WO1996032274A1 - Structure et procede de fabrication de chauffage de tetes d'impression monolithiques - Google Patents

Structure et procede de fabrication de chauffage de tetes d'impression monolithiques Download PDF

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Publication number
WO1996032274A1
WO1996032274A1 PCT/US1996/005022 US9605022W WO9632274A1 WO 1996032274 A1 WO1996032274 A1 WO 1996032274A1 US 9605022 W US9605022 W US 9605022W WO 9632274 A1 WO9632274 A1 WO 9632274A1
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WIPO (PCT)
Prior art keywords
ink
nozzles
heater
drop
nozzle
Prior art date
Application number
PCT/US1996/005022
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English (en)
Inventor
Kia Silverbrook
Original Assignee
Eastman Kodak Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Eastman Kodak Company filed Critical Eastman Kodak Company
Priority to US08/765,017 priority Critical patent/US5905517A/en
Priority to JP8531187A priority patent/JPH10501772A/ja
Priority to EP96912694A priority patent/EP0765234A1/fr
Publication of WO1996032274A1 publication Critical patent/WO1996032274A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B41J2/1621—Manufacturing processes
    • B41J2/164—Manufacturing processes thin film formation
    • B41J2/1646—Manufacturing processes thin film formation thin film formation by sputtering
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/14—Structure thereof only for on-demand ink jet heads
    • B41J2/14451—Structure of ink jet print heads discharging by lowering surface tension of meniscus
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B41J2/1621—Manufacturing processes
    • B41J2/1623—Manufacturing processes bonding and adhesion
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B41J2/1621—Manufacturing processes
    • B41J2/1626—Manufacturing processes etching
    • B41J2/1628—Manufacturing processes etching dry etching
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B41J2/1621—Manufacturing processes
    • B41J2/1626—Manufacturing processes etching
    • B41J2/1629—Manufacturing processes etching wet etching
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B41J2/1621—Manufacturing processes
    • B41J2/1631—Manufacturing processes photolithography
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B41J2/1621—Manufacturing processes
    • B41J2/1635—Manufacturing processes dividing the wafer into individual chips
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/16—Production of nozzles
    • B41J2/1621—Manufacturing processes
    • B41J2/164—Manufacturing processes thin film formation
    • B41J2/1642—Manufacturing processes thin film formation thin film formation by CVD [chemical vapor deposition]

Definitions

  • the present invention is in the field of computer controlled printing devices.
  • the field is constructions and manufacturing processes for thermally activated drop on demand (DOD) printing heads which integrate multiple nozzles on a single substrate.
  • DOD thermally activated drop on demand
  • Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing.
  • ink jet printing mechanisms have been invented. These can be categorized as either continuous ink jet (CIJ) or drop on demand (DOD) ink jet Continuous ink jet printing dates back to at least 1929: Hansell, US Pat. No. 1,941,001. Sweet et al US Pat. No. 3,373,437, 1967, discloses an array of continuous ink jet nozzles where ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection CU, and is used by several manufacturers, including Elmjet and Scitex. Hertz et al US Pat. No.
  • 3,416,153, 1966 discloses a method of achieving variable optical density of printed spots in CIJ printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture.
  • This technique is used in ink jet printers manufactured by Iris Graphics.
  • Kyser et al US Pat. No. 3,946,398, 1970 discloses a DOD ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand.
  • Many types of piezoelectric drop on demand printers have subsequently been invented, which utilize piezoelectric crystals in bend mode, push mode, shear mode, and squeeze mode.
  • Piezoelectric DOD printers have achieved commercial success using hot melt inks (for example, Tektronix and Dataproducts printers), and at image resolutions up to 720 dpi for home and office printers (Seiko Epson). Piezoelectric DOD printers have an advantage in being able to use a wide range of inks. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance.
  • Endo et al GB Pat. No. 2,007,162, 1979 discloses an electrothermal DOD ink jet printer which applies a power pulse to an electrothermal transducer (heater) which is in the ⁇ nal contact with ink in a nozzle.
  • the heater rapidly heats water based ink to a high temperature, whereupon a small quantity of ink rapidly evaporates, forming a bubble.
  • the formation of these bubbles results in a pressure wave which cause drops of ink to be ejected from small apertures along the edge of the heater substrate.
  • BubblejetTM trademark of Canon K.K. of Japan
  • No. 4,490,728, 1982 discloses an electrothermal drop ejection system which also operates by bubble formation.
  • drops are ejected in a direction normal to the plane of the heater substrate, through nozzles formed in an aperture plate positioned above the heater.
  • This system is known as The ⁇ nal Ink Jet, and is manufactured by Hewlett-Packard.
  • Thermal Ink Jet is used to refer to both the Hewlett- Packard system and systems commonly known as BubblejetTM.
  • the ⁇ nal Ink Jet printing typically requires approximately 20 ⁇ J over a period of approximately 2 ⁇ s to eject each drop.
  • the 10 Watt active power consumption of each heater is disadvantageous in itself and also necessitates special inks, complicates the driver electronics and precipitates deterioration of heater elements.
  • U.S. Patent No. 4,275,290 discloses a system wherein the coincident address of predetermined print head nozzles with heat pulses and hydrostatic pressure, allows ink to flow freely to spacer-separated paper, passing beneath the print head.
  • U.S. Patent Nos. 4,737,803; 4,737,803 and 4,748,458 disclose ink jet recording systems wherein the coincident address of ink in print head nozzles with heat pulses and an electrostatically attractive field cause ejection of ink drops to a print sheet
  • one object of the invention is to provide improved heater constructions for ink nozzles of thermally activated drop on demand printing heads.
  • the present invention constitutes a thermally activated, drop on demand printing head comprising a substrate, a heater formed on said substrate and including a planar annulus of heater material and an ink nozzle having an axis that is generally normal to the plane of said heater material, and passes substantially through the center of said heater annulus.
  • the present invention constitutes a method for manufacturing heaters for ink nozzles of thermally activated drop on demand printing heads, comprising the steps of (a) forming on a substrate, a planar region of heater material which has a predetermined outline and which covers at least the peripheral regions of the desired nozzle tip; and (b) creating a nozzle hole along an axis which is generally normal to the plane of said heater and which is substantially concentric with said heater outline.
  • a further preferred aspect of the invention is that the planar region of heater material is substantially circular.
  • a further preferred aspect of the invention is that the passivation material is silicon dioxide.
  • the passivation material is silicon nitride.
  • a further preferred aspect of the invention is that the nozzle is fabricated with a radius less than 50 microns.
  • a further preferred aspect of the invention is that drive circuitry is fabricated on the same substrate as the nozzles.
  • Figure 1 shows a simplified block schematic diagram of one exemplary printing apparatus according to the present invention.
  • Figure 1(b) shows a cross section of one variety of nozzle tip in accordance with the invention.
  • Figures 2(a) to 2(f) show fluid dynamic simulations of drop selection.
  • Figure 3(a) shows a finite element fluid dynamic simulation of a nozzle in operation according to an embodiment of the invention.
  • Figure 3(b) shows successive meniscus positions during drop selection and separation.
  • Figure 3(c) shows the temperatures at various points during a drop selection cycle.
  • Figure 3(d) shows measured surface tension versus temperature curves for various ink additives.
  • Figure 3(e) shows the power pulses which are applied to the nozzle heater to generate the temperature curves of figure 3(c)
  • Figure 4 shows a block schematic diagram of print head drive circuitry for practice of the invention.
  • Figure 5 shows projected manufacturing yields for an A4 page width color print head embodying features of the invention, with and without fault tolerance.
  • Figure 6 shows a generalized block diagram of a printing system using a print head
  • Figure 7 shows a single silicon substrate with a multitude of nozzles etched in it.
  • Figures 8(a) to 8(d) shows an exemplary nozzle layout for a small section of a print head.
  • Figure 8(b) is a detail of figure 8(a).
  • Figures 9(a) to 9(o) show simplified manufacturing steps for combination with standard integrated circuit fabrication.
  • Figures 10(a) and 10(b) show heater layouts which do not use the present invention.
  • Figures 11(a) to 11(c) show one preferred manufacturing and construction sequence for the heater structures in accordance with this invention.
  • Figures 12(a) to 12(c) show another preferred manufacturing and construction sequence for a heater structure in accordance with this invention.
  • Figures 13(a), 13(c), 13(e), 13(g), 13(i), 13(k), and 13(m) are graphs of the position of the centre of the meniscus versus time for various heater widths.
  • Figures 13(b), 13(d), 13(f), 13(h), 13(j), 13(1), and 13(n) are plots of the meniscus shape at various instants for various heater widths.
  • the invention constitutes a drop-on-demand printing mechanism wherein the means of selecting drops to be printed produces a difference in position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an alternative means is provided t ⁇ cause separation of the selected drops from the body of ink.
  • the separation of drop selection means from drop separation means significantly reduces the energy required to select which ink drops are to be printed. Only the drop selection means must be driven by individual signals to each nozzle.
  • the drop separation means can be a field or condition applied simultaneously to all nozzles.
  • the drop selection means may be chosen from, but is not limited to, the following list: 1 ) Electrothermal reduction of surface tension of pressurized ink
  • the drop separation means may be chosen from, but is not limited to, the following list:
  • a drop velocity of approximately 10 meters per second is prefe ⁇ ed to ensure that the selected ink drops overcome ink surface tension, separate from the body of the ink, and strike the recording medium.
  • These systems have a very low efficiency of conversion of electrical energy into drop kinetic energy.
  • the efficiency of ⁇ J systems is approximately 0.02%).
  • This means that the drive circuits for TTJ print heads must switch high currents.
  • the drive circuits for piezoelectric inkjet heads must either switch high voltages, or drive highly capacitive loads.
  • the total power consumption of pagewidth TTJ printheads is also very high.
  • An 800 dpi A4 full color pagewidth TU print head printing a four color black image in one second would consume approximately 6 kW of electrical power, most of which is converted to waste heat. The difficulties of removal of this amount of heat precludes the production of low cost, high speed, high resolution compact pagewidth TTJ systems.
