Patent Publication Number: US-5838339-A

Title: Data distribution in monolithic print heads

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to my commonly assigned, co-pending U.S. patent applications Ser. No. 08/701,021 entitled CMOS PROCESS COMPATIBLE FABRICATION OF PRINT HEADS filed Aug. 21, 1996; Ser. No. 08/733,711 entitled CONSTRUCTION AND MANUFACTURING PROCESS FOR DROP ON DEMAND PRINT HEADS WITH NOZZLE HEATERS filed Oct. 17, 1996; Ser. No. 08/734,822 entitled A MODULAR PRINT HEAD ASSEMBLY filed Oct. 22, 1996; Ser. No. 08/736,537 entitled PRINT HEAD CONSTRUCTIONS FOR REDUCED ELECTROSTATIC INTERACTION BETWEEN PRINTED DROPLETS filed Oct. 24, 1996; Ser. No. 08/750,320 entitled NOZZLE DUPLICATION FOR FAULT TOLERANCE IN INTEGRATED PRINTING HEADS and Ser. No. 08/750,312 entitled HIGH CAPACITY COMPRESSED DOCUMENT IMAGE STORAGE FOR DIGITAL COLOR PRINTERS both filed Nov. 26, 1996; Ser. No. 08/753,718 entitled NOZZLE PLACEMENT IN MONOLITHIC DROP-ON-DEMAND PRINT HEADS and Ser. No. 08/750,606 entitled A COLOR VIDEO PRINTER AND A PHOTO CD SYSTEM WITH INTEGRATED PRINTER both filed on Nov. 27, 1996; Ser. No. 08/750,438 entitled A LIQUID INK PRINTING APPARATUS AND SYSTEM, Ser. No. 08/750,599 entitled COINCIDENT DROP SELECTION, DROP SEPARATION PRINTING METHOD AND SYSTEM, Ser. No. 08/750,435 entitled MONOLITHIC PRINT HEAD STRUCTURE AND A MANUFACTURING PROCESS THEREFOR USING ANISTROPIC WET ETCHING, Ser. No. 08/750,436 entitled POWER SUPPLY CONNECTION FOR MONOLITHIC PRINT HEADS, Ser. No. 08/750,437 entitled MODULAR DIGITAL PRINTING, Ser. No. 08/750,439 entitled A HIGH SPEED DIGITAL FABRIC PRINTER, Ser. No. 08/750,763 entitled A COLOR PHOTOCOPIER USING A DROP ON DEMAND INK JET PRINTING SYSTEM, Ser. No. 08/765,756 entitled PHOTOGRAPH PROCESSING AND COPYING SYSTEMS, Ser. No. 08/750,646 entitled FAX MACHINE WITH CONCURRENT DROP SELECTION AND DROP SEPARATION INK JET PRINTING, Ser. No. 08/759,774 entitled FAULT TOLERANCE IN HIGH VOLUME PRINTING PRESSES, Ser. No. 08/750,429 entitled INTEGRATED DRIVE CIRCUITRY IN DROP ON DEMAND PRINT HEADS, Ser. No. 08/750,433 entitled HEATER POWER COMPENSATION FOR TEMPERATURE IN THERMAL PRINTING SYSTEMS, Ser. No. 08/750,640 entitled HEATER POWER COMPENSATION FOR THERMAL LAG IN THERMAL PRINTING SYSTEMS, and Ser. No. 08/750,642 entitled PRESSURIZABLE LIQUID INK CARTRIDGE FOR COINCIDENT FORCES PRINTERS all filed Dec. 3, 1996; Ser. No. 08/750,647 entitled MONOLITHIC PRINTING HEADS AND MANUFACTURING PROCESSES THEREFOR, Ser. No. 08/750,604 entitled INTEGRATED FOUR COLOR PRINT HEADS, Ser. No. 08/750,605 entitled A SELF-ALIGNED CONSTRUCTION AND MANUFACTURING PROCESS FOR MONOLITHIC PRINT HEADS, Ser. No. 08/682,603 entitled A COLOR PLOTTER USING CONCURRENT DROP SELECTION AND DROP SEPARATION INK JET PRINTING TECHNOLOGY, Ser. No. 08/750,603 entitled A NOTEBOOK COMPUTER WITH INTEGRATED CONCURRENT DROP SELECTION AND DROP SEPARATION COLOR PRINTING SYSTEM, Ser. No. 08/765,130 entitled PRINTING MECHANISMS; Ser. No. 08/750,431 entitled BLOCK FAULT TOLERANCE IN INTEGRATED PRINTING HEADS, Ser. No. 08/750,607 entitled FOUR LEVEL INK SET FOR BI-LEVEL COLOR PRINTING, Ser. No. 08/750,430 entitled A NOZZLE CLEARING PROCEDURE FOR LIQUID INK PRINTING, Ser. No. 08/750,600 entitled METHOD AND APPARATUS FOR ACCURATE CONTROL OF TEMPERATURE PULSES IN PRINTING HEADS, Ser. No. 08/750,608 entitled A PORTABLE PRINTER USING A CONCURRENT DROP SELECTION AND DROP SEPARATION PRINTING SYSTEM, and Ser. No. 08/750,602 entitled IMPROVEMENTS IN IMAGE HALFTONING all filed Dec. 4, 1996; Ser. No. 08/765,127 entitled PRINTING METHOD AND APPARATUS EMPLOYING ELECTROSTATIC DROP SEPARATION, Ser. No. 08/750,643 entitled COLOR OFFICE PRINTER WITH A HIGH CAPACITY DIGITAL PAGE IMAGE STORE, and Ser. No. 08/765,035 entitled HEATER POWER COMPENSATION FOR PRINTING LOAD IN THERMAL PRINTING SYSTEMS all filed Dec. 5, 1996; Ser. No. 08/765,036 entitled APPARATUS FOR PRINTING MULTIPLE DROP SIZES AND FABRICATION THEREOF, Ser. No. 08/765,017 entitled HEATER STRUCTURE AND FABRICATION PROCESS FOR MONOLITHIC PRINT HEADS, Ser. No. 08/750,772 entitled DETECTION OF FAULTY ACTUATORS IN PRINTING HEADS, Ser. No. 08/765,037 entitled PAGE IMAGE AND FAULT TOLERANCE CONTROL APPARATUS FOR PRINTING SYSTEMS all filed Dec. 9, 1996; and Ser. No. 08/765,038 entitled CONSTRUCTIONS AND MANUFACTURING PROCESSES FOR THERMALLY ACTIVATED PRINT HEADS filed Dec. 10, 1996. 
     1. Field of the Invention 
     The present invention is in the field of computer controlled printing devices. In particular, the field is thermally activated drop on demand (DOD) printing systems. 
     2. 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, U.S. Pat. No. 1,941,001. 
     Sweet et al U.S. 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 CIJ, and is used by several manufacturers, including Elmjet and Scitex. 
     Hertz et al U.S. Pat. No. 3,416,153, 1966, discloses a method of achieving variable optical density of printed spots in CIJ printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris Graphics. 
     Kyser et al U.S. 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 U.S. 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. Pat. 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. Pat. 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. 
     The printing mechanism is based on a new printing principle called &#34;Liquid Ink Fault Tolerant&#34; (LIFT) Drop on Demand printing 
     SUMMARY OF THE INVENTION 
     My concurrently filed applications, entitled &#34;Liquid Ink Printing Apparatus and System&#34; and &#34;Coincident Drop-Selection, Drop-Separation Printing Method and System&#34; 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. 
     One important purpose of the invention is to provide a print head control circuit with a reduced number of transistors in the latches in each nozzle driving circuit than would normally be required. In preferred embodiment, this is achieved by using dynamic shift registers instead of static shift registers, and by creating a gated clocking scheme which holds the data valid for the period that the heaters in the corresponding nozzle group are energized. This gating scheme can eliminate the transfer registers that would otherwise be required. 
     In one aspect, the present invention constitutes a monolithic print head device having a plurality of print actuators and a plurality of driver circuits, if respective binary data so indicate, integrated thereon, said device further comprising: (a) shift register means for transferring binary data in response to clock signals; (b) means for coupling said shift register means to said drivers to said drivers to receive enable pulses, if respective binary data so indicate; (c) clock means for providing clock signals to said shift register means; (d) means for stopping the clock signals to avoid data transfer during a predetermined stable period; and (e) means for holding the binary data in the shift register valid for the duration of an enable pulse. 
     A preferred aspect of the invention is that the print head operates in the coincident forces printing mode. 
     A further preferred aspect of the invention is that the clock generator is a non-overlapping two phase clock generator. 
     A further preferred aspect of the invention is that the shift register is a dynamic shift register. 
     A further preferred aspect of the invention is that the means of holding the binary data in the shift register valid is positive feedback from the drive circuit, this positive feedback being enabled during the active periods of the enable pulse. 
     An alternative form of the invention provides for an integrated thermal printing head, data transfer apparatus comprising: 
     1) a plurality of gated clock generators which stop the clocking of data transfer shift registers during the period that the data is to be held stable; 
     2) a plurality of data transfer shift registers which are clocked by the clock generator; 
     3) a plurality of drive circuits connected to the stages of the shift registers which energize the corresponding heaters during an enable pulse if the binary content of the corresponding shift register stage is in the active state; and 
     4) a means of holding the binary data in the shift register valid for the duration of the enable pulse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1(a) shows a simplified block schematic diagram of one exemplary printing apparatus according to the present invention. 
     FIG. 1(b) shows a cross section of one variety of nozzle tip in accordance with the invention. 
     FIGS. 2(a) to 2(f) show fluid dynamic simulations of drop selection. 
     FIG. 3(a) shows a finite element fluid dynamic simulation of a nozzle in operation according to an embodiment of the invention. 
     FIG. 3(b) shows successive meniscus positions during drop selection and separation. 
     FIG. 3(c) shows the temperatures at various points during a drop selection cycle. 
     FIG. 3(d) shows measured surface tension versus temperature curves for various ink additives. 
     FIG. 3(e) shows the power pulses which are applied to the nozzle heater to generate the temperature curves of FIG. 3(c) 
     FIG. 4 shows a block schematic diagram of print head drive circuitry for practice of the invention. 
     FIG. 5 shows projected manufacturing yields for an A4 page width color print head embodying features of the invention, with and without fault tolerance. 
     FIG. 6 shows a generalized block diagram of a printing system using a print head. 
     FIG. 7 shows a block diagram of a large print head with integrated drive circuitry 
     FIG. 8(a) shows a logic diagram of a single heater drive module. 
     FIG. 8(b) shows a circuit diagram of a single drive module. 
     FIG. 9 shows a circuit diagram of a gated two phase clock generator 230. 
     FIG. 10 shows a timing diagram for a single clock generator 230. 
     FIG. 11 shows the timing relationship of the sixteen clock generators used in a sixteen phase print head with 19,440 active nozzles. 
    
