Patent Publication Number: US-6336714-B1

Title: Fully integrated thermal inkjet printhead having thin film layer shelf

Description:
This is a continuation-in-part of U.S. Application Ser. No. 09/033,504, filed Mar. 2, 1998, now U.S. Pat. No. 6,126,276, entitled, “Fluid Jet Printhead With Integrated Heat Sink, ” by Colin Davis et al., a continuation-in-part of U.S. Patent Application Ser. No. 09/314,551, May 19, 1999, entitled, “Solid State Ink Jet Printhead And Method Of Manufacture, ” by Timothy Weber et al., which is a continuation of U.S. Patent Application Ser. No. 08/597,746, filed Feb. 7, 1996, now U.S. Patent No. 6,000,787, and a continuation-in-part of U.S. Patent Application Ser. No. 09/033,987 filed Mar. 2, 1998, now U.S. Patent No. 6,162,589, entitled “Direct Imaging Polymer Fluid Jet Orifice, ” by Chien-Hua Chen, Naoto Kamamura et al. These applications are assigned to the present assignee and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to inkjet printers and, more particularly, to a monolithic printhead for an inkjet printer. 
     BACKGROUND 
     Inkjet printers typically have a printhead mounted on a carriage that scans back and forth across the width of a sheet of paper feeding through the printer. Ink from an ink reservoir, either on-board the carriage or external to the carriage, is fed to ink ejection chambers on the printhead. Each ink ejection chamber contains an ink ejection element, such as a heater resistor or a piezoelectric element, which is independently addressable. Energizing an ink ejection element causes a droplet of ink to be ejected through a nozzle for creating a small dot on the medium. The pattern of dots created forms an image or text. 
     As dot resolutions (dots per inch) increase along with the firing frequencies, more heat is generated by the firing elements. This heat needs to be dissipated. Heat is dissipated by a combination of the ink being ejected and the printhead substrate sinking heat from the ink ejection elements. The substrate may even be cooled by the supply of ink flowing to the printhead. Additional information regarding one particular type of printhead and inkjet printer is found in U.S. Pat. No. 5,648,806, entitled, “Stable Substrate Structure For A Wide Swath Nozzle Array In A High Resolution Inkjet Printer,” by Steven Steinfield et al., assigned to the present assignee and incorporated herein by reference. 
     As the resolutions and printing speeds of printheads increase to meet the demanding needs of the consumer market, new printhead manufacturing techniques and structures are required. Hence, there is a need for an improved printhead that has at least the following properties: adequately sinks heat from the ink ejection elements at high operating frequencies; provides an adequate refill speed of the ink ejection chambers with minimum blowback; minimizes cross-talk between nearby ink ejection chambers; is tolerant to particles within the ink; provides a high printing resolution; enables precise alignment of the nozzles and ink ejection chambers; provides a precise and predictable drop trajectory; is relatively easy and inexpensive to manufacture; and is reliable. 
     SUMMARY 
     Described herein is a monolithic printhead formed using integrated circuit techniques. Thin film layers, including a resistive layer, are formed on a top surface of a silicon substrate. The various layers are etched to provide conductive leads to the heater resistor elements. Piezoelectric elements may be used instead of the resistive elements. An optional thermally conductive layer below the heater resistors sinks heat from the heater resistors and transfers the heat to a combination of the silicon substrate and the ink. 
     At least one ink feed hole is formed through the thin film layers for each ink ejection chamber. 
     A trench is etched in the bottom surface of the substrate so that ink can flow into the trench and into each ink ejection chamber through the ink feed holes formed in the thin film layers. The trench completely etches away portions of the substrate near the ink feed holes so that the thin film layers form a shelf in the vicinity of the ink feed holes. In one embodiment, the shelf supports the ink ejection elements. 
     An orifice layer is formed on the top surface of the thin film layers to define the nozzles and ink ejection chambers. In one embodiment, a photodefinable material is used to form the orifice layer. 
     Various thin film structures are described as well as various ink feed arrangements and orifice layers. 
     The resulting fully integrated thermal inkjet printhead can be manufactured to a very precise tolerance since the entire structure is monolithic, meeting the needs for the next generation of printheads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of one embodiment of a print cartridge that may incorporate any one of the printheads described herein. 