  • One important feature of embodiments of the invention is a means of significantly reducing the energy required to select which ink drops are to be printed. This is achieved by separating the means for selecting ink drops from the means for ensuring that selected drops separate from the body of ink and form dots on the recording medium. Only the drop selection means must be driven by individual signals to each nozzle.
  • the drop separation means can be a field or condition applied simultaneously to all nozzles.
  • Drop selection means shows some of the possible means for selecting drops in accordance with the invention.
  • the drop selection means is only required to create sufficient change in the position of selected drops that the drop separation means can discriminate between selected and unselected drops.
  • Drop selection means is only required to create sufficient change in the position of selected drops that the drop separation means can discriminate between selected and unselected drops.
  • the preferred drop selection means for water based inks is method 1 : "Electrothermal reduction of surface tension of pressurized ink”.
  • This drop selection means provides many advantages over other systems, including; low power operation (approximately 1% of TU), compatibility with CMOS VLSI chip fabrication, low voltage operation (approx. 10 V), high nozzle density, low temperature operation, and wide range of suitable ink formulations.
  • the ink must exhibit a reduction in surface tension with increasing temperature.
  • the preferred drop selection means for hot melt or oil based inks is method 2: ' ⁇ lectrothe ⁇ nal reduction of ink viscosity, combined with oscillating ink pressure".
  • This drop selection means is particularly suited for use with inks which exhibit a large reduction of viscosity with increasing temperature, but only a small reduction in surface tension. This occurs particularly with non-polar ink carriers with relatively high molecular weight This is especially applicable to hot melt and oil based inks.
  • the table “Drop separation means” shows some of the possible methods for separating selected drops from the body of ink, and ensuring that the selected drops form dots on the printing medium.
  • the drop separation means discriminates between selected drops and unselected drops to ensure that unselected drops do not form dots on the printing medium.
  • drop separation means may also be used.
  • the preferred drop separation means depends upon the intended use. For most applications, method 1: “Electrostatic attraction”, or method 2: “AC electric field” are most appropriate. For applications where smooth coated paper or film is used, and very high speed is not essential, method 3: “Proximity” may be appropriate. For high speed, high quality systems, method 4: 'Transfer proximity” can be used. Method 6: “Magnetic attraction” is appropriate for portable printing systems where the print medium is too rough for proximity printing, and the high voltages required for electrostatic drop separation are undesirable. There is no clear 'best' drop separation means which is applicable to all circumstances.
  • FIG. 1 A simplified schematic diagram of one preferred printing system according to the invention appears in Figure 1(a).
  • An image source 52 may be raster image data from a scanner or computer, or outline image data in the form of a page description language (PDL), or other forms of digital image representation.
  • This image data is converted to a pixel-mapped page image by the image processing system 53.
  • This may be a raster image processor (RIP) in the case of PDL image data, or may be pixel image manipulation in the case of raster image data.
  • Continuous tone data produced by the image processing unit 53 is halftoned. Halftoning is performed by the Digital
  • Halftoning unit 54 Halftoned bitmap image data is stored in the image memory 72.
  • the image memory 72 may be a full page memory, or a band memory.
  • Heater control circuits 71 read data from the image memory 72 and apply time- varying electrical pulses to the nozzle heaters (103 in figure 1 (b)) that are part of the print head 50. These pulses are applied at an appropriate time, and to the appropriate nozzle, so that selected drops will form spots on the recording medium 51 in the appropriate position designated by the data in the image memory 72.
  • the recording medium 51 is moved relative to the head 50 by a paper transport system 65, which is electronically controlled by a paper transport control system 66, which in turn is controlled by a microcontroller 315.
  • the paper transport system shown in figure 1(a) is schematic only, and many different mechanical configurations are possible. In the case of pagewidth print heads, it is most convenient to move the recording medium 51 past a stationary head 50.
  • the microcontroller 315 may also control the ink pressure regulator
  • ink is contained in an ink reservoir 64 under pressure.
  • the ink pressure is insufficient to overcome the ink surface tension and eject a drop.
  • a constant ink pressure can be achieved by applying pressure to the ink reservoir 64 under the control of an ink pressure regulator 63.
  • the ink pressure can be very accurately generated and controlled by situating the top surface of the ink in the reservoir 64 an appropriate distance above the head 50. This ink level can be regulated by a simple float valve (not shown).
  • ink is contained in an ink reservoir 64 under pressure, and the ink pressure is caused to oscillate.
  • the means of producing this oscillation may be a piezoelectric actuator mounted in the ink channels (not shown).
  • the ink is distributed to the back surface of the head 50 by an ink channel device 75.
  • the ink preferably flows through slots and/or holes etched through the silicon substrate of the head 50 to the front surface, where the nozzles and actuators are situated.
  • the nozzle actuators are electrothermal heaters.
  • an external field 74 is required to ensure that the selected drop separates from the body of the ink and moves towards the recording medium 51.
  • a convenient external field 74 is a constant electric field, as the ink is easily made to be electrically conductive.
  • the paper guide or platen 67 can be made of electrically conductive material and used as one electrode generating the electric field.
  • the other electrode can be the head 50 itself.
  • Another embodiment uses proximity of the print medium as a means of discriminating between selected drops and unselected drops.
  • Figure 1(b) is a detail enlargement of a cross section of a single microscopic nozzle tip embodiment of the invention, fabricated using a modified CMOS process.
  • the nozzle is etched in a substrate 101, which may be silicon, glass, metal, or any other suitable material. If substrates which are not semiconductor materials are used, a semiconducting material (such as amorphous silicon) may be deposited on the substrate, and integrated drive transistors and data distribution circuitry may be formed in the surface semiconducting layer.
  • Single crystal silicon (SCS) substrates have several advantages, including: 1 ) High performance drive transistors and other circuitry can be fabricated in SCS;
  • Print heads can be fabricated in existing facilities (fabs) using standard VLSI processing equipment;
  • SCS has high mechanical strength and rigidity; and 4) SCS has a high thermal conductivity.
  • the nozzle is of cylindrical form, with the heater 103 forming an annulus.
  • the nozzle tip 104 is fo ⁇ ned from silicon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry.
  • the nozzle tip is passivated with silicon nitride.
  • the protruding nozzle tip controls the contact point of the pressurized ink 100 on the print head surface.
  • the print head surface is also hydrophobized to prevent accidental spread of ink across the front of the print head.
  • Many other configurations of nozzles are possible, and nozzle embodiments of the invention may vary in shape, dimensions, and materials used.
  • Monolithic nozzles etched from the substrate upon which the heater and drive electronics are fo ⁇ ned have the advantage of not requiring an orifice plate.
  • the elimination of the orifice plate has significant cost savings in manufacture and assembly.
  • Recent methods for eliminating orifice plates include the use of 'vortex' actuators such as those described in Domoto et al US Pat. No. 4,580,158, 1986, assigned to Xerox, and Miller et al US Pat. No. 5,371,527, 1994 assigned to
  • This type of nozzle may be used for print heads using various techniques for drop separation.
  • Figure 2 shows the results of energy transport and fluid dynamic simulations performed using FIDAP, a commercial fluid dynamic simulation software package available from Fluid Dynamics Inc., of Illinois, USA.
  • FIDAP Fluid Dynamics Inc.
  • This simulation is of a thermal drop selection nozzle embodiment with a diameter of 8 ⁇ m, at an ambient temperature of 30°C.
  • the total energy applied to the heater is 276 nJ, applied as 69 pulses of 4 nJ each.
  • the ink pressure is 10 kPa above ambient air pressure, and the ink viscosity at 30°C is 1.84 cPs.
  • the ink is water based, and includes a sol of 0.1% palmitic acid to achieve an enhanced decrease in surface tension with increasing temperature.
  • a cross section of the nozzle tip from the central axis of the nozzle to a radial distance of 40 ⁇ m is shown.
  • Heat flow in the various materials of the nozzle including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials.
  • the time step of the simulation is 0.1 ⁇ s.
  • Figure 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus.
  • Figure 2(b) shows thermal contours at 5°C intervals 5 ⁇ s after the start of the heater energizing pulse. When the heater is energized, the ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes the heated portion of the meniscus to rapidly expand relative to the cool ink meniscus.
  • Figure 2(c) shows thermal contours at 5°C intervals 10 ⁇ s after the start of the heater energizing pulse.
  • the increase in temperature causes a decrease in surface tension, disturbing the equilibrium of forces. As the entire meniscus has been heated, the ink begins to flow.
  • Figure 2(d) shows thermal contours at 5°C intervals 20 ⁇ s after the start of the heater energizing pulse.
  • the ink pressure has caused the ink to flow to a new meniscus position, which protrudes from the print head.
  • the electrostatic field becomes concentrated by the protruding conductive ink drop.
  • Figure 2(e) shows thermal contours at 5°C intervals 30 ⁇ s after the start of the heater energizing pulse, which is also 6 ⁇ s after the end of the heater pulse, as the heater pulse duration is 24 ⁇ s.
  • the nozzle tip has rapidly cooled due to conduction through the oxide layers, and conduction into the flowing ink.
  • the nozzle tip is effectively 'water cooled' by the ink. Electrostatic attraction causes the ink drop to begin to accelerate towards the recording medium. Were the heater pulse significantly shorter (less than 16 ⁇ s in this case) the ink would not accelerate towards the print medium, but would instead return to the nozzle.
  • Figure 2(f) shows thermal contours at 5°C intervals 26 ⁇ s after the end of the heater pulse.
  • the temperature at the nozzle tip is now less than 5°C above ambient temperature. This causes an increase in surface tension around the nozzle tip.
  • the rate at which the ink is drawn from the nozzle exceeds the viscously limited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks', and the selected drop separates from the body of ink.
  • the selected drop then travels to the recording medium under the influence of the external electrostatic field.
  • the meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the next ink drop.
