Abstract:
The invention provides a method for reducing ink corrosion of exposed metal layers on a chip surface of a semiconductor chip for an ink jet printhead. The method includes depositing a protective layer in a plasma process to the chip surface, the protective layer being deposited adjacent ink ejectors so that the protective layer substantially circumscribes an ink via in the chip. A thick film layer is applied to the protective layer and chip, whereby the protective layer and thick film layer are sufficient to promote increased adhesion between the thick film layer and a nozzle plate attached to the thick film layer thereby substantially reducing a tendency for the nozzle plate and thick film layer to delaminate from one another during printhead manufacture or use and interrupting contact between ink and the exposed metal layers on the chip surface.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 10/135,251, filed Apr. 30, 2002, now U.S. Pat. No. 6,540,334 now allowed. 
    
    
     TECHNICAL FIELD 
     This invention relates to the field of ink jet printheads and in particular, to printheads having enhanced corrosion protection. 
     BACKGROUND OF THE INVENTION 
     Ink jet printheads contain semiconductor chips which are electrically activated to eject ink droplets on demand through nozzle holes in a nozzle plate attached to the chips. In a “roof shooter” type printhead, ink is provided to the active surface of the chips for ink droplet ejection through ink vias or ink feed slots formed through the thickness dimension of the silicon chips. The ink ejection devices are typically located in close proximity to the ink feed via or slot along opposing sides thereof for the length of the ink feed via or slot. Metal conducting traces or lines are provided on the chip adjacent the ink feed slots to provide power to the ink ejection devices. Because of the corrosive nature of the ink, the ink ejection devices and metal traces should be protected from the ink. A variety of layers of protective material may be used to provide protection against corrosion for the ink ejection devices and metal conducting layers. However, despite the use of protective layers over the ejection devices and metal layers, ink often gets between the nozzle plate and a planarizing layer on the chip causing delamination between the nozzle plate and planarizing layer. Once delamination has occurred, the ink may find its way to the chip surface thereby corroding unprotected metal conducting layers. There is a need therefore for improved methods for protecting the metal conducting layers on an ink jet chip from ink corrosion and damage. 
     SUMMARY OF THE INVENTION 
     The foregoing and other needs are provided by a method for reducing ink corrosion of exposed metal layers on a chip surface of a semiconductor chip for an ink jet printhead, the chip having an elongate ink feed via and ink ejectors adjacent the ink feed via. The method includes depositing a thin film protective layer to the chip surface, the protective layer being deposited adjacent the ink ejectors so that the protective layer substantially circumscribes the ink via. A thick film layer is applied to the protective layer and chip, whereby the protective layer and thick film layer are sufficient to promote increased adhesion between the thick film layer and a nozzle plate attached to the thick film layer thereby substantially reducing a tendency for the nozzle plate and thick film layer to delaminate from one another during printhead manufacture or use and interrupting contact between ink and the exposed metal layers on the chip surface. 
     In another aspect the invention provides a semiconductor chip for an ink jet printhead, the chip having a chip surface, an elongate ink via therein, ink ejectors on the chip surface adjacent the ink via, metal conductive traces attached to the ink ejectors and a protective layer deposited adjacent the ink ejectors. The protective layer substantially circumscribes the ink via and provides an improved seal between a thick film layer and a nozzle plate attached to the thick film layer sufficient to inhibit delamination and ink flow between the thick film layer and the nozzle plate. 
     An important aspect of the invention is that the protective layer extends completely around the ink via region thereby forming a seal “ring” providing a raised surface for improved adhesion of the thick film layer to the nozzle plate. The protective layer is advantageously wide enough to reduce instances of delamination between the nozzle plate and thick film layer and subsequent ink corrosion of exposed metal outside of the seal ring area. Because the seal ring may be deposited by typical semiconductor processing techniques, an improved adhesion between the nozzle plate and the thick film adjacent the ink via area may be provided without resorting to exotic adhesives or other multi-step methods for improving adhesion. Furthermore, the width of the seal ring may be easily adjusted to provide more or less adhesion promotion surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with the drawings, which are not to scale, wherein like reference characters designate like or similar elements throughout the several drawings as follows: 
     FIG. 1 is a plan view, not to scale, of a portion of a conventional printhead chip; 
     FIG. 2 is a cross-sectional view, not to scale, of a portion of a conventional printhead chip through lines  2 — 2  of FIG. 1; 
     FIG. 3 is a cross-sectional view, not to scale, of portion of a conventional printhead chip through lines  3 — 3  of FIG. 1; 
     FIG. 4 is a plan view, not to scale, of a conventional printhead chip; 
     FIG. 5 is a plan view, not to scale, of a printhead chip according to the invention; 
     FIG. 6 is a plan view, not to scale, of a portion of a printhead chip according to the invention; 
     FIG. 7 is a cross-sectional view, not to scale, of a portion of a printhead chip through lines  7 — 7  of FIG. 6; 
     FIG. 8 is a plan view, not to scale, of a portion of a printhead chip according to a second embodiment of the invention; 
     FIG. 9 is a cross-sectional view, not to scale, of a portion of a printhead chip through lines  9 — 9  of FIG. 8; and 
     FIG. 10 is a cross-sectional view, not to scale, of a portion of a printhead chip according to a third embodiment of the invention. 
