BLOCK FAULT TOLERANCE IN INTEGRATED PRINTING HEADS

Abstract: Single faults in shift registers incorporated on monolithic printing heads can render inoperable large numbers of printing actuators, as data will either be stuck high or stuck low for subsequent shift register and actuator stages. This can reduce the effectiveness of other means of fault tolerance, and increase the device sensitivity to faults in individual, normally redundant, actuators. A printing head is disclosed which provides block fault tolerance in the shift registers, limiting the effect of shift register fabrication faults to small numbers of redundant actuators. This allows a high probability of defect correction by other forms of fault tolerance integrated on the chip, thereby increasing overall device yield.

Description: BLOCK FAULT TOLERANCE IN INTEGRATED PRINTING HEADS Field of the Invention The present invention is in the field of computer controlled printing devices. In particular, the field is fault tolerance for drop on demand (DOD) printing systems. Background of the Invention Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical printing presses, even though this conventional method requires very expensive setup and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems, for example, being able to produce high quality color images at a high-speed and low cost, using standard paper. 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. Many types of 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 CD, 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 CU 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 thermal 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. This technology is known as Bubblejet™ (trademark of Canon K.K. of Japan), and is used in a wide range of printing systems from Canon, Xerox, and other manufacturers. Vaught et al US Pat. No. 4,490,728, 1982, discloses an electrothermal drop ejection system which also operates by bubble formation. In this system, 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 Thermal Ink Jet, and is manufactured by Hewlett-Packard. In this document, the term Thermal Ink Jet is used to refer to both the Hewlett- Packard system and systems commonly known as Bubblejet™. Thermal Ink Jet printing typically requires approximately 20 μJ over a period of approximately 2 μs 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. Other ink jet printing systems have also been described in technical literature, but are not currently used on a commercial basis. For example, 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 Each of the above-described inkjet printing systems has advantages and disadvantages. However, there remains a widely recognized need for an improved inkjet printing approach, providing advantages for example, as to cost speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables. The printing mechanism is based on a new printing principle called "Liquid Ink Fault Tolerant" (LIFT) Drop on Demand printingln this document the term "optical density" refers to a human perceived visual image darkness, and not to spectroscopic optical density OD - A = logjo (Io I). Snmmarv of the invention My concurrently filed applications, entitled "Liquid Ink Printing Apparatus and System" and "Coincident Drop-Selection, Drop-Separation Printing Method and System" describe new methods and apparatus that afford significant improvements toward overcoming the prior art problems discussed above. Those inventions offer important advantages, e.g., in regard to drop size and placement accuracy, as to printing speeds attainable, as to power usage, as to durability and operative thermal stresses encountered and as to other printer performance characteristics, as well as in regard to manufacturability and the characteristics of useful inks. One important purpose of the present invention is to further enhance the structures and methods described in those applications and thereby contribute to the advancement of printing technology. Single faults in shift registers incorporated on monolithic printing heads can render inoperable large numbers of printing actuators, as data will either be stuck high or stuck low for subsequent shift register and actuator stages. This can reduce the effectiveness of other means of fault tolerance, and increase the device sensitivity to faults in individual, normally redundant actuators. The current invention is a means of limiting the effect of a fault in the shift registers of a printing head to a short length of shift registers. This is achieved by providing redundant shift registers which can be switched in to replace faulty segments of the main shift registers. The shift registers are tested by an external process, and the print head is programmed to replace shift register segments containing faulty nodes with redundant shift registers. The redundant shift register does not directly control any printing actuators. If used in isolation, this method cannot fully correct a printing head, as printing actuators associated the shift register segment that are replaced will not be activated. However, the effect of a fault in the shift register is limited to a short section of that shift register. This can dramatically reduce the probability that a fault in the shift register cannot be corrected by other fault tolerance mechanisms which provide redundant printing actuators. The faults in the shift registers may occur as the result of paniculate contamination during the manufacturing process, in which case the inclusion of the block fault tolerance circuitry disclosed herein, in conjunction with other circuits which provide redundant printing actuators, can improve manufacturing yield. The faults may also occur as a failure of the integrated electronic components in the field. In this case, the inclusion of fault tolerance circuitry can improve the operating life of the printing head. In one aspect the present invention constitutes in an integrated printing head having a plurality of printing actuators, apparatus for correcting faults in the data transfer to such actuators, said apparatus comprising: (a) a plurality of data transfer devices which, in the absence of faults, transfer data to the printing actuators; (b) at least one redundant data transfer device; (c) means for determining which of the data transfer device contain faults; (d) means for connecting the output of an operational data transfer device which precedes such faulty data transfer device, in terms of data flow, to the input of a corresponding redundant data transfer device; and (e) means for connecting the output of said corresponding redundant data transfer device to the input of the data transfer device which normally is connected to the output of said faulty data transfer device, in terms of data flow. A preferred aspect of the invention is that the data transfer devices are shift registers. A further preferred aspect of the invention is that the redundant data transfer devices are shift registers. A further preferred aspect of the invention is that the means of determining which of the data transfer devices contain faults is a test which applies data to the inputs of the shift registers and determines if the same data appears at the outputs of the data transfer devices an appropriate number of clock cycles later. A further preferred aspect of the invention is that the test is applied by an external microprocessor. A further preferred aspect of the invention is that the test is applied by an on-chip test circuit A further preferred aspect of the invention is that the means of connecting the output of an operational data transfer device to the input of a redundant data transfer mechanism is a multiplexer. A further preferred aspect of the invention is that the multiplexer is programmed by an external microprocessor. An alternative preferred aspect of the invention is that the multiplexer is programmed by an on-chip test and repair circuit A further preferred aspect of the invention is that the means of connecting the output of an operational data transfer mechanism to the input of a redundant data transfer device is an integrated fusible link. An alternative preferred aspect of the invention is that the means of connecting the output of the redundant data transfer device to the input of the data transfer device which normally is connected to the output of the faulty data transfer device in terms of data flow is a multiplexer. A further preferred aspect of the invention is that the marking means of the integrated printing head is a coincident forces printing head. A further preferred aspect of the invention is that the marking means of the integrated printing head is a thermal drop on demand printing head. A further alternative preferred aspect of the invention is that the marking means of the integrated printing head is a thermal wax printer actuator. A further alternative preferred aspect of the invention is that the marking means of the integrated printing head is a dye sublimation printer actuator. A further alternative preferred aspect of the invention is that the marking means of the integrated printing head is a heater element that is part of a heater bar of a thermal paper printer. Brief Description of the Drawings Figure 1 (a) 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 block diagram of a large print head with integrated drive circuitry. Figure 8 shows a block diagram of block fault tolerance in the shift registers of a large print head. Detailed Description of Preferred Embodiments In one general aspect, 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 to 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 2) Electrothermal bubble generation, with insufficient bubble volume to cause drop ejection 3) Piezoelectric, with insufficient volume change to cause drop ejection 4) Electrostatic attraction with one electrode per nozzle The drop separation means may be chosen from, but is not limited to, the following list: 1 ) Proximity (recording medium in close proximity to print head) 2) Proximity with oscillating ink pressure 3) Electrostatic attraction 4) Magnetic attraction The table "DOD printing technology targets" shows some desirable characteristics of drop on demand printing technology. The table also lists some methods by which some embodiments described herein, or in other of my related applications, provide improvements over the prior art. DOD prinύng technology targets In thermal ink jet (TU) and piezoelectric ink jet systems, a drop velocity of approximately 10 meters per second is preferred 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 TU systems is approximately 0.02%). This means that the drive circuits for TU 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 ΗJ 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 TU 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. The table "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 Other drop selection means may also be used. 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: "Electrothermal 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 Other 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 die high voltages required for electrostatic drop separation are undesirable. There is no clear 'best' drop separation means which is applicable to all circumstances. Further details of various types of printing systems according to die present invention are described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference: 'A Liquid ink Fault Tolerant (LIFT) printing mechanism' (Filing no.: PN2308); 'Electrothermal drop selection in LIFT printing' (Filing no.: PN2309); 'Drop separation in LIFT printing by print media proximity' (Filing no.: PN2310); 'Drop size adjustment in Proximity LIFT printing by varying head to media distance' (Filing no.: PN2311); 'Augmenting Proximity LIFT printing with acoustic ink waves' (Filing no.: PN2312); 'Electrostatic drop separation in LIFT printing' (Filing no.: PN2313); 'Multiple simultaneous drop sizes in Proximity LIFT printing' (Filing no.: PN2321); 'Self cooling operation in thermally activated print heads' (Filing no.: PN2322); and 'Thermal Viscosity Reduction LIFT printing' (Filing no.: PN2323). 