Abstract:
The invention provides a printhead for an ink jet printer and a method for making a printhead for an ink jet printer. The printhead includes a printhead body having a chip surface side, an ink surface side opposite the chip surface side and a first coefficient of thermal expansion (CTE). A semiconductor chip containing ink ejector devices is adhesively attached to the chip surface side of the printhead body. A stiffener is adhesively attached to the ink surface side to provide body stiffening during curing of the adhesive. The semiconductor chip has a second CTE and the stiffener has a third CTE wherein the second and third CTE&#39;s have a similar value. The invention provides an improved structure for printheads which resist warpage and/or breakage of the semiconductor chips during the manufacturing process used to make the printheads.

Description:
FIELD OF THE INVENTION 
     The invention relates to ink jet printers, particularly to semiconductor chips used for ink ejection and to the structure and construction of the chips which provide reliable, long-life ink jet pens. 
     BACKGROUND 
     Ink jet printers continue to be improved as the technology for making the printheads continues to advance. New techniques are constantly being developed to provide low cost, highly reliable printers which approach the speed and quality of laser printers. An added benefit of ink jet printers is that color images can be produced at a fraction of the cost of laser printers with as good or better quality than laser printers. All of the foregoing benefits exhibited by ink jet printers have also increased the competitiveness of suppliers to provide comparable printers in a more cost efficient manner than their competitors. 
     One area of improvement in the printers is in the print engine or printhead itself. This seemingly simple device is a microscopic marvel containing electrical circuits, ink passageways and a variety of tiny parts assembled with precision to provide a powerful, yet versatile ink jet pen. The printhead components of the pen must cooperate with each other and with an endless variety of ink formulations to provide the desired print properties. Accordingly, it is important to match the printhead components to the ink and the duty cycle demanded by the printer. Slight variations in production quality can have a tremendous influence on the product yield and resulting printer performance. 
     The primary components of the ink jet printhead are a semiconductor chip, a nozzle plate and a flexible circuit attached to the chip. The semiconductor chip is typically made of silicon and contains various passivation layers, conductive metal layers, resistive layers, insulative layers and protective layers deposited on a device surface thereof. For thermal ink jet printers, individual heater resistors are defined in the resistive layers and each heater resistor corresponds to a nozzle hole in the nozzle plate for heating and ejecting ink toward a print media. In a top-shooter type printhead, nozzle plates are attached to the chips and there are ink chambers and ink feed channels for directing ink to each of the ejection devices on the semiconductor chip either formed in the nozzle plate material or in a separate thick film layer. In a center feed design for a top-shooter type printhead, ink is supplied to the ink channels and ink chambers from a slot or single ink via which is conventionally formed by chemically etching or grit blasting through the thickness of the semiconductor chip. The chip, nozzle plate and flexible circuit assembly is typically bonded to a thermoplastic body using a heat curable and/or radiation curable adhesive to provide an ink jet pen. 
     Individual chips are fabricated from a silicon wafer containing many chips. The chips are cut from the wafer during the pen fabrication process and are attached to the pen body. Chips typically measure 2 to 8 mm wide by 10 to 20 mm long by 0.6 to 0.65 mm thick. The chips are delicate and require special care to prevent cracking, breaking or warping during the assembly process. 
     In order to increase print speed, larger chips are being designed. By increasing the size of the chips, the chips are capable of containing more ink ejectors thereby providing more ink per print swath. However, larger chips also increase the difficulty associated with handling the chips without damage or breakage and larger chips require more care when attaching the chips to a thermoplastic body so as to minimize chip cracking and warpage. 
     As advances are made in print quality and speed, a need arises for an increased number of ink ejectors which are more closely spaced on the silicon chips. The advances in print speed and quality encourage increases in printhead complexity resulting in a need for long-life printheads which can be produced in high yield while meeting more demanding manufacturing tolerances. Thus, there continues to be a need for improved manufacturing processes and techniques which provide improved printhead components. 
     SUMMARY OF THE INVENTION 
     With regard to the foregoing, the invention provides an improved ink jet printhead and method for making a printhead for an ink jet pen. The printhead includes a printhead body having a chip surface side, an ink surface side opposite the chip surface side and a first coefficient of thermal expansion (CTE). A semiconductor chip containing ink ejector devices is adhesively attached to the chip surface side of the printhead body. A stiffener is adhesively attached to the ink surface side to provide body stiffening during curing of the adhesive. The semiconductor chip has a second CTE and the stiffener has a third CTE wherein the second and third CTE&#39;s have a similar value. 
