MONOLITHIC PRINT HEAD STRUCTURE AND A MANUFACTURING PROCESS THEREFOR USING ANISOTROPIC WET ETCHING

Abstract: A manufacturing process and construction for printing heads which operate using coincident forces, drop on demand printing principles. The print head integrates many nozzles into a single monolithic silicon structure. Semiconductor processing methods such as photolithography and chemical etching are used to simultaneously fabricate a multitude of nozzles into the monolithic head. The nozzles are etched through the silicon substrate, allowing two dimensional arrays of nozzles for color printing. The manufacturing process can be based on existing CMOS, nMOS and bipolar semiconductor manufacturing processes, allowing fabrication in existing semiconductor fabrication facilities. Drive transistors, shift registers, and fault tolerance circuitry can be fabricated on the same wafer as the nozzles. The manufacturing process uses anisotropic wet etching to etch ink channels and nozzle barrels from the back surface of the wafer to the front surface of the wafer. The etching follows the crystallographic planes of the silicon, which result in highly accurate and consistent etch angles using simple etching equipment.

Description: MONOLITHIC PRINT HEAD STRUCTURE AND A MANUFACTURING PROCESS THEREFOR USING ANISOTROPIC WET ETCHING Field of the Invention The present invention is in the field of computer controlled printing devices. In particular, the field is manufacturing processes and constructions for thermally activated drop on demand (DOD) printing heads which integrate multiple nozzles on a single substrate. 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 CD 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 ink jet printing approach, providing advantages for example, as to cost speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables. Summary 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. Thus, one important object of the invention is to provide a process for manufacturing a drop on demand printing head. In one aspect the invention constitutes the steps of: 1) forming a surface layer on the front surface of a substrate; 2) forming a plurality of nozzle tip holes through the surface layer; and 3) etching channels from the back surface of the substrate to form regions where the minimum thickness of the substrate in the region of the channels is less than half the normal thickness of the substrate, and 4) etching a plurality of apertures from the regions of minimum substrate thickness in the channels which provide communicable passage with the nozzle tip holes. A further preferred aspect of the invention is that said channels and passages are etched anisotropically. A further preferred aspect of the invention is that the etching step comprises wet etching. A further preferred aspect of the invention is that the substrate is composed of single crystal silicon. A further preferred aspect of the invention is that the substrate is a single crystal silicon wafer of (100) crystallographic orientation. A further preferred aspect of the invention is that the surface layer is composed of silicon dioxide. A further preferred aspect of the invention is that the nozzle tip hole is fabricated with a radius less than 50 microns. A further preferred aspect of the invention is that the ink channels are etched exposing { 111 } crystallographic planes of the substrate. A further preferred aspect of the invention is that the barrel holes are etched exposing { 111 } crystallographic planes of the substrate. A further preferred aspect of the invention is that drive circuitry is fabricated on the same substrate as the nozzles. In another aspect the present invention constitutes a monolithic print head structure for a drop on demand printer, said structure comprising (a) a silicon substrate; (b) a silicon dioxide layer formed on a front surface of said substrate; (c) a plurality of nozzle tip hole arrays formed through said silicon dioxide layer; (d) a plurality of ink ingress channels extending from the back surface of said substrate partially therethrough in respective alignments with said nozzle tip hole arrays; and (e) means defining ink passages from the bottom surfaces of said channels through the silicon substrate to respective nozzle tip holes. 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 single silicon substrate with a multitude of no zzles etched in accordance with one aspect of the invention. Figure 8(a) shows an example embodiment of a small section of a print head in accordance with the invention. Figure 8(b) is a detail of figure 8(a). Figures 9(a) to 9(k) show simplified manufacturing steps in accordance with the present invention for the combination with processes of a standard integrated circuit fabrication. 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 printing 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 ink jet heads must either switch high voltages, or drive highly capacitive loads. The total power consumption of pagewidth TU 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 the 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 the present invention are described in the foUowing Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated 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 (RIP) in the case of PDL image data, or may be pixel image manipulation in the case of raster image data. Continuous tone data produced by the image processing unit 53 is half oned. 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 the appropriate nozzle, so that selected drops will form spots on the recording medium 51 in the appropriate position designated by the data in the image memory 72. The recording medium 51 is moved relative to the head 50 by a paper transport system 65, which is electronically controlled by a paper transport control system 66, which in turn is controlled by a microcontroller 315. The paper transport system shown in figure 1(a) is schematic only, and many different mechanical configurations are possible. In the case of pagewidth print heads, it is most convenient to move the recording medium 51 past a stationary head 50. 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 the recording medium 51 along the orthogonal axis (the main scanning direction), in a relative raster motion. The microcontroUer 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 controUed 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 the ink pressure is caused to oscillate. The means of producing this oscillation may be a piezoelectric actuator mounted in the ink channels (not shown). When properly arranged with the drop separation means, selected drops proceed to form spots on the recording medium 51, whUe unselected drops remain part of the body of ink. The ink is distributed to the back surface of the head 50 by an ink channel device 75. The ink preferably flows through slots and/or holes etched through the silicon substrate of the head 50 to the front surface, where the no7zles and actuators are situated. In the case of thermal selection, the nozzle actuators are electrothermal heaters. In some types of printers according to the invention, an external field 74 is required to ensure that the selected drop separates from the body of the ink and moves towards the recording medium 51. A convenient external field 74 is a constant electric field, as the ink is easily made to be electrically conductive. In this case, the paper guide or platen 67 can be made of electricaUy 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 smaU drop sizes gravitational force on the ink drop is very small, approximately 10 " * of the surface tension forces, so gravity can be ignored in most cases. This aUows the print head 50 and recording medium 51 to be oriented in any direction in relation to the 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 amoφhous silicon) may be deposited on the substrate, and integrated drive transistors and data distribution circuitry may be formed in the surface semiconducting layer. Single crystal silicon (SCS) substrates have several advantages, including: 1) High performance drive transistors and other circuitry can be fabricated in SCS; 2) Print heads can be fabricated in existing faciUties (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 siUcon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry. The nozzle tip is passivated with siUcon 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. MonoUthic nozzles etched from the substrate upon which the heater and drive electronics are formed have the advantage of not requiring an orifice plate. The ehmination of the orifice plate has significant cost savings in manufacture and assembly. Recent methods for eliminating orifice plates include the use of 'vortex' actuators such as those described in Domoto et al US Pat. No. 4,580,158, 1986, assigned to Xerox, and MiUer 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 method for elimination of orifice plates for print heads of the invention is incoφoration of the orifice into the actuator substrate. This type of nozzle may be used for print heads using various techniques for drop separation. Operation 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 FIDAP, 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 apphed to the heater is 276 nJ, applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and the ink viscosity at 30°C is 1.84 cPs. The ink is water based, and includes a sol of 0.1 % palmitic acid to achieve an enhanced decrease in surface tension with increasing temperature. A cross section of the nozzle tip from the central axis of the nozzle to a radial distance of 40 μm is shown. Heat flow in the various materials of the nozzle, including siUcon, siUcon nitride, amoφhous siUcon dioxide, crystalline siUcon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials. The time step of the simulation is 0.1 μs. Figure 2(a) shows a quiescent state, just before the heater is actuated. An equiUbrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus. Figure 2(b) shows thermal contours at 5°C intervals 5 μs after the start of the heater energizing pulse. When the heater is energized, the ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes the heated portion of the meniscus to rapidly expand relative to the cool ink meniscus. This drives a convective flow which rapidly transports this heat over part of the free surf ace of the ink at the nozzle tip. It is necessary for the heat to be distributed over the ink surface, and not just where the ink is in contact with the heater. This is because viscous drag against the solid heater prevents the ink directly 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 the equiUbrium of forces. As the entire meniscus has been heated, the ink begins to flow. Figure 2(d) shows thermal contours at 5°C intervals 20 μs after the start of the heater energizing pulse. The ink pressure has caused the ink to flow to a new meniscus position, which