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 (REP) 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. Depending upon the printer and system configuration, 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 die appropriate nozzle, so that selected drops will form spots on the recording medium 51 in the appropriate position designated by die 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. However, in the case of scanning print systems, it is usually most convenient to move the head 50 along one axis (the sub-scanning direction) and die recording medium 51 along the orthogonal axis (the main scanning direction), in a relative raster motion. The microcontroller 315 may also control the ink pressure regulator 63 and the heater control circuits 71. For printing using surface tension reduction, ink is contained in an ink reservoir 64 under pressure. In the quiescent state (with no ink drop ejected), 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. Alternatively, for larger printing systems, 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). For printing using viscosity reduction, ink is contained in an ink reservoir 64 under pressure, and die ink pressure is caused to oscillate. The means of producing diis oscillation may be a piezoelectric actuator mounted in the ink channels (not shown). When properly arranged with the drop separation means, selected drops proceed to form spots on the recording medium 51 , while unselected drops remain part of the body of ink. The ink is distributed to die back surface of the head 50 by an ink channel device 75. The ink preferably flows through slots and/or holes etched dirough the silicon substrate of the head 50 to die front surface, where the nozzles and actuators are situated. In the case of thermal selection, the nozzle actuators are electrothermal heaters. In some types of printers according to die invention, an external field 74 is required to ensure that the selected drop separates from the body of die ink and moves towards die recording medium 51. A convenient external field 74 is a constant electric field, as the ink is easily made to be electrically conductive. In this case, 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. For small drop sizes gravitational force on die ink drop is very small; approximately 10 "4 of the surface tension forces, so gravity can be ignored in most cases. This allows the print head 50 and recording medium 51 to be oriented in any direction in relation to die local gravitational field. This is an important requirement for portable printers. 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 sihcon) may be deposited on die 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 otiier circuitry can be fabricated in SCS; 2) Print heads can be fabricated in existing facilities (fabs) using standard VLSI processing equipment; 3) SCS has high mechanical strength and rigidity; and 4) SCS has a high thermal conductivity. In this example, the nozzle is of cylindrical form, with the heater 103 forming an annulus. The nozzle tip 104 is formed from sihcon dioxide layers 102 deposited during the fabrication of die 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 formed 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 die 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 Hewlett-Packard. These, however are complex to actuate, and difficult to fabricate. The preferred metiiod for elimination of orifice plates for print heads of die invention is incorporation of d e orifice into the actuator substrate. This type of nozzle may be used for print heads using various techniques for drop separation. peration with Electrostatic Drop Separation As a first example, operation using thermal reduction of surface tension and electrostatic drop separation is shown in figure 2. Figure 2 shows the results of energy transport and fluid dynamic simulations performed using EODAP, a commercial fluid dynamic simulation software package available from Fluid Dynamics Inc., of Illinois, USA. This simulation is of a thermal drop selection nozzle embodiment with a diameter of 8 μm, at an ambient temperature of 30°C. The total energy applied to die heater is 276 nJ, applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and die 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 die nozzle tip from die central axis of the nozzle to a radial distance of 40 μm is shown. Heat flow in the various materials of the nozzle, including silicon, silicon nitride, amorphous sihcon dioxide, crystalline sihcon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of die materials. The time step of the simulation is 0.1 μs. 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 die meniscus. Figure 2(b) shows thermal contours at 5°C intervals 5 μs after the start of the heater energizing pulse. When the heater is energized, die 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 die cool ink meniscus. This drives a convective flow which rapidly transports this heat over part of the free surface of die ink at the nozzle tip. It is necessary for the heat to be distributed over the ink surface, and not just where die ink is in contact with the heater. This is because viscous drag against the solid heater prevents the ink directiy in contact with the heater from moving. Figure 2(c) shows thermal contours at 5°C intervals 10 μs after the start of the heater energizing pulse. The increase in temperature causes a decrease in surface tension, disturbing die equilibrium of forces. As the entire meniscus has been heated, die ink begins to flow. Figure 2(d) shows tiiermal contours at 5°C intervals 20 μs after the start of the heater energizing pulse. The ink pressure has caused die ink to flow to a new meniscus position, which protrudes from the print head. The electrostatic field becomes concentrated by die protruding conductive ink drop. Figure 2(e) shows tiiermal contours at 5°C intervals 30 μs after the start of the heater energizing pulse, which is also 6 μs after the end of die heater pulse, as the heater pulse duration is 24 μs. The nozzle tip has rapidly cooled due to conduction dirough the oxide layers, and conduction into the flowing ink. The nozzle tip is effectively 'water cooled' by the ink. Electrostatic attraction causes die ink drop to begin to accelerate towards die recording medium. Were me heater pulse significantiy shorter (less than 16 μs in this case) the ink would not accelerate towards die print medium, but would instead return to the nozzle. Figure 2(f) shows thermal contours at 5°C intervals 26 μs after the end of die heater pulse. The temperature at the nozzle tip is now less dian 5°C above ambient temperature. This causes an increase in surface tension around die nozzle tip. When die rate at which the ink is drawn from the nozzle exceeds die viscously limited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks', and die selected drop separates from the body of ink. The selected drop then travels to the recording medium under die influence of the external electrostatic field. The meniscus of the ink at the nozzle tip tiien returns to its quiescent position, ready for die 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 inkjet operation can be achieved. Figure 3(a) shows successive meniscus positions during die drop selection cycle at 5 μs 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 μs 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 μs. The temperature curve shown in figure 3(b) was calculated by FIDAP, using 0.1 μs 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 die passivation layer, die ink, and air. B - Meniscus midpoint: This is at a circle on the ink meniscus midway between the nozzle tip and die centre of the meniscus. C - Chip surface: This is at a point on the print head surface 20 μm from the centre of the nozzle. The temperature only rises a few degrees. This indicates diat 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 htde below the boiling point of the ink for the duration of die pulse, and a rapid fall in temperature at the end of die pulse. To achieve this, the average energy applied to the heater is varied over die duration of die pulse. In this case, the variation is achieved by pulse frequency modulation of 0.1 μs sub-pulses, each with an energy of 4 nJ. The peak power applied to die heater is 40 mW, and die average power over the duration of die heater pulse is 11.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significandy affecting the operation of die print head. A higher sub-pulse frequency allows finer control over the power applied to die 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). Inks with a negative temperature coefficient of surface tension The requirement for the surface tension of the ink to decrease with increasing temperature is not a major restriction, as most pure hquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary hquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many hquids: Where γris the surface tension at temperature T, & is a constant T c is the critical temperature of the hquid, M is the molar mass of the hquid, x is the degree of association of die hquid, and p is die density of the hquid. This equation indicates diat die surface tension of most hquids falls to zero as the temperamre reaches the critical temperature of the hquid. For most hquids, die critical temperature is substantially above the boiling point at atmospheric pressure, so to achieve an ink with a large change in surface tension with a small change in temperature around a practical ejection temperamre, the admixture of surfactants is recommended. The choice of surfactant is important For example, water based ink for thermal inkjet printers often contains isopropyl alcohol (2-propanol) to reduce die surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4°C, lower than that of water. As the temperature rises, the alcohol evaporates faster than the water, decreasing die alcohol concentration and causing an increase in surface tension. 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. However, 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 htde as lOmN/m can be used to achieve operation of d e print head according to die present invention. Inks With Large -Δy x Several methods may be used to achieve a large negative change in surface tension with increasing temperature. Two such methods are: 1) 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. 2) The ink may contain an oil/water microemulsion with a phase inversion temperamre (PIT) which is above the maximum ambient temperature, but below the boiling point of the ink. For stability, the PIT of the microemulsion is preferably 20°C or more above the maximum non-operating temperature encountered by die ink. A PIT of approximately 80°C is suitable. Inks with Surfactant Sols Inks can be prepared as a sol of small particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as: As the melting point of sols with a small particle size is usually shghdy less than of the bulk material, it is preferable to choose a carboxyhc acid with a melting point shghdy above the desired drop selection temperature. A good example is Arachidic acid. These carboxyhc acids are available in high purity and at low cost. The amount of surfactant required is very small, so the cost of adding diem to the ink is insignificant A mixture of carboxyhc acids with shghdy varying chain lengths can be used to spread die melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid. 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 die surfactant. It is desirable diat die polar end be ionizable in water, so that die surface of the surfactant particles can be charged to aid dispersion and prevent flocculation. In the case of carboxyhc acids, diis can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide. Preparation of Inks with Surfactant Sols The surfactant sol can be prepared separately at high concentration, and added to die ink in die required concentration. An example process for creating the surfactant sol is as follows: 1) Add die carboxyhc acid to purified water in an oxygen free atmosphere. 