane  100 . The path of the continuous flow of liquid  52  flowing within each pore  110  is parallel to a path of the continuous flow of liquid  52  within each of the nozzles  50 . Reference axis X and Y are provided for convenience. In this case, axis Y is oriented along the axis of the array of nozzles  50  and axis X is arranged orthogonally to this direction. In some example embodiments, axis X is arranged along a relative movement direction between recording medium  32  and printhead  30 . The relative movement direction can be associated with the direction of a moving web, for example. 
     Referring additionally to  FIGS. 5A and 5B , pores  110  are grouped together in various pore clusters  120 . Each of the pores clusters  120  is associated with a respective one of the nozzles  50 . A pore cluster  120  can include a plurality of pore sub-clusters  125  associated with each of the nozzles  50 . The pores  110  within a pore cluster  120  can be arranged in either a regular or a random pattern. Each cluster  120  is positioned to allow fluid  52  to flow under pressure through the pores  110  of the cluster  120  into an associated fluid chamber  53  and finally into an associated nozzle  50  from which the fluid  52  is jetted. It is understood that each cluster  120  is not limited to two pore sub-clusters  125  and can include other suitable numbers of pore sub-clusters  125  in other embodiments of the invention. 
     Pores  110  in each pore cluster  120  are regularly arranged. As shown in  FIG. 5A , one or more of the pore clusters  120  is positioned such that a pore  110  overlaps a nozzle  50  when viewed in the direction of fluid flow through the nozzle  50 . As shown in  FIGS. 4B and 5B , each pore cluster  120  is separated from another of the pore clusters  120  in an associated sub-cluster  125  by a non-porous portion  130  of filter membrane  100 . The non-porous portions  130  are positioned collinearly with the associated one of the nozzles  50  while none of the pores  110  in each sub-cluster  125  are positioned collinearly with the associated one of the nozzles  50 . Each of the pore clusters  120  in a given sub-cluster  125  is symmetrically located relative to an associated nozzle  50 . 
     The number and size of the pores  110  employed in each pore cluster  120  can vary in various embodiments of the invention. Typically, each of the pore clusters  120  includes a sufficient number of pores  110  to allow a small number of pores in the pore cluster to become obstructed during filtering without adversely affecting the flow of liquid from the nozzle  50 . The number of pores  110  employed can be tailored to account for the flow impedance through the pores  110  and therefore the pressure drop across the thermal stimulation membrane  100  even if a small number of pores in the pore cluster become obstructed. A suitable number of the pores  110  can be determined on the basis of a measured or predicted quantity of particulates in liquid  52 . Pressure drops will arise as the continuous flow of liquid  52  flows through the pores  110  of filter membrane  100 . It is desired that these pressure drops be reduced as much a possible. Factors including the number and size of the pores  110  employed, the number of pores  110  that are expected to be obstructed during filtering, and the thickness of filter membrane  110  can have a bearing on the pressure drops that are encountered during the operation of printhead  30 . In some example embodiments, a size of the pores  110  when viewed in a plane perpendicular to a direction of the path of the continuous flow of the liquid  52  through each pore  110  in a sub-cluster  125  is selected so that a pressure drop through the pores  110  of the sub-cluster  125  is less than ⅕ th  of a pressure drop through an associated nozzle  50 . In some example embodiments, a thickness of filter membrane  100  is selected so that a pressure drop through the pores  110  of a sub-cluster  125  is less than ⅕ th  of a pressure drop through an associated nozzle  50 . 
     A degree to which a jet of liquid  52  that is emitted from a nozzle  50  maintains a desired orientation is typically referred to as “jet straightness”. Jet straightness is of paramount importance as it pertains to the quality of images produced by continuous inkjet printing systems. In some cases, a jet deflection no greater than 0.50 degrees is preferred. In other cases, a jet deflection no greater than 0.25 degrees is preferred. In yet other cases, a jet deflection no greater than 0.05 degrees or less is most preferred. Various factors can cause undesired jet deflections deviations from a desired jet straightness requirement. For example, an obstruction of the various pores  110  of filter membrane  100  can lead to undesired deflections in the jets of liquid  52  that are emitted from various ones of the nozzles  50 . It has been determined that the separation between filter membrane  100  and nozzle plate  49  can have a significant effect on jet straightness when various ones of the pores  110  become obstructed by particulate matter in fluid  52 . This effect can become especially pronounced when these separations are on the order of several microns as would be the case when the nozzle plate  49  and filter membrane  100  are formed as an integrated unit by the use of MEMS techniques. 