     FIG. 2 is a perspective cutaway view of a portion of one embodiment of a printhead in accordance with the present invention. 
     FIG. 3 is a perspective view of the underside of the printhead shown in FIG.  2 . 
     FIG. 4 is a cross-sectional view along line  4 — 4  in FIG.  2 . 
     FIG. 5 is a top-down view of the printhead of FIG. 2 with a transparent orifice layer. 
     FIG. 6 is a top-down view of a portion of an alternative embodiment printhead. 
     FIG. 7 is a perspective cutaway view taken along line  7 - 7  in FIG.  6 . 
     FIG. 8 is a cross-sectional view taken along line  8 — 8  in FIG.  7 . 
     FIG. 9 is a top-down view showing in greater detail a portion of a single ink ejection chamber in the printhead embodiment of FIG.  8 . 
     FIGS. 10A-10F are cross-sectional views of the printhead of FIG. 8 during various stages of the manufacturing process. 
     FIG. 11 is a cross-sectional view of a second alternative embodiment of a printhead. 
     FIG. 12 is a perspective view of a conventional inkjet printer into which the printheads of the present invention may be installed for printing on a medium. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is a perspective view of one type of inkjet print cartridge  10  which may incorporate the printhead structures of the present invention. The print cartridge  10  of FIG. 1 is the type that contains a substantial quantity of ink within its body  12 , but another suitable print cartridge may be the type that receives ink from an external ink supply either mounted on the printhead or connected to the printhead via a tube. 
     The ink is supplied to a printhead  14 . Printhead  14 , to be described in detail later, channels the ink into ink ejection chambers, each chamber containing an ink ejection element. Electrical signals are provided to contacts  16  to individually energize the ink ejection elements to eject a droplet of ink through an associated nozzle  18 . The structure and operation of conventional print cartridges are very well known. 
     The present invention relates to the printhead portion of a print cartridge, or a printhead that can be permanently installed in a printer, and, thus, is independent of the ink delivery system that provides ink to the printhead. The invention is also independent of the particular printer into which the printhead is incorporated. 
     FIG. 2 is a cross-sectional view of a portion of the printhead of FIG. 1 taken along line  2 — 2  in FIG.  1 . Although a printhead may have 300 or more nozzles and associated ink ejection chambers, detail of only a single ink ejection chamber need be described in order to understand the invention. It should also be understood by those skilled in the art that many printheads are formed on a single silicon wafer and then separated from one another using conventional techniques. 
     In FIG. 2, a silicon substrate  20  has formed on it various thin film layers  22 , to be described in detail later. The thin film layers  22  include a resistive layer for forming resistors  24 . Other thin film layers perform various functions, such as providing electrical insulation from the substrate  20 , providing a thermally conductive path from the heater resistor elements to the substrate  20 , and providing electrical conductors to the resistor elements. One electrical conductor  25  is shown leading to one end of a resistor  24 . A similar conductor leads to the other end of the resistor  24 . In an actual embodiment, the resistors and conductors in a chamber would be obscured by overlying layers. 
     Ink feed holes  26  are formed completely through the thin film layers  22 . 
     An orifice layer  28  is deposited over the surface of the thin film layers  22  and etched to form ink ejection chambers  30 , one chamber per resistor  24 . A manifold  32  is also formed in the orifice layer  28  for providing a common ink channel for a row of ink ejection chambers  30 . The inside edge of the manifold  32  is shown by a dashed line  33 . Nozzles  34  may be formed by laser ablation using a mask and conventional photolithography techniques. 
     The silicon substrate  20  is etched to form a trench  36  extending along the length of the row of ink feed holes  26  so that ink  38  from an ink reservoir may enter the ink feed holes  26  for supplying ink to the ink ejection chambers  30 . 
     In one embodiment, each printhead is approximately one-half inch long and contains two offset rows of nozzles, each row containing 150 nozzles for a total of 300 nozzles per printhead. The printhead can thus print at a single pass resolution of 600 dots per inch (dpi) along the direction of the nozzle rows or print at a greater resolution in multiple passes. Greater resolutions may also be printed along the scan direction of the printhead. Resolutions of 1200 or greater dpi may be obtained using the present invention. 