  • One ink drop is selected, separated and forms a spot on the recording medium for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved.
  • Figure 3(a) shows successive meniscus positions during the drop selection cycle at 5 ⁇ s intervals, starting at the beginning of the heater energizing pulse.
  • Figure 3(b) is a graph of meniscus position versus time, showing the movement of the point at the centre of the meniscus. The heater pulse starts 10 ⁇ s into the simulation.
  • Figure 3(c) shows the resultant curve of temperature with respect to time at various points in the nozzle.
  • the vertical axis of the graph is temperature, in units of 100°C.
  • the horizontal axis of the graph is time, in units of 10 ⁇ s.
  • the temperature curve shown in figure 3(b) was calculated by FIDAP, using 0.1 ⁇ s time steps.
  • the local ambient temperature is 30 degrees C. Temperature histories at three points are shown:
  • a - Nozzle tip This shows the temperature history at the circle of contact between the passivation layer, the ink, and air.
  • B - Meniscus midpoint This is at a circle on the ink meniscus midway between the nozzle tip and the centre of the meniscus.
  • C - Chip surface This is at a point on the print head surface 20 ⁇ m from the centre of the nozzle. The temperature only rises a few degrees. This indicates that active circuitry can be located very close to the nozzles without experiencing performance or lifetime degradation due to elevated temperatures.
  • Figure 3(e) shows the power applied to the heater.
  • Optimum operation requires a sharp rise in temperature at the start of the heater pulse, a maintenance of the temperature a little below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of the pulse.
  • the average energy applied to the heater is varied over the duration of the pulse.
  • the variation is achieved by pulse frequency modulation of 0.1 ⁇ s sub-pulses, each with an energy of 4 nJ.
  • the peak power applied to the heater is 40 mW, and the average power over the duration of the heater pulse is 11.5 mW.
  • the sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significantly affecting the operation of the print head.
  • a higher sub-pulse frequency allows finer control over the power applied to the heater.
  • a sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable for minimizing the effect of radio frequency interference (RFI).
  • RFID radio frequency
  • ⁇ *r is the surface tension at temperature T
  • k is a constant
  • 7 is the critical temperature of the liquid
  • M is the molar mass of the liquid
  • x is the degree of association of the liquid
  • p is the density of the liquid.
  • surfactant is important.
  • water based ink for thermal ink jet printers often contains isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying.
  • Isopropyl alcohol has a boiling point of 82.4°C, lower than that of water.
  • a surfactant such as 1-Hexanol (b.p. 158°C) can be used to reverse this effect, and achieve a surface tension which decreases slightly with temperature.
  • a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude.
  • a surface tension decrease of 20 mN/m over a 30°C temperature range is preferred to achieve large operating margins, while as little as lOmN/m can be used to achieve operation of the print head according to the present invention.
  • the ink may contain a low concentration sol of a surfactant which is solid at ambient temperatures, but melts at a threshold temperature. Particle sizes less than 1,000 A are desirable. Suitable surfactant melting points for a water based ink are between 50°C and 90°C, and preferably between 60°C and 80°C.
  • the ink may contain an oil/water microemulsion with a phase inversion temperature (PIT) which is above the maximum ambient temperature, but below the boiling point of the ink.
  • PIT phase inversion temperature
  • the PIT of the microemulsion is preferably 20°C or more above the maximum non-operating temperature encountered by the ink. A PIT of approximately 80°C is suitable.
  • Inks can be prepared as a sol of small particles of a surfactant which melts in the desired operating temperature range.
  • surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as:
  • the melting point of sols with a small particle size is usually slightly less than of the bulk material, it is preferable to choose a carboxyhc acid with a melting point slightly above the desired drop selection temperature.
  • a good example is Arachidic acid.
  • carboxyhc acids are available in high purity and at low cost.
  • the amount of surfactant required is very small, so the cost of adding them to the ink is insignificant
  • a mixture of carboxyhc acids with slightly varying chain lengths can be used to spread the melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid.
  • surfactant it is not necessary to restrict the choice of surfactant to simple unbranched carboxyhc acids.
  • Surfactants with branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxyhc acid.
  • Many highly polar moieties are suitable for the hydrophilic end of the surfactant It is desirable that the polar end be ionizable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent flocculation.
  • carboxylic acids this can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide.
  • the surfactant sol can be prepared separately at high concentration, and added to the ink in the required concentration.
  • An example process for creating the surfactant sol is as follows:
  • Inks made with anionic surfactant sols are generally unsuitable for use with catiomc dyes or pigments. This is because the cationic dye or pigment may precipitate or flocculate with the anionic surfactant.
  • a cationic surfactant sol is required. The family of alkylamines is suitable for this purpose.
  • the method of preparation of cationic surfactant sols is essentially similar to that of anionic surfactant sols, except that an acid instead of an alkali is used to adjust the pH balance and increase the charge on the surfactant particles.
  • a pH of 6 using HC1 is suitable.
  • Microemulsinn Based Inks An alternative means of achieving a large reduction in surface tension as some temperature threshold is to base the ink on a microemulsion.
  • a microemulsion is chosen with a phase inversion temperature (PIT) around the desired ejection threshold temperature. Below the PIT, the microemulsion is oil in water (O/W), and above the PIT the microemulsion is water in oil (W/O).
  • PIT phase inversion temperature
  • O/W oil in water
  • W/O water in oil
  • the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperatures significantly above the PIT, the surfactant prefers a high curvature surface around water.
  • the microemulsion forms a continuous 'sponge' of topologically connected water and oil.
  • the surfactant prefers surfaces with very low curvature.
  • surfactant molecules migrate to the ink/air interface, which has a curvature which is much less than the curvature of the oil emulsion. This lowers the surface tension of the water.
  • the microemulsion changes from O/W to W/O, and therefore the ink/air interface changes from water/air to oil/air.
  • the oil air interface has a lower surface tension.
  • water is a suitable polar solvent
  • polar solvents with a high surface tension should be chosen, so that a large decrease in surface tension is achievable.
  • the surfactant can be chosen to result in a phase inversion temperature in the desired range.
  • surfactants of the group poly(oxyethylene)alkylphenyl ether ethoxylated alkyl phenols, general formula: can be used.
  • the hydrophihcity of the surfactant can be increased by increasing m, and the hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable.
  • the HLB is 13.6, the melting point is 7°C, and the cloud, point is 65°C.
  • ethoxylated alkyl phenols include those listed in the following table:
  • Microemulsion based inks have advantages other than surface tension control: 1) Microemulsions are thermodynamically stable, and will not separate.
  • the storage time can be very long. This is especially significant for office and portable printers, which may be used sporadically.
  • microemulsion will form spontaneously with a particular drop size, and does not require extensive stirring, centrifuging, or filtering to ensure a particular range of emulsified oil drop sizes.
  • the amount of oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or both, can be used. It is also possible to use a mixture of dyes, one soluble in water, and the other soluble in oil, to obtain specific colors. 4) Oil miscible pigments are prevented from flocculating, as they are trapped in the oil microdroplets. 5) The use of a microemulsion can reduce the mixing of different dye colors on the surface of the print medium.
  • Oil in water mixtures can have high oil contents - as high as 40% - and still form O/W microemulsions. This allows a high dye or pigment loading.
  • the ninth combination is useful for printing transparent coatings, UV ink, and selective gloss highhghts.
  • dyes are amphiphiUc, large quantities of dyes can also be solubilized in the oil-water boundary layer as this layer has a very large surface area.
  • the color of the ink may be different on different substrates. If a dye and a pigment are used in combination, the color of the dye will tend to have a smaller contribution to the printed ink color on more absorptive papers, as the dye will be absorbed into the paper, while the pigment wiU tend to 'sit on top' of the paper. This may be used as an advantage in some circumstances.
  • a surfactant should be chosen with a Krafft point which is near the top of the range of temperatures to which the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperatures, and the concentration of surfactant in solution at the drop selection temperature.
  • the concentration of surfactant should be approximately equal to the CMC at the Krafft point In this manner, the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient temperatures.
  • Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension faUs with increasing temperature.
  • the POE chain is hydrophilic, and maintains the surfactant in solution.
  • the structured water around the POE section of the molecule is disrupted, and the POE section becomes hydrophobic.
  • the surfactant is increasingly rejected by the water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension.
  • the temperature at which the POE section of a nonionic surfactant becomes hydrophiUc is related to the cloud point of that surfactant POE chains by themselves are not particularly suitable, as the cloud point is generally above 100°C
  • Polyoxypropylene (POP) can be combined with POE in POE POP block copolymers to lower the cloud point of POE chains without introducing a strong hydrophobicity at low temperatures.
  • POP polyoxypropylene
  • Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between 40°C and 100°C, and preferably between 60°C and 80°C.
  • Meroxapol IHO(CHCH3CH 2 O),(CH 2 CH 2 O) y (CHCH3CH 2 O) z OH] varieties where the average x and z are approximately 4, and the average y is approximately 15 may be suitable. If salts are used to increase the electrical conductivity of the ink, then the effect of this salt on the cloud point of the surfactant should be considered.
  • the cloud point of POE surfactants is increased by ions that disrupt water structure (such as I " ), as this makes more water molecules available to form hydrogen bonds with the POE oxygen lone pairs.
  • the cloud point of POE surfactants is decreased by ions that form water structure (such as Cl " , OH " ), as fewer water molecules are available to form hydrogen bonds.
  • the ink composition can be 'tuned' for a desired temperature range by altering the lengths of POE and POP chains in a block copolymer surfactant, and by changing the choice of salts (e.g Cl ' to Br " to T) that are added to increase electrical conductivity.
  • salts e.g Cl ' to Br " to T
  • NaCl is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity.
  • SoUd 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink.
  • the hot melt ink must be formulated so that the surface tension of the molten ink decreases with temperature. A decrease of approximately
  • the temperature difference between quiescent temperature and drop selection temperature may be greater for a hot melt ink than for a water based ink, as water based inks are constrained by the boiling point of the water.