     In another aspect, the invention provides a method for making a printhead for an ink jet printer. The method includes the steps of providing a printhead body having a chip surface side, an ink surface side opposite the chip surface side and a first coefficient of thermal expansion (CTE). An adhesive is applied to the chip surface side of the printhead body. A semiconductor chip containing ink ejector devices and having a second CTE is adhesively attached to the chip surface side of the printhead body using the adhesive. A stiffener having a third CTE is adhesively attached to the ink surface side of the printhead body using the adhesive to provide body stiffening during curing of the adhesive and the adhesive is cured. The second and third CTE&#39;s preferably have a similar value. 
     In yet another aspect the invention provides an ink jet pen for an ink jet printer. The pen includes an ink container, ink in the ink container and a printhead body attached to the ink container having a chip surface side, an ink surface side opposite the chip surface side and a first coefficient of thermal expansion (CTE). A semiconductor chip containing ink ejector devices and having a second CTE is adhesively attached to the chip surface side of the printhead body. A stiffener having a third CTE is adhesively attached to the ink surface side to provide body stiffening during curing of the adhesive, wherein the second and third CTE&#39;s have a similar value. 
     An advantage of the invention is that it provides an improved structure for printheads which resist warpage and/or breakage of the semiconductor chips during the manufacturing process used to make the printheads. It has been observed that the chip side of the printhead body is substantially constrained from contracting by the chip and adhesive during the cooling process after curing the chip adhesive, while the unconstrained side of the printhead body is free to expand and contract. This unequal constraint on the printhead body material induces bowing of the printhead body during the curing process sufficient to warp or crack the chip. The invention solves the bowing problem by providing a stiffener on the opposite side of the printhead body from the chip. It is preferred that the stiffener be attached to the printhead body with the same adhesive used to attach the chip and that the stiffener be placed substantially opposite the chip on the opposing surface of the printhead body. Another advantage of the invention is that the printheads exhibit improved impact resistance due to the presence of the stiffener thereby improving product yield and decreasing chip failure during printhead handling in manufacturing or by consumers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will become apparent by reference to the detailed description when considered in conjunction with the FIGS., which are not to scale, wherein like reference numbers indicate like elements through the several views, and wherein: 
     FIG. 1 is an end cross-sectional view through a portion of an ink jet printhead including a printhead body and semiconductor chip; 
     FIG. 1A is an enlarged cross-sectional view of a portion of an ink jet printhead including a printhead body and a semiconductor chip; 
     FIG. 2 is a perspective view of an ink jet pen according to the invention; 
     FIG. 3 is a side cross-sectional view through a portion of a printhead body and semiconductor chip; 
     FIG. 4 is a diagrammatic representation of expansion of a printhead body and semiconductor chip as an adhesive between the body and chip is being cured; 
     FIG. 5 is a diagrammatic representation of bowing of a printhead body and semiconductor chip during a cooling process after curing an adhesive used to attached the chip to the body; 
     FIG. 6 is a side cross-sectional view through a portion of a printhead body, semiconductor chip and stiffener during a heating step for curing an adhesive for the printhead according to the invention; 
     FIG. 7 is a side-cross-sectional view through a portion of a printhead body, semiconductor chip and stiffener during a cooling step after curing an adhesive used to attach a chip and stiffener to a printhead body according to the invention; and 
     FIG. 8 is an end cross-sectional view through a portion of an ink jet printhead including a printhead body, semiconductor chip and stiffener assembly according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIGS. 1 and 1A, there is shown a portion of an ink jet  10  printhead  10  viewed in cross-section from a narrow end thereof. The printhead  10  includes a printhead body  12  having a chip surface  14  and an ink surface  16  opposite the chip surface  14 . The printhead body  12  is preferably made of a polymeric material selected from the group consisting of amorphous thermoplastic polyetherimide available from G.E. Plastics of Huntersville, N.C. under the trade name ULTEM 1010, glass filled thermoplastic polyethylene terephthalate resin available from E. I. du Pont de Nemours and Company of Wilmington, Del. under the trade name RYNITE, syndiotactic polystyrene containing glass fiber available from Dow Chemical Company of Midland, Mich. under the trade name QUESTRA, polyphenylene ether/polystyrene alloy resin available from G.E. Plastics under the trade names NORYL SE1 and NORYL 300X and polyamide/polyphenylene ether alloy resin available from G.E. Plastics under the trade name NORYL GTX. A preferred material for making the printhead body is ULTEM 1010 polymer. A printhead body made of ULTEM 1010 polymer has a coefficient of thermal expansion (CTE) of about 42 microns/meter per ° C. as determined by ASTM E-831. 