protrudes from the print head. The electrostatic field becomes concentrated by the protruding conductive ink drop. Figure 2(e) shows thermal contours at 5°C intervals 30 μs after the start of the heater energizing pulse, which is also 6 μs after the end of the heater pulse, as the heater pulse duration is 24 μs. The nozzle tip has rapidly cooled due to conduction through the oxide layers, and conduction into the flowing ink. The nozzle tip is effectively 'water cooled' by the ink. Electrostatic attraction causes the ink drop to begin to accelerate towards the recording medium. Were the heater pulse significantly shorter Qess than 16 μs in this case) the ink would not accelerate towards the print medium, but would instead return to the nozzle. Figure 2(f) shows thermal contours at 5°C intervals 26 μs after the end of the heater pulse. The temperature at the nozzle tip is now less than 5°C above ambient temperature. This causes an increase in surface tension around the nozzle tip. When the rate at which the ink is drawn from the nozzle exceeds the viscously Umited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks', and the selected drop separates from the body of ink. The selected drop then travels to the recording medium under the influence of the external electrostatic field. The meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the next ink drop. One ink drop is selected, separated and forms a spot on the recording medium for each heat pulse. As the heat pulses are electricaUy controUed, drop on demand ink jet operation can be achieved. Figure 3(a) shows successive meniscus positions during the 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 the passivation layer, the ink, and air. B - Meniscus midpoint: This is at a circle on the ink meniscus midway between the nozzle tip and the centre of the meniscus. C - Chip surface: This is at a point on the print head surface 20 μm from the centre of the nozzle. The temperature only rises a few degrees. This indicates that active circuitry can be located very close to the nozzles without experiencing performance or lifetime degradation due to elevated temperatures. Figure 3(e) shows the power appUed to the heater. Optimum operation requires a shaφ rise in temperature at the start of the heater pulse, a maintenance of the temperature a Uttle below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of the pulse. To achieve this, the average energy appUed to the heater is varied over the duration of the 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 appUed to the heater is 40 mW, and the average power over the duration of the heater pulse is 11.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significantly affecting the operation of the print head. A higher sub-pulse frequency allows finer control over the power appUed to the heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable for minimizing the effect of radio frequency interference (RFI). 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 Uquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary Uquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many Uquids: Where fris the surface tension at temperature T, k is a constant, T c is the critical temperature of the Uquid, M is the molar mass of the Uquid, x is the degree of association of the Uquid, and p is the density of the Uquid. This equation indicates that the surface tension of most Uquids falls to zero as the temperature reaches the critical temperature of the Uquid. For most Uquids, the 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 temperature, the admixture of surfactants is recommended. The choice of surfactant is important. For example, water based ink for thermal ink jet printers often contains isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4°C, lower than that of water. As the temperature rises, the alcohol evaporates faster than the water, decreasing the 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 sUghtly 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 Uttle as lOmN/m can be used to achieve operation of the print head according to the present invention. Inks With Large -Aγ.. 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 temperature (PIT) which is above the maximum ambient temperature, but below the boiling point of the ink. For stabiUty, the PIT of the microemulsion is preferably 20°C or more above the maximum non-operating temperature encountered by the ink. A PIT of approximately 80°C is suitable. Inks with Surfactant Sols Inks can be prepared as a sol of smaU particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxyhc acids with between 14 and 30 carbon atoms, such as: As the melting point of sols with a small particle size is usually stightly less than of the bulk material, it is preferable to choose a carboxyUc acid with a melting point sUghtly above the desired drop selection temperature. A good example is Arachidic acid. These carboxyUc acids are available in high purity and at low cost. The amount of surfactant required is very small, so the cost of adding them to the ink is insignificant A mixture of carboxyUc acids with sUghtly varying chain lengths can be used to spread the melting points over a range of temperatures. Such mixtures wiU typicaUy cost less than the pure acid. It is not necessary to restrict the choice of surfactant to simple unbranched carboxyUc acids. Surfactants with branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxylic acid. Many highly polar moieties are suitable for the hydrophilic end of the surfactant. It is desirable that the polar end be ionizable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent flocculation. In the case of carboxyUc acids, this 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 the ink in the required concentration. An example process for creating the surfactant sol is as foUows: 1) Add the carboxylic acid to purified water in an oxygen free atmosphere. 2) Heat the mixture to above the melting point of the carboxyUc acid. The water can be brought to a boil. 3) Ultrasonicate the mixture, until the typical size of the carboxyUc acid droplets is between lOOA and l.OOOA. 4) Allow the mixture to cool. 5) Decant the larger particles from the top of the mixture. 6) Add an alkaU such as NaOH to ionize the carboxyUc 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 the carboxyUc acid is lower than water, smaUer particles wiU accumulate at the outside of the centrifuge, and larger particles in the centre. 8) Filter the sol using a microporous filter to eliminate any particles above 5000 A. 9) Add the surfactant sol to the ink preparation. The sol is required only in very dilute concentration. The ink preparation wiU also contain either dye(s) or pigment^, bactericidal agents, agents to enhance the electrical conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required. Anti-foaming agents wiU generaUy not be required, as there is no bubble formation during the drop ejection process. Cationic surfactant sols Inks made with anionic surfactant sols are generaUy unsuitable for use with catiomc dyes or pigments. This is because the cationic dye or pigment may precipitate or flocculate with the 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 the 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 the desired ejection threshold temperature. Below the PIT, the microemulsion is oil in water (O/W), and above the PIT the microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperatures significantly above the PIT, the surfactant prefers a high curvature surface aroimd 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 the 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 therefore 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 possibiUties for the 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 these cases, polar solvents with a high surface tension should be chosen, so that a large decrease in surface tension is achievable. The surfactant can be chosen to result in a phase inversion temperature in the desired range. For example, surfactants of the group poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general formula: C n H2n+ιC 4 H6(CH 2 CH 2 O) m OH) can be used. The hydrophiUcity of the surfactant can be increased by increasing m, and the hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable. Low cost commercial preparations are the result of a polymerization of various molar ratios of ethylene oxide and alkyl phenols, and the exact number of oxyethylene groups varies around the chosen mean. These commercial preparations are adequate, and highly pure surfactants with a specific number of oxyethylene groups are not required. The formula for this surfactant is C 8 Hι 7 C 4 H 6 (CH 2 CH 2 O) I1 OH (average n=10). Synonyms include Octoxynol-10, PEG- 10 octyl phenyl ether and POE (10) octyl phenyl ether The HLB is 13.6, the melting point is 7°C, and the cloud point is 65°C. Commercial preparations of this surfactant are available under various brand names. SuppUers and brand names are listed in the foUowing table: These are available in large volumes at low cost (less than one doUar per pound in quantity), and so contribute less than 10 cents per Uter to prepared microemulsion ink with a 5% surfactant concentration. Other suitable ethoxylated alkyl phenols include those listed in the foUowing table: Microemulsion based inks have advantages other than surface tension control: 1) Microemulsions are thermodynamically stable, and wiU 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 wiU 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 amoimt of oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or both, can be used. It is also possible to use a mixture of dyes, one soluble in water, and the other soluble in oil, to obtain specific colors. 4) Oil miscible pigments are prevented from flocculating, as they are trapped in the oil microdroplets. 5) The use of a microemulsion can reduce the mixing of different dye colors on the surface of the print medium. 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 aUows 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 foUows: 1) 70% water 2) 5% water soluble dye 3) 5% surfactant 4) 10% oU 5) 10% oil miscible pigment The foUowing 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 hightights. As many dyes are amphiphiUc, large quantities of dyes can also be solubilized in the oil-water boundary layer as this layer has a very large surface area. 