     Referring to  FIGS. 5A and 5B , sectional plan and side views of a nozzle  50  and a portion of a filter membrane  100  having a particular configuration of pore cluster  120  are shown. Each of the sectional plan views are referenced by axis X and Y which are arranged as previously defined.  FIG. 5A  shows a pore cluster  120  configuration including a plurality of pores  110  arranged in a uniform fashion over a liquid chamber  53  and nozzle  50 . In this case, the pores  110  are uniformly arranged across a distance L along the X axis and a distance W along the Y axis. In  FIG. 5A , one or more of pores  110  in pore cluster  120  overlap nozzle  50  (shown in broken lines). In  FIG. 5B , a pore cluster  120  configuration includes two pore sub-clusters  125  separated from each other along the X axis by a non-porous portion  130  of the filter membrane  100 . In this case, the pores  110  arranged across a distance L along the X axis and a distance W along the Y axis. In this case, the two pore sub-clusters  125  are positioned such that non-porous portion  130  overlaps nozzle  50  (shown in broken lines in the plan view). 
     Experimental results included the following observations. Larger jet deflections (for example, in the X direction) are associated with a smaller separation distance H when compared to a larger separation distance H when one or more pores  110  of the pore cluster  120  become obstructed by particles. For a given separation distance H, the jet deflections associated with the pore cluster arrangement of  FIG. 5B  are generally lower in magnitude than the jet deflections associated with the pore cluster configuration of  FIG. 5A . These lower levels are especially prevalent in the X direction which is typically associated with a relative movement direction of a recording medium  32  printed by printheads of the present invention. These lower levels are especially prevalent when a smaller separation distance H is used. In some cases, the jet deflections associated with the pore cluster  120  configuration of  FIG. 5B  are less than half of the jet deflections associated with the pore cluster  120  configuration of  FIG. 5A . As a result, the pore cluster  120  configuration of  FIG. 5B  can be especially effective in reducing jet deflection levels when very small nozzle plate  49  to filter membrane  100  distances H are used. Whether using the pore cluster configuration shown in  FIG. 5A  or  FIG. 5B , small nozzle plate  49  to filter membrane  100  separations includes nozzles having a width D N  being spaced apart from the filter membrane by a distance H, where 0.5 D N &lt;H&lt;5 D N  (i.e. D N  being a size of a nozzle  50  as previously defined). 
     Although the present invention is not to be bound by any particular theories, observations as to why the pore cluster  120  configuration of  FIG. 5B  can reduce jet deflections caused by obstructions of pores  110  are discussed below. It is believed that perturbations in the continuous flow of liquid  52  have increased time and distance to settle out since the flow of liquid  52  approaching the non-porous portion  130  bends and travels a longer path to pass through the pores  110  of the adjacent pore sub-clusters  125 . 
     Referring to  FIG. 6 , it is believed that the continuous flow of liquid  52  is directed towards filter membrane  100  such that a portion of the liquid  52  flows along a first path  140  as the liquid portion approached the filter membrane  100 . In this case, the first path  140  extends along a first direction  142  that intersects an inlet of nozzle  50 . Non-porous portion  130  is positioned to intercept the continuous flow of liquid  52  and redirect the portion of liquid  52  away from first path  140  and cause the portion of liquid  52  to enter various ones of the pores  110  in the filter membrane  100 . The portion of the liquid  52  enters liquid chamber  53  and is redirected along a second path  150  that has a directional component  152  that intersects first direction  142 . Accordingly, a symmetrical positioning of the pore sub-clusters  125  relative to nozzle  50  can cause substantially equal and opposing directional flows of liquid  52  within liquid chamber  53 . The opposing directional flows can create a strong bias in the flow characteristics which overcomes any perturbations in the flow caused by an obstruction of one or more of the pores  110 . 
     Without limitation, other causes can additionally or alternatively contribute to these effects. The use of particular pore cluster  120  configuration in example embodiments of the invention can be motivated by different reasons including a desired nozzle plate  49  to filter membrane  100  separation distance H. In some example embodiments, a particular pore cluster  120  configuration is employed based at least on a nozzle plate  49  to filter membrane  100  separation, H where H is selected from a range defined by 0.5 D N &lt;H&lt;5 D N  (i.e. D N  being a size of a nozzle  50  as previously defined). 
