Patent Publication Number: US-7901057-B2

Title: Thermal inkjet printhead on a metallic substrate

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. patent application Ser. No. 11/516,134 filed Sep. 6, 2006, entitled “LARGE AREA ARRAY PRINT HEAD EJECTOR ACTUATION” in the name of Stanley W. Stephenson, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates generally to the field of thermal liquid ejector printheads, and in particular to printheads formed on metallic substrates. 
     BACKGROUND OF THE INVENTION 
     Ink jet printing systems apply ink to a substrate. The inks are typically dyes and pigments in a fluid. The ink-receiving substrate can be comprised of a material or object. Most typically, the substrate is a flexible sheet that can be a paper, polymer or a composite of either type of material. The surface of the substrate and the ink are formulated to optimize the ink lay down. 
     Ink drops can be applied to the substrate by modulated deflection of a stream of ink (continuous) or by selective ejection from a drop generator (drop-on-demand). The drop-on-demand (DOD) systems eject ink using either a thermal pulse delivered by a resistor or a mechanical deflection of a cavity wall by a piezoelectric actuator. Ejection of the droplet is synchronized to motion of the substrate by a controller, which electrical signals to each ejector with appropriate timing to form an image. 
     U.S. Pat. No. 6,491,385 describes a continuous ink jet head and it&#39;s operation. An linear array of ejectors is disposed on a substrate. Each nozzle has a unique supply bore through the substrate. The supply bore ejects fluid through a nozzle in a membrane across the front surface of the supply bore. The membrane supports layers that form a pair of semi-circular resistive elements around each nozzle. Each resistor pair is pulsed to break the stream of fluid into discrete droplets. Asymmetric heating of the resistors can selectively direct the droplets into different pathways. A gutter can be used to filter out select droplets, providing a stream of selected droplets useful for printing. The modulated stream printing system requires significant additional apparatus to manage fluid flow. 
     Piezoelectric actuated heads use an electrically flexed membrane to pressurize a fluid-containing cavity. The membranes can be oriented in parallel or perpendicular to the ejection direction. U.S. Pat. No. 6,969,158 describes a piezoelectric drop-on-demand ink jet head having an electrically responsive piezo membrane that forces fluids through a nozzle. The ink jet head is formed of a numerous, stacked metallic plates, which includes the piezoelectric membrane. The metallic membranes require a large amount of surface area, and multiple rows of ejectors are arrayed in depth across the head. Ejectors are arranged across the printing direction at a pitch of 50 dpi and are arrayed in the printing direction 12 ejectors deep on an angle theta to form a head having an effective pitch of 600 dpi. Such heads are complex, requiring multiple layers that must be bonded together to form passages to the nozzle. The materials comprising the head and the structures do not lend themselves to incorporating semiconductor switching elements. 
     U.S. Pat. No. 6,926,384 discloses a piezoelectric drop-on-demand inkjet head permitting single-pass printing. A single pass print head comprises 12 linear array module assemblies that are attached to a common manifold/orifice plate assembly. Droplets are ejected from the orifice by twelve staggered linear array assemblies that support piezoelectric body assemblies to provide drop-on-demand ejection of ink through the orifice array. The piezoelectric system cannot pitch nozzles closely together; in the example, each swath module has a pitch of 50 dpi. The twelve array assemblies are necessary to provide 600 dpi resolution in a horizontally and vertically staggered fashion. 
     The orifice array on the plate can be a single two-dimensional array of orifices or a combination of orifices to form an array of nozzles. In the printing application, the orifices must be positioned such that the distance between orifices in adjacent line is at last an order of magnitude (more than ten times) the pitch between print lines. The assembly is quite complex, requiring many separate array assemblies to be attached to the orifice plate thorough the use of sub frames, stiffeners, clamp bar, washers and screws. It would be advantageous to provide a staggered array in a unitary assembly with an integral orifice plate. It would be useful for the spacing between nozzles to be less than an order of magnitude deeper than is disclosed in this patent. 
     U.S. Pat. No. 6,722,759 describes a common thermal drop-on-demand inkjet head structure. The drop generator consists of ink chamber, an inlet to the ink chamber, a nozzle to direct the drop out of the cavity and a resistive element for creating an ink ejecting bubble. Linear arrays of drop generators are positioned on either side of an ink feed passage. Two linear arrays are fed by a common ink feed passage. Ink from the slot passes through a flow restricting ink channels to the ink chamber. A heater resistor at the bottom of the ink chamber is energized to form a bubble in the chamber and eject a drop of ink through a nozzle in the top of the chamber. A matching set of transistors is formed adjacent to each resistor to provide a three-terminal switching device to each resistor. Sets of traces are provided adjacent to the transistors to provide power, power return and switching logic to each transistor. The structure limits nozzles to be placed in linear rows on either side of the ink jet supply slot. The patent uses both power supply and return lines, increasing the complexity of the device. 