    
     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 &#34;DOD printing technology targets&#34; 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                                           
Target      Method of achieving improvement over prior art                
______________________________________                                    
High speed operation                                                      
            Practical, low cost, pagewidth printing heads with            
            more than 10,000 nozzles. Monolithic A4                       
            pagewidth print heads can be manufactured using               
            standard 300 mm (12&#34;) silicon wafers                          
High image quality                                                        
            High resolution (800 dpi is sufficient for most               
            applications), six color process to reduce image              
            noise                                                         
Full color operation                                                      
            Halftoned process color at 800 dpi using                      
            stochastic screening                                          
Ink flexibillty                                                           
            Low operating ink temperature and no                          
            requirement for bubble formation                              
Low power   Low power operation results from drop selection               
requirements                                                              
            means not being required to fully eject drop                  
Low cost    Monolithic print head without aperture plate, high            
            manufacturing yield, small number of electrical               
            connections, use of modified existing CMOS                    
            manufacturing facilities                                      
High manufacturing                                                        
            Integrated fault tolerance in printing head                   
yield                                                                     
High reliability                                                          
            Integrated fault tolerance in printing head.                  
            Elimination of cavitation and kogation. Reduction             
            of thermal shock.                                             
Small number of                                                           
            Shift registers, control logic, and drive circuitry           
electrical  can be electrical connections integrated on a                 
connections monolithic print head using standard CMOS                     
            processes                                                     
Use of existing VLSI                                                      
            CMOS compatibility. This can be achieved                      
manufacturing                                                             
            because the manufacturing heater drive power is               
facilities  less is than 1% of Thermal Ink Jet facilities heater          
            drive power                                                   
Electronic collation                                                      
            A new page compression system which can                       
            achieve 1000:1 compression with insignificant                 
            image degradation, resulting in a compressed data             
            rate low enough to allow real-time printing of any            
            combination of thousands of pages stored on a                 
            low cost magnetic disk drive.                                 
______________________________________                                    
 
    
     In thermal ink jet (TIJ) 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 TIJ systems is approximately 0.02%). This means that the drive circuits for TIJ 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 TIJ printheads is also very high. An 800 dpi A4 full color pagewidth TIJ 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 TIJ 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 &#34;Drop selection means&#34; 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                                                      
Method     Advantage      Limitation                                      
______________________________________                                    
1. Electrothermal                                                         
           Low temperature                                                
                          Requires ink pressure                           
reduction of surface                                                      
           increase and low drop                                          
                          regulating mechanism.                           
tension of selection energy. Can be                                       
                          Ink surface tension must                        
pressurized ink                                                           
           used with many ink                                             
                          reduce substantially as                         
           types. Simple fabrication.                                     
                          temperature increases                           
           CMOS drive circuits can                                        
           be fabricated on same                                          
           substrate                                                      
2. Electrothermal                                                         
           Medium drop selection                                          
                          Requires ink pressure                           
reduction of ink                                                          
           energy, suitable for hot                                       
                          oscillation mechanism.                          
viscosity, combined                                                       
           melt and oil based inks.                                       
                          Ink must have a large                           
with oscillating ink                                                      
           Simple fabrication.                                            
                          decrease in viscosity as                        
pressure   CMOS drive circuits can                                        
                          temperature increases                           
           be fabricated on same                                          
           substrate                                                      
3. Electrothermal                                                         
           Well known technology,                                         
                          High drop selection                             
bubble generation,                                                        
           simple fabrication,                                            
                          energy, requires water                          
with insufficient                                                         
           bipolar drive circuits can                                     
                          based ink, problems with                        
bubble volume to                                                          
           be fabricated on same                                          
                          kogation, cavitation,                           
cause drop ejection                                                       
           substrate      thermal stress                                  
4. Piezoelectric,                                                         
           Many types of ink base                                         
                          High manufacturing cost,                        
with insufficient                                                         
           can be used    incompatible with                               
volume change to          integrated circuit                              
cause drop ejection       processes, high drive                           
                          voltage, mechanical                             
                          complexity, bulky                               
5. Electrostatic                                                          
           Simple electrode                                               
                          Nozzle pitch must be                            
attraction with one                                                       
           fabrication    relatively large. Crosstalk                     
electrode per nozzle      between adjacent electric                       
                          fields. Requires high                           
                          voltage drive circuits                          
______________________________________                                    
 
    
     Other drop selection means may also be used. 
     The preferred drop selection means for water based inks is method 1: &#34;Electrothermal reduction of surface tension of pressurized ink&#34;. This drop selection means provides many advantages over other systems, including; low power operation (approximately 1% of TIJ), 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: &#34;Electrothermal reduction of ink viscosity, combined with oscillating ink pressure&#34;. 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 &#34;Drop separation means&#34; 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                                                     
Means      Advantage      Limitation                                      
______________________________________                                    
1. Electrostatic                                                          
           Can print on rough                                             
                          Requires high voltage                           
attraction surfaces, simple                                               
                          power supply                                    
           implementation                                                 
2. AC electric field                                                      
           Higher field strength is                                       
                          Requires high voltage                           
           possible than electro-                                         
                          AC power supply                                 
           static, operating margins                                      
                          synchronized to drop                            
           can be increased, ink                                          
                          ejection phase. Multiple                        
           pressure reduced, and                                          
                          drop phase operation is                         
           dust accumulation is                                           
                          difficult                                       
           reduced                                                        
3. Proximity                                                              
           Very small spot sizes can                                      
                          Requires print medium to                        
(print head in close                                                      
           be achieved. Very low                                          
                          be very close to print                          
proximity to, but                                                         
           power dissipation. High                                        
                          head surface, not suitable                      
not touching,                                                             
           drop position accuracy                                         
                          for rough print media,                          
recording medium)         usually requires transfer                       
                          roller or belt                                  
4. Transfer                                                               
           Very small spot sizes can                                      
                          Not compact due to size                         
Proximity (print                                                          
           be achieved, very low                                          
                          of transfer roller or                           
head is in close                                                          
           power dissipation, high                                        
                          transfer belt.                                  
proximity to a                                                            
           accuracy, can print on                                         
transfer roller or                                                        
           rough paper                                                    
belt                                                                      
5. Proxiniity with                                                        
           Useful for hot melt inks                                       
                          Requires print medium to                        
oscillating ink                                                           
           using viscosity reduction                                      
                          be very close to print                          
pressure   drop selection method,                                         
                          head surface, not suitable                      
           reduces possibility of                                         
                          for rough print media.                          
           nozzle clogging, can use                                       
                          Requires ink pressure                           
           pigments instead of dyes                                       
                          oscillation apparatus                           
6. Magnetic                                                               
           Can print on rough                                             
                          Requires uniform high                           
attraction surfaces. Low power if                                         
                          magnetic field strength,                        
           permanent magnets are                                          
                          requires magnetic ink                           
           used                                                           
______________________________________                                    
 
