Abstract:
A printhead has a substrate with a first surface, an opposite substantially parallel second surface, and an edge surface extending from the first surface to the second surface. The substrate further includes a fluid channel that extends from the edge surface inward away from the edge surface and through a recess in the first surface. The printhead further has a heating element disposed over the first surface, and a firing chamber disposed over the heating element. The fluid channel directs fluid to the firing chamber through the recess, where the fluid is heated by the heating element and ejected therefrom.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to inkjet printhead fabrication processes and more particularly to methods for fabricating fully integrated inkjet printheads on a substrate. 
     There are known and available commercial printing devices such as computer printers, graphics plotters and facsimile machines which employ inkjet technology, such as inkjet pens. An inkjet pen typically includes an ink reservoir and an array of inkjet printing elements. The array is formed by an inkjet printhead. Each printing element includes a nozzle chamber, a firing resistor and a nozzle opening. Ink is stored in the reservoir and passively loaded into respective firing chambers of the printhead via an ink refill channel and respective ink feed channels. Capillary action moves the ink from the reservoir through the refill channel and ink feed channels into the respective firing chambers. Printer control circuitry outputs respective signals to the printing elements to activate corresponding firing resistors. In response an activated firing resistor heats ink within the surrounding nozzle chamber causing an expanding vapor bubble to form. The bubble forces ink from the nozzle chamber out the nozzle opening. An orifice plate adjacent to the barrier layer defines the nozzle openings. The geometry of the nozzle chamber, ink feed channel and nozzle opening defines how quickly a corresponding nozzle chamber is refilled after firing. 
     To achieve high quality printing ink drops or dots are accurately placed at desired locations at designed resolutions. Printing at resolutions of 300 dots per inch and 600 dots per inch is known. Higher resolutions also are being sought. 
     A monolithic structure for an inkjet printhead is described in copending U.S. patent application Ser. No. 08/597,746 filed Feb. 7, 1996 for “Solid State Ink Jet Print Head and Method of Manufacture.” The process described therein includes photoimaging techniques similar to those used in semiconductor device manufacturing. The printing elements of a monolithic printhead are formed by applying layers to a silicon die. The firing resistors, wiring lines and nozzle chambers are formed by applying various passivation, insulation, resistive and conductive layers on the silicon die. Such layers are referred to collectively as a thin film structure. An orifice plate overlays the thin film structure opposite the die. Nozzle openings are formed in the orifice plate in alignment with the nozzle chambers and firing resistors. The geometry of the orifice openings affect the size, trajectory and speed of ink drop ejection. Orifice plates often are formed of nickel and fabricated by lithographic and electroforming processes. 
     SUMMARY 
     In one embodiment of the present invention, a printhead has a substrate with a first surface, an opposite substantially parallel second surface, and an edge surface extending from the first surface to the second surface. The substrate further includes a fluid channel that extends from the edge surface inward away from the edge surface and through a recess in the first surface. The printhead further has a heating element disposed over the first surface, and a firing chamber disposed over the heating element. The fluid channel directs fluid to the firing chamber through the recess, where the fluid is heated by the heating element and ejected therefrom. 
     These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an inkjet pen having a printhead fabricated according to an embodiment of this invention; 
     FIG. 2 is a block diagram of an embodiment of the inkjet printhead; 
     FIG. 3 is a partial cross-sectional view of an embodiment of the inkjet printhead fabricated according to one methodology of this invention; 
     FIG. 4 is a partial plan view of one embodiment of a die having a patterned layer of field oxide; 
     FIG. 5 is a cross-sectional view taken along line V—V of FIG. 4; 
     FIG. 6 is a partial plan view of a printhead in one process embodiment with the thin film structure layers applied and patterned; 
     FIG. 7 is a cross-sectional view along line VII—VII of FIG. 6; 
     FIG. 8 is a cross-sectional view along line VIII—VIII of FIG. 6; 
     FIG. 9 is a partial plan view of a printhead in another process embodiment with the feed channel and fill channels etched out of the die; 
     FIG. 10 is a cross-sectional view along line X—X of FIG. 9; 
     FIG. 11 is a cross-sectional view along line XI—XI of FIG. 9; 
     FIG. 12 is a partial cross-sectional view of a printhead in process with filler material added to the structure of FIG. 9; 
     FIG. 13 is a partial cross-sectional view of a printhead in process after polishing and a plasma etching the structure of FIG. 12; 
     FIG. 14 is another partial cross-sectional view of a printhead in process after polishing and a plasma etching the structure of FIG. 12; 
     FIG. 15 is a partial cross-sectional view of a printhead in process after applying a sacrificial mandrel to the structure of FIGS. 13 and 14; 
     FIG. 16 is a partial cross-sectional view of a printhead in process after applying an orifice plate around the sacrificial mandrel of FIG. 15; and 
     FIG. 17 a partial cross-sectional view of a completed printhead with the sacrificial mandrel of FIG.  15  and filler material removed. 