     FIG. 11 is a cross-sectional view, not to scale, of a portion of a printhead chip and nozzle plate assembly; 
     FIG. 12 is a plan view, not to scale, of a portion of a printhead chip; 
     FIG. 13 is a cross-sectional view, not to scale of a portion of a printhead chip and nozzle plate assembly according to another embodiment of the invention; and 
     FIG. 14 is a plan view, not to scale of a portion of a nozzle plate according to one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Ink jet printheads of the thermal drop on demand type typically use semiconductor silicon chips containing a plurality of insulating, conductive, resistive, passivation and/or cavitation layers which together provide an active layer for ejecting ink. The conductive layers typically include metal materials which are susceptible to ink corrosion. Accordingly, a variety of passivating and insulating layers are applied to the metal layers to reduce or prevent damage to the metal layers caused by ink corrosion. However, not all of the layers are adequately protected from ink as ink may find a path between the nozzle plate and thick film layer applied to the chip surface. FIGS. 1-3 illustrate portions of a prior art printhead chip  10  and FIG. 4 is a plan view of a prior art chip viewed from a nozzle plate side thereof. In FIG. 1, the chip  10  is also viewed from the nozzle plate side thereof while FIGS. 2 and 3 are selected cross-sectional views through portions of the layers on the chip surface. The chip  10  includes a first metal conductive layer  12  which provides an electrical conductive path to both sides of a heater resistor  14 . A second metal conductive layer  16  is electrically connected to one side of the resistor  14  by connections  18  which pass through an insulation or passivation layer  20  disposed between the conductive layers  12  and  16 . A cavitation layer  22  is typically applied to the chip  10  to cover at least the heater resistor  14  and may be extended to cover other portions of the first metal layer  12 . 
     The metal layers  12  and  16  are spaced from an ink via  24  (a portion of which is shown in FIG.  1 ). The ink via  24  is typically a slot which is formed through the chip  10  so as to provide a flow path for ink from an ink reservoir to the surface of the chip  10  containing the heater resistors  14 . For convenience only, the printhead chip will be described as containing a single ink via  24 . However, the invention is applicable to printhead chips containing more than one ink via  24 . 
     The chip  10  is preferably made of a silicon material having a thickness ranging from about 200 to about 800 microns. The insulative, conductive and resistive layers on the surface of the chip  10  preferably have an overall thickness ranging from about 1 micron to about 5 microns, most preferably from about 2 to about 3 microns. Such layers are deposited on the chip surface by conventional semiconductor processing techniques. 
     With regard to depositing the insulative, conductive and resistive layers to provide the printhead chip  10 , generally, the silicon substrate is first insulated with a layer of material which is preferably an oxide layer, most preferably silicon dioxide having a thickness ranging from about 8,000 Angstroms to about 10,000 Angstroms. A phosphorous silicon glass (PSG) or boron impregnated PSG layer having a thickness ranging from about 7,500 to about 9,500 Angstroms may be preferably deposited over the insulating layer. A resistive material of tantalum/aluminum, or tantalum is next deposited on at least a portion of the PSG layer or on the silicon dioxide layer. The resistive material provides the heater resistors  14  which upon activation urge ink to be ejected through the nozzle holes in the nozzle plate attached to the chip. The resistive material preferably has a thickness ranging from about 900 to about 1100 Angstroms. 
     Metal conductive layer  12  made of an aluminum/copper alloy, gold, aluminum and the like is deposited on one or more portions of the resistive layer to provide electrical connection between the resistors  14  and a printer controller. The conductive layer  12  preferably has a thickness ranging from about 5000 to about 6000 Angstroms. 
     In order to protect the conductive layers  12  and resistors  14  from ink corrosion, a passivation layer  20  is preferably deposited over the resistors  14  and the first metal conductive layer  12 . The passivation layer  20  may be a composite layer of silicon nitride and silicon carbide, or may be individual layers of silicon nitride and silicon carbide, respectively. The passivation layer  20  is preferably deposited directly on the first metal conductive layer  12  and the resistors  14 . It is preferred that the silicon carbide layer have a thickness ranging from about 1,200 to about 3,000 Angstroms, most preferably from about 2,600 Angstroms. The silicon nitride layer preferably has a thickness ranging from about 2,600 to about 5,000 Angstroms, most preferably about 4,400 Angstroms. 