  • the ink must be Uquid at the quiescent temperature.
  • the quiescent temperature should be higher than the highest ambient temperature likely to be encountered by the printed page. T he quiescent temperature should also be as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop ejection temperatures.
  • a quiescent temperature between 60°C and 90°C is generally suitable, though other temperatures may be used.
  • a dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the drop ejection temperature, can be added to the hot melt ink while in the Uquid phase.
  • a polar/non-polar microemulsion with a PIT which is preferably at least 20°C above the melting points of both the polar and non-polar compounds.
  • the hot melt ink carrier have a relatively large surface tension
  • Figure 3(d) shows the measured effect of temperature on the surface tension of various aqueous preparations containing the following additives:
  • operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink is as foUows.
  • sohd ink Prior to operation of the printer, sohd ink is melted in the reservoir 64.
  • the reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperature at which the ink 100 is Uquid, but exhibits a relatively high viscosity (for example, approximately 100 cP).
  • the Ink 100 is retained in the nozzle by the surface tension of the ink.
  • the ink 100 is formulated so that the viscosity of the ink reduces with increasing temperature.
  • the ink pressure oscillates at a frequency which is an integral multiple of the drop ejection frequency from the nozzle.
  • the ink pressure oscillation causes osculations of the ink meniscus at the nozzle tips, but this osciUation is smaU due to the high ink viscosity. At the normal operating temperature, these os ⁇ Uations are of insufficient amphtude to result in drop separation.
  • the heater 103 When the heater 103 is energized, the ink forming the selected drop is heated, causing a reduction in viscosity to a value which is preferably less than 5 cP. The reduced viscosity results in the ink meniscus moving further during the high pressure part of the ink pressure cycle.
  • the recording medium 51 is arranged sufficiently close to the print head 50 so that the selected drops contact the recording medium 51, but sufficiently far away that the unselected drops do not contact the recording medium 51.
  • part of the selected drop freezes, and attaches to the recording medium.
  • ink pressure faUs ink begins to move back into the nozzle.
  • the body of ink separates from the ink which is frozen onto the recording medium.
  • the meniscus of the ink 100 at the nozzle tip then returns to low amphtude oscillation.
  • the viscosity of the ink increases to its quiescent level as remaining heat is dissipated to the bulk ink and print head.
  • One ink drop is selected, separated and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electricaUy controUed, drop on demand inkjet operation can be achieved.
  • An objective of printing systems according to the invention is to attain a print quaUty which is equal to that which people are accustomed to in quality color pubUcations printed using offset printing.
  • This can be achieved using a print resolution of approximately 1,600 dpi.
  • 1,600 dpi printing is difficult and expensive to achieve.
  • Similar results can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and magenta, and one bit per pixel for yellow and black.
  • This color model is herein caUed CC'MM' YK. Where high quaUty monochrome image printing is also required, two bits per pixel can also be used for black.
  • This color model is herein caUed CC'MM'YKK'. Color models, halftoning, data compression, and real-time expansion systems suitable for use in systems of this invention and other printing systems are described in the foUowing AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference:
  • Printing apparatus and methods of this invention are suitable for a wide range of appUcations, including (but not Umited to) the foUowing: color and monochrome office printing, short run digital printing, high speed digital printing, process color printing, spot color printing, offset press supplemental printing, low cost printers using scanning print heads, high speed printers using pagewidth print heads, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimile and copying machines, label printing, large format plotters, photographic dupUcation, printers for digital photographic processing, portable printers incorporated into digital 'instant' cameras, video printing, printing of PhotoCD images, portable printers for 'Personal Digital Assistants', wallpaper printing, indoor sign printing, billboard printing, and fabric printing.
  • drop on demand printing systems have consistent and predictable ink drop size and position. Unwanted variation in ink drop size and position causes variations in the optical density of the resultant print, reducing the perceived print quaUty. These variations should be kept to a smaU proportion of the nominal ink drop volume and pixel spacing respectively. Many environmental variables can be compensated to reduce their effect to insignificant levels. Active compensation of some factors can be achieved by varying the power appUed to the nozzle heaters.
  • An optimum temperature profile for one print head embodiment involves an instantaneous raising of the active region of the nozzle tip to the ejection temperature, maintenance of this region at the ejection temperature for the duration of the pulse, and instantaneous cooling of the region to the ambient temperature.
  • One way to achieve this is by 'broadcasting' a variety of different digital pulse trains across the print head chip, and selecting the appropriate pulse train for each nozzle using multiplexing circuits.
  • An example of the environmental factors which may be compensated for is Usted in the table "Compensation for environmental factors”. This table identifies which environmental factors are best compensated globaUy (for the entire print head), per chip (for each chip in a composite multi-chip print head), and per nozzle.
  • Figure 4 is a block schematic diagram showing electronic operation of an example head driver circuit in accordance with this invention.
  • This control circuit uses analog modulation of the power supply voltage appUed to the print head to achieve heater power modulation, and does not have individual control of the power applied to each nozzle.
  • Figure 4 shows a block diagram for a system using an 800 dpi pagewidth print head which prints process color using the CC'MM'YK color model.
  • the print head 50 has a total of 79,488 nozzles, with 39,744 main nozzles and 39,744 redundant nozzles.
  • the main and redundant nozzles are divided into six colors, and each color is divided into 8 drive phases.
  • Each drive phase has a shift register which converts the serial data from a head control ASIC 400 into parallel data for enabling heater drive circuits.
  • Each shift register is composed of 828 shift register stages 217, the outputs of which are logicaUy anded with phase enable signal by a nand gate 215.
  • the output of the nand gate 215 drives an inverting buffer 216, which in turn controls the drive transistor 201.
  • the drive transistor 201 actuates the electrothermal heater 200, which may be a heater 103 as shown in figure 1(b).
  • the clock to the shift register is stopped the enable pulse is active by a clock stopper 218, which is shown as a single gate for clarity, but is preferably any of a range of weU known glitch free clock control circuits. Stopping the clock of the shift register removes the requirement for a paraUel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400. Data is routed to either the main nozzles or the redundant nozzles by the data router 219 depending on the state of the appropriate signal of the fault status bus.
  • the print head shown in figure 4 is simplified, and does not show various means of improving manufacturing yield, such as block fault tolerance.
  • Drive circuits for different configurations of print head can readily be derived from the apparatus disclosed herein.
  • Digital information representing patterns of dots to be printed on the recording medium is stored in the Page or Band memory 1513, which may be the same as the Image memory 72 in figure 1 (a).
  • Data in 32 bit words representing dots of one color is read from the Page or Band memory 1513 using addresses selected by the address mux 417 and control signals generated by the Memory Interface 418.
  • Address generators 411 which forms part of the
  • the addresses are generated based on the positions of the nozzles in relation to the print medium. As the relative position of the nozzles may be different for different print heads, the Address generators 411 are preferably made programmable. The Address generators 411 normally generate the address corresponding to the position of the main nozzles. However, when faulty nozzles are present, locations of blocks of nozzles containing faults can be marked in the Fault Map RAM 412. The Fault Map RAM 412 is read as the page is printed. If the memory indicates a fault in the block of nozzles, the address is altered so that the Address generators 411 generate the address corresponding to the position of the redundant nozzles.
  • Data read from the Page or Band memory 1513 is latched by the latch 413 and converted to four sequential bytes by the multiplexer 414. Timing of these bytes is adjusted to match that of data representing other colors by the FIFO 415.
  • This data is then buffered by the buffer 430 to form the 48 bit main data bus to the print head 50.
  • the data is buffered as the print head may be located a relatively long distance from the head control ASIC.
  • Data from the Fault Map RAM 412 also forms the input to the FIFO 416. The timing of this data is matched to the data output of the FIFO 415, and buffered by the buffer 431 to form the fault status bus.
  • the programmable power supply 320 provides power for the head 50.
  • the voltage of the power supply 320 is controlled by the DAC 313, which is part of a RAM and DAC combination (RAMDAC) 316.
  • the RAMDAC 316 contains a dual port RAM 317.
  • the contents of the dual port RAM 317 are programmed by the MicrocontroUer 315. Temperature is compensated by changing the contents of the dual port RAM 317. These values are calculated by the microcontroUer 315 based on temperature sensed by a the ⁇ nal sensor 300.
  • the the ⁇ nal sensor 300 signal connects to the Analog to Digital Converter (ADC) 311.
  • ADC 311 is preferably incorporated in the MicrocontroUer 315.
  • the Head Control ASIC 400 contains control circuits for thermal lag compensation and print density.
  • Thermal lag compensation requires that the power supply voltage to the head 50 is a rapidly time- varying voltage which is synchronized with the enable pulse for the heater. This is achieved by programming the programmable power supply 320 to produce this voltage.
  • An analog time varying programming voltage is produced by the DAC 313 based upon data read from the dual port RAM 317. The data is read according to an address produced by the counter 403.
  • the counter 403 produces one complete cycle of addresses during the period of one enable pulse. This synchronization is ensured, as the counter 403 is clocked by the system clock 408, and the top count of the counter 403 is used to clock the enable counter 404.
  • the count from the enable counter 404 is then decoded by the decoder 405 and buffered by the buffer 432 to produce the enable pulses for the head 50.
  • the counter 403 may include a prescaler if the number of states in the count is less than the number of clock periods in one enable pulse. Sixteen voltage states are adequate to accurately compensate for the heater thermal lag. These sixteen states can be specified by using a four bit connection between the counter 403 and the dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To aUow non-linear timing of these states the counter 403 may also include a ROM or other device which causes the counter 403 to count in a non-linear fashion. Alternatively, fewer than sixteen states may be used.
  • the printing density is detected by counting the number of pixels to which a drop is to be printed ('on' pixels) in each enable period.
  • the 'on' pixels are counted by the On pixel counters 402.
  • the number of enable phases in a print head in accordance with the invention depend upon the specific design. Four, eight, and sixteen are convenient numbers, though there is no requirement that the number of enable phases is a power of two.