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 absoφtion spectrum of the resultant ink wiU be the weighted average of the absoφtion spectra of the different colorants used. This presents two problems: 1) The absoφtion spectrum wiU tend to become broader, as the absoφtion peaks of both colorants are averaged. This has a tendency to 'muddy' the colors. To obtain brilUant color, careful choice of dyes and pigments based on their 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, the color of the dye wiU tend to have a smaller contribution to the 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 the paper. This may be used as an advantage in some circumstances. Surfactants with a Krafft point in the drop selection temperature range For ionic surfactants there is a temperature (the Krafft point) below which the solubiUty is quite low, and the solution contains essentially no miceUes. Above the Krafft temperature miceUe formation becomes possible and there is a rapid increase in solubiUty of the surfactant If the critical miceUe concentration (CMC) exceeds the solubiUty of a surfactant at a particular temperature, then the minimum surface tension wiU be achieved at the point of maximum solubiUty, rather than at the CMC. Surfactants are usuaUy much less effective below the 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 the surfactant is in solution. When the nozzle heater is turned on, the temperature rises, and more of the surfactant goes into solution, decreasing the surface tension. A surfactant should be chosen with a Krafft point which is near the top of the range of temperatures to which the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperatures, and the concentration of surfactant in solution at the drop selection temperature. The concentration of surfactant should be approximately equal to the CMC at the Krafft point. In this manner, the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient temperatures. The foUowing table shows some commerciaUy available surfactants with Krafft points in the desired range. Surfactants with a cloud point in the drop selection temperature range Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension faUs with increasing temperature. At low temperatures, the POE chain is hydrophilic, and maintains the surfactant in solution. As the temperature increases, the structured water around the POE section of the molecule is disrupted, and the POE section becomes hydrophobic. The surfactant is increasingly rejected by the water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension. The temperature at which the POE section of a nonionic surfactant becomes hydrophiUc is related to the cloud point of that surfactant POE chains by themselves are not particularly suitable, as the cloud point is generaUy 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 the 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 the molecules, and a POE segment in the centre, such as the meroxapol class of surfactants (genericaUy also CAS 9003-11-6) Some commerciaUy 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 foUowing table: Other varieties of poloxamer and meroxapol can readily be synthesized using weU 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 CH 2 O) x (CH 2 CH 2 O) y (CHCH 3 CH 2 O) z OH] varieties where the average x and z are approximately 4, and the average y is approximately 15 may be suitable. If salts are used to increase the electrical conductivity of the ink, then the effect of this salt on the cloud point of the surfactant should be considered. The cloud point of POE surfactants is increased by ions that disrupt water structure (such as I " ), as this makes more water molecules available to form hydrogen bonds with the POE oxygen lone pairs. The cloud point of POE surfactants is decreased by ions that form water structure (such as Cl ' , OH " ), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively Uttle 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 sUghtly lowers the cloud point of nonionic surfactants. Hot Melt Inks The ink need not be in a Uquid state at room temperature. SoUd 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink. The hot melt ink must be formulated so that the surface tension of the molten ink decreases with temperature. A decrease of approximately 2 mN/m wiU be typical of many such preparations using waxes and other 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 temperature and drop selection temperature may be greater for a hot melt ink than for a water based ink, as water based inks are constrained by the boiling point of the water. The ink must be Uquid at the quiescent temperature. The quiescent temperature should be higher than the highest ambient temperature likely to be encountered by the printed page. T he quiescent temperature should also be as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop ejection temperatures. A quiescent temperature between 60°C and 90°C is generaUy suitable, though other temperatures may be used. A drop ejection temperature of between 160°C and 200°C is generaUy 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 temperature, but substantiaUy below the drop ejection temperature, can be added to the hot melt ink while in the liquid 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 temperature, it is desirable that the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperature. This generaUy excludes alkanes such as waxes. Suitable materials will generaUy have a strong intermolecular 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 the measured effect of temperature on the surface tension of various aqueous preparations containing the foUowing 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 foUowing AustraUan 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' (Fiting 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 foUows. Prior to operation of the printer, soUd ink is melted in the reservoir 64. The reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperature at which the ink 100 is Uquid, but exhibits a relatively high viscosity (for example, approximately 100 cP). The Ink 100 is retained in the nozzle by the surface tension of the ink. The ink 100 is formulated so that the viscosity of the ink reduces with increasing temperature. The ink pressure osciUates at a frequency which is an integral multiple of the drop ejection frequency from the nozzle. The ink pressure osciUation causes osci ations of the ink meniscus at the nozzle tips, but this osciUation is smaU due to the high ink viscosity. At the normal operating temperature, these osciUations are of insufficient ampUtude to result in drop separation. When the heater 103 is energized, the ink forming the selected drop is heated, causing a reduction in viscosity to a value which is preferably less than 5 cP. The reduced viscosity results in the ink meniscus moving further during the high pressure part of the ink pressure cycle. The recording medium 51 is arranged sufficiently close to the print head 50 so that the selected drops contact the recording medium 51, but sufficiently far away that the unselected drops do not contact the recording medium 51. Upon contact with the recording medium 51, part of the selected drop freezes, and attaches to the recording medium. As the ink pressure faUs, ink begins to move back into the nozzle. The body of ink separates from the ink which is frozen onto the recording medium. The meniscus of the ink 100 at the nozzle tip then returns to low ampUtude osciUation. The viscosity of the ink increases to its quiescent level as remaining heat is dissipated to the bulk ink and print head. One ink drop is selected, separated and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electricaUy controlled, drop on demand ink jet operation can be achieved. Manufacturing of Print Heads Manufacturing processes for monoUthic print heads in accordance with the present invention are described in the following AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'A monoUthic LIFT printing head' (Filing no.: PN2301); 'A manufacturing process for monoUthic 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 monoUthic LIFT printing heads' (Filing no.: PN2305); 'A manufacturing process for monoUthic LIFT print heads using anisotropic wet etching' (Filing no.: PN2306); 'Nozzle placement in monoUthic drop-on-demand print heads' (Filing no.: PN2307); 'Heater structure for monoUthic LIFT print heads' (Filing no.: PN2346); 'Power supply connection for monoUthic LIFT print heads' (Filing no.: PN2347); 'External connections for Proximity LIFT print heads' (FUing no.: PN2348); and 'A self- aligned manufacturing process for monoUthic LIFT print heads' (FUing 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' (FUing 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 the present invention is described in the following AustraUan 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 temperature 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 temperature pulses in printing heads' (Filing no.: PN2317); 'Data distribution in monoUthic LIFT print heads' (Filing no.: PN2318); 'Page image and fault tolerance routing device for LIFT printing systems' (Filing no.: PN2319); and 'A removable pressurized Uquid ink cartridge for LEFT printers' (FUing no.: PN2320). Image Processing for Print Heads An objective of printing systems according to the invention is to attain a print quality which is equal to that which people are accustomed to in quaUty color pubUcations printed using offset printing. This can be achieved using a print resolution of approximately 1 ,600 dpi. However, 1 ,600 dpi printing is di ficult and expensive to achieve. Similar results can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and magenta, and one bit per pixel for yellow and black This color model is herein caUed CC'MM' YK. Where high quaUty monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein caUed CC'MM' YKK' . Color models, halftoning, data compression, and real-time expansion systems suitable for use in systems of this invention and other printing systems are described in the foUowing AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby 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 methods of this invention are suitable for a wide range of applications, including (but not limited to) the foUowing: color and monochrome office printing, short run digital printing, high speed digital printing, process color printing, spot color printing, offset press supplemental printing, low cost printers using scanning print heads, high speed printers using pagewidth print heads, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsiπύle and copying machines, label printing, large format plotters, photographic dupUcation, printers for digital photographic processing, portable printers incoφorated into digital 'instant' cameras, video printing, printing of PhotoCD images, portable printers for 'Personal Digital Assistants', waUpaper printing, indoor sign printing, billboard printing, and fabric printing. Printing systems based on this invention are described in the foUowing AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'A high speed color office printer with a high capacity digital page image store' (FUing no.: PN2329); 'A short run digital color printer with a high capacity digital page image store' (FUing 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' (FUing 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' (FUing