    
     Other drop separation means may also be used. 
     The preferred drop separation means depends upon the intended use. For most applications, method 1: &#34;Electrostatic attraction&#34;, or method 2: &#34;AC electric field&#34; are most appropriate. For applications where smooth coated paper or film is used, and very high speed is not essential, method 3: &#34;Proximity&#34; may be appropriate. For high speed, high quality systems, method 4: &#34;Transfer proximity&#34; can be used. Method 6: &#34;Magnetic attraction&#34; 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 following Australian patent specifications filed on 12 Apr. 1995, the disclosure of which are hereby incorporated by reference: 
     `A Liquid ink Fault Tolerant (LIFT) printing mechanism` (Filing no.: PN2308); 
     `Electrothermal drop selection in LIFT printing` (Filing no.: PN2309); 
     `Drop separation in LIFT printing by print media proximity` (Filing no.: PN2310); 
     `Drop size adjustment in Proximity LIFT printing by varying head to media distance` (Filing no.: PN2311); 
     `Augmenting Proximity LIFT printing with acoustic ink waves`(Filing no.: PN2312); 
     `Electrostatic drop separation in LIFT printing` (Filing no.: PN2313); 
     `Multiple simultaneous drop sizes in Proximity LIFT printing` (Filing no.: PN2321); 
     `Self cooling operation in thermally activated print heads` (Filing no.: PN2322); and 
     `Thermal Viscosity Reduction LIFT printing` (Filing no.: PN2323). 
     A simplified schematic diagram of one preferred printing system according to the invention appears in FIG. 1 (a). 
     An image source 52 may be raster image data from a scanner or computer, or outline image data in the form of a page description language (PDL), or other forms of digital image representation. This image data is converted to a pixel-mapped page image by the image processing system 53. This may be a raster image processor (RIP) in the case of PDL image data, or may be pixel image manipulation in the case of raster image data. Continuous tone data produced by the image processing unit 53 is halftoned. Halftoning is performed by the Digital Halftoning unit 54. Halftoned bitmap image data is stored in the image memory 72. 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 FIG. 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 FIG. 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 microcontroller 315 may also control the ink pressure regulator 63 and the heater control circuits 71. 
     For printing using surface tension reduction, ink is contained in an ink reservoir 64 under pressure. In the quiescent state (with no ink drop ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a drop. A constant ink pressure can be achieved by applying pressure to the ink reservoir 64 under the control of an ink pressure regulator 63. Alternatively, for larger printing systems, the ink pressure can be very accurately generated and controlled by situating the top surface of the ink in the reservoir 64 an appropriate distance above the head 50. This ink level can be regulated by a simple float valve (not shown). 
     For printing using viscosity reduction, ink is contained in an ink reservoir 64 under pressure, and 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, while 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 nozzles 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 electrically conductive material and used as one electrode generating the electric field. The other electrode can be the head 50 itself. Another embodiment uses proximity of the print medium as a means of discriminating between selected drops and unselected drops. 
     For small drop sizes gravitational force on the ink drop is very small; approximately 10 -4  of the surface tension forces, so gravity can be ignored in most cases. This allows the print head 50 and recording medium 51 to be oriented in any direction in relation to the local gravitational field. This is an important requirement for portable printers. 
     FIG. 1(b) is a detail enlargement of a cross section of a single microscopic nozzle tip embodiment of the invention, fabricated using a modified CMOS process. The nozzle is etched in a substrate 101, which may be silicon, glass, metal, or any other suitable material. If substrates which are not semiconductor materials are used, a semiconducting material (such as amorphous silicon) may be deposited on the substrate, and integrated drive transistors and data distribution circuitry may be formed in the surface semiconducting layer. Single crystal silicon (SCS) substrates have several advantages, including: 
     1) High performance drive transistors and other circuitry can be fabricated in SCS; 
     2) Print heads can be fabricated in existing facilities (fabs) using standard VLSI processing equipment; 
     3) SCS has high mechanical strength and rigidity; and 
     4) SCS has a high thermal conductivity. 
     In this example, the nozzle is of cylindrical form, with the heater 103 forming an annulus. The nozzle tip 104 is formed from silicon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry. The nozzle tip is passivated with silicon nitride. The protruding nozzle tip controls the contact point of the pressurized ink 100 on the print head surface. The print head surface is also hydrophobized to prevent accidental spread of ink across the front of the print head. 
     Many other configurations of nozzles are possible, and nozzle embodiments of the invention may vary in shape, dimensions, and materials used. Monolithic nozzles etched from the substrate upon which the heater and drive electronics are formed have the advantage of not requiring an orifice plate. The elimination of the orifice plate has significant cost savings in manufacture and assembly. Recent methods for eliminating orifice plates include the use of `vortex` actuators such as those described in Domoto et al U.S. Pat. No. 4,580,158, 1986, assigned to Xerox, and Miller et al U.S. 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 incorporation 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 FIG. 2. 
     FIG. 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 applied to the heater is 276 nJ, applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and the ink viscosity at 30° C. is 1.84 cPs. The ink is water based, and includes a sol of 0.1% palmitic acid to achieve an enhanced decrease in surface tension with increasing temperature. A cross section of the nozzle tip from the central axis of the nozzle to a radial distance of 40 μm is shown. Heat flow in the various materials of the nozzle, including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials. The time step of the simulation is 0.1 μs. 
     FIG. 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus. 
     FIG. 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 surface 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. 
     FIG. 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 equilibrium of forces. As the entire meniscus has been heated, the ink begins to flow. 
     FIG. 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. 
     FIG. 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 (less than 16 μs in this case) the ink would not accelerate towards the print medium, but would instead return to the nozzle. 
     FIG. 2(f) shows thermal contours at 5° C. intervals 26 μs is 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 limited rate of ink flow through the nozzle, the ink in the region of the nozzle tip `necks`, and the selected drop separates from the body of ink. The selected drop then travels to the recording medium under the influence of the external electrostatic field. The meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the next ink drop. One ink drop is selected, separated and forms a spot on the recording medium for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved. 
     FIG. 3(a) shows successive meniscus positions during the drop selection cycle at 5 μs intervals, starting at the beginning of the heater energizing pulse. 
     FIG. 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. 
     FIG. 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 FIG. 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. 
     FIG. 3(e) shows the power applied to the heater. Optimum operation requires a sharp rise in temperature at the start of the heater pulse, a maintenance of the temperature a little below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of the pulse. To achieve this, the average energy applied 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 applied to the heater is 40 mW, and the average power over the duration of the heater pulse is 11.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significantly affecting the operation of the print head. A higher sub-pulse frequency allows finer control over the power applied to the heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable for minimizing the effect of radio frequency interference (RFI). 
     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 liquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary liquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many liquids: ##EQU1## 
     Where γ T  is the surface tension at temperature T,k is a constant,T c  is the critical temperature of the liquid, M is the molar mass of the liquid, x is the degree of association of the liquid, and ρ is the density of the liquid. This equation indicates that the surface tension of most liquids falls to zero as the temperature reaches the critical temperature of the liquid. For most liquids, 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 slightly with temperature. However, a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude. A surface tension decrease of 20 mN/m over a 30° C. temperature range is preferred to achieve large operating margins, while as little as 10 mN/m can be used to achieve operation of the print head according to the present invention. 
     Inks With Large -Δγ T   
     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 Å 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 stability, 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 small particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as: 
     
         ______________________________________                                    
Name       Formula       m.p.    Synonym                                  
______________________________________                                    
Tetradecanoic acid                                                        
           CH.sub.3 (CH.sub.2).sub.12 COOH                                
                         58° C.                                    
                                 Myristic acid                            
Hexadecanoic acid                                                         
           CH.sub.3 (CH.sub.2).sub.14 COOH                                
                         63° C.                                    
                                 Palmitic acid                            
Octadecanoic acid                                                         
           CH.sub.3 (CH.sub.2).sub.15 COOH                                
                         71° C.                                    
                                 Stearic acid                             
Eicosanoic acid                                                           
           CH.sub.3 (CH.sub.2).sub.16 COOH                                
                         77° C.                                    
                                 Arachidic acid                           
Docosanoic acid                                                           
           CH.sub.3 (CH.sub.2).sub.20 COOH                                
                         80° C.                                    
                                 Behenic acid                             
______________________________________                                    
 
    
     As the melting point of sols with a small particle size is usually slightly less than of the bulk material, it is preferable to choose a carboxylic acid with a melting point slightly above the desired drop selection temperature. A good example is Arachidic acid. 
     These carboxylic 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 carboxylic acids with slightly varying chain lengths can be used to spread the melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid. 
     It is not necessary to restrict the choice of surfactant to simple unbranched carboxylic 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 carboxylic 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 follows: 
     1) Add the carboxylic acid to purified water in an oxygen free atmosphere. 
     2) Heat the mixture to above the melting point of the carboxylic acid. The water can be brought to a boil. 
     3) Ultrasonicate the mixture, until the typical size of the carboxylic acid droplets is between 100 Å and 1,000 Å. 
     4) Allow the mixture to cool. 
     5) Decant the larger particles from the top of the mixture. 
     6) Add an alkali such as NaOH to ionize the carboxylic 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 carboxylic acid is lower than water, smaller particles will 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 Å. 
     9) Add the surfactant sol to the ink preparation. The sol is required only in very dilute concentration. 
     The ink preparation will also contain either dye(s) or pigment(s), 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 will generally not be required, as there is no bubble formation during the drop ejection process. 
     Cationic surfactant sols 
     Inks made with anionic surfactant sols are generally unsuitable for use with cationic 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: 
     
         ______________________________________                                    
Name         Formula        Synonym                                       
______________________________________                                    
Hexadecylamine                                                            
             CH.sub.3 (CH.sub.2).sub.14 CH.sub.2 NH.sub.2                 
                            Palmityl amine                                
Octadecylamine                                                            
             CH.sub.3 (CH.sub.2).sub.16 CH.sub.2 NH.sub.2                 
                            Stearyl amine                                 
Eicosylamine CH.sub.3 (CH.sub.2).sub.18 CH.sub.2 NH.sub.2                 
                            Arachidyl amine                               
Docosylamine CH.sub.3 (CH.sub.2).sub.20 CH.sub.2 NH.sub.2                 
                            Behenyl amine                                 
______________________________________                                    
 
    
     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 HCl 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 around water. At temperatures close to the PIT, the microemulsion forms a continuous `sponge` of topologically connected water and oil. 
     There are two mechanisms whereby this reduces the surface tension. Around the 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 possibilities 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  H 2n+1  C 4  H 6  (CH 2  CH 2  O) m  OH) can be used. The hydrophilicity 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  H 17  C 4  H 6  (CH 2  CH 2  O) n  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. Suppliers and brand names are listed in the following table: 
     
         ______________________________________                                    
Trade name    Supplier                                                    
______________________________________                                    
Akyporox OP100                                                            
              Chem-Y GmbH                                                 
Alkasurf OP-10                                                            
              Rhone-Poulenc Surfactants and Specialties                   
Dehydrophen POP 10                                                        
              Pulcra SA                                                   
Hyonic OP-10  Henkel Corp.                                                
Iconol OP-10  BASF Corp.                                                  
Igepal O      Rhone-Poulenc France                                        
Macol OP-10   PPG Industries                                              
Malorphen 810 Huls AG                                                     
Nikkol OP-10  Nikko Chem. Co. Ltd.                                        
Renex 750     ICI Americas Inc.                                           
Rexol 45/10   Hart Chemical Ltd.                                          
Synperonic OP10                                                           
              ICI PLC                                                     
Teric X10     ICI Australia                                               
______________________________________                                    
 
    
     These are available in large volumes at low cost (less than one dollar per pound in quantity), and so contribute less than 10 cents per liter to prepared microemulsion ink with a 5% surfactant concentration. 
     Other suitable ethoxylated alkyl phenols include those listed in the following table: 
     
         ______________________________________                                    
Trivial name                                                              
         Formula           HLB    Cloud point                             
______________________________________                                    
Nonoxynol-9                                                               
         C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about
         .9 OH             13     54° C.                           
Nonoxynol-10                                                              
         C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about
         .10 OH            13.2   62° C.                           
Nonoxynol-11                                                              
         C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about
         .11 OH            13.8   72° C.                           
Nonoxynol-12                                                              
         C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about
         .12 OH            14.5   81° C.                           
Octoxynol-9                                                               
         C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about
         .9 OH             12.1   61° C.                           
Octoxynol-10                                                              
         C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about
         .10 OH            13.6   65° C.                           
Octoxynol-12                                                              
         C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about
         .12 OH            14.6   88° C.                           
Dodoxynol-10                                                              
         C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..abou
         t.10 OH           12.6   42° C.                           
Dodoxynol-11                                                              
         C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..abou
         t.11 OH           13.5   56° C.                           
Dodoxynol-14                                                              
         C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..abou
         t.14 OH           14.5   87° C.                           
______________________________________                                    
 
    
     Microemulsion based inks have advantages other than surface tension control: 
     1) Microemulsions are thermodynamically stable, and will not separate. Therefore, the storage time can be very long. This is especially significant for office and portable printers, which may be used sporadically. 
     2) The microemulsion will form spontaneously with a particular drop size, and does not require extensive stirring, centrifuging, or filtering to ensure a particular range of emulsified oil drop sizes. 
     3) The amount of oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or both, can be used. It is also possible to use a mixture of dyes, one soluble in water, and 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. 
     Dyes and pigments in microemulsion based inks 
     Oil in water mixtures can have high oil contents--as high as 40%--and still form O/W microemulsions. This allows a high dye or pigment loading. 
     Mixtures of dyes and pigments can be used. An example of a microemulsion based ink mixture with both dye and pigment is as follows: 
     1) 70% water 
     2) 5% water soluble dye 
     3) 5% surfactant 
     4) 10% oil 
     5) 10% oil miscible pigment 
     The following table shows the nine basic combinations of colorants in the oil and water phases of the microemulsion that may be used. 
     