     In order to eject ink from an ink jet pen  18  (FIG.  2 ), the pen  18  contains one or more of the ink jet printheads  10 . The one or more printheads  10  each include a semiconductor substrate  20 , preferably a silicon semiconductor substrate  20  having a CTE of about 2 to about 3 microns/meter per ° C. as determined by ASTM C-372. The semiconductor substrate  20  contains a plurality of heater resistors  22  thereon (FIG. 1A) for heating ink supplied through an ink via  24  in the semiconductor chip  20  and an ink feed slot  26  in the printhead body  12 . The ink via  24  may be provided conventionally, as by grit blasting through the silicon substrate. The heater resistors  22  are formed in the device side  28  of the chip  20  by well known semiconductor manufacturing techniques. The printhead body  12  preferably includes a recess or chip pocket  30  therein for attachment of a semiconductor chip  20  to body  12 . 
     The semiconductor chips  20  are relatively small in size and typically have overall dimensions ranging from about 2 to about 8 millimeters wide by about 10 to about 20 millimeters long and from about 0.6 to about 0.65 mm thick. In conventional semiconductor chips  20  containing slot-type ink vias  24  which are grit blasted in the chips  20 , the ink via slots  24  have dimensions of about 9.7 millimeters long and 0.39 millimeters wide. In the alternative, the ink via  24  may be provided by a slot or a plurality of holes adjacent the heater resistors  22  made by a deep reactive ion or inductively coupled plasma process. 
     The ink feed vias  24  are etched through the entire thickness of the semiconductor chip  20  and are in fluid communication with ink supplied from an ink supply container  32 , ink cartridge or remote ink supply. In FIGS. 1 and 2, the ink supply container  32  is integral with the printhead body  12 , however the invention is not limited to such an ink supply arrangement. The ink vias  24  direct ink from an ink supply area  34  which is located adjacent the ink surface  16  of the printhead body  12  through the chip  20  to the device side  28  of the chip  20  containing heater resistors  22  (FIG.  1 A). The device side  28  of the chip also preferably contains electrical tracing from the heater resistors to contact pads used for connecting the chip  20  to a flexible circuit or TAB circuit for supplying electrical impulses from a printer controller to activate one or more heater resistors  22 . 
     Prior to attaching the chip to the printhead body  12 , a nozzle plate  36  is attached to the device side  28  of the chip  20  by use of one or more adhesives such as a UV-curable or heat curable epoxy adhesive. The adhesive used to attach the nozzle plate  36  to the chip  20  is preferably a heat curable adhesive such as a B-stageable thermal cure resin, including, but not limited to phenolic resins, resorcinol resins, epoxy resins, ethylene-urea resins, furane resins, polyurethane resins and silicone resins. A particularly preferred adhesive for attaching the nozzle plate  36  to the chip is a phenolic butyral adhesive which is cured by heat and pressure. The nozzle plate adhesive is preferably cured before attaching the chip  20  to the printhead body  12 . 
     As shown in detail in FIG. 1A, the nozzle plate  36  contains a plurality of nozzle holes  38  each of which are in fluid flow communication with an ink chamber  40  and an ink supply channel  42  which are formed in the nozzle plate material by means such as laser ablation. A preferred nozzle plate material is polyimide which may contain an ink repellent coating on a surface  44  thereof. 
     The nozzle plate  36  and semiconductor chip  20  are preferably aligned optically so that the nozzle holes  38  in the nozzle plate  36  align with heater resistors  22  on the semiconductor chip  20 . Misalignment between the nozzle holes  38  and the heater resistor  22  may cause problems such as misdirection of ink droplets from the printhead  10 , inadequate droplet volume or insufficient droplet velocity. 
     After attaching the nozzle plate  36  to the chip  20 , the semiconductor chip  20  of the nozzle plate/chip assembly  36 / 20  is electrically connected to the flexible circuit or TAB circuit and the nozzle plate/chip assembly  36 / 20  is attached to the printhead body  12  using a die bond adhesive  46 . The nozzle plate/chip assembly  36 / 20  is preferably attached to the printhead body  12  in the chip pocket  30 . The die bond adhesive  46  seals around edges  48  of the semiconductor chip  20  to provide a substantially liquid tight seal to inhibit ink from flowing between edges  48  of the chip  10  and the chip pocket  30 . 