  • Counters 402 can be composed of combinatorial logic pixel counters 420 which determine how many bits in a nibble of data are on. This number is then accumulated by the adder 421 and accumulator 422. A latch 423 holds the accumulated value vahd for the duration of the enable pulse. The multiplexer 401 selects the output of the latch 423 which corresponds to the current enable phase, as determined by the enable counter 404. The output of the multiplexer 401 forms part of the address of the dual port RAM 317. An exact count of the number of 'on' pixels is not necessary, and the most significant four bits of this count are adequate. Combining the four bits of thermal lag compensation address and the four bits of print density compensation address means that the dual port RAM 317 has an 8 bit address.
  • the dual port RAM 317 contains 256 numbers, which are in a two dimensional array. These two dimensions are time (for thermal lag compensation) and print density. A third dimension - temperature - can be included. As the ambient temperature of the head varies only slowly, the microcontroUer 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print density at the current temperature. Periodically (for example, a few times a second), the microcontroUer senses the current head temperature and calculates this matrix.
  • the clock to the print head 50 is generated from the system clock 408 by the Head clock generator 407, and buffered by the buffer 406.
  • JTAG test circuits 499 may be included.
  • Invention compares the aspects of printing in accordance with the present invention with thermal inkjet printing technology.
  • Thermal ink jet printers use the foUowing fundamental operating principle.
  • a the ⁇ nal impulse caused by electrical resistance heating results in the explosive formation of a bubble in Uquid ink. Rapid and consistent bubble formation can be achieved by superheating the ink, so that sufficient heat is transferred to the ink before bubble nucleation is complete.
  • ink temperatures of approximately 280°C to 400°C are required.
  • the bubble formation causes a pressure wave which forces a drop of ink from the aperture with high velocity.
  • the bubble then coUapses, drawing ink from the ink reservoir to re-fiU the nozzle.
  • Thermal inkjet printing has been highly successful commerciaUy due to the high nozzle packing density and the use of weU established integrated circuit manufacturing techniques.
  • thermal inkjet printing technology faces significant technical problems including multi-part precision fabrication, device yield, image resolution, 'pepper' noise, printing speed, drive transistor power, waste power dissipation, satelhte drop formation, thermal stress, differential the ⁇ nal expansion, kogation, cavitation, rectified diffusion, and difficulties in ink formulation.
  • Printing in accordance with the present invention has many of the advantages of thermal ink jet printing, and completely or substantiaUy eliminates many of the inherent problems of thermal ink jet technology.
  • yield The percentage of operational devices which are produced from a wafer run is known as the yield. Yield has a direct influence on manufacturing cost. A device with a yield of 5% is effectively ten times more expensive to manufacture than an identical device with a yield of 50%.
  • Figure 5 is a graph of wafer sort yield versus defect density for a monoUthic fuU width color A4 head embodiment of the invention.
  • the head is 215 mm long by 5 mm wide.
  • the non fault tolerant yield 198 is calculated according to
  • Murphy's method which is a widely used yield prediction method. With a defect density of one defect per square cm, Murphy's method predicts a yield less than
  • Figure 5 also includes a graph of non fault tolerant yield 197 which exphcitly models the clustering of defects by introducing a defect clustering factor.
  • the defect clustering factor is not a controllable parameter in manufacturing, but is a characteristic of the manufacturing process.
  • the defect clustering factor for manufacturing processes can be expected to be approximately 2, in which case yield projections closely match Murphy's method.
  • a solution to the problem of low yield is to incorporate fault tolerance by including redundant functional units on the chip which are used to replace faulty functional units.
  • redundant sub-units In memory chips and most Wafer Scale Integration (WSI) devices, the physical location of redundant sub-units on the chip is not important. However, in printing heads the redundant sub-unit may contain one or more printing actuators. These must have a fixed spatial relationship to the page being printed. To be able to print a dot in the same position as a faulty actuator, redundant actuators must not be displaced in the non-scan direction. However, faulty actuators can be replaced with redundant actuators which are displaced in the scan direction. To ensure that the redundant actuator prints the dot in the same position as the faulty actuator, the data timing to the redundant actuator can be altered to compensate for the displacement in the scan direction.
  • the minimum physical dimensions of the head chip are determined by the width of the page being printed, the fragiUty of the head chip, and manufacturing constraints on fabrication of ink channels which supply ink to the back surface of the chip.
  • the minimum practical size for a full width, fuU color head for printing A4 size paper is approximately 215 mm x 5 mm. This size allows the inclusion of 100% redundancy without significantly increasing chip area, when using 1.5 ⁇ m CMOS fabrication technology. Therefore, a high level of fault tolerance can be included without significantly decreasing primary yield.
  • Figure 5 shows the fault tolerant sort yield 199 for a full width color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation.
  • This graph shows projected yield as a function of both defect density and defect clustering.
  • the yield projection shown in figure 5 indicates that thoroughly implemented fault tolerance can increase wafer sort yield from under 1% to more than 90% under identical manufacturing conditions. This can reduce the manufacturing cost by a factor of 100.
  • fault tolerance is highly recommended to improve yield and reliability of print heads containing thousands of printing nozzles, and thereby make pagewidth printing heads practical.
  • fault tolerance is not to be taken as an essential part of the present invention.
  • FIG. 6 A schematic diagram of a digital electronic printing system using a print head of this invention is shown in Figure 6.
  • This shows a monoUthic printing head 50 printing an image 60 composed of a multitude of ink drops onto a recording medium 51.
  • This medium will typically be paper, but can also be overhead transparency film, cloth, or many other substantiaUy flat surfaces which wiU accept ink drops.
  • the image to be printed is provided by an image source 52, which may be any image type which can be converted into a two dimensional array of pixels.
  • Typical image sources are image scanners, digitaUy stored images, images encoded in a page description language (PDL) such as Adobe Postscript, Adobe Postscript level 2, or Hewlett-Packard PCL 5, page images generated by a procedure-caU based rasterizer, such as Apple QuickDraw, Apple Quickdraw GX, or Microsoft GDI, or text in an electronic form such as ASCII.
  • PDL page description language
  • This image data is then converted by an image processing system 53 into a two dimensional array of pixels suitable for the particular printing system. This may be color or monochrome, and the data wiU typically have between 1 and 32 bits per pixel, depending upon the image source and the specifications of the printing system.
  • the image processing system may be a raster image processor (RIP) if the source image is a page description, or may be a two dimensional image processing system if the source image is from a scanner. If continuous tone images are required, then a halftoning system 54 is necessary. Suitable types of halftoning are based on dispersed dot ordered dither or error diffusion. Variations of these, commonly known as stochastic screening or frequency modulation screening are suitable. The halftoning system commonly used for offset printing - clustered dot ordered dither - is not recommended, as effective image resolution is unnecessarily wasted using this technique. The output of the halftoning system is a binary monochrome or color image at the resolution of the printing system according to the present invention.
  • RIP raster image processor
  • the binary image is processed by a data phasing circuit 55 (which may be incorporated in a Head Control ASIC 400 as shown in figure 4) which provides the pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and the movement of the paper.
  • the driver circuits 57 When the data has been loaded into the shift registers 56, it is presented in paraUel to the heater driver circuits 57. At the correct time, the driver circuits 57 will electronically connect the corresponding heaters 58 with the voltage pulse generated by the pulse shaper circuit 61 and the voltage regulator 62. The heaters 58 heat the tip of the nozzles 59, affecting the physical characteristics of the ink.
  • Ink drops 60 escape from the nozzles in a pattern which corresponds to the digital impulses which have been apphed to the heater driver circuits.
  • the pressure of the ink in the ink reservoir 64 is regulated by the pressure regulator 63.
  • Selected drops of ink drops 60 are separated from the body of ink by the chosen drop separation means, and contact the recording medium 51.
  • the recording medium 51 is continually moved relative to the print head 50 by the paper transport system 65. If the print head 50 is the fuU width of the print region of the recording medium 51, it is only necessary to move the recording medium 51 in one direction, and the print head 50 can remain fixed. If a smaUer print head 50 is used, it is necessary to implement a raster scan system. This is typicaUy achieved by scanning the print head 50 along the short dimension of the recording medium 51 , while moving the recording medium 51 along its long dimension.
  • a printing speed of 60 A4 pages per minute (one page per second) wiU generaUy be adequate for many appUcations.
  • achieving an electronicaUy controUed print speed of 60 pages per minute is not simple.
  • the minimum time taken to print a page is equal to the number of dot positions on the page times the time required to print a dot, divided by the number of dots of each color which can be printed simultaneously.
  • the image quality that can be obtained is affected by the total number of ink dots which can be used to create an image.
  • approximately 800 dots per inch (31.5 dots per mm) are required.
  • the spacing between dots on the paper is 31.75 ⁇ m.
  • a standard A4 page is 210 mm times 297 mm. At 31.5 dots per mm, 61,886,632 dots are required for a monochrome fuU bleed A4 page.
  • High quaUty process color printing requires four colors - cyan, magenta, yellow, and black. Therefore, the total number of dots required is 247,546,528. WhUe this can be reduced somewhat by not allowing printing in a smaU margin at the edge of the paper, the total number of dots required is stiU very large. If the time taken to print a dot is 144 ms, and only one nozzle per color is provided, then it will take more than two hours to print a single page.
  • printing heads with many smaU nozzles are preferred.
  • the printing of a 800 dpi color A4 page in one second can be achieved if the printing head is the fiill width of the paper.
  • the printing head can be stationary, and the paper can travel past it in the one second period.
  • a four color 800 dpi printing head 210 mm wide requires 26,460 nozzles.
  • Such a print head may contain 26,460 active nozzles, and 26,460 redundant (spare) nozzles, giving a total of 52,920 nozzles. There are 6,615 active nozzles for each of the cyan, magenta, yeUow, and black process colors.