     In operation, an electrical signal is provided to heater resistance  24 , which vaporizes a portion of the ink to form a bubble within an ink ejection chamber  30 . The bubble propels an ink droplet through an associated nozzle  34  onto a medium. The ink ejection chamber is then refilled by capillary action. 
     FIG. 3 is a perspective view of the underside of the printhead of FIG. 2 showing trench  36  and ink feed holes  26 . In the particular embodiment of FIG. 3, a single trench  36  provides access to two rows of ink feed holes  26 . 
     In one embodiment, the size of each ink feed hole  26  is smaller than the size of a nozzle  34  so that particles in the ink will be filtered by the ink feed holes  26  and will not clog a nozzle  34 . The clogging of an ink feed hole  26  will have little effect on the refill speed of a chamber  30  since there are multiple ink feed holes  26  supplying ink to each chamber  30 . In one embodiment, there are more ink feed holes  26  than ink ejection chambers  30 . 
     FIG. 4 is a cross-sectional view along line  4 — 4  of FIG.  2 . FIG. 4 shows the individual thin film layers. In the particular embodiment of FIG. 4, the portion of the silicon substrate  20  shown is about 10 microns thick. This portion is referred to as the bridge. The bulk silicon is about 675 microns thick. 
     A field oxide layer  40 , having a thickness of 1.2 microns, is formed over silicon substrate  20  using conventional techniques. A phosphosilicate glass (PSG) layer  42 , having a thickness of 0.5 microns, is then applied over the layer of oxide  40 . 
     A boron PSG or boron TEOS (BTEOS) layer may be used instead of layer  42  but etched in a manner similar to the etching of layer  42 . 
     A resistive layer of, for example, tantalum aluminum (TaAl), having a thickness of 0.1 microns, is then formed over the PSG layer  42 . Other known resistive layers can also be used. The resistive layer, when etched, forms resistors  24 . The PSG and oxide layers,  42  and  40 , provide electrical insulation between the resistors  24  and substrate  20 , provide an etch stop when etching substrate  20 , and provide a mechanical support for the overhang portion  45 . The PSG and oxide layers also insulate polysilicon gates of transistors (not shown) used to couple energization signals to the resistors  24 . 
     It is difficult to perfectly align the backside mask (for forming trench  36 ) with the ink feed holes  26 . Thus, the manufacturing process is designed to provide a variable overhang portion  45  rather than risk having the substrate  20  interfere with the ink feed holes  26 . 
     Not shown in FIG. 4, but shown in FIG. 2, is a patterned metal layer, such as an aluminum-copper alloy, overlying the resistive layer for providing an electrical connection to the resistors. Traces are etched into the AlCu and TaAl to define a first resistor dimension (e.g., a width). A second resistor dimension (e.g., a length) is defined by etching the AlCu layer to cause a resistive portion to be contacted by AlCu traces at two ends. This technique of forming resistors and electrical conductors is well known in the art. 
     Over the resistors  24  and AlCu metal layer is formed a silicon nitride (Si 3 N 4 ) layer  46 , having a thickness of 0.5 microns. This layer provides insulation and passivation. Prior to the nitride layer  46  being deposited, the PSG layer  42  is etched to pull back the PSG layer  42  from the ink feed hole  26  so as not to be in contact with any ink. This is important because the PSG layer  42  is vulnerable to certain inks and the etchant used to form trench  36 . 
     Etching back a layer to protect the layer from ink may also apply to the polysilicon and metal layers in the printhead. 
     Over the nitride layer  46  is formed a layer  48  of silicon carbide (SiC), having a thickness of 0.25 microns, to provide additional insulation and passivation. The nitride layer  46  and carbide layer  48  now protect the PSG layer  42  from the ink and etchant. Other dielectric layers may be used instead of nitride and carbide. 
     The carbide layer  48  and nitride layer  46  are etched to expose portions of the AlCu traces for contact to subsequently formed ground lines (out of the field of FIG.  4 ). 