     The die bond adhesive  46  used to attach the nozzle plate/chip assembly  36 / 20  to the printhead body  12  is preferably an epoxy adhesive such as a die bond adhesive available from Emerson &amp; Cuming of Monroe Township, N.J. under the trade name ECCOBOND 3193-17. In the case of a thermally conductive printhead body  12 , the die bond adhesive  46  is preferably a resin filled with thermal conductivity enhancers such as silver or boron nitride. A preferred thermally conductive die bond adhesive  46  is POLY-SOLDER LT available from Alpha Metals of Cranston, R.I. A suitable die bond adhesive  46  containing boron nitride fillers is available from Bryte Technologies of San Jose, Calif. under the trade designation G0063. The thickness of adhesive  46  preferably ranges from about 25 microns to about 125 microns. Heat is typically required to cure adhesive  46  and fixedly attach the nozzle plate/chip assembly  36 / 20  to the printhead body  12 . 
     Once the nozzle plate/chip assembly  36 / 20  is attached to the printhead body  12 , the flexible circuit or TAB circuit is attached to the printhead body  12  by use of a heat activated or pressure sensitive adhesive. Preferred pressure sensitive adhesives include, but are not limited to phenolic butyral adhesives, acrylic based pressure sensitive adhesives such as AEROSET 1848 available from Ashland Chemicals of Ashland, Ky. and phenolic blend adhesives such as SCOTCH WELD 583 available from 3M Corporation of St. Paul, Minn. The pressure sensitive adhesive preferably has a thickness ranging from about 25 to about 200 microns. 
     Ejection of ink through the nozzle holes  38  is controlled by a print controller in the printer to which the printhead  10  is attached. Connections between the print controller and the heater resistors  22  of printhead  10  are provided by electrical traces which terminate in contact pads on the device side  28  of the chip  20 . Electrical TAB bond or wire bond connections are made between the flexible circuit or TAB circuit and the contact pads on the semiconductor chip  20 . 
     During a printing operation, an electrical impulse is provided from the printer controller to activate one or more of the heater resistors  22  thereby heating ink in the ink chamber  40  to vaporize a component of the ink thereby forcing ink through nozzle hole  38  toward a print media. Ink is caused to refill the ink channel  42  and ink chamber  40  by collapse of the bubble in the ink chamber once ink has been expelled through nozzle  38 . The ink flows from the ink supply area  34  (FIG. 1) through an ink feed slot  26  in the printhead body  12  to the ink feed vias  24  in the chip  20 . 
     One step in the manufacture of an ink jet pen is the curing of the adhesives used to attach the nozzle plate  36  to the chip  20  and to attach the nozzle plate/chip assembly  36 / 20  to the printhead body  12 . During the curing step, the printhead body  12  and chip  20  are heated to a temperature ranging from about 80° to about 120° C. or higher. The expansion of the nozzle plate/chip assembly  36 / 20  is shown schematically in cross-sectional side views in FIGS. 3-7. For simplicity and clarity, only the chip  20  and printhead body  12  are shown. Furthermore, it will be recognized that expansion of the printhead body  12  and chip  20  occur in all directions upon heating during the curing step. In FIG. 3, the chip  20  and printhead body  12  are at room temperature after placing the nozzle plate/chip assembly  36 / 20  in the chip pocket  30  (FIG.  1 ). The die bond adhesive  46  is disposed on the chip  20  or in the chip pocket  30  to fixedly attach the chip  20  to the printhead body  12 . The printhead body  12  thickness to which the chip  20  is attached preferably ranges from about 0.5 to about 3 mm. 
     In FIG. 4, the chip  20  and printhead  12  are heated to the curing temperature as described above. As the chip  20  and printhead  12  are heated, the chip  20 , printhead  12  and adhesive  46  expand proportional to their respective CTE&#39;s. For example, a printhead body made of ULTEM 1010 polymer having a CTE of about 42 microns/meter per ° C. may increase as much as 53.3 microns in length for a printhead body length of about 12 to about 13 millimeters at a temperature of 100° C. as indicated diagrammatically by long arrows  50 . In contrast a silicon chip  20  having a CTE of about 2.6 microns/meter per ° C. may increase in length only 3.2 microns at 100° C. as indicated diagrammatically by relatively short arrows  52 . The adhesive  46  having a CTE of about 114 microns/meter per ° C. may increase as much as 145 microns in length. 