  • Print heads with large numbers of nozzles can be manufactured at low cost This can be achieved by using semiconductor manufacturing processes to simultaneously fabricate many thousands of nozzles in a sihcon wafer. To eliminate problems with mechanical alignment and differential thermal expansion that would occur if the print head were to be manufactured in several parts and assembled, the head can be manufactured from a single piece of sihcon. Nozzles and ink channels are etched into the sihcon. Heater elements are formed by evaporation of resistive materials, and subsequent photoUthography using standard semiconductor manufacturing processes. To reduce the large number of connections that would be required on a print head with thousands of nozzles, data distribution circuits and drive circuits can also be integrated on the print head.
  • Figure 7 is a simplified view of a portion of a print head, seen from the back surface of the chip, and cut through some of the nozzles.
  • the substrate 120 can be made from a single sihcon crystal.
  • Nozzles 121 are fabricated in the substrate, e.g., by semiconductor photolithography and chemical wet etch or plasma etching processes. Ink enters the nozzle at the top surface of the head, passes through the substrate, and leaves via the nozzle tip 123. Planar fabrication of the heaters and the drive circuitry is on the underside of the wafer; that is, the print head is shown 'upside down' in relation the surface upon which active circuitry is fabricated.
  • the substrate thickness 124 can be that of a standard sihcon wafer, approximately 650 ⁇ m.
  • the head width 125 is related to the number of colors, the arrangement of nozzles, the spacing between the nozzles, and the head area required for drive circuitry and interconnections. For a monochrome head, an appropriate width would be approximately 2 mm. For a process color head, an appropriate width would be approximately 5 mm. For a CC'MM'YK color print head, the appropriate head width is approximately 8 mm. The length of the head
  • 126 depends upon the appUcation. Very low cost appUcations may use short heads, which must be scanned over a page. High speed appUcations can use fixed page- width monoUthic or multi-chip print heads. A typical range of lengths for print heads is between 1 cm and 21 cm, though print heads longer than 21 cm are appropriate for high volume paper or fabric printing.
  • the process described herein is based on standard semiconductor manufacturing processes, and can use equipment designed for 1.5 ⁇ m line widths.
  • the use of Uthographic equipment which is essentiaUy obsolete (at the time of writing, the latest production IC manufacturing equipment is capable of 0.25 ⁇ m line widths) can substantially reduce the cost of estabUshing factories for the production of heads.
  • CMOS complementary metal-oxide-semiconductor
  • VLSI CMOS Low power, high speed process
  • the speeds required are moderate, and the power consumption is dominated by the heater power required for the inkjet nozzles. Therefore, a simple technology such as nMOS is adequate.
  • CMOS is likely to be the most practical production solution, as there is a significant amount of idle CMOS manufacturing capability available with line widths between 1 ⁇ m and 2 ⁇ m.
  • the manufacturing steps required for fabricating nozzles can be incorporated into many different semiconductor processing systems. For example, it is possible to manufacture print heads by modifying the foUowing technologies: 1) nMOS 2) pMOS
  • TFT Thin Film Transistors
  • the choice of the base technology is largely independent of the ability to fabricate nozzles.
  • the method of incorporation of nozzle manufacturing steps into semiconductor processing procedures which have not yet been invented is also likely to be obvious to those skilled in the art.
  • the simplest fabrication process is to manufacture the nozzles using sihcon micromechanical processing, without fabricating active semiconductor devices on the same wafer.
  • this approach is not practical for heads with large numbers of nozzles, as at least one external connection to the head is required for each nozzle.
  • CMOS is currently the most popular integrated circuit process. At present, many CMOS processes are in commercial use, with line widths as smaU as 0.35 ⁇ m being in common use. CMOS offers the foUowing advantages for the fabrication of heads:
  • the substrate can be grounded from the front side of the wafer.
  • CMOS has, however, some disadvantages over nMOS and other technologies in the fabrication of heads which include integrated drive circuitry. These include:
  • CMOS is susceptible to latchup. This is of particular concern due to the high currents at a voltage typically greater than Vdd that are required for the heater circuits.
  • CMOS is susceptible to electrostatic discharge damage. This can be minimized by including protection circuits at the inputs, and by careful handling.
  • Layout example Figure 8(a) shows an example layout for a section of an 800 dpi four color head.
  • the nozzle pitch for 800 dpi printing is 31.75 ⁇ m.
  • Figure 8(a) shows four rows of nozzles, for cyan, magenta, yellow, and black inks. Each of these four rows contains four paraUel ink channels.
  • the ink channels are etched almost through the wafer and each contains 64 nozzles. Two ink channels are for the main nozzles, and two ink channels are for the redundant nozzles.
  • the nozzles are spaced by two pixel widths (63.5 ⁇ m) along each ink channel.
  • the nozzles in one of the two main ink channels for each color are offset by one pixel width (31.75 ⁇ m) from the nozzles in the other main ink channel.
  • the redundant nozzles are arranged in an identical manner, but offset in the print direction.
  • the ink channels do not extend the entire length of the print region of the print head, as this would mechanicaUy weaken the print head too much. Instead, ink channels containing 64 nozzles are staggered in the print direction. Using a staggered array of nozzles such as this requires that the data be provided to drive the nozzles in such a manner as to compensate for the nozzle offsets. This can be achieved by digital circuitry which reads the page image from memory in the appropriate order and suppUes the data to the print head.
  • Rectangular regions 100 ⁇ m wide and 200 ⁇ m long are shown along the short edge chip layout. These are bonding pads for data, clocks, and logic power and ground.
  • the V " and V " bonding pads extend along the entire two long edges of the chip, and are 200 ⁇ m wide.
  • Figure 8(b) is a detail enlargement of the ink channels and nozzles for one color of the print head shown in figure 8(a).
  • the distance 4,064 mm is 64 times the nozzle spacing in a channel (63.5 ⁇ m).
  • the distance 8,128 ⁇ m is 128 times the nozzle spacing in a channel.
  • the distance 6790.6 ⁇ m is 4064 ⁇ m plus 2 * (1260 ⁇ m + (50 ⁇ m / tan 70.52°) + (50 ⁇ m / tan 54.74°) + 50 ⁇ m tolerance).
  • the wafer thickness may vary by as much as 25 ⁇ m.
  • Figure 8(c) is a detail enlargement of the end of a single ink channel.
  • the angles shown are due to the anisotropic etching process, and result from the orientation of the ⁇ 111 ⁇ crystaUographic planes.
  • an extra length of 1260 ⁇ m must be provided at the ends of the slot to be etched.
  • Figure 8(d) is a detail enlargement of two of the nozzles shown in figure 8(c).
  • the nozzle radius is 10 ⁇ m, therefore the nozzle diameter is 20 ⁇ m.
  • the nozzle barrel is shown as a dotted line.
  • the nozzle barrel does not have a well defined radius, as it is formed by a boron diffusion etch stop for KOH etching.
  • the distance from the edge of the nozzle to the edge of the ink channel is 15 ⁇ m. This is because the surface of the wafer is typicaUy not perfectly aUgned to the (110) crystaHographic plane, but may vary by as much as ⁇ 1 °.
  • the line from A to B in figure 8(d) is the line through which the cross section diagrams of figure 9 are taken. This line includes a heater connection on the "A" side, and goes through a 'normal' section of the heater on the "B" side.
  • the manufacturing process described herein uses the crystaUographic planes inherent in the single crystal sihcon wafer to control etching.
  • the orientation of the masking procedures to the ⁇ 111 ⁇ planes must be precisely controUed.
  • the orientation of the primary flats on a sihcon wafer are normally only accurate to within ⁇ 1° of the appropriate crystal plane. It is essential that this angular tolerance be taken into account in the design of the mask and manufacturing processes.
  • a groove is to be etched along the long edges of a 215 mm print head, then a 1 ° error in the ahgnment of the wafer to the ⁇ 111 ⁇ planes controlling the etch rates wiU result in a 3,752 ⁇ m error in the width of the groove, given sufficient etch time.
  • An ahgnment error of ⁇ 0.1 ° or less is required. This can be achieved by etching a test groove in an area of the wafer which is imused. The groove should be long, and aligned to a (111) plane using the primary flat to align the wafer. The test groove is then over-etched using a solution of 500 grams of
  • a ⁇ 1° e ⁇ or in ahgnment of the surface contributes a maximum of 11.3 ⁇ m of positional inaccuracy when etching through the entire wafer thickness. This is accommodated in the design of the etch masks.
  • the first manufacturing step is the deU very of the wafers .
  • Sihcon wafers are highly recommended over other materials such as gallium arsenide, due to the availabiUty of large, high quaUty wafers at low cost, the strength of sihcon as a substrate, and the general maturity of fabrication processes and equipment
  • the example manufacturing process described herein uses n-type wafers with (110) crystaUographic orientation.
  • the wafers should not be mechanicaUy or laser gettered, as this wiU affect back surface etching processes.
  • FIG. 9(a) shows a (110) n-type 300 mm wafer. The wafer shows
  • Each print head chip is 215 mm long - 8 mm wide. These print heads can be used for US letter or A4 size printing, or as components in multi-chip print heads for A3 printing, sheet fed or web fed digital printing presses, and cloth printing.
  • the boundary of each chip is etched with a deep groove. This groove can be etched before or after the fabrication of the active devices, depending upon process flow for the active devices. However, it is recommended that the grooves be etched after most fabrication steps are complete to avoid problems with resist edge beading at the grooves.
  • Figure 9(b) shows a cross section of the boundary groove along the short edges of the chip. CrystaUographic planes of the ⁇ 111 ⁇ family control the etch direction, resulting in a slope of 26.56° in the groove.
  • Figure 9(c) shows a cross section of the boundary groove along the long edges of the chip.
  • CrystaUographic planes of the ⁇ 111 ⁇ family control the etch direction, resulting in vertical sidewaUs in the groove.
  • the grooves are only required for proximity print heads, and are formed so that the electrical connections to the print head do not protrude beyond the surface of the chip.