    
    
     DETAILED DESCRIPTION 
     Overview 
     FIG. 1 shows a scanning-type thermal inkjet pen  10  according to an embodiment of this invention. The pen  10  is formed by a pen body  12 , an internal reservoir  14  and a printhead  16 . The pen body  12  serves as a housing for the reservoir  14 . The reservoir  14  is for storing ink to be ejected from the printhead  16  onto a media sheet. The printhead  16  defines an array  22  of printing elements  18  (i.e., nozzle array). The nozzle array  22  is formed on a die. The reservoir  14  is in physical communication with the nozzle array enabling ink to flow from the reservoir  14  into the printing elements  18 . Ink is ejected from a printing element  18  through an opening toward a media sheet to form dots on the media sheet. 
     The openings are formed in an orifice layer. In one embodiment the orifice layer is a plate attached to the underlying layers. In another embodiment the orifice layer is formed integrally with the underlying layers. In an exemplary embodiment of a printhead having an orifice plate, openings also are formed in a flex circuit  20 . The flex circuit  20  is a printed circuit made of a flexible base material having multiple conductive paths and a peripheral connector. Conductive paths run from the peripheral connector to the nozzle array  22 . The flex circuit  20  is formed from a base material made of polyimide or other flexible polymer material (e.g., polyester, poly-methyl-methacrylate) and conductive paths made of copper, gold or other conductive material. The flex circuit  20  with only the base material and conductive paths is available from the 3M Company of Minneapolis, Minn. The nozzle openings and peripheral connector then are added. The flex circuit  20  is coupled to off-circuit printer control electronics via an edge connector or button connector. Windows  17 ,  19  within the flex circuit  20  facilitate mounting of the printhead  16  to the pen  10 . During operation signals are received from the printer control circuitry and activate select printing elements  18  to eject ink at specific times causing a pattern of dots to be output onto a media sheet. The pattern of dots forms a desired symbol, character or graphic. 
     Although a scanning-type inkjet pen is shown in FIG. 1, the fabrication processes for the printhead  16  to be described below also apply to printheads for a wide-array printhead, such as a non-scanning page-wide array printhead. 
     As shown in FIG. 2, the printhead  16  includes multiple rows of printing elements  18 . In the embodiment shown two rows  22 ,  24  form one set of rows  21 , while another two rows  22 ,  24  form another set of rows  23 . In alternative embodiments fewer or more rows are included. Associated with each printing element  18  is a driver for generating the current level to achieve the desired power levels for heating the element&#39;s firing resistor (or heating element). Also included is logic circuitry for selecting which printing element is active at a given time. Driver arrays  43  and logic arrays  44  are depicted in block format. The firing resistor (or heating element) of a given printing element is connected to a driver by a wiring line. Also included in the printhead  16  are contacts pad arrays  46  for electrically coupling the integrated portion of the printhead to a flex circuit or to off-pen circuitry. 
     FIG. 3 shows a printing element  18  of a printhead  16 . The printhead includes a silicon die  25 , a thin film structure  27  and an orifice layer  30 . The silicon die  25  provides rigidity and in effect serves as a chassis for other portions of the printhead  16 . An ink feed channel  29  is formed in the die  25 . In one embodiment an ink feed channel  29  is formed for each printing element  18 . The thin film structure  27  is formed on the die  25 , and includes various passivation, insulation and conductive layers. A firing resistor  26  and conductive traces  28  (see FIGS. 9 and 17) are formed in the thin film structure  27  for each printing element  18 . The orifice layer  30  is formed on the thin film structure  27  opposite the die  25 . The orifice layer  30  has an exterior surface  34  which during operation faces a media sheet on which ink is to be printed. The orifice layer is either an integral layer formed with the thin film structure  27  or is a plate overlaid on the thin film structure. In some embodiments the flex circuit  20  overlays the orifice layer  30 . Nozzle chambers  36  and nozzle openings  38  are formed in the orifice layer  30 . 
     Each printing element  18  includes a firing resistor  26 , a nozzle chamber  36 , a nozzle opening  38 , and one or more fill channels  40 . A center point of the firing resistor  26  defines a normal axis about which components of the printing element  18  are aligned. Specifically it is preferred that the firing resistor  26  be centered within the nozzle chamber  36  and be aligned with the nozzle opening  38 . The nozzle chamber  36  in one embodiment is frustoconical in shape. One or more fill channels  40  or vias are formed in the thin film structure  27  to couple the nozzle chamber  36  to the feed channel  29 . The fill channels  40  are encircled by the nozzle chamber lower periphery  43  so that the ink flowing through a given fill channel  40  flows exclusively into a corresponding nozzle chamber  36 . 