     On top of the carbide layer  48  is formed an adhesive layer  50  of tantalum (Ta), having a thickness of 0.6 microns. The tantalum also functions as a bubble cavitation barrier over the resistor elements. This layer  50  contacts the AlCu conductive traces through the openings in the nitride/carbide layers. 
     Gold (not shown) is deposited over the tantalum layer  50  and etched to form ground lines electrically connected to certain ones of the AlCu traces. Such conductors may be conventional. 
     The AlCu and gold conductors may be coupled to transistors formed on the substrate surface. Such transistors are described in U.S. Pat. No. 5,648,806, previously mentioned. The conductors may terminate at electrodes along edges of the substrate  20 . 
     A flexible circuit (not shown) has conductors which are bonded to the electrodes on the substrate  20  and terminate in contact pads  16  (FIG. 1) for electrical connection to the printer. 
     The ink feed holes  26  are formed by etching through the thin film layers. In one embodiment, a single feed hole mask is used. In another embodiment, several masking and etching steps are used as the various thin film layers are formed. 
     The orifice layer  28  is then deposited and formed, followed by the etching of the trench  36 . In another embodiment, the trench etch is conducted before the orifice layer fabrication. The orifice layer  28  may be formed of a spun-on epoxy called SU 8 . The orifice layer in one embodiment is about 20 microns. 
     A backside metal may be deposited if necessary to better conduct heat from substrate  20  to the ink. 
     FIG. 5 is a top-down view of the structure of FIG.  2 . The dimensions of the elements may be as follows: ink feed holes  26  are 10 microns×20 microns; ink ejection chambers  30  are 20 microns×40 microns; nozzles  34  have a diameter of 16 microns; heater resistors  24  are 15 microns×15 microns; and manifold  32  has a width of about 20 microns. The dimensions will vary depending on the ink used, the operating temperature, the printing speed, the desired resolution, and other factors. 
     FIG. 6 is a top-down view of a portion of an alternative embodiment printhead. In this printhead, there is no ink manifold. Ink to each ink ejection chamber is provided by two dedicated ink feed holes. Other views of this printhead are shown in FIGS. 7,  8 , and  9 . In the embodiment shown, there are twice as many ink feed holes as heater resistors. In another embodiment, there are one or more dedicated ink feed holes for each chamber. 
     In FIG. 6, the outline of an ink ejection chamber  60  is shown along with a heater resistor  62 , a nozzle  64  (with the smaller diameter of the nozzle shown in dashed outline), and ink feed holes  66  and  67 . Ink feed holes  66  and  67  are designed to be smaller than nozzle  64  so as to filter any particles before reaching chamber  60 . If a particle clogs one ink feed hole, the size of the other ink feed hole is adequate to refill chamber  60  at close to the operating frequency. 
     FIG. 7 is a cross-sectional perspective view along line  7 — 7  in FIG. 6 illustrating a single ink ejection chamber  60 . 
     In FIG. 7, a silicon substrate  70  has formed on it a plurality of thin film layers  72  (to be identified in FIG.  8 ), including a resistive layer and an AlCu layer that are etched to form the heater resistors  62 . AlCu conductors  63  are shown leading to the resistors  62 . 
     Ink feed holes  67  are formed through the thin film layers  72  to extend to the surface of the silicon substrate  70 . An orifice layer  74  is then formed over the thin film layers  72  to define ink ejection chambers  60  and nozzles  64 . The silicon substrate  70  is etched to form a trench  76  extending the length of the row of ink ejection chambers. The trench  76  may be formed prior to the orifice layer. Ink  78  from an ink reservoir is shown flowing into trench  76 , through ink feed hole  67 , and into chamber  60 . 
     FIG. 8 is a cross-sectional view along line  8 — 8  in FIG. 7 showing one-half of chamber  60 . The other half is symmetrical with FIG.  8 . Unlike the first embodiment, where a portion of the silicon substrate  20  was located directly beneath the heater resistors to sink heat from the resistors, the structure of FIG. 8 uses a metal layer beneath the heater resistors to draw heat away from the resistors and transfer the heat to the substrate and to the ink itself. 