  • the etching of these grooves is best performed after the fabrication of the active devices on the chip, and is described in steps 5) and 6) below. 2)
  • a boron etch stop is then diffused into the sihcon. The etch stop is required only in the regions of the bottoms of the ink channels, and is masked from the nozzles by an oversize mask.
  • FIG. 9(d) shows a cross section of wafer in the region of a nozzle tip after the boron doping stage.
  • the active devices are then fabricated using a prior art integrated circuit fabrication process with double layer metal.
  • the prior art process may be nMOS, pMOS, CMOS, Bipolar, or other process.
  • the active circuits can be fabricated using unmodified processes.
  • some processes wiU need modification to aUow for the large currents which may flow though a head.
  • a large head may have in excess of 28 Amperes flowing through the heater circuits when fuUy energized, it is essential to prevent electromigration.
  • Molybdenum can be used instead of aluminum for first level metal, as it is resistant to electromigration.
  • care must be taken not to damage underlying MOS or CMOS structures.
  • the preferred method of preventing electromigration is the provision of very wide aluminum traces which form a grid over the surface of the print head. This approach does not require modification of the manufacturing process, but must be considered in the mask pattern design. The prior-art manufacturing process proceeds unaltered up to the stage of appUcation of the inter-level dielectric.
  • Etch the bonding pad grooves The etch can be performed by an anisotropic wet etch, which etches the [100] crystaUographic direction preferentiaUy to the [ 111 ] direction.
  • a solution of 440 grams of potassium hydroxide (KOH) per Uter of water can be used for a very high preferential etch rate (approximately 400:1).
  • Figure 9(b) shows a cross section of V groove at the short edge of the heads after this etching step.
  • Figure 9(c) shows a cross section of the boundary groove along the long edges of the chip. CrystaUographic planes of the ⁇ 111 ⁇ family control the eich direction, resulting in vertical sidewaUs in the groove.
  • a 0.5 ⁇ m layer of CVD SiO 2 should be apphed after etching the V grooves to insulate the bonding pads from the substrate. 7) Etch the inter-metal vias. In some cases, this step may be able to be combined with the etching of the SiO 2 to form the mask for V groove etching. As the inter-metal SiO 2 is much thicker than normal, tapering of the via sidewaUs is recommended.
  • Second level metal As with the first level metal, electromigration must be taken into account. Electromigration can be minimized by using large line-widths for aU high current traces, and by using an aluminum alloy containing 2% copper. Molybdenum is not recommended due to the difficulty in bonding to molybdenum thin films.
  • the step coverage of the second level metal is important as the inter-level oxide is thicker than normal. Also, the vertical sidewaUs of the V " and V * grooves along the long edges of the chips must be coated. Adequate step coverage is possible by using low pressure evaporation. Via step coverage can be improved by placing vias only to areas where the first level metal covers field oxide.
  • the preferred process is the deposition by low pressure evaporation of 1mm of 98% aluminum, 2% copper. 9) Mask and etch second level metal. Special attention to masking and etching of the bonding pads is required if the print head is to be used for close proximity printing, as they are fabricated in the V grooves. This introduces two problems: the resist thickness wiU be greater in the bottom of the V grooves, and the mask wiU be out of focus. This does not pose a problem for the long edges of the chip, as these are dedicated to the V " and N " power rails, and are not patterned. Bonding pads fabricated on the short edges of the chip should be separated by at least 100 ⁇ m. No active circuitry or fine geometry lines should be located in the V grooves.
  • Figure 9(e) shows a cross section of the wafer in the region of a nozzle after this step.
  • the heater material for example 0.05 ⁇ m of TaAl aUoy, or refractory materials such as HfB 2 or ZrB 2
  • the heater is planar, masking and etching is straightforward.
  • the heater is masked as a disk rather than an annulus.
  • the centre of the disk is later etched during the nozzle formation step. This is to ensure excellent ahgnment between the heater and the nozzle.
  • Heater radius should be controUed to finer tolerance than is generally available in a 1.5 ⁇ m process, and the use of a stepper for 0.5 ⁇ m process is recommended.
  • Figure 9(f) shows a cross section of the wafer in the region of a nozzle after this step.
  • 11) Apply a protective coating of Si 3 N 4 . This is applied to the front face of the wafer only, and should be at least 0.1 ⁇ m thick to protect the front face of the wafer from attack by the long wet-etch of the back face of the wafer.
  • Figure 9(g) shows a cross section of the wafer in the region of a nozzle after this step. 12) Mask the back surface of the wafer.
  • Si 3 N 4 is used as a mask, as resist is attacked by the wet etching solution, and the etch rate of SiO 2 is too high (approx. 20 A/ minute) for effective use as a mask.
  • the etch rate of Si 3 N is approximately 14 A/hour.
  • the ink channels are oriented paraUel to two of these paraUel planes so that the ⁇ 111 ⁇ planes define the vertical sidewaUs of the ink channels.
  • a further two ⁇ 111 ⁇ planes are oriented at an angle of 26.56° to the wafer surface in the plane of the ink channels, and limit the etch depth of the ink channels. For this reason, the ink channel mask must be made longer than the required channel length, so that the full etch depth is attained where required in the ink channel.
  • FIG. 9(h) is a perspective view of some of the ink channels after etching. This view is from the back surface of the wafer.
  • Figure 9(i) shows a cross section of the wafer in the region of a nozzle after this step.
  • the first step is the etching of the Si 3 N 4 layer.
  • the second step is etching the heater. As the heater is very thin, a wet etch can be used.
  • the third step is the etching of the SiO 2 forming the nozzle tip. This should be etched with an anisotropic etch, for example an RIE etch using CF - H 2 gas mixture. The etch is down to sihcon in the nozzle region. The resist is then stripped.
  • Figure 9(k) shows a cross section of the wafer in the region of a nozzle after this step.
  • Etch the nozzle barrels This is also performed by a wet etch of the sihcon using KOH. Etching proceeds from both sides of the wafer at the same time, with etching from the rear occurring through the ink channels, and etching from the front occurring through the nozzle tip. Approximately 20 ⁇ m of sihcon thickness must be etched, 10 ⁇ m from each side. However, as the boron etch stop controls the geometry of the final nozzle barrel, etch time is not critical, and should be substantiaUy longer than the minimum etch time to accommodate 25 ⁇ m variations in wafer thickness and variable etch rates. Etch the wafer in a 50% solution of KOH in water at 80°C for 1 hour.
  • Figure 9Q is a perspective view of some of the nozzle barrels in two of the ink channels after this step. This view is from the back surface of the wafer, looking down into two adjacent ink channels.
  • the circular apertures are the nozzle tips.
  • the arrangement is for a 800 dpi printer with 31.75 ⁇ m pixel spacing.
  • the nozzles in each channel are spaced at 63.5 ⁇ m, and are offset between the two channels by 31.75 ⁇ m.
  • the diameter of the nozzle tip is 20 ⁇ m.
  • the line A to B is the line of the cross sections in figure 9., as shown in figure 8(d).
  • Figure 9(m) shows a cross section of the wafer in the region of a nozzle after this step. 17) Form the passivation layer.
  • a hydrophobic surface coating may be appUed at this stage, if the coating chosen can survive the subsequent processing steps. Otherwise, the hydrophobic coating should be apphed after TAB bonding.
  • hydrophobic coatings which may be used, and many methods which may be used to apply them.
  • one such suitable coating is fluorinated diamond ⁇ like carbon (F*DLC), an amorphous carbon film with the outer surface substantiaUy saturated with fluorine.
  • F*DLC fluorinated diamond ⁇ like carbon
  • PECVD plasma enhanced chemical vapor deposition
  • the exposed dielectric layer can be treated with a hydrophobising agent
  • a hydrophobising agent For example, if SiO 2 is used as the passivation layer in place of Si 3 N , the device can be treated with dimethyldichlorosilane to make the exposed SiO 2 hydrophobic. This wiU affect the entire nozzle, unless the regions which are to remain hydrophilic are masked, as dimethyldichlorosUane fumes wiU affect any exposed SiO 2 .
  • the appUcation of a hydrophobic layer is required if the ink is water based, or based on some other polar solvent If the ink is wax based or uses a non- polar solvent, then the front surface of the head should be Upophobic. In summary, the front surface of the head should be fabricated or treated in such a manner as to repel the ink used.
  • the hydrophobic layer need not be Umited to the front surface of the device.
  • the entire device may be coated with a hydrophobic layer (or hpophobic layer is non- polar ink is used) without significantly affecting the performance of the device.
  • Bond, package and test The bonding, packaging, and testing processes can use standard manufacturing techniques. Bonding pads must be opened out from the Si 3 N 4 passivation layer. Although the bonding pads are fabricated at an angle in the V groove, no special care is required to mask them, as the entire V groove area can be stripped of Si 3 N . After the bonding pads have been opened, the resist must be stripped, and the wafer cleaned. Then wafer testing can proceed. Then the wafer is diced.
  • FIG. 9(0) shows a cross section of the wafer in the region of a nozzle after this step.
  • 100 is ink
  • 101 is sihcon
  • 102 is CVD
  • SiO ,103 is the heater material
  • 105 is boron doped silicon
  • 106 is the second layer metal interconnect (aluminum)
  • 107 is resist
  • 108 is sihcon nitride (Si 3 N )
  • 109 is the hydrophobic surface coating.
  • the use of l ⁇ m line widths (or even finer geometries) aUows more circuitry to be integrated on the wafer, and aUows a reduction in either the size or the on resistance (or both) of the drive transistors.
  • the smaUer device geometries can be used in the foUowing, or a combination of the following, ways: 1) To reduce the width of the monolithic head 2) To increase the yield of the head, by incorporating more sophisticated fault tolerance circuitry
  • data phasing circuits can be incorporated on chip, and the head can be suppUed with a standard memory interface, via which it acquires the printing data by direct memory access. It is possible to alter the nozzle formation processes in many ways.