     Upon cooling after curing adhesive  46 , the ink surface  16  of the printhead body  12  contracts substantially more than the chip surface  14  which is substantially constrained by the chip  20  and cured adhesive  46 . Because of the relatively large difference in the CTE&#39;s of the chip  20  and printhead body  12 , the ink surface  16  of the printhead body  12  will tend to contract to a greater degree as represented diagrammatically by arrows  54  than the contraction of the chip  20  represented diagrammatically by arrows  56 . Unequal contraction of the printhead  10  upon cooling induces bowing of the printhead  10  as shown in diagrammatic representation in FIG.  5 . It will be recognized that the bowing of the printhead  10  may not be as dramatic as shown in FIG. 5, however it may be sufficient to substantially bow or crack the chip  20  resulting in pen failure or production loss of useable parts. For example, a chip  20  having a length of about 16 millimeters had a bow height of about 32 microns above the plane of the chip  20  in the center of the chip  20  upon cooling the chip  20  and printhead body  12  after curing the die bond adhesive  46 . 
     The invention provides a novel solution to the problem associated with printhead bowing and chip cracking described above. According to the invention, a stiffener  58  is attached to the ink surface  16  of the printhead body  12 . The stiffener  58  preferably has a CTE similar to the CTE of the chip  20  and the stiffener  58  is preferably attached to surface  16  of the printhead body  16  using an adhesive  60  similar to adhesive  46 . The stiffener  58  may be selected from the group consisting of aluminum oxide and various glasses and ceramic materials having a CTE similar to the CTE of the silicon substrate  20 . In a particularly preferred embodiment, the stiffener  58  is made of silicon or is a silicon chip having similar dimensions to the semiconductor chip  20 . The dimensions of the stiffener preferably range from about 2 to about 8 mm wide, from about 10 to about 20 mm long and from about 0.6 to about 0.65 mm thick. It is also preferred, though not required that adhesive  60  and adhesive  46  be the same or at least have similar CTE&#39;s. 
     The stiffener  58  is preferably positioned substantially opposite the semiconductor chip  20  so that upon heating and cooling of a printhead  62  as shown diagrammatically in FIGS. 6 and 7, both surfaces  14  and  16  of the printhead body  12  are equally constrained. The heating step is indicated diagrammatically in FIG. 6 by long and short arrows  64  and  66  respectively and the cooling step is indicated diagrammatically in FIG. 7 by long and short arrows  68  and  70  respectively. The bow height in the center of a chip  20  of a printhead  62  according to the invention is less than about 2 microns for a 16 millimeter long chip  20 . The dramatic decrease in bowing of the printhead body  12  and chip  20  compared to conventional printheads as a result of the use of stiffener  58  is significant and provides improved fabrication techniques for printheads  10  which result in higher production yields and longer ink jet pen life. Because there is less bowing of the chip  20 , ink droplet placement from ink ejected from the printheads  10  tends to be more accurate resulting in higher quality printing. 
     The stiffener  58  may be attached to the printhead body  12  before or after attaching the chip  20  to the printhead body. In one process, the stiffener  58  is attached with an adhesive  60  to ink surface  16  of the printhead body  12 . Adhesive  60  is then cured using heat. The assembly is cleaned to assure that no debris or excessive adhesive is present on the chip surface  14  or in the ink feed slot  26 . A filter, if any, is next attached to the ink surface side  16  of the printhead body  12  to provide filtered ink through the stiffener  58  and ink feed slot  26  to the chip  20 . Next the nozzle plate/chip assembly  36 / 20  is attached with adhesive  46  to the chip surface  14  of the printhead body  12 . Adhesive  46  is then cured. 
     It will be recognized that the stiffener  58  and chip  20  may be attached to the printhead body  12  before curing either adhesive  60  or adhesive  46 . In this case, both adhesives  46  and  60  are cured at essentially the same time. Regardless of the process sequence selected, the stiffener  58  is effective to prevent excessive bowing or warping of the chip during the adhesive  46  curing process. As with the semiconductor chip  20 , the stiffener  58  also contains an ink via  72  as shown in FIG. 8 for flow of ink therethrough to the heater resistors  22  on the device side  28  of the chip  20 . 
     Having described various aspects and embodiments of the invention and several advantages thereof, it will be recognized by those of ordinary skills that the invention is susceptible to various modifications, substitutions and revisions within the spirit and scope of the appended claims.