     An insulating layer of field oxide  90 , having a thickness of 1.2 microns, is formed over the silicon substrate  70  (FIG. 7) prior to the trench  76  being formed. The portion of the printhead in FIG. 8 is shown after the trench  76  is formed so the substrate  70  is not shown in the field of view. 
     A PSG layer  92  having a thickness of 0.5 microns is then deposited over oxide  90 . As described with respect to FIG. 4, the oxide and PSG layers provide electrical insulation and thermal conductivity between the heater resistor and the underlying conductive layers, as well as provide increased mechanical support of the bridge extending between the remaining silicon substrate portions after the trench  76  is etched. Also, as previously mentioned, the PSG layer  92  is pulled back from the ink feed hole  67  to prevent contact with the ink which would otherwise react with the PSG. 
     Formed over the PSG layer  92  is a resistive layer of tantalum aluminum, having a thickness of 0.1 microns. An AlCu layer (not shown) is formed over the TaAl layer. The TaAl layer and AlCu layer are etched as previously described to form the various heater resistors  62  and conductors  63  (FIG.  7 ). 
     A layer of nitride  96 , having a thickness of 0.5 microns, is then formed over the resistors  62  and AlCu conductors, followed by a layer of silicon carbide  98 , having a thickness of 0.25 microns. The nitride/carbide layers are etched to expose portions of the AlCu conductors. 
     An adhesive layer  100  of tantalum, having a thickness of 0.6 microns, is then deposited, followed by a conductive layer of gold. Both layers are then etched to form gold conductors electrically contacting certain AlCu conductors leading to heater resistors  62  and ultimately terminating in bonding pads along edges of the substrate. In one embodiment, the gold conductors are ground lines. 
     The ink feed holes  67  are then etched through the thin film layers (or patterned during fabrication of the thin film layers). The orifice layer  74  is deposited and etched to form chambers  60  and nozzles  64 . Nozzles  64  may also be formed by laser ablation. 
     The back side of the substrate  70  (FIG. 7) is then masked and etched using a TMAH etch to form the trench  76 , extending the length of a row of ink ejection chambers  60 . Any one of several etch techniques could be used, wet or dry. Examples of dry etches include XeF 2  and SiF 6 . Examples of appropriate wet etches include Ethylene Diamine Pyrocatechol (EDP), Potassium Hydroxide (KOH), and TMAH. Other etches may also be used. Any one of these or a combination thereof could be used for this application. 
     The trench  76  may have a width of approximately one ink ejection chamber or may have a width that encompasses multiple rows of ink ejection chambers. The trench may be formed at any time during the fabrication process. 
     After the trench  76  is formed, an adhesion layer  101  of tantalum (Ta), having a thickness of 0.1 microns, is formed on the back side of the wafer overlying the field oxide  90 . A heat conducting layer  102  of, for example, gold (Au), having a thickness of 1.5 microns, is then formed over the adhesion layer  101 . Another adhesion layer  103  of tantalum, having a thickness of 0.1 microns, is then formed over the heat conducting layer  102 . 
     FIG. 9 is a top-down view of one-half of an ink ejection chamber  60  in the printhead of FIG.  6 . FIG. 9 illustrates the etching of the various layers and is to be taken in conjunction with FIG.  8 . Starting with the ink feed hole  67 , the oxide and passivation layers  90 ,  96 , and  98  form a shelf approximately 2 microns long. The shelf length could be other sizes, for example, 1-100 microns. The tantalum layer  100  (used as an adhesive layer for gold conductors) is shown extending 1 micron beyond the PSG layer  92 , and the PSG layer  92  is shown extending 2 microns beyond the resistor  62 . 
     FIGS. 10A-10F are cross-sectional views of a portion of the wafer during various steps during the manufacturing of the printhead of FIG.  8 . Conventional deposition, masking, and etching steps are used unless otherwise noted. 
     In FIG. 10A, a silicon substrate  70  with a crystalline orientation of ( 111 ) is placed in a vacuum chamber. Field oxide  90  is grown in a conventional manner. PSG layer  92  is then deposited using conventional techniques. FIG. 10A shows mask  110  being formed over the PSG layer  92  using conventional photolithographic techniques. The PSG layer  92  is then etched using conventional Reactive Ion Etching (RIE) to pull back the PSG layer  92  from the subsequently formed ink feed hole. 