         ______________________________________                                    
Combination                                                               
         Colorant in water phase                                          
                          Colorant in oil phase                           
______________________________________                                    
1        none             oil miscible pigment                            
2        none             oil soluble dye                                 
3        water soluble dye                                                
                          none                                            
4        water soluble dye                                                
                          oil miscible pigment                            
5        water soluble dye                                                
                          oil soluble dye                                 
6        pigment dispersed in water                                       
                          none                                            
7        pigment dispersed in water                                       
                          oil miscible pigment                            
8        pigment dispersed in water                                       
                          oil soluble dye                                 
9        none             none                                            
______________________________________                                    
 
    
     The ninth combination, with no colorants, is useful for printing transparent coatings, UV ink, and selective gloss highlights. 
     As many dyes are amphiphilic, large quantities of dyes can also be solubilized in the oil-water boundary layer as 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 absorption spectrum of the resultant ink will be the weighted average of the absorption spectra of the different colorants used. This presents two problems: 
     1) The absorption spectrum will tend to become broader, as the absorption peaks of both colorants are averaged. This has a tendency to `muddy` the colors. To obtain brilliant color, careful choice of dyes and pigments based on their absorption 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 will tend to have a smaller contribution to the printed ink color on more absorptive papers, as the dye will be absorbed into the paper, while the pigment 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 solubility is quite low, and the solution contains essentially no micelles. Above the Krafft temperature micelle formation becomes possible and there is a rapid increase in solubility of the surfactant. If the critical micelle concentration (CMC) exceeds the solubility of a surfactant at a particular temperature, then the minimum surface tension will be achieved at the point of maximum solubility, rather than at the CMC. Surfactants are usually 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 following table shows some commercially available surfactants with Krafft points in the desired range. 
     
         ______________________________________                                    
Formula                Krafft point                                       
______________________________________                                    
C.sub.16 H.sub.33 SO.sub.3.sup.- Na.sup.+                                 
                       57° C.                                      
C.sub.18 H.sub.37 SO.sub.3.sup.- Na.sup.+                                 
                       70° C.                                      
C.sub.16 H.sub.33 SO.sub.4.sup.- Na.sup.+                                 
                       45° C.                                      
Na.sup.+- O.sub.4 S(CH.sub.2).sub.16 SO.sub.4.sup.- Na.sup.+              
                       44.9° C.                                    
K.sup.+- O.sub.4 S(CH.sub.2).sub.16 SO.sub.4.sup.- K.sup.+                
                       55° C.                                      
C.sub.16 H.sub.33 CH(CH.sub.3)C.sub.4 H.sub.6 SO.sub.3.sup.- Na.sup.+     
                       60.8° C.                                    
______________________________________                                    
 
    
     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 falls 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 hydrophilic is related to the cloud point of that surfactant. POE chains by themselves are not particularly suitable, as the cloud point is generally above 100° C. 
     Polyoxypropylene (POP) can be combined with POE in POE/POP block copolymers to lower the cloud point of POE chains without introducing a strong hydrophobicity at low temperatures. 
     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 (generically also CAS 9003-11-6) 
     Some commercially available varieties of poloxamer and meroxapol with a high surface tension at room temperature, combined with a cloud point above 40° C. and below 100° C. are shown in the following table: 
     
         __________________________________________________________________________
                                      Surface                             
       BASF Trade                     Tension                             
                                          Cloud                           
Trivial name                                                              
       name  Formula                  (mN/m)                              
                                          point                           
__________________________________________________________________________
Meroxapol                                                                 
       Pluronic                                                           
             HO(CHCH.sub.3 CH.sub.2 O).sub.˜7 (CH.sub.2 CH.sub.2    
             O).sub.˜22 (CHCH.sub.3 CH.sub.2 O).sub.˜7        
                                      50.9                                
                                          69° C.                   
105    10R5                                                               
Meroxapol                                                                 
       Pluronic                                                           
             HO(CHCH.sub.3 CH.sub.2 O).sub.˜7 (CH.sub.2 CH.sub.2    
             O).sub.˜91 (CHCH.sub.3 CH.sub.2 O).sub.˜7        
                                      54.1                                
                                          99° C.                   
108    10R8                                                               
Meroxapol                                                                 
       Pluronic                                                           
             HO(CHCH.sub.3 CH.sub.2 O).sub.˜12 (CH.sub.2 CH.sub.2   
             O).sub.˜136 (CHCH.sub.3 CH.sub.2 O).sub.˜12      
                                      47.3                                
                                          81° C.                   
178    17R8                                                               
Meroxapol                                                                 
       Pluronic                                                           
             HO(CHCH.sub.3 CH.sub.2 O).sub.˜18 (CH.sub.2 CH.sub.2   
             O).sub.˜163 (CHCH.sub.3 CH.sub.2 O).sub.˜18      
                                      46.1                                
                                          80° C.                   
258    25R8                                                               
Poloxamer 105                                                             
       Pluronic L35                                                       
             HO(CH.sub.2 CH.sub.2 O).sub.˜11 (CHCH.sub.3 CH.sub.2   
             O).sub.˜16 (CH.sub.2 CH.sub.2 O).sub.˜11         
                                      48.8                                
                                          77° C.                   
Poloxamer 124                                                             
       Pluronic L44                                                       
             HO(CH.sub.2 CH.sub.2 O).sub.˜11 (CHCH.sub.3 CH.sub.2   
             O).sub.˜21 (CH.sub.2 CH.sub.2 O).sub.˜11         
                                      45.3                                
                                          65° C.                   
__________________________________________________________________________
 