       FIG. 7  shows a flow chart representing a method  300  for manufacturing an integrated nozzle plate  49 /filter membrane  100  unit in accordance with an example embodiment of the invention. Various processes steps associated with the method represented by the  FIG. 7  flow chart are additionally schematically illustrated in  FIGS. 10A ,  10 B,  10 C,  10 D,  10 E, and  10 F for convenience. In step  310 , a substrate  160  is provided as illustrated in  FIG. 8A . In this example embodiment, substrate  160  includes a semiconductor material (e.g. silicon). Substrate  160  includes an etch stop layer  162  positioned between the two semi-conductor layers  164 A and  164 B. One example of such an integrated substrate is a silicon-on-insulator substrate (SOI). In step  315 , patterning and etching techniques are used to form liquid chambers  53 A in semiconductor layer  164 A and associated pore clusters  120  in etch stop layer  162 . This can include masking layer  164 A to define pore structure using a positive resist. DRIE etching layer  164 A for a period of time. Then expose and develop the same photoresist to define the larger liquid chamber regions. DRIE etch the chamber regions. The regions that previously had been etched with the pore structure will continue to be etched at about the same rate as the chamber regions to keep about the same height differential. The DRIE etching continues until the pore regions have been etched through to the insulator layer. Layer  162  can then be etched through the DRIE etched pores in layer  164 A, to define pores in layer  162 . The wafer can then be returned to DRIE etch the liquid chambers down to the insulator layer. The photoresist is then removed from layer  164 A. 
     In step  320 , the regions of substrate  160  that were etched in step  315  are filled with filler material  166 , for example, polyimide, and planarized as illustrated in  FIG. 8C . In step  325 , a material layer  170  is deposited on the planarized surface of substrate  160 . The deposited material layer  170  is subsequently patterned and etched to form a plurality of nozzles  50  as shown in  FIG. 8D . Step  325  can also include the fabrication of drop forming devices  28 , which can include heaters  51 , adjacent to the nozzles  50 . Exemplary steps for depositing the material layer  170  and forming the nozzles  50  and associated drop forming devices  28  are described in U.S. Pat. No. 6,943,037, which is incorporated by reference herein. 
     In step  330 , one or more secondary liquid chambers  53 B are patterned and etched into semiconductor layer  164 B. Liquid chambers  53 B are positioned upstream of pore clusters  120  relative to anticipated flow direction of liquid within the printhead. Liquid channels  53 B provide fluid communication between the liquid source, for example, ink source, and the filter membrane, while the walls  55 B in layer  164 B provide structural support. In some embodiments, a single liquid chamber  53 B spans the entire nozzle array and provides fluid communication between the ink source and the pore clusters  120  associated with each of the nozzles. In step  335 , filler material  166  is removed to complete the integrated nozzle plate/filter membrane unit as shown in  FIG. 8F . It is noted that manufacturing method  300  is presented by way of example only and additional and/or alternate steps or additional and/or alternate sequences of steps are within the scope of the present invention. 
     Referring to  FIG. 8F , and back to  FIG. 4A , another example embodiment of the present invention is shown. Jetting module  48  includes a filter  100  adapted for filtering particulate matter from the continuous flow of liquid  52 . In particular, jetting module  48  includes filter membrane  100 . Filter membrane  100  is adapted for filtering portions of the continuous flow of liquid  52  that is provided by channel  47  (shown in  FIG. 4A ). Filter membrane  100  includes a plurality of pores  110  positioned relative to each other to create pore cluster  120 . Pores  110  and pore cluster  120  are adapted for filtering particulate matter in the continuous flow of liquid  52 . 
     Jetting module  48  includes a plurality of liquid chambers  53 A, each of the liquid chambers  53 A providing a portion of liquid  52  to a respective one of nozzles  50 . In this example embodiment, filter membrane  100  is separated from nozzles  50  by the plurality of liquid chambers  53 A. The liquid chambers  53 A provide for fluid communication between nozzles  50  and pores  110  of pore cluster  120 . Each liquid chamber  53  can be positioned for fluid communication with a different one of the plurality of nozzles  50 . 
     In this example embodiment, filter  100  includes a first side  100 A and a second side  100 B that is upstream relative to a direction of fluid flow and first side  100 A. In this embodiment, the plurality of walls  55  are a first plurality of walls  55 A that extend to the first side  100 A of the filter  100 . A second plurality of walls  55 B extend from the second side  100 B of the filter  100  toward channel  47  (shown in  FIG. 4A ). 
     Referring to  FIG. 8F , each liquid chamber  53 A is positioned for fluid communication with a single different one of the nozzles  50 . Each liquid chamber  53 A is defined by a walled enclosure at least partially defined by wall(s)  55 A. Each wall  55 A extends from substrate  85  to filter membrane  100  and helps define liquid chambers  53 A that are positioned between substrate  85  and filter membrane  100 . In addition to being in fluid communication with a respective one of the plurality of nozzles  50 , each liquid chamber  53 A of the plurality of liquid chambers  53 A is in fluid communication with a plurality of pores  110  of a respective one of the plurality of pore clusters  120 , described in more detail above, of filter  100 . 
