Abstract:
An improved integral ink filter for an ink jet printhead has decreased flow resistance and minimal machine cost to implement. The integral ink filter includes both a first filter array patterned in a silicon channel plate of the printhead and a second filter array patterned in an insulative layer located between the channel plate and a heater plate. As the insulative layer already requires photolithographic patterning, such as to remove portions over heating elements and between an ink manifold and nozzle channels, there are no additional processing steps necessary. As such, the present invention achieves improved filtration by doubling the filtration rate, while retaining a small pore size corresponding to a channel size of the nozzles. Thus, a desired ink flow to the nozzles can be maintained even if the ink channels are made increasingly smaller.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to an improved integral ink filter for an ink jet printhead that has decreased flow resistance and minimal machine cost to implement. The integral ink filter includes both a first filter portion patterned in a silicon channel plate of the printhead and a second filter portion patterned in a polyimide layer located between the channel plate and a heater plate. 
     2. Description of Related Art Integral ink filters for inkjet printheads are known. Examples of such are U.S. Pat. No. 4,639,748 to Drake et al., U.S. Pat. No. 5,124,717 to Campanelli et al., U.S. Pat. No. 5,141,596 to Hawkins et al., and U.S. Pat. No. 5,204,690 to Lorenze, Jr. et al. 
     Of these, only U.S. Pat. No. 4,639,748 includes an integral, internal ink filter positioned within the channel plate before the individual ink channel nozzes. However, while its ink filter fabrication costs are small, it suffers an undesirable problem in that the filter pore open area is on the same order as the channel array density. Accordingly, such an ink filter induces significant ink flow resistance, which is detrimental to ink refill frequency. 
     The other cited references include a membrane filter fabricated over an ink fill opening between an ink supply cartridge and the ink manifold of the printhead (i.e., external to the channel plate and affixed to an outer face thereof). As such, these latter patents require additional fabrication costs and time to pattern and implement the ink filter assembly. Further, such a filter is quite removed from the nozzes. 
     Another known ink filter is the integral filter provided on the Hewlett Packard HP722 color printhead. 
     As technology moves towards smaller drop capability and thus increased resolution, there is a need for a better integral ink filter that can be fabricated with only minimal impact on manufacturing costs, while achieving a desired small pore size and decreased flow resistance. 
     SUMMARY OF THE INVENTION 
     In ink jet printers, very small nozzles having correspondingly small flow areas are required to produce small ink droplets for printing. Current ink jet trends are requiring smaller and smaller ink droplets. This necessitates the use of a very fine filtration system to prevent contaminating particles from clogging the small printhead nozzles. To maximize effectiveness, the filtration system should be located close to the nozzles so as to not restrict ink flow. However, as pore sizes decrease, flow rates of ink to the nozzles accordingly decrease. 
     The present invention overcomes the above problems by providing an integral ink filter having two separate filter portions fabricated on separate elements of an ink jet printhead. In particular, the invention in exemplary embodiments provides a first filter array portion formed in a channel plate of the printhead and a second filter array portion patterned and formed in an insulating layer sandwiched between the channel plate and a heater plate of the printhead. The filter arrays have a pore size equal to or smaller than the width of the individual ink channels. 
     Applicants have found that the additional filtering, in addition to the filtering achieved by an internal filter as used in U.S. Pat. No. 4,639,748, can be achieved by creating filtration pores in the already existing thick film structure interposed between the channel plate and the heater plate. Moreover, as this thick film structure already requires photolithographic patterning, such as to remove portions over heating elements and between the ink manifold and the nozzle channels, there is no additional processing steps necessary. As such, the present invention achieves improved filtration, while retaining a small pore size corresponding to a channel size of the nozzles, by doubling the filtration rate and maintaining a desired ink flow to the nozzles. 
     In a first exemplary embodiment of the invention, a plurality of ink jet printheads with integral, internal ink filters are fabricated from two separate substrates, such as silicon ( 100 ) wafers. The printheads are preferably of the thermal, drop-on-demand type and adapted for carriage printing. However, the invention is applicable to other ink jet printheads. 