2) Heat the mixture to above the melting point of the carboxyhc acid. The water can be brought to a boil. 3) Ultrasonicate the mixture, until the typical size of the carboxyhc acid droplets is between 100 A and l,OOθA. 4) Allow the mixture to cool. 5) Decant the larger particles from die top of die mixture. 6) Add an alkali such as NaOH to ionize the carboxyhc acid molecules on the surface of the particles. A pH of approximately 8 is suitable. This step is not absolutely necessary, but helps stabilize the sol. 7) Centrifuge the sol. As the density of die carboxyhc acid is lower dian water, smaller particles will accumulate at the outside of die centrifuge, and larger particles in the centre. 8) Filter the sol using a microporous filter to eliminate any particles above 5000 A. 9) Add die surfactant sol to the ink preparation. The sol is required only in very dilute concentration. The ink preparation will also contain eitiier dye(s) or pigment(s), bactericidal agents, agents to enhance the electrical conductivity of die ink if electrostatic drop separation is used, humectants, and otiier agents as required. Anti-foaming agents will generally not be required, as there is no bubble formation during die drop ejection process. Cationic surfactant sols Inks made with anionic surfactant sols are generally unsuitable for use with catiomc dyes or pigments. This is because die cationic dye or pigment may precipitate or flocculate with die anionic surfactant. To allow the use of cationic dyes and pigments, a cationic surfactant sol is required. The family of alkylamines is suitable for this purpose. Various suitable alkylamines are shown in the following table: 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 die pH balance and increase the charge on the surfactant particles. A pH of 6 using HC1 is suitable. Microemulsion 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 die desired ejection tiireshold temperamre. Below the PIT, the microemulsion is oil in water (O W), and above die PIT the microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperamres significandy above the PIT, the surfactant prefers a high curvature surface around water. At temperatures close to the PIT, the microemulsion forms a continuous 'sponge' of topologically connected water and oil. There are two mechanisms whereby this reduces die surface tension. Around die PIT, the surfactant prefers surfaces with very low curvature. As a result 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. Above the phase inversion temperature, the microemulsion changes from O W to W/O, and tiierefore the ink/air interface changes from water/air to oil/air. The oil/air interface has a lower surface tension. There is a wide range of possibilities for die preparation of microemulsion based inks. For fast drop ejection, it is preferable to chose a low viscosity oil. In many instances, water is a suitable polar solvent. However, in some cases different polar solvents may be required. In diese cases, polar solvents with a high surface tension should be chosen, so diat a large decrease in surface tension is achievable. The surfactant can be chosen to result in a phase inversion temperature in the desired range. For example, surfactants of the group poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general formula: C n H 2n+ ιCH6(CH 2 CH 2 O) m OH) can be used. The hydrophilicity of the surfactant can be increased by increasing m, and die hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable. Low cost commercial preparations are the result of a polymerization of various molar ratios of ed ylene oxide and alkyl phenols, and die exact number of oxyethylene groups varies around die chosen mean. These commercial preparations are adequate, and highly pure surfactants with a specific number of oxyediylene groups are not required. The formula for this surfactant is C 8 Hι 7 C4H6(CH 2 CH 2 O) n OH (average n-10). Synonyms include Octoxynol-10, PEG-10 octyl phenyl etiier and POE (10) octyl phenyl ether The HLB is 13.6, the melting point is 7°C, and die cloud point is 65°C. Commercial preparations of this surfactant are available under various brand names. Suppliers and brand names are listed in the following table: These are available in large volumes at low cost Qtss than one dollar per pound in quantity), and so contribute less than 10 cents per hter to prepared microemulsion ink with a 5% surfactant concentration. Otiier suitable ethoxylated alkyl phenols include tiiose listed in the following table: Microemulsion based inks have advantages otiier than surface tension control: 1) Microemulsions are thermodynamically stable, and will not separate. Therefore, the storage time can be very long. This is especially significant for office and portable printers, which may be used sporadically. 2) The 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. 3) 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 die otiier 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 die mixing of different dye colors on die surface of the print medium. 6) The viscosity of microemulsions is very low. 7) The requirement for humectants can be reduced or eliminated. Dves and pigments in microemulsion based inks 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. Mixtures of dyes and pigments can be used. An example of a microemulsion based ink mixture with both dye and pigment is as follows: 1) 70% water 2) 5% water soluble dye 3) 5% surfactant 4) 10% oil 5) 10% oil miscible pigment The following table shows the nine basic combinations of colorants in the oil and water phases of the microemulsion that may be used. The ninth combination, with no colorants, is useful for printing transparent coatings, UV ink, and selective gloss highlights. As many dyes are amphiphilic, large quantities of dyes can also be solubilized in the oil-water boundary layer as diis layer has a very large surface area. It is also possible to have multiple dyes or pigments in each phase, and to have a mixture of dyes and pigments in each phase. When using multiple dyes or pigments the absorption spectrum of the resultant ink will be the weighted average of die absorption spectra of the different colorants used. This presents two problems: 1) The absorption spectrum will tend to become broader, as die absorption peaks of both colorants are averaged. This has a tendency to 'muddy' die colors. To obtain brilliant color, careful choice of dyes and pigments based on dieir absoφtion spectra, not just their human-perceptible color, needs to be made. 2) The color of the ink may be different on different substrates. If a dye and a pigment are used in combination, die color of die dye will tend to have a smaller contribution to die printed ink color on more absoφtive papers, as the dye will be absorbed into the paper, while the pigment will tend to 'sit on top' of die paper. This may be used as an advantage in some circumstances. Surfactants with a Krafft noint in the drop selection temnerature rang e. For ionic surfactants there is a temperature (the Krafft point) below which the solubility is quite low, and die solution contains essentially no micelles. Above the Krafft temperature micelle formation becomes possible and tiiere is a rapid increase in solubility of the surfactant If the critical micelle concentration (CMC) exceeds die solubility of a surfactant at a particular temperature, then die minimum surface tension will be achieved at die point of maximum solubility, rather than at die CMC. Surfactants are usually much less effective below die Krafft point This factor can be used to achieve an increased reduction in surface tension with increasing temperature. At ambient temperatures, only a portion of die surfactant is in solution. When the nozzle heater is turned on, die temperature rises, and more of the surfactant goes into solution, decreasing die surface tension. A surfactant should be chosen with a Krafft point which is near the top of die range of temperatures to which the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperamres, and die concentration of surfactant in solution at die 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 temperamres, and is reduced to a minimum amount at ambient temperatures. The following table shows some commercially available surfactants with Krafft points in the desired range. Surfactants with a cloud point in the drop selection temperamre range Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension falls with increasing temperature. At low temperatures, the POE chain is hydrophilic, and maintains die surfactant in solution. As the temperature increases, the strucmred water around die POE section of die molecule is disrupted, and die POE section becomes hydrophobia The surfactant is increasingly rejected by die 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 hydrophilic is related to die 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. Two main configurations of symmetrical POE POP block copolymers are available. These are: 1 ) Surfactants with POE segments at the ends of die molecules, and a POP segment in the centre, such as the poloxamer class of surfactants (generically CAS 9003-11-6) 2) Surfactants with POP segments at the ends of die molecules, and a POE segment in the centre, such as the meroxapol class of surfactants (generically also CAS 9003-11-6) Some commercially available varieties of poloxamer and meroxapol with a high surface tension at room temperature, combined with a cloud point above 40°C and below 100°C are shown in the following table: Other varieties of poloxamer and meroxapol can readily be synthesized using well known techniques. 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 [HO(CHCH 3 CH2θ) I (CH2CH2θ) y (CHCH 3 CH 2 O) I OH] varieties where the average x and z are approximately 4, and die average y is approximately 15 may be suitable. If salts are used to increase the electrical conductivity of die ink, then die effect of this salt on the cloud point of the surfactant should be considered. The cloud point of POE surfactants is increased by ions diat disrupt water structure (such as T), as this makes more water molectdes available to form hydrogen bonds with die 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. Bromide ions have relatively htde effect 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 I " ) that are added to increase electrical conductivity. NaCl is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity. NaCl shghdy lowers the cloud point of nonionic surfactants. Hot Melt Inks The ink need not be in a hquid state at room temperature. Solid 'hot melt' inks can be used by heating die printing head and ink reservoir above the melting point of the ink. The hot melt ink must be formulated so diat die surface tension of the molten ink decreases with temperamre. A decrease of approximately 2 mN/m will be typical of many such preparations using waxes and otiier substances. However, a reduction in surface tension of approximately 20 mN/m is desirable in order to achieve good operating margins when relying on a reduction in surface tension rather than a reduction in viscosity. The temperature difference between quiescent temperamre 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 die boiling point of the water. The ink must be hquid at die quiescent temperature. The quiescent temperamre should be higher than die highest ambient temperamre likely to be encountered by die printed page. T he quiescent temperamre should also be as low as practical, to reduce die power needed to heat die print head, and to provide a maximum margin between the quiescent and die drop ejection temperamres. A quiescent temperature between 60°C and 90°C is generally suitable, though otiier temperamres may be used. A drop ejection temperamre of between 160°C and 200°C is generally suitable. There are several methods of achieving an enhanced reduction in surface tension with increasing temperature. 1) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperamre, but substantially below the drop ejection temperamre, can be added to die hot melt ink while in the hquid phase. 2) 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. To achieve a large reduction in surface tension with temperamre, it is desirable diat the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperature. This generally excludes alkanes such as waxes. Suitable materials will generally have a strong inteπnolecular attraction, which may be achieved by multiple hydrogen bonds, for example, polyols, such as Hexanetetrol, which has a melting point of 88°C. Surface tension reduction of various solutions Figure 3(d) shows die measured effect of temperamre on the surface tension of various aqueous preparations containing the following additives: 1) 0.1% sol of Stearic Acid 2) 0.1% sol of Palmitic acid 3) 0.1 % solution of Pluronic 10R5 (trade mark of BASF) 4) 0.1 % solution of Pluronic L35 (trade mark of BASF) 5) 0.1 % solution of Pluronic L44 (trade mark of BASF) Inks suitable for printing systems of the present invention are described in the following Australian patent specifications, the disclosure of which are hereby incoφorated by reference: 'Ink composition based on a microemulsion' (Filing no.: PN5223, filed on 6 September 1995); 'Ink composition containing surfactant sol' (Filing no.: PN5224, filed on 6 September 1995); 'Ink composition for DOD printers with Krafft point near the drop selection temperature sol' (Filing no.: PN6240, filed on 30 October 1995); and 'Dye and pigment in a microemulsion based ink' (Filing no.: PN6241, filed on 30 October 1995). Operation Using Reduction of Viscosity As a second example, operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink, is as follows. Prior to operation of the printer, solid 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 hquid, 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 temperamre. 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 oscillations of die ink meniscus at the nozzle tips, but diis oscillation is small due to die high ink viscosity. At the normal operating temperamre, these oscillations are of insufficient amplitude to result in drop separation. When the heater 103 is energized, die 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 die high pressure part of the ink pressure cycle. The recording medium 51 is arranged sufficiendy close to the print head 50 so diat die selected drops contact the recording medium 51, but sufficiendy far away that die unselected drops do not contact the recording medium 51. Upon contact with the recording medium 51 , part of the selected drop freezes, and attaches to the recording medium. As the ink pressure falls, 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 amplitude 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 die recording medium 51 for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved. Manufacturing of Print Heads Manufacturing processes for monolithic print heads in accordance widi die present invention are described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'A monolitiiic LIFT printing head' (Filing no.: PN2301); 'A manufacturing process for monolithic LIFT printing heads' (Filing no.: PN2302); 'A self-aligned heater design for LIFT print heads' (Filing no.: PN2303); 'Integrated four color LIFT print heads' (Filing no.: PN2304); 'Power requirement reduction in monolithic LIFT printing heads' (Filing no.: PN2305); 'A manufacturing process for monolithic LIFT print heads using anisotropic wet etching' (Filing no.: PN2306); 'Nozzle placement in monolithic drop-on-demand print heads' (Filing no.: PN2307); 'Heater structure for monolithic LIFT print heads' (Filing no.: PN2346); 'Power supply connection for monolithic LIFT print heads' (Filing no.: PN2347); 'External connections for Proximity LIFT print heads' (Filing no.: PN2348); and 'A self-aligned manufacturing process for monolithic LIFT print heads' (Filing no.: PN2349); and 'CMOS process compatible fabrication of LIFT print heads' (Filing no.: PN5222, 6 September 1995). 'A manufacturing process for LIFT print heads with nozzle rim heaters' (Filing no.: PN6238, 30 October 1995); 'A modular LIFT print head' (Filing no.: PN6237, 30 October 1995); 'Method of increasing packing density of printing nozzles' (Filing no.: PN6236, 30 October 1995); and 'Nozzle dispersion for reduced electrostatic interaction between simultaneously printed droplets' (Filing no.: PN6239, 30 October 1995). Control of Print Heads Means of providing page image data and controlling heater temperature in print heads of die present invention is described in the foUowing Austrahan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'Integrated drive circuitry in LIFT print heads' (Filing no.: PN2295); ' A nozzle clearing procedure for Liquid Ink Fault Tolerant (LIFT) printing' (Filing no.: PN2294); 'Heater power compensation for temperamre in LIFT printing systems' (Filing no.: PN2314); 'Heater power compensation for thermal lag in LIFT printing systems' (Filing no.: PN2315); 'Heater power compensation for print density in LIFT printing systems' (Filing no.: PN2316); 'Accurate control of temperamre pulses in printing heads' (Filing no.: PN2317); 'Data distribution in monolithic LEFT print heads' (Filing no.: PN2318); 'Page image and fault tolerance routing device for LIFT printing systems' (Filing no.: PN2319); and 'A removable pressurized hquid ink cartridge for LIFT printers' (Filing no.: PN2320). Imape Processing for Print Heads An objective of printing systems according to die invention is to attain a print quahty which is equal to that which people are accustomed to in quahty color pubhcations printed using offset printing. This can be achieved using a print resolution of approximately 1 ,600 dpi. However, 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 called CC'MM' YK. Where high quahty monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein called CC'MM'YKK' . Color models, halftoning, data compression, and real-time expansion systems suitable for use in systems of this invention and otiier printing systems are described in the following Austrahan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'Four level ink set for bi-level color printing ' (Filing no. : PN2339) ; 'Compression system for page images' (Filing no.: PN2340); 'Real-time expansion apparatus for compressed page images' (Filing no.: PN2341); and 'High capacity compressed document image storage for digital color printers' (Filing no.: PN2342); 'Improving JPEG compression in the presence of text' (Filing no.: PN2343); 'An expansion and halftoning device for compressed page images' (Filing no.: PN2344); and 'Improvements in image halftoning' (Filing no.: PN2345). Applications Using Print Heads According to this Invention Printing apparatus and mediods of this invention are suitable for a wide range of applications, including (but not limited to) die 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 pagewidtii 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 duplication, printers for digital photographic processing, portable printers incoφorated 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. Printing systems based on this invention are described in the following Austrahan patent specifications filed on 12 April 1995, die disclosure of which are hereby incoφorated by reference: 'A high speed color office printer with a high capacity digital page image store' (Filing no.: PN2329); 'A short run digital color printer with a high capacity digital page image store' (Filing no.: PN2330); 'A digital color printing press using LIFT printing technology' (Filing no.: PN2331); 'A modular digital printing press' (Filing no.: PN2332); 'A high speed digital fabric printer' (Filing no.: PN2333); 'A color photograph copying system' (Filing no.: PN2334); 'A high speed color photocopier using a LIFT printing system' (Filing no.: PN2335); 'A portable color photocopier using LIFT printing technology' (Filing no.: PN2336); 'A photograph processing system using LIFT printing technology' (Filing no.: PN2337); 'A plain paper facsimile machine using a LIFT printing system' (Filing no.: PN2338); 'A PhotoCD system with integrated printer' (Filing no.: PN2293); 'A color plotter using LIFT printing technology' (Filing no.: PN2291); 'A notebook computer with integrated LIFT color printing system' (Filing no.: PN2292); 'A portable printer using a LIFT printing system' (Filing no.: PN2300); 'Fax machine with on-line database interrogation and customized magazine printing' (Filing no.: PN2299); 'Miniature portable color printer' (Filing no.: PN2298); 'A color video printer using a LIFT printing system' (Filing no.: PN2296); and 'An integrated printer, copier, scanner, and facsimile using a LIFT printing system' (Filing no.: PN2297) Compensation of Print Heads for Environmental Conditions It is desirable diat 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 die resultant print reducing the perceived print quahty. These variations should be kept to a small proportion of the nominal ink drop volume and pixel spacing respectively. Many environmental variables can be compensated to reduce dieir effect to insignificant levels. Active compensation of some factors can be achieved by varying the power applied 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 die pulse, and instantaneous cooling of the region to the ambient temperature. This optimum is not achievable due to die stored heat capacities and tiiermal conductivities of the various materials used in die fabrication of die nozzles in accordance with die invention. However, improved performance can be achieved by shaping the power pulse using curves which can be derived by iterative refinement of finite element simulation of the print head. The power applied to die heater can be varied in time by various techniques, including, but not limited to: 1) Varying the voltage apphed to die heater 2) Modulating die widtii of a series of short pulses (PWM) 3) Modulating die frequency of a series of short pulses (PFM) To obtain accurate results, a transient fluid dynamic simulation with free surface modeling is required, as convection in the ink, and ink flow, significandy affect on the temperamre achieved with a specific power curve. By the incoφoration of appropriate digital circuitry on the print head substrate, it is practical to individually control the power apphed to each nozzle. 