no.: PN2338); 'A PhotoCD system with integrated printer' (FUing no.: PN2293); 'A color plotter using LIFT printing technology' (Filing no.: PN2291); 'A notebook computer with integrated LIFT color printing system' (FiUng 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' (FUing 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' (FUing no.: PN2297) Compensation of Print Heads for Environmental Conditions It is desirable that drop on demand printing systems have consistent and predictable ink drop size and position. Unwanted variation in ink drop size and position causes variations in the optical density of the resultant print, reducing the perceived print quaUty. These variations should be kept to a smaU proportion of the nominal ink drop volume and pixel spacing respectively. Many environmental variables can be compensated to reduce their effect to insignificant levels. Active compensation of some factors can be achieved by varying the power appUed to the nozzle heaters. An optimum temperature profile for one print head embodiment involves an instantaneous raising of the active region of the nozzle tip to the ejection temperature, maintenance of this region at the ejection temperature for the duration of the pulse, and instantaneous cooling of the region to the ambient temperature. This optimum is not achievable due to the stored heat capacities and thermal conductivities of the various materials used in the fabrication of the nczzles in accordance with the 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 the heater can be varied in time by various techniques, including, but not Umited to: 1 ) Varying the voltage appUed to the heater 2) Modulating the width of a series of short pulses (PWM) 3) Modulating the 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 temperature achieved with a specific power curve. By the incoφoration of appropriate digital circuitry on the print head substrate, it is practical to individuaUy 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 Usted in the table "Compensation for environmental factors". This table identifies which environmental factors are best compensated globaUy (for the entire print head), per chip (for each chip in a composite multi-chip print head), and per nozzle. Co pensation for environmental factors Most appUcations wiU not require compensation for aU of these variables. Some variables have a minor effect and compensation is only necessary where very high image quaUty 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 applied to the print head to achieve heater power modulation, and does not have individual control of the power appUed to each nozzle. Figure 4 shows a block diagram for a system using an 800 dpi pagewidtii 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 paraUel data for enabling heater drive circuits. 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 logicaUy anded with phase enable signal by a nand gate 215. The output of the nand gate 215 drives an inverting buffer 216, which in turn controls the drive transistor 201. The drive transistor 201 actuates the electrothermal heater 200, which may be a heater 103 as shown in figure 1(b). To maintain the shifted data vahd during the enable pulse, the clock to the shift register is stopped the enable pulse is active by a clock stopper 218, which is shown as a single gate for clarity, but is preferably any of a range of weU known gUtch free clock control circuits. Stopping the clock of the shift register removes the requirement for a paraUel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400. Data is routed to either the main nozzles or the redundant nozzles by the data router 219 depending on the state of the appropriate signal of the fault status bus. The print head shown in figure 4 is simplified, and does not show various means of improving manufacturing yield, such as block fault tolerance. Drive circuits for different configurations of print head can readUy be derived from the apparatus disclosed herein. Digital information representing patterns of dots to be printed on the recording medium is stored in the Page or Band memory 1513, which may be the same as the Image memory 72 in figure 1(a). Data in 32 bit words representing dots of one color is read from the Page or Band memory 1513 using addresses selected by the address mux 417 and control signals generated by the Memory Interface 418. 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 the positions of the nozzles in relation to the print medium. As the relative position of the nozzles may be different for different print heads, the Address generators 411 are preferably made programmable. The Address generators 411 normally generate the address corresponding to the position of the main nozzles. However, when faulty nozzles are present, locations of blocks of nozzles containing faults can be marked in the Fault Map RAM 412. The Fault Map RAM 412 is read as the page is printed. If the memory indicates a fault in the block of nozzles, the address is altered so that the Address generators 411 generate the address corresponding to the position of the redundant nozzles. Data read from the Page or Band memory 1513 is latched by the latch 413 and converted to four sequential bytes by the multiplexer 414. Timing of these bytes is adjusted to match that of data representing other colors by the FIFO 415. This data is then buffered by the buffer 430 to form the 48 bit main data bus to the print head 50. The data is buffered as the print head may be located a relatively long distance from the head control ASIC. Data from the Fault Map RAM 412 also forms the input to the FIFO 416. The timing of this data is matched to the data output of the FIFO 415, and buffered by the buffer 431 to form the fault status bus. The programmable power supply 320 provides power for the head 50. The voltage of the power supply 320 is controlled by the DAC 313, which is part of a RAM and DAC combination (RAMDAC) 316. The RAMDAC 316 contains a dual port RAM 317. The contents of the dual port RAM 317 are programmed by the MicrocontroUer 315. Temperature is compensated by changing the contents of the dual port RAM 317. These values are calculated by the microcontroUer 315 based on temperature sensed by a thermal sensor 300. The thermal sensor 300 signal connects to the Analog to Digital Converter (ADC) 311. The ADC 311 is preferably incoφorated in the MicrocontroUer 315. The Head Control ASIC 400 contains control circuits for thermal lag compensation and print density. Thermal lag compensation requires that the power supply voltage to the head 50 is a rapidly time- varying voltage which is synchronized with the enable pulse for the heater. This is achieved by programming the programmable power supply 320 to produce this voltage. An analog time varying programming voltage is produced by the DAC 313 based upon data read from the dual pott RAM 317. The data is read according to an address produced by the counter 403. The counter 403 produces one complete cycle of addresses during the period of one enable pulse. This synchronization is ensured, as the counter 403 is clocked by the system clock 408, and the top count of the counter 403 is used to clock the enable counter 404. The count from the enable counter 404 is then decoded by the decoder 405 and buffered by the buffer 432 to produce the enable pulses for the head 50. The counter 403 may include a prescaler if the number of states in the count is less than the number of clock periods in one enable pulse. Sixteen voltage states are adequate to accurately compensate for the heater thermal lag. These sixteen states can be specified by using a four bit connection between the counter 403 and the dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To aUow non-linear timing of these states the counter 403 may also include a ROM or other device which causes the counter 403 to count in a non-linear fashion. Alternatively, fewer than sixteen states may be used. 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 the 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 the invention depend upon the specific design. Four, eight and sixteen are convenient numbers, though there is no requirement that the number of enable phases is a power of two. 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 the adder 421 and accumulator 422. A latch 423 holds the accumulated value vaUd for the duration of the enable pulse. The multiplexer 401 selects the output of the latch 423 which corresponds to the current enable phase, as determined by the enable counter 404. The output of the multiplexer 401 forms part of the address of the dual port RAM 317. An exact count of the number of 'on' pixels is not necessary, and the most significant four bits of this count are adequate. Combining the four bits of thermal lag compensation address and the four bits of print density compensation address means that the dual port RAM 317 has an 8 bit address. 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 temperature of the head varies only slowly, the microcontroUer 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print density at the current temperature. Periodically (for example, a few times a second), the microcontroller senses the current head temperature and calculates this matrix. The clock to the print head 50 is generated from the system clock 408 by the Head clock generator 407, and buffered by the buffer 406. 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 with thermal ink jet printing technology. A direct comparison is made between the present invention and thermal In jet technology because both are drop on demand systems which operate using thermal actuators and Uquid ink. Although they may appear similar, the two technologies operate on different principles. Thermal ink jet printers use the foUowing fundamental operating principle. A thermal impulse caused by electrical resistance heating results in the explosive formation of a bubble in Uquid ink. Rapid and consistent bubble formation can be achieved by superheating the ink, so that sufficient heat is transferred to the ink before bubble nucleation is complete. 