     A plurality of sets of heating elements and their individual addressing electrodes are formed on a surface of one of the wafers (i.e., heater wafer) and a corresponding plurality of sets of parallel channels are etched in a surface of the other wafer (i.e., channel wafer), with each channel communicating with a recessed manifold through a first integral filter array formed in the channel wafer between the manifold and channels. A fill hole and alignment openings are etched in the other surface of the channel wafer. The heating elements and channels are aligned and bonded together with a thick film organic structure, such as polyimide, interposed therebetween. Prior to bonding, the thick film organic structure is patterned and etched to form a second filter array. A plurality of individual printheads are obtained by dicing the two bonded wafers. 
     Each printhead is fixedly positioned on a daughterboard with the manifold fill hole exposed so that the channel nozzles are parallel to the daughterboard edge. The printhead and daughterboard are mounted on an ink supply cartridge so that the printhead fill hole is coincident with an aperture in the cartridge to fill and maintain ink in the printhead manifold and associated ink channels. 
     The printhead and cartridge may be mounted on a carriage of an ink jet printer adapted for reciprocation across the surface of a recording medium, such as paper. Current pulses are selectively applied to the heating elements in each channel from a controller in the printer in response to receipt of data signals by a controller in a known fashion. In a pagewidth configuration, the printhead array can be fixed and oriented perpendicular to a direction of movement of the recording medium. During the printing operation, the recording medium continually moves at a constant velocity in a known fashion. 
     The current pulses cause the heating elements to transfer thermal energy to the ink, vaporizing the ink as known in the art to produce a bubble. The heating element cools after the passage of the current and the bubble collapses. This bubble formation forms an ink droplet and propels it towards the recording medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial isometric view of a printhead and daughterboard according to the invention; 
     FIG. 2 is an enlarged cross-sectional view of FIG. 1 as viewed along line  2 — 2  showing electrode passivation, an ink flow path between the manifold and ink channels, and internal filter structure according to an embodiment of the invention; 
     FIG. 3 is an enlarged cross-sectional view of FIG. 2 taken along line  3 — 3  showing the two part internal filter structure according to the invention; 
     FIG. 4 is a partial top view of a patterned and formed insulating layer having columns and openings forming a second filter array portion of the printhead according to an embodiment of the invention; 
     FIG. 5 is a schematic plan view of a wafer having a plurality of ink manifold recesses, each manifold having an array of channels and a filtration wall in the manifold concurrently etched therein, one enlarged manifold recess with its filtration wall and associated array of channels being shown, as well as one enlarged alignment opening; 
     FIG. 6 is a flow chart of an exemplary manufacturing process for forming an ink jet printhead according to the invention; 
     FIG. 7 is an enlarged cross-sectional view of FIG. 1 as viewed along line  7 — 7  showing an ink flow path between the internal chamber and ink channels through the internal filter structure according to an alternative embodiment of the invention; 
     FIG. 8 is an enlarged cross-sectional view of FIG. 1 as viewed along line  8 — 8  showing an ink flow path between the internal chamber and ink channels through the internal filter structure according to yet another alternative embodiment of the invention; and 
     FIG. 9 is a partial view showing the etched structure on the channel plate of the FIG. 8 embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 1-2, a first exemplary embodiment is illustrated in which a front face of a printhead  10  includes an array of droplet emitting nozzles  27 . A heater plate  28 , preferably formed from silicon, has an electrically insulating surface  30  on which heating elements  34 , addressing electrodes  33  and terminals  37 ,  32  are patterned on, while channel plate  31 , also preferably formed from silicon, has parallel grooves  20  that extend in one direction and penetrate through the channel plate front face  29 . The other end of grooves  20  terminate in slanted wall  21 . The grooves  20  form ink channels  27 . An ink supply manifold  45  is adjacent the channels  27 . An internal chamber  56  is adjacent ink supply manifold  45  and receives ink therein through a fill hole  25 , which is connected to a source of ink such as an ink cartridge. The terminals  32  are exposed and available for wire bonding to daughterboard  19 , on which printhead  10  is mounted. 