     U.S. Pat. No. 5,134,425 discloses a passive two-dimensional array of heater resistors. The structure and arrangement of the droplet generators is not disclosed. The patent discloses the problem of power cross talk between resistors in two-dimensional arrays of heater resistors. Voltages firing a resistor also apply partial voltages across unfired resistors. The parasitic voltage increases as the number of rows is increased to a maximum of 5 rows. The patent applies partial voltages on certain lines to reduce the voltage cross talk. The partial energy does not eject a droplet, but maintains a common elevated temperature for both fired and unfired nozzles. Passive matrix arrays of resistors are limited in the depth of the array because of the parasitic resistance. The patent suggests that the number of rows is limited to less than five rows. 
     U.S. Pat. No. 6,921,156 discloses forming inkjet heads on non-silicon flat-panel substrates. Thin film transistors are coupled to an array of ink jet drop generators. The monolithic substrate is described as being made of any suitable material (preferably having a low coefficient of expansion) and discloses a preferred embodiment of being ceramic. The device is multiplexed driven using flip chip devices bonded to conductors using solder. Ink feed channels supply two rows of nozzles. The resistors and chambers are formed using thin film processes. Multiple feedholes can supply each ejector from a single, common manifold for the two rows of ejectors. Reference to the thin film transistors on the substrate is limited, describing them as driving the resistors. The thin-film devices are formed over barrier and/or smoothing layers to isolate the thin-film devices from the substrate. 
     U.S. Pat. No. 6,911,666 discloses a display on a flexible metal substrate. The patent discloses stainless steel, titanium, Inconel or Kovar alloys as possible substrates to support thin film transistors to drive OLED displays. The substrate thickness is in the range of 100 to 500 microns in thickness to create a flexible display. Via can be formed through a thick silicon oxide film that electrically isolates the thin film transistor from the conductive metallic substrate. Connections can be provided from the TFT to the substrate so that the substrate acts as part of an electrical circuit. No mention is made of what alloy operates optimally within the processing temperatures. Displays on non-silicon alloys do not require via through the substrate. 
     U.S. Pat. No. 6,663,221 discloses page wide ink jet printing. The substrate is pagewide, described as being more than 4 inches wide. The substrate can be formed of metal, such as stainless steel, ceramic, glass or resinous material such as polyimide. A nozzle array is formed on a first surface and supplied with ink from a bore formed through the substrate. Actuating elements and drive circuitry are formed on the surface of the substrate supporting the nozzle array. The suggested drive circuit is formed using thin film transistors. No process is described for forming the ink channel through the substrate. 
     U.S. Pat. No. 4,528,070 discloses a method for manufacturing an orifice plate. The patent cites prior art that used copper substrates and metallic masks on both sides of the wafer to form a passage through the substrate. Crystalline nickel was used previously as the metal mask for etching through copper substrates. The patent is directed to etching though more durable substrates, such as stainless steel, titanium zirconium and titanium. The patent discloses the use of electroplated nickel or cobalt having phosphorous as masks on both sides of the improved substrate materials. Etching is done by masking and etching though both surfaces of the substrate. No other structures are formed on the nozzle plate. 
     It would be useful to have an inkjet printhead and a method for forming an inkjet printhead on a metallic substrate. Additionally, it would be useful if the method created accurately aligned structures and efficiently etched ink supply channels through the substrate. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a method of forming an ink feed passage through a print head substrate includes providing a metallic substrate having a first surface and a second surface; providing an ink ejector structure on a first surface of the metallic substrate; providing a mask over the second surface of the metallic substrate to define the ink feed passage; and forming the ink feed passage from the second surface of the metallic substrate using a liquid etchant. 
     According to another aspect of the present invention, a method of forming a print head substrate includes providing a metallic alloy layer having a coefficient of thermal expansion; providing an isolation layer in contact with the metallic alloy layer, the isolation layer having a coefficient of thermal expansion that is substantially equivalent to the coefficient of thermal expansion of the metallic alloy layer; and curing the metallic alloy layer and the isolation layer by heating to over 200° C., wherein a negligible amount of thermally induced stress exists between the metallic alloy layer and the isolation layer. 
     According to another aspect of the present invention, a print head substrate includes a metallic alloy layer having a coefficient of thermal expansion and an isolation layer in contact with the metallic alloy layer. The isolation layer has a coefficient of thermal expansion that is substantially equivalent to the coefficient of thermal expansion of the metallic alloy layer such that a negligible amount of thermally induced stress exists between the metallic alloy layer and the isolation layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  is a top schematic view of an ejector in accordance with the present invention; 