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 coUapses, drawing ink from the ink reservoir to re-fiU the nozzle. Thermal ink jet printing has been highly successful commerciaUy due to the high nozzle packing density and die use of weU established integrated circuit manufacturing techniques. However, thermal ink jet 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, sateUite drop formation, thermal 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 Yield and Fault Tolerance In most cases, monoUthic integrated circuits cannot be repaired if they are not completely functional when manufactured. The percentage of operational devices which are produced from a wafer run is known as the yield. Yield has a direct influence on manufacturing cost. A device with a yield of 5% is effectively ten times more expensive to manufacture than an identical device with a yield of 50%. There are three major yield measurements: 1) Fab yield 2) Wafer sort yield 3) Final test yield For large die, it is typicaUy the wafer sort yield which is the most serious limitation on total yield. FuU pagewidth color heads in accordance with this invention are very large in comparison with typical VLSI circuits. Good wafer sort yield is critical to the cost-effective manufacture of such heads. Figure 5 is a graph of wafer sort yield versus defect density for a monoUthic fiiU width color A4 head embodiment of the invention. The head is 215 mm long by 5 mm wide. The non fault tolerant yield 198 is calculated according to Muφhy's method, which is a widely used yield prediction method. With a defect density of one defect per square cm, Muφhy's method predicts a yield less than 1 %. This means mat more than 99% of heads fabricated would have to be discarded. This low yield is highly undesirable, as the print head manufacturing cost becomes unacceptably high. Muφhy's method approximates the effect of an uneven distribution of defects. Figure 5 also includes a graph of non fault tolerant yield 197 which explicitly models die clustering of defects by introducing a defect clustering factor. The defect clustering factor is not a controUable parameter in manufacturing, but is a characteristic of the manufacturing process. The defect clustering factor for manufacturing processes can be expected to be approximately 2, in which case yield projections closely match Muφhy's method. A solution to the problem of low yield is to incoφorate fault tolerance by including redundant functional units on the chip which are used to replace faulty functional units. In memory chips and most Wafer Scale Integration (WSI) devices, die physical location of redundant sub-units on the chip is not important However, in printing heads the redundant sub-unit may contain one or more printing actuators. These must have a fixed spatial relationship to the page being printed. To be able to print a dot in the same position as a faulty actuator, redundant actuators must not be displaced in the non-scan direction. However, faulty actuators can be replaced with redundant actuators which are displaced in the scan direction. To ensure that the redundant actuator prints the dot in the same position as the faulty actuator, the data timing to the redundant actuator can be altered to compensate for the displacement in the scan direction. To aUow replacement of aU nozzles, there must be a complete set of spare nozzles, which results in 100% redundancy. The requirement for 100% redundancy would noπnaUy more than double the chip area, dramaticaUy reducing die primary yield before substituting redundant units, and tiius eliminating most of the advantages of fault tolerance. However, with print head embodiments according to this invention, me minimum physical dimensions of the head chip are determined by die widtii of the page being printed, the fragility of the head chip, and manufacturing constraints on fabrication of ink channels which supply ink to the back surface of the chip. The minimum practical size for a fuU widtii, fuU color head for printing A4 size paper is approximately 215 mm x 5 mm. This size aUows the inclusion of 100% redundancy without significandy increasing chip area, when using 1.5 μm CMOS fabrication technology. Therefore, a high level of fault tolerance can be included wid out significandy decreasing primary yield. When fault tolerance is included in a device, standard yield equations cannot be used. Instead, die mechanisms and degree of fault tolerance must be specificaUy analyzed and included in the yield equation. Figure 5 shows the fault tolerant sort yield 199 for a fuU widtii color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation. This graph shows projected yield as a function of both defect density and defect clustering. The yield projection shown in figure 5 indicates that thoroughly implemented fault tolerance can increase wafer sort yield from under 1% to more than 90% under identical manufacturing conditions. This can reduce the manufacturing cost by a factor of 100. Fault tolerance is highly recommended to improve yield and reliabiUty of print heads containing thousands of printing nozzles, and thereby make pagewidtii printing heads practical. However, fault tolerance is not to be taken as an essential part of the present invention. Fault tolerance in drop-on-demand printing systems is described in the foUowing AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'Integrated fault tolerance in printing mechanisms' (FUing no.: PN2324); 'Block fault tolerance in integrated printing heads' (FUing no.: PN2325); 'Nozzle dupUcation for fault tolerance in integrated printing heads' (FUing no.: PN2326); 'Detection of faulty nozzles in printing heads' (Filing no.: PN2327); and 'Fault tolerance in high volume printing presses' (FUing no.: PN2328). Printing System Embodiments A schematic diagram of a digital electronic printing system using a print head of this invention is shown in Figure 6. This shows a monoUthic printing head 50 printing an image 60 composed of a multitude of ink drops onto a recording medium 51. This medium wiU typically be paper, but can also be overhead transparency film, cloth, or many other substantiaUy flat surfaces which wUl accept ink drops. The image to be printed is provided by an image source 52, which may be any image type which can be converted into a two dimensional array of pixels. Typical image sources are image scanners, digitaUy stored images, images encoded in a page description language (PDL) such as Adobe Postscript Adobe Postscript level 2, or Hewlett-Packard PCL 5, page images generated by a procedure-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 then converted by an image processing system 53 into a two dimensional array of pixels suitable for the particular printing system. This may be color or monochrome, and the data wiU typicaUy have between 1 and 32 bits per pixel, depending upon the image source and die specifications of the printing system. The image processing system may be a raster image processor (RIP) if the source image is a page description, or may be a two dimensional image processing system if the source image is from a scanner. If continuous tone images are required, then a halftoning system 54 is necessary. Suitable types of halftoning are based on dispersed dot ordered dither or error diffusion. Variations of these, commonly known as stochastic screening or frequency modulation screening are suitable. The halftoning system commonly used for offset printing - clustered dot ordered dither - is not recommended, as effective image resolution is unnecessarily wasted using this technique. The output of th 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 the pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and the movement of the paper. When the data has been loaded into the shift registers 56, it is presented in paraUel to die heater driver circuits 57. At the correct time, the driver circuits 57 wiU electronically connect die corresponding heaters 58 with the voltage pulse generated by die pulse shaper circuit 61 and the voltage regulator 62. The heaters 58 heat the tip of die nozzles 59, affecting die physical characteristics of the ink. Ink drops 60 escape from die nozzles in a pattern which corresponds to the digital impulses which have been appUed 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 the 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 the paper transport system 65. If the print head 50 is the fuU 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 smaUer print head 50 is used, it is necessary to implement a raster scan system. This is typicaUy achieved by scanning the print head 50 along d e short dimension of the recording medium 51 , whUe moving the recording medium 51 along its long dimension. Multiple nozzles in a single monolithic print head It is desirable that a new printing system intended for use in equipment such as office printers or photocopiers is able to print quickly. A printing speed of 60 A4 pages per minute (one page per second) wdl generaUy be adequate for many appUcations. However, achieving an electronicaUy controUed print speed of 60 pages per minute is not simple. The minimum time taken to print a page is equal to the number of dot positions on die page times the time required to print a dot, divided by the number of dots of each color which can be printed simultaneously. The image quaUty mat can be obtained is affected by die total number of ink dots which can be used to create an image. For full color magazine quaUty printing using dispersed dot digital halftoning, approximately 800 dots per inch (31.5 dots per mm) are required. The spacing between dots on the paper is 31.75 μm. A standard A4 page is 210 mm times 297 mm. At 31.5 dots per mm, 61,886,632 dots are required for a monochrome full bleed A4 page. High quaUty process color printing requires four colors - cyan, magenta, yellow, and black. Therefore, die total number of dots required is 247,546,528. While tiiis can be reduced somewhat by not allowing printing in a smaU margin at the edge of die paper, the total number of dots required is stiU very large. If the time taken to print a dot is 144 ms, and only one nozzle per color is provided, tiien it wiU take more than two hours to print a single page. To achieve high speed, high quality printing with my printing system described above, printing heads witii many smaU nozzles are preferred. The printing of a 800 dpi color A4 page in one second can be achieved if die printing head is die ftdl widtii of die paper. The printing head can be stationary, and die paper can travel past it in the one second period. A four color 800 dpi printing head 210 mm wide requires 26,460 nozzles. Such a print head may contain 26,460 active nozzles, and 26,460 redundant (spare) nozzles, giving a total of 52,920 nozzles. There are 6,615 active nozzles for each of die cyan, magenta, yeUow, and black process colors. Print heads witii large numbers of nozzles can be manufactured at low cost This can be achieved by using semiconductor manufacturing processes to simultaneously fabricate many thousands of nozzles in a siUcon wafer. To eliminate problems with mechanical aUgnment and differential tiiermal expansion that would occur if the print head were to be manufactured in several parts and assembled, the head can be manufactured from a single piece of siUcon. Nozzles and ink channels are etched into the siUcon. Heater elements are formed by evaporation of resistive materials, and subsequent photoUmography using standard semiconductor manufacturing processes. To reduce the large number of connections that would be required on a print head with thousands of nozzles, data distribution circuits and drive circuits can also be integrated on die print head. Figure 7 is a simplified view of a portion of a print head, seen