     In FIG. 10B, mask  110  is removed and a resistive layer  111  of TaAl is deposited over the surface of the wafer. A conductive layer  112  of AlCu is then deposited over the TaAl. A first mask  113  is deposited and patterned using conventional photolithographic techniques, and the conductive layer  112  and the resistive layer  111  are etched using conventional IC fabrication techniques. Another masking and etching step (not shown) is used to remove the portions of the AlCu over the heater resistors  62 , as previously described. The resulting AlCu conductors are outside the field of view of FIGS. 10A-10F. 
     In FIG. 10C, the passivation layers, nitride  96  and carbide  98 , are then deposited on the surface of the wafer using conventional techniques. The passivation layers are then masked (outside the field of view) and etched using conventional techniques to expose portions of the AlCu conductive traces for electrical contact to a subsequent gold conductive layer. 
     An adhesive layer  100  of tantalum and a conductive layer of gold  114  are then deposited over the wafer, masked, using a first mask  115 , and etched, using conventional techniques to form the ground lines, terminating in bond pads along edges of the substrate. A second mask (not shown) removes portions of the gold over the Ta adhesive layer  100 , such as over the heater resistor area. 
     FIG. 10D illustrates the resulting structure, after the steps of FIG. 10C, having a mask  116  exposing a portion of the thin film layers to be etched to form the ink feed holes. Alternatively, multiple masking and etching steps may be used as the various thin film layers are formed to etch the ink feed holes. 
     FIG. 10E illustrates the structure after etching the thin film layers. The thin film layers are etched using an anisotropic etch. This ink feed etch process can be a combination of several types of etches (RIE or wet). The ink feed holes  67  could be fabricated with an etch in combination with the films being patterned during fabrication. The holes  67  could be defined with one mask and etch step or with a series of etches. All the etches may use conventional IC fabrication techniques. 
     The back side of the wafer is then masked using conventional techniques to expose the ink trench portion  76  (see FIG.  7 ). The trench  76  is etched using a wet-etching process using tetramethyl ammonium hydroxide (TMAH) as an etchant to form the angled profile. Other wet anisotropic etchants may also be used. (See U. Schnakenberg et al., TMAHW Etchants for Silicon Micromachining, Tech Digest, 6th Int. Conf. Solid State Sensors and Actuators (Transducers &#39; 91 ), San Francisco, Calif., Jun. 24-28, 1991, pp. 815-818.) Such a wet etch will form the angled trench  76 . The trench  76  may extend the length of the printhead or, to improve the mechanical strength of the printhead, only extend a portion of the length of the printhead beneath the ink ejection chambers  60 . A passivation layer may be deposited on the substrate if reaction of the substrate with the ink is a concern. 
     In FIG. 10F, a tantalum adhesive layer  101  is then flash evaporated or sputtered over the bottom surface of the substrate followed by a gold heat conductive layer  102  and another tantalum layer  103 . These layers act as thermally conductive layers and provide mechanical strength to the bridge portion. 
     FIG. 10F also shows the formation of the orifice layer  74 . Orifice layer  74 , in one embodiment, is a photo-imagible material, such as SU 8 . Orifice layer  74  may be laminated, screened, or spun-on. The ink chambers and nozzles are formed through photolithography. 
     The resulting structure after etching of the orifice layer  74  is shown in FIG.  8 . The orifice layer  74  may also be formed in a two-stage process, with a first layer being formed to define the ink chambers and the second layer being formed to define the nozzles. 
     The resulting wafer is then sawed to form the individual printheads, and a flexible circuit (not shown) used to provide electrical access to the conductors on the printhead is then connected to the bonding pads at the edges of the substrate. The resulting assembly is then affixed to a plastic print cartridge, such as that shown in FIG. 1, and the printhead is sealed with respect to the print cartridge body to prevent ink seepage. 