     The channel plate  31  also includes etched openings  55  forming an ink filter array in the channel plate. The openings  55  preferably extend at least in one row parallel to front face  29  between internal chamber  56  and supply manifold  45  to filter ink as it passes from the internal chamber  56  to supply manifold  45 . However, multiple row arrays or offset arrays can be used. 
     Channel plate  31  can be formed by providing a masking layer, such as a pyrolytic CVD silicon nitride layer, deposited on one or both sides of the channel plate  31 . Preferably, channel plate  31  can be formed by using a single masking layer of a first side of the wafer. This side is then photolithographically patterned to form vias in the silicon nitride layer for subsequent anisotropic etching of the ink manifold  45 , internal chamber  56 , channels  27 , and openings  55 . By allowing the etch to continue through the entire thickness of the channel plate  31  in the area of the internal chamber  56 , fill hole  25  can be formed by the etched through hole. 
     Thus far, the formation of and description of the channel plate  31  is consistent with and similar to the methods of U.S. Pat. No. 4,639,748, assigned to the same assignee as the present invention and incorporated herein by reference in its entirety. 
     Heater plate  28  is formed with an underglaze layer  39 . The electrodes  33  and heating elements  34  are formed over the underglaze layer  39 . A passivation layer  16  is then formed over the underglaze layer  39 , electrodes  33  and heating elements  34 . A protective layer  17  is provided over heating elements  34 . A thick film type insulative layer  18  is then formed on passivation layer  16 . Layer  18  can be formed, for example, from Riston®, Vacrel®, Probimer  52 ®, or more preferably polyimide. Thick film insulating layer  18  is photolithographically patterned and etched to remove selected areas  26  over the heating elements  34  to expose the heating elements  34 , and at areas  38  to provide an ink flow path from ink supply manifold  45  to ink channels  27 . Thus far, the formation of the heater plate  31  is conventional, such as that described in U.S. Pat. No. 4,774,530 to Hawkins, assigned to the same assignee as the present invention and incorporated herein by reference in its entirety. However, the invention is not limited to such heater plate structure and is adaptable to other heater plate configurations. 
     The invention differs from typical heater plates, such as that disclosed in either U.S. Pat. No. 4,774,530 or U.S. Pat. No. 4,639,748, in that typical heater plates do not include any internal filter structure. However, Applicant has found that filtration effectiveness can be greatly increased by providing a second filter array portion in addition to a filter array provided in the channel plate  31 . This second filter array can be added with minimal cost and no additional processing by adding a second filter array structure in the insulating layer  18 , which is formed between channel plate  31  and heater plate  28 . 
     To achieve this second filter array, the insulating layer  18  is deposited on the upper surface of heater plate  28  to a suitable thickness of between 5-100 micrometers. In addition to the structures conventionally formed in the insulating layer, such as areas  26  and  38 , insulating layer  18  is further provided with a series of apertures  60  defined by columns  61 . The apertures  60  can be formed by a photolithographic pattern and subsequently etched during formation of the areas  26  and  38 . As insulating layer  18  already requires photolithography and etching steps to form areas  26  and  38 , the additional filter array can be formed without increasing processing steps or machine cost. 
     Apertures  60  and columns  61  are preferably aligned in a linear row that extends parallel to the front face  29  and most preferably opposed to etched openings  55  provided on the channel plate  31 . When opposed to etched openings  55 , the apertures  60  are offset laterally from the openings  55  (columns  61  directly oppose etched openings  55 ) so that they form first and second filter array portions of a particular pore size to filter out particulates or contaminants that may agglomerate in the internal chamber prior to the contaminants reaching the ink nozzles  27 . See FIG.  3 . 
     It is preferable for both filter arrays to have substantially the same pore size. By making the pore size small, a fine filter structure can be provided that removes even the smallest of contaminants, with the two filter portions doubling the effective filter area of the filter without increasing pore size. This increases the flow of ink through the filter. The particular pore size chosen will depend on the size/width of the ink channels forming ink nozzles of the printhead. The pore size of the filter arrays should be selected to be equal to or smaller than the size/width of the ink channels so that contaminants will not pass through the filter and clog the individual ink channels. 