     The second plurality of walls  55 B define a plurality of liquid feed channels  53 B with each of the liquid feed channels  53 B being in fluid communication through one of the plurality of pore clusters  120  with a respective one of the plurality of liquid chambers  53 A. The liquid feed channels  53 B and the liquid chambers  53 A can be substantially co-linear with the respective one of the plurality of nozzles  50 . Liquid feed channels  53 B are also in fluid communication with feed channel  47  (shown in  FIG. 4A ). Alternatively, each liquid feed channel  53 B can be in fluid communication with a plurality of liquid chambers  53 A through the pore cluster  120  associated with each liquid chamber  53 A. 
     Referring to  FIGS. 11A and 11B , and back to  FIGS. 10F and 4A , additional example embodiments of the present invention. The nozzles  50  are arranged in an array, typically, a one or two dimensional linear array. As shown in  FIGS. 11A and 11B , the array of nozzles  50  extends into and out of each figure. Liquid chamber  53 A includes a first width  350  that is measured perpendicular to an axis  358  of nozzles  50 . Liquid feed channel  53 B includes a second width  352  measured perpendicular to the nozzle axis  358 . The first width  350  is different when compared to the second width  352 . The first width  350  is smaller than the second width  352  which helps to define supports  356  that provide additional stability and rigidity to filter  100 . As shown in  FIG. 9A , liquid chamber  53 A also includes a third width  354  that is measured perpendicular to the nozzle axis  358  and is downstream relative to the first width  352 . Third width  354  is larger than first width  350 . This helps to define supports  356  that provide adequate flow characteristics and increased contact area that contacts filter  100  (for example, when compared to the supports  356  shown in  FIG. 9B ). The liquid chamber  53 A shown in  FIG. 9A  can be formed to produce the sloping walls  55 A by means of an anisotropic etching of the silicon material by such etchants as KOH or tetramethylammonium (TMAH). While the example embodiments shown in  FIGS. 10F ,  11 A, and  11 B include the filter type shown in  FIGS. 4A and 5A , alternative example embodiments include, for example, the filter type shown in  FIGS. 4B and 5B . 
     Embodiments of the present invention advantageously allow for the formation of integrated nozzle plate/filter membrane units formed from a single substrate. Embodiments of the present invention advantageously allow for the use of MEMS fabrication techniques which can substantially lower particulate contamination associated with other manufacturing techniques. Embodiments of the present invention advantageously allow for the formation of integrated nozzle plate/filter membrane units with acceptable jet straightness. 
     The invention has been described in detail with particular reference to certain example embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
         
           
               20  continuous inkjet printer system 
               22  image source 
               24  image processing unit 
               26  mechanism control circuits 
               28  drop forming device 
               30  printhead 
               32  recording medium 
               34  recording medium transfer system 
               36  recording medium control transfer system 
               38  micro-controller 
               40  reservoir 
               42  catcher 
               44  recycling unit 
               46  pressure regulator 
               47  channel 
               48  jetting module 
               49  nozzle plate 
               50  plurality of nozzles 
               51  heater 
               52  liquid 
               53  liquid chamber 
               53 A liquid chamber 
               53 B liquid channel 
               54  drops 
               55 A wall 
               55 B wall 
               56  drops 
               57  trajectory 
               58  drop stream 
               60  gas flow deflection mechanism 
               61  positive pressure gas flow structure 
               62  gas flow 
               63  negative pressure gas flow structure 
               64  deflection zone 
               66  small drop trajectory 
               68  large drop trajectory 
               72  first gas flow duct 
               74  lower wall 
               76  upper wall 
               78  second gas flow duct 
               82  upper wall 
               84  seals 
               85  substrate 
               86  liquid return duct 
               87  substrate 
               88  plate 
               90  face 
               92  positive pressure source 
               94  negative pressure source 
               96  wall 
               98  semiconductor material 
               100  filter membrane 
               110  pores 
               120  pore cluster 
               125  pore sub-cluster 
               130  non-porous portion 
               140  first path 
               142  first direction 
               150  second path 
               152  directional component 
               160  substrate 
               162  etch stop layer 
               164 A semiconductor layer 
               164 B semiconductor layer 
               166  filler material 
               170  material layer 
               200  conventional continuous inkjet printhead 
               249  nozzle plate 
               250  nozzles 
               252  liquid 
               253  streams 
               255  liquid chamber 
               260  liquid supply manifold 
               270  filter 
               280  pores 
               300  method 
               310  provide a substrate 
               315  form liquid chambers and associated pore clusters 
               320  fill and planarize etched regions 
               325  provide material layer on planarized surface 
               330  form secondary liquid chambers 
               335  remove filler material 
               350  first width 
               352  second width 
               354  third width 
               356  support 
             X axis 
             Y axis 
             W distance 
             L distance 
             D N  nozzle size 
             H separation