         FIG. 2  is a side sectional view through the ejector shown in  FIG. 1 ; 
         FIG. 3  is a top view of an array of ink ejectors according to prior art; 
         FIG. 4  is a top view of an inkjet print head assembly in accordance with prior art; 
         FIG. 5  is a top view of an ejector in accordance with the present invention; 
         FIG. 6  is a side sectional view of a transistor on a substrate in accordance with the invention; 
         FIG. 7  is a schematic representation of an ejector array in accordance one example embodiment of the invention; 
         FIG. 8   a  is a side sectional view of a device being constructed in accordance with the invention; 
         FIG. 8   b  is a side sectional view of the device shown in  FIG. 8   a  after backside masking; 
         FIG. 8   c  is side sectional views of the device shown in  FIG. 8   b  after ink feed passage etch; 
         FIG. 8   d  is a side sectional view of the device shown in  FIG. 8   c  after etch; 
         FIG. 8   e  is a side sectional view of the device shown in  FIG. 8   d  after chamber clearing; 
         FIG. 9  is an electrical schematic of an inkjet head in accordance with the present invention; 
         FIG. 10  is a schematic view of a head assembly in accordance with the present invention; and 
         FIG. 11  is a side view of a printer using a head in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
       FIG. 1  is a top schematic view of an ejector  10  in accordance with the present invention.  FIG. 2  is a side sectional view through the ejector shown in  FIG. 1 . Substrate  3  is formed of a metallic alloy. A polymer layer  5  is patterned to create an ink chamber  12  to hold a printing ink. A nozzle layer  7  over ink chamber  12  can be formed directly over polymer layer  5  using a vacuum deposited ceramic, polymer or metal. Nozzle layer  7  over ink chamber  12  can also be a separate plate formed of ceramic, polymer or metal that is bonded to the polymer layer  5  defining ink chamber  12 . Nozzle layer  7  has an opening to form a nozzle  14  to direct an ejected droplet of ink in a specified direction when ink chamber  12  is pressurized. 
     The invention is directed to forming drop on demand (DOD) inkjet ejectors using flat panel display (FPD) manufacturing processes. FPD manufacturing has the advantage of forming microelectronic devices over large areas at costs lower than devices formed on mono-crystalline silicon wafers. FPD glass substrates are inexpensive and have transparency that is needed for displays operating on transmitted light. The majority of flat panel displays incorporates thin film transistor (TFT) driving and control circuitry over glass substrates. 
     Ejectors  10  use thermal energy to heat ink and form a gas bubble that expels ink through a directing orifice. The ink ejecting process requires energy for each ejection and a significant portion of the energy is retained within the ejecting structure. The heat must be dissipated into substrate  3  before a subsequent ejection event. A key property for a thermal DOD heads is the ability of substrate  3  to conduct heat from ejector  10 . Currently, DOD thermal inkjet heads are built on silicon substrates that have a thermal conductivity of 124 W/m-k. This permits heat to be efficiently transferred away from ejector  10  and cooling of the ejector prior to a subsequent ejection event. Rapid cooling permits high frequency pulsing of the ejectors for fast printing. Display glass, such as Corning 1737 glass or Eagle glass, used in flat panel display manufacturing has thermal conductivity of 1.2 W/m-K, approximately one percent of the conductivity of monolithic silicon. 
     Glass etching can only be done using chemical solutions at slow rates. The etching process is isotropic, requiring a large area on the backside when etching and opening on the front side. The backside area increases as the thickness of substrate  3  increases in thickness. Glass is fragile, limiting the thickness of material that can be used in FPD processes to greater than 200 microns. Using a metal alloy foil for substrate  3  permits significantly higher thermal conductivity and faster discharge rates for a thermal ejector. Metal foils can be much thinner that 200 microns, down to 125 microns, permitting faster chemical etching through the substrate. The thinner foil reduces the etch area used on the rear of the wafer. Metal foils can be much thinner that 200 microns, down to 125 microns, permitting faster chemical etching through the substrate. 
     Polymers have been suggested as substrates to be used on FPD manufacturing equipment. Polymers have even lower thermal conductivities than display glass. Building thermal DOD inkjet heads on either glass or polymeric substrates creates inkjet heads with very slow firing speeds due to the low thermal conductivity. 
     In the invention, the thermal DOD substrate is metal alloy with thermal conductivity significantly greater than glass or polymers. Research on forming organic LED arrays on flexible substrates has looked at the use of stainless steel alloy 304 (SS 304) for flexible displays. Table 1 lists properties of metal alloys that have thermal conductivities ten times greater than glass. The thermal conductivity of SS 304 creates thermal DOD inkjet heads with faster firing rates than thermal DOD heads formed on other polymeric or glass substrates. Other metal alloys also have significantly higher thermal conductivities than glass or polymers. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Coefficient of 
                   