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 listed in the table "Compensation for environmental factors". This table identifies which environmental factors are best compensated globally (for the entire print head), per chip (for each chip in a composite multi-chip print head), and per nozzle. Compensation for environmental factors Most applications will not require compensation for all of diese variables. Some variables have a minor effect, and compensation is only necessary where very high image quahty is required. Print head drive circuits 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 apphed to die print head to achieve heater power modulation, and does not have individual control of the power apphed to each nozzle. Figure 4 shows a block diagram for a system using an 800 dpi pagewiddi 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 circtiits. There is a total of 96 shift registers, each providing data for 828 nozzles. Each shift register is composed of 828 shift register stages 217, the outputs of which are logically anded with phase enable signal by a nand gate 215. The output of the nand gate 215 drives an inverting buffer 216, which in m 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). To maintain the shifted data valid during die enable pulse, the clock to die shift register is stopped die 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 well known glitch free clock control circuits. Stopping the clock of the shift register removes the requirement for a parallel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400. Data is routed to eitiier the main nozzles or the redundant nozzles by the data router 219 depending on die 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 e 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 die address mux 417 and control signals generated by die Memory Interface 418. These addresses are generated by Address generators 411, which forms part of the 'Per color circuits' 410, for which there is one for each of the six color components. The addresses are generated based on die positions of the nozzles in relation to the print medium. As the relative position of die nozzles may be different for different print heads, die Address generators 411 are preferably made programmable. The Address generators 411 normally generate the address corresponding to die position of die 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 die page is printed. If die memory indicates a fault in the block of nozzles, the address is altered so diat die Address generators 411 generate the address corresponding to die position of die redundant nozzles. Data read from me Page or Band memory 1513 is latched by die 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 tiien buffered by die buffer 430 to form the 48 bit main data bus to die 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 die 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 die 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 Microcontroller 315. Temperature is compensated by changing the contents of the dual port RAM 317. These values are calculated by die microcontroller 315 based on temperamre sensed by a tiiermal sensor 300. The thermal sensor 300 signal connects to the Analog to Digital Converter (ADC) 311. The ADC 311 is preferably incoφorated in the Microcontroller 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 diis voltage. An analog time varying programming voltage is produced by die DAC 313 based upon data read from die dual port RAM 317. The data is read according to an address produced by die counter 403. The counter 403 produces one complete cycle of addresses during die period of one enable pulse. This synchronization is ensured, as the counter 403 is clocked by the system clock 408, and die top count of the counter 403 is used to clock die enable counter 404. The count from the enable counter 404 is then decoded by die decoder 405 and buffered by die buffer 432 to produce die 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 die dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To allow 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. For print density compensation, 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 die On pixel counters 402. There is one On pixel counter 402 for each of the eight enable phases. The number of enable phases in a print head in accordance with die invention depend upon die specific design. Four, eight, and sixteen are convenient numbers, though there is no requirement that die number of enable phases is a power of two. The On Pixel 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 die adder 421 and accumulator 422. A latch 423 holds the accumulated value valid for die duration of die enable pulse. The multiplexer 401 selects the output of the latch 423 which corresponds to the current enable phase, as determined by die enable counter 404. The output of the multiplexer 401 forms part of the address of die dual port RAM 317. An exact count of the number of 'on' pixels is not necessary, and die most significant four bits of this count are adequate. Combining the four bits of thermal lag compensation address and die four bits of print density compensation address means that the dual port RAM 317 has an 8 bit address. This means that 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 temperamre of the head varies only slowly, die microcontroller 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print density at die current temperamre. Periodically (for example, a few times a second), die microcontroller senses the current head temperature and calculates this matrix. The clock to the print head 50 is generated from die system clock 408 by the Head clock generator 407, and buffered by die buffer 406. To facilitate testing of the Head control ASIC, JTAG test circuits 499 may be included. Comparison with thermal ink iet technology The table "Comparison between Thermal ink jet and Present Invention" compares the aspects of printing in accordance with the present invention widi thermal inkjet printing technology. A direct comparison is made between the present invention and thermal ink jet technology because both are drop on demand systems which operate using thermal actuators and hquid ink. Although they may appear similar, the two technologies operate on different principles. Thermal ink jet printers use the following fundamental operating principle. A thermal impulse caused by electrical resistance heating results in the explosive formation of a bubble in hquid ink. Rapid and consistent bubble formation can be achieved by superheating the ink, so that sufficient heat is transferred to die ink before bubble nucleation is complete. For water based ink, 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 collapses, drawing ink from the ink reservoir to re-fill the nozzle. Thermal ink jet printing has been highly successful commercially due to die high nozzle packing density and die use of well established integrated circuit manufacturing techniques. However, 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, satellite drop formation, tiiermal stress, differential thermal 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 substantially eliminates many of the inherent problems of thermal ink jet technology. Comparison between Thermal ink jet and Present Invention When fault tolerance is included in a device, standard yield equations cannot be used. Instead, die mechanisms and degree of fault tolerance must be specifically analyzed and included in die yield equation. Figure 5 shows the fault tolerant sort yield 199 for a full widtii color A4 print 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 diat thoroughly implemented fault tolerance can increase wafer sort yield from under 1% to more than 90% under identical manufacturing conditions. This can reduce the manufacmring cost by a factor of 100. Fault tolerance approaches in drop-on-demand printing systems are described in die following Austrahan patent specifications filed on 12 April 1995, die disclosure of which are hereby incoφorated by reference: 'Integrated fault tolerance in printing mechanisms' (Filing no.: PN2324); 'Block fault tolerance in integrated printing heads' (Filing no.: PN2325); 'Nozzle duplication for fault tolerance in integrated printing heads' (Filing no.: PN2326); 'Detection of faulty nozzles in printing heads' (Filing no.: PN2327); and 'Fault tolerance in high volume LIFT printing presses' (Filing no.: PN2328). The Effect of Fault Tolerance on Device Yield Electronic fabrication processes are inexact and not all devices are functional after fabrication. The scale of modern electronic devices is so small that contaminants smaller than 1 micron can cause catastrophic device failure. These contaminants may be airborne dust particles which settle on the lithography mask or on the photoresist causing point defects in die manufacmring process. Pinholes in the resist layer may also cause device defects. The contaminants may also be larger, such as thin residues left by an impure chemical process, or dislodged particles of resist or otiier parts of the processing environment. Impurities and micro-fractures in the sihcon wafer itself may also cause device defects. Process parameters, such as etching times, temperatures, gas densities, plasma excitation energies and so forth, which are not correctly adjusted can cause device failure. There are many other causes of defects in integrated circuit manufacture. The percentage of devices which are operational is known as the yield. Yield has a direct influence on manufacmring cost. A device with a yield of 5% is effectively ten times more expensive to manufacture than a similar device with a manufacturing yield of 50%. The semiconductor manufacturing industry has made significant improvements in device yield by establishing cleaner processing environments, purer substances, more accurate processes, and electronic designs more tolerant of processing variations. Yield Estimation It is important to know approximately what yield can be expected before beginning manufacture of a new device. This information is used for planning the economics of the device, setting targets for production yield, and finding ways to improve the production process and device. There are three major yield measurements: 1) Fab yield: This is the percentage of the wafers which are started on die wafer fabrication line that reach the end of wafer fabrication. Causes for rejection during manufacture include breakage, waφing, incorrect processing order, process out of tolerance, and large area contamination. The fab yield Y FΛ is typically low for a new process. However, with a mature process on an automated fab line, a fab yield of better than 90% can usually be achieved. 2) Wafer sort yield: This is percentage of die which pass wafer test Before the wafer is diced, die individual die are tested with a wafer probe. The wafer sort yield Y^ is usually affected primarily by the number of point defects caused by dust and other contaminants per unit area (the defect density, D), and die chip area, A. Only die which pass wafer sort are packaged. 3) Final test yield: This is the percentage of packaged die which pass final functional and parametric tests. Final test yield Y Tt „ is usually 95% or more in a mature process. Total Yield The total yield Yτotai is the percentage of functional dice (in this case, print heads) as compared with the number of whole dice on die starting wafers. This is calculated as: 'Total = ^Fab X ^Sort X jta All three major yield factors must be high to achieve a good total yield. Wafer Sort Yield In a mature process, it is typically the wafer sort yield which is the most serious limitation on total yield. This is particularly true for large dice. Full page width print heads are large in comparison with typical VLSI circuits. Good wafer sort yield is critical to die cost effective manufacture of print heads. There are several techniques in use for wafer sort yield estimation. An early method assumes that defects are randomly distributed at a specific defect density. The device yield is calculated according to probabilities based on Boltzmann distribution: 'Sort e where Ysert is the wafer sort yield, D is the defect density, and A is the chip area. This method was shown to be generally pessimistic for large size chips, as the defect density is usually not perfectly even. Rather, there is a distribution of defect densities. One of the most widely used yield prediction methods is Muφhy's method, which has proven to be a good predictor for LSI and VLSI circuits. Muφhy's method approximates the distribution of defect densities, calculating the yield as: Figure 5 is a graph of wafer sort yield versus defect density for a monolithic full width color A4 print head. This graph compares the non fault- tolerant yield 198 with die fault tolerant yield 199. The non fault tolerant yield is calculated according to Muφhy's metiiod. The head is 215 mm long by 5 mm wide. It is possible to fabricate such print heads using current technology by using sihcon wafers cut axially from the sihcon crystal, rather than radial cut wafers. With a defect density of one defect per square cm, Muφhy's method predicts a yield less dian 1%. This means that more than 99% of heads fabricated would have to be discarded. This low