     The cavitation layer  22  or additional passivation layer made from a material selected from the group consisting of tantalum, diamond-like carbon, silicon carbide, silicon nitride, titanium, and tantalum nitride is preferably deposited over at least a portion of the passivation layer  20 , most preferably adjacent the heater resistor  14 . The cavitation layer  22  provides protection to the heater resistors  14  during ink ejection operations which could cause mechanical damage to the heater resistors  14  in the absence of the cavitation layer  22 . The cavitation layer  22  is believed to absorb energy from a collapsing ink bubble after ejection of ink from the nozzle holes. The cavitation layer  22  thickness may range from about 2,500 to about 7,000 Angstroms or more, preferably from about 4,000 to about 6,000 Angstroms. 
     An adhesion promotion layer (not shown) may be provided to enhance the adhesion between a thick film layer  26  and the surface of the chip  10  containing the insulative, conductive and resistive layers. A preferred adhesion promotion layer is derived from a silane material provided by a spin-coat process from a solution of the silane material in alcohol. When used, the adhesion promotion layer should be of a thickness sufficient to promote adhesion between the thick film layer  26  and the surface of the chip  10 . 
     The thick film layer  26  is preferably applied to the chip  10  to provide a surface for attachment of a nozzle plate to the chip  10 . The thick film layer  26  may be derived from a radiation and/or heat curable polymeric film material preferably containing a difunctional epoxy material, a polyfunctional epoxy material and suitable cure initiators and catalyst. A particularly preferred thick film layer  26  is a polymeric photoresist material described in U.S. Pat. No. 5,907,333 to Patil et al., the disclosure of which is incorporated herein by reference as if fully set forth. The thick film layer  26  has a thickness ranging from about 2 to about 3 microns. 
     With reference to FIG. 4, a plan view of the chip  10  as viewed from the nozzle plate side thereof is illustrated. Each chip  10  includes a plurality of heater resistors  14  and first metal conductors layer  12  connected to the heater resistors  14 . As seen in FIG. 4, the first metal conductor layer  12  is typically adequately protected by the passivation/cavitation layer  20 / 22 . However, the second metal layer  16  as well as bond pads  28  adjacent longitudinal edges  29  of the chip may be exposed to ink from the ink via  24 . The bond pads  28  are typically not covered by the thick film layer  26  as illustrated in FIG.  3 . Accordingly, if there is poor adhesion or slight delamination between the thick film layer  26  and a nozzle plate attached to the thick film layer  26 , ink may be able to contact and corrode the second metal layer  16  and/or unprotected bond pads  28 . 
     In view of the problems associated with conventional printhead manufacturing techniques, a printhead chip  30  is provided according to one embodiment of the invention as illustrated in FIGS. 5-7. As with a conventional chip  10 , chip  30  also contains the first metal conductive layer  12 , heater resistors  14 , a second metal conductive layer  16 , connections  18  between the metal layers  12  and  16 , passivation layer  20  and cavitation layer  22 . However, unlike a conventional printhead chip  10 , the cavitation layer  22  is extended to provide a ring  32  of cavitation layer material circumscribing the ink via  24 . The ring  32  effectively promotes increased adhesion between the thick film layer  26  and a nozzle plate attached to the thick film layer  26  to reduce instances of delamination between the nozzle plate and thick film layer  26  so that ink is inhibited from contacting the second metal layer  16  and unprotected bond pads  28  adjacent longitudinal edges  29  of the chip. 
     The ring  32  is preferably a raised area provided by the cavitation layer material. Accordingly, the ring  32  may be comprised of a material selected from the group consisting of tantalum, diamond-like-carbon, silicon carbide, silicon nitride, titanium, tantalum nitride, and the like. It is particularly preferred that the ring  32  be the uppermost layer between the passivation layer  20  and the thick film layer  26 . 
     In order to provide the seal ring  32  with sufficient adhesion promoting characteristics, the seal ring width W preferably ranges from about 20 to about 60 microns and may extend as wide as cavitation layer  22  as it circumscribes ink via  24 . The thickness of the seal ring  32  is preferably the same as the thickness of the cavitation layer  22  described above and thus may be formed by extending the cavitation layer  22  during the cavitation layer  22  deposition process. Because the seal ring  32  is deposited on the passivation layer  20 , the seal ring  32  provides a raised topography above the plane of the passivation layer  20  thereby promoting a raised topography of the thick film layer  26  applied to the seal ring  32  and passivation layer  20  as shown in FIG.  7 . While the width of the seal ring  32  preferably ranges from 20 to 60 microns, the seal ring may also be extended toward the ink via  24  so that edge  34  of the seal ring closest to the ink via  24  does not cause the raised topography of the thick film layer  26  to be disposed in a nozzle plate flow feature area as described in more detail below. 