    
     Other varieties of poloxamer and meroxapol can readily be synthesized using well known techniques. Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between 40° C. and 100° C., and preferably between 60° C. and 80° C. 
     Meroxapol  HO(CHCH 3  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 31  , OH - ), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively little 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 slightly lowers the cloud point of nonionic surfactants. 
     Hot Melt Inks 
     The ink need not be in a liquid state at room temperature. Solid `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 will 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 liquid at the quiescent temperature. The quiescent temperature should be higher than the highest ambient temperature likely to be encountered by the printed page. The quiescent temperature should also be as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop ejection temperatures. A quiescent temperature between 60° C. and 90° C. is generally suitable, though other temperatures may be used. A drop ejection temperature of between 160° C. and 200° C. is generally suitable. 
     There are several methods of achieving an enhanced reduction in surface tension with increasing temperature. 
     1) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially 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 generally excludes alkanes such as waxes. Suitable materials will generally 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 
     FIG. 3(d) shows the measured effect of temperature on the surface tension of various aqueous preparations containing the following additives: 
     1) 0.1% sol of Stearic Acid 
     2) 0.1% sol of Palmitic acid 
     3) 0.1% solution of Pluronic 10R5 (trade mark of BASF) 
     4) 0.1% solution of Pluronic L35 (trade mark of BASF) 
     5) 0.1% solution of Pluronic L44 (trade mark of BASF) 
     Inks suitable for printing systems of the present invention are described in the following Australian patent specifications, the disclosure of which are hereby incorporated by reference: 
     `Ink composition based on a microemulsion` (Filing no.: PN5223, filed on 6 Sep. 1995); 
     `Ink composition containing surfactant sol` (Filing no.: PN5224, filed on 6 Sep. 1995); 
     `Ink composition for DOD printers with Krafft point near the drop selection temperature sol` (Filing no.: PN6240, filed on 30 Oct. 1995); and 
     `Dye and pigment in a microemulsion based ink` (Filing no.: PN6241, filed on 30 Oct. 1995). 
     Operation Using Reduction of Viscosity 
     As a second example, operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink, is as follows. Prior to operation of the printer, solid ink is melted in the reservoir 64. The reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperature at which the ink 100 is liquid, but exhibits a relatively high viscosity (for example, approximately 100 cP). The Ink 100 is retained in the nozzle by the surface tension of the ink. The ink 100 is formulated so that the viscosity of the ink reduces with increasing temperature. The ink pressure oscillates at a frequency which is an integral multiple of the drop ejection frequency from the nozzle. The ink pressure oscillation causes oscillations of the ink meniscus at the nozzle tips, but this oscillation is small due to the high ink viscosity. At the normal operating temperature, these oscillations are of insufficient amplitude 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 falls, ink begins to move back into the nozzle. The body of ink separates from the ink which is frozen onto the recording medium. The meniscus of the ink 100 at the nozzle tip then returns to low amplitude oscillation. The viscosity of the ink increases to its quiescent level as remaining heat is dissipated to the bulk ink and print head. One ink drop is selected, separated and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved. 
     Manufacturing of Print Heads 
     Manufacturing processes for monolithic print heads in accordance with the present invention are described in the following Australian patent specifications filed on 12 Apr. 1995, the disclosure of which are hereby incorporated by reference: 
     `A monolithic LIFT printing head` (Filing no.: PN2301); 
     `A manufacturing process for monolithic LIFT printing heads` (Filing no.: PN2302); 
     `A self-aligned heater design for LIFT print heads` (Filing no.: PN2303); 
     `Integrated four color LIFT print heads` (Filing no.: PN2304); 
     `Power requirement reduction in monolithic LIFT printing heads` (Filing no.: PN2305); 
     `A manufacturing process for monolithic LIFT print heads using anisotropic wet etching` (Filing no.: PN2306); 
     `Nozzle placement in monolithic drop-on-demand print heads` (Filing no.: PN2307); 
     `Heater structure for monolithic LIFT print heads` (Filing no.: PN2346); 
     `Power supply connection for monolithic LIFT print heads` (Filing no.: PN2347); 
     `External connections for Proximity LIFT print heads` (Filing no.: PN2348); and 
     `A self-aligned manufacturing process for monolithic LIFT print heads` (Filing no.: PN2349); and 
     `CMOS process compatible fabrication of LIFT print heads` (Filing no.: PN5222, 6 Sep. 1995). 
     `A manufacturing process for LIFT print heads with nozzle rim heaters` (Filing no.: PN6238, 30 Oct. 1995); 
     `A modular LIFT print head` (Filing no.: PN6237, 30 Oct. 1995); 
     `Method of increasing packing density of printing nozzles` (Filing no.: PN6236, 30 Oct. 1995); and 
     `Nozzle dispersion for reduced electrostatic interaction between simultaneously printed droplets` (Filing no.: PN6239, 30 Oct. 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 Australian patent specifications filed on 12 Apr. 1995, the disclosure of which are hereby incorporated 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 monolithic LIFT print heads` (Filing no.: PN2318); 
     `Page image and fault tolerance routing device for LIFT printing systems` (Filing no.: PN2319); and 
     `A removable pressurized liquid ink cartridge for LIFT printers` (Filing 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 quality color publications printed using offset printing. This can be achieved using a print resolution of approximately 1,600 dpi. However, 1,600 dpi printing is difficult and expensive to achieve. Similar results can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and magenta, and one bit per pixel for yellow and black. This color model is herein called CC&#39;MM&#39;YK. Where high quality monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein called CC&#39;MM&#39;YKK&#39;. 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 following Australian patent specifications filed on 12 Apr. 1995, the disclosure of which are hereby incorporated 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 following: color and monochrome office printing, short run digital printing, high speed digital printing, process color printing, spot color printing, offset press supplemental printing, low cost printers using scanning print heads, high speed printers using pagewidth print heads, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimile and copying machines, label printing, large format plotters, photographic duplication, printers for digital photographic processing, portable printers incorporated into digital `instant` cameras, video printing, printing of PhotoCD images, portable printers for `Personal Digital Assistants`, wallpaper printing, indoor sign printing, billboard printing, and fabric printing. 
     Printing systems based on this invention are described in the following Australian patent specifications filed on 12 Apr. 1995, the disclosure of which are hereby incorporated by reference: 
     `A high speed color office printer with a high capacity digital page image store` (Filing no.: PN2329); 
     `A short run digital color printer with a high capacity digital page image store` (Filing no.: PN2330); 
     `A digital color printing press using LIFT printing technology` (Filing no.: PN2331); 
     `A modular digital printing press` (Filing no.: PN2332); 
     `A high speed digital fabric printer` (Filing no.: PN2333); 
     `A color photograph copying system` (Filing no.: PN2334); 
     `A high speed color photocopier using a LIFT printing system` (Filing no.: PN2335); 
     `A portable color photocopier using LIFT printing technology` (Filing no.: PN2336); 
     `A photograph processing system using LIFT printing technology` (Filing no.: PN2337); 
     `A plain paper facsimile machine using a LIFT printing system` (Filing no.: PN2338); 
     `A PhotoCD system with integrated printer` (Filing no.: PN2293); 
     `A color plotter using LIFT printing technology` (Filing no.: PN2291); 
     `A notebook computer with integrated LIFT color printing system` (Filing no.: PN2292); 
     `A portable printer using a LIFT printing system` (Filing no.: PN2300); 
     `Fax machine with on-line database interrogation and customized magazine printing` (Filing no.: PN2299); 
     `Miniature portable color printer` (Filing no.: PN2298); 
     `A color video printer using a LIFT printing system` (Filing no.: PN2296); and 
     `An integrated printer, copier, scanner, and facsimile using a LIFT printing system` (Filing no.: PN2297) 
     Compensation of Print Heads for Environmental Conditions 
     It is desirable 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 quality. These variations should be kept to a small proportion of the nominal ink drop volume and pixel spacing respectively. 
     There are many factors which can affect drop volume and position. In some cases, the variation can be minimized by appropriate head design. In other cases, the variation can compensated by active circuitry. 
     1) Ambient temperature: Changes in ambient temperature can affect the quiescent meniscus position, and the temperature achieved by the heater pulse. Changes in the quiescent meniscus position can be compensated by altering the ink pressure or the strength of the external electric or magnetic field. Changes in the temperature achieved by the heater pulse can be compensated by altering the power supplied to the heater. 
     2) Nozzle temperature: It is not practical to compensate for temperature independently for each nozzle. Reliable operation of heads requires that the difference between the nozzle temperature and the ambient temperature measured at the substrate is small. This can be achieved using a substrate with high thermal conductivity (such a silicon), and allowing adequate time between pulses for the waste heat to dissipate. 
     3) Nozzle radius: The variation in nozzle radius for nozzles supplied from a single ink reservoir should be minimized, as it its difficult to supply different electric field strengths or ink pressures on a nozzle by nozzle basis. Fortunately, the variation in nozzle radius can readily be kept below 0.5 μm using modern semiconductor manufacturing equipment. 
     4) Print density: Different numbers of ink drops may be ejected in each cycle. As a result, the load resistance of the head may vary widely and rapidly, causing voltage fluctuations due to the finite resistance of the power supply and wiring. This can be accurately compensated by digital circuitry which determines the number of drops to be ejected in each cycle, and alters the power supply voltage to compensate for load resistance changes. 
     5) Ink contaminants: The ink must be free of contaminants larger than approximately 5 μm, which may lodge against each other and clog the nozzle. This can be achieved by placing a 5 μm absolute filter between the ink reservoir and the head. 
     6) Ink surface tension characteristics: The most important requirement of the ink is the surface tension characteristics. The ink must be formulated so that the surface tension is high enough to retain the ink in the nozzle at ambient temperatures within the design limits, and falls below the ejection threshold at temperatures achievable by the heater. Many ink formulations can meet these criteria, but care must be taken to control contaminants which affect surface tension. 
     7) Ink drying: If the period between drop ejections from a nozzle becomes too long, then the ink at the exposed meniscus may dry out to the extent that drop ejection is affected or prevented. This can be compensated by ejecting one or more drops from each nozzle between each printed page, and capping the printhead during idle periods. 
     8) Pulse width: Heater pulse width can be accurately controlled, and may be set very close to the minimum pulse width. Higher reliability can be achieved by making the pulse width considerably longer than the minimum. For a 7 μm nozzle using water based ink as herein described, the minimum pulse width is approximately 10 μs. The nominal pulse width is set at 18 μs to give a wide operating margin. Pulse width has almost no effect on drop size. 
     9) Clogged or defective nozzles: In many cases, clogged nozzles may be cleared by providing a rapid sequence of pulses to the heater, raising the ink above the boiling point. The vapor bubbles thus formed can dislodge the `crust` of dried ink. Persistent clogged nozzles may be periodically cleared using a solvent. Nozzles which are defective or permanently clogged can be automatically replaced by redundant nozzles using inbuilt fault tolerance. 
     10) Print media roughness: This is particularly significant for proximity printing, where media roughness may be a significant fraction of the head to media distance. Protruding fibers in a paper medium may cause the ink drop to wick into the paper sooner than intended, resulting in less ink transferred to the paper, and a smaller drop size. This can be compensated by using coated paper, compressing the paper fibers with rollers before printing, and/or coating or wetting the paper immediately prior to printing. 
     Nozzle temperature control 
     The performance of nozzles is sensitive to the temperature and duration of thermal pulses applied to the nozzle tip. 
     If too little energy is supplied to the heater, the temperature at the nozzle tip will not rise fast enough for a drop to be ejected in the allotted time, or the ejected ink drop may be smaller than required. If too much energy is supplied to the heater, too much ink may be ejected, the ink may boil, and the energy used by the print head will be greater than required. This energy may then exceed the limit for self-cooling operation. The amount of energy required to activate a nozzle can be determined by dynamic finite element analysis of the nozzle. This method can determine the required ejection energy of the nozzle under various static and dynamic environmental circumstances. 
     An optimum temperature profile for a head 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 nozzles. 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. To obtain accurate results, a transient fluid dynamic simulation with free surface modeling is required, as convection in the ink, and ink flow, significantly affect on the temperature achieved with a specific power curve. 
     Compensation techniques 
     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 applied to the nozzle heaters. 
     An optimum temperature profile for a print head 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 nozzles. 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 limited to: 
     1) Varying the voltage applied 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, significantly affect on the temperature achieved with a specific power curve. 
     By the incorporation of appropriate digital circuitry on the print head substrate, it is practical to individually control the power applied to each nozzle. One way to achieve this is by `broadcasting` a variety of different digital pulse trains across the print head chip, and selecting the appropriate pulse train for each nozzle using multiplexing circuits. 
     An example of the environmental factors which may be compensated for is listed in the table &#34;Compensation for environmental factors&#34;. This table identifies which environmental factors are best compensated globally (for the entire print head), per chip (for each chip in a composite multi-chip print head), and per nozzle. 
     