               
               
                   
                   
                 Thermal 
                 Thermal 
               
               
                   
                 MATERIAL 
                 Expansion (CTE) 
                 Conductivity 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Silicon 
                 3.6 
                 124.0 
               
               
                   
                 Glass (SiO 2 ) 
                 4.0 
                 1.1 
               
               
                   
                 SS 304 
                 17.8 
                 16.2 
               
               
                   
                 Invar 36 
                 4.2 
                 10.1 
               
               
                   
                 Kovar 
                 5.1 
                 17.3 
               
               
                   
                 NILO Alloy 42 
                 4.5-6.5 
                 10.5 
               
               
                   
                   
                 um/m-° C., 250° C. 
                 W/m-°K 
               
               
                   
                   
               
            
           
         
       
     
     Thin film transistors on alloys require an isolation layer  78  between the metal alloy and subsequent layers. Isolation layer  78  is formed of silicon dioxide and provides a surface that is smoother than the surface of substrate  3  and electrically isolated from substrate  3 . Isolation layer  78  can be a silicon dioxide layer that is deposited by electron beam evaporation or by depositing a liquid organo-silane over the surface and baking out organic components. The preferred embodiment is to apply 5000 angstroms of an organo-silane liquid (spin-on-glass, SOG), such as Honeywell Accuglass T-5121B as isolation layer  78 , which is then over-deposited with 2500 angstroms of evaporated SiO2. 
     Depositing the first layer as a fluid creates an optically smooth surface that fills in discontinuities in the surface of the metal foil. Metallic alloys can be mechanically rolled into foils 50 to 500 microns thick with a surface roughness of 1500 angstroms. In the exemplary embodiment, a 5,000 angstroms layer of the T-512B is applied over the metallic substrate to cover the roughness from the metallic alloy foil. The 5,000 angstroms of spin-on-glass is sufficient to smooth the 1,500 angstroms rough metal surface to semiconductor grade surfaces. As part of the curing process of the spin-on-glass, the substrate and isolation layer must be raised to a temperature of 250 degrees centigrade to form a stable layer. 
     Alloys that do not match the COE of the spin-on-glass will curl after the heating process. Wafers with thermal stress-induced curl are difficult to process, and may require carrier wafers and attachment processes. The SEMI standard M1.8-89 for wafers 150 nm um diameter is 50 microns of bow. Wafers must have less than maximum bow to be processed on automated equipment. Metal alloy wafers should meet this specification to be processed on automated equipment. In this sense, a negligible amount of thermally induced stress needs to exist between the metallic alloy layer and the isolation layer in order to meet bow standards and be processed on automated equipment. 
     In order to ensure that a negligible amount of thermally induced stress exists between the metallic alloy layer and the isolation layer, the isolation layer must have a coefficient of thermal expansion that is substantially equivalent to the coefficient of thermal expansion of the metallic alloy layer. For example, referring back to Table 1, the coefficient of thermal expansion (COE) of SS 304 is four times that of the COE of a glass isolation layer. This difference induces excessive bow in thin metal wafers which makes SS 304 unsuitable for use when forming an inkjet printhead on a metallic substrate. However, still referring to Table 1, the COE of Invar 36, Kovar, and NILO Alloy 42, for example, is less than four times that of the COE of a glass isolation layer. This difference does not induce excessive bow in thin metal wafers which makes these alloys suitable for use when forming an inkjet printhead on a metallic substrate. 
     In an experiment, wafers NILO alloy 42 supporting a 5,000 angstrom thick cured spin-on-glass layer were found to have curl below the SEMI standard for a set of processing steps that generated thin film transistors at high temperature. Iron can be alloyed with various concentrations of nickel or other metals to create various coefficients of thermal expansion. In general, a set of layers formed over a metallic substrate at high temperature can induce a given stress within the wafer. In general, the metallic alloy should have a CTE that matched CTE of the composite layers to minimize warp in the wafer. 
     A heater resistor  20  lies over an isolation layer  78  on substrate  3 . A pulse of electrical energy to heater resistor  20  causes ink within ink chamber  12  to momentarily be converted into a gaseous state. A gas bubble is formed over heater resistor  20  within ink chamber  12 , and pressurizes ink chamber  12 . Pressure within ink chamber  12  acts on ink within ink chamber  12  and forces a droplet of ink to be ejected through nozzle  14 . Inlet  16  supplies ink to ink chamber  12 . Restriction  18  can be formed at inlet  16  to improve firing efficiency by restricting the majority of the pressure pulse to ink chamber  12 . Restriction  18  can be in the form of one or more pillars formed within inlet  16 , or by a narrowing of the sidewalls in polymer layer  5  at inlet  16  of ink chamber  12 . 