     If it is desired to further increase the height of the seal ring  32  and thick film layer  26  without increasing the thickness of the cavitation layer  22 , a seal promoting layer  35  selected from a third metal layer  36 , a polycrystalline layer  38  or both the third metal layer  36  and the polycrystalline layer  38  may be deposited on the surface of the chip  30  before depositing the passivation layer  20  on the chip as illustrated in FIGS. 8-10. The third metal layer  36  may the same as the metal conductive layer  12 , or may independently be selected from aluminum/copper alloys, gold and aluminum. A preferred metal layer  36  is an aluminum/copper alloy and a preferred polycrystalline layer  38  is polysilicon. 
     Each of the polycrystalline layer  38  and third metal layer  36  may be deposited by conventional semiconductor processing techniques such as sputter, spin coating and the like, followed by etching if necessary to provide the desired shape and width thereof. It is preferred that the third metal layer  36  and/or polycrystalline layer  38  be deposited only in an area which is underneath the seal ring  32 . Accordingly, the third metal layer  36  and polycrystalline layer  38  have a width similar to the width of the seal ring  32 . The thickness of each of the third metal layer  36  and polycrystalline layer  38  preferably ranges from about 3000 Angstroms to about 6000 Angstroms. Accordingly, these layers further increase the height of the seal ring  32  and provide additional sealing area for the thick film layer  26  to the nozzle plate. 
     With reference now to FIGS. 11-13, a nozzle plate  40  is then preferably adhesively attached to the thick film layer  26  to provide a nozzle plate/chip assembly. The nozzle plate  40  may be made of metals or plastics and is preferably made of a polyimide polymer containing an adhesive layer which materials are laser ablated to provide flow features, namely, ink chambers  42 , nozzle holes  44  and ink supply channels  46  therein. 
     The adhesive used to attach the nozzle plate  40  to the thick film layer  26  is any B-stageable material, including some thermoplastics. Examples of B-stageable thermal cure resins include phenolic resins, resorcinol resins, urea resins, epoxy resins, ethylene-urea resins, furane resins, polyurethanes, and silicone resins. Suitable thermoplastic, or hot melt, materials include ethylene-vinyl acetate, ethylene ethylacrylate, polypropylene, polystyrene, polyamides, polyesters and polyurethanes. The adhesive is preferably applied with a thickness ranging from about 5 to about 15 microns and the polyimide has a thickness preferably ranging from about 25 to about 50 microns. In the most preferred embodiment, the adhesive is a phenolic butyral adhesive such as that used in RFLEX R1100 or RFLEX R1000 films, commercially available from Rogers of Chandler, Ariz. 
     As shown in FIG. 11, a portion of the ink feed channel  46  may overlap a region  48  which is adjacent the ink via  24 . The overlap of the ink channel  46  into the ink via region  48  may provide areas of increased susceptibility to delamination between the nozzle plate  40  and the thick film layer  26  as this area is susceptible to forming air pockets during assembly and use of the printheads. However, if the nozzle plate is attached to the chip  30  so that the ink flow channel  46  does not overlap via region  48 , there is decreased likelihood that gaps or air pockets will develop between the nozzle plate and the thick film layer  26  in the area adjacent the ink via region  48 . 
     As an additional provision to enhance the adhesion between the nozzle plate  40  and the thick film layer  26 , vent holes  50  may be formed in the nozzle plate  40  to provide release of trapped air which may cause delamination. A plan view of a portion of a nozzle plate  40  viewed from the chip side thereof is illustrated in FIG. 14 showing the preferred location of vent holes  50  which are formed through the entire thickness of the nozzle plate  40 . The vent holes  50  may be formed by conventional micromachining techniques such as etching, laser ablation and the like. 
     To complete the printhead chip  30 , a flexible circuit or tape automated bonding (TAB) circuit is attached to the nozzle plate/chip assembly to provide a nozzle plate/chip/circuit assembly. The nozzle plate/chip/circuit assembly is preferably adhesively attached to a printhead body portion to provide a printhead for an ink jet printer. The nozzle plate/chip assembly may be attached as by means of a die bond adhesive, preferably a conventional die bond adhesive such as a substantially transparent phenolic polymer adhesive which is commercially available from Emerson &amp; Cuming of Monroe Township, N.J. under the trade name ECCOBOND 3193-17, preferably in a chip pocket of a printhead body portion. The flexible circuit or TAB circuit is adhesively attached to surface of the printhead body portion after attaching the nozzle plate/chip assembly in the chip pocket. 
     It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings, that modifications and changes may be made in the embodiments of the invention. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of preferred embodiments only, not limiting thereto, and that the true spirit and scope of the present invention be determined by reference to the appended claims.