     FIG. 11 is a cross-sectional view of a portion of a second alternative embodiment printhead similar to that shown in FIG. 4, except the trench in the silicon is not etched all the way to the thin film. Rather, the bulk silicon  120  is partially etched to form a thin silicon bridge below the heater resistors  24 . To accomplish this, before the thin film layers are deposited, the front side of the wafer is patterned with a mask to expose those silicon areas in the trench area which are not to be completely etched through. The exposed portions are then doped with a P-type dopant, such as boron, to an approximate depth of 1 to 2 microns. The depth could be as deep as 15 microns or deeper. The mask is then removed. A backside hardmask is used to define where the trench etch will occur. The back of the wafer is then subjected to a TMAH etch process, which only etches the un-doped silicon portions. Silicon portions in the trench area having a thickness of about 10 microns now underlie the resistors  24 . 
     A similar process may be used to form the thin silicon bridge in FIG.  4 . 
     Thin film layers identified with the same numbers in FIG. 4 may be identical and are subsequently formed using processes similar to those previously described. The orifice layer  122  may be identical to that shown in FIG.  8 . 
     One advantage of the printhead of FIG. 11 is that the silicon below the resistors  24  conducts heat away from the resistors  24 . 
     One skilled in the art of integrated circuit manufacturing would understand the various techniques used to form the printhead structures described herein. The thin film layers and their thicknesses may be varied, and some layers deleted, while still obtaining the benefits of the present invention. 
     FIG. 12 illustrates one embodiment of an inkjet printer  130  that can incorporate the invention. Numerous other designs of inkjet printers may also be used along with this invention. More detail of an inkjet printer is found in U.S. Pat. No. 5,852,459, to Norman Pawlowski et al., incorporated herein by reference. 
     Inkjet printer  130  includes an input tray  132  containing sheets of paper  134  which are forwarded through a print zone  135 , using rollers  137 , for being printed upon. The paper  134  is then forwarded to an output tray  136 . A moveable carriage  138  holds print cartridges  140 - 143 , which respectively print cyan (C), black (K), magenta (M), and yellow (Y) ink. 
     In one embodiment, inks in replaceable ink cartridges  146  are supplied to their associated print cartridges via flexible ink tubes  148 . The print cartridges may also be the type that hold a substantial supply of fluid and may be refillable or non-refillable. In another embodiment, the ink supplies are separate from the printhead portions and are removeably mounted on the printheads in the carriage  138 . 
     The carriage  138  is moved along a scan axis by a conventional belt and pulley system and slides along a slide rod  150 . In another embodiment, the carriage is stationery, and an array of stationary print cartridges print on a moving sheet of paper. 
     Printing signals from a conventional external computer (e.g., a PC) are processed by printer  130  to generate a bitmap of the dots to be printed. The bitmap is then converted into firing signals for the printheads. The position of the carriage  138  as it traverses back and forth along the scan axis while printing is determined from an optical encoder strip  152 , detected by a photoelectric element on carriage  138 , to cause the various ink ejection elements on each print cartridge to be selectively fired at the appropriate time during a carriage scan. 
     The printhead may use resistive, piezoelectric, or other types of ink ejection elements. 
     As the print cartridges in carriage  138  scan across a sheet of paper, the swaths printed by the print cartridges overlap. After one or more scans, the sheet of paper  134  is shifted in a direction towards the output tray  136 , and the carriage  138  resumes scanning. 
     The present invention is equally applicable to alternative printing systems (not shown) that utilize alternative media and/or printhead moving mechanisms, such as those incorporating grit wheel, roll feed, or drum or vacuum belt technology to support and move the print media relative to the printhead assemblies. With a grit wheel design, a grit wheel and pinch roller move the media back and forth along one axis while a carriage carrying one or more printhead assemblies scans past the media along an orthogonal axis. With a drum printer design, the media is mounted to a rotating drum that is rotated along one axis while a carriage carrying one or more printhead assemblies scans past the media along an orthogonal axis. In either the drum or grit wheel designs, the scanning is typically not done in a back and forth manner as is the case for the system depicted in FIG.  12 . 
     Multiple printheads may be formed on a single substrate. Further, an array of printheads may extend across the entire width of a page so that no scanning of the printheads is needed; only the paper is shifted perpendicular to the array. 
     Additional print cartridges in the carriage may include other colors or fixers. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.