     Heater resistor  20  and associated layers are formed over isolation layer  78 , followed by polymer layer  5 . Polymer layer  5  is patterned, followed by nozzle layer  7 , which is patterned to form nozzle  14 . After those layers have been formed ink feed passage  22  can be formed through substrate  3  to supply ink to ejector  10 . Processes used to form feed passage  22  must not induce stress into substrate  3  neither should they damage the ejectors  10 . Substrate  3  is bonded to head holder  31  that has one or more cavities for supplying ink to some or all of ejectors  10  formed on substrate  3 . 
     In one embodiment, electrical current used to power resistors  20  is returned through substrate  3 . Head holder  31  is formed of an electrically conductive metal with the same CTE as the metal used in substrate  3 . In the case that substrate  3  is formed of NILO alloy 42, then head holder  31  should be formed of the same material. The common CTE permits high temperature cure when bonding head holder  231  to substrate  3 . An electrically conductive adhesive  33  can provide an electrical path between the two components. Conductive adhesive  33  should have low electrical resistance, less than 0.1 ohms resistance, to current flow between head holder  31  and substrate  3 . Alternatively, power can be removed from substrate  3  using contacts along the perimeter of substrate  3 . Each ejector  10  is fed by a cavity in head holder  31  through its ink feed passage  22  in substrate  3 . Individual ink feed passages  22  are associated with individual ejectors  10  and are physically separated from other ejectors  10  by the material forming substrate  3 . 
       FIG. 3  is a top view of an array of ink ejectors according to prior art. Ejectors  10  must be supplied by ink from the rear side of substrate  3 . U.S. Pat. No. 6,722,759 is an excellent recitation of prior art associated with thermal drop-on-demand print heads. Ejectors  10  are arranged in two closely packed rows that share common ink feed passage  22 . Ink feed passage  22  passes through substrate  3 , which supplies to ink to multiple ejectors  10 . Arranging two linear rows of ejectors  10  on either side of ink feed passage  22  provides for a compact ink jet head. Because the nozzles are adjacent to each other, fluidic cross-talk can occur between ejectors  10 . Close packing of the nozzles makes the head susceptible to thermal cross talk between adjacent nozzles. Overheating can become more pronounced if substrate  3  is not silicon, but a less thermally conductive material such as metal alloy, glass, ceramic or polymer. It is useful then to separate individual ejectors in accordance with this invention. 
       FIG. 4  is a top view of an inkjet print head in accordance with prior art. The recitation again generally follows the structures found in U.S. Pat. No. 6,722,759. A print head  32  has two ink feed passages  22 , each feed passage feeding two rows of ejectors  10 . A set of ejector drivers  52  is formed adjacent to each row of ejectors  10 . Each ejector driver  52  is a semiconductor-switching element that is attached to each heater resistor  20  within each ejector  10 . The power requirements for thermal drop on demand inkjet are high, typically over 1 watt of power for approximately 1 microsecond. Ejector drivers  52  are typically formed of PMOS or NMOS transistors that are diffused into a silicon substrate  3 . Ejector drivers  52  are activated to electively apply power to heater resistors  20 . Prior art discloses that ejector drivers  52  can also be thin-film-transistor elements having characteristics capable of meeting the power and switching times required to thermally eject a droplet from an ejector  10 . 
     Power to ejector drivers  52  is provided by conductor lines  54  disposed on the sides and down the center of substrate  3 . Conductor lines  54  supply power and data for ejector drivers  52 . Control logic  58  is disposed on both sides of the substrate  3  to decode data signals from printer controller  38  (not shown in figure). Data and power are delivered to control logic  58  through bond pads  60 . Wire bonds  62  provide connection between bond pads  60  on substrate  3  and flex circuit  64 . Data from control logic  58  is delivered through flex circuit  64  through wire bonds  62  to control logic  58 . Control logic  58  responds to control data from printer controller  38  (not shown in this figure). 
       FIG. 5  is a top schematic view of an ejector in accordance with the present invention. In the invention, an ejector  10  comprises an ink chamber  12  actuated by heater resistor  20 . Ink chamber  12  is fed by inlet  16  and ejects fluid through nozzle  14  (not shown) over resistor  20 . Dedicated ink feed passage  22  is integral with ejector  10 . Ink feed passage  22  shares a common cavity in head holder  31  facing the back of substrate  3 . Ejector  10  in accordance with the invention provides a complete assembly that can be positioned at any distance from adjacent ejectors  10  to eliminate fluidic cross talk and improve cooling efficiency. In the case that substrate  3  is not silicon, the greater distance prevents overheating that would result from closely spaced ejectors  10  on lower conductivity substrates  3 . 