from the back surface of the chip, and cut tiirough some of the nozzles. The substrate 120 can be made from a single siUcon crystal. Nozzles 121 are fabricated in die substrate, e.g., by semiconductor photoUdiography and chemical wet etch or plasma etching processes. Ink enters the nozzle at the top surface of the head, passes tiirough the substrate, and leaves via the nozzle tip 123. Planar fabrication of the heaters and the drive circuitry is on the underside of the wafer, that is, die print head is shown 'upside down' in relation die surface upon which active circuitry is fabricated. The substrate thickness 124 can be tiiat of a standard siticon wafer, approximately 650 μm. The head width 125 is related to die number of colors, the arrangement of nozzles, the spacing between die nozzles, and die head area required for drive circuitry and interconnections. For a monochrome head, an appropriate widtii would be approximately 2 mm. For a process color head, an appropriate widtii would be approximately 5 mm. For a CC'MM'YK color print head, d e appropriate head widtii is approximately 8 mm. The length of the head 126 depends upon die appUcation. Very low cost appUcations may use short heads, which must be scanned over a page. High speed appUcations can use fixed page- widtii monoUthic or multi-chip print heads. A typical range of lengths for print heads is between 1 cm and 21 cm, though print heads longer than 21 cm are appropriate for high volume paper or fabric printing. Print head manufacturing using anisotropic wet etches The manufacture of monoUdiic printing heads in accordance witii the invention is simUar to standard siUcon integrated circuit manufacture. However, the normal process flow is modified in several ways. This is essential to form the nozzles, the barrels for the nozzles, the heaters, and die nozzle tips. There are many different semiconductor processes upon which monoUthic head production can be based. For each of diese semiconductor processes, there are many different ways the basic process can be modified to form the necessary structures. To reduce the cost of estabUshing factories to produce heads, it is desirable to base die production on a simple process. It is also desirable to use a set of design rules which is as coarse as practical. This is because equipment to produce fine line widtiis is more expensive, and requires a cleaner environment to achieve equivalent yields. The minimum length of a monoUdiic head is determined by d e width of die required printing capabiUty. The minimum widtii of a monoUdiic head is determined by die mechanical strength requirements, and by die abUity to provide ink supply channels to die back of the siUcon chip. As an example, the minimum size of a photograph type fuU widtii four color head is at least 100 mm long by approximately 5 mm wide. This gives an area of approximately 5 square cm. However, less than 300,000 transistors are required for the shift registers and drive circuitry. It is therefore not necessary to use recent Uthographic equipment The process described herein is based on standard semiconductor manufacturing processes, and can use equipment designed for 2 μm line widtiis. The use of Uthographic equipment which is essentia y obsolete (at the time of writing, the latest production IC manufacturing equipment is capable of 0.25 μm line widtiis) can substantiaUy reduce die cost of estabUshing factories for the production of heads. It is also not necessary to use a low power, high speed process such as VLSI CMOS. The speeds required are moderate, and die power consumption is dominated by he heater power required for the ink jet nozzles. Therefore, a simple technology such as nMOS is adequate. However, CMOS is likely to be die most practical production solution, as there is a significant amount of idle CMOS manufacturing capabiUty avaUable with line widtiis between 1 μm and 2 μm Suitable basic manufacturing processes The manufacturing steps required for fabricating nozzles can be incoφorated into many different semiconductor processing systems. For example, it is possible to manufacture print heads by modifying die foUowing technologies: 1) nMOS 2) pMOS 3) CMOS 4) Bipolar 5) ECL 6) Various galUum arsenide processes 7) Thin Film Transistors (TFT) on glass substrates 8) Micromechanical fabrication without active semiconductor circuits The choice of the base technology is largely independent of die abtiity to fabricate nozzles. The method of incoφoration of nozzle manufacturing steps into semiconductor processing procedures which have not yet been invented is also likely to be obvious to tiiose skilled in the art. The simplest fabrication process is to manufacture the nozzles using siUcon micromechanical processing, without fabricating active semiconductor devices on the same wafer. However, this approach is not practical for heads with large numbers of nozzles, as at least one external connection to the head is required for each nozzle. For large print heads, it is highly advantageous to fabricate drive transistors and data distribution circuits on the same chip as the nozzles. CMOS is currently the most popular integrated circuit process. At present, many CMOS processes are in commercial use, with line widtiis as smaU as 0.35 μm being in common use. CMOS offers the foUowing advantages for the fabrication of print heads in accordance with the invention: 1) WeU known and well characterized production process. 2) Quiescent current is almost zero 3) High reUabUity 4) High noise immunity 5) Wide power supply operating range 6) Reduced electromigration in metal lines 7) Simpler circuit design of shift registers and fault tolerance logic 8) The substrate can be grounded from the front side of die wafer. CMOS has, however, some disadvantages over nMOS and other technologies in the fabrication of heads which include integrated drive circuitry. These include: 1) A large number of processing steps are required to simultaneously manufacture high quality NMOS and PMOS devices on the same chip. 2) CMOS is susceptible to latchup. This is of particular concern due to the high currents at a voltage typicaUy greater than Vdd that are required for die heater circuits. 3) Like other MOS technologies, CMOS is susceptible to electrostatic discharge damage. This can be minimized by including protection circuits at the inputs, and by careful handling. There is no absolute 'best' base manufacturing process which is appUcable to all possible configurations of printing heads. Instead, the manufacturing steps which are specific to the fabrication of nozzles in accordance witii the invention should be incoφorated into the manufacturer's preferred process. In most cases, there wiU need to be minor alterations to die specific details of nozzle manufacturing steps to be compatible with the existing process flow, equipment used, preferred photoresists, and preferred chemical processes. These modifications are obvious to those skilled in the ait, and can be made without departing from me scope of the invention. Layout example Figure 8(a) shows an example layout for a smaU section of an 800 dpi head. This shows two columns of nozzles. One of tiiese columns contains the main printing nozzles. The other column contains the redundant nozzles for fault tolerance. The nozzle 200 and drive transistor 201 are shown. Figure 8(b) is a detaU enlargement of a section of figure 8(a). The layout is for 1.5 micron nMOS, though Uttie change is required for CMOS, as the drive transistor of a CMOS design would be fabricated as an nMOS transistor. The layout shows three nozzles 200, with their drive transistors 201 and inverting drivers 216. The three nozzles are in a staggered (zig-zag) pattern to increase the distance between the nozzles, and thereby increase the strength of die sihcon wafer after the nozzles have been etched tiirough the substrate. The large V* and V currents are carried by a matrix of wide first and second level metal lines which covers the chip. The V * and V " terminals extend along me entire two long edges of the chip. The line from A to B in figure 8(b) is the Une through which the cross section diagrams of figure 9 are taken. This line includes a heater connection on die "A" side, and goes tiirough a 'normal' section of the heater on the "B" side. Alignment to crvstaUographic planes The manufacturing process described herein uses the crystallographic planes inherent in the single crystal siUcon wafer to control etching. The orientation of the masking procedures to the { 111 } planes must be precisely controHed. The orientation of the primary flats on a siUcon wafer are noπnaUy only accurate to within ±1° of the appropriate crystal plane. It is essential that this angular tolerance be taken into account in the design of d e mask and manufacturing processes. For example, if a groove is to be etched along the long edges of a 105 mm print head, then a 1° error in the aUgnment of the wafer to the { 111 } planes controlling the etch rates wiU result in a 1,833 μm error in the widtii of the groove, given sufficient etch time. An aUgnment error of ±0.1 ° or less is required. This can be achieved by etching a test groove in an area of the wafer which is unused. The groove should be long, and aUgned to a (111) plane using the primary flat to aUgn the wafer. The test groove is then over-etched using a solution of 500 grams of KOH per Uter of water at 50°C to expose the { 111 } planes. This solution etches siUcon approximately 400 times faster in <100> directions than <111> directions. Subsequent angular aUgnment can be made optically to this groove. Alternatively, the wafer can be etched clean through at the groove, which may extend to die edges of the wafer. This wiU produce another flat on the wafer, aUgned with high accuracy to the chosen (111) plane. This flat can then be used for mechanical angular alignment. The surface orientation of the wafer is also only accurate to ±1°. However, since the wafer thickness is only approximately 650 μm, a ±1 ° error in aUgnment of the surface contributes a maximum of 11.3 μm of positional inaccuracy when etching through the entire wafer thickness. This is accommodated in die design of the etch masks. Manufacturing process summary A summary of the preferred manufacturing method is shown in Figure 9(a) to Figure 9(k). This consists of the following major steps: 1 ) The first manufacturing step is the deUvery of die wafers . SiUcon wafers are highly recommended over other materials such as galUum arsenide, due to die avatiabitity of large, high quaUty wafers at low cost the strength of siUcon as a substrate, and the general maturity of fabrication processes and equipment. The example manufacturing process described herein uses an n-type wafer with (100) crystaUographic orientation. The wafers should not be mechanicaUy or laser gettered, as this wiU affect back surface etching processes. 