  • the process described herein is a preferred process for production of printing heads as it allows high resolution, full color heads to incorporate drive circuitry, data distribution circuitry, and fault tolerance, and can be manufactured with relatively low cost extensions to standard CMOS production processes. Many simpler head manufacturing processes can be derived. In particular, heads which do not include active circuitry may be manufactured using much simpler processes.
  • Heater structure Figures 10(a) and 10(b) show heater structures which do not use the invention disclosed herein.
  • the heater is planar, with the plane being that of the diagram.
  • the nozzle is substantiaUy cylindrical, with the axis of symmetry being a line normal to the centre of the heater.
  • the completed nozzle diameter is 20 ⁇ m, being formed as a 21 ⁇ m diameter hole with 0.5 ⁇ m thick passivation layer on the inside.
  • the heaters are fabricated using 2 ⁇ m linewidths, with 2 ⁇ m spacing.
  • Figure 10(a) shows a heater 103 formed as an open loop around a nozzle.
  • the nozzle is filled with ink 100, and is coated by a passivation layer 108 protecting the heater and active devices from the ink.
  • This nozzle structure has several problems: there is a substantial 'cold' spot where the circle is broken to form electrodes for connection to the drive electronics. This may affect both the direction and the reliability of drop ejection.
  • FIG. 10(b) shows a heater 103 formed as a closed loop around a nozzle.
  • the nozzle is filled with ink 100, and is coated by a passivation layer 108 protecting the heater and active devices from the ink.
  • This nozzle structure reduces the 'cold spot' associated with the nozzle structure of figure 10(a), but retains the other problems.
  • the electrical resistance of the heater is also reduced by a factor of approximately 4, which can result in design restraints on the drive circuitry or heater material.
  • the invention is a structure and manufacturing process for electrothermal heaters for integrated printing heads where the axes of the ink nozzles are substantiaUy normal to the plane of the heaters, and pass substantiaUy through the centers of the heaters.
  • the completed heater forms a loop, with the centre of the loop being formed by the nozzle etching process.
  • a disk of heater material is formed on the substrate.
  • the radius of the disk is equal to the radius of hole to be etched for the nozzle, plus the required heater width.
  • the nozzle hole is subsequently etched from the centre of the heater disk.
  • the remaining heater material is in the form of an annulus, with the internal radius being equal to the nozzle hole radius.
  • the structure is subsequently coated with a layer of passivation material.
  • This fabrication procedure has the advantage of reducing the distance between the heater and the ink.
  • a linewidth of the optical pattern would be required between the heater and a nozzle formed through the centre of the heater. For a 2 ⁇ m process, this is 2 ⁇ m (or 3 ⁇ m if the heater material is opticaUy reflective).
  • This construction disclosed reduces the distance between the inner radius of the heater and the etched nozzle hole to zero.
  • the heater is only separated from the ink by the passivation layer or layers. This increases the thermal coupling between the heater and the ink, thereby increasing the print head efficiency. Also, ahgnment variations between the inner radius of the heater and the nozzle are eliminated.
  • FIG 11 (a) shows a heater pattern that can be used with this invention.
  • the heater 103 is patterned as a filled disk with two electrodes on opposite sides. It is not necessary for the disk to be completely fiUed, it is sufficient that the inner radius of the disk be less than the radius of the nozzle hole minus mask exposure, ahgnment, and etching tolerances.
  • Figure 11(b) shows the heater 103 after the nozzle hole has been etched. The nozzle hole is etched through the centre of the heater. In this case, the diameter of the nozzle hole is 21 ⁇ m, leaving an annulus of 2 ⁇ m width of heater material with an outside diameter of 25 ⁇ m.
  • Figure 11 (c) shows the nozzle after application of a 0.5 ⁇ m passivation layer 108, and after filling with ink 100.
  • the distance between the inner radius of the heater and the ink is equal to the thickness of the passivation layer.
  • FIG 12(a) shows an alternative heater pattern that can be used with this invention.
  • the heater 103 is patterned as a filled disk with two electrodes on the same side, and with a slot which separates the electrodes and extends towards the centre of the disk.
  • Figure 12(b) shows the alternative heater 103 after the nozzle hole has been etched.
  • the nozzle hole is etched through the centre of the heater.
  • the diameter of the nozzle hole is 21 ⁇ m, leaving an annulus of 2 ⁇ m width of heater material with an outside diameter of 25 ⁇ m.
  • Figure 12(c) shows the nozzle after application of a 0.5 ⁇ m passivation layer 108, and after filling with ink 100.
  • the distance between the inner radius of the heater and the ink is also equal to the thickness of the passivation layer.
  • This pattern has an advantage of approximately four times higher resistance than the pattern of figure 11. It can therefore be used with heater materials of lower resistivity, thicker heaters, higher drive voltages, lower drive currents, or some combination of the above. It may be more appropriate that the pattern of figure 11 in some print head designs.
  • the width of the heater can be controUed accurately, but not exactly.
  • the width wiU vary depending upon the characteristics of the mask exposure system, the exposure time, the type of resist used, resist pre-bake and postbake temperature and time, resist developer temperature and time, and the characteristics of the etchant.
  • the cumulative variation in the width of the heater line can readily be reduced to less than ⁇ 0.1 ⁇ m using modem equipment, but higher tolerance requirements often leads to higher manufacturing costs.
  • Fluid dynamic simulations of nozzles have been performed of variations of ⁇ 0.6 ⁇ m from a nominal heater width of 2 ⁇ m. These simulations assume that the variations occur on the same print head, so the effects of the heater variation cannot be compensated by altering drive voltage, ink pressure, or other parameters without adversely affecting other nozzles on the same print head. For this reason, all parameters other than heater width are kept constant
  • the main effect of variation in heater width is a change to the amount of energy dehvered to the nozzle when using the same drive voltage.
  • FIGS. 13(a) to 13(n) show summarized results of fluid dynamic simulations performed using the FIDAP simulation software. In each case the simulation is over a duration of 100 ⁇ s, in 0.1 ⁇ s steps.
  • the nozzle tip is cylindrical, with a radius of 10 ⁇ m.
  • the ink pressure is 7.7 kPa, and the ambient temperature is 30°C.
  • a time varying power pulse is applied to the heater, starting at 20 ⁇ s, for a duration of 18 ⁇ s. The pulse starts at 20 ⁇ s to allow time for the ink meniscus to reach the quiescent position before the drop selection pulse.
  • the drop separation process may be proximity, electrostatic, or other means. Separation of the selected drop from unselected drops rehes upon a physical difference in meniscus position between the selected drop and the unselected drops. A n axial difference of 15 ⁇ m between the position of the centre of the meniscus before and after the drop selection pulse is adequate for drop separation.
  • Figures 13(a), 13(c), 13(e), 13(g), 13(i), 13(k), and 13(m) are graphs of the position of the centre of the meniscus versus time for heater widths of 1.4 ⁇ m, 1.6 ⁇ m, 1.8 ⁇ m, 2.0 ⁇ m, 2.2 ⁇ m, 2.4 ⁇ m, and 2.6 ⁇ m respectively.
  • Figures 13(b), 13(d), 13(f), 13(h), 13(j), 130), and 13(n) are plots of the meniscus shape at various instants for heater widths of 1.4 ⁇ m, 1.6 ⁇ m, 1.8 ⁇ m, 2.0 ⁇ m, 2.2 ⁇ m, 2.4 ⁇ m, and 2.6 ⁇ m respectively.
  • the meniscus positions are shown at 2 ⁇ s intervals from the start of the drop selection pulse at 20 ⁇ s to 4 ⁇ s after the end of the 18 ⁇ s pulse, at 42 ⁇ s.
  • 100 is ink
  • 101 is the sihcon substrate
  • 102 is SiO
  • 103 marks the position of one side of the annular heater
  • 108 is a Si 3 N passivation layer
  • 109 is a hydrophobic surface coating.
  • the heater width can vary by ⁇ 30% from the nominal width of 2.0 ⁇ m while maintaining operation between acceptable hmits.
  • Other nominal heater widths can also result in satisfactory performance, especially if drive voltage, heater thickness, heater resistivity, nozzle diameter, thermal coupUng to the ink, pulse duration, ink pressure, or other factors are altered to compensate for the difference in heater width.
  • the simulation results shown in figure 13 indicate that readily available semiconductor manufacturing equipment can be used to fabricate the heaters using the method disclosed herein.

Abstract

Cette structure et ce procédé de fabrication de chauffages électrothermiques sont destinés à des têtes d'impression intégrées, les axes de buses d'éjection d'encre étant sensiblement perpendiculaires au plan des chauffages et passant sensiblement par les centres de ces derniers. Un chauffage, une fois terminé, constitue une boucle dont le centre est formé par le processus de gravure de buse. Un disque de matériau chauffant est formé sur le substrat et son rayon égale celui de l'orifice à graver dans la buse, plus la largeur requise pour le chauffage. L'orifice de buse est ensuite gravé à partir du centre du disque de chauffage. Le reste du matériau propre à ce chauffage prend la forme d'un anneau dont le rayon interne égale le rayon d'orifice de buse. Cette structure est ensuite recouverte d'une couche d'un matériau de passivation.
PCT/US1996/005022 1995-04-12 1996-04-09 Structure et procede de fabrication de chauffage de tetes d'impression monolithiques WO1996032274A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
US08/765,017 US5905517A (en) 1995-04-12 1996-04-09 Heater structure and fabrication process for monolithic print heads
JP8531187A JPH10501772A (ja) 1995-04-12 1996-04-09 モノリシック印刷ヘッドのヒータ構造体および製造プロセス
EP96912694A EP0765234A1 (fr) 1995-04-12 1996-04-09 Structure et procede de fabrication de chauffage de tetes d'impression monolithiques

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AUPN2346 1995-04-12
AUPN2346A AUPN234695A0 (en) 1995-04-12 1995-04-12 Heater structure for monolithic lift print heads

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EP0765234A1 (fr) 1997-04-02
JPH10501772A (ja) 1998-02-17

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