         __________________________________________________________________________
Compensation for environmental factors                                    
Factor         Sensing or user                                            
                          Compensation                                    
compensated                                                               
          Scope                                                           
               control method                                             
                          mechanism                                       
__________________________________________________________________________
Ambient   Global                                                          
               Temperature sensor                                         
                          Power supply                                    
Temperature    mounted on print                                           
                          voltage or global                               
               head       PFM patterns                                    
Power supply                                                              
          Global                                                          
               Predictive active                                          
                          Power supply voltage                            
voltage fluctuation                                                       
               nozzle count based on                                      
                          or global PFM patterns                          
with number of print data                                                 
active nozzles                                                            
Local heat build-                                                         
          Per  Predictive active                                          
                          Selection of                                    
up with successive                                                        
          nozzle                                                          
               nozzle count based on                                      
                          appropriate PFM                                 
nozzle actuation                                                          
               print data pattern for each printed                        
                          drop                                            
Drop size control                                                         
          Per  Image data Selection of                                    
for multiple bits                                                         
          nozzle          appropriate PFM                                 
per pixel                 pattern for each printed                        
                          drop                                            
Nozzle geometry                                                           
          Per  Factory measurement,                                       
                          Global PFM patterns                             
variations between                                                        
          chip datafile supplied with                                     
                          per print head chip                             
wafers         print head.                                                
Heater resistivity                                                        
          Per  Factory measurement,                                       
                          Global PFM patterns                             
variations between                                                        
          chip datafile supplied with                                     
                          per print head chip                             
wafers         print head.                                                
User image                                                                
          Global                                                          
               User selection                                             
                          Power supply voltage,                           
intensity                 electrostatic                                   
adjustment                acceleration voltage, or                        
                          ink pressure                                    
Ink surface tension                                                       
          Global                                                          
               Ink cartridge sensor or                                    
                          Global PFM patterns                             
reduction method                                                          
               user selection                                             
and threshold                                                             
temperature                                                               
Ink viscosity                                                             
          Global                                                          
               Ink cartridge sensor or                                    
                          Global PFM patterns                             
               user selection                                             
                          and/or clock rate                               
Ink dye or pigment                                                        
          Global                                                          
               Ink cartridge sensor or                                    
                          Global PFM patterns                             
concentration  user selection                                             
Ink response time                                                         
          Global                                                          
               Ink cartridge sensor or                                    
                          Global PFM patterns                             
               user selection                                             
__________________________________________________________________________
 
    
     Most applications will not require compensation for all of these variables. Some variables have a minor effect, and compensation is only necessary where very high image quality is required. 
     Print head drive circuits 
     FIG. 4 is a block schematic diagram showing electronic operation of the print head driver circuits. FIG. 4 shows a block diagram for a system using an 800 dpi pagewidth print head which prints process color using the CC&#39;MM&#39;YK color model. The print head 50 has a total of 79,488 nozzles, with 39,744 main nozzles and 39,744 redundant nozzles. The main and redundant nozzles are divided into six colors, and each color is divided into 8 drive phases. Each drive phase has a shift register which converts the serial data from a head control ASIC 400 into parallel data for enabling heater drive circuits. There is a total of 96 shift registers, each providing data for 828 nozzles. Each shift register is composed of 828 shift register stages 217, the outputs of which are logically anded with phase enable signal by a nand gate 215. The output of the nand gate 215 drives an inverting buffer 216, which in turn controls the drive transistor 201. The drive transistor 201 actuates the electrothermal heater 200, which may be a heater 103 as shown in FIG. 1 (b). To maintain the shifted data valid 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 well known glitch free clock control circuits. Stopping the clock of the shift register removes the requirement for a parallel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400. Data is routed to 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 FIG. 4 is simplified, and does not show various means of improving manufacturing yield, such as block fault tolerance. Drive circuits for different configurations of print head can readily be derived from the apparatus disclosed herein. 
     Digital information representing patterns of dots to be printed on the recording medium is stored in the Page or Band memory 1513, which may be the same as the Image memory 72 in FIG. 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 Microcontroller 315. Temperature is compensated by changing the contents of the dual port RAM 317. These values are calculated by the microcontroller 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 incorporated in the Microcontroller 315. 
     The Head Control ASIC 400 contains control circuits for thermal lag compensation and print density. Thermal lag compensation requires that the power supply voltage to the head 50 is a rapidly time-varying voltage which is synchronized with the enable pulse for the heater. This is achieved by programming the programmable power supply 320 to produce this voltage. An analog time varying programming voltage is produced by the DAC 313 based upon data read from the dual port RAM 317. The data is read according to an address produced by the counter 403. The counter 403 produces one complete cycle of addresses during the period of one enable pulse. This synchronization is ensured, as the counter 403 is clocked by the system clock 408, and the top count of the counter 403 is used to clock the enable counter 404. The count from the enable counter 404 is then decoded by the decoder 405 and buffered by the buffer 432 to produce the enable pulses for the head 50. The counter 403 may include a prescaler if the number of states in the count is less than the number of clock periods in one enable pulse. Sixteen voltage states are adequate to accurately compensate for the heater thermal lag. These sixteen states can be specified by using a four bit connection between the counter 403 and the dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To allow non-linear timing of these states the counter 403 may also include a ROM or other device which causes the counter 403 to count in a non-linear fashion. Alternatively, fewer than sixteen states may be used. 
     For print density compensation, the printing density is detected by counting the number of pixels to which a drop is to be printed (`on` pixels) in each enable period. The `on` pixels are counted by the On pixel counters 402. There is one On pixel counter 402 for each of the eight enable phases. The number of phases in a head depend upon the specific design. Four, eight, and sixteen are convenient numbers, though there is no requirement that the number of 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 valid 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 microcontroller 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 following equation can be used to calculate the matrix of numbers to be stored in the dual port RAM 317: ##EQU2## Where: V PS  is the voltage specified to the programmable power supply 320; 
     R OUT  is the output resistance of the programmable power supply 320, including the connections to the head 50; 
     R H  is the resistance of a single heater; 
     p is a number representing the number of heaters that are turned on in the current enable period, as provided by the multiplexer 401; 
     n is a constant equal to the number of heaters represented by one least significant bit of p; 
     t is time, divided into number of steps over the period of a single enable pulse; 
     P(t) is a function defining the power input to a single heater required to achieve improved drop ejection. This function depends upon the specific geometry and materials of the nozzle, as well as various characteristics of the ink. It is best determined by comprehensive computer simulation, combined with experimentation; 
     T E  is the temperature required for drop ejection in °C.; and 
     T A  is the `ambient` temperature of the head as measured by the temperature sensor in °C. 
     To reduce execution time and simplify programming for the microcontroller, most or all of the factors can be pre-calculated, and simply looked up in a table stored in the microcontroller&#39;s ROM. 
     Comparison with thermal ink jet technology 
     The table &#34;Comparison between Thermal ink jet and Present Invention&#34; 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 ink jet technology because both are drop on demand systems which operate using thermal actuators and liquid ink. Although they may appear similar, the two technologies operate on different principles. 
     Thermal ink jet printers use the following fundamental operating principle. A thermal impulse caused by electrical resistance heating results in the explosive formation of a bubble in liquid 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 collapses, drawing ink from the ink reservoir to re-fill the nozzle. Thermal ink jet printing has been highly successful commercially due to the high nozzle packing density and the use of well 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, satellite 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                  
          Thermal Ink-Jet                                                 
                       Present Invention                                  
__________________________________________________________________________
Drop selection                                                            
          Drop ejected by pressure                                        
                       Choice of surface tension                          
mechanism wave caused by thermally                                        
                       or viscosity reduction                             
          induced bubble                                                  
                       mechanisms                                         
Drop separation                                                           
          Same as drop selection                                          
                       Choice of proximity,                               
mechanism mechanism    electrostatic, magnetic, and                       
                       other methods                                      
Basic ink carrier                                                         
          Water        Water, microemulsion,                              
                       alcohol, glycol, or hot melt                       
Head construction                                                         
          Precision assembly of                                           
                       Monolithic                                         
          nozzle plate, ink channel,                                      
          and substrate                                                   
Per copy printing                                                         
          Very high due to limited                                        
                       Can be low due to                                  
cost      print head life and                                             
                       permanent print heads and                          
          expensive inks                                                  
                       wide range of possible inks                        
Satellite drop                                                            
          Significant problem which                                       
                       No satellite drop formation                        
formation degrades image quality                                          
Operating ink                                                             
          280° C. to 400° C. (high                          
                       Approx. 70° C. (depends                     
temperature                                                               
          temperature limits dye use                                      
                       upon ink formulation)                              
          and ink formulation)                                            
Peak heater                                                               
          400° C. to 1,000° C. (high                        
                       Approx. 130° C.                             
temperature                                                               
          temperature reduces device                                      
          life)                                                           
Cavitation (heater                                                        
          Serious problem limiting                                        
                       None (no bubbles are                               
erosion by bubble                                                         
          head life    formed)                                            
collapse)                                                                 
Kogation (coating                                                         
          Serious problem limiting                                        
                       None (water based ink                              
of heater by ink                                                          
          head life and ink                                               
                       temperature does not exceed                        
ash)      formulation  100° C.)                                    
Rectified diffusion                                                       
          Serious problem limiting                                        
                       Does not occur as the ink                          
(formation of ink                                                         
          ink formulation                                                 
                       pressure does not go                               
bubbles due to         negative                                           
pressure cycles)                                                          
Resonance Serious problem limiting                                        
                       Very small effect as                               
          nozzle design and                                               
                       pressure waves are small                           
          repetition rate                                                 
Practical resolution                                                      
          Approx. 800 dpi max.                                            
                       Approx. 1,600 dpi max.                             
Self-cooling                                                              
          No (high energy required)                                       
                       Yes: printed ink carries                           
operation              away drop selection energy                         
Drop ejection                                                             
          High (approx. 10 m/sec)                                         
                       Low (approx. 1 m/sec)                              
velocity                                                                  
Crosstalk Serious problem requiring                                       
                       Low velocities and                                 
          careful acoustic design,                                        
                       pressures associated with                          
          which limits nozzle refill                                      
                       drop ejection make crosstalk                       
          rate.        very small.                                        
Operating thermal                                                         
          Serious problem limiting                                        
                       Low: maximum temperature                           
stress    print-head life.                                                
                       increase approx. 90° C. at                  
                       centre of heater.                                  
Manufacturing                                                             
          Serious problem limiting                                        
                       Same as standard CMOS                              
thermal stress                                                            
          print-head size.                                                
                       manufacturing process.                             
Drop selection                                                            
          Approx. 20 μJ                                                
                       Approx. 270 nJ                                     
energy                                                                    
Heater pulse period                                                       
          Approx. 2-3 μs                                               
                       Approx. 15-30 μs                                
Average heater                                                            
          Approx. 8 Watts per                                             
                       Approx. 12 mW per heater.                          
pulse power                                                               
          heater.      This is more than 500 times                        
                       less than Thermal Ink-Jet.                         
Heater pulse                                                              
          Typically approx. 40 V.                                         
                       Approx. 5 to 10 V.                                 
voltage                                                                   
Heater peak pulse                                                         
          Typically approx. 200 mA                                        
                       Approx. 4 mA per heater.                           
current   per heater. This requires                                       
                       This allows the use of small                       
          bipolar or very large MOS                                       
                       MOS drive transistors.                             
          drive transistors.                                              
Fault tolerance                                                           
          Not implemented. Not                                            
                       Simple implementation                              
          practical for edge shooter                                      
                       results in better yield and                        
          type.        reliability                                        
Constraints on ink                                                        
          Many constraints including                                      
                       Temperature coefficient of                         
composition                                                               
          kogation, nucleation, etc.                                      
                       surface tension or viscosity                       
                       must be negative.                                  
Ink pressure                                                              
          Atmospheric pressure or                                         
                       Approx. 1.1 atm                                    
          less                                                            
Integrated drive                                                          
          Bipolar circuitry usually                                       
                       CMOS, nMOS, or bipolar                             
circuitry required due to high drive                                      
          current                                                         
Differential                                                              
          Significant problem for                                         
                       Monolithic construction                            
thermal expansion                                                         
          large print heads                                               
                       reduces problem                                    
Pagewidth print                                                           
          Major problems with yield,                                      
                       High yield, low cost and                           
heads     cost, precision                                                 
                       long life due to fault                             
          construction, head life, and                                    
                       tolerance. Self cooling due                        
          power dissipation                                               
                       to low power dissipation.                          
__________________________________________________________________________
 