yield is highly undesirable, as die print head manufacturing cost becomes unacceptably high. As commercial pressure to introduce larger devices increases, the quahty of clean rooms, processes, and raw materials has steadily improved to reduce die defect density. However, single chip devices as large as full width print heads remain uneconomic due to low wafer sort yield. Defect Clustering Muφhy's method approximates the effect of an uneven distribution of defects. To explicitly model diis uneven distribution, a defect clustering factor C can be introduced. The defect clustering factor is a measure of the proportion that defects are clustered (eitiier by area on a wafer, or by wafer), thereby affecting fewer chips. Defect clustering is advantageous for non-fault tolerant designs, but can adversely affect fault tolerance. The yield for a non-fault tolerant device, with explicit modeling for clustering factor, can be calculated as: -DA v _ e- DAC + Ce c Sβ " c+ι Figure 5 includes a graph of non fault tolerant yield with explicit clustering factor 197. The defect clustering factor is not a controllable parameter in manufacturing, but is a characteristic of the manufacmring process. The clustering factor for manufacmring processes can be expected to be approximately 2, in which case yield projections closely match Muφhy's method. Fault tolerance A solution to the problem of low yield is to incoφorate fault tolerance. Fault tolerance techniques have been used for some time in large memory chips and in wafer scale integration (WSI). Fault tolerance usually operates by providing redundancy. If some functional unit of the chip contains a defect, it is replaced by a 'redundant' or spare functional unit First, the faulty sub-units are determined (usually by external testing), then routing paths to connect redundant sub-units to replace the faulty sub-units are determined. Then the chip is programmed with these new connections. This programming may be achieved by various means, such as laser programming of connections, fused links, anti-fuses, or on-chip configuration registers. In memory chips and most WSI devices, the physical location of redundant sub-units has no intrinsic relevance. However, in printing heads the redundant sub-unit contains one or more printing actuators. These must have a fixed spatial relationship to the page being printed. In general, it is not effective to replace a faulty acmator with another acmator which is in a different position in the non-scan direction. Such an acmator cannot print a dot in the correct position to replace the faulty acmator. However, it is possible to replace faulty acmators with actuators which are displaced in the scan direction. To ensure that the redundant acmator prints the dot in the same position as die faulty acmator, the data timing to the redundant actuator can be altered to compensate for the displacement in the scan direction. To allow replacement of all nozzles, there must be a complete set of spare nozzles, which results in 100% redundancy. 100% redundancy is typically not required in memory chips or WSI devices, as a small number of redundant sub-units can be connected to faulty sub-units in many positions. The requirement for 100% redundancy would normally more than double die chip area, dramatically reducing the primary yield before fault tolerance programming. However, in such print heads, minimum physical dimensions of the head chip are set by the width of die page being printed, die fragility of the print 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 widtii, full color print head for printing A4 size paper is approximately 215 mm x 5 mm. This size allows the inclusion of 100% redundancy without increasing chip area, when using 1.5 micron CMOS fabrication technology. Therefore, a high level of fault tolerance can be included without decreasing primary yield. Yield calculation for fault tolerance Yield projections for wafer sort yield versus defect density for a full widtii color A4 print head which includes various forms of fault tolerance are shown in Figure 5. This graph shows projected yield as a function of both defect density and defect clustering. Defect clustering models die non-uniform distribution of defects. If a defect occurs at a particular location, the probabihty of another defect being nearby is typically higher than that implied by the defect density. This is because physical defects tend to cluster, both spatially and temporally. A defect cluster factor of 1 is equivalent to a Boltzmann probabihty distribution. When fault tolerance is included in a device, standard yield equations cannot be used. Instead, die mechanisms and degree of fault tolerance must be specifically analyzed and included in any equation. The main equation used for this wafer sort yield projection is: NO X I- is the yield from defects in the nozzles and nozzle drive circuits. It models die fault tolerant situation where a fault must occur in both a nozzle or drive circuit and in the matching redundant nozzle or drive circuit before a system fault occurs. It is calculated according to die following equation: Where: D is the defect density NN is the number of main nozzles [19,840] AN is the area of one main nozzle and drive circuit [8,400 μm 2 ] C is the defect clustering factor (Values shown in square brackets D are specific for the A4 full color LIFT head with yield projections shown in Figure 5.) Ysn is the yield from defects in the shift register circuits. The shift 5 register circuits include redundant shift registers and data routing multiplexers. A fault in a shift register block will have no system level effect if there is no fault in either the matching redundant shift register, or any one of the nozzles driven by die matching redundant shift register. This case is described by the following equation: Where: 0 Y a = 1 _(i- e- w»A« )(l - g - DCL " (A»+A ^) NSR is the number of main shift register stages [19,840] AS K is the area of one shift register stage [4,200 μm 2 ] LS R is the length of fault tolerant shift register blocks [64] 5 Ycioek is the yield from defects in the fault tolerant clock circuits. This yield is described by die following equation Where: l Oo k = Aα is the area of one clock generator [1,600 μm 2 ] 20 Y NFT is the yield from defects in die non fault tolerant input circuits. This does not include input pads, which usually have very low defect densities. This yield is described by die following equation: Where: 2 -"5 J Y I NFΓ = β e -^*- +A - ) Aj„ p ut is the area of non fault tolerant input circuits [80,000 μm 2 ] A M X is the area of non fault tolerant multiplexer select controller circuits [1,600,000 μm 2 ] Y B S is the yield from defects in the non fault tolerant multiplexer control bus. While this is simply a 9 bit bus on one metal layer, it is not fault tolerant in the current design. The defect density is divided by tiiree because only the top metal layer is defect sensitive. In a two level metal device, a single level of metal usually contributes less than 33% of the chip defects. The multiplexer control bus can be made fault tolerant with a small increase chip complexity. This yield is described by die following equation: Where: Lutoά is the length of the print head [215 mm] WBUI is widtii of die bus [108 μm] These equations combine to form the following equation for fault tolerant sort yield: -Dλ N C' ' ■ Sort = (> - (>- '* -«-**)) M- ■ DCL u (A st +A M ) * - )) The fault tolerant yield projection 199 shown in figure 5 is calculated according to diis equation. It indicates diat 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. Total practical yield for diis device at a defect density of 1 defect per square cm can be calculated as: Yτ m = Y F mb * Ysor, x YT- I - 90% x 90% x 95% = 77% This is a practical total yield for volume production. Printing System Embodiments A schematic diagram of a digital electronic printing system using a print head of diis invention is shown in Figure 6. This shows a monolithic 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 substantially flat surfaces which will 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, digitally 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-call based rasterizer, such as Apple QuickDraw, Apple Quickdraw GX, or Microsoft GDI, or text in an electronic form such as ASCII. This image data is tiien 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 die data will typically have between 1 and 32 bits per pixel, depending upon die image source and die specifications of die 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. The binary image is processed by a data phasing circuit 55 (which may be incoφorated in a Head Control ASIC 400 as shown in figure 4) which provides die pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and die movement of die paper. When the data has been loaded into the shift registers 56, it is presented in parallel 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 die pulse shaper circuit 61 and die 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 die heater driver circuits. The pressure of the ink in the ink reservoir 64 is regulated by die pressure regulator 63. Selected drops of ink drops 60 are separated from die body of ink by the chosen drop separation means, and contact the recording medium 51. During printing, the recording medium 51 is continually moved relative to the print head 50 by die paper transport system 65. If the print head 50 is the full widtii of the print region of the recording medium 51, it is only necessary to move the recording medium 51 in one direction, and die print head 50 can remain fixed. If a smaller print head 50 is used, it is necessary to implement a raster scan system. This is typically achieved by scanning the print head 50 along the short dimension of die recording medium 51, while moving the recording medium 51 along its long dimension. Integrated Drive Circuitry Figure 7 shows one preferred embodiment of die invention comprising a print head with integrated drive circuitry. This print head has 19,840 nozzles, which are connected using eight shift registers, each of which contains 2,480 drive modules 220. For simplicity of me drawing, only eight of die 2,480 drive modules 220 in each shift register are shown. Also, only four of the eight shift registers are shown. The preferred circuit for integrated nozzle drivers on large print heads incoφorates fault tolerance. This is omitted from this diagram for simplicity. The clock generation module 230 generates a gated two phase clock for the shift registers. This gated two phase clock allows the elimination of the parallel registers that would odierwise be required to hold die data constant during die heater enable pulse. The two clock phases allow the use of dynamic shift registers instead of static shift registers, further reducing the number of integrated transistors required for each nozzle driver. The three EnPhase signals are the input of a three line to eight line decoder 260. The Eight outputs of the decoder 260 are connected to die enable controls of the drive modules 220. As each output of the decoder 260 drives 2,480 loads distributed over die length of the print head, the output transistors of the decoder must be either very large, or buffered multiple times, to obtain fast switching. The inclusion of the decoder 260 reduces die number of external connections required to control which of the eight groups is activated from eight to four. The print head has only a small number of connections. There are: 1 ) V * , which is the positive power connection to the heaters. 2) V, which is the return power (ground) connection to the heater drive transistors. 3) V M , which is the positive power connection to the shift registers and data enable circuits. 4) V β , which is the return power (ground) connection for the shift registers and data enable circuits. 5) Clock, which is the main system clock, used for clocking the shift registers. 6) EnPhase, which is firing phase enable selection. 