     U.S. Pat. No. 5,134,425 discloses a passive two-dimensional array of heater resistors. The patent discloses the problem of power cross talk between resistors in two-dimensional arrays of heater resistors. A voltage applied to one resistor applies partial voltages across unfired resistors, significantly increasing the current and power demand. In the present invention, a three-terminal device, generally referred to as a transistor  24 , permits multiple ejectors  10  to be attached to a matrix of row conductors  26  and column conductors  28  and eliminates power cross talk. Row conductor  26  provides a digital logic signal to gate power supplied by column conductor  28 . In this way, transistors  24  provide both a power and logic multiplexing using either row conductor  26  or column conductor  28  to provide power to resistor  20  when a gating voltage is applied on the other conductor. Transistors  24  and individual ink feed passages  22  permit ejectors  10  to be organized on substrate  10  in large numbers of both columns and rows that are separated for thermal efficiency on non-silicon substrates. 
     Transistor  24  can be arranged over isolation layer  78  over substrate  3  and be formed by a plurality of thin film material layers.  FIG. 6  is a side sectional view of transistor on a substrate in accordance with the invention. Because substrate  3  is metallic, semiconductor devices cannot be formed directly in the substrate. In the example, transistors  24  are thin-film transistors formed over isolation layer  78  over substrate  3 . Two doped areas  70  provide pools of charge in a semiconductor material, such as polysilicon. Channel  72  is disposed between doped areas  70  and is responsive to a field applied to gate electrode  74 . The presence of a field on gate electrode  74  permits current to flow between doped areas  70 . Various levels and types of n or p dopants can be applied to doped areas  70  and channel  72  to change the characteristics of transistor  24 . Transistor contacts  76  are applied through isolation layer  78  to supply power through transistor  24 . In the invention, transistor contacts  76 , for example, a first electrical contact and a second electrical contact, are formed of the material comprising row conductors  26  to minimize layers. 
     In the exemplary embodiment, gate electrode  74  and transistor contacts  76  are isolated areas of the material providing row conductor  26 . An opening is made through isolation layer  78  to provide substrate contact  80  between one transistor contact  76  and substrate  3 . In the invention, two of the device terminals provide switching and power means, which are through gate electrode  74  and the transistor contact  76  not connected to substrate  3 . The power return is through the substrate using substrate contact  80 . In the invention, it is important that the substrate provide sufficient conductivity that the power delivered to multiple ejectors  10  be transmitted through substrate  3 . The conductivity can be provided by a metallic alloy, but not by glass or polymeric substrates. In the case of very wide heads, the number of ejectors can be large, and applied power can be high. Substrate  3  is formed of metal and power is conducted through the substrate to eliminate additional conductors for power return. The structure permits row conductors  26  and column conductors  28  to be thin, and ejectors  10  can be packed closely together to minimize device cost. 
     Column conductors  28  are formed over isolation layer  78  and have through via to connect to isolated areas of conductor  26  that forms a transistor contact  76  to complete the circuit. The structure of the matrix electrical backplane of the invention uses two metal layers spaced from substrate  3  by isolation layer  78  and spaced from each other by isolation layer  78 . The structure provides a logic and power matrix inkjet array backplane with a minimal number of layers. 
       FIG. 7  is a schematic representation of an ejector array in accordance one example embodiment of the invention. A coordinate system is shown and includes a first direction X with X an axis of motion between the printhead and an ink-receiving surface, commonly referred to as a printing direction. A second direction Y is also shown with Y being a cross printing direction. A direction Z is also shown with Z being a direction perpendicular to the printhead. This is commonly referred to as the direction of ink drop ejection from the printhead. 
     Ejectors  10  are shown schematically as a box having individual supply ports  22  and nozzles  14  and transistors  24 . Ejectors  10  have been attached to a matrix of row conductors  26  and column conductors  28  to form laterally staggered columns of ejectors  10 . Each ejector  10  of a column of ejectors is staggered at a desired pitch, typically expressed in dpi or microns, which is finer than the pitch of the ejector columns. For example, each column can be pitched 600 microns apart due to the area required for each ejector. If the required printing pitch is 40 microns, each ejector in the column can be laterally staggered 40 microns to a depth of 15 ejectors (40×15=600) to achieve the required 40 micron printing pitch. The invention permits the staggered matrix array to be placed on a single substrate. Transistors  24  attached to ejectors  10  using row conductors  26  as the gate lines and column conductors  28  as power supply lines permit thermal Drop-On-Demand print heads having a large number of rows along printing direction X with close packing. 