    
     Yield and Fault Tolerance 
     In most cases, monolithic 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 typically the wafer sort yield which is the most serious limitation on total yield. Full 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. 
     FIG. 5 is a graph of wafer sort yield versus defect density for a monolithic full width color A4 head embodiment of the invention. The head is 215 mm long by 5 mm wide. The non fault tolerant yield 198 is calculated according to Murphy&#39;s method, which is a widely used yield prediction method. With a defect density of one defect per square cm, Murphy&#39;s method predicts a yield less than 1%. This means that 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. 
     Murphy&#39;s method approximates the effect of an uneven distribution of defects. FIG. 5 also includes a graph of non fault tolerant yield 197 which explicitly models the clustering of defects by introducing a defect clustering factor. The defect clustering factor is not a controllable parameter in manufacturing, but is a characteristic of the manufacturing process. The defect clustering factor for manufacturing processes can be expected to be approximately 2, in which case yield projections closely match Murphy&#39;s method. 
     A solution to the problem of low yield is to incorporate fault tolerance by including redundant functional units on the chip which are used to replace faulty functional units. 
     In memory chips and most Wafer Scale Integration (WSI) devices, the physical location of redundant sub-units on the chip is not important. However, in printing heads the redundant sub-unit may contain one or more printing actuators. These must have a fixed spatial relationship to the page being printed. To be able to print a dot in the same position as a faulty actuator, redundant actuators must not be displaced in the non-scan direction. However, faulty actuators can be replaced with redundant actuators which are displaced in the scan direction. To ensure that the redundant actuator prints the dot in the same position as the faulty actuator, the data timing to the redundant actuator can be altered to compensate for the displacement in the scan direction. 
     To allow replacement of all nozzles, there must be a complete set of spare nozzles, which results in 100% redundancy. The requirement for 100% redundancy would normally more than double the chip area, dramatically reducing the primary yield before substituting redundant units, and thus eliminating most of the advantages of fault tolerance. 
     However, with print head embodiments according to this invention, the minimum physical dimensions of the head chip are determined by the width 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 full width, full color head for printing A4 size paper is approximately 215 mm×5 mm. This size allows the inclusion of 100% redundancy without significantly increasing chip area, when using 1.5 μm CMOS fabrication technology. Therefore, a high level of fault tolerance can be included without significantly decreasing primary yield. 
     When fault tolerance is included in a device, standard yield equations cannot be used. Instead, the mechanisms and degree of fault tolerance must be specifically analyzed and included in the yield equation. FIG. 5 shows the fault tolerant sort yield 199 for a full width color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation. This graph shows projected yield as a function of both defect density and defect clustering. The yield projection shown in FIG. 5 indicates that thoroughly implemented fault tolerance can increase wafer sort yield from under 1% to more than 90% under identical manufacturing conditions. This can reduce the manufacturing cost by a factor of 100. 
     Fault tolerance is highly recommended to improve yield and reliability of print heads containing thousands of printing nozzles, and thereby make pagewidth printing heads practical. 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 following Australian patent specifications filed on 12 Apr. 1995, the disclosure of which are hereby incorporated by reference: 
     `Integrated fault tolerance in printing mechanisms` (Filing no.: PN2324); 
     `Block fault tolerance in integrated printing heads` (Filing no.: PN2325); 
     `Nozzle duplication for fault tolerance in integrated printing heads` (Filing no.: PN2326); 
     `Detection of faulty nozzles in printing heads` (Filing no.: PN2327); and 
     `Fault tolerance in high volume printing presses` (Filing no.: PN2328). 
     Printing System Embodiments 
     A schematic diagram of a digital electronic printing system using a print head of this invention is shown in FIG. 6. This shows a monolithic printing head 50 printing an image 60 composed of a multitude of ink drops onto a recording medium 51. This medium will typically be paper, but can also be overhead transparency film, cloth, or many other substantially flat surfaces which will accept ink drops. The image to be printed is provided by an image source 52, which may be any image type which can be converted into a two dimensional array of pixels. Typical image sources are image scanners, digitally stored images, images encoded in a page description language (PDL) such as Adobe Postscript, Adobe Postscript level 2, or Hewlett-Packard PCL 5, page images generated by a procedure-call based rasterizer, such as Apple QuickDraw, Apple Quickdraw GX, or Microsoft GDI, or text in an electronic form such as ASCII. This image data is 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 will typically have between 1 and 32 bits per pixel, depending upon the image source and the specifications of the printing system. The image processing system may be a raster image processor (RIP) if the source image is a page description, or may be a two dimensional image processing system if the source image is from a scanner. 
     If continuous tone images are required, then a halftoning system 54 is necessary. Suitable types of halftoning are based on dispersed dot ordered dither or error diffusion. Variations of these, commonly known as stochastic screening or frequency modulation screening are suitable. The halftoning system commonly used for offset printing--clustered dot ordered dither--is not recommended, as effective image resolution is unnecessarily wasted using this technique. The output of the halftoning system is a binary monochrome or color image at the resolution of the printing system according to the present invention. 
     The binary image is processed by a data phasing circuit 55 (which may be incorporated in a Head Control ASIC 400 as shown in FIG. 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 parallel to the heater driver circuits 57. At the correct time, the driver circuits 57 will electronically connect the corresponding heaters 58 with the voltage pulse generated by the pulse shaper circuit 61 and the voltage regulator 62. The heaters 58 heat the tip of the nozzles 59, affecting the physical characteristics of the ink. Ink drops 60 escape from the nozzles in a pattern which corresponds to the digital impulses which have been applied to the heater driver circuits. The pressure of the ink in the ink reservoir 64 is regulated by the pressure regulator 63. Selected drops of ink drops 60 are separated from the body of ink by the chosen drop separation means, and contact the recording medium 51. 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 full width of the print region of the recording medium 51, it is only necessary to move the recording medium 51 in one direction, and the print head 50 can remain fixed. If a smaller print head 50 is used, it is necessary to implement a raster scan system. This is typically achieved by scanning the print head 50 along the short dimension of the recording medium 51, while moving the recording medium 51 along its long dimension. 
     Context of the invention 
     FIG. 7 shows a print head formed in accordance with one embodiment of the invention with integrated drive circuitry. This print head has 19,840 nozzles, which are connected using eight shift registers, each of which contains 2,480 drive modules 220. For simplicity of the drawing, only eight of the 2,480 drive modules 220 in each shift register are shown. Also, only four of the eight shift registers are shown. The preferred circuit for integrated nozzle drivers on large print heads incorporates fault tolerance. This is omitted from this diagram for simplicity. 
     The clock generation module 230 generates a gated two phase clock for the shift registers. This gated two phase clock allows the elimination of the parallel registers that would otherwise be required to hold the data constant during the heater enable pulse. The two clock phases allow the use of dynamic shift registers instead of static shift registers, further reducing the number of integrated transistors required for each nozzle driver. 
     The three EnPhase signals are the input of a three line to eight line decoder 260. The Eight outputs of the decoder 260 are connected to the enable controls of the drive modules 220. As each output of the decoder 260 drives 2,480 loads distributed over the length of the head, the output transistors of the decoder must be either very large, or buffered multiple times, to obtain fast switching. 
     The inclusion of the decoder 260 reduces the number of external connections required to control which of the eight groups is activated from eight to four. 
     The head has only a small number of connections. There are: 
     1) V + , which is the positive power connection to the heaters. 
     2) V - , which is the return power (ground) connection to the heater drive transistors. 
     3) V dd , which is the positive power connection to the shift registers and data enable circuits. 
     4) V ss , which is the return power (ground) connection for the shift registers and data enable circuits. 
     5) Clock, which is the main system clock, used for clocking the shift registers. 
     6) EnPhase, which is firing phase enable selection. 
     7) Enable, which is a global enable signal. If this signal is inactive, no printing can occur. 
     8) Data&lt;0-7&gt;, which are the eight serial data input signals which control which nozzles are to be energized. 
     9) Test, which is an Or function of the data at the output of the shift registers. The eight outputs are wired to the inputs of a eight input Or gate 270. This output can be used for testing the integrity of the shift registers in the head. Only one shift register can be tested at a time. More sophisticated test circuitry can be included on the head using well known techniques. 
     As with most manufactured products, the cost of manufacture is important. If the device costs too much to manufacture, it will not succeed commercially. 
     When a large number of nozzles are integrated into a single monolithic device, the connection of the heaters to external drive circuitry becomes difficult. If no active circuitry is incorporated on the same device, the number of external connections must be at least equal to the number of heaters plus one. Should the head contain many thousands of nozzles, many thousands of external connections are required. This is clearly difficult using currently available technology. 
     A simple data transfer system uses two latches for each nozzle drive circuit. One latch is a stage of a serial shift register which is used to shift data into the printing head. The other latch is used to hold the data valid during the active period of the printing actuator. Each latch will typically require at least eight transistors, requiring a total of 16 transistors for each nozzle to implement the storage elements. This can have a significant effect on chip size and yield for heads with many thousands of nozzles. 
     Data timing and clocks 
     The table &#34;LIFT head type A4-4-600&#34; (see Appendix A) is a summary of some characteristics of one preferred full color monolithic printing head capable of printing an color A4 page at 600 dpi in one second. 
     FIG. 8(a) shows a logic representation of a drive module 220. The data is clocked into the shift register stage 217 by the signal `Clock`. The clock to the shift register stops when the corresponding enable pulse is active. This ensures that the data at the output of the shift register stage 217 is stable for the duration of the enable pulse. This data is gated with the enable pulse by the Nand gate 215. An inverting buffer 216 amplifies the signal to enable it to quickly drive the high capacitance of the gate of the drive transistor 201. When the gate of the drive transistor 201 is raised to a sufficient voltage, the transistor will turn on, providing a conduction path for current which flows the heater 200, thus energizing it. 
     FIG. 8(b) is a circuit representation of a drive module 220, as it may be implemented in an nMOS process. In this example, a dynamic shift register is used to reduce the number of transistors required for each shift register stage. The dynamic shift registers are of standard nMOS construction, being composed of two inverter stages connected by pass transistors. The dynamic shift registers require a two phase clock being Clockφ1 and Clockφ2. When the enable pulse for the nozzle group is activated, the data in the shift register is enabled by the And gate 215, buffered by the inverter 216, and used to control the drive transistor 201. When the enable pulse is active, the two clock phases Clockφ1 and Clockφ2 are disabled. Data stability is maintained by turning on the pass transistor 222 with the enable signal. This pass transistor connects the output of the And gate 215 to the gate of the second inverter in the shift register 221, creating a stable data loop. 
     This example uses only twelve transistors for each drive module. It is important that the design of the drive module is simple, as this head example requires 19,840 drive modules. If fault tolerance is included on the print head, then this number may be significantly increased. 
     FIG. 9 is a schematic of the clock generation module 230 which generates a gated two phase clock for the shift registers. The head has one of these clock generators for each nozzle group. In this example, there are 8 nozzle groups, so there are 8clock generators. The clock generators create a two phase non-overlapping clock Clockφ1 n! and Clockφ2 n! based on an input clock signal Clock and an enable signal Enable n!. The Enable n! signal is first inverted by the inverter 231 before being applied to an input of the three input Nor gates 233 and 234. The Clock is applied directly to another input of the Nor gate 233, and is inverted by the inverter 232 before being applied to an input of the Nor gate 234. The outputs of the Nor gates 233 and 234 are buffered by the inverters 235 and 236 respectively. The outputs of these inverters are again buffered by the high current inverters 237 and 238 respectively. The outputs of the inverters 237 and 238 form the Clockφ1 n! and Clockφ2 n! signals. The inverters 237 and 238 must use large transistors as they must directly drive several hundred loads. the outputs of the inverters 237 and 238 are cross-coupled to the inputs of the Nor gates 234 and 233 respectively, thus ensuring that the two clock signals Clockφ1 n! and Clockφ2 n! do not overlap. 
     FIG. 10 is a timing diagram of the output of a single clock generation module. The two non-overlapping clock signals Clockφ1 n! and Clockφ2 n! are inactive during the period of the enable pulse. In this example, the enable pulse has a duration of 355 clock cycles of a 19.7 Hz clock, for a total of 18 μs. 
     FIG. 11 is a timing diagram which relates the timing of the eight clock generation modules 230 which are included in the example head shown in FIG. 4. Each of the generated clocks Clockφ1 0-7! and Clockφ2 0-7! has repeating cycle of 2,480 clocks followed by an active low period of 355 clocks which is synchronous with its corresponding enable signal Enable 0-7!. The enable signals are offset in time so that Enable 1! starts directly after Enable 0! finishes. Once Enable 7! finishes, Enable 0! recommences. The entire cycle lasts 2,835 clock cycles before repeating. 
     By the use of this special clocking scheme, parallel transfer registers are eliminated, and the complexity of the drive circuitry on the head is significantly reduced. 
     However, the complexity of data timing to the head is increased. A cost effective compatible interface to the head can be achieved by using an ASIC which provides the data at the correct time required by the head. 
     The foregoing describes several preferred embodiments of the present invention. Modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the invention. 
     