     The fully fabricated heater plate  28  with insulating layer  18  is aligned and bonded to channel plate  31  to form printhead  10  as shown in FIGS. 1-3. 
     FIG. 5 illustrates a silicon wafer substrate on which a plurality of channel plates  31  are formed and subsequently separated by dice cuts  49 , 50 . An enlarged top view of channel plate  31  is also illustrated and shows the linear array of nozzles  27  adjacent ink manifold  45  and etched openings  55  forming the first filter array portion between ink manifold  45  and ink chamber  56 . 
     The inventive ink jet printhead with integral filter arrays can be formed according to the exemplary manufacturing process set forth in the flow chart of FIG.  6 . In particular, the manufacturing process starts at Step S 600  and proceeds to step S 610  where first and second substrates, such as silicon ( 100 ) wafers, are provided. Then, at step S 620 , a masking layer, such as a pyrolytic CVD silicon nitride is deposited on one surface of the first substrate, which will eventually form channel plate  31 . Then, the first substrate is suitably photolithographic patterning on the one surface of the first substrate and ink channels  27 , ink manifold  45  and ink internal chamber  56  are etched at step S 630 . This etching can be achieved by an anisotropic etch, such as KOH. Preferably, concurrent with step S 630 , the one surface of the first substrate is photolithographically patterned and etched to form the first filter array  55  between ink manifold  45  and internal chamber  26 . While this reduces processing steps, it is possible to pattern and etch the first filter array  55  in a separate preceding or subsequent step. To further reduce processing steps, ink fill hole  25  can be formed by allowing the etching of ink internal chamber  56  to continue until it forms a through hole on a second, opposite surface of the first substrate. 
     At step S 640 , heating element array  34  and addressing elements  30 ,  32  are formed on one surface of the second substrate, which can also be a silicon wafer. This second substrate will eventually form heater plate  28 . At step S 650 , insulating layer  18 , preferably a polyimide layer, is deposited over the one surface of the second substrate. At step S 660 , the insulating layer  18  is patterned and select portions are removed to expose heating elements  34 . To minimize processing steps, this patterning and removal step preferably includes patterning openings in the insulating layer to form the second filter array  60  within the insulating layer  18 . However, array  60  could be patterned and formed in a separate step. 
     Once the channel plate  31  and heater plate  28  have been formed from the first and second substrates, the two substrates are aligned, mated and bonded at step S 670  so that the one surface of the first substrate (channel plate  31 ) is mated to the one surface of the second substrate (heater plate  28 ) with the insulating layer  18  sandwiched therebetween. At this time, each heating element  34  of heater plate  28  will be opposed to a corresponding ink channel  27  in the channel plate  31  to form an ink jet printhead assembly. 
     FIG. 7 is an enlarged cross-sectional view of FIG. 1 as viewed along line  7 — 7  showing electrode passivation, an ink flow path between the manifold and ink channels, and internal filter structure according to an alternate embodiment of the invention in which the internal chamber  56  and ink manifold  45  are replaced with an internal ink chamber  56  directly connected to the ink fill hole  25 . In this embodiment, the etched openings  55  are formed between the internal ink chamber  56  and ink channels  27 . 
     FIGS. 8-9 show a further alternative embodiment of the invention in which the first and second filter array portions remain as in the previous embodiment. However, in this embodiment, ink channels  27 ′ are defined in the insulative layer  18  rather than etched in channel plate  31 . Grooves  20 ′ are provided to form a fluid bypass to allow the ink to travel from the first and second filter array portions to ink channels  27 ′. Thus, grooves  20 ′ do not form ink nozzles extending from channel face  29  as in the other embodiments. Instead, ink channels  27 ′ defined in the insulative layer form the ink nozzles. 
     Although the invention has been described in detail above with respect to several preferred embodiments, various modifications can be implemented without departing from the spirit and scope of the invention.