     In one embodiment there is one feed channel  29  for each printing element  18 . The feed channels  29  for a given set of rows  21  or  23  receive ink from a refill channel that is adjacent an interface between the substrate and the reservoir  14  (not shown). In an edge feed construction there is a refill channel  101  on each of two opposing side edges of the printhead. The feed channels  29  from one set of printing elements  21  are in communication with one refill channel, while the feed channels  29  from the other set of printing elements  23  are in communication with the other refill channel. In a center feed construction, there is a refill channel trough in communication with the feed channels. Such refill channel trough serves both sets of printing elements  21 ,  23 . In one embodiment, the trough receives ink from a pen cartridge reservoir at an edge of the printhead. Thus, in the embodiments described the refill channel  101  does not extend through to the bottom surface  55  of the die  25 . 
     In an exemplary embodiment, the die  25  is a silicon die approximately 675 microns thick. Glass or a stable polymer are used in place of the silicon in alternative embodiments. The thin film structure  27  is formed by one or more passivation or insulation layers formed by silicon dioxide, silicon carbide, silicon nitride, tantalum, poly silicon glass, or another suitable material. The thin film structure also includes a conductive layer for defining the firing resistor and for defining the conductive traces. The conductive layer is formed by tantalum, tantalum-aluminum or another metal or metal alloy. In an exemplary embodiment the thin film structure is approximately 3 microns thick. The orifice layer  30  has a thickness of approximately 10 to 30 microns. The nozzle opening  38  has a diameter of approximately 10-30 microns. In an exemplary embodiment the firing resistor  26  is approximately square with a length on each side of approximately 10-30 microns. The base surface  43  of the nozzle chamber  36  supporting the firing resistor  26  has a diameter approximately twice the length of the resistor  26 . In one embodiment an anisotropic silicon etch defines 54° wall angles for the feed slot  29 . Although exemplary dimensions and angles are given, such dimensions and angles mary vary for alternative embodiments. 
     Single-Side Fabrication 
     For naming convention purposes the die  25  has two sides, a top side  19  and a bottom side  55 . The top side defines a top surface and the bottom side defines a bottom surface. For a rectilinear die  25 , the die  25  also includes four edges extending between the top side and bottom side. The shape and number of edges of the die may vary in alternative embodiments. According to the invention, a monolithic inkjet printhead  16  is formed with fabrication processes acting from a single side of the substrate. In some embodiments the fabrication processes also act from an edge during at least one step of the fabrication. According to the invention, however, the fabrication processes need not act from the bottom side of the die  25 . The term substrate as used herein refers to the in-process structure of the die  25  and thin film structure  27 , and when present, the orifice layer  30 . 
     Starting with a planar die  25 , a layer of field oxide  31  is applied (e.g., grown) to a first side  19 . The field oxide layer  25  then is masked and etched as shown in FIGS. 4 and 5 to delimit areas  33  for respective feed channels. In addition a membrane region  39  is formed within each feed channel area  33 . The feed channel area  33  extends from an edge  35  of the die  25  toward an opposite edge  37 . Once the feed channel is etched in the area  33  at a later stage, the feed channel  29  will extend from the side edge  35  toward the opposite edge  37 . The resulting printhead is to be an edge feed printhead with ink entering the feed channel  29  from the reservoir  14  at the edge  35  (see FIG.  3 ). A shelf is formed at the edge and serves as the refill channel  101 . 
     The membrane region  39  occurs within the feed channel area  33  and marks regions of the field oxide to remain overlaying the corresponding feed channel  29 . At this stage in the fabrication there is no feed channel etched into the die  25 , just an area  33  delimited by the field oxide layer  31 . 
     The field oxide is a first layer of the thin film structure  27 . With the field oxide layer  31  patterned as desired, additional layers of the thin film structure  27  are applied to the same side  19  of the die  25  having the field oxide  31 . The additional layers are patterned to form firing resistors  26 , wiring lines  28  and passivation  45  as shown in FIGS. 6-8. Deposition, masking and etching processes as known in the art are used to apply and pattern the firing resistors  26 , wiring lines  28  and passivation material  45 . In one embodiment the firing resistors  26  are formed of tantalum-aluminum and the wiring lines  28  are formed of aluminum. In another embodiment different or additional conductive metals, alloys or stacks of metals and/or alloys are used. FIG. 6 shows a plan view of a portion of the printhead  16 . The entire surface of the substrate is covered with passivation material  45  other than the areas labeled as the die  25 . In FIG. 6 the wiring lines  28  and firing resistor  26  are shown hidden underlying the passivation layer  45 . At this stage of the fabrication, the feed channel  29  still has not been etched in the area  33 . 