                                           APPENDIX A                              
__________________________________________________________________________
Monolithic LIFT head type A4-4-600                                        
This is a four color print head for A4 size printing. The print head is   
fixed, and is the full width of the A4                                    
paper. Resolution is 600 dpi bi-level for medium quality output.          
                       Derivation                                         
__________________________________________________________________________
Basic specifications                                                      
Resolution     600 dpi Specification                                      
Print head length                                                         
               215 mm  Width of print area, plus 5 mm                     
Print head width                                                          
               5 mm    Derived from physical and layout constraints of    
                       head                                               
Ink colors     4       CMYK                                               
Page size      A4      Specification                                      
Print area width                                                          
               210 mm  Pixels per line/Resolution                         
Print area length                                                         
               297 mm  Total length of active printing                    
Page printing time                                                        
               1.3 seconds                                                
                       Derived from fluid dynamics, number of nozzles,    
                       etc.                                               
Pages per minute                                                          
               45 ppm  Per head, for full page size                       
Recording medium speed                                                    
               22.0 cm/sec                                                
                       1/(resolution * actuation period times phases)     
Basic IC process                                                          
               1.5 μm CMOS                                             
                       Recommendation                                     
Bitmap memory requirement                                                 
               16.6 MBytes                                                
                       Memory required when compression is not used       
Pixel spacing  42.33 μm                                                
                       Reciprocal of resolution                           
Pixels per line                                                           
               4,960   Active nozzles/Number of colors                    
Lines per page 7,015   Scan distance * resolution                         
Pixels per page                                                           
               34,794,400                                                 
                       Pixels per line * lines per page                   
Drops per page 139,177,600                                                
                       Pixels per page * simultaneous ink colors          
Average data rate                                                         
               12.3 MByte/sec                                             
                       Pixels per second * ink colors/8 MBits             
Yield and cost                                                            
Number of chips per head                                                  
               1       Recommendation                                     
Wafer size     300 mm (12&#34;)                                               
                       Recommendation for full volume production          
Chips per wafer                                                           
               36      From chip size and recommended wafer size          
Print head chip area                                                      
               10.7 cm.sup.2                                              
                       Chip width * length                                
Sort yield without fault tolerance                                        
               0.87%   Using Murphy&#39;s method, defect density = 1 per      
                       cm.sup.2                                           
Sort yield with fault tolerance                                           
               90%     See fault tolerant yield calculations (D =         
                       l/cm.sup.3, F = 2)                                 
Total yield with fault tolerance                                          
               72%     Based on mature process yield of 80%               
Functional print heads per month                                          
               260,208 Assuming 10,000 wafer starts per month             
Print head assembly cost                                                  
               $10     Estimate                                           
Factory overhead per print head                                           
               $13     Based on $120 m, cost for refurbished 1.5 μm    
                       Fab line                                           
                       amortised over 5 years, plus $16 m. P.A. operating 
                       cost                                               
Wafer cost per print head                                                 
               $23     Based on materials costs of $600 per wafer         
Approx. total print head cost                                             
               $46     Sum of print head assembly, overhead, and wafer    
                       costs                                              
Nozzle and actuation specifications                                       
Nozzle radius  14 μm                                                   
                       Specification                                      
Number of actuation phases                                                
               8       Specification                                      
Nozzles per phase                                                         
               2,480   From page width, resolution and colors             
Active nozzles per head                                                   
               19,840  Actuation phases * nozzles per phase               
Redundant nozzles per head                                                
               19,840  Same as active nozzles for 100% redundancy         
Total nozzles per head                                                    
               39,680  Active plus redundant nozzles                      
Drop rate per nozzle                                                      
               5,208 Hz                                                   
                       1/(heater active period * number of phases)        
Heater radius  14.5 μm                                                 
                       From nozzle geometry and radius                    
Heater thin film resistivity                                              
               2.3 μΩm                                           
                       For heater formed from TaAl                        
Heater resistance                                                         
               2,095 Ω                                              
                       From heater dimensions and resistivity             
Average heater pulse current                                              
               5.6 mA  From heater power and resistance                   
Heater active period                                                      
               24 μs                                                   
                       From finite element simulations                    
Settling time petween pulses                                              
               168 μs                                                  
                       Active period * (actuation phases-I)               
Clock pulses per line                                                     
               2,834   Assuming multiple clocks and no transfer register  
Clock frequency                                                           
               14.8 MHz                                                   
                       From clock pulses per line, and lines per second   
Drive transistor on resistance                                            
               42 Ω                                                 
                       From recommended device geometry                   
Average head drive voltage                                                
               12.0 V  Heater current * (heater + drive transistor        
                       resistance)                                        
Drop selection temperature                                                
               75° C.                                              
                       m.p. of surfactant sol of PIT of microemulsion     
Heater peak temperature                                                   
               120° C.                                             
                       From finite element simulations                    
Ink specifications                                                        
Basic ink carrier                                                         
               Water   Specification                                      
Surfactant     Arachidic acid                                             
                       Suggested method of achieving temperature          
                       threshold                                          
Ink drop volume                                                           
               18 pl   From finite element simulations                    
Ink density    1.030 g/cm.sup.3                                           
                       Black ink density at 60° C.                 
Ink drop mass  18.5 ng Ink drop volume * ink density                      
Ink specific heat capacity                                                
               4.2 J/Kg/°C.                                        
                       Ink carrier characteristic                         
Max. energy for self cooling                                              
               2,715 nJ/drop                                              
                       Ink drop heat capacity * temperature increase      
Ejection energy per drop                                                  
               1,587 nJ                                                   
                       Energy applied to heater in finite element         
                       simulations                                        
Energy to print full black page                                           
               221 J   Drop ejection energy * drops per page              
Total ink per color per page                                              
               0.63 ml Drops per page per color * drop volume             
Maximum ink flow rate per color                                           
               0.47 ml/sec                                                
                       Ink per color per page/page print time             
Full black ink coverage                                                   
               40.2 ml/m.sup.2                                            
                       Ink drop volume * colors * drops per square meter  
Ejection ink surface tension                                              
               38.5 mN/m                                                  
                       Surface tension required for ejection              
Ink pressure   5.5 kPa 2 * Ejection ink surface tension/nozzle radius     
Ink column height                                                         
               545 mm  Ink column height to achieve ink                   
__________________________________________________________________________
                       pressure