7) Enable, which is a global enable signal. If this signal is inactive, no printing can occur. 8) Data<0-7>, which are the eight serial data input signals which control which nozzles are to be energized. 9) Test, which is an Or function of the data at die output of the shift registers. The eight outputs are wired to die inputs of a eight input Or gate 270. This output can be used for testing the integrity of the shift registers in the print head. Only one shift register can be tested at a time. More sophisticated test circuitry can be included on die print head using well known techniques. As with most manufactured products, die cost of manufacture is important If the device costs too much to manufacture, it will not succeed commercially. Block fault tolerance The invention consists of block fault tolerance circuitry which corrects faults in the data transfer mechanisms of an integrated printing head comprising: 1 ) a plurality of data transfer mechanisms which, in the absence of faults, transfers data to die printing acmators; 2) one or more redundant data transfer mechanisms; 3) a means of determining which of the data transfer mechanisms contain faults; 4) a means of connecting the output of an operational data transfer mechanism, which precedes a faulty data transfer mechanism in terms of data flow, to die input of a redundant data transfer mechanism; and 5) a means of connecting the output of the redundant data transfer mechanism to the input of the data transfer mechanism which normally is connected to die ouφut of die faulty data transfer mechanism in terms of data flow. The invention is applicable to many types of printing mechanisms which consist of a plurality of dot marking means integrated into a single structure. Examples of such printing mechanisms include, but are not limited to, coincident forces drop on demand printing heads, tiiermal inkjet print heads, tiiermal wax printer heads, dye sublimation print heads, and tiiermal paper print heads. The table "LIFT head type A4-4-600" (see Appendix A) is a summary of some characteristics of an example full color monolithic printing head capable of printing an color A4 page at 600 dpi in approximately one second. Block fault tolerance implementation Figure 8 shows a block diagram of a system implementing block fault tolerance in the data distribution system of a print head with integrated drive circuitry. In this example, the data distribution mechanisms are shift registers. There are as many shift registers operating in parallel as there are operational phases of the print head. This is indicated by die number n, which in the example of a high speed full color print head is eight Each stage of each shift register provides parallel data to a printer actuator driver. The shift registers are divided into segments 241. Individual segments 241 can be replaced by a redundant shift register segment of the same length 242. The number of segments m that each shift register is divided into is not critical. Decreasing the length of each segment results in a required increase in the number of segments for a given number of actuators in the print head. This increases the number of multiplexers required on die chip, and therefore the redundancy overhead. However, it also decreases die number of actuators that are de-activated by die fault and therefore increases the probabihty that a fault in the shift register can be compensated for by redundant actuator circuits. For the example high speed color print head, each shift register contains 2,480 stages. These can be divided into 38 segments, each containing 64 shift register stages, with a 39dι segment containing 48 shift register stages. By this means, a single fault in a shift register can affect a maximum of 64 acmators, instead of 2,480 acmators. Many other configurations are possible. As in this example, the shift register segments can be of differing lengtiis, so the number of segments that a shift register is divided into does not need to be a factor of the number of stages in the shift register. All of the actuators which are driven by die faulty shift register segment should also be disabled. This can be simply achieved by gating the enable pulse for the appropriate actuator divers. This is done to simplify the redundancy circuit which replaces the faulty acmators. If all of the actuators in a faulty segment are deactivated, tiiere is no requirement to determine die actual shift register stage in a segment which is faulty. All of the actuators in a segment are replaced by die redundancy circuit Also, if the shift register fault is 'stuck active', then disabling die acmator drivers for die section of shift register prevents spurious dots from being printed. The same signal that is used to control the multiplexer 244 to select the redundant shift register segment 242 can be used to disable die acmators controlled by die faulty shift register segment. The redundant shift register 242 does not direcdy control any printer acmators. The redundant shift register simply maintains the overall shift register lengths, therefore resulting in the correct data being apphed to shift registers segments 241 subsequent to the faulty shift register segment. The replacement of the dot printing function of die acmators controlled by die faulty shift register segment is performed by redundancy circuitry disclosed in an Austrahan patent specification lodged concurrently herewith entided 'Nozzle duplication for fault tolerance in integrated printing heads'. Figure 9 discloses a block diagram of a system employing redundant actuators. Under normal operation, die image data 281 controls the drive circuit 282 which energizes the normally active (main) printing actuators 283. The main printing acmators are energized with electrical pulses which are timed so diat die recording medium is marked in die correct positions corresponding to die image data as die printing head containing the printing acmators scans the recording medium. At various times, certain printing actuators may become faulty. These are detected by die fault detection unit 289. The design of die fault detection unit depends upon die circumstance in which the faults must be detected. Three major categories for fault detection are: 1) After fabrication of the printing head. In this case, printing heads can be tested by especially constructed equipment which detects die presence of marks on a recording medium, or directly detects die presence of the ink or other marking material as it leaves the printing actuator. Such equipment may detect die marking material optically, electrically, or by other means. 2) After installation of the printing head in equipment containing drive circuitry and image generation circuitry, but before this equipment leaves the factory which manufactures the equipment. In this case, special test equipment can also be used. The cost of this equipment is not tightly constrained, as very few pieces of such equipment would be required. This allows many different methods of detection to be used. One appropriate method is to cause die print head to print a particular pattern of dots, which includes dots printed by all of the printing acmators. The medium upon which these dots is recorded can then be scanned and analyzed by digital electronic equipment for the presence of dots from each printing actuator. If the dots from a particular printing actuator are missing, then that printing actuator is recorded as being faulty. 3) During use of the equipment containing the printing head by die 'end user'. In this case, the cost of the fault detection equipment is important. If the equipment is a photocopier, it will typically include a scanner and a microprocessor. In this case, one possible method is to print a test page which includes dots printed by all of the printing acmators. This page can then be scanned by die user in a special 'calibration' operating mode. The microprocessor then analyses the scanned data and calculates a 'map' of faulty printing acmators. If the unit is a printer, it will typically not incoφorate a scanner. In this case, the printer may include a single photodetector which is scanned across die printed test page while in 'calibration' mode. Using this technique, a low cost detector can be constructed. A 'map' of faulty acmators is stored in the faulty acmator memory 288. A simple method is to use one bit of information to store die status of each acmator. In the example printing head, 19,840 bits (2,480 bytes) are required to independendy store the status of each main nozzle. This amount of memory can readily be provided using semiconductor memory of various types. It is convenient to store the fault map in a semiconductor memory which will not lose data when die power is turned off. Suitable memory devices are EEPROMs, EPROMs, battery- backed SRAMs, or FLASH memory devices. Otiier device types may also be used. The 'map' of faulty printing acmators is used to control a gating circuit 284 which suppressed print data which is directed to functional main printing acmators, and allows print data directed towards faulty printing acmators to pass to the redundant printing actuators. The timing of the print data is adjusted by a timing adjustment circuit 285 so that a dot printed by die redundant printing acmator will be at the same location as the dot would have been had it been printed by die main printing actuator. In the printing head example described herein, the timing adjustment is a delay of two line periods. The timing adjusted image data for the redundant printing acmators controls the drive circuit 286 which energizes the redundant printing acmators 287. A faulty shift register segment is detected by applying data at die inputs of the shift register segments 241, and detecting die data at die outputs of the shift register segments. If the shift register segment is operational, die data at die output should be identical to the input data after a number of clock cycles equal to the segment length. The outputs of the shift register segments can be determined by routing the appropriate output to a test circuit by controlling the multiplexers 243 and 245. The test function will typically be performed by an external microprocessor, but may be an on chip test circuit The test function may also be provided by test equipment during wafer probe. However, if this latter method is used in exclusion, fabrication faults can be corrected, but field failures cannot be corrected. If a faulty shift register segment is found, die multiplexer select control circuitry 246 is programmed to control the appropriate multiplexer 243 to select the ouφut of the shift register segment 241, the output of which is normally connected to die input of the faulty shift register segment as the input of the redundant shift register segment 242. The multiplexer select control circuitry 246 is also programmed to control a multiplexer 244 which normally selects data from die faulty shift register segment to instead select die output of the redundant shift register segment and connect the data to die input of the shift register segment subsequent (in terms of data flow) to die faulty shift register segment The multiplexer select control circuitry 246 may be implemented in many different ways. One of the most flexible ways is to implement it as static registers which are programmed every time the head is tested by an external microprocessor. This would typically be every time that power is applied to die unit, but could also be at other times, such as upon user request. To reduce wiring on the chip, the static registers should be distributed along the printing head near the multiplexers that they control. Another possible implementation of the multiplexer select control circuitry 246 is as programmable fuses or anti-fuses. This will typically use less gates on the chip, but will also usually require extra wafer processing steps. The multiplexer select control circuitry 246 may also be implemented by laser programming of the print head during wafer