     The embodiment shown in  FIG. 7  is particularly well suited for print heads having large area arrays, for example, print heads having a print width across the Y direction of over of 100 millimeters and a print depth dimension Y of 18 millimeters. However, the large area array print head can have other length and width dimensions. One head (or a plurality of large area array print heads stitched together) can be used to form a pagewide print head. In a pagewide print head, the length of the printhead is preferably at least equal to the width of the receiver and does not “scan” during printing. The length of the page wide printhead is scalable depending on the specific application contemplated and, as such, can range from less than one inch to lengths exceeding twenty inches. 
       FIGS. 8   a - 8   e  are sectional views of a device being constructed in accordance with the invention.  FIG. 8   a  is a side sectional view of a device at the beginning of the ink feed passage etch process. Layers forming transistors  24 , heater resistors  20 , row conductors  26  and column conductors have been formed on a first surface of substrate  3 . Isolation layer  78  is been etched prior to the application of polymer layer  5  to expose metallic substrate  3  through clear area  92 . Clear area  92  will open into ink fed slot  22  that will be formed through substrate  3 . Polymer layer  5  has been patterned to provide a block of material filling in an area that will correspond to ink chamber  12 . Polymer layer  5  can be a photo-imaged epoxy or an oxygen-plasma etched layer of polyimide. Nozzle layer  7  has been applied over polymer layer  5  and a nozzle  14  has been formed in nozzle layer  7 . Nozzle layer  7  can be formed of a photo-imaged epoxy, a plasma-enhanced chemical vapor deposited layer of silicon dioxide or a metallic layer. Polymer layer  5  and nozzle layer  7  have the property that polymer layer  5  can be removed without harm to nozzle layer  7 . 
       FIG. 8   b  is a side sectional view of a device after backside masking. An etch mask  94  is applied opposite to the surface carrying ejectors  10 . The masking material can be a thick polymer layer, an evaporated dielectric layer or a metal layer or combinations thereof. Etch mask openings  96  are formed through etch mask  96  opposite clear area  92 . Protective tape  98  is applied over the ejector surface of substrate  3 . Protective tape  98  can be conventional “dicing tape” which is applied under pressure over microelectronic surfaces prevent damage during wafer dicing and backside grinding operations. Protective tape  98  can be released after operations by exposure to actinic ultraviolet radiation to degrade the attachment adhesive and release the inkjet head without damage to nozzle layer  7 . 
       FIG. 8   c  is a side sectional views of a device after ink feed passage etch. In the invention, a conventional ferric chloride etching solution and circuit board processing equipment is used to etch feed passage  22  through substrate  3 . An example solution is a 50% ferric chloride solution with 2% hydrochloric acid. The solution is effective in etching most iron, nickel or copper materials and alloys thereof. The etching solution is pumped against etch mask  94  to provide a continuous flushing of ink feed passage  22  during the etching process. The etching process is isotropic, being wider at the initial area and narrower at the area that opens at clear area  92 , typically forming a tapered opening with a sidewall angle of about 30 degrees. Ejectors  10  are positioned apart from each other so that enough material in substrate  3  exists to form a mechanically sound structure for ejectors  3 . Clear area  92  and etch mask opening  96  are adjusted in size to create an ink feed passage of approximately the same size as clear area  92 . An important part of the invention is the presence of polymer  5  in the area that becomes chamber  12 . The presence of the polymer provides a strong layer to resist the impact of the etching fluid at the end of etch as metal is removed away from clear area  92 . Protective tape  98  protects the ejector-bearing surface of the device from damage during the etching process. 
       FIG. 8   d  is a side sectional view of a device after etch. Etch mask  94  is removed using a solvent wash to expose the back surface of substrate  3 . In this embodiment, the area that becomes chamber  12  is filled with polymer  5 . This can occur if polymer  5  is a solvent resistant epoxy or polyimide and etch mask  94  is a conventional photo-resist. This can occur if etch mask  94  is a metal and a metal solvent is used for removal. This condition can occur if etch mask  94  is a dielectric material that is removed with a fluorine plasma or chemical etch. In the case nozzle layer  7  is sensitive to processes that remove etch mask  94 . Protective tape  98  is removed after etch mask  94  to protect nozzle layer  7 . 
       FIG. 8   e  is a side sectional view of a device after clearance of the ink chamber. Actinic ultraviolet radiation has been used to break down the adhesive that held protective tape  98  to the ejector-bearing surface of the device. In one case, a polymer layer  5  and polymeric etch mask  94  are removed by a common solvent to create chamber  12  and remove etch mask  94 . In the case that nozzle layer  7  is an inorganic structure, formed of either a dielectric such a silicon dioxide or a metal such as nickel, polymer  5  can be removed by a plasma-oxygen etch. A device having an open chamber  12  is capable of passing ink through substrate  3  using ink feed passage  22  that is in connection with chamber  12 . 