     With the firing resistors  26  and wiring lines  28  patterned, the next step is to etch the feed channel  29  and the fill channels  40 . An etchant is applied to the top side  19 . The die  25  is etched using tetra-methyl ammonium hydroxide, potassium hydroxide or another anisotropic silicon etchant which acts upon the exposed die  25  regions and not upon the passivation  45 . In one embodiment the etchant works upon the &lt;100&gt; plane of the silicon die to etch the silicon at an angle. The etching process continues with the silicon etched away downward at an angle until the angled lines intersect at a given depth. The result is a triangular trench for the feed channel  29  as shown in FIGS. 9-11. At this stage a trench has been created in the die  25  using a process acting from the top side  19  of the die  25 . The trench defines the feed channel  29 . 
     At this stage of the fabrication the feed channels  29 , the fill channels  40 , the firing resistors  26  and the wiring lines  28  have been formed, but the nozzle chambers  36  (see FIG. 3) have not yet been formed. The nozzle chambers  36  are to be formed with an orifice plate, with an orifice film or by direct imaging. For any of such methods the presence of the feed channel  29  and fill channels  40  can adversely impact the formation of the nozzle chambers  36  due to the varied topography introduced by such voids. Such voids are filed up to enable continued processing from the top surface. Thus, according to an aspect of this invention, a material  50  of photoresist or polyimide is spun and baked onto the substrate as shown in FIG.  12 . The material  50  fills in the feed channel  29  and fill channels  40  and covers the passivation layer  45 . Next, a chemical-mechanical polishing process is applied to the substrate to remove the material  50  in areas other than the feed channels  29  and fill channels  40 , as shown in FIGS. 13 and 14. In one embodiment an O 2  plasma etch also is performed so that the filler material  50  is removed without removing the passivation material  45 . The result is a planar surface with bumps of passivation material  45  over the firing resistors  26  (see FIGS.  13  and  14 ). The top side  19  of the substrate now has areas of passivation material  45  and filler material  50 . At this stage of the fabrication the substrate is ready for processes to form the nozzle chambers  36 . 
     In one embodiment (see FIG. 15) a frustoconical sacrificial mandrel  52  is formed over each resistor  26  in the shape of the desired nozzle chamber. Such sacrificial mandrel  52  is formed by depositing a suitable material, such as photoresist or polyimide, then patterning and etching the material to the desired shape. Next an orifice layer  30  is applied as shown in FIG. 16 to a thickness flush with the sacrificial mandrel  52 . In one embodiment the orifice layer is applied by an electroplating process, in which the substrate is dipped into an electroplating tank. Material (e.g., nickel, gold) forms on the substrate around the sacrificial mandrel  52 . Other deposition processes also may be used, but may be accompanied by an additional polishing step to level the layer  30  to the sacrificial mandrel  52 . Next, the sacrificial mandrel  52  is etched or dissolved away from the orifice layer  30 , leaving the remaining nozzle chamber  36  as shown in FIG.  17 . In the same step or in another etching step, the filler material  50  is etched out of the fill channels  40  and the feed channels  29  resulting in a printhead  16  as shown in FIGS. 3 and 17. The filler material  50  is etched from the top side  19  of the substrate or from the top side  19  and the edge fill side  35  of the substrate. For either case, the fabrication processes do not act from the bottom surface  55  (see FIGS. 3 and 17) opposite side  19 . 
     Although the nozzle chambers  36  are described as being formed by applying a sacrificial mandrel and orifice layer then etching out the sacrificial mandrel, other processes also may be used. In one alternative embodiment, an orifice film is applied to the substrate as the substrate appears in FIG.  14 . Patterning and etching processes then are performed to define the nozzle chamber  36 . An etching process as described above then is performed to remove the filler material  50  from the feed channel(s)  29  and fill channels  40 . In still another embodiment material is spun onto the substrate, masked and exposed to form the nozzle chambers  36 . Again an etching process as described above is performed afterward to remove the filler material  50  from the feed channels  29  and fill channels  40 . 
     Upon completion there is a printhead  16  without any ink channel openings in the bottom surface of the bottom side  55 . More specifically, no portion of the bottom side  55  has been removed for ink channel openings. 
     Although preferred embodiments of the invention have been illustrated and described, various alternatives, modifications and equivalents may be used. Therefore, the foregoing description should not be taken as limiting the scope of the inventions which are defined by the appended claims.