probe. However, this requires extra processing steps during fabrication, and cannot easily be used to compensate for field failures. The foregoing describes several preferred embodiments of me present invention. Modifications, obvious to those skilled in the art, can be made tiiereto without departing from die scope of die invention. Appendix A Appendix A (cont'd.) Monolithic LIFT head type A4-4-600 Nozzle and actuation specifications Derivation Nozzle radius 14 μm Specification Number of actuation phases 8 Specification Nozzles per phase 2.480 From page width, resolution and colors Active nozzles per bead 19,840 Actuation phases • nozzles per phase Redundant nozzlet per bead- 19,840 Same as active nozzles for 100% redundancy Total nozzlet per bead 39,680 Active plus redundant nozzles Drop rate per nozzle 5,208 Hz I /(heater active period * number of phases) Heater radius 143 μm From nozzle geometry and radius Heater thin film resistivity 23 μflm For heater formed from ToAl Heater resistance 2,095 Ω From heater dimensions and resistivity Average heater pulse current 5.6 mA From heater power and resistance Heater active period 24 μs From finite element simulations Sealing time petween pulses 168 μs Active period * (actuation phases-]) Clock pulses per line 2,834 Assuming multiple clocks and no transfer register Clock frequency 14.8 MHz From clock pulses per line, and lines per second Drive transistor on resistance 42 Ω From recommended device geometry Average head drive voltage 12.0 V Heater current * (healer +drive transistor resistance) Drop selection temperature 75 "C m.p. of surfactant sol or PIT of microemulsion Heater peak temperature 120 S C From finite element simulations Ink specifications Derivation Basic ink carrier Water Specification Surfactant Aracbidic acid Suggested method of achieving temperature threshold Ink drop volume 18 pi From finite element simulations Ink density 1.030 g/cm' Black ink density at 60°C Ink drop mass 18.5 ng Ink drop volume • ink density Ink specific beat capacity 42 J/KgΛC Ink carrier characteristic Max. energy for self cooling 2,715 αl/drop Ink drop heat capacity * temperature increase E j ection energy per drop 1,587 nJ Energy applied to heater in finite element simulations Energy to print full black page 221 J Drop ejection energy * drops per page Total ink per color per page 0.63 ml Drops per page per color * drop volume Maximum ink flow rate per color 0.47 ml sec Ink per color per page /page print time Full black ink coverage 402 ml m s Ink drop volume * colors * drops per square metre Ejection ink surface tension. 38.5 mN/m Surface tension required for ejection Ink pressure 53 Pa 2 * Ejection ink surface tension / nozzle radius Ink column height 545 mm Ink column height to achieve ink pressure

Claims: I Claim: 1. In an integrated printing head having a plurality of printing acmators, apparams for correcting faults in the data transfer to such acmators, said apparams comprising: (a) a plurality of data transfer devices which, in the absence of faults, transfer data to die printing acmators; (b) at least one redundant data transfer device; (c) means for determining which of the data transfer device contain faults; (d) means for connecting die output of an operational data transfer device which precedes such faulty data transfer device, in terms of data flow, to the input of a corresponding redundant data transfer device; and (e) means for connecting the output of said corresponding redundant data transfer device to die input of the data transfer device which normally is connected to die output of said faulty data transfer device, in terms of data flow. 2. An apparams as claimed in Claim 1 wherein said data transfer devices are shift registers. 3. An apparams as claimed in Claim 1 wherein said redundant data transfer devices are shift registers 4. An apparatus as claimed in Claim 1 wherein said means for determining which of the data transfer devices contain faults including a test means for applying data to die inputs of the devices and means for determining if the same data appears at the outputs of the data transfer devices an appropriate number of clock cycles later. 5. An apparatus as claimed in Claim 4 wherein said test means comprises an external microprocessor. 6. An apparatus as claimed in Claim 4 wherein said test means comprises an on-chip test circuit. 7. An apparams as claimed in Claim 1 wherein said means for connecting the output of an operational data transfer device to die input of a redundant data transfer device is a multiplexer. 8. An apparams as claimed in Claim 7 wherein said multiplexer is constructed to be programmed by an external microprocessor. 9. An apparams as claimed in Claim 7 further comprising an on- chip test and repair circuit for programming said multiplexer. 10. An apparatus as claimed in Claim 1 wherein said means for connecting the output of an operational data transfer device to die input of a redundant data transfer device comprises an integrated fusible link. 11. An apparams as claimed in Claim 1 wherein said means for connecting the output of said redundant data transfer device to die input of the data transfer device which normally is connected to die output of said faulty data transfer, in terms of data flow, is a multiplexer. 12. An apparams as claimed in Claim 11 wherein said multiplexer is adapted for programming by an external microprocessor. 13. An apparatus as claimed in Claim 11 further comprising an on- chip test and repair circuit means for programming said multiplexer. 14. An apparams as claimed in Claim 1 wherein said means for connecting the output of said redundant data transfer device to die input of the data transfer device which normally is connected to die output of said faulty data transfer device, in terms of data flow, is an integrated fusible link 15. An apparatus as claimed in Claim 1 wherein the marking means of said integrated printing head is a thermal printing element. 16. An apparatus as claimed in Claim 1 wherein the marking means of said integrated printing head is a tiiermal ink jet nozzle. 17. An apparams as claimed in Claim 1 wherein the marking means of said integrated printing head is a tiiermal wax printer acmator. 18. An apparams as claimed in Claim 1 wherein the marking means of said integrated printing head is a dye sublimation printer actuator. 19. An apparatus as claimed in Claim 1 wherein the marking means of said integrated printing head is a heater element that is part of a heater bar of a thermal paper printer. 20. The invention according to Claim 1 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (d) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, while allowing ink to be retained in non-selected nozzles. 21. The invention according to Claim 1 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means. 22. The invention according to Claim 1 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperamre range; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 23. An apparams which corrects faults in the data transfer mechanisms of an integrated printing head comprising: (a) a plurality of groups of normal data transfer devices which, in the absence of faults, transfer data to die printing acmators; (b) a plurality of redundant data transfer devices, with at least one redundant data transfer device for each group of normal data transfer devices, except for a last said group; (c) a means for determining which of die data transfer devices within a group contain faults; and (d) multiplexer means for each group, such multiplexer means having a number of inputs at least equal to the number of data transfer devices in its respective group, and being programmable to select the output of a data transfer device which would normally be connected to die input of said faulty data transfer device, and direct such output to the input of the redundant data transfer device for said group. 24. The invention defined in Claim 23 further comprising multiplexer means for each group for directing die output of the group redundant transfer device to die input of the transfer device downstream from such faulty device. 25. An apparams as claimed in Claim 23 wherein said data transfer devices are shift registers. 26. An apparams as claimed in Claim 23 wherein said redundant data transfer devices are shift registers. 27. An apparams as claimed in Claim 23 wherein the marking means of said integrated printing head is a tiiermal printing nozzle. 28. An apparatus as claimed in Claim 23 wherein the marking means of said integrated printing head is a thermal inkjet nozzle. 29. An apparatus as claimed in Claim 23 wherein the marking means of said integrated printing head is a tiiermal wax printer actuator. 30. An apparatus as claimed in Claim 23 wherein the marking means of said integrated printing head is a dye sublimation printer acmator. 31. An apparatus as claimed in Claim 23 wherein the marking means of said integrated printing head is a heater element that is part of a heater bar of a thermal paper printer. 32. The invention according to Claim 23 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (d) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 33. The invention according to Claim 23 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, while allowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means. 34. The invention according to Claim 23 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperature range; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, while allowing ink to be retained in non-selected nozzles. 35. A fault tolerant printing system comprising: (1) an integrated printing head having a plurality of normally active printing acmators and: (a) a plurality of data transfer devices which, in the absence of faults, transfer data to die printing acmators; (b) at least one redundant data transfer device; (c) means for determining which of the data transfer device contain faults; (d) means for connecting the output of an operational data transfer device which precedes such faulty data transfer device, in terms of data flow, to die input of a corresponding redundant data transfer device; and (e) means for connecting the output of said corresponding redundant data transfer device to die input of the data transfer device which normally is connected to die output of said faulty data transfer device, in terms of data flow; (2) means for signaling the identity of a normally and active printing actuator that is ineffective due to faulty data transfer; (3) a plurality of redundant printing actuators having print capability correspondence to said normally active acmators; and (4) means, responsive to said signaling means, for energizing a redundant printing acmator that corresponds die acmator ineffective due to faulty data transfer, to operate under control of data diat normally would be transferred to said ineffective actuator. 36. The invention defined in Claim 35 wherein said redundant acmators are located in printing alignment upstream or downstream, of dieir respective corresponding normally active acmators and further comprising control means to synchronize operation of the redundant acmators to print in proper spacial register. 37. The invention according to Claim 34 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (d) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, while allowing ink to be retained in non-selected nozzles. 38. The invention according to Claim 34 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, while allowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means. 39. The invention according to Claim 34 wherein said printhead comprises (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperature range; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, while allowing ink to be retained in non-selected nozzles.

Inventors: SILVERBROOK KIA, SILVERBROOK, KIA
Applicants: EASTMAN KODAK CO, SILVERBROOK KIA, EASTMAN KODAK COMPANY, SILVERBROOK, KIA
Application year: 1996
Main classification: B41J2
IPC codes: B41J2