     A first part of the invention is the construction of a complete ejector surface on the front surface of substrate  3 . A second part of the invention is providing a stress-free etch through substrate  3  using a metal solvent. Another part of the invention is filling chamber  12  with a polymer during the etching process. A final part of the invention is removal of etch mask  94  simultaneously with removal of the material filling chamber  12 . 
       FIG. 9  is an electrical schematic of an ink jet head in accordance with the present invention. Print head  32  includes a plurality of drivers electrically connected to the plurality of row conductors and the plurality of column conductors. The plurality of drivers is operable to provide current to each resistive element row sequentially. In  FIG. 9 , each column conductor  28  is connected to a column driver  36 . Column driver  36  can be, for example, an ST Microelectronics STV 7612 Plasma Display Panel Diver chip that is connected to each column conductor  28 . The chip responds to digital signals to either apply a drive voltage or ground to each column conductors. Each row conductor  26  is connected to a row driver  34 . Row driver  34  can be the same ST Microelectronics STV 7612 Plasma Display Panel Diver chip to provide either a gating voltage (Vdd) or ground to each row conductor  26 . Transistor  24 , provided with each ejector  10 , responds to the logic and power states to permit print head  32  to be logically driven in a row sequential fashion without parasitic resistance effects. 
     Print head  32  is fired row sequentially. Digital signals apply a drive voltage (Vdd) or ground voltage to each column conductor  28 . Column conductors  28  having an applied drive voltage provide energy to the ejector attached to column conductor  28  and the grounded row conductor  26 . Column conductors  28  at ground voltage are not fired. Row driver  34  applies a Gate voltage (Vdd) to a row of ejectors  10  to enable firing of powered ejectors  10  of a given row, while the remaining rows remain at ground voltage regardless of power applied to their associated column conductor  28 . This process is repeated to apply an image wise pattern of ink droplets from print head  32 . 
     Only a single ejector  10  on any given column conductor  28  is active at any one time, which permits column conductor  28  to be thin. However, all ejectors  10  on the selected row conductor  26  can be fired, which represents a large amount of current and power that must be returned through substrate  3 . In a head having thirty activated heater resistors  20  on a line, each sinking 50 milli-amperes, 1.5 amps will pass through substrate  3 . Power from each ejector  10  must pass through contact  80 , substrate  3  and through conductive adhesive  33  in the case that power is transmitted through head holder  31 . The edges of substrate  3  can provide a large amount of surface area to transmit the power, in particular wide print heads will have large contact areas that will scale with width. 
       FIG. 10  is a schematic view of a head assembly in accordance with the present invention. Print head  32  has been mounted to head holder  31 , which holds a supply of ink in a cavity behind substrate  3  to supply ink through substrate  3  to ejectors  10  mounted on the front of substrate  3 . Row driver  34  and column driver  36  are attached to head holder  31  and wire bonds are made between the flex circuit for the drivers to the row and column conductors on print head  32 . The width of the head is not limited to a single column driver  36 . The width can be extended and additional column drivers  36  added to provide power to additional columns. 
       FIG. 11  is a schematic side view of a printer using a head in accordance with the present invention. Controller  38  moves an ink receiver  40  using receiver driver  42 . Receiver driver  42  is a motor that operates on a plate or roller to drive ink receiver  40  under print head  32 . Controller  38  provides drive signals to row driver  34  and column driver  36  connected to print head  32  to apply an image-wise pattern of ink droplets onto ink receiver  40  in synchronization with the motion of ink receiver  40 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
         
           
               3  substrate 
               5  polymer layer 
               7  nozzle layer 
               10  ejector 
               12  ink chamber 
               14  nozzle 
               16  inlet 
               18  restriction 
               20  heater resistor 
               22  ink feed passage 
               24  transistor 
               26  row conductor 
               28  column conductor 
               30  spacing distance 
               31  head holder 
               32  print head 
               33  conductive adhesive 
               34  row drivers 
               36  column drivers 
               38  printer controller 
               40  ink receiver 
               42  receiver driver 
               52  ejector drivers 
               54  conductor lines 
               58  control logic 
               60  bond pads 
               62  wire bonds 
               64  flex circuit 
               70  doped areas 
               72  channel 
               74  gate electrode 
               76  transistor contacts 
               78  isolation layer 
               80  substrate contact 
               92  clear area 
               94  etch mask 
               96  etch mask opening 
               98  protective tape