150 mm wafers manufactured to standard Semiconductor Equipment and Materials Institute (SEMI) specifications aUow 25 μm total thickness variation. It is desirable to obtain wafers with less thickness variation to simpUfy back-etch control. However, standard tolerance wafers can be used if the etch depth of the ink channel back-etching step is carefully controUed. At the time of writing, 200 mm (8") wafers are in use, and international standards are being set for 300 mm (12") siUcon wafers. 300 mm wafers are especiaUy useful for manufacturing print heads, as pagewidth A4 (also US letter) print heads can be fabricated as a single chip on tiiese wafers. Figure 9(a) shows a (100) n-type 150 mm wafer. The wafer shows 12 print heads, in accordance witii the invention, of 100mm print length, which can be used for photo or A6 size printing, high speed scanning printers, or as components in multi-chip pagewidth printers. The boundary of each chip is etched witii a deep groove. This groove can be etched before or after the fabrication of the active devices, depending upon process flow for the active devices. However, it is recommended that the grooves be etched after most fabrication steps are complete to avoid problems witii resist edge beading at the grooves. 2) The active devices are then fabricated using a prior art integrated circuit fabrication process with double layer metal. The prior art process may be nMOS, pMOS, CMOS, Bipolar, or other process. In general, the active circuits can be fabricated using unmodified processes. However, some processes wiU need modification to aUow for the large currents which may flow though a print head. As a large print head may have in excess of 8 Amperes of current flowing through the heater circuits when fuUy energized, it is essential to prevent electromigration. Molybdenum can be used instead of aluminum for first level metal, as it is resistant to electromigration. The prior-art manufacturing process proceeds unaltered up to the stage of appUcation of the inter-level dielectric. 3) Apply the inter-level dielectric. This can be 3 μm of CVD SiO_. 4) Mask and etch the SiO 2 at the borders of the chips. A region of approximately 100 μm inside the edge of the chips is etched. This is d e bonding pad region. Deep V grooves are etched in the bonding region. When the wafer is diced, these grooves are sawn lengthways, resulting in a chip with beveled edges. The bonding pads are formed on these bevels, aUowing the chip to be bonded without bonding wires or TAB bonding extending above the chip front surface. This is important for close proximity printing, as the print head must be in close proximity (approximately 20 μm) to the recording medium or transfer roUer. Conventional bonding methods would interfere with this proximity. 5) Etch the bonding pad grooves. The etch can be performed by an anisotropic wet etch, which etches the [100] crystallographic direction preferentiaUy to the [111] direction. The result is an exposed crystal face at an angle equal to tan "1 (20.5), or 54.74°. A solution of 440 grams of potassium hydroxide (KOH) per Uter of water can be used for a very high preferential etch rate (approximately 400: 1). However, the recommended etchant is EDP, made using 120 grams of pyrocatechol per Uter of three parts ethylene diamine to one part water. This has a lower preferential etch rate of 35:1, but etches SiO ∑ very slowly, at 2 Angstrom per minute. Etch the wafer in EDP at 115°C for 4 hours to form the V groove. The SiO 2 forms an effective mask, and loses only approximately 0.05 μm of its 3 μm thickness during the etch process. Figure 9(b) shows a cross section of V groove at the boundary between two chips after this etching step. A 0.5 μm layer of CVD SiO 2 should be appUed after etching the V grooves to insulate the bonding pads from die substrate. 6) Etch the inter-metal vias. In some cases, this step may be able to be combined witii the etching of the SiO 2 to form the mask for V groove etching. As the inter-metal SiO ∑ is much thicker than normal, tapering of the via sidewaUs is recommended. 7) AppUcation of second level metal. As witii the first level metal, electromigration must be taken into account. However, the difficulty of bonding to molybdenum thin films requires that molybdenum is not used for die second level metal where the bonding pads are located. Instead, this level can be formed from aluminum. Electromigration can be minimized by using large line- widths for aU high current traces, and by using an aluminum aUoy containing 2% copper. The step coverage of the second level metal is important as the inter-level oxide is thicker than normal. Adequate step coverage is possible by using low pressure evaporation. Via step coverage can be improved by placing vias only to areas where the first level metal covers field oxide. At tiiese points the thickness of the inter-metal oxide is less due to the previous planarization steps. The preferred process is the deposition by low pressure evaporation of 1mm of 98% aluminum, 2% copper. 8) Mask and etch second level metal. Special attention to masking and etching of the bonding pads is required, as they are fabricated in the V grooves. This introduces two problems: the resist thickness wiU be greater in the bottom of the V grooves, and die mask wiU be out of focus. This does not pose a problem for the long edges of the chip, as these are dedicated to die V * and V " power ratis, and are not patterned. Bonding pads fabricated on die short edges of die chip should be larger than usual, and be separated by at least 100 μm. No active circuitry or fine geometry lines should be located in the V grooves. Figure 9(c) shows a cross section of the wafer in the region of a nozzle after this step. 9) Form the heater. The heater material (for example 0.05 μm of TaAl alloy, or refractory materials such as HfB 2 or ZrB 2 ) can be appUed by low pressure evaporation. As the heater is planar, masking and etching is straightforward. The heater is masked as a disk rather than an annulus. The centre of the disk is later etched during die nozzle formation step. This is to ensure exceUent aUgnment between the heater and die nozzle. Heater radius should be controUed to finer tolerance than is generaUy avaUable in a 1.5 μm process, and die use of a stepper for 0.5μm process is recommended. Figure 9(d) shows a cross section of the wafer in the region of a nozzle after this step. 10) Mask the nozzle tip using resist. This must be performed accurately, as the aUgnment of the nozzle tip to the heater, and die radius of die nozzle tip, both affect drop ejection performance. These parameters should be controUed to an accuracy of better than 0.5 μm, and preferably better than 0.3 μm. Figure 9(d) shows a cross section of the wafer in the region of a nozzle after this step. 11) Etch the nozzle tip. The first step is etching the heater. As the heater is very thin, a wet etch can be used. The SiO 2 forming the nozzle tip should be etched witii an anisotropic etch, for example an RIE etch using CF 4 - H 2 gas mixture. The etch is down to siUcon in die nozzle region. The resist is then stripped. 12) Apply a protective coating of Si 3 N 4 . This is appUed to die front face of die wafer only, and should be at least 0.1 μm thick to protect the front face of the wafer from attack by the long wet-etch of the back face of the wafer. The etch rate of Si 3 N 4 in EDP is approximately 1 Angstrom per minute. Figure 9(e) shows a cross section of the wafer in the region of a nozzle after this step. 13) Mask the back surface of the wafer. SiO 2 is used as a mask, as resist is attacked by die wet etching solution. Apply a 0.5 μm coat of spin-on glass (SOG), foUowed by spin coating with 0.5 μm of resist Expose and develop die resist on the back surface of the wafer using a mask of the ink channels. AUgnment is taken from the front surface of the wafer by modified alignment optics of the Uthography equipment. AUgnment of this step is not critical, and can be performed to an accuracy of approximately 10 μm. 14) Etch the ink channels. This is performed by a wet etch of the siUcon using a solution of ethylene diamine, pyrocatechol and water (EDP). The advantage of a wet etch over an anisotropic plasma etch is very low equipment cost, combined with highly accurate etch angles determined by crystaUographic planes. The etchant exposes the { 111 } planes, at an angle of 54.74°. Although the crystal plane is 'perfectly' accurate, a tolerance of 1° should be assumed, as die cut wafer face wiU not be perfectly aUgned to the [100] plane. Etch die wafer in EDP at 115°C for approximately 8 hours and 20 minutes, until the thickness of the wafer at the bottom of the ink channels is approximately 25 μm. Note that if the nozzle barrel etch is also an EDP wet etch, that a diffused boron etch stop cannot be used, as it wiU prevent the etching of the nozzle barrels. Figure 9(g) is a perspective view of some of the ink channels after etching. This view is from the back surface of the wafer. This arrangement is for a four color print head. Two rows of ink channels are used for each color. This is to retain the strength of the siUcon crystal. If ink channels were to be etched die entire length of the printing widtii, the head would be severely weakened. The two rows are staggered so that nozzles are present the entire length of the print region. Using a staggered array of nozzles such as this requires that the data be provided to drive the nozzles in such a manner as to compensate for the nozzle offsets. This can be achieved by digital circuitry which reads die page image from memory in the appropriate order and supplies the c ata to the print head. There is no need to strip the oxide resist used for the ink channel etching step. 15) Mask the nozzle barrels. Apply a 0.1 μm coat of CVD SiO 2 , followed by spin coating with 0.2 μm of resist to the back surface of the wafer. The nozzle barrels are also masked from the back surface of the wafer, though the active pattern of the mask is at the bottom of the ink channels. Expose and develop die resist on the back surface of the wafer using a mask of the nozzle barrels. The exposure system must be focused o die plane of the bottom of the ink channels. A relatively low NA lens should be used to avoid shadowing by the waUs of the ink channels. This is practical as the feature size of the mask is large compared to the feature sizes of modern semiconductor processes requiring high NA lenses. AUgnment is taken from the front surface of the wafer by modified alignment optics of the Uthography equipment. AUgnment of this step should be performed to an accuracy of approximately 1 μm or better. 16) Etch the nozzle barrels. This is also performed by a wet etch of the siUcon using EDP. Etch the wafer in EDP at 115°C for approximately 25 minutes. The etch wiU stop automaticaUy when it reaches the SiO 2 on the front surface of the wafer, so accurate control of etch timing is not required. Figure 9(h) is a perspective view of some of the ink channels after etching. This view is from the back surface of the wafer, looking down into one of the ink channel pits. The circular apertures are the nozzle tips etched in step 11. The arrangement is for a 800 dpi printer with 31.75 μm pixel spacing. The rows are spaced at 31.75 μm, and individual nozzles in a row are spaced at 63.5 μm. Rows are offset in a zig-zag pattern so that there is both a main nozzle and a redundant nozzle for every pixel position. The diameter of the nozzle tip is 20 μm. The nozzles are in a zig-zag pattern instead of in a single line to increase mechanical strength and allow space for die heaters and electrical connections. Two zig-zag rows are provided, one being for the main nozzles and the other being for redundant nozzles. This arrangement requires 'ears' at the convex corners of the mask regions to prevent etching of the {331 } crystal planes at the corners. These ears can be exaggerated to preserve more siUcon in the diagonal regions between nozzles. An alternative is to space the rows of nozzles further apart. If the rows are spaced at 63.5 μm, men there is no etch interference between adjacent nozzles, and aU nozzle mask apertures are simple squares, and aU corners are concave. This simplifies the mask design, and preserves more siUcon between nozzles, and decreases crosstalk between nozzles. (note that crosstalk between the nozzles would prevent a design such as this from being used with conventional thermal ink jet or piezoelectric heads, but is not a significant problem for coincident force print heads constructed in accordance witii my above-described printing systems). The disadvantage is that the extra row spacing adds 508 μm to the width of a four color 800 dpi head witii nozzle redundancy. There is no need to strip the SiO 2 resist used in the nozzle barrel etching step. 17) Strip the Si 3 N protective layer from the front surface of the wafer. Figure 9(i) shows a cross section of the wafer in the region of a nozzle after this step. 18) Form the passivation layer. As die monoUthic print head is in contact with heated water based ink during operation, effective passivation is essential. A 0.5 μm conformal layer of Si 3 N 4 appUed by PECVD can be used. Use SI^ at 200 seem and NH 3 at 2000 seem, pressure of 1.6 torr, temperature of 250 °C, at 46 watts for 50 minutes. Figure 9(j) shows a cross section of the wafer in the region of a nozzle after this step. 19) A hydrophobic surface coating may be appUed at tiiis stage, if the coating chosen can survive the subsequent processing steps. Otherwise, the hydrophobic coating should be appUed after TAB bonding. There are many hydrophobic coatings which may be used, and many methods which may be used to apply them. By way of iUustration, one such suitable coating is fluorinated diamond¬ like carbon (F*DLC), an amoφhous carbon film with the outer surface substantiaUy saturated witii fluorine. A method of applying such a film using plasma enhanced chemical vapor deposition (PECVD) equipment is described in US patent number 5,073,785. It is not essential to apply a separate hydrophobic layer. Instead, die exposed dielectric layer can be treated witii a hydrophobising agent For example, if SiO 2 is used as die insulation layer in place of Si 3 N , the device can be treated witii dimethyldichlorosUane to make the exposed SiO ∑ hydrophobic. This wiU affect die entire nozzle, unless the regions which are to remain hydrophihc are masked, as dimethyldichlorosUane fumes wtil affect any exposed SiO 2 . The apphcation of a hydrophobic layer is required if the ink is water based, or based on some other polar solvent. If the ink is wax based or uses a non- polar solvent, then the front surface of the print head should be Upophobic. In summary, the front surface of the head should be fabricated or treated in such a manner as to repel the ink used. When using the physical device configuration disclosed herein, the hydrophobic layer need not be Umited to the front surface of the device. The entire device may be coated witii a hydrophobic layer (or Upophobic layer is non-polar ink is used) without significandy affecting the performance of the device. If the entire device is treated with an ink repeUent layer, tiien the nozzle radius should be taken as the inside radius of the nozzle tip, instead of die outside radius. 20) Bond, package and test The bonding, packaging, and testing processes can use standard manufacturing techniques. Bonding pads must be opened out from the Si 3 N 4 passivation layer. Although the bonding pads are fabricated at an angle in the V groove, no special care is required to mask them, as the entire V groove area can be stripped of Si 3 N 4 . After the bonding pads have been opened, die resist must be stripped, and the wafer cleaned. Then wafer testing can proceed. Then the wafer is diced. The wafers should be cut instead of scribed and snapped, to prevent breakage of long heads, and because the wafer is weakened along the nozzle rows. The diced wafers (chips) are then mounted in the ink channels. For color heads, the separate ink channels are sealed to die chip at this stage. After mounting, the chip is bonded, and dry device tests performed. The device is then be connected to die ink supply, ink pressure is apphed, and functional testing can be performed. Figure 9(k) shows a cross section of the wafer in the region of a nozzle after this step. In Figure 9(a) to Figure 9(k), 100 is ink, 101 is stiicon, 102 is CVD SiO 2 ,103 is the heater material, 106 is the second layer metal interconnect (aluminum), 107 is resist 108 is siUcon nitride (Si 3 N 4 ) and 109 is die hydrophobic surface coating. Alternative fabrication processes Many otiier manufacturing processes are possible. The above manufacturing process is not the simplest process that can be employed, and is not the lowest cost practical process. However, the above process has the advantage of simultaneous fabrication of high performance devices on the same wafer. The process is also readily scalable, and 1mm line widths can be used if desired. The use of lμm line widtiis (or even finer geometries) aUows more circuitry to be integrated on die wafer, and aUows a reduction in either the size or the on resistance (or both) of the drive transistors. The smaUer device geometries can be used in die foUowing, or a combination of the foUowing, ways: 1 ) To reduce the widtii of the monoUthic head 2) To increase the yield of die head, by incoφorating more sophisticated fault tolerance circuitry 3) To increase the number of nozzles on the head without increasing chip area. 4) To increase the resolution of the print head by more closely spacing the nozzles in terms of the linear dimensions. 5) To incoφorate more of the total system circuitry on the chip. For example, data phasing circuits can be incoφorated on chip, and die head can be supphed witii a standard memory interface, via which it acquires the printing data by direct memory access. It is possible to alter the nozzle formation processes in many ways. For example, it is possible to create the heater using a self-aUgned vertical technique instead of die planar heater formation described herein. The process described herein is a preferred process for production of printing heads as it aUows high resolution, fuU color heads to incoφorate drive circuitry, data distribution circuitry, and fault tolerance, and can be manufactured witii relatively low cost extensions to standard CMOS production processes. Many simpler head manufacturing processes can be derived. In particular, heads which do not include active circuitry may be manufactured using much simpler processes. The foregoing describes a number of prefeπed embodiments of the present invention. Modifications, obvious to those skiUed in die art, can be made thereto without departing from the scope of the invention.

Claims: I Claim: 1. A method for manufacturing a drop on demand printing head, said process including the steps of: (a) forming a surface layer on the front surface of a substrate; (b) forming a plurahty of nozzle tip holes through said surface layer; (c) etching channels from the back surface of said substrate to form regions where the minimum thickness of said substrate in the region of said channels is less than half the normal thickness of said substrate; and (d) etching a plurality of apertures fromthe regions of minimum substrate thickness in said channels which provide communicable passage with said nozzle tip holes. 2. The method defined in Claim 1 wherein said channels and passages are etched anisotropicaUy. 3. The method defined in Claim 2 wherein said etching step comprises a wet etching process. 4. A method as claimed in Claim 1 wherein said substrate is composed of single crystal siUcon. 5. A method as claimed in Claim 1 wherein said substrate is a single crystal siUcon wafer of (100) crystallographic orientation. 6. A method as claimed in Claim 1 wherein said surface layer is composed of siUcon dioxide. 7. A method as claimed in Claim 1 wherein said nozzle tip hole is fabricated witii a radius less than 50 microns. 8. A method as claimed in Claim 1 wherein said substrate is composed of single crystal siUcon, and said ink channels are etched exposing { 111 } crystallographic planes of said substrate. 9. A method as claimed in Claim 1 further including the step of fabricating drive circuitry on the same substrate as the nozzles. 10. A monoUdiic print head structure for a drop on demand printer, said structure comprising: (a) a siUcon substrate; (b) a siUcon dioxide layer formed on a front surface of said substrate; (c) a pluraUty of nozzle tip hole arrays formed through said siUcon dioxide layer; (d) a pluraUty of ink ingress channels extending from die back surface of said substrate partiaUy therethrough in respective aUgnments with said nozzle tip hole arrays; and (e) means defining ink passages from the bottom surfaces of said channels through the sUicon substrate to respective nozzle tip holes. 11. The invention defined in Claim 10 wherein said channels are defined by exposed [111] planes of a siUcon crystal substrate. 12. The invention defined in Claim 10 further comprising heater elements and heater drive circuits formed on the surface of said crystal for each nozzle of said print head. 13. The invention defined in Claim 10 wherein said structure further comprises: (a) a pluraUty 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, whUe aUowing ink to be retained in non-selected nozzles. 14. The invention defined in Claim 10 wherein said structure further comprises: (a) a pluraUty of drop-emitter nozzles; (b) a body of ink associated witii 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 aUowing 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. 15. The invention defined in Claim 10 wherein said structure further comprises: (a) a pluraUty of drop-emitter nozzles; (b) a body of ink associated